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Revision 2
June 2016
Sensors and Detectors
Part 1
Student Guide
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ii
Table of Contents
INTRODUCTION ..................................................................................................................... 2
TLO 1 TEMPERATURE DETECTORS ....................................................................................... 3
Overview .......................................................................................................................... 3
ELO 1.1 Temperature Detector Functions ....................................................................... 4
ELO 1.2 Resistance Temperature Detector Construction ................................................ 7
ELO 1.3 Temperature Resistance Relationship ............................................................... 8
ELO 1.4 Temperature Detection Circuits ...................................................................... 10
ELO 1.5 Environmental Effects ..................................................................................... 13
ELO 1.6 Circuit Faults ................................................................................................... 15
ELO 1.7 Alternate Temperature Detection .................................................................... 17
ELO 1.8 Thermocouples ................................................................................................ 18
TLO 1 Summary ............................................................................................................ 23
TLO 2 PRESSURE DETECTORS ............................................................................................ 24
Overview ........................................................................................................................ 24
ELO 2.1 Pressure Detector Functions ............................................................................ 25
ELO 2.2 Pressure Detector Theory and Operation ........................................................ 25
ELO 2.3 Factors Affecting Accuracy and Detector Failure Modes............................... 35
TLO 2 Summary ............................................................................................................ 40
TLO 3 LEVEL DETECTORS .................................................................................................. 42
Overview ........................................................................................................................ 42
ELO 3.1 Level Detection Functions .............................................................................. 43
ELO 3.2 Operation of Level Detectors .......................................................................... 44
ELO 3.3 Density Compensation .................................................................................... 50
ELO 3.4 Level Detection Circuits ................................................................................. 54
ELO 3.5 Environmental Effects on Level...................................................................... 55
ELO 3.6 Failure Modes.................................................................................................. 58
ELO 3.7 Detector Transients ......................................................................................... 60
TLO 3 Summary ............................................................................................................ 64
TLO 4 FLOW DETECTORS ................................................................................................... 67
Overview ........................................................................................................................ 67
ELO 4.1 Flow Meter Theory of Operations................................................................... 68
ELO 4.2 Flow Meter Construction ................................................................................ 71
ELO 4.3 Steam Flow Density Compensation ................................................................ 78
ELO 4.4 Failure Modes.................................................................................................. 82
ELO 4.5 Environmental Effects ..................................................................................... 84
TLO 4 Summary ............................................................................................................ 86
TLO 5 POSITION DETECTORS.............................................................................................. 88
Overview ........................................................................................................................ 88
ELO 5.1 Switch Type Detectors .................................................................................... 89
ELO 5.2 Variable Output Detectors............................................................................... 92
ELO 5.3 Environmental Effects ..................................................................................... 94
ELO 5.5 Failure Modes.................................................................................................. 96
TLO 5 Summary ............................................................................................................ 98
SENSORS AND DETECTORS PART 1 SUMMARY .................................................................... 99
iii
KNOWLEDGE CHECK ANSWER KEY ..................................................................................... 1
ELO 1.1 Temperature Detector Functions ...................................................................... 1
ELO 1.2 Resistance Temperature Detector Construction ............................................... 1
ELO 1.3 Temperature Resistance Relationship .............................................................. 2
ELO 1.4 Temperature Detection Circuits........................................................................ 3
ELO 1.5 Environmental Effects ...................................................................................... 3
ELO 1.6 Circuit Faults .................................................................................................... 5
ELO 1.7 Alternate Temperature Detection ..................................................................... 6
ELO 1.8 Thermocouples ................................................................................................. 7
ELO 2.1 Pressure Detector Functions ............................................................................. 8
ELO 2.2 Pressure Detector Theory ................................................................................. 9
ELO 2.3 Factors Affecting Accuracy and Detector Failure Modes .............................. 11
ELO 3.1 Level Detection Functions .............................................................................. 14
ELO 3.2 Operation of Level Detectors ......................................................................... 14
ELO 3.3 Density Compensation .................................................................................... 16
ELO 3.4 Level Detection Circuits ................................................................................. 17
ELO 3.5 Environmental Effects on Level ..................................................................... 17
ELO 3.6 Failure Modes ................................................................................................. 18
ELO 3.7 Detector Transients ......................................................................................... 20
ELO 4.1 Flow Meter Theory of Operations .................................................................. 22
ELO 4.2 Flow Meter Construction ................................................................................ 23
ELO 4.3 Steam Flow Density Compensation ............................................................... 24
ELO 4.4 Failure Modes ................................................................................................. 26
ELO 4.5 Environmental Effects .................................................................................... 29
ELO 5.1 Switch Type Detectors.................................................................................... 29
ELO 5.2 Variable Output Detectors .............................................................................. 31
ELO 5.3 Position Detector Circuits ............................................................................... 32
ELO 5.4 Failure Modes ................................................................................................. 33
iv
Sensors and Detectors Part 1
Revision History
Revision
Date
Version
Number
Purpose for Revision
Performed
By
10/31/2014
0
New Module
OGF Team
12/11/2014
1
Added signature of OGF
Working Group Chair
OGF Team
06/28/2016
2
Updated as part of PPT
Upgrade Project
OGF Team
Rev 2
1
Introduction
Proper operation of an industrial plant, such as a nuclear power generating
station, requires the measurement of many plant parameters. Operator and
automatic actions rely on accurate information provided by sensors and
detectors installed within the plant systems for controlling plant parameters.
Nuclear facility operators are required to monitor key parameters that can
affect plant operation and public safety on a regular schedule and analyze
those parameters for trends and abnormal conditions.
Sensors, detectors, and their associated circuitry measure and indicate
parameters including temperature, pressure, level, flow, position, radiation,
and reactor power level.
It is important to have an understanding of how these sensors and detectors
measure plant parameters and how they are prone to failure. Recognizing
the indications associated with failed sensors and detectors is an essential
skill for plant operators. Familiarity with instrument failure modes will
ensure proper interpretation of plant parameters during abnormal operating
events, allowing operators to take appropriate mitigating actions.
Objectives
At the completion of this training session, the trainee will demonstrate
mastery of this topic by passing a written exam with a grade of 80 percent
or higher on the following Terminal Learning Objectives (TLOs):
1. Describe the operation of temperature detectors and conditions that
effect their accuracy and reliability.
2. Describe the operation of pressure detectors and conditions that affect
their accuracy and reliability.
3. Describe the operation of level detectors and conditions that affect
their accuracy and reliability.
4. Describe the operation of flow detectors and conditions that affect
their accuracy and reliability.
5. Describe the operation of position detectors and conditions that affect
their accuracy and reliability.
Rev 2
2
TLO 1 Temperature Detectors
Overview
The hotness or coldness of a piece of plastic, wood, metal, or other material
depends upon the molecular activity of the material. Kinetic energy is a
measure of the activity of the atoms that make up the molecules of any
material. Temperature, therefore, is a measure of the average molecular
kinetic energy of any material.
Temperature detectors provide an important indication of the condition of
equipment and material. An operator uses temperature-monitoring data to
prevent equipment problems due to temperatures that are either too high or
too low. Whether attempting to determine the temperature of the
surrounding air, the temperature of coolant in a car’s engine, or the
temperature of components of an industrial facility, it is necessary to have
some means of measuring the kinetic energy of the material. Most
temperature measuring devices use the energy of the material or system
they are monitoring to raise (or lower) the kinetic energy of the device in
order to provide an indication of temperature.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. State the three basic functions of temperature detectors.
2. Describe the construction of a basic resistance temperature detector
(RTD), including:
a. Component arrangement
b. Materials used
3. Describe how RTD resistance varies for temperature changes.
4. State the purpose of basic temperature instrument detection and
control system blocks:
a. RTD
b. Bridge circuit
c. DC-AC converter
d. Amplifier
e. Balancing motor/mechanical linkage
5. Describe bridge circuit compensation for changes in ambient
temperature and environmental conditions that can affect temperature
detection instrumentation.
6. Describe the effect on temperature indication(s) for the following
circuit faults:
a. Short circuit
b. Open circuit
7. Describe alternate methods of determining temperature when the
normal sensing devices are inoperable.
8. Describe the construction and operation of a thermocouple.
Rev 2
3
ELO 1.1 Temperature Detector Functions
Introduction
Although different facility design details require monitoring varying
specific temperatures, temperature detectors usually provide the following
three basic functions in industrial applications:



Indications
Alarms
Control
Display of the monitored temperatures may be local or in a central location,
such as a control room, and may have audible and/or visual alarms that
trigger when specified preset limits are exceeded. The monitored
temperatures may have control functions tied to them so that equipment is
started or stopped to support a given temperature condition or so that a
protective action occurs.
An ordinary household thermometer is an example of a simple temperature
detector. The mercury, or other liquid, in the bulb of the thermometer
expands as heat increases its average molecular kinetic energy level. By
measuring this expansion against a scale calibrated to indicate temperature,
the temperature of the object in contact with the bulb can be determined.
Temperature is an important parameter in many industrial processes and
many types of instruments measure it.
Filled System Thermometer
A filled system thermometer is a type of temperature detector that can
provide both local and remote indication and/or a record of temperature
some distance from the point of measurement. The detector consists of a
sensing element, which is a bulb containing gas or liquid, and an indicator
scale, as shown in the figure below.
Figure: Filled System Thermometer
Rev 2
4
As the temperature surrounding the sensing bulb changes the pressure of the
fill gas or liquid inside the bulb changes. This change in pressure acts on a
receiving element (spiral bourdon tube) via capillary tubing connected to
the bulb. The spiral tube responds to the changing pressure in the sensing
bulb and produces motion that is proportional to the temperature of the
sensing bulb. This motion can drive a pointer on an indicator, a pen on a
recorder, or actuate a switch for control response (e.g. a thermostat). Filled
system thermometers are available to detect temperatures ranging from
approximately -400°F to 1,000°F, depending on the filling medium used in
the detector bulb. These types of detectors can detect temperature from
distances of up to 400 feet.
Bimetallic Strip Thermometer
A bimetallic strip thermometer is a simple, rugged device for monitoring
temperature. The temperature-measuring element is comprised of two
strips of metal that have different coefficients of thermal expansion,
fastened together throughout their length, as shown in the figure below.
One end is fixed and the other is free to move. Since the two strips of metal
act as one, they will both always be at the same temperature. If heated, the
bimetallic will bend to adapt to the increased length of the metal with the
greater temperature coefficient of expansion. Conversely, if cooled, the
bimetallic strip will bend to adapt to the decreased length of the metal with
the greater temperature coefficient of expansion.
Figure: Bimetallic Strip
Often, the thermometer is wound into a spiral-formed bimetallic element
with one end fixed. A pointer attached to the free end of the element will
rotate with temperature changes to provide temperate indication as shown in
figure below. The general range of operation for bimetallic strip
thermometers is from -200°F to 1,000°F.
Rev 2
5
Figure: Bimetallic Strip Thermometer
Knowledge Check (Answer Key)
Temperature detection is used to provide the following:
(select all that apply)
A.
Interlocks
B.
Indications
C.
Alarms
D.
Automatic trips
Knowledge Check (Answer Key)
Which of the following is not a function of a temperature
detector?
Rev 2
A.
Indication
B.
Control functions
C.
Alarm functions
D.
Amplification
6
ELO 1.2 Resistance Temperature Detector Construction
Introduction
Resistance temperature detector (RTD) circuits act somewhat like electrical
transducers, converting temperature changes to voltage changes through the
measurement of changing resistance. The RTD itself is a pure metal or
alloy that increases its resistance to electrical current flow as its temperature
increases. Conversely, the RTD will decrease its resistance to electrical
current flow as its temperature decreases.
Resistance Temperature Detectors Construction
The RTD elements are usually long, spring-like wires surrounded by an
insulator and enclosed in a sheath of metal. Therefore, the material used to
fabricate the RTD element must be drawn into fine wire so that the element
can be long, yet compactly constructed. The figure below shows the
internal construction of an RTD.
Figure: Internal Construction of a Typical RTD
The design shown in the figure has a platinum wire element surrounded by
a porcelain insulator. The insulator prevents a short circuit between the
wire and the metal sheath when an electric current is applied.
The RTD sheath is normally comprised of Inconel, a nickel-iron-chromium
alloy, because of its temperature response time and its inherent corrosion
resistance. When placed in a liquid or gas medium, the Inconel sheath
quickly reaches the temperature of the medium. The change in temperature
will cause the platinum wire to heat or cool, resulting in a proportional
change in resistance. A precision resistance-measuring device calibrated to
give the proper temperature reading then measures this change in resistance.
Rev 2
7
The figure below shows a cross-section view of a RTD protective well and
terminal head. The well protects the RTD from damage by the gas or liquid
measured by the RTD. Protecting wells are normally made of stainless
steel, carbon steel, Inconel, or cast iron, and protect the RTD from
temperatures up to 1,100°C.
Figure: RTD Protective Well and Terminal Head
Knowledge Check (Answer Key)
A resistance temperature detector operates on the
principle that the change in electrical __________ of a
metal is ________ proportional to its change in
temperature.
A.
conductivity; directly
B.
conductivity; indirectly
C.
resistance; indirectly
D.
resistance; directly
ELO 1.3 Temperature Resistance Relationship
Introduction
The resistance to electrical current flow (resistivity) of certain metals will
change as temperature changes. Some of these metals exhibit a linear
coefficient of resistivity (change in resistance) as temperature changes.
This characteristic is the basis for the operation of RTD equipment. An
RTD operates on the principle that the change in electrical resistance of a
metal is directly proportional to its change in temperature.
Rev 2
8
Temperature vs. Resistance
The metals that are best suited for RTD sensors are pure, uniform in quality,
stable within a given range of temperature, and able to give reproducible
resistance-temperature readings. Only a few metals have the properties
necessary for use in RTD elements. The figure below shows temperatureresistance graphs of three of the most commonly used metals.
Figure: Resistance vs. Temperature Graph
Platinum, copper, or nickel, typically comprise RTD elements. These
metals are best suited for RTD applications because of their linear
resistance-temperature characteristics, their high coefficient of resistance,
and their ability to withstand repeated temperature cycles. The coefficient
of resistance is the change in resistance per degree change in temperature,
usually expressed as a percentage per degree of temperature. Additionally,
the material used to fabricate the RTD element must be drawn into a fine
wire so that the element can be long and compactly constructed. RTD
elements are usually long, spring-like wires surrounded by an insulator and
enclosed in a sheath of metal.
Rev 2
9
Knowledge Check (Answer Key)
A resistance temperature detector operates on the
principle that the change in electrical resistance of...
A.
a metal is inversely proportional to its change in
temperature.
B.
two dissimilar metals is inversely proportional to the
temperature change measured at their junction.
C.
two dissimilar metals is directly proportional to the
temperature change measured at their junction.
D.
a metal is directly proportional to its change in
temperature.
Knowledge Check (Answer Key)
What happens to the resistance of a resistance
temperature detector (RTD) when the temperature of the
substance it is measuring increases?
A.
Resistance of the RTD decreases and then increases.
B.
Resistance of the RTD decreases.
C.
Resistance of the RTD increases.
D.
Resistance of the RTD remains the same.
ELO 1.4 Temperature Detection Circuits
Introduction
A temperature detection circuit consists of components with specific
functions to detect temperature changes and condition the signal so that it is
in a readable form for operators to monitor or for control circuits to
interpret. Each component is necessary for the temperature monitoring
circuit because of the range and environmental conditions in which the
system is required to function.
Temperature Detection Circuits
Bridge Circuit
A bridge circuit is used with RTD elements to obtain accurate temperature
measurements. A bridge circuit consists of three known resistances (R1, R2
and R3), one unknown variable resister (Rx); a voltage source, and a
sensitive voltmeter.
Rev 2
10
Figure: Typical Bridge Circuit
R1 and R2 form the ratio arms of the bridge and R3, the standard arm, is a
variable resister adjusted to match the unknown resistance.
Unbalanced Bridge Circuit
An unbalance bridge circuit uses a millivolt meter calibrated in units of
temperature that correspond to RTD resistance. (See figure below.) A
battery connects to two opposite points of the bridge circuit, while a
millivolt meter connects to the other two opposite points. A rheostat
balances the bridge circuit, while regulated current divides between two
branches. One branch consists of Rx and R1, while the other consists of the
RTD and resister R2.
Figure: Unbalanced Bridge Circuit
Rev 2
11
Balanced Bridge Circuit
A balanced bridge circuit uses a galvanometer to compare RTD resistance
to a fixed resister. (See figure below.) The galvanometer pointer deflects to
either side of zero when the resistance arms are not equal. The slidewire
resister balances the arms of the bridge, such that no current will flow when
the circuit is balanced. The resistance of the slidewire adjusts until the
galvanometer indicates zero, the value of the slidewire determines
temperature of the monitored system. As temperature changes, there is a
new value of resistance developed to balance the circuit.
Figure: Balanced Bridge Circuit
The figure below is a block diagram of a typical temperature detection
circuit. This represents a balanced bridge temperature detection circuit
modified to eliminate the galvanometer.
Figure: Basic Temperature Detection Circuit
Rev 2
12
The temperature measuring steps are as follows:





The resistance temperature detector (RTD) reacts to the temperature.
The detector reaction modifies resistance to the bridge network.
The bridge network converts this resistance to a DC voltage signal.
The DC-AC converter is an electronic instrument that converts the
DC voltage of the potentiometer, or the bridge, to an AC voltage.
An amplifier increases the AC voltage to a higher (usable) voltage
that is used to drive a bi-directional balancing motor.
The bi-directional balancing motor positions the slider on the
slidewire to balance the circuit resistance.
Knowledge Check (Answer Key)
Typical temperature bridge circuits use low voltage
(millivolt) signals. How does this low voltage drive a
remote meter indication?
A.
The signal is amplified, which raises the voltage.
B.
The signal is converted from AC to DC, which raises the
voltage.
C.
The signal is amplified, which lowers the voltage.
D.
The signal is converted from DC to AC, which raises the
voltage.
ELO 1.5 Environmental Effects
Introduction
Resistance temperature circuits measure the resistance of a metal in a
process, and correlate the measured resistance changes to temperature.
These circuits operate at very low voltages (millivolt) and amperage
(milliamp). At these very low voltages and currents, it is necessary to
consider environmental effects on the circuit itself because ambient
temperature and humidity changes affect the circuit's resistance. These
changes can affect the circuit output signal and give a false indication of
temperature; therefore, the circuitry design includes compensation features.
Rev 2
13
Ambient Temperature
Ambient temperature variations will affect the accuracy and reliability of
temperature detection instrumentation. Variations in ambient temperature
can directly affect the resistance of components in a bridge circuit and the
resistance of the reference junction for a thermocouple. In addition,
ambient temperature variations can affect the calibration of
electric/electronic equipment. Circuitry design and maintaining the
temperature detection instrumentation in the proper environment will
reduce the effects of temperature variations.
Humidity
The presence of ambient humidity will also affect most electrical
equipment, especially electronic equipment. High humidity causes
moisture to collect on the equipment. This moisture can cause short
circuits, grounds, and corrosion, which, in turn, may damage components.
Maintaining electronic equipment in the proper environment will control the
detrimental effects of humidity. The proper use of heating, ventilation, and
air conditioning equipment controls humidity in plant electrical equipment.
Design Compensation
Proper electronic circuitry design will compensate for ambient temperature
changes in the equipment cabinet. It is also possible for the resistance of
the detector leads to change due to a change in ambient temperature. To
compensate for these ambient temperature changes, three and four wire
RTD circuits are used. In this way, both branches of the bridge circuit use
the same amount of lead wire, and both branches will feel a change in
resistance, thus negating the effects of the change in ambient temperature.
Knowledge Check (Answer Key)
To compensate for ambient temperature change, both
three and four wire resistance temperature detector
circuits use the same amount of lead wire in both
branches of the bridge circuit because...
Rev 2
A.
the change in resistance will be felt on neither branch.
B.
the change in resistance is not an important factor in
temperature measurement.
C.
the change in resistance will be felt on both branches.
D.
the change in resistance is important only when
calibrating temperature circuits.
14
Knowledge Check – NRC Bank (Answer Key)
A simple two-wire resistance temperature detector
(RTD) is being used to measure the temperature of a
water system. Copper extension wires run from the RTD
to a temperature instrument 40 feet away.
If the temperature of the extension wires decreases, the
electrical resistance of the extension wires will
__________; and the temperature indication will
__________ unless temperature compensation is
provided.
A.
increase; increase
B.
increase; decrease
C.
decrease; increase
D.
decrease; decrease
ELO 1.6 Circuit Faults
Introduction
Electrical faults affect the indication because RTD circuits actually measure
the changes in electrical circuit performance. Short circuits and open
circuits are two electrical faults that can result in faulty indication. In a
short circuit, the short diverts the signal, precluding a complete circuit; in an
open circuit, the open halts the signal, also precluding a complete circuit.
Circuit Fault
In an RTD:


Rev 2
If either an unbalanced or balanced bridge circuit becomes open, the
resistance will be infinite, and the temperature-indicating meter will
indicate a very high temperature.
If there is a short circuit, resistance will be zero, and the temperatureindicating meter will indicate a very low temperature.
15
Knowledge Check
Consider the circuit below, what would the meter read if
the lead between Y and the resistance temperature
detector developed an open circuit?
A.
300°
B.
600°
C.
0°
D.
Dependent on measured temperature
Knowledge Check – NRC Bank (Answer Key)
If shorting occurs within a resistance temperature
detector, the associated indication will fail...
Rev 2
A.
low.
B.
high.
C.
as is.
D.
to midscale.
16
ELO 1.7 Alternate Temperature Detection
Introduction
In the event that primary temperature sensing instruments become
inoperative, several alternate methods may be used to obtain temperature
indications. Some methods use the temperature detection circuit even
though there may be a failure within the circuit.
Alternate Temperature Detection
The design of the circuit, the nature of the circuit or detector failure, and the
components that remain functional will determine the viable alternate
method of temperature indication.



Rev 2
Some temperature detecting circuit applications utilize installed spare
temperature detectors or dual-element RTD's. The dual-element
RTD has two sensing elements, only one of which is normally
connected. If the operating element becomes faulty, connect the
second element to provide temperature indication.
If there is no installed spare, use a contact pyrometer (portable
thermocouple) or an optical pyrometer to obtain temperature readings
on those pieces of equipment or systems that are accessible.
If the malfunction is in the circuitry and the detector itself is still
functional, it may be possible to obtain temperatures by connecting
an external bridge circuit to the detector. Record resistance readings
and obtain a corresponding temperature from the detector calibration
curves.
17
Knowledge Check (Answer Key)
In the circuit below, a dual-element resistance
temperature detector (RTD) indicates temperature. If the
RTD develops an internal open circuit (bridge circuit
remains intact), temperature indication could be obtained
by…
A.
connecting a spare RTD into the circuit.
B.
doing nothing, the existing circuit will still measure
temperature with an open circuit.
C.
direct resistance measurements.
D.
surface resistor.
ELO 1.8 Thermocouples
Introduction
A thermocouple is a device that converts thermal energy into electrical
energy. The thermocouple operates on the principle that when two
dissimilar metals form two junctions at different temperatures, they produce
a measurable voltage. Because of their construction, thermocouples are
capable of measuring temperatures in much harsher environments than
RTDs, but are not as accurate as the RTD. High-temperature applications
often use thermocouples, and thermocouples often serve as a backup means
of measuring temperature when other temperature detection methods fail.
Rev 2
18
Thermocouples
A thermocouple is comprised of two dissimilar metal wires joined together
at one end (the measuring junction). When the other end of each wire
connects to a measuring instrument (the reference junction), the
thermocouple becomes a sensitive and accurate temperature-measuring
device.
Voltage produced across the reference junction, based on the temperature at
the measuring (sensing) junction, is in the millivolt range. A meter
connected across the reference junction measures voltage, which is
proportional to temperature. The figure below shows an example of a
simple thermocouple.
Figure: Simple Thermocouple Circuit
Several different combinations of materials may comprise thermocouples.
The most important factor when selecting a pair of materials is the
"thermoelectric difference" between the two materials. A higher
thermoelectric difference between the two materials will result in better
thermocouple performance. Thermocouples often use platinum as one of
the paired materials; a combination of another material paired with platinum
serves as the performance standard when evaluating other possible
thermocouple materials.
The figure on the next page shows the internal construction of a typical
thermocouple. A rigid metal sheath encases the leads of the thermocouple.
The bottom of the thermocouple housing normally contains the measuring
junction. Magnesium oxide surrounds the thermocouple wires to prevent
vibration that could damage the fine wires and to enhance heat transfer
between the measuring junction and the medium surrounding the
thermocouple.
Rev 2
19
Figure: Internal Construction of a Typical Thermocouple
The dissimilar thermocouple wires lead to a reference junction; a sealed
aluminum block normally protects the reference junction, and the junction
is temperature controlled. Changes in temperature at the reference junction
will affect the temperature reading. If the temperature at the reference
junction were to decrease, the indicated temperature would increase. Many
thermocouple circuits include a reference junction panel to ensure that
temperature changes away from the thermocouple-measuring junction do
not affect thermocouple temperature indication.
Thermocouple Operation Example
Thermocouples will cause an electric current to flow in the attached circuit
when subjected to changes in temperature. The amount of current produced
depends on the temperature difference between the measurement and
reference junction, the characteristics of the two metals used, and the
characteristics of the attached circuit. The figure below shows a basic
thermocouple circuit.
Figure: Simple Thermocouple Circuit
Rev 2
20
Heating the measuring junction of the thermocouple produces a voltage that
is greater than the voltage across the reference junction. The voltmeter
measures the difference between the two voltages (in millivolts); the
voltage is proportional to the difference in temperature.
𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 𝐼𝑛𝑑𝑖𝑐𝑎𝑡𝑒𝑑
∝ 𝐻𝑜𝑡 𝐽𝑢𝑛𝑐𝑡𝑖𝑜𝑛 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 − 𝐶𝑜𝑙𝑑 𝐽𝑢𝑛𝑐𝑡𝑖𝑜𝑛 𝑉𝑜𝑙𝑡𝑎𝑔𝑒
If temperature at the reference (cold) junction were to decrease, indicated
temperature would increase and vice versa. For ease of operator use, some
voltmeters are set up to read out directly in temperature through use of
electronic circuitry. Other applications provide only the millivolt readout.
In order to convert the millivolt reading to its corresponding temperature,
the operator must refer to vendor-supplied thermocouple tables. The
thermocouple manufacturer supplies these tables, and they list the specific
temperature corresponding to a series of millivolt readings.
Thermocouple Failures and Disadvantages
Thermocouples generally fail in several common modes. If a break occurs
in a wire, and there is no current flow, the device normally fails low. If a
break or open occurs in the detector, the indicated temperature fails to the
reference junction temperature.
A change in reference junction temperature causes an indication change.
The indication is proportional to the signal difference between the measured
temperature and the reference temperature; therefore, the reference junction
temperature should be controlled or accounted for. By comparison, a
thermocouple is less accurate than a resistance temperature detector.
Rev 2
21
Knowledge Check – NRC Bank (Answer Key)
Refer to the drawing of a simple thermocouple circuit
below.
A thermocouple temperature indication is initially 410°F
with the reference (cold) junction at 125°F. An ambient
temperature decrease lowers the reference junction
temperature to 110°F, while the measuring junction
temperature remains constant. Without temperature
compensation for the reference junction, the new
thermocouple temperature indication will be...
A.
380°F.
B.
395°F.
C.
410°F.
D.
425°F.
Knowledge Check – NRC Bank (Answer Key)
An open circuit in a thermocouple detector causes the
affected temperature indication to fail...
Rev 2
A.
high.
B.
low.
C.
to reference junction temperature.
D.
as-is.
22
TLO 1 Summary
Temperature detectors uses are as follows:



Indication
Alarm functions
Control functions
An RTD operates on the principle that change in electrical resistance of a
metal is directly proportional to its change in temperature.




As temperature increases, resistance increases.
As temperature decreases, resistance decreases.
An open circuit in a temperature instrument is indicated by a very
high temperature.
A short circuit in a temperature instrument is indicated by a very low
temperature.
If a temperature detector becomes inoperative:


A spare detector may be used (if installed).
Substitute a contact or optical pyrometer for temporary use.
A thermocouple consists of two dissimilar wires joined at one end encased
in a metal sheath.




The other end of each wire connects to a meter or measuring circuit.
The measuring junction produces a voltage greater than voltage
across the reference junction.
They are less accurate than the RTD.
An open circuit in a detector is indicated by temperature failing to the
reference junction temperature.
Now that you have completed this lesson, you should be able to:
1. State the three basic functions of temperature detectors.
2. Describe the construction of a basic RTD, including:
a. Component arrangement
b. Materials used
3. Describe how RTD resistance varies for temperature changes.
4. State the purpose of basic temperature instrument detection and
control system blocks:
a. RTD
b. Bridge circuit
c. DC-AC converter
d. Amplifier
e. Balancing motor/mechanical linkage
5. Describe bridge circuit compensation for changes in ambient
temperature and environmental conditions that can affect temperature
detection instrumentation.
Rev 2
23
6. Describe the effect on temperature indication(s) for the following
circuit faults:
a. Short circuit
b. Open circuit
7. Describe alternate methods of determining temperature when the
normal sensing devices are inoperable.
8. Describe the construction and operation of a thermocouple.
TLO 2 Pressure Detectors
Overview
Pressure measurements control many processes. Gauge pressure (Pgauge,
psig) is the pressure felt by a pressure detector and equivalent to the
pressure of the system less the atmospheric pressure (Patm, psia). The
equation below expresses this relationship:
𝑃𝑔𝑎𝑢𝑔𝑒 = 𝑃𝑠𝑦𝑠𝑡𝑒𝑚 − 𝑃𝑎𝑡𝑚
Knowing this relationship is important to understanding how pressure
changes can affect pressure gauges.
Solving Problems with Pressure
Caution
Some pressure instruments or indicators read out in psia
versus psig, so always ensure you check the detector
values and convert where necessary.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. State the three functions of pressure measuring instrumentation.
2. Describe the theory and operation of the following differential
pressure detectors:
a. Bellows
b. Diaphragm
c. Bourdon tube
d. Strain Gauge
3. Describe the factors that affect accuracy and instrumentation of
differential pressure detectors, including their failure modes.
Rev 2
24
ELO 2.1 Pressure Detector Functions
Functions for Pressure Detectors
Regardless of the pressures monitored (they vary slightly depending on the
details of facility design), all pressure detectors provide up to the following
three basic functions:



Indication
Alarm
Control
Display of monitored pressures may be local, or in a central location, such
as a control room, and may have audible and/or visual alarms associated
with them when specified preset limits are exceeded. These pressures may
have control functions associated with them so that equipment is started or
stopped to support a given pressure condition or so that a certain protective
action occurs. Since a fluid system may operate at both saturation and subcooled conditions, accurate pressure indication must be available to
maintain proper system parameters. Some pressure detectors have audible
and visual alarms associated with them to alert an operator when specified
preset limits are exceeded. Some pressure detector applications provide
inputs for protective features and control functions.
Knowledge Check (Answer Key)
Pressure detectors provide the following: (select all that
apply)
A.
Indications
B.
Automatic trips
C.
Interlocks
D.
Alarms
ELO 2.2 Pressure Detector Theory and Operation
Introduction
Pressure detectors are devices that convert changes in pressure energy to
physical movement that can change the characteristics of a circuit. The
circuit develops signals proportional to the pressure and/or pressure changes
that can provide indication, alarms, or control of a process.
Rev 2
25
Pressure detection is accomplished by connecting a bellows, diaphragm, or
bourdon tube device to a system so that system pressure is exerted on the
inside of the device while the external surface of the device is exposed to
atmospheric pressure. Pressure detection devices actually measure
differential pressure between a system and atmospheric pressure. The
device will respond to a difference in pressure across the internal to external
boundary. The pressure difference produces movement of the bellows,
diaphragm, or bourdon tube. This movement is directly proportional to the
differential pressure change. The pressure detector converts the movement
to an electrical signal or mechanical movement of an indicator proportional
to the pressure change.
Bellows
The need for a pressure-sensing element that is extremely sensitive to low
pressures and provides power for activating recording and indicating
mechanisms resulted in the development of the metallic bellows pressuresensing element. The metallic bellows is most accurate when measuring
pressures from 0.5 psig to 75 psig. However, when used in conjunction
with a heavy range spring, some bellows can measure pressures of over
1,000 psig.
The figure below shows a basic metallic bellows pressure-sensing element.
The bellows is a one-piece, collapsible, seamless metallic unit that has deep
folds formed from very thin-walled tubing. The diameter of the bellows
ranges from 0.5 inch to 12 inches, and may have as many as 24 folds.
System pressure acts on the area surrounding the bellows. The pressure acts
upon the moveable wall and as the inlet pressure to the instrument varies,
the bellows will expand or contract. The moving end of the bellows
connects to a mechanical linkage assembly. As the bellows and linkage
assembly moves, the linkage provides a direct pressure indication, as shown
in the figure below. Though not shown, movement of the bellows and
linkage can alternately provide an electrical signal.
Figure: Basic Metallic Bellows
Rev 2
26
The flexibility of a metallic bellows is similar in character to that of a
helical, coiled compression spring. Up to the elastic limit of the bellows,
the relationship between changes in load and deflection is linear. However,
this relationship exists only when the bellows is under compression. It is
necessary to construct the bellows such that all of the travel occurs on the
compression side of the point of equilibrium. A spring must always oppose
the bellows, and the deflection characteristics will be the difference in the
forces of the spring and bellows.
Knowledge Check (Answer Key)
A bellows pressure transmitter with its low-pressure side
vented to containment atmosphere measures reactor
coolant system (RCS) pressure. A decrease in the
associated pressure indication could be caused by either a
containment pressure ____________ or an RCS pressure
____________.
A.
decrease; increase
B.
increase; decrease
C.
decrease; decrease
D.
increase; increase
Diaphragm Detector
Diaphragm detectors are used in low-pressure applications. The high
pressure side is connected to system pressure (sensing pressure) and the low
pressure side is connected to a reference pressure (e.g. atmospheric
pressure). Diaphragm detectors are available in two types – metallic and
non-metallic. Corrugated designs provide additional strength and
sensitivity.
The figure on the next page shows a diagram of a diaphragm pressure
gauge. When a force acts against a thin stretched diaphragm, it causes a
deflection of the diaphragm with its center deflecting the most. This results
in an output from the detector.
Rev 2
27
Figure: Diaphragm Pressure Gauge
Bourdon Tube-Type Detectors
The bourdon tube consists of a thin-walled tube, partially flattened to a
cross-sectional area elliptical in shape, having two long flat sides and two
short round sides. The manufacturing process bends the tube lengthwise
into an arc of a circle ranging from 270 to 300 degrees. The figure below
shows basic parts of a Bourdon tube detector.
Figure: Bourdon Tube Detector Construction
Rev 2
28
There are many different bourdon tube designs for different applications,
but all operate in the same manner. Pressure applied to the inside of the
tube causes distention of the flat sections and tends to restore its original
round cross-section. This change in cross-section causes the tube to
straighten slightly. Additionally, greater force acts on the outer radius of
the tube due to its larger surface area. Atmospheric pressure opposes the
tube expansion.
With one end of the tube fixed in place, the other end of the tube traces a
curve that is the result of the change in angular position with respect to the
center. Within limits, the movement of the tip of the tube can position a
pointer or develop an equivalent electrical signal to indicate the value of the
applied internal pressure. When pressure is removed from the tube, it tends
to coil and return to its original shape. The spring action of the metal and
atmospheric pressure overcomes the force on the inside of the tube.
The normal distance of travel for the tip of the tube, depending on
application, is approximately 1/4 inch to 3/8 inch. A series of gears
translates and amplifies this small amount of tip movement, causing the
indicator needle to rotate, moving the indicator needle (pointer) across the
scale. Calibration of the scale on the gauge face of the detector allows the
gauge to accurately indicate pressure based on tip movement.
Changes in atmospheric pressure will affect the indication from a bourdon
tube detector. Consider atmospheric pressure that acts on the outside of the
bourdon tube. If that pressure was to change significantly, it would change
the indicated pressure output of the detector.
For example, a bourdon tube pressure detector measuring system pressure
in an isolated room in which the pressure rises rapidly due to an event such
as a steam line rupture would indicate lower than actual system pressure. In
this case, a 10-psi increase in atmospheric pressure (14.7 psi to 24.7 psi)
acting inside the room would result in a 10-psig decrease in indicated
system pressure. Note that under normal operating conditions, large
pressure changes do not occur and small deviations in atmospheric pressure
have a negligible effect on the output of a bourdon tube detector.
Knowledge Check (Answer Key)
If the pressure sensed by a bourdon tube increases, the
curvature (amount of curve) of the detector will
____________ because the greater force is being applied
to the ____________ curve of the detector.
Rev 2
A.
increase; outer
C.
increase; inner
B.
decrease; outer
D.
decrease; inner
29
Strain Gauge
A strain gauge measures the external force (pressure) applied to a fine wire.
The fine wire, in an accordion pattern, forms a grid on a flexible backing, as
shown in the figure below. The pressure change causes movement in the
flexible backing, and a resistance change due to the distortion (shortening or
lengthening) of the wire. Measuring the change in resistance of the wire
grid will yield the change in pressure.
Figure: Strain Gauge
As the wire grid stretches by elastic deformation, its length increases, and
its cross-sectional area decreases. These changes cause an increase in the
resistance of the strain gauge wire. This change in resistance is the variable
resistance in a bridge circuit that provides an electrical signal for indication
of pressure. The figure below shows a common strain gauge application.
Figure: Strain Gauge Pressure Transducer
In the figure above, an increase in pressure at the inlet of the bellows causes
the bellows to expand. The expansion of the bellows moves a flexible beam
with an attached strain gauge. As the beam deflects, the resistance of the
strain gauge changes. The temperature-compensating gauge compensates
for the heat produced by current flowing through the fine wire of the strain
gauge.
Rev 2
30
Strain Gauge Example
Strain gauges act as resistors in bridge circuits, as shown in figure below.
Figure: Strain Gauge Used in a Bridge Circuit
An exciter provides alternating current, replacing the battery and
eliminating the need for a galvanometer. When a change in resistance in the
strain gauge causes an unbalanced condition, an error signal enters the
amplifier and actuates the balancing motor. The balancing motor moves the
slider along the slidewire, restoring the bridge to a balanced condition. The
slider indicates pressure on a scale marked in units of pressure.
Strain gauges often monitor pressure in transmitters for reactor coolant
pressure instruments.
Knowledge Check – NRC Bank (Answer Key)
Semiconductor strain gages are often used in transmitters
for...
Rev 2
A.
control rod position instruments.
B.
reactor coolant pressure instruments.
C.
reactor coolant temperature instruments.
D.
steam generator level instruments.
31
Pressure Detection Circuitry
Introduction
A pressure transducer is comprised of pressure detectors joined to an
electrical device. Transducers produce a change in resistance, inductance,
or capacitance in order to produce a signal representative of pressure.
Sensing Element
Various types of sensing elements have just been discussed. These
elements sense the pressure of a monitored system, convert the pressure to a
mechanical signal and then supply the mechanical signal to a transducer
Resistance Type Transducers
Some resistance-type transducers combine a bellows or a bourdon tube with
a variable resistor. As pressure changes, the bellows will either expand or
contract. This expansion and contraction causes the attached slider to move
along the slidewire, increasing or decreasing the resistance, and thereby
indicating an increase or decrease in pressure. The figure below shows an
example of a slidewire resistance transducer.
Figure: Slidewire Resistance Type Transducer
Rev 2
32
Inductance Type Transducers
The inductance-type transducer consists of the following three parts:



Coil
Moveable magnetic core
Pressure-sensing element
The pressure-sensing element attaches to the magnetic core, and, as pressure
varies, the element causes the core to move inside the coil. An AC voltage
acts on the coil, and, as the core moves, the inductance of the coil changes.
The current through the coil will increase as the inductance decreases. For
increased sensitivity, designs use a coil separated into two coils by utilizing
a center tap. As the core moves within the coils, the inductance of one coil
will increase, while the inductance of the other will decrease. The figure
below shows an example of an inductance type transducer.
Figure: Inductance Type Transducer
Differential Transformer Type Transducers
A differential transformer pressure transducer is another type of inductance
transducer. The differential transformer pressure transducer uses two coils
wound on a single tube. The primary coil winds around the center of the
tube. The secondary coil splits, with one-half wound around each end of
the tube. Each end winds in the opposite direction, which causes the
induced voltages to oppose one another. A core, positioned by a pressure
element, is able to move within the tube. When the core is in the lower
position, the lower half of the secondary coil provides the output. When the
core is in the upper position, the upper half of the secondary coil provides
the output. The magnitude and direction of the output depends on the
amount the core has moved from its center position. When the core is in the
mid-position, there is no secondary output. The figure on the next page
shows a cross-section of a differential transformer type transducer.
Rev 2
33
Figure: Differential Transformer
Variable Capacitive-Type Transducers
Variable capacitive-type transducers consist of two flexible conductive
plates and a dielectric. In this case, the transducer measures the pressure of
the dielectric fluid. As pressure increases, the flexible conductive plates
(C1, C2) will move farther apart, changing the capacitance of the transducer.
This change in capacitance is measurable and is proportional to the change
in pressure. The figure below shows a cross-section of a variable
capacitive-type transducer.
Figure: Variable Capacitive Type Transducer
Rev 2
34
Detector Circuitry
The detector circuitry amplifies and/or transmits the signal to the pressure
indicator. The electrical signal generated by the detection circuitry is
proportional to system pressure. The exact operation of detector circuitry
depends upon the type of transducer used.
Pressure Indicator
The pressure indicator provides remote indication of the measured pressure.
Display of pressure may be local or at a remote location, depending on the
application of the detector. Some applications use both a local and a remote
indication.
Knowledge Check (Answer Key)
In a typical pressure detection circuit, the __________
senses the pressure of the monitored system and converts
the pressure to a mechanical signal.
A.
pressure indicator
B.
transducer
C.
slidewire
D.
sensing element
ELO 2.3 Factors Affecting Accuracy and Detector Failure Modes
Introduction
Pressure detection circuits sense small changes in process pressure by
directly measuring the difference in pressure of a process system compared
to atmospheric. These circuits operate at very low voltages (millivolt) and
amperage (milliamp). At these low voltages and currents, it is important to
consider environmental effects on the circuit itself because temperature and
humidity effects change the circuit resistance. These changes can modify
the circuit output signal and give a false indication of pressure.
Rev 2
35
Ambient Pressure
Pressure instruments are sensitive to variations in the atmospheric pressure
surrounding the detector. This is especially apparent when the detector is
located within an enclosed space. Variations in the pressure surrounding
the detector will cause the indicated pressure from the detector to change
when there may not have been an actual pressure change at the detector.
Pressure variations surrounding the detector will greatly reduce the
accuracy of the pressure instrument; minimizing these variations when
installing and maintaining these instruments will improve their accuracy.
Ambient Temperature
Ambient temperature variations will affect the accuracy and reliability of
pressure detection instrumentation. Variations in ambient temperature can
directly affect the resistance of components in the instrumentation circuitry,
and, therefore, affect the calibration of electric/electronic equipment.
Proper circuitry design and maintaining the pressure detection
instrumentation in the proper environment will reduce the effects of
temperature variations.
Humidity
Humidity will also affect most electrical equipment, especially electronic
equipment. High humidity causes moisture to collect on the equipment.
This moisture can cause short circuits, grounds, and corrosion, which, in
turn, may damage components. Maintaining electronic equipment in the
proper environment controls the effects due to humidity.
Penetrating Radiation
Radiation levels can affect detector reliability. Extremely high radiation
environments can permanently embrittle the metal in the detectors resulting
in changes to the characteristics and elasticity of sensing mechanisms which
introduce errors. High radiation levels can also affect the sensitive
electronic circuits housed in detectors.
Detector Failure or Over-Ranging
Pressure instruments are designed and selected to withstand pressure that is
above the normal design pressure for the application. However, sudden
over pressure events can cause over-range conditions that could straighten
bourdon tubes or weaken the bellows spring. If the sensing element of a
detector is stretched or stressed, pressure indications may be erroneously
high. If a sensing element has a leak or rupture, the indication will fail low.

0 – if calibrated to psig

14.7 – if calibrated to psia
Rev 2
36
Example
Consider a typical pressurized water reactor, depicted below, that
experiences a steam leak in the primary containment. As containment
pressure rises, pressure sensors located inside the containment will feel the
effects of the changing atmospheric pressure. The rise in atmospheric
pressure will reduce the difference in pressure between the primary system
and atmospheric. It is necessary to reduce the resultant pressure indication
by the exact amount that the primary containment pressure increased to
yield an accurate pressure rise. Pressure transducers located outside the
containment will not feel this effect.
Figure: Typical Pressurized Water Reactor
Knowledge Check (Answer Key)
A pressure-sensing element located inside a primary
containment will be subject to which of the following
environmental effects during a steam leak inside
containment? Select all that apply.
Rev 2
A.
Humidity
B.
Atmospheric pressure
C.
Temperature
D.
Alpha radiation
37
Alternate Pressure Detection
In the event that primary pressure sensing instruments become inoperative,
there are alternate methods to obtain pressure indications. Some methods
use the detection circuit even though there may be a failure within the
circuit.
Pressure Detector Failure
 If a pressure instrument fails, use spare detector elements, if
installed. If there are no spare detectors installed, read the pressure
with an independent local mechanical gauge, if available, or install a
precision pressure gauge (Heise gauge, for example) in the system at
a convenient point.
 If the detector is functional, it may be possible to obtain pressure
readings by measuring voltage or current values across the detector
leads and comparing this reading with calibration curves.
 Pressure instruments include a safety factor above normal design
pressure. However, sudden overpressurization causing over-range
conditions could permanently straighten bourdon tubes and bellows,
damaging the sensing element. If overpressurization stretches or
stresses the sensing element beyond its design operating range, the
indications may be erroneously high.
 If the sensing element has a leak or rupture, the instrument would fail
with a low indication.
Rev 2
38
Knowledge Check - NRC Bank (Answer Key)
Refer to the drawing of a bellows-type differential
pressure (D/P) detector below. The spring in this
detector (shown in a compressed state) has weakened
from long-term use. If the actual D/P is constant, how
will indicated D/P respond as the spring weakens?
A.
Increase, because the spring will expand more.
B.
Decrease, because the spring will expand more.
C.
Increase, because the spring will compress more.
D.
Decrease, because the spring will compress more.
Knowledge Check (Answer Key)
If a bourdon-tube pressure detector is over-ranged
sufficiently to permanently distort the bourdon tube,
subsequent pressure measurement will be inaccurate
because the ____________ of the detector tube will be
inaccurate.
Rev 2
A.
change in the volume
B.
change in the length
C.
expansion of the cross-sectional area
D.
distance moved by the tip
39
Knowledge Check (Answer Key)
A cooling water system pressure detector uses a bourdon
tube as the sensing element. Which one of the following
explains how the indicated system pressure will be
affected if a local steam leak raises the temperature of the
bourdon tube by 50°F? (Assume the cooling water
system pressure does not change.)
A.
Indicated pressure will decrease because the bourdon
tube will become more flexible.
B.
Indicated pressure will increase because the bourdon tube
will become more flexible.
C.
Indicated pressure will decrease because the bourdon
tube internal pressure will increase.
D.
Indicated pressure will increase because the bourdon tube
internal pressure will increase.
TLO 2 Summary
Pressure detector basic functions are as follows:



Indication
Alarm
Control
In a bellows-type detector:


System pressure acts on the external area surrounding a bellows.
As pressure changes, the bellows and linkage assembly move and
cause production of an electrical signal or movement of a gauge
pointer.
In a bourdon tube-type detector:


System pressure acts on the inside of a slightly flattened, arc-shaped
tube.
Pressure increases tend to restore the tube to its original round crosssection, causing the tube to straighten.
Operation of strain gauge:



Rev 2
The operation of a strain gauge measures pressure applied to a fine
wire, usually arranged in the form of a grid.
A pressure change causes a resistance change due to distortion of the
wire grid.
These are often used in transmitters for reactor coolant pressure
instruments.
40
Slidewire pressure transducer operation:


Operation consists of a bellows or a bourdon tube with a variable
resistor.
Expansion or contraction of bellows causes attached slider to move
along the slidewire, increasing or decreasing the resistance, thereby
indicating an increase or decrease in pressure.
Inductance-type pressure transducer operation:




Inductance-type pressure transducer operation consists of the
following three parts: a coil, a movable magnetic core, and a
pressure-sensing element.
The sensing element and magnetic core are tied together; as pressure
varies, the element and the core move inside the coil.
An AC voltage acts on the coil; as the core moves, the inductance of
the coil changes.
The current through the coil will increase as the inductance
decreases.
Differential transformer pressure transducer operation:


This operation utilizes two coils wound on a single tube.
 The primary coil winds around the center of the tube; the
secondary coil splits, with one-half wound around each end of
the tube.
 Each end winds in the opposite direction, which causes the
induced voltages to oppose one another.
 A core, positioned by a pressure element, is movable within the
tube.
The magnitude and direction of the output depends on the amount the
core moves from its center position.
Capacitive-type transducer operation:



The transducer consists of two flexible conductive plates with a
dielectric separating them.
As pressure increases, the flexible conductive plates will move
farther apart, changing the capacitance of the transducer.
This change in capacitance is measurable and is proportional to the
change in pressure.
Pressure instrument failure:




Rev 2
A spare detector element may be utilized if installed.
Pressure may be read at an independent local mechanical gauge.
A precision pressure gauge may be installed in the system.
If the detector is functional, it may be possible to obtain pressure
readings by measuring voltage or current values across the detector
leads and comparing this reading with calibration curves.
41
Now that you have completed this lesson, you should be able to do the
following:
1. State the three functions of pressure measuring instrumentation.
2. Describe the theory and operation of the following differential
pressure detectors:
a. Bellows
b. Diaphragm
c. Bourdon tube
d. Strain Gauge
3. Describe the factors that affect accuracy and instrumentation of
differential pressure detectors, including their failure modes.
TLO 3 Level Detectors
Overview
Accurate indication of tank and other process-related vessel level is vital to
the control of any industrial process. Without accurate level indication,
tanks could overflow resulting in spills of hazardous materials or tank levels
could fall to a low level where equipment damage will result. Level
detectors provide operators with both local and remote indication of levels
associated with the process at a particular facility.
Remote level indication is necessary to provide transmittal of vital tank and
vessel level information to a central location, such as the control room,
where all level information associated with an industrial process can be
coordinated and evaluated.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. Describe the three functions for using remote level indicators.
2. Describe the operation of the following types of level
instrumentation:
a. Gauge glass
b. Magnetic bond
c. Conductivity probe
d. Differential Pressure (D/P)
3. Describe density compensation used in level detection systems, why
systems need it, and how it is accomplished.
4. State the purpose of basic differential pressure detector-type level
instrument blocks in a basic block diagram:
a. D/P transmitter
b. Amplifier
c. Indication
5. Describe the environmental conditions that can affect the accuracy
and reliability of level detection instrumentation.
6. State the various failure modes of level detection instrumentation.
Rev 2
42
7. Analyze detector installation and applications to determine the effects
of transients on level indication.
ELO 3.1 Level Detection Functions
Introduction
Although different facility designs require monitoring varying system and
process levels, all level detectors provide one or more of the following basic
functions:



Indication
Alarm
Control
Liquid level measuring devices fall into the following two groups:


Direct method
Inferred method
An example of the direct method is the dipstick in a car, which measures the
height of the oil in the oil pan. An example of the inferred method is a
pressure gauge at the bottom of a tank, which measures the hydrostatic head
pressure from the height of the liquid.
Level Detector Functions
The following are the three major reasons for using remote level indication:



It is possible to monitor and record level measurements at locations
far from the main facility.
Controlled level may be a long distance from the control room or
control station.
Measured level may be in an unsafe/hazardous area.
Knowledge Check (Answer Key)
Level detection provides the following: (select all that
apply)
Rev 2
A.
Interlocks
B.
Alarms
C.
Automatic trips
D.
Indications
43
ELO 3.2 Operation of Level Detectors
Introduction
There are various ways to detect levels in tanks, steam generators,
pressurizers, and other plant components. The system conditions determine
the best level detector. For example, in a high-pressure and hightemperature application, a D/P cell that provides remote signals may be
appropriate, while a simple gauge glass may work fine in a low-pressure
tank vented to atmosphere. Each level detector has advantages and
disadvantages and it is up to the designer to choose the appropriate detector
for a specific application.
Gauge Glass
A very simple liquid level measuring device (direct method) in a vessel is
the gauge glass. In the gauge glass device, a transparent tube is attached to
the bottom and top (top connection is not needed in a tank open to
atmosphere) of the tank that is monitored. The height of the liquid in the
tube will be equal to the height of water in the tank. The figure below
shows two possible applications of a gauge glass.
Figure: Gauge Glass
Figure (a) above shows a gauge glass used for vessels where the liquid is at
ambient temperature and pressure conditions. Figure (b) shows a gauge
glass used for vessels where the liquid is at an elevated pressure or a partial
vacuum. Notice that gauge glasses in effect form a "U" tube manometer
where the liquid seeks its own level due to the pressure of the liquid in the
vessel.
Gauge glasses made from tubular glass or plastic suffice for service up to
450 psig and 400°F. If measuring the level of a vessel at higher
temperatures and pressures, a different type of gauge glass is required. The
type of gauge glass used in these conditions has a body made of metal with
a heavy glass or quartz section for visual observation of the liquid level.
The glass section is usually flat to provide strength and safety.
Rev 2
44
Another type of gauge glass is the reflex gauge glass where one side of the
glass section is prism-shaped. The glass is flat on the outside, with molded
90-degree angles that run lengthwise (prisms) on the inside. Light rays
strike the outer surface of the glass at a 90-degree angle. The light rays
travel through the glass striking the inner side of the glass at a 45-degree
angle. The light rays refract into the chamber, or reflect back to the outer
surface of the glass, depending on the presence or absence of liquid in the
chamber. The figure below shows a front view and a cross-section of a
reflex gauge glass.
Figure: Reflex Gauge Glass
When the liquid is at an intermediate level in the gauge glass, the light rays
encounter an air-glass interface in one portion of the chamber and a waterglass interface in the other portion of the chamber. Where an air-glass
interface exists, the light rays reflect back to the outer surface of the glass
since the critical angle for light to pass from air to glass is 42 degrees. This
causes the gauge glass to appear silvery-white. In the portion of the
chamber with the water-glass interface, the light prisms refract into the
chamber. No reflection of the light back to the outer surface of the gauge
glass occurs because the critical angle for light to pass from glass to water is
62 degrees. This results in the glass appearing black, since it is possible to
see through the water to the black-painted walls of the chamber.
Rev 2
45
Magnetic Bond Level Detector
The magnetic bond method of level detection overcomes the problems of
cages and stuffing boxes. The magnetic bond mechanism consists of a
magnetic float that rises and falls with changes in level. The float travels
outside of a nonmagnetic tube, which houses an inner magnet connected to
a level indicator. When the float rises and falls, the outer magnet will
attract the inner magnet, causing the inner magnet to follow the level within
the vessel. The figure below shows the basic elements of a magnetic bond
level detector.
Figure: Magnetic Bond Level Detector
Conductivity Probe Level Detector
A conductivity probe level detection system consists of one or more level
detectors, an operating relay, and a controller. When the liquid makes
contact with any of the electrodes, an electric current will flow between the
electrode and ground. The current energizes a relay, which causes the relay
contacts to open or close depending on the state of the process involved.
The relay in turn will actuate an alarm, a pump, a control valve, or a
combination of the three. The figure below shows a typical system with
three probes: a low-level probe, a high-level probe, and a high-level alarm
probe. The system below would indicate a high level; however, the alarm
would not yet be active.
Figure: Conductivity Probe Level Detection System
Rev 2
46
Open Tank Differential Pressure Level Detector
The differential pressure (D/P) detector method of liquid level measurement
uses a D/P detector connected to the bottom of the monitored tank. The
fluid level in the tank creates a pressure (high), from which a lower
reference pressure (usually atmospheric) is subtracted. This subtraction
takes place in the D/P detector. The figure below illustrates a typical
differential pressure detector attached to an open tank.
Figure: Open Tank Differential Pressure Detector
Three basic types of D/P level detectors are used (the NRC uses a fourth typ
for testing (#3). Refer to the figure below for each.
1. Open reference (D/P Detector #2 below)
2. Open reference with loop seal (D/P Detector #3 below)
 Basically like open reference, but with less D/P
3. Dry reference (D/P Detector #4 below)
4. Wet reference (D/P Detector #1 below)
Rev 2
47

For each of the detectors, D/P = high pressure – low pressure. For
D/P detectors 2, 3, and 4:
 High pressure is the tank
 If D/P increases, indicated level will rise

Only D/P detectors 2 and 3 are affected by atmospheric pressure
changes.

For D/P detectors 1 and 4 the gas or vapor pressure cancels out since
it is sensed on both sides.
Knowledge Check (Answer Key)
A calibrated differential pressure (D/P) level detector
measures the level in a vented tank inside the auxiliary
building, shown in the figure below. If building pressure
increases with no change in temperature, the associated
level indication will...
Rev 2
A.
decrease, then increase and stabilize at the actual level.
B.
increase and stabilize above the actual level.
C.
decrease and stabilize below the actual level.
D.
remain at the actual level.
48
Knowledge Check – NRC Bank (Answer Key)
Refer to the drawing of a differential pressure (D/P) level
detection system (see figure below) for a pressurizer at
normal operating temperature and pressure. The level
detector has just been calibrated.
The high pressure side of the detector is connected to the
__________; and if the equalizing valve is opened, the
indicated pressurizer level will be __________ than the
actual level.
Rev 2
A.
condensing pot; lower
B.
condensing pot; higher
C.
pressurizer; lower
D.
pressurizer; higher
49
ELO 3.3 Density Compensation
Introduction
If a vapor with a significant density exists above the liquid in a particular
tank or vessel, the vapor adds hydrostatic pressure to the liquid surface.
Accurate level transmitter output must account for the hydrostatic pressure
added by the vapor.
Specific Volume
Specific volume equals volume per unit of mass, as shown in equation
below.
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑣𝑜𝑙𝑢𝑚𝑒(𝑣) =
𝑉𝑜𝑙𝑢𝑚𝑒
𝑀𝑎𝑠𝑠
Specific volume is the reciprocal of density as shown in equation below.
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑣𝑜𝑙𝑢𝑚𝑒(𝑣) =
1
𝑑𝑒𝑛𝑠𝑖𝑡𝑦
Specific volume is the standard unit used when working with vapors and
steam that have low density values. For applications that involve water and
steam, specific volume values are in "Saturated Steam Tables," which list
the specific volumes for water and saturated steam at different pressures and
temperatures.
Effects of Vapor Density on Level Detection
The density of steam (or vapor) above the liquid level will have an effect on
the weight of the steam or vapor bubble and the hydrostatic head pressure.
As the density of the steam or vapor increases, the weight increases and
causes an increase in hydrostatic head even though the actual level of the
tank has not changed. The larger the steam bubble, the greater the change
in hydrostatic head pressure. The figure below illustrates a vessel in which
the water is at saturated boiling conditions.
Figure: Effects of Fluid Density
Rev 2
50
A condensing pot or chamber at the top of the reference leg condenses the
steam and maintains the reference leg filled. Because steam vapor pressure
acts equally on both the low and high sides of the transmitter, there is no
effect of the steam vapor pressure at the D/P transmitter. The differential
pressure seen by the transmitter is due only to hydrostatic head pressure, as
shown in equation below.
𝐻𝑦𝑑𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐 𝐻𝑒𝑎𝑑 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒 = 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 × 𝐻𝑒𝑖𝑔ℎ𝑡
If the reference leg containing saturated water has a pressure drop below the
saturation pressure, liquid could flash to steam. A condensing pot located
in the reference leg condenses this steam. In order to enhance heat
dissipation, the reference leg is located away from the vessel and is
uninsulated so it readily gives up heat to atmosphere. This helps keep the
liquid reference level steady by minimizing flashing. The flashing action
may result in minor level indication fluctuations.
Reference Leg Temperature Considerations
When measuring the level in a pressurized tank at elevated temperatures, a
number of additional factors affect the measurements. As the temperature
of the fluid in the tank increases, the density of the fluid decreases. As the
fluid’s density decreases, the fluid expands, occupying more volume. Even
though the density is less, the mass of the fluid in the tank is the same. The
issue is that as the fluid in the tank heats and cools, the density of the fluid
changes, but the reference leg temperature and density remain relatively
constant, which causes the indicated level to remain constant.
The density of the fluid in the reference leg depends on the ambient
temperature of the room in which the leg is located; therefore, it is relatively
constant and independent of tank temperature. An accurate tank level
indication requires some means of density compensation to account for
fluid temperature changes, and therefore density. This is the problem
encountered when measuring steam generator water levels.
Compensating for Reference Leg Temperature Changes
Calibration charts are available that allow manual level corrections for
changes in level indication due to reference leg temperatures. It is possible
to account for changes in reference leg density during instrument
alignments and calibrations; however, this is not an actual method of
density compensation.
Electronic circuitry may perform density compensation. Some systems
compensate for density changes automatically through the design of the
level detection circuitry. Other applications compensate for density by
having operators manually adjust inputs to the level detection circuitry as
the affected vessel cools down and depressurizes, or heats up and
pressurizes.
Rev 2
51
Steam Generator Level Density Compensation
The figure below illustrates a typical steam generator level detection
arrangement. The D/P detector measures actual differential pressure. A
separate pressure detector measures the pressure of the saturated steam.
Since saturation pressure is proportional to saturation temperature, a
pressure signal can correct the differential pressure for liquid density. An
electronic circuit uses the pressure signal to compensate for the difference
in density between the reference leg water and the steam generator fluid.
As the saturation temperature and pressure increase, the density of the
steam generator water decreases. The level instrument should now indicate
a higher level, even though the actual D/P has not changed. The increase in
pressure feeds into the level instrument to compensate for the change in the
density of the liquid so that the level instrument will reflect the change in
actual liquid level.
Figure: Steam Generator Level Detection System
Rev 2
52
Knowledge Check (Answer Key)
Many steam generator water level instruments include a
condensing chamber in the reference leg. The purpose of
the condensing chamber is to...
A.
ensure the reference leg temperature remains close to the
temperature of the variable leg.
B.
maintain a constant water level in the reference leg
during normal operations.
C.
provide reference leg compensation for the steam
generator pressure exerted on the variable leg.
D.
prevent reference leg flashing during a rapid
depressurization of the steam generator.
Knowledge Check (Answer Key)
Refer to the drawing of a pressurizer differential pressure
(D/P) level detection system below. With the nuclear
power plant at normal operating conditions, a pressurizer
level D/P instrument that had been calibrated while the
plant was in a cold condition would indicate _________
than actual level because of a ___________ D/P sensed
by the D/P detector at normal operating conditions.
Rev 2
A.
higher; smaller
B.
lower; smaller
C.
higher; larger
D.
lower; larger
53
ELO 3.4 Level Detection Circuits
Introduction
A typical level detection circuit consists of a D/P detector, a transducer, an
amplifier, and an indicator. These components together sense the D/P in a
system and convert that signal into an electrical signal proportional to the
D/P. The electrical signal then provides indication, alarm, or control.
Level Detection Circuit
The figure below illustrates a block diagram of a typical differential
pressure detector. It consists of the following:



D/P transmitter (transducer)
Amplifier
Level indication
Figure: Differential Pressure Level Detection Circuit
The D/P transmitter consists of a diaphragm with the high-pressure (HP)
and low-pressure (LP) inputs on opposite sides. As the differential pressure
changes, the diaphragm will move. The transducer changes this mechanical
motion into an electrical signal. The electrical signal generated by the
transducer is amplified, and passed on to the level indicator for display at a
remote location. Using relays, this system provides alarms for high and low
levels. It may also provide control functions such as repositioning a valve
and protective features such as tripping a pump.
Knowledge Check (Answer Key)
Place the following components in order starting with
level sensing to output signal.
Rev 2
A.
Alarm
B.
Transducer
C.
Amplifier
D.
Bourdon tube
54
ELO 3.5 Environmental Effects on Level
Introduction
Level detection circuits sense small changes in levels by measuring the
actual pressure difference between the height of the fluid and atmospheric
pressure or a reference leg level. The conditions surrounding the reference
legs and process can affect the properties of the fluid and thereby effect the
indication. In addition, circuits operate at very low voltages (millivolt) and
amperage (milliamp). At these low voltages and currents, temperature and
humidity changes will affect the resistance in the circuit itself. These
changes can affect the circuit output signal and result in a false level
indication.
Environmental Effects
Fluid Density
When measuring the level of a fluid, the fluid density can have a large
effect on level detection instrumentation. Fluid density affects level sensing
instruments that utilize either wet or dry reference legs. In a wet reference
leg instrument, it is possible for the reference leg's fluid temperature to be
different from the vessel fluid temperature where the level is measured.
An example of this is the level detection instrumentation for a boiler steam
drum. The water in the reference leg is at a lower temperature than the
water in the steam drum. Therefore, the water in the reference leg is denser,
and level indicators must adjust the reference leg level for the density
difference to ensure the indicated steam drum level is accurate.
Ambient Temperature
Ambient temperature variations will affect the accuracy and reliability of
level detection instrumentation. Variations in ambient temperature can
directly affect the resistance of components in the instrumentation circuitry,
and therefore can affect the calibration of electric/electronic equipment.
Proper circuitry design and maintaining the level detection instrumentation
in the proper environment reduces the effects of temperature variations.
Ambient temperature will change the density of the water in the reference
leg and will affect level indication. If the ambient temperature around a wet
reference leg rises, the density of the reference leg liquid will decrease
causing the reference leg to overflow into the tank. The mass in the
reference leg will decrease and therefore the hydrostatic pressure in the
reference leg will decrease, which will cause indicated level to increase
when actual tank level has not changed.
Rev 2
55
Calibrations of level transmitters use the ambient conditions where the
transmitters will perform. Calibrations for some transmitters will use cold
or shutdown conditions, while calibrations for others will use hot or normal
operating conditions, to reflect density and pressure conditions that the
instrument will see during its operation. This is necessary because
instruments will not read correctly under conditions that differ from their
calibration conditions.
Humidity
Humidity will also affect most electrical equipment, especially electronic
equipment. High humidity causes moisture to collect on the equipment.
This moisture can cause short circuits, grounds, and corrosion, which, in
turn, may damage components. Maintaining the electrical equipment in the
proper environment controls the effects due to humidity.
Example
Refer to the drawing of two tank differential pressure (D/P) level indicators
(see figure below). A large water storage tank has two D/P level indicators
installed. Calibration of Indicator No. 1 took place at 100°F water
temperature and calibration of Indicator No. 2 took place at 200°F water
temperature. Assuming both indicators are on scale, which one will
indicate the higher level?
Figure: Tank Differential Level Detectors
The instrument calibrated at a higher temperature reflects the liquid at a
higher expansion state, or lower density. In this open tank, the expansion
will equal a high column of water pushing down on the detector. Indicator
2 will indicate a higher level at all water temperatures.
Rev 2
56
Knowledge Check (Answer Key)
Consider the level indicator for a steam generator below.
A steam leak has occurred and the temperature of the
area around the reference leg is increasing. What effect
would this have on the indicated level?
Rev 2
A.
Indicate higher than actual because resistance of the D/P
cell components is increasing.
B.
Indicate higher than actual because reference leg density
is decreasing.
C.
No effect.
D.
Indicate lower than actual because reference leg density
is increasing.
57
ELO 3.6 Failure Modes
Introduction
Level detection systems are extremely reliable for long-term operation. The
indirect level detector failure mode depends on the high-pressure and lowpressure connection setup. For most level detectors if the D/P decreases
because of the malfunction, the indicated level will also decrease.
Conversely, if D/P increases because of the malfunction, the indicated level
will increase. This is true with the exception of the wet reference leg level
detection.
Failure Modes
In the wet reference leg arrangement, the reference leg connects to the highpressure side of the D/P cell causing the opposite reaction. With the wet
reference leg connected to the high-pressure detector, a break in the variable
leg or low-pressure side will cause a low-level indication. If the break was
on the high-pressure reference leg side, then a lower D/P results and the
indicated level is higher than the true level.
However, for D/P cell problems, the set-up must be analyzed carefully to
determine the high and low-pressure sides of the detector to correctly
answer the question.
Knowledge Check (Answer Key)
The level indication for a reference leg differential
pressure (D/P) level instrument will fail low because of...
Rev 2
A.
a break on the variable leg.
B.
closing the equalizing valve in the D/P cell.
C.
the reference leg flashing to steam.
D.
a break on the reference leg.
58
Knowledge Check – NRC Bank (Answer Key)
Refer to the drawing of a steam generator (SG)
differential pressure (D/P) level detection system below.
The SG is at normal operating temperature and pressure
with accurate level indication. Which one of the
following events will result in an SG level indication that
is greater than actual level?
Rev 2
A.
The external pressure surrounding the D/P detector
increases by 2 psi.
B.
SG pressure increases by 50 psi with no change in actual
water level.
C.
Actual SG level increases by 6 inches.
D.
The temperature of the reference leg increases by 20°F.
59
ELO 3.7 Detector Transients
Introduction
Open Tank Differential Pressure Level Detector
The tank in the figure below is open to the atmosphere; it is necessary to use
only the high-pressure (HP) connection on the D/P transmitter. The lowpressure (LP) side vents to the atmosphere; the pressure differential is the
hydrostatic head, or weight, of the liquid in the tank. The maximum
detectable level for the D/P transmitter depends on the maximum height of
liquid above the transmitter. The minimum detectable level depends on the
tank height above the transmitter connection to the tank (usually close to the
bottom).
Figure: Open Tank Differential Pressure Detector
Closed Tank Dry Reference Leg Level Detector
Not all tanks or vessels are open to the atmosphere. Many are totally
enclosed to prevent vapors or steam from escaping, or to allow pressurizing
the contents of the tank. When measuring the level in a tank that is
pressurized or that can become pressurized by vapor pressure from the
liquid, both the high-pressure and low-pressure sides of the D/P transmitter
must connect as shown in the figure below.
Figure: Closed Tank Dry Reference Leg Level Detector
Rev 2
60
Dry Reference Leg
The high-pressure connection joins the tank at or below the lower range
value measured. The low-pressure side connects to a "reference leg" that is
connected at or above the upper range value to be measured. The gas or
vapor pressure in the vessel pressurizes the reference leg and no liquid
remains in the reference leg. The reference leg must stay dry so that there is
no liquid head pressure on the low-pressure side of the transmitter. The
hydrostatic head of the liquid and the gas or vapor pressure exerted on the
liquid surface both act on the high-pressure side. The gas or vapor pressure
acts equally on the low and high-pressure sides. Therefore, the output of
the D/P transmitter is directly proportional to the hydrostatic head pressure,
that is, the level in the tank.
Wet Reference Leg
Where the tank contains a condensable fluid, such as steam, a slightly
different arrangement is used, shown in the figure below. Because ambient
temperature surrounds the reference leg, the fluid vapor condenses in the
leg. Filling the reference leg with the same liquid that occupies the tank
prevents vapor condensation in the reference leg. The liquid in the
reference leg applies a hydrostatic head to the high-pressure side of the
transmitter, and the value of this level is constant as long as the reference
leg is full. If this pressure remains constant, any change in D/P is due to a
change on the low-pressure side of the transmitter.
Figure: Closed Tank Wet Reference Leg Differential Pressure Detector
The filled reference leg applies a hydrostatic pressure to the high-pressure
side of the transmitter, which is equal to the maximum detectable level.
The D/P transmitter receives equal pressure on the high and low-pressure
sides when the liquid level is at its maximum; therefore, the differential
pressure is zero. As the tank level decreases, the pressure applied to the
low-pressure side decreases also, and the differential pressure increases. As
a result, the differential pressure and the transmitter output are inversely
proportional to the tank level.
Rev 2
61
Guidelines
Consider the basic designs of level detectors. A differential pressure level
detector measures the difference in force exerted between a reference and a
variable leg across a diaphragm. When there are factors in addition to the
actual level changes that affect these force differences, it is necessary to
account for these non-level related forces to obtain the true liquid level.
When a transient acts on a differential pressure level detector, determine the
effect that the transient has on the force exerted by either the reference leg
or the variable leg. The direction and magnitude of the force change will
determine the direction and magnitude of the indication mis-match. For
most differential level detectors, a higher D/P results in a lower indicated
level and a lower D/P (the closer to equal) results in a higher indicated
level.
Loss of Reference Leg Force
Reference leg force can be lost or reduced by temperature increases, leaks
or by open or leaking equalizer valves. When this occurs, the reference leg
force decreases. When compared to the variable leg, the difference in
pressure decreases resulting in the indicated level being higher than the true
level.
Loss of Variable Leg Force
Variable leg force can be lost or reduced by temperature increases, leaks or
by open or leaking vent valves. When this occurs, the variable leg force
decreases. When compared to the reference leg, the difference in pressure
increases, resulting in the indicated level being lower than the true level.
Equalization
Equalization of a differential level detector occurs when the equalization
valve is either open or leaking. When this occurs, it is similar to losing the
reference leg force. The difference in pressure decreases, resulting in the
indicated level being higher than the true level.
Example
Refer to the following drawing of a D/P level detection system for a
pressurizer at normal operating temperature and pressure. Calibration of the
level detector took place under normal conditions. The high-pressure side
of the detector connects to the reference leg and upon opening the
equalizing valve, the indicated pressurizer level will be greater than the
actual level because the forces exerted by the reference leg and the variable
leg approach each other. This results in a minimum D/P and a maximum
indicated level.
Rev 2
62
Figure: Steam Generator Level Detector
Now consider a transient condition where the reference leg temperature
decreases. This will result in higher density of the reference leg fluid. The
force exerted on the reference leg side of the D/P detector is a result of the
height of the fluid and the density. If the density increases, the resultant
force will increase, resulting in a higher differential pressure and lower
indicated level than the true fluid level.
Knowledge Check (Answer Key)
Refer to the drawing of a differential pressure (D/P) level
detection system below for a pressurizer at normal
operating temperature and pressure. Assume that the
level detector was just calibrated. The low-pressure side
of the detector is connected to the __________; if a leak
develops on the variable leg, the indicated pressurizer
level will be ___________ than the true level.
Rev 2
A.
condensing pot; higher
B.
pressurizer; higher
C.
condensing; lower
D.
pressurizer; lower
63
Knowledge Check (Answer Key)
Refer to the drawing of a water storage tank with a
differential pressure (D/P) level detection system (see
figure). The level detector has just been calibrated.
How will the indicated level be affected if condensation
partially fills the normally dry reference leg?
A.
Indicated level will not be affected.
B.
Indicated level will be lower than actual level.
C.
Indicated level will be higher than actual level.
D.
Indicated level may be higher or lower than actual level
depending on the pressure in the upper volume of the
tank.
TLO 3 Summary
The three major reasons for utilizing remote level indication are as follows:



It may be necessary to take level measurements at locations far from
the main facility.
The level to be controlled may be a long distance from the point of
control.
The measured level may be in an unsafe/hazardous area.
Gauge glass:


Rev 2
A transparent tube is attached to the bottom and top (the top
connection is not needed in a tank open to atmosphere) of the tank
that is monitored.
The liquid height in the tube will be equal to the height of the liquid
in the tank.
64
Magnetic bond level detector:


The detector consists of a magnetic float that rises and falls with
changes in level. The float travels outside of a nonmagnetic tube,
which houses an inner magnet connected to a level indicator.
When the float rises and falls, the outer magnet will attract the inner
magnet, causing the inner magnet to follow the level within the
vessel and actuate the level indicator.
Conductivity probe:


The probe consists of one or more level detectors, an operating relay,
and a controller. When the liquid makes contact with any of the
electrodes, an electric current will flow between the electrode and
ground.
The current energizes a relay, which causes the relay contacts to open
or close depending on the state of the process involved. The relay in
turn will actuate an alarm, a pump, a control valve, or a combination
of the three.
D/P detector:


A D/P detector uses a pressure detector connected to the bottom of
the monitored tank. The difference between the higher pressure in
the tank and a lower reference pressure (usually atmospheric) yields
the tank pressure.
This pressure comparison takes place in the D/P detector.
Density compensation:

If a vapor with a significant density exists above the liquid, it is
necessary to add the vapor hydrostatic pressure to the liquid
hydrostatic pressure to obtain accurate transmitter output.
The three options for density compensation are as follows:



Electronic circuitry
Pressure detector manual input
Instrument calibration
The environmental effects on level detection are as follows:



Rev 2
Density of the fluid
Ambient temperature changes
Humidity
65
The basic block diagram of a D/P level instrument:




A D/P transmitter consists of a diaphragm with the high-pressure
(HP) and low-pressure (LP) inputs on opposite sides. As the
differential pressure changes, the diaphragm will move.
The transducer changes this mechanical motion into an electrical
signal.
An amplifier amplifies the electrical signal generated by the
transducer and sends it to the level indicator.
A level indicator displays the level indication at a remote location.
Failure mode of an indirect level detector depends on details of the HP and
LP D/P cell connections.
Now that you have completed this lesson, you should be able to do the
following:
1. Describe the three functions for using remote level indicators.
2. Describe the operation of the following types of level instrumentation:
a. Gauge glass
b. Magnetic bond
c. Conductivity probe
d. Differential Pressure (D/P)
3. Describe density compensation used in level detection systems, why
systems need it, and how it is accomplished.
4. State the purpose of basic differential pressure detector-type level
instrument blocks in a basic block diagram:
a. D/P transmitter
b. Amplifier
c. Indication
5. Describe the environmental conditions that can affect the accuracy and
reliability of level detection instrumentation.
6. State the various failure modes of level detection instrumentation.
7. Analyze detector installation and applications to determine the effects
of transients on level indication.
Rev 2
66
TLO 4 Flow Detectors
Overview
Flow measurement is an important process measurement in operating a
facility’s fluid systems. Flow measurement is necessary for efficient and
economic operation of these fluid systems. Flow detecting instruments and
circuitry (like temperature, pressure, and level detection instruments) can be
designed and configured to provide either local or remote indication and can
be used to control process parameters and provide alarm functions.
To control plant systems, an operator must determine mass flow rates
through various processes. Flow measurements provide important data that
operators use in their plant process adjustments. Flow rate is critical when
determining heat transfer rates and total power through heat balance.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. Describe the theory of operation of a basic head flow meter.
2. Describe the basic construction of the following types of head flow
detectors:
a. Orifice plates
b. Venturi tube
c. Dall flow tube
d. Flow nozzle
e. Elbow meter
f. Pitot tube
3. Describe density compensation of a steam flow instrument to
include the reason density compensation is required and the
parameters used.
4. State the typical failure modes for head flow meters including the
effects of vapor on a flow instrument.
5. Describe the environmental conditions that can affect the accuracy
and reliability of flow sensing instrumentation.
Rev 2
67
ELO 4.1 Flow Meter Theory of Operations
Introduction
Head flow meters operate on the principle that placing a restriction in a line
will cause a pressure drop from the upstream side of the restriction to the
downstream side. Head flow meters operate by quantifying the pressure
drop, and converting the drop to a flow rate. Industrial applications of head
flow meters incorporate a pneumatic or electrical transmitting system for
remote readout of flow rate. Generally, the indicating instrument extracts
the square root of the differential pressure and displays the flow rate on a
linear indicator.
Flow Meter Theory of Operation
There are two elements in a head flow meter; the primary element is the
restriction in the line, and the secondary element is the differential pressuremeasuring device. The figure below shows the basic operating
characteristics of a head flow meter.
Figure: Head Flow Instrument
Flow path restriction results in a differential pressure across the restriction.
A mercury manometer or a differential pressure detector measures this
pressure differential. From this measurement, flow rate is determined from
known physical laws. The restriction will cause a downstream increase in
fluid velocity and decrease in pressure. The volumetric flow rate remains
unchanged the same amount of fluid passes through per unit time both
upstream and downstream of the restriction. The change in fluid pressure is
proportional to the square of volumetric flow rate.
𝐷/𝑃 ∝ 𝑉̇ 2
Where:
D/P = differential pressure caused by restriction
𝑉̇ = Volumetric flow rate
Rev 2
68
To find the volumetric flow rate the following equation is used based on the
relationship between pressure and volumetric flow.
𝑉̇ = 𝐾√𝐷/𝑃
Where:
𝑉̇ = volumetric flow rate
K = flow constant for the meter
D/P = differential pressure caused by restriction
The head flow meter actually measures volumetric flow rate rather than
mass flow rate. Mass flow rate is easily calculated or computed from
volumetric flow rate by knowing or sensing temperature and/or pressure.
Temperature and pressure affect the density of the fluid and, therefore, the
mass of fluid flowing past a certain point. If the volumetric flow rate signal
compensates for changes in temperature and/or pressure, a true mass flow
rate signal results. Thermodynamics describes that temperature and density
are inversely proportional, while pressure and density are directly
proportional. To show the relationship between temperature and pressure,
one of two forms of the mass flow rate equation is used:
𝑚̇ = 𝐾𝐴√𝐷/𝑃(𝑃)
𝐷/𝑃
𝑚̇ = 𝐾𝐴√
𝑇
Where:
𝑚̇ = mass flow rate
A = area
D/P = differential pressure
P = pressure
T = temperature
K = flow coefficient
The flow coefficient is constant for the system based mainly on the
construction characteristics of the pipe and type of fluid flowing through the
pipe. The flow coefficient in each equation contains the appropriate units to
balance the equation and provide the proper units for the resulting mass
flow rate. Calculating volumetric flow rate uses the area of the pipe and
differential pressure. As stated above, compensating for system
temperature or pressure converts this volumetric flow rate to mass flow rate.
Rev 2
69
Example
A cooling water system is operating at steady-state conditions indicating
900 gpm with 60 psid across the flow transmitter venturi. If cooling water
flow rate is increased to 1,800 gpm, flow transmitter venturi delta-P will be
approximately________ psid?
We know that the flow meter described operates on the principle of 𝐷/𝑃 ∝
𝑉̇ 2 . The volumetric flow was increased by a factor of 2. Therefore, taking
that factor and squaring it means that the D/P must increase by a factor of 4.
To calculate the actual change:
𝐹𝑖𝑛𝑎𝑙 𝐹𝑙𝑜𝑤 2
𝐹𝑖𝑛𝑎𝑙 𝐷/𝑃
(
) =
𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝐹𝑙𝑜𝑤
𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝐷/𝑃
1800 𝑔𝑝𝑚 2 𝐹𝑖𝑛𝑎𝑙 𝐷/𝑃
(
) =
900 𝑔𝑝𝑚
60 𝑝𝑠𝑖𝑑
1800 𝑔𝑝𝑚 2
𝐹𝑖𝑛𝑎𝑙 𝐷/𝑃 = 60 𝑝𝑠𝑖𝑑 (
)
900 𝑔𝑝𝑚
𝐹𝑖𝑛𝑎𝑙 𝐷/𝑃 = 60 𝑝𝑠𝑖𝑑(2)2
𝐹𝑖𝑛𝑎𝑙 𝐷/𝑃 = 60 𝑝𝑠𝑖𝑑(4)
𝐹𝑖𝑛𝑎𝑙 𝐷/𝑃 = 240 𝑝𝑠𝑖𝑑
Knowledge Check – NRC Bank (Answer Key)
Flow detectors (such as an orifice, flow nozzle, and
venturi tube) measure flow rate using the principle that
flow rate is...
Rev 2
A.
inversely proportional to the D/P squared.
B.
inversely proportional to the square root of the D/P.
C.
directly proportional to the square root of the D/P.
D.
directly proportional to the D/P squared.
70
ELO 4.2 Flow Meter Construction
Introduction
There are several designs of flow meters that work on the theory that flow is
proportional to the square root of the D/P. This section discusses some of
those designs, including:







Orifice plates
Venturi tubes
Dall flow tube
Flow nozzle
Elbow flow meter
Pitot tube
Manometer
Orifice Plates
The orifice plate is the simplest of the flow path restrictions used in flow
detection, as well as the most economical. Orifice plates are flat plates 1/16
to 1/4 inch thick. They are normally located between a pair of flanges and
in a straight run of smooth pipe to avoid disturbance of flow patterns from
fittings and valves. The figure below shows key dimensions of an orifice
plate.
Figure: Orifice Plate
Rev 2
71
When the fluid reaches the orifice plate fluid is forced to converge through
the small hole; the point of maximum convergence actually occurs slightly
downstream of the physical orifice, at the vena contracta point. The
velocity increases and pressure decreases. Beyond the vena contracta, the
fluid expands and the velocity and pressure change once again. The
difference in fluid pressure between the normal pipe section and at the vena
contracta provides the necessary data to determine the volumetric and mass
flow rates.
Three kinds of orifice plates are used: concentric, eccentric, and segmental;
the figure below shows their flow sections. Segmental and eccentric orifice
plates are functionally identical to the concentric orifice.
Figure: Orifice Plate Types
Concentric Orifice Plate
The concentric orifice plate is the most common of the three types. As
shown above, the orifice is equidistant (concentric) to the inside diameter of
the pipe. Flow through a sharp-edged orifice plate results in a velocity
change. As the fluid passes through the orifice, the fluid converges, and the
velocity of the fluid increases to a maximum value. At this point, the
pressure is at its minimum value. As the fluid diverges to fill the entire pipe
area, the velocity decreases back to the original value, however, the pressure
increases only to about 60 percent to 80 percent of the original input value.
This pressure loss is irrecoverable; therefore, the output pressure will
always be less than the input pressure. The pressures on both sides of the
orifice are measured; the measured differential pressure is proportional to
the flow rate.
Eccentric Orifice Plates
Eccentric orifice plates shift the edge of the orifice to the inside of the pipe
wall. This design also prevents upstream damming in the same way as the
segmental orifice plate.
Rev 2
72
Segmental Orifice Plate
The circular section of the segmental orifice is concentric with the pipe.
The segmental portion of the orifice eliminates damming of foreign
materials on the upstream side of the orifice when mounted in a horizontal
pipe. Depending on the type of fluid, the segmental section is located on
either the top or bottom of the horizontal pipe to increase the accuracy of
the measurement.
Disadvantages of Orifice Plates
Orifice plates have two distinct disadvantages; they cause a high permanent
pressure drop of 20 percent to 40 percent (outlet pressure will be 60 percent
to 80 percent of inlet pressure), and they are subject to erosion, which will
eventually cause inaccuracies in the measured differential pressure. They
yield inaccurate readings for fluids that may have gases or vapors in
solution. The gases and vapors tend to collect at the top of the upstream
face. This could cause changes in the density thereby causing erroneous
readings. When gas or steam passes the orifice flow detector, the fluid
density, and corresponding pressure fluctuates. These fluctuations cause
transients on the D/P cell and make the reading very difficult and
inaccurate.
Venturi Tube
The venturi tube is the most accurate flow-sensing element when properly
calibrated. The figure below shows a typical venturi tube, with a
converging conical inlet, a cylindrical throat, and a diverging recovery cone.
It has no projections into the fluid, no sharp corners, and no sudden changes
in contour.
Figure: Venturi Tube
Rev 2
73
The inlet section decreases the area of the fluid stream, causing the velocity
to increase and the pressure to decrease. In the center of the cylindrical
throat, the pressure will be at its lowest value, and neither the pressure nor
the velocity is changing; low-pressure measurements occur here. The
recovery cone allows for some pressure recovery such that total pressure
loss is only 10 percent to 25 percent. This is the lowest pressure drop of
any of the head flow meters. The high-pressure measurements occur
upstream of the entrance cone. The major disadvantages of this type of
flow detection are the high initial costs for installation and difficulty in
installation and inspection.
Flow Nozzle
The flow nozzle is similar to the venturi and normally used for high velocity
flow. The figure below shows a cross-section of a flow nozzle. It has a
smooth contoured flow restriction, but does have a relatively high
permanent pressure loss similar to the orifice.
Figure: Flow Nozzle
Flow nozzles are common measuring elements for air and gas flow in
industrial applications. Because of their relatively smooth contoured flow
restriction, flow nozzles are appropriate for measuring flow of fluids
containing particulates. The Steam Flow Detection section includes more
detail on the flow nozzle.
Elbow Meter
The elbow meter is another head flow meter using a developed differential
pressure to determine flow, as shown in the following figure. When fluid
encounters a piping bend, the fluid traveling on the inner radius does not
have to travel as far as the fluid next to the outer radius, which creates a
slight differential pressure within the bend. The bend's difference in surface
area will create a low-pressure area on the inner pipe wall and a higherpressure area on the outer pipe wall. This change in pressure created by the
elbow is a small change. This pressure difference is proportional to the
volumetric flow rate squared.
Rev 2
74
Figure: Elbow Meter
The small pressure difference created by the elbow meter allows high
accuracy even at high flow rates. The differential pressure instrument used
is more costly than some other head flow meters. The elbow meter is a
simple design and can measure flow in either direction, which is a big
advantage.
Pitot Tube
The Pitot tube is another primary flow element used to produce a
differential pressure for flow detection. In its simplest form, it consists of a
tube with an opening at the end. The small hole in the end is located such
that it faces the flowing fluid. The velocity of the fluid at the opening of the
tube decreases to zero. This provides for the high-pressure input to a
differential pressure detector. A pressure tap provides the low-pressure
input, as shown in the figure below.
Figure: Pitot Tube
The Pitot tube actually measures fluid velocity instead of fluid flow rate.
However, the equation below shows the volumetric flow rate calculation.
𝑉̇ = 𝐾𝐴𝑣
Where:
𝑉̇ = volumetric flow rate
A = area of flow cross-section
v = velocity of flowing fluid
K = flow coefficient (normally about 0.8)
Rev 2
75
Calibration is required for Pitot tubes for each specific application, as there
is no standardization. Pitot tubes are versatile instruments; they can
measure fluid velocity even when the fluid is outside a confined pipe or
duct, such as the exterior of an airplane.
Knowledge Check – NRC Bank (Answer Key)
Refer to the drawing of a venturi flow element below,
with direction of fluid flow indicated by the arrow.
Where should the high-pressure tap of a differential
pressure flow detector be connected?
A.
Point D
B.
Point B
C.
Point C
D.
Point A
Other Types of Flow Detectors (Additional information)
Introduction
Area Flow Meters
Displacement Flow Meter
The head causing the flow through
an area meter is relatively constant
such that the rate of flow is directly
proportional to the metering area.
The rise and fall of a floating
element produces the variation in
area. Mounting of this type of flow
meter must be such that the floating
element moves vertically and
friction is minimal.
In a displacement flow meter, all of
the fluid passes through the meter in
almost completely isolated
quantities. A register counts the
number of these quantities and
indicates them in terms of volume or
weight units.
Rev 2
76
Ultrasonic Flow Equipment
Ultrasonic flow devices use the Doppler frequency shift of ultrasonic
signals reflected from discontinuities in the fluid stream to obtain flow
measurements. These discontinuities can be suspended solids, bubbles, or
interfaces generated by turbulent eddies in the flow stream. The sensor
clamps on the outside of the pipe, and an ultrasonic beam from a
piezoelectric crystal passes through the pipe wall into the fluid at an angle
to the flow stream, shown in the figure below. A second piezoelectric
crystal located in the same sensor detects signals reflected off flow
disturbances. An electrical circuit compares transmitted and reflected
signals, and the corresponding frequency shift is proportional to the flow
velocity.
Figure: Ultrasonic Flow Detector
Knowledge Check (Answer Key)
What type of flow meter is depicted in the cross-section
below?
Rev 2
A.
Analog
B.
Ultrasonic
C.
Nutating disk
D.
Rotameter
77
ELO 4.3 Steam Flow Density Compensation
Introduction
Measurements of steam flow normally use a steam flow nozzle, as shown in
cross-section in the figure below. The flow nozzle is most applicable for
the measurement of steam flow and other high-velocity fluid flow
measurements where erosion may occur. It is capable of measuring
approximately 60 percent higher flow rates than an orifice plate with the
same diameter. This is due to the streamlined contour of the throat, which
is a distinct advantage for the measurement of high velocity fluids. The
flow nozzle requires less straight run piping than an orifice plate. However,
the pressure drop is about the same for both.
Figure: Steam Flow Nozzle
Density Compensation
Because steam behaves like a gas, changes in pressure and temperature
greatly affect its density. The equations below illustrate the fundamental
relationship for volumetric flow and mass flow.
𝐷/𝑃
𝑉̇ = 𝐾√
𝜌
and
𝑚̇ = 𝑉̇ 𝜌
Where:
𝑉̇ = volumetric flow
K = constant relating to the ratio of pipe to orifice
D/P = differential pressure
ρ = density
𝑚̇ = mass flow
Rev 2
78
It is possible to substitute for density in the relationship using:
𝜌=
𝑝𝑚
𝑅𝜃
Where:
ρ = density
p = upstream pressure
m = molecular weight of the gas
θ = absolute temperature
R = gas constant
By substituting measured values and gas characteristics for density, the
electronic circuit will calculate the density automatically. Since steam
temperature is relatively constant in most steam systems, upstream pressure
is the only variable in the above equation that changes as the system
operates. If the electronic circuit has other variables hardwired into it,
measuring system pressure is all that is required for the circuit to calculate
the fluid’s density.
Mass Flow Detection System
Electronic circuitry uses measured temperature and pressure values with the
above equations to calculate gas flow, including compensating the flow for
changes in density. The figure below illustrates a simple mass flow
detection system where commonly used instruments make temperature and
pressure measurements.
Figure: Simple Mass Flow Detection
Rev 2
79
Gas Flow Computer
For the precise measurement of gas flow (steam) at varying pressures and
temperatures, it is necessary to determine the density (which depends on
pressure and temperature), and using the density value, calculate the actual
flow. The use of a computer is essential to measure flow with changing
pressure or temperature. The figure below shows a block diagram of a
computer specifically designed for the measurement of gas flow. The
computer accepts input signals from commonly used differential pressure
detectors, or from density or pressure plus temperature sensors, and
provides an output, which is proportional to the actual rate of flow. The
computer has an accuracy of better than +0.1 percent at flow rates of 10
percent to 100 percent.
Figure: Gas Flow Computer
Rev 2
80
Knowledge Check (Answer Key)
Density input is normally used in steam flow instruments
to convert ______________ into ______________.
A.
differential pressure; volumetric flow rate
B.
volumetric flow rate; mass flow rate
C.
mass flow rate; volumetric flow rate
D.
mass flow rate; differential pressure
Knowledge Check – NRC Bank (Answer Key)
A main steam flow rate measuring instrument uses a
steam pressure input to produce main steam mass flow
rate indication. Assuming steam volumetric flow rate
does not change, a steam pressure decrease will cause
indicated steam mass flow rate to...
A.
increase, because the density of the steam has increased.
B.
decrease, because the density of the steam has decreased.
C.
remain the same, because steam pressure does not affect
the mass flow rate of steam.
D.
remain the same, because the steam pressure input
compensates for changes in steam pressure.
Knowledge Check – NRC Bank (Answer Key)
If the steam pressure input to a density-compensated
steam flow instrument fails low, the indicated flow rate
will...
Rev 2
A.
decrease because the density input has decreased.
B.
decrease because the density input has increased.
C.
increase because the density input has increased.
D.
increase because the density input has decreased.
81
ELO 4.4 Failure Modes
Introduction
The head flow meters are reliable for long-term continuous operation. The
leakage of differential pressure cell connections is one of the most common
problems with head flow meters.
Failure Modes
Condition
Indication
Discussion
1.
Leak on highpressure
connection
Indicated flow less
than actual
Leak on the high-pressure tap
would result in a lower D/P,
which corresponds to lower
indicated flow.
2.
Leak on lowpressure
connection
Indicated flow
more than actual
Leak on the low-pressure tap
would result in a higher D/P,
which corresponds to higher
indicated flow.
3.
Orifice plate
erosion
Indicated flow less
than actual
Orifice size will increase due to
erosion. This results in a lower
D/P for the same flows.
4.
Loss of density
Indicated flow less
compensation input than actual
Density compensation adjusts
the indication to take into
account the effect of pressure
change on the gas being
measured. Without density
compensation, the D/P will be
less.
5.
Steam pressure
input fail low
Indicated flow less
than actual
Apparent density has decreased;
less mass is sensed passing the
flow detector.
6.
Steam pressure
input fail high
Indicated flow
more than actual
Apparent density has increased;
more mass is sensed passing the
flow detector.
7.
Vapor in a liquid
Erratic unstable
flow indication
As vapor goes through the
measuring device, the
difference in pressure is
dependent on the density of the
fluid. Gas has much less
density that liquid and therefore
the D/P will change rapidly as
the vapor goes through the
detector.
Rev 2
82
Knowledge Check (Answer Key)
The most probable cause for fluctuating indication from
a liquid flow rate differential pressure detector is...
A.
unequal temperature gradients in the liquid.
B.
gas or steam being trapped in the liquid.
C.
vortexing of the liquid passing through the flow device.
D.
the valve on the high-pressure sensing line being
partially closed.
Knowledge Check (Answer Key)
Which one of the following will cause indicated
volumetric flow rate to be lower than actual volumetric
flow rate using a differential pressure flow detector
connected to a calibrated orifice?
A.
The orifice erodes over time.
B.
Debris becomes lodged in the orifice.
C.
System pressure decreases.
D.
A leak develops in the low-pressure sensing line.
Knowledge Check – NRC Bank (Answer Key)
If the orifice in a differential pressure (D/P) flow sensor
erodes such that the orifice opening becomes larger,
indicated flow rate will __________ due to a
__________ D/P across the orifice. (Assume actual flow
rate remains the same.)
Rev 2
A.
increase; larger
B.
increase; smaller
C.
decrease; larger
D.
decrease; smaller
83
Knowledge Check – NRC Bank (Answer Key)
Refer to the drawing of a pipe elbow used for flow
measurement in a cooling water system below. A
differential pressure (D/P) flow detector connects to
instrument lines A and B. If instrument line B develops
a leak, indicated flow rate will ______________ due to a
______________ measured D/P.
A.
increase; smaller
B.
decrease; larger
C.
increase; larger
D.
decrease; smaller
ELO 4.5 Environmental Effects
Introduction
Flow detection circuits sense small changes in flow by measuring the
difference in pressure that results from a pressure drop across an orifice or
other device that causes the fluid to undergo a velocity change.
Environmental conditions surrounding the instruments and circuitry can
affect the indication. Circuits operate at very low voltages (millivolt) and
amperage (milliamp); therefore, it is necessary to include temperature and
humidity change effects on the circuit resistance. These environmental
conditions can modify the circuit output signal and give a false indication of
flow.
Rev 2
84
Environmental Effects
Fluid Density
When measuring fluid flow, the density of the fluid can have a large effect
on flow sensing instrumentation. The effect of density is most important
when the flow sensing instrumentation is measuring gas flows, such as
steam. Because temperature and pressure directly affect gas density, any
changes in either of these parameters will have a direct effect on the
measured gas flow. To obtain an accurate flow value, it is necessary to
include the effects of fluid temperature, pressure, and density in flow
computations.
Ambient Temperature
Ambient temperature variations will affect the accuracy and reliability of
flow sensing instrumentation. Variations in ambient temperature can
directly affect the resistance of components in the instrumentation circuitry
and affect the calibration of electric/electronic equipment. Proper circuitry
design and maintaining the flow sensing instrumentation in the proper
environment will reduce the effects of temperature variations.
Humidity
Humidity will also affect most electrical equipment, especially electronic
equipment. High humidity causes moisture to collect on the equipment.
This moisture can cause short circuits, grounds, and corrosion, which, in
turn, may damage components. Maintaining electrical equipment in the
proper environment will control humidity effects.
Knowledge Check (Answer Key)
The __________ of the fluid whose flow is to be
measured can have a large effect on flow sensing
instrumentation. The effect of _______ is most
important when the flow sensing instrumentation is
measuring gas flows, such as steam.
Rev 2
A.
mass; mass
B.
flow rate; flow rate
C.
density; density
D.
volume; volume
85
TLO 4 Summary
Head flow meter:

This meter operates on the principle of placing a restriction in the
line to cause a pressure drop. The head restriction causes a D/P;
sensors measure the D/P and convert it to a flow measurement.
Orifice plate:


This is a flat plate 1/16 to 1/4 inch thick, mounted between a pair of
flanges.
They are installed in a straight run of smooth pipe to avoid
disturbance of flow patterns due to fittings and valves.
Venturi tube:


This tube is comprised of a converging conical inlet, a cylindrical
throat, and a diverging recovery cone.
There are no projections into the fluid, no sharp corners, and no
sudden changes in contour.
Dall flow tube:


This tube consists of a short, straight inlet section followed by an
abrupt decrease in the inside diameter of the tube (inlet shoulder),
followed by a converging inlet cone and a diverging exit cone.
A slot gap between the two cones separates them.
Pitot tube:

This tube has an opening at the end which is positioned so that it
faces the flowing fluid.
Rotameter:





A rotameter consists of a metal float and a conical glass tube.
The tube diameter increases with height.
High-density float will remain on the bottom of tube with no flow.
Space between the float and the tube allows for flow past the float.
As flow increases, the pressure drop increases; when the pressure
drop is sufficient, the float rises to indicate the amount of flow.
Nutating disc:




Rev 2
This is a circular disk attached to a central ball.
A shaft protrudes from the ball and a cam or roller holds the shaft in
an inclined position.
Fluid enters an opening in the spherical wall on one side of the
partition and leaves through the other side.
As the fluid flows through the chamber, the disk wobbles, or
executes a nutating motion.
86
Hot-wire anemometer:



This is an electrically heated, fine platinum wire immersed in flow.
The wire cools as flow is increased.
Operators can measure either change in wire resistance or heating
current to determine flow.
Electromagnetic flowmeter:



This is a magnetic field established around system pipe.
An electromotive force is induced in fluid as it flows through the
magnetic field.
The electromotive force is measured with electrodes and is
proportional to flow rate.
Ultrasonic flow equipment:

This equipment uses Doppler frequency shift of ultrasonic signals
reflected off discontinuities in fluid.
Density compensation:


Changes in temperature and pressure greatly affect the indicated
steam flow.
By measuring temperature and pressure, a computerized system can
electronically compensate steam or gas flow indication for changes
in fluid density.
Environmental effects on flow detection:



Density of the fluid
Ambient temperature
Humidity
Now that you have completed this lesson, you should be able to:
1. Describe the theory of operation of a basic head flow meter.
2. Describe the basic construction of the following types of head flow
detectors:
a. Orifice plates
b. Venturi tube
c. Dall flow tube
d. Flow nozzle
e. Elbow meter
f. Pitot tube
3. Describe density compensation of a steam flow instrument to
include the reason density compensation is required and the
parameters used.
4. State the typical failure modes for head flow meters including the
effects of vapor on a flow instrument.
5. Describe the environmental conditions that can affect the accuracy
and reliability of flow sensing instrumentation.
Rev 2
87
TLO 5 Position Detectors
Overview
Industrial facilities use position indicating instrumentation to provide
remote equipment indication such as for the indication of open or shut
valves. There are several types of position detection devices. They include
switches that are "on-off" type devices and variable output devices.
Remote indication is necessary to monitor vital components located in
remote or inaccessible areas, or to obtain indication data from a piece of
equipment where there is a personnel safety concern.
Objectives
Upon completion of this lesson, you will be able to do the following:
1. Describe the following switch position indicators to include basic
construction and theory of operation.
a. Limit switches
b. Reed switches
c. Coil stacks
2. Describe the following variable output position indicators to include
basic construction and theory of operation.
a. Potentiometer
b. Linear variable differential transformer (LVDT)
3. Describe the environmental conditions that can affect the accuracy
and reliability of position indication equipment.
4. Describe the failure modes for the following position detectors:
a. Reed switch
b. Limit switch
c. Potentiometer
d. LVDT
Rev 2
88
ELO 5.1 Switch Type Detectors
Introduction
Mechanical limit switches and reed switches provide valve open and valve
shut indications, and show the physical position of equipment.
Limit Switches
A limit switch is a mechanical device that reflects the physical position of
equipment. For example, an extension on a valve shaft mechanically trips a
limit switch as it moves from open to shut or from shut to open. The limit
switch is the most commonly used sensor for an on/off output that
corresponds to valve position. Normally, limit switches provide full open
or full shut indications as illustrated in figure below.
Figure: Limit Switches
Many limit switches are the push-button variety. When the valve extension
contacts the limit switch, the switch depresses to complete, or turn on, the
electrical circuit. As the valve extension moves away from the limit
switches, spring pressure opens the switch, which turns off the circuit.
Reed Switches
Reed switches are more reliable than limit switches because they use a
magnetic field and have fewer mechanical parts to wear. Reed switches
consist of flexible ferrous strips (reeds) placed near the intended travel of a
valve stem or component shaft, for example. When using reed switches, the
valve stem or shaft extension is a permanent magnet, as shown in the figure
below.
Rev 2
89
Figure: Reed Switches
As the magnet approaches the reed switch, the reeds contact one another
and the switch shuts. When the magnet moves away, the reeds no longer
contact one another, and the switch opens. This on/off indicator is similar
to mechanical limit switches. It is possible to show incremental position
during valve travel by using a large number of magnetic reed switches, at
incremental travel positions. In the control rod drive mechanism, reed
switches provide position indication for the control rod. A permanent
magnet on the control rod drive shaft attracts the moveable contact arm of
each reed switch as the drive passes by.
A common example of where reed switches are in the plant is for position
indication of the control rod drive monitors. A permanent magnet located
on the control rod drive shaft attracts the movable contact arm of each reed
switch as the control rod drive passes by their location. As the rod is
withdrawn contacts (S1, the S2, etc) are closed which shorts out the
resistors. As the control rod is withdrawn, current increases.
Figure: Reed Switch – Control Rod Position
Rev 2
90
Coil Stacks
A coil stack consists of coils wired in sets of three. With the control rod at
the bottom position, coil impedance is balanced and all detector sets send a
0.0V signal. As the rod drive shaft passes through Coil A, impedance
increases and current decreases in ammeter A. As rod withdrawal
continues, the current in ammeter A stays the same and current in ammeter
B decreases.
Figure: Coil Stacks
Knowledge Check (Answer Key)
What is the most common type of sensor used to provide
remote position indication of a valve that is normally
either fully open or fully closed?
Rev 2
A.
Linear variable differential transformer
B.
Limit switch
C.
Reed switch
D.
Servo transmitter
91
Knowledge Check (Answer Key)
In an electrical measuring circuit, reed switches monitor
the position of a control rod in a nuclear reactor. The
reed switches mount to a column above the reactor vessel
such that the control rod drive shaft passes by the reed
switches as the control rod is withdrawn.
Which one of the following describes the action that
causes the electrical output of the measuring circuit to
change as the control rod is withdrawn?
A.
An AC coil on the control rod drive shaft induces a
voltage into each reed switch as the drive shaft passes by.
B.
A metal tab on the control rod drive shaft mechanically
closes each reed switch as the drive shaft passes by.
C.
The primary and secondary coils of each reed switch
attain maximum magnetic coupling as the drive shaft
passes by.
D.
A permanent magnet on the control rod drive shaft
attracts the movable contact arm of each reed switch as
the drive shaft passes by.
ELO 5.2 Variable Output Detectors
Introduction
Variable output devices provide an accurate and reliable position indication
for a particular piece of equipment, such as a valve. These devices include
potentiometers and linear variable differential transformers (LVDTs).
Potentiometer
Potentiometer valve position indicators provide an accurate indication of
position throughout the travel of a valve. A variable resistor is attached to
an extension on the valve. As the extension moves up or down with the
valve, the resistance of the attached circuit changes, changing the amount of
current flow in the circuit. The following figure shows key elements of a
potentiometer valve position indicator.
Rev 2
92
Figure: Potentiometer
The amount of current is proportional to the valve position. When
potentiometer valve position indicators fail, the failures are normally
electrical in nature. An electrical short or open will cause the indication to
fail at one extreme or the other. If an increase or decrease in the
potentiometer resistance occurs, valve position indication will become
erratic.
Linear Variable Differential Transformer (LVDT)
Another device that provides accurate position indication throughout the
range of valve travel is a linear variable differential transformer (LVDT).
Unlike the potentiometer position indicator, no physical connection to the
valve extension is required, as shown in the figure below. The valve
extension is made of a metal suitable for acting as the movable core of a
transformer. Moving the extension between the primary and secondary
windings of a transformer causes the inductance between the two windings
to vary, thereby varying the output voltage proportional to the position of
the valve extension.
Figure: Linear Variable Differential Transformer
Rev 2
93
The previous figure illustrates a valve with an LVDT position indicator. If
only the open and shut positions are required, place two small secondary
coils at each end of the extension’s travel. Typically, LVDTs exhibit high
reliability. As a rule, failures are limited to rare electrical faults, which
cause erratic or erroneous indications. An open primary winding will cause
the indication to fail to some predetermined value equal to zero differential
voltage. This normally corresponds to mid-stroke of the valve. A failure of
either secondary winding will cause the output to indicate either full open or
full closed, regardless of actual valve position.
Knowledge Check (Answer Key)
Which one of the following devices is commonly used to
provide remote indication of valve position on an analog
meter in units of "percent of full open"?
A.
Limit switch
B.
Reed switch
C.
Linear variable differential transformer
D.
Resistance temperature detector
ELO 5.3 Environmental Effects
Introduction
Position detectors for a valve have exposure to the same environment as the
valve whose position they are monitoring. Therefore, position detector
design considers the same environmental conditions as the valves. Position
detectors are normally part of an electrical circuit, much like level, pressure,
or flow detectors so their circuits are sensitive to temperature and humidity
changes.
Environmental Effects
Ambient Temperature
Ambient temperature variations can affect the accuracy and reliability of
certain types of position indication instrumentation. Variations in ambient
temperature can directly affect the resistance of components in the
instrumentation circuitry and affect the calibration of electric/electronic
equipment. Proper circuitry design and maintaining the position indication
instrumentation in the proper environment where possible will reduce the
effects of temperature variations.
Rev 2
94
Humidity
Humidity will also affect most electrical equipment, especially electronic
equipment. High humidity causes moisture to collect on the equipment.
This moisture can cause short circuits, grounds, and corrosion, which, in
turn, may damage components. Maintaining the equipment in the proper
environment, where possible will control the effects due to humidity.
Example
Environmental conditions or in some cases, potential environmental
conditions are taken into consideration when designing remotely operated
equipment. Consider an emergency injection valve that is located in the
primary containment. This valve must operate under accident conditions
when local humidity and temperature may be elevated due to a loss of
coolant from a leak. Since this valve must operate during such an incident,
valve design and operating criteria include environmental qualification
requirements. Design criteria for components and circuitry include the
ability to withstand the worst-case environmental conditions and still
function for the duration of the transient.
Knowledge Check (Answer Key)
Variations in __________________ can directly affect
the____________of components in the instrumentation
circuitry.
Rev 2
A.
ambient temperature; voltage
B.
voltage; resistance
C.
voltage; temperature
D.
ambient temperature; resistance
95
ELO 5.5 Failure Modes
Introduction
Since position indication devices are mechanical-electrical devices, they are
susceptible to both mechanical and electrical failures. In general, position
indicators are highly reliable devices and normally have alternate devices or
methods to determine position.
Failure Modes
Each position detection indicator has unique qualities and therefore is
susceptible to unique failures. Below are some examples of each detector
and their most probable failure.




Rev 2
Limit switch failures are normally mechanical in nature. If the
position indication or control function fails, the limit switch is
probably faulty. In this case, use local position indication to verify
equipment position.
Reed switch failures are normally limited to a reed switch that is
stuck open or stuck shut. If a reed switch is stuck shut, the indication
(open or closed) will remain lit regardless of valve position. If a reed
switch is stuck open, the position indication for that switch remains
extinguished regardless of valve position.
Potentiometer valve position indicator failures are normally electrical
in nature. An electrical short or open will cause the indication to fail
at one extreme or the other. If an increase or decrease in the
potentiometer resistance occurs, valve position indication will
become erratic.
LVDTs are extremely reliable. As a rule, LVDT failures are limited
to rare electrical faults, which cause erratic or erroneous indications.
An open primary winding will cause the indication to fail to some
predetermined value equal to zero differential voltage. This normally
corresponds to mid-stroke of the valve. A failure of either secondary
winding will cause the output to indicate either full open or full
closed, regardless of actual valve position.
96
Knowledge Check (Answer Key)
In the figure below there are 4 sets of reed switches. One
set is for full closed, another full open and the other sets
are for intermediate positions. If each reed switch set
completes a circuit to indicating lights what would be the
indication if the bottom set of reed switches failed closed
as the valve goes from closed to open?
Rev 2
A.
The intermediate lights would remain lit in addition to
the full open light as the valve travels full stroke.
B.
The full open light would remain lit in addition to the
intermediate and full open light as the valve travels full
stroke.
C.
No effect, all indicating lights would work as designed.
D.
The full closed light would remain lit in addition to the
intermediate and full open light as the valve travels full
stroke.
97
TLO 5 Summary
Limit switch:



This switch is a mechanical device used to determine the physical
position of valves.
An extension on a valve shaft mechanically trips the switch as it
moves from open to shut or shut to open.
The limit switch gives on/off output, which corresponds to the valve
position.
Reed switch:



This switch consists of flexible ferrous strips affixed to a stationary
point adjacent to the intended travel of the valve stem.
The extension used is a permanent magnet.
As the magnet approaches the reed switch, the switch shuts. When
the magnet moves away, the reed switch opens.
Potentiometer valve position indicator:


This indicator uses an extension with an attached variable resistor.
As the extension moves up or down, the resistance of the attached
circuit changes, changing the amount of current flow in the circuit.
Linear variable differential transformer (LVDT):


This uses the extension shaft of a valve as a movable core of a
transformer.
Moving the extension between the primary and secondary windings
of a transformer causes the inductance between the two windings to
vary, thereby varying the output voltage proportional to the position
of the valve extension.
The environmental conditions below affect the accuracy and reliability of
position indication instrumentation:


Ambient temperature
Humidity
Indicating and control circuitry provides for remote indication of valve or
component position and/or various control functions.
Now that you have completed this lesson, you should be able to:
1. Describe the following switch position indicators to include basic
construction and theory of operation.
a. Limit switches
b. Reed switches
c. Coil stacks
Rev 2
98
2. Describe the following variable output position indicators to include
basic construction and theory of operation.
a. Potentiometer
b. Linear variable differential transformer (LVDT)
3. Describe the environmental conditions that can affect the accuracy
and reliability of position indication equipment.
4. Describe the failure modes for the following position detectors:
a. Reed switch
b. Limit switch
c. Potentiometer
d. LVDT
Sensors and Detectors Part 1 Summary
Now that you have completed this module, you should be able to
demonstrate mastery of this topic by passing a written exam with a grade of
80 percent or higher on the following TLOs:
1. Describe the operation of temperature detectors and conditions that
effect their accuracy and reliability.
2. Describe the operation of pressure detectors and conditions that
affect their accuracy and reliability.
3. Describe the operation of level detectors and conditions that affect
their accuracy and reliability.
4. Describe the operation of flow detectors and conditions that affect
their accuracy and reliability.
5. Describe the operation of position detectors and conditions that
affect their accuracy and reliability.
Rev 2
99
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check Answer Key
ELO 1.1 Temperature Detector Functions
Knowledge Check - Answer
Temperature detection is used to provide the following:
(select all that apply)
A.
Interlocks
B.
Indications
C.
Alarms
D.
Automatic trips
Knowledge Check - Answer
Which of the following is not a function of a temperature
detector?
A.
Indication
B.
Control functions
C.
Alarm functions
D.
Amplification
ELO 1.2 Resistance Temperature Detector Construction
Knowledge Check - Answer
A resistance temperature detector operates on the
principle that the change in electrical __________ of a
metal is ________ proportional to its change in
temperature.
Rev 2
A.
conductivity; directly
B.
conductivity; indirectly
C.
resistance; indirectly
D.
resistance; directly
1
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 1.3 Temperature Resistance Relationship
Knowledge Check - Answer
A resistance temperature detector operates on the
principle that the change in electrical resistance of...
A.
a metal is inversely proportional to its change in
temperature.
B.
two dissimilar metals is inversely proportional to the
temperature change measured at their junction.
C.
two dissimilar metals is directly proportional to the
temperature change measured at their junction.
D.
a metal is directly proportional to its change in
temperature.
Knowledge Check - Answer
What happens to the resistance of a resistance
temperature detector (RTD) when the temperature of the
substance it is measuring increases?
Rev 2
A.
Resistance of the RTD decreases and then increases.
B.
Resistance of the RTD decreases.
C.
Resistance of the RTD increases.
D.
Resistance of the RTD remains the same.
2
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 1.4 Temperature Detection Circuits
Knowledge Check
Typical temperature bridge circuits use low voltage
(millivolt) signals. How does this low voltage drive a
remote meter indication?
A.
The signal is amplified, which raises the voltage.
B.
The signal is converted from AC to DC, which raises the
voltage.
C.
The signal is amplified, which lowers the voltage.
D.
The signal is converted from DC to AC, which raises the
voltage.
ELO 1.5 Environmental Effects
Knowledge Check - Answer
To compensate for ambient temperature change, both
three and four wire resistance temperature detector
circuits use the same amount of lead wire in both
branches of the bridge circuit because...
Rev 2
A.
the change in resistance will be felt on neither branch.
B.
the change in resistance is not an important factor in
temperature measurement.
C.
the change in resistance will be felt on both branches.
D.
the change in resistance is important only when
calibrating temperature circuits.
3
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
A simple two-wire resistance temperature detector
(RTD) is being used to measure the temperature of a
water system. Copper extension wires run from the RTD
to a temperature instrument 40 feet away.
If the temperature of the extension wires decreases, the
electrical resistance of the extension wires will
__________; and the temperature indication will
__________ unless temperature compensation is
provided.
A.
increase; increase
B.
increase; decrease
C.
decrease; increase
D.
decrease; decrease
Analysis:
The ability of a metal to conduct is dependent on its composition and
temperature. As temperature rises, the ability of the metal to conduct
electricity becomes somewhat diminished. An RTD employs this relationship
to measure temperature. In order to calculate temperature based on
resistance, the detector must be calibrated to this relationship. A true linear
relationship for a metal such as platinum makes this calibration simple;
cheaper metals such as nickel or copper aren’t quite as linear, but are still
used due to relative cost.
Rev 2
4
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 1.6 Circuit Faults
Knowledge Check - Answer
Consider the circuit below, what would the meter read if
the lead between Y and the resistance temperature
detector developed an open circuit?
A.
300°
B.
600°
C.
0°
D.
Dependent on measured temperature
Knowledge Check - Answer
If shorting occurs within a resistance temperature
detector, the associated indication will fail...
Rev 2
A.
low.
B.
high.
C.
as is.
D.
to midscale.
5
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 1.7 Alternate Temperature Detection
Knowledge Check - Answer
In the circuit below, a dual-element resistance
temperature detector (RTD) indicates temperature. If the
RTD develops an internal open circuit (bridge circuit
remains intact), temperature indication could be obtained
by…
Rev 2
A.
connecting a spare RTD into the circuit.
B.
doing nothing, the existing circuit will still measure
temperature with an open circuit.
C.
direct resistance measurements.
D.
surface resistor.
6
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 1.8 Thermocouples
Knowledge Check – Answer
Refer to the drawing of a simple thermocouple circuit
below.
A thermocouple temperature indication is initially 410°F
with the reference (cold) junction at 125°F. An ambient
temperature decrease lowers the reference junction
temperature to 110°F, while the measuring junction
temperature remains constant. Without temperature
compensation for the reference junction, the new
thermocouple temperature indication will be...
A.
380°F.
B.
395°F.
C.
410°F.
D.
425°F.
Analysis:
The output (or measured) voltage produced by a thermocouple is
proportional to the temperature of the measuring (hot) junction compared to
the reference (cold) junction.
When the cold junction temperature is decreased by 15ºF, a larger differential
temperature exists, therefore a larger output signal.
Recall:
Indication = Measuring – Reference + Calibrated (Initial conditions were,
410 – 125 + 125 = 410)
Final Conditions: 400 – 110 + 125 = 425
Therefore, “D” is correct.
Rev 2
7
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
An open circuit in a thermocouple detector causes the
affected temperature indication to fail...
A.
high.
B.
low.
C.
to reference junction temperature.
D.
as-is.
Analysis:
A. WRONG. A thermocouple will never fail HIGH. Even on a SHORT, a
thermocouple fails LOW.
B. WRONG. Even though an open circuit will cause a thermocouple to fail
in the “low” direction, it doesn’t necessarily mean to ZERO (which is
what I am assuming this choice references).
C. CORRECT. If the junction between the dissimilar metals is interrupted
by an open circuit, no path for current flow exists, and thus the
temperature indication will fail low.
Keep in mind that a thermocouple also employs a reference junction. When
this junction is calibrated to some temperature above 0°F, depending on
where the “open” exists, the thermocouple would fail to the reference
junction calibrated temperature (which is still in the “low” direction).
D.
WRONG.
Temperature
indications
can never fail AS IS.
ELO
2.1 Pressure
Detector
Functions
Knowledge Check - Answer
Pressure detectors provide the following: (select all that
apply)
Rev 2
A.
Indications
B.
Automatic trips
C.
Interlocks
D.
Alarms
8
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 2.2 Pressure Detector Theory
Knowledge Check – Answer
A bellows pressure transmitter with its low-pressure side
vented to containment atmosphere measures reactor
coolant system (RCS) pressure. A decrease in the
associated pressure indication could be caused by either a
containment pressure ____________ or an RCS pressure
____________.
A.
decrease; increase
B.
increase; decrease
C.
decrease; decrease
D.
increase; increase
Knowledge Check - Answer
If the pressure sensed by a bourdon tube increases, the
curvature (amount of curve) of the detector will
____________ because the greater force is being applied
to the ____________ curve of the detector.
A.
increase; outer
B.
increase; inner
C.
decrease; outer
D.
decrease; inner
Analysis:
When system pressure is applied to a Bourdon tube, a force is applied to both
the inner and outer walls of the tube, which is the product of pressure and
area:
P = F/A, or, F=PA
The outer curve of the bourdon tube detector has a higher surface area
because it has a larger radius from the center of the tube. Therefore, when
system pressure is increased, a larger force is applied to the outer wall of the
tube, which results in decreased curvature of the tube.
Rev 2
9
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
Semiconductor strain gages are often used in transmitters
for...
A.
control rod position instruments.
B.
reactor coolant pressure instruments.
C.
reactor coolant temperature instruments.
D.
steam generator level instruments.
Analysis:
Semiconductor strain gages measure the amount of deformation per unit
length when a tensile stress is applied.
Recall from 193010 – Brittle Fracture and Vessel Thermal Stress, that strain
(e) equals change in length divided by original length. This “strain” or
movement in the wire changes the resistance. As pressure is increased, a
strain is produced on the reactor coolant system boundary (piping, vessels,
etc) such that it undergoes mild plastic deformation. Recall when a wire gets
longer (or its cross-sectional area gets smaller) its resistance gets larger. This
change in resistance (and current) is related to the change in pressure in the
system where it is being used.
Knowledge Check - Answer
A type of pressure sensor that is constructed of two
conductive plates separated by a dielectric substance is a
_______________ pressure detector.
Rev 2
A.
bellows-type
B.
bourdon-tube
C.
capacitive-type
D.
inductance-type
10
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 2.3 Factors Affecting Accuracy and Detector Failure Modes
Knowledge Check – Answer
A pressure-sensing element located inside a primary
containment will be subject to which of the following
environmental effects during a steam leak inside
containment? Select all that apply.
Rev 2
A.
Humidity
B.
Atmospheric pressure
C.
Temperature
D.
Alpha radiation
11
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
Refer to the drawing of a bellows-type differential
pressure (D/P) detector below. The spring in this
detector (shown in a compressed state) has weakened
from long-term use. If the actual D/P is constant, how
will indicated D/P respond as the spring weakens?
A.
Increase, because the spring will expand more.
B.
Decrease, because the spring will expand more.
C.
Increase, because the spring will compress more.
D.
Decrease, because the spring will compress more.
Analysis:
For a typical bellows-type D/P detector, the fluid on the high pressure (RCS)
connection exerts a force against the moveable wall. This force is opposed by
both the low pressure fluid and the force of the spring, which will increase as
it is compressed until all forces cancel out. The linear deflection is then
measured and converted into a differential pressure.
A spring which has weakened will have to compress further in order to
provide the same amount of counterforce against the high pressure fluid.
Additional compression of the spring will result in further axial deflection,
and thus a higher indicated D/P (pressure).
Rev 2
12
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
If a bourdon-tube pressure detector is over-ranged
sufficiently to permanently distort the bourdon tube,
subsequent pressure measurement will be inaccurate
because the ____________ of the detector tube will be
inaccurate.
A.
change in the volume
B.
change in the length
C.
expansion of the cross-sectional area
D.
distance moved by the tip
Knowledge Check - Answer
A cooling water system pressure detector uses a bourdon
tube as the sensing element. Which one of the following
explains how the indicated system pressure will be
affected if a local steam leak raises the temperature of the
bourdon tube by 50°F? (Assume the cooling water
system pressure does not change.)
A.
Indicated pressure will decrease because the bourdon
tube will become more flexible.
B.
Indicated pressure will increase because the bourdon tube
will become more flexible.
C.
Indicated pressure will decrease because the bourdon
tube internal pressure will increase.
D.
Indicated pressure will increase because the bourdon tube
internal pressure will increase.
Analysis:
A 50ºF increase in building temperature will slightly alter the material
properties of the Bourdon tube.
Unlike pressure, building temperature changes will not have a significant
impact on Bourdon tube operation. Even though the impact on the bourdon
tube by this temperature change is insignificant, the only correct answer is
that the temperature rise will cause the tube to become more flexible causing
it to move more for a given pressure difference. This will cause indicated
pressure to increase.
This makes Choice “B” the correct answer.
Rev 2
13
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 3.1 Level Detection Functions
Knowledge Check
Level detection provides the following: (select all that
apply)
A.
Interlocks
B.
Alarms
C.
Automatic trips
D.
Indications
ELO 3.2 Operation of Level Detectors
Knowledge Check – Answer
A calibrated differential pressure (D/P) level detector
measures the level in a vented tank inside the auxiliary
building, shown in the figure below. If building pressure
increases with no change in temperature, the associated
level indication will...
Rev 2
A.
decrease, then increase and stabilize at the actual level.
B.
increase and stabilize above the actual level.
C.
decrease and stabilize below the actual level.
D.
remain at the actual level.
14
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check – Answer
Refer to the drawing of a differential pressure (D/P) level
detection system (see figure below) for a pressurizer at
normal operating temperature and pressure. The level
detector has just been calibrated.
The high pressure side of the detector is connected to the
__________; and if the equalizing valve is opened, the
indicated pressurizer level will be __________ than the
actual level.
A.
condensing pot; lower
B.
condensing pot; higher
C.
pressurizer; lower
D.
pressurizer; higher
Analysis:
Because a pressurizer contains a two-phase fluid, a wet reference D/P cell
must be used. Remember that for a wet reference only, the high pressure tap
is on the reference leg, while the low pressure tap is on the tank side.
The high pressure tap is on the reference leg because this water is much
cooler (hence significantly more dense) than the water on the variable leg,
and thus a higher pressure will be developed on the reference leg
(condensing pot).
This results in indicated level being inversely proportional to differential
pressure.
If the equalizing valve is opened, this will result in a minimum differential
pressure, thus a higher indicated level.
Rev 2
15
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 3.3 Density Compensation
Knowledge Check
Many steam generator water level instruments include a
condensing chamber in the reference leg. The purpose of
the condensing chamber is to...
A.
ensure the reference leg temperature remains close to the
temperature of the variable leg.
B.
maintain a constant water level in the reference leg
during normal operations.
C.
provide reference leg compensation for the steam
generator pressure exerted on the variable leg.
D.
prevent reference leg flashing during a rapid
depressurization of the steam generator.
Knowledge Check
Refer to the drawing of a pressurizer differential pressure
(D/P) level detection system below. With the nuclear
power plant at normal operating conditions, a pressurizer
level D/P instrument that had been calibrated while the
plant was in a cold condition would indicate _________
than actual level because of a ___________ D/P sensed
by the D/P detector at normal operating conditions.
Rev 2
A.
higher; smaller
B.
lower; smaller
C.
higher; larger
D.
lower; larger
16
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 3.4 Level Detection Circuits
Knowledge Check - Answer
Place the following components in order starting with
level sensing to output signal.
4 A.
Alarm
2 B.
Transducer
3 C.
Amplifier
1 D.
Bourdon tube
ELO 3.5 Environmental Effects on Level
Knowledge Check - Answer
Consider the level indicator for a steam generator below.
A steam leak has occurred and the temperature of the
area around the reference leg is increasing. What effect
would this have on the indicated level?
Rev 2
A.
Indicate higher than actual because resistance of the D/P
cell components is increasing.
B.
Indicate higher than actual because reference leg density
is decreasing.
C.
No effect.
D.
Indicate lower than actual because reference leg density
is increasing.
17
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 3.6 Failure Modes
Knowledge Check - Answer
The level indication for a reference leg differential
pressure (D/P) level instrument will fail low because of...
Rev 2
A.
a break on the variable leg.
B.
closing the equalizing valve in the D/P cell.
C.
the reference leg flashing to steam.
D.
a break on the reference leg.
18
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check – Answer
Refer to the drawing of a steam generator (SG)
differential pressure (D/P) level detection system below.
The SG is at normal operating temperature and pressure
with accurate level indication. Which one of the
following events will result in an SG level indication that
is greater than actual level?
A.
The external pressure surrounding the D/P detector
increases by 2 psi.
B.
SG pressure increases by 50 psi with no change in actual
water level.
C.
Actual SG level increases by 6 inches.
D.
The temperature of the reference leg increases by 20°F.
Analysis:
Recall, on a wet reference leg DP level detector, for indicated level to be
greater than actual level, DP would need to DECREASE.
A. WRONG. External pressure has no impact on a wet reference DP
level detector.
B. WRONG. If SG pressure increases, temperature increases, and
density decreases. This causes LP pressure to decrease, causing DP
to INCREASE. We are looking for a decrease in DP.
C. WRONG. If actual level increased by 6 inches, indicated level would
also increase by 6 inches (Indicated equals Actual).
CORRECT. If reference leg temperature increases, density decreases,
causing HP pressure to decrease. This results in a DECREASE in DP
(which is what we are looking for).
Rev 2
19
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 3.7 Detector Transients
Knowledge Check - Answer
Refer to the drawing of a differential pressure (D/P) level
detection system below for a pressurizer at normal
operating temperature and pressure. Assume that the
level detector was just calibrated. The low-pressure side
of the detector is connected to the __________; if a leak
develops on the variable leg, the indicated pressurizer
level will be ___________ than the true level.
Rev 2
A.
condensing pot; higher
B.
pressurizer; higher
C.
condensing; lower
D.
pressurizer; lower
20
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
Refer to the drawing of a water storage tank with a
differential pressure (D/P) level detection system (see
figure). The level detector has just been calibrated.
How will the indicated level be affected if condensation
partially fills the normally dry reference leg?
Rev 2
A.
Indicated level will not be affected.
B.
Indicated level will be lower than actual level.
C.
Indicated level will be higher than actual level.
D.
Indicated level may be higher or lower than actual level
depending on the pressure in the upper volume of the
tank.
21
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 4.1 Flow Meter Theory of Operations
Knowledge Check - Answer
Flow detectors (such as an orifice, flow nozzle, and
venturi tube) measure flow rate using the principle that
flow rate is...
A.
inversely proportional to the D/P squared.
B.
inversely proportional to the square root of the D/P.
C.
directly proportional to the square root of the D/P.
D.
directly proportional to the (D/P squared.
Analysis:
Differential pressure is measured across the venturi. Volumetric flow rate is
proportional to the square root of the differential pressure.
If the knowledge item is difficult to remember, this relationship can be
derived using Bernoulli’s equation given on the equation sheet (change in
flow energy and kinetic energy).
Rev 2
22
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 4.2 Flow Meter Construction
Knowledge Check - Answer
Refer to the drawing of a venturi flow element below,
with direction of fluid flow indicated by the arrow.
Where should the high-pressure tap of a differential
pressure flow detector be connected?
A.
Point A
B.
Point B
C.
Point C
D.
Point D
Analysis:
The highest pressure in a venture tube is upstream of the device. Normally
the upstream tap is placed approximately a distance upstream of the
convergence equal to ½ the diameter of the pipe.
For example, if the pipe is a 6” pipe, the tap is 3” upstream of where it starts
to converge. This minimizes any fluctuations of the upstream side by the
turbulence caused by the converging pipe
Rev 2
23
Sensors and Detectors Part 1
Knowledge Check Answer Key
Other Types of Flow Detectors
Knowledge Check
What type of flow meter is depicted in the cross-section
below?
A.
Analog
B.
Ultrasonic
C.
Nutating disk
D.
Rotameter
ELO 4.3 Steam Flow Density Compensation
Knowledge Check - Answer
Density input is normally used in steam flow instruments
to convert ______________ into ______________.
Rev 2
A.
differential pressure; volumetric flow rate
B.
volumetric flow rate; mass flow rate
C.
mass flow rate; volumetric flow rate
D.
mass flow rate; differential pressure
24
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
A main steam flow rate measuring instrument uses a
steam pressure input to produce main steam mass flow
rate indication. Assuming steam volumetric flow rate
does not change, a steam pressure decrease will cause
indicated steam mass flow rate to...
A.
increase, because the density of the steam has increased.
B.
decrease, because the density of the steam has decreased.
C.
remain the same, because steam pressure does not affect
the mass flow rate of steam.
D.
remain the same, because the steam pressure input
compensates for changes in steam pressure.
Analysis:
NOTE: This question is basically discussing how a steam flow detection is
supposed to work!
It is important to note that for saturated steam, changes in temperature and
pressure will impact the mass flow rate of the steam. For example, if the
pressure of the steam were increased, this would increase the density of the
steam. However, the differential pressure measured by the venturi would not
change, so there would be no indicated change in mass flow rate of the
steam. Therefore, steam pressure (which is proportional to density) is also
measured and used to density compensate the signal to provide accurate mass
flow rate.
If the steam pressure decreases, this will result in a lower density, therefore
lower flow rate indication (with a constant volumetric flow rate).
Rev 2
25
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
If the steam pressure input to a density-compensated
steam flow instrument fails low, the indicated flow rate
will...
A.
decrease because the density input has decreased.
B.
decrease because the density input has increased.
C.
increase because the density input has increased.
D.
increase because the density input has decreased.
Analysis:
It is important to note that for saturated steam, changes in temperature and
pressure will impact the mass flow rate of the steam. For example, if the
pressure of the steam were increased, this would increase the density of the
steam. However, the differential pressure measured by the venturi would not
change, so there would be no indicated change in mass flow rate of the
steam. Therefore, steam pressure (which is proportional to density) is also
measured and used to density compensate the signal to provide accurate mass
flow rate.
If the steam pressure input fails low, this will result in a lower perceived
density, therefore lower flow rate indication.
Remember, as goes density compensation signal, so goes indication!
ELO 4.4 Failure Modes
Knowledge Check - Answer
The most probable cause for fluctuating indication from
a liquid flow rate differential pressure detector is...
Rev 2
A.
unequal temperature gradients in the liquid.
B.
gas or steam being trapped in the liquid.
C.
vortexing of the liquid passing through the flow device.
D.
the valve on the high-pressure sensing line being
partially closed.
26
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
Which one of the following will cause indicated
volumetric flow rate to be lower than actual volumetric
flow rate using a differential pressure flow detector
connected to a calibrated orifice?
A.
The orifice erodes over time.
B.
Debris becomes lodged in the orifice.
C.
System pressure decreases.
D.
A leak develops in the low-pressure sensing line.
Knowledge Check – Answer
If the orifice in a differential pressure (D/P) flow sensor
erodes such that the orifice opening becomes larger,
indicated flow rate will __________ due to a
__________ D/P across the orifice. (Assume actual flow
rate remains the same.)
A.
increase; larger
B.
increase; smaller
C.
decrease; larger
D.
decrease; smaller
Analysis:
Erosion of the orifice will result in a larger cross-sectional area, therefore
producing a smaller pressure drop (D/P).
A D/P cell measures high pressure minus low pressure and calculates a flow
rate. Flow is proportional to the square root of differential pressure.
Thus, anything that reduces differential pressure across the orifice will cause
indicated volumetric flow rate to decrease.
Rev 2
27
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
Refer to the drawing of a pipe elbow used for flow
measurement in a cooling water system below. A
differential pressure (D/P) flow detector connects to
instrument lines A and B. If instrument line B develops
a leak, indicated flow rate will ______________ due to a
______________ measured D/P.
A.
increase; larger
B.
increase; smaller
C.
decrease; larger
D.
decrease; smaller
Analysis:
The pipe elbow can be used to measure differential pressure (which is
proportional to flow). As the water flows through the pipe elbow/bend, a low
pressure area is created on instrument line B. This is because the velocity of
the flow tends to flow directly toward instrument line A, creating a higher
pressure. A D/P cell measures high pressure minus low pressure and
calculates a flow rate. Flow is proportional to the square root of differential
pressure.
If instrument line B (low pressure) develops a leak, D/P will increase,
causing an increase in indicated flow.
Rev 2
28
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 4.5 Environmental Effects
Knowledge Check - Answer
The __________ of the fluid whose flow is to be
measured can have a large effect on flow sensing
instrumentation. The effect of _______ is most
important when the flow sensing instrumentation is
measuring gas flows, such as steam.
A.
mass; mass
B.
flow rate; flow rate
C.
density; density
D.
volume; volume
ELO 5.1 Switch Type Detectors
Knowledge Check - Answer
What is the most common type of sensor used to provide
remote position indication of a valve that is normally
either fully open or fully closed?
Rev 2
A.
Linear variable differential transformer
B.
Limit switch
C.
Reed switch
D.
Servo transmitter
29
Sensors and Detectors Part 1
Knowledge Check Answer Key
Knowledge Check - Answer
In an electrical measuring circuit, reed switches monitor
the position of a control rod in a nuclear reactor. The
reed switches mount to a column above the reactor vessel
such that the control rod drive shaft passes by the reed
switches as the control rod is withdrawn.
Which one of the following describes the action that
causes the electrical output of the measuring circuit to
change as the control rod is withdrawn?
A.
An AC coil on the control rod drive shaft induces a
voltage into each reed switch as the drive shaft passes by.
B.
A metal tab on the control rod drive shaft mechanically
closes each reed switch as the drive shaft passes by.
C.
The primary and secondary coils of each reed switch
attain maximum magnetic coupling as the drive shaft
passes by.
D.
A permanent magnet on the control rod drive shaft
attracts the movable contact arm of each reed switch as
the drive shaft passes by.
Analysis:
Reed switches are commonly used to provide control rod position indication.
As the control rod is withdrawn, permanent magnets in the control rod cause
the reeds (made out of ferrous material) to move toward the rod and makes
up a switch. The electrical switches/contacts are then arranged in a voltage
divider network to change resistance/current in a circuit to determine control
rod position.
Rev 2
30
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 5.2 Variable Output Detectors
Knowledge Check - Answer
Which one of the following devices is commonly used to
provide remote indication of valve position on an analog
meter in units of "percent of full open"?
A.
Limit switch
B.
Reed switch
C.
Linear variable differential transformer
D.
Resistance temperature detector
Analysis:
The key words in the stem are “percent of full open “. Linear variable
differential transformers (LVDT’s) operate on the principal that as valve
stem position changes, a moveable core will change the magnetic coupling
between the primary and secondary windings.
It is important to note that the two secondary windings are wired such that
the electrical currents oppose each other. If these currents were additive,
there would be no way to determine valve position in mid-stroke (i.e. as the
stem moves up and the moveable core uncouples part of the lower secondary
winding, it in turn couples part of the upper secondary winding that was not
previously coupled). The output ranges from -10 volts (Full closed) to +10
volts (full open). If the primary winding power supply was to lose power, the
secondary output indication would go to 50% open (0 volts).
Rev 2
31
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 5.3 Position Detector Circuits
Knowledge Check - Answer
Variations in __________________ can directly affect
the____________of components in the instrumentation
circuitry.
Rev 2
A.
ambient temperature; voltage
B.
voltage; resistance
C.
voltage; temperature
D.
ambient temperature; resistance
32
Sensors and Detectors Part 1
Knowledge Check Answer Key
ELO 5.4 Failure Modes
Knowledge Check
In the figure below there are 4 sets of reed switches. One
set is for full closed, another full open and the other sets
are for intermediate positions. If each reed switch set
completes a circuit to indicating lights what would be the
indication if the bottom set of reed switches failed closed
as the valve goes from closed to open?
Rev 2
A.
The intermediate lights would remain lit in addition to
the full open light as the valve travels full stroke.
B.
The full open light would remain lit in addition to the
intermediate and full open light as the valve travels full
stroke.
C.
No effect, all indicating lights would work as designed.
D.
The full closed light would remain lit in addition to the
intermediate and full open light as the valve travels full
stroke.
33