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Operator Generic Fundamentals
Components - Sensors and Detectors 1
© Copyright 2016
Operator Generic Fundamentals
2
Terminal 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 on the following Terminal Learning Objectives (TLOs):
1. Describe the operation of temperature detectors and conditions
that affect 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.
© Copyright 2016
Intro
Operator Generic Fundamentals
3
Temperature Detectors
TLO 1 – Describe the operation of temperature detectors and conditions
that affect their accuracy and reliability.
1.1 State the three basic functions of temperature detectors.
1.2 Describe the construction of a basic RTD including:
a.
Component arrangement
b.
Materials used
1.3 Describe how RTD resistance varies for temperature changes.
1.4 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
© Copyright 2016
TLO 1
Operator Generic Fundamentals
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Enabling Learning Objectives for TLO 1
1.5 Describe bridge circuit compensation for changes in ambient
temperature and environmental conditions that can affect
temperature detection instrumentation.
1.6 Describe the effect on temperature indication(s) for the following
circuit faults:
a. Short circuit
b. Open circuit
1.7 Describe alternate methods of determining temperature when the
normal sensing devices are inoperable.
1.8 Describe the construction and operation of a thermocouple.
© Copyright 2016
TLO 1
Operator Generic Fundamentals
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Functions of Temperature Detectors
ELO 1.1 – State the three basic functions of temperature detectors.
• The three basic functions of temperature detectors are:
– Indication
– Alarm
– Control (in the form of both protective interlocks and automatic trip
functions)
• Temperature is a measure of molecular activity
• Kinetic energy is a measure of the activity of atoms which make up
molecules of any material
• Temperature ⇒ measure of kinetic energy of a material
• Examples of temperature detectors
– Bulb thermometer, bimetallic strip
– RTD, thermocouple (explained in detail later)
© Copyright 2016
ELO 1.1
Operator Generic Fundamentals
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Filled System Thermometer
• Can provide local and remote
indication
• Consists of a sensing element
(a bulb containing gas or liquid)
and an indicator scale
• As temperature changes,
– Gas or liquid pressure
changes
– Can act on a spiral bourdon
tube for more displacement
– Motion can drive a
o Pointer or indicator, or,
o actuate a switch for
control response
© Copyright 2016
Figure: Filled System Thermometer
ELO 1.1
Operator Generic Fundamentals
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Bimetallic Strip Thermometer
• A simple, rugged device for
monitoring temperature
• Two strips of metal fastened
together throughout their length
Figure: Bimetallic Strip
– One end fixed, the other free
to move
– Two different coefficients of
thermal expansion
– Two metals will both always
be at the same temperature
Figure: Bimetallic Strip Thermometer
© Copyright 2016
ELO 1.1
Operator Generic Fundamentals
8
Resistance Temperature Detector
Construction
ELO 1.2 – Describe the construction of a basic RTD including the
following: component arrangement and materials used.
• RTDs act like electrical transducers
– Convert changes in temperature to changes in voltage
– Usually constructed of pure metal or alloy
– Increase resistance as temperature increases
– Decrease resistance as temperature decreases
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Resistance Temperature Detector
Construction
• Metals best suited for RTD use are as follows:
– Pure
– Uniform quality
– Stable within a given range of temperature
– Able to give reproducible resistance-temperature readings
– Capable of being drawn into fine wire
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
10
Resistance Temperature Detector
Construction
• Elements are usually:
– Long, spring-like wires
– Surrounded by insulator
– Enclosed in metal (inconel)
sheath
– Inconel normally used
because of inherent
corrosion resistance
Figure: Internal Construction of a Typical RTD
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Resistance Temperature Detector
Construction
• RTD inserted into protective measuring well
• Change in temperature causes platinum wire to heat or cool
• Resistance change measured by precision resistance measuring
device
Figure: RTD Protective Well and Terminal Head
© Copyright 2016
ELO 1.2
Operator Generic Fundamentals
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Temperature Resistance Relationship –
Resistance Temperature Detector
ELO 1.3 – Describe how RTD resistance varies for temperature changes.
• Normally constructed of
platinum, copper, or nickel
– Linear resistancetemperature characteristics
o Temperature increases,
resistance increases
– High coefficient of
resistance
– Ability to withstand
repeated temperature
cycles
© Copyright 2016
Figure: Resistance vs. Temperature Graph
ELO 1.3
Operator Generic Fundamentals
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Temperature Resistance Relationship –
Resistance Temperature Detector
Knowledge Check
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.
Correct answer is C.
© Copyright 2016
ELO 1.3
Operator Generic Fundamentals
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RTD Temperature Detection Circuit
ELO 1.4 – State the purpose of basic temperature instrument detection
and control system blocks: RTD, bridge circuit, DC-AC converter
amplifier, and balancing motor/mechanical linkage.
• Basic bridge circuit components
– Three known resistances, R1,
R2, and R3 (variable)
– Unknown variable resistor Rx
o RTD
– Source of voltage
– Sensitive ammeter
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ELO 1.4
Figure: Typical Bridge Circuit
Operator Generic Fundamentals
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RTD Bridge Circuit
• Ratio arms of bridge: R1 and R2
• Standard arm: R3 (variable
resistor)
– Adjusted to match unknown
resistor (Rx)
• Sensing ammeter visually
displays current that is flowing
through bridge circuit
© Copyright 2016
Figure: Typical Bridge Circuit
ELO 1.4
Operator Generic Fundamentals
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Unbalanced RTD Bridge Circuit
• Regulated current is divided
between two branches
– One branch has fixed
resistor Rx and range
resistor R1,
– Other branch with RTD and
range resistor R2
• As resistance of RTD changes
– voltages at points X and Y
change
• Millivolt meter detects change
in voltage
– Calibrated for temperature
© Copyright 2016
Figure: Unbalanced Bridge Circuit
ELO 1.4
Operator Generic Fundamentals
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Balanced Bridge Circuit
• Similar to unbalanced, except:
– Slidewire resistor used to
balance arms of bridge
• As RTD resistance changes
– Resistance of slide wire
adjusted until galvanometer
indicates zero
• Value of slide resistance
determines temperature of RTD
© Copyright 2016
Figure: Balanced Bridge Circuit
ELO 1.4
Operator Generic Fundamentals
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Typical Temperature Detection Circuit
• RTD measures temperature
– Detector is felt as resistance
to bridge network
• Bridge network converts
resistance to DC voltage signal
• Electronic instrument converts
DC to AC
• AC voltage amplified to drive
bi-directional motor
• Bi-directional motor positions
slider
– balance circuit resistance
– provide temperature
© Copyright 2016
ELO 1.4
Figure: Basic Temperature Detection Circuit
Operator Generic Fundamentals
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Environmental Effects On Temperature
Detection
ELO 1.5 – Describe bridge circuit compensation for changes in ambient
temperature and environmental conditions which can affect temperature
detection instrumentation.
Ambient Temperature
• Variations in ambient temperature directly affect
– Resistance of components in bridge circuit
– Resistance of thermocouple reference junction
o Explained later
• If temperature surrounding wiring increases
– Resistance increases
– Indication increases
© Copyright 2016
ELO 1.5
Operator Generic Fundamentals
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Environmental Effects On Temperature
Detection
Humidity
• Humidity causes moisture to collect on the equipment
– Leads to short circuits, grounds, and corrosion
– Could damage components
• The proper use of HVAC equipment controls humidity
© Copyright 2016
ELO 1.5
Operator Generic Fundamentals
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Environmental Effects On Temperature
Detection
Knowledge Check
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
Correct answer is D.
© Copyright 2016
ELO 1.5
Operator Generic Fundamentals
22
RTD Circuit Failures
ELO 1.6 – Describe the effect on temperature indication(s) for the
following circuit faults: Short circuit, Open circuit.
• If RTD in either unbalanced or balanced bridge circuit becomes open:
– Resistance becomes infinite
– Temperature will indicate high temperature (fail high)
• If RTD becomes shorted:
– Resistance becomes zero
– Temperature will indicate a low temperature (fail low)
© Copyright 2016
ELO 1.6
Operator Generic Fundamentals
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RTD Circuit Failures
Knowledge Check – NRC Bank
If shorting occurs within a resistance temperature detector, the
associated indication will fail...
A. low.
B. high.
C. as is.
D. to midscale.
Correct answer is A.
© Copyright 2016
ELO 1.6
Operator Generic Fundamentals
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Alternate Temperature Indications
ELO 1.7 – Describe alternate methods of determining temperature when
the normal sensing devices are inoperable.
• Installed spare/dual-element RTDs
– Dual-element RTD has two sensing elements
– If operating element becomes faulty, second element may be
used
• Contact pyrometer (portable thermocouple) or optical pyrometer
• If detector itself is still functional, connect an external bridge circuit to
detector
– Temperature obtained by comparing resistance readings to
detector calibration curves
© Copyright 2016
ELO 1.7
Operator Generic Fundamentals
25
Alternate Temperature Indications
Knowledge Check
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.
Correct answer is A.
© Copyright 2016
ELO 1.7
Operator Generic Fundamentals
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Thermocouples
ELO 1.8 – Describe the construction and operation of a thermocouple.
• A thermocouple converts thermal energy into electrical energy
• Constructed of two dissimilar metal wires joined together at one end
(junction)
• Other end of each wire is connected to a measuring instrument
Figure: Simple Thermocouple Circuit
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ELO 1.8
Operator Generic Fundamentals
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Thermocouple Construction
• Leads encased in a rigid metal
sheath
• Measuring junction at bottom of
thermocouple housing
• Magnesium oxide surrounds
thermocouple wires
– Prevents vibration that could
damage fine wires
– Enhances heat transfer
between measuring junction
and medium surrounding
thermocouple
© Copyright 2016
Figure: Internal Construction of a Typical Thermocouple
ELO 1.8
Operator Generic Fundamentals
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Thermocouple Operation
• Heating measuring junction of
thermocouple produces a
voltage
• Temperature indicated equals
hot junction voltage minus cold
junction voltage
• Reference junction calibrated to
“normal” environment
temperature
Figure: Simple Thermocouple Circuit
Mathematical representation:
Measuring Junction – Reference Junction = Voltmeter
Voltmeter + Reference Calibration = Indication
Assume: Measuring Junction = 400°F; Reference Junction Calibration = 100°F
Therefore, 400°F - 100°F = 300°F; 300°F + 100°F = 400°F
© Copyright 2016
ELO 1.8
Operator Generic Fundamentals
29
Thermocouple Failures and
Disadvantages
• Change in reference junction temperature causes a change in
indication
– If the temperature at the reference junction were to decrease, the
indicated temperature would increase
– If the temperature at the reference junction were to increase, the
indicated temperature would decrease
• Reference junction temperature should be controlled
• If break occurs in wire and there is no current flow, the device fails
low
• If break or open occurs in the detector, the indicated temperature fails
to the reference junction temperature
• Thermocouple is less accurate than a resistance temperature
detector
© Copyright 2016
ELO 1.8
Operator Generic Fundamentals
30
Thermocouples
Knowledge Check – NRC Bank
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.
Correct answer is D.
© Copyright 2016
ELO 1.8
Operator Generic Fundamentals
31
Thermocouples
Knowledge Check – NRC Bank Question
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.
Correct answer is C.
© Copyright 2016
ELO 1.8
Operator Generic Fundamentals
32
Pressure Detectors
TLO 2 – Explain the operation of pressure detectors and conditions that
affect their accuracy and reliability.
2.1 State the three functions of pressure measuring instrumentation.
2.2 Describe the theory and operation of the following differential
pressure detectors:
a. Bellows
b. Diaphragm
c.
Bourdon tube
d. Strain Gauge
2.3 Describe the factors that affect accuracy and instrumentation of
differential pressure detectors, including their failure modes.
© Copyright 2016
TLO 2
Operator Generic Fundamentals
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Pressure Detector Functions
ELO 2.1 – State the three functions of pressure measuring
instrumentation.
• Pressure detectors are used to provide three basic functions:
– Indication
o local and/or remote
– Alarm
o audible and/or visual
– Control
o Such that equipment is started or stopped as needed
© Copyright 2016
ELO 2.1
Operator Generic Fundamentals
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Pressure Detector Theory and Operation
ELO 2.2 – Describe the theory and operation of the following differential
pressure detectors: Bellows, Diaphragm, Bourdon tube, Strain gauge.
• Normally system pressure is exerted on one side while atmospheric
pressure exerted on the other
– The pressure difference produces movement
– This movement is directly proportional to the pressure differential
change
• Regardless of type of pressure detector, all operate on the concept:
– D/P = High pressure – Low pressure
o High pressure side is the sensing pressure (variable)
o Low pressure side is the reference pressure (atmospheric)
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
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Bellows Detector
• Sensitive to low pressures
• Most accurate when measuring
pressures from 0.5 to 75 psig
• When used in conjunction with
a heavy range spring, some
bellows can be used to
measure pressures of over
1,000 psig
Figure: Basic Metallic Bellows
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
36
Bellows Detector
• Bellows
– One-piece
– Collapsible
– Seamless
– Deep folds formed from very
thin-walled tubing
Figure: Basic Metallic Bellows
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
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Bellows Detector
• System pressure applied to
area surrounding the bellows
• Bellows will expand or contract
• Moving end of the bellows is
connected to a mechanical
linkage assembly
• As bellows and linkage
assembly move, either:
– Electrical signal is
generated
Figure: Basic Metallic Bellows
– Direct pressure indication
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
38
Bellows Detectors
Knowledge Check
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 ____________.
decrease; increase
A. decrease; increase
B. increase; decrease
C. decrease; decrease
D. increase; increase
Correct answer is B.
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
39
Diaphragm Detector
• Used in low pressure applications
• High pressure side – sensing pressure
• Low pressure side – some reference pressure (atmospheric)
• Diaphragm material types
– Metallic or non-metallic
• When system pressure changes
– Causes axial deflection of diaphragm
• Corrugated designs provide additional strength and sensitivity
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
40
Bourdon Tube Detector
• Thin-walled tube
– flattened diametrically on
opposite sides
• Pressure applied to the inside of
the tube
– causes tube to straighten
slightly
• Tip of the tube used to position
a pointer
• Pressure increase causes tube
to “straighten out”
– less curvature
– causes meter deflection
© Copyright 2016
ELO 2.2
Figure: Bourdon Tube Detector Construction
Operator Generic Fundamentals
41
Bourdon Tube Detector
Knowledge Check
If the pressure sensed by a bourdon tube increases, the curvature 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
Correct answer is C.
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
42
Strain Gauge
• Sometimes used for RCS pressure instruments
• Increase in pressure at inlet of bellows causes bellows to expand
• Expansion of bellows moves flexible beam
• Movement of beam causes resistance of strain gauge to change
• Temperature compensating gauge compensates for heat produced
by current flowing through fine wire of strain gauge
Figure: Strain Gauge Pressure Transducer
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
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Strain Gauge
• Value of pressure found by
measuring change in resistance
of wire grid
𝐿
𝑅=𝐾
𝐴
• R = resistance of wire grid in
ohms
Figure: Strain Gauge
• K = resistivity constant for
particular type of wire grid
• L = length of wire grid
• A = cross sectional area of wire
grid
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
44
Strain Gauge
• Wire grid is distorted by elastic
deformation of pressure change
• For an increase in pressure
– Length increases
– Cross-sectional area
decreases
Figure: Strain Gauge
– Resistance increases
• Change in resistance used as
variable resistance in bridge
circuit that provides an electrical
signal for indication of pressure
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
45
Strain Gauge
• When change in resistance in
strain gauge causes an
unbalanced condition
– Error signal enters amplifier
– Actuates balancing motor
– Moves slider along
slidewire, restoring bridge to
balanced condition
Figure: Strain Gauge Used in a Bridge Circuit
• Slider’s position is noted on
scale marked in units of
pressure
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
46
Strain Gauge
Knowledge Check
Semiconductor strain gages are often used in transmitters for...
A. reactor coolant pressure instruments.
B. reactor coolant temperature instruments.
C. control rod position instruments.
D. steam generator level instruments.
Correct answer is A.
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
47
Pressure Detection Circuitry
Sensing Element (various types just discussed)
• Senses pressure of monitored system
• Converts pressure to a mechanical signal
• Supplies mechanical signal to transducer
Figure: Pressure Detection Circuit Block Diagram
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
48
Pressure Detection Circuitry
Transducer
• Converts mechanical signal to electrical signal that is proportional to
system pressure
• Various types include:
– Slidewire, inductance-type, differential transformer, and variable
capacitance
• If mechanical signal from sensing element is used directly, a
transducer is not required
– For example, bourdon tube local pressure gage
Figure: Pressure Detection Circuit Block Diagram
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
49
Pressure Transducer Types - Slidewire
• Some resistance-type
transducers combine a bellows
or a bourdon tube with a
variable resistor
• Expansion and contraction
causes the attached slider to
move along the slidewire,
increasing or decreasing the
resistance
Figure: Slidewire Resistance Type Transducer
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
50
Pressure Transducer Types - Inductance
• The inductance-type transducer
consists of the following three
parts:
– Coil
– Movable magnetic core
– Pressure-sensing element
Figure: Inductance Type Transducer
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
51
Pressure Transducer Types - Transformer
• Differential transformer
pressure transducer utilizes two
coils wound on a single tube
• Magnitude and direction of the
output depends on distance
core is displaced from its center
position
• Changes coupling from Primary
coil to Secondary coil (output)
Figure: Differential Transformer
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
52
Pressure Transducer Types - Capacitance
• Two flexible conductive plates
and a dielectric
• Dielectric is the fluid whose
pressure is being measured
• As pressure increases,
– Flexible conductive plates
move farther apart
– Changes capacitance of
transducer
© Copyright 2016
Figure: Variable Capacitive Type Transducer
ELO 2.2
Operator Generic Fundamentals
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Pressure Detection Circuitry
Pressure Detection Circuitry
• More common term - Transmitter
• Will amplify and/or transmit signal to pressure indicator
• Electrical signal generated by detection circuitry is proportional to
system pressure
Figure: Pressure Detection Circuit Block Diagram
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
54
Pressure Detection Circuitry
Pressure Indication
• Provides indication of system pressure
– May either be read locally or at remote location
Figure: Pressure Detection Circuit Block Diagram
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
55
Pressure Detection Circuitry
Knowledge Check
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
Correct answer is D.
© Copyright 2016
ELO 2.2
Operator Generic Fundamentals
56
Pressure Detector Environmental Effects
ELO 2.3 – Describe the factors that affect accuracy and instrumentation
of differential pressure detectors, including their failure modes.
Ambient Pressure
• Pressure instruments are usually sensitive to variations in
atmospheric pressure (reference pressure)
– If reference pressure increases, indication decreases by same
amount
Ambient Temperature
• Ambient temperature affects resistance of components in circuitry
• Slightly impacts material properties of metals used
– Temperature increase can cause metal to become more flexible
• Temperature compensation may be used, if necessary
© Copyright 2016
ELO 2.3
Operator Generic Fundamentals
57
Pressure Detector Environmental Effects
Humidity
• Presence of humidity will affect electrical equipment, especially
electronic components
• High humidity causes moisture to collect on equipment
• Moisture can cause short circuits, grounds, and corrosion, which
effect component performance
• Effects due to humidity are controlled by maintaining equipment in
proper environment
© Copyright 2016
ELO 2.3
Operator Generic Fundamentals
58
Pressure Detector Environmental Effects
Penetrating Radiation
• Radiation levels can affect detector reliability
• Extremely high radiation environments permanently embrittle the
metal in detectors
• Changes characteristics and elasticity of sensing mechanisms,
introducing errors
• High radiation levels can also affect the sensitive electronic circuits
housed in detectors
© Copyright 2016
ELO 2.3
Operator Generic Fundamentals
59
Pressure Detector Environmental Effects
Detector Failure or Overranging
• Pressure instruments are designed and selected to withstand
pressure above normal design pressure
• However, sudden overpressurizations causing over-range conditions
could straighten bourdon tubes or weaken bellows spring
• If sensing element of detector is stretched or stressed, indications
may be erroneously high
• If sensing element has a leak or rupture
– Indication will fail low
o 0# – if calibrated to psig
o 15# – if calibrated to psia
© Copyright 2016
ELO 2.3
Operator Generic Fundamentals
60
Pressure Detector Environmental Effects
Knowledge Check – NRC Bank
Refer to the drawing of a bellows-type differential pressure (D/P)
detector.
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
Correct answer is C.
© Copyright 2016
ELO 2.3
Operator Generic Fundamentals
61
Pressure Detector Environmental Effects
Knowledge Check – NRC Bank
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.
Correct answer is B.
© Copyright 2016
ELO 2.3
Operator Generic Fundamentals
62
Level Detectors
TLO 3 – Explain the operation of level detectors and conditions that affect
their accuracy and reliability.
3.1 Describe the three functions for using remote level indicators.
3.2 Describe the operation of the following types of level
instrumentation:
a. Gauge glass
b. Magnetic bond
c.
Conductivity probe
d. Differential pressure (D/P)
3.3 Describe density compensation in level detection systems to
include:
a. Why needed
b. How accomplished
© Copyright 2016
TLO 3
Operator Generic Fundamentals
63
Enabling Learning Objectives for TLO 3
3.4 State the purpose of basic differential pressure detector-type level
instrument blocks in a basic block diagram:
a. Differential pressure (D/P) transmitter
b. Amplifier
c.
Indication
3.5 Describe the environmental conditions which can affect the
accuracy and reliability of level detection instrumentation.
3.6 State the various failure modes of level detection instrumentation.
3.7 Analyze detector installation and applications to determine the
effects of transients on level indication.
© Copyright 2016
TLO 3
Operator Generic Fundamentals
64
Level Detector Functions
ELO 3.1 – Describe the three functions for using remote level indicators.
• Level detectors are used to provide the following basic functions:
– Indication
– Alarm
– Control
• Liquid level measuring devices are classified into the following groups:
– Direct method
o Dipstick in car which measures height of oil in oil pan
– Inferred method
o Pressure gauge at bottom of tank which measures hydrostatic
head pressure from height of liquid
© Copyright 2016
ELO 3.1
Operator Generic Fundamentals
65
Operation of Level Detectors
ELO 3.2 – Describe the operation of the following types of level
instrumentation: gauge glass, magnetic bond, conductivity probe, and
differential pressure (D/P).
• Multiple methods of monitoring and detecting level in plant systems
and components
• Each type of level detector has advantages and disadvantages
• Different types of D/P level detectors
– Wet-Reference type used for PZR and SG
© Copyright 2016
ELO 3.2
Operator Generic Fundamentals
66
Gauge Glass
• Transparent tube attached to bottom and top of tank
– Top connection not needed in tank open to atmosphere
– Height of liquid in tube will be equal to height of water in tank
Figure: Gauge Glass
© Copyright 2016
ELO 3.2
Operator Generic Fundamentals
67
Gauge Glass
• (a) shows a gauge glass used for vessels where liquid is at ambient
temperature and pressure conditions
• (b) shows a gauge glass used for vessels where liquid is at elevated
pressure or partial vacuum
Figure: Gauge Glass
© Copyright 2016
ELO 3.2
Operator Generic Fundamentals
68
Magnetic Bond Level Detector
• Developed to overcome
problems of cages and stuffing
boxes
• Magnetic bond mechanism
consists of magnetic float which
rises and falls with changes in
level
Figure: Magnetic Bond Level Detector
© Copyright 2016
ELO 3.2
Operator Generic Fundamentals
69
Conductivity Probe Level Detector
• Consists of:
– One or more level detectors
– Operating relay
– Controller
• When liquid makes contact with
any of electrodes, electric
current will flow between
electrode and ground
© Copyright 2016
Figure: Conductivity Probe Level Detection System
ELO 3.2
Operator Generic Fundamentals
70
Differential Pressure Level Detectors
• D/P detector connected to bottom of tank being monitored
• Higher pressure, caused by fluid in tank, is compared to lower
reference pressure (usually atmospheric)
– Comparison takes place in D/P detector
Figure: Open Tank Differential Pressure Detector
© Copyright 2016
ELO 3.2
Operator Generic Fundamentals
71
Differential Pressure Level Detectors
• Three basic types of D/P level detectors
(NRC uses a 4th for testing - #3)
– Open Reference (#2)
– Open Reference with loop seal (#3)
o Basically like #2, but with less
D/P
– Dry Reference (#4)
– Wet reference (#1)
• D/P = HP – LP (all 4 tanks)
• Tanks 2, 3, 4
– HP is tank
– If D/P increases, indicated level
increases
• Tank 1 (wet reference)
– HP is reference leg
– If D/P increases, indicated level
decreases
© Copyright 2016
ELO 3.2
Operator Generic Fundamentals
72
Differential Pressure Level Detectors
• Only Tanks 2 and 3 affected by
atmospheric pressure changes
• Tanks 1 and 4 gas pressure
cancels out
– Sensed on both sides
• Tank #1 D/P (HP – LP)
– Pref level + Pgas – (Ptank level + Pgas)
• Tank #2 D/P (HP – LP)
– (Ptank level + Pgas) – Patm
• Tank #3 D/P (HP – LP)
– (Ptank level + Pgas) – (Patm + Ploop
seal)
• Tank #4 D/P (HP – LP)
– (Ptank level + Pgas) – Pgas
© Copyright 2016
ELO 3.2
Operator Generic Fundamentals
73
Operation of Level Detectors
Knowledge Check
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
Correct answer is B.
© Copyright 2016
ELO 3.2
Operator Generic Fundamentals
74
Density Compensation In Level Detection
ELO 3.3 – Describe density compensation in level detection systems to
include: why needed and how accomplished.
• Density compensation considers hydrostatic pressure added by
vapor needs
• Considered when vapor with significant density exists above liquid in
tank or vessel
– Ensures accurate transmitter output
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
75
Specific Volume
• Specific volume is the standard unit used when working with vapors
and steam that have low values of density
• For applications that involve water and steam, specific volume can be
found using "Saturated Steam Tables”
• Specific volume is volume per unit mass:
𝑉𝑜𝑙𝑢𝑚𝑒
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑉𝑜𝑙𝑢𝑚𝑒 =
𝑀𝑎𝑠𝑠
• Specific volume is the reciprocal of density:
1
𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 𝑉𝑜𝑙𝑢𝑚𝑒 =
𝐷𝑒𝑛𝑠𝑖𝑡𝑦
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
76
Vessel with Water at Saturated Boiling
Conditions
• Condensing pot at top of
reference leg
– Condenses steam
– Maintains reference leg
filled
• Effect of steam vapor pressure
is cancelled at D/P transmitter
– Pressure is equally applied
to both LP and HP sides
© Copyright 2016
Figure: Effects of Fluid Density
ELO 3.3
Operator Generic Fundamentals
77
Vessel with Water at Saturated Boiling
Conditions
• Differential pressure seen by
transmitter is due only to
hydrostatic head pressure
𝐻𝑦𝑑𝑟𝑜𝑠𝑡𝑎𝑡𝑖𝑐 𝐻𝑒𝑎𝑑 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒
= 𝐷𝑒𝑛𝑠𝑖𝑡𝑦 × 𝐻𝑒𝑖𝑔ℎ𝑡
Figure: Effects of Fluid Density
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
78
Reference Leg Temperature
Considerations
• When level to be measured is in pressurized tank at elevated
temperatures, a number of additional consequences must be
considered
– Temperature of fluid in tank increases, density of fluid decreases
o As density decreases, fluid expands, occupying more volume
o Even though density is less, mass of fluid in tank is same
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
79
Reference Leg Temperature
Considerations
• As fluid in tank is heated and cooled, density of fluid in tank changes
• Reference leg density remains relatively constant
– Density of fluid in reference leg is dependent upon ambient
temperature
o Relatively constant and independent of tank temperature
– Causes indicated level to remain constant
• If fluid in tank changes temperature (and density) compensation must
be provided to have accurate indication
• Problem is encountered when measuring steam generator water
levels
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
80
Compensating for Reference Leg
Temperature Changes
• Density compensation may be accomplished through electronic
circuitry
– Compensate for density changes automatically
– Compensate for density by having operators manually adjust
inputs to level detection circuitry
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
81
Calibrated vs Actual Temperature (Tank)
• Consider a Wet Reference D/P Level Detector
– If tank temperature increases (above calibrated value)
o Density decreases, LP pressure decreases, D/P increases,
indicated level decreases
– In other words, if Tcal(tank) < Tact(tank), then, Lind < Lact
• Consider the other three D/P Level Detector types
– If tank temperature increases (above calibrated value)
o Density decreases, LP pressure decreases, D/P decreases,
indicated level decreases
– In other words, if Tcal(tank) < Tact(tank), then, Lind < Lact
• You can see this thumbrule works for ALL four tank types
– If the reference leg temperature increases, the opposite effect
occurs
o If Tcal(ref) < Tact(ref), then, Lind > Lact
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
82
Operation of Level Detectors
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.
A. higher, smaller
B. lower, smaller
C. higher, larger
D. lower, larger
Correct answer is D.
© Copyright 2016
ELO 3.3
Operator Generic Fundamentals
83
Level Detection Circuitry
ELO 3.4 – State the purpose of basic differential pressure detector-type
level instrument blocks in a basic block diagram: Differential pressure
(D/P) transmitter, Amplifier, Indication
• Diaphragm with HP and LP inputs
on opposite sides
• As D/P changes, diaphragm
moves
• Transducer changes mechanical
motion into electrical signal
• Signal is amplified for level
indication at remote location
• System provides alarms on high
and low level via relays and may
provide control functions
– Valve repositioning
– Pump tripping
© Copyright 2016
ELO 3.4
Figure: Differential Pressure Level Detection Circuit
Operator Generic Fundamentals
84
Environmental Effects On Level
ELO 3.5 – Describe the environmental conditions which can affect the
accuracy and reliability of level detection instrumentation.
Fluid Density
• Primarily affects wet reference leg devices (PZR and SG)
– Saturated systems
Ambient Temperature
• Effects on reference leg of wet reference D/P level detector
Humidity
• Can cause short circuits, grounds, and corrosion which may damage
components
© Copyright 2016
ELO 3.5
Operator Generic Fundamentals
85
Environmental Effects On Level - Fluid
Density
Knowledge Check
Refer to the drawing of a water storage tank with a differential pressure
(D/P) level detector (see figure below).
The level instrument has just been calibrated to read actual tank water
level. If the reference leg subsequently experiences high ambient
temperature, indicated level will...
A. equal the actual level.
B. read less than the actual level.
C. read greater than the actual level.
D. drift above and below the actual level.
Correct answer is C.
© Copyright 2016
ELO 3.5
Operator Generic Fundamentals
86
Level Detection Failure Modes
ELO 3.6 – State the various failure modes of level detection
instrumentation.
Failure mode depends on high-pressure and low-pressure
connection setup
• For most level detectors:
– If D/P decreases, indicated level will decrease
– If D/P increases, indicated level will increase
• Exception is wet reference leg level detection, which has the opposite
effect
– If D/P decreases, indicated level will increase
– If D/P increases, indicated level will decrease
© Copyright 2016
ELO 3.6
Operator Generic Fundamentals
87
Failures - Wet Reference Leg
• Reference leg is connected to the high-pressure side of the D/P cell,
causing the opposite reaction
– A break in variable leg or low-pressure side would cause low-level
indication
– A break on high-pressure (reference leg) side results in a lower
D/P and higher level than indicated
© Copyright 2016
ELO 3.6
Operator Generic Fundamentals
88
Level Detection Failure Modes
Knowledge Check – NRC Bank
Refer to the drawing of a steam generator (SG)
differential pressure (D/P) level detection system.
The SG is at normal operating temperature and
pressure with accurate level indication. Which one
of the following events will result in a 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.
Correct answer is D.
© Copyright 2016
ELO 3.6
Operator Generic Fundamentals
89
Detector Transients
ELO 3.7 – Analyze detector installation and applications to determine the
effects of transients on level indication.
• Wet reference D/P transients have big impacts
– Changes in SG or PZR pressure
o Impacts temperature and density of tank
– Reference leg flashing
o Occurs on lowering of SG or PZR pressure
• Temperature change in tank temperatures impact ALL tank types
• Dry Reference Leg types also impacted by condensation in dry leg
– Pressure of dry (LP) side increases, D/P decreases
– Indicated level decreases
© Copyright 2016
ELO 3.7
Operator Generic Fundamentals
90
Detector Transients – Wet Reference Leg
Wet Reference Leg Level Detector
• Liquid in reference leg applies a hydrostatic head to high-pressure
side of the transmitter
– value of this level is constant as long as reference leg is full
• If pressure remains constant, any change in D/P is due to a change
on low-pressure side of transmitter
Figure: Closed Tank Wet Reference Leg Differential Pressure Detector
© Copyright 2016
ELO 3.7
Operator Generic Fundamentals
91
Detector Transients – Wet Reference Leg
Loss of Reference Leg Pressure
• Reference leg pressure can be lost or reduced by
‒ Temperature increases
‒ Leaks or reference leg flashing
‒ Open or leaking equalizer valves
• Results in indicated level being higher than true level
Loss of Variable Leg Pressure
• Variable leg pressure can be lost or reduced by
– Temperature increases
– Leaks
– Open or leaking vent valves
• Results in indicated level being lower than true level
© Copyright 2016
ELO 3.7
Operator Generic Fundamentals
92
Detector Transients – Wet Reference Leg
Equalization
• Occurs when the equalization valve is either open or leaking
• Similar to losing the reference leg pressure
• Results in the indicated level being higher than actual
© Copyright 2016
ELO 3.7
Operator Generic Fundamentals
93
Detector Transients – Wet Reference Leg
Example
Refer to the figure for a
pressurizer at normal operating
temperature and pressure.
• Calibration at normal operation
temperature and pressure
• High-pressure side connects to
the reference leg
• If equalizing valve is opened,
indicated pressurizer level will
be greater than the actual level
• Results in a minimum D/P and a
maximum indicated level
© Copyright 2016
ELO 3.7
Figure: Steam Generator Level Detector
Operator Generic Fundamentals
94
Detector Transients – Wet Reference Leg
Example
• Now consider a transient where
reference leg temperature
decreases
• Results in higher density of
reference leg fluid
• Force exerted on reference leg
side of D/P detector is a result
of the height of the fluid and the
density
• If density increases, resultant
force will increase, resulting in a
higher differential pressure and
lower indicated level than the
true fluid level
© Copyright 2016
ELO 3.7
Figure: Steam Generator Level Detector
Operator Generic Fundamentals
95
Detector Transients
Knowledge Check
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.
Correct answer is B.
© Copyright 2016
ELO 3.7
Operator Generic Fundamentals
96
Flow Detectors Overview
TLO 4 – Describe the operation of flow detectors and conditions which
effect their accuracy and reliability.
4.1 Describe the theory of operation of a basic head flow meter.
4.2 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.
© Copyright 2016
Pitot tube
TLO 4
Operator Generic Fundamentals
97
Enabling Learning Objectives for TLO 4
4.3 Describe density compensation of a steam flow instrument to
include the reason density compensation is required and the
parameters used.
4.4 State the typical failure modes for head flow meters including the
effects of vapor on a flow instrument.
4.5 Describe the environmental conditions which can affect the
accuracy and reliability of flow sensing instrumentation.
© Copyright 2016
TLO 4
Operator Generic Fundamentals
98
Head Flow Meters
ELO 4.1 – Explain the theory of operation of a basic head flow meter.
• Head flow meters operate on principle of placing restriction in line to
cause differential pressure
• Differential pressure
– converted to flow measurement
• Types include:
– Orifice, flow venturi, pitot tube, flow nozzle, elbow flow meter
© Copyright 2016
ELO 4.1
Operator Generic Fundamentals
99
Head Flow Meter Construction
• Two elements in head flow
meter are:
– Primary element - restriction
in line
– Secondary element differential pressure
measuring device
Figure: Head Flow Instrument
© Copyright 2016
ELO 4.1
Operator Generic Fundamentals
100
Head Flow Meter Theory of Operation
V  DP
• Where:
D/P = differential pressure caused by restriction
V = volumetric flow rate
• Also,
V  K DP
• Where:
K = flow constant for the meter
© Copyright 2016
ELO 4.1
Operator Generic Fundamentals
101
Head Flow Meter Theory of Operation
• Head flow meter actually
measures volumetric flow rate
• Mass flow rate is required for
certain flow systems
– Calculated by knowing
o Temperature/pressure
– Recall
m  V
© Copyright 2016
Figure: Head Flow Instrument
ELO 4.1
Operator Generic Fundamentals
102
Head Flow Meter Theory of Operation
• To show the relationship between temperature or pressure, the mass
flow rate equation is often written as follows:
𝑚 = 𝐾𝐴 𝐷/𝑃(𝑃)
𝑚 = 𝐾𝐴
𝐷/𝑃
𝑇
• Where:
𝑚 = mass flow rate
A = area
D/P = differential pressure
P = pressure
T = temperature
K = flow coefficient
© Copyright 2016
ELO 4.1
Operator Generic Fundamentals
103
Head Flow Meter
Knowledge Check
Flow detectors (such as an orifice, flow nozzle, and venturi tube)
measure flow rate using the principle that flow rate is...
A. directly proportional to the differential pressure (D/P) squared.
B. inversely proportional to the D/P squared.
C. directly proportional to the square root of the D/P.
D. inversely proportional to the square root of the D/P.
Correct answer is C.
© Copyright 2016
ELO 4.1
Operator Generic Fundamentals
104
Flow Meter Construction
ELO 4.2 – Describe the basic construction of the following types of head
flow detectors: orifice plates, venturi tube, dall flow tube, flow nozzle,
elbow meter, and pitot tube.
• There are several designs of flow meters that work on the theory
that flow is proportional to the square root of the D/P
© Copyright 2016
ELO 4.2
Operator Generic Fundamentals
105
Orifice Plates
• Fluid forced to converge
through small hole
• Velocity and pressure change
• Point of maximum convergence
– Called vena contracta
• Beyond vena contracta, fluid
expands; velocity and pressure
change back close to original
• Measuring D/P between normal
pipe section and vena contracta
to find flow
© Copyright 2016
ELO 4.2
Figure: Orifice Plate
Operator Generic Fundamentals
106
Orifice Plates
• Three kinds of orifice plates used are:
– Concentric
– Eccentric
– Segmental
Figure: Orifice Plate Types
• Disadvantages
– Cause high permanent pressure drop
o Outlet pressure will be 60% to 80% of inlet pressure
– Subject to clogging or erosion
o Clogging – high D/P, higher indicated flow
o Erosion – low D/P, lower indicated flow
© Copyright 2016
ELO 4.2
Operator Generic Fundamentals
107
Venturi Tube
• Most accurate flow-sensing element when properly calibrated
• Inlet section decreases area of fluid stream
– Velocity increase
– Pressure decrease
• Low pressure measured in center of cylindrical throat
– Pressure will be at its lowest value
– Neither pressure nor velocity is changing
Figure: Venturi Tube
© Copyright 2016
ELO 4.2
Operator Generic Fundamentals
108
Flow Nozzle
• Used for high-velocity flow
• Smooth contoured flow restriction
• High permanent pressure loss similar to the orifice
Figure: Flow Nozzle
© Copyright 2016
ELO 4.2
Operator Generic Fundamentals
109
Elbow Meter
• Small D/P allows for high accuracy
• D/P a function of:
– centripetal force throws the fluid
to the "outside of the curve",
increasing pressure there
• Difference in surface area creates
– Low-pressure on the inner pipe
wall
– High-pressure on the outer pipe
wall
• Can measure flow in either
direction
• Some elbow meters have three
inner taps
– redundancy
© Copyright 2016
ELO 4.2
Figure: Elbow Meter
Operator Generic Fundamentals
110
Pitot Tube
• Pitot tube actually measures fluid velocity instead of fluid flow rate
– High pressure side = static pressure+dynamic pressure (velocity)
– Low pressure side = static pressure
– D/P is dynamic pressure (velocity)
• Must be calibrated for each specific application
– No standardization
• Can be used even when fluid is not enclosed in pipe or duct
Figure: Pitot Tube
© Copyright 2016
ELO 4.2
Operator Generic Fundamentals
111
Flow Meter Construction
Knowledge Check
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
Correct answer is A.
© Copyright 2016
ELO 4.2
Operator Generic Fundamentals
112
Steam Flow - Density Compensation
ELO 4.3 – Describe density compensation of a steam flow instrument to
include the reason density compensation is required and the parameters
used.
• Recall – steam flow detectors provide volumetric flow rate
• Pressure changes affect density of steam
• Flow measurement of steam systems requires compensation for
density
– Gasses are compressible, while liquids are not
• When pressure decreases, so does density
• Less mass per unit of volume (density)
• Change in density takes place between high- and low-pressure taps
• Density compensation converts volumetric flow rate to mass flow rate
© Copyright 2016
ELO 4.3
Operator Generic Fundamentals
113
Density Compensation
• The following equations are used to describe the fundamental
relationship for volumetric flow and mass flow
V  K DP
𝑚 = 𝑉𝜌
• Where:
𝑉 = volumetric flow
K = constant relating to the ratio of pipe to orifice
D/P = differential pressure
ρ = density
𝑚 = mass flow
© Copyright 2016
ELO 4.3
Operator Generic Fundamentals
114
Steam Mass Flow Detection System
Figure: Simple Mass Flow Detection
© Copyright 2016
ELO 4.3
Operator Generic Fundamentals
115
Steam Mass Flow Detection System
Density Compensation Example
𝑚Act = 𝑚Ind, or, 𝜌𝑉 Act = 𝜌𝑉 Ind
• If steam pressure increases (with constant volumetric flow rate)
– ↑ 𝜌 ↔ 𝑉 Act=↑ 𝑚Act (actual mass flow rate increases)
• With density compensation
– ↑ 𝜌 ↔ 𝑉 Ind = ↑ 𝑚Ind (indicated mass flow rate also increases)
• Without density compensation
– All 𝑚Ind sees is 𝑉Ind, therefore, since ↔ 𝑉 Ind
– ↑ 𝑚Act = ↔ 𝑚Ind
– Actual > Indicated
© Copyright 2016
ELO 4.3
Operator Generic Fundamentals
116
Steam Mass Flow Detection System
Density Compensation Failure Example
𝜌𝑉 Act = 𝜌𝑉 Ind
• If density compensation fails high
– ↔ 𝑚Act (no change in steam pressure or volumetric flow rate)
– However, ↑ 𝜌𝑉Ind, therefore, ↑ 𝑚Ind
• If density compensation fails low
– Opposite effect (indicated flow decreases)
© Copyright 2016
ELO 4.3
Operator Generic Fundamentals
117
Density Compensation
Knowledge Check
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.
Correct answer is B.
© Copyright 2016
ELO 4.3
Operator Generic Fundamentals
118
Density Compensation
Knowledge Check
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.
Correct answer is A.
© Copyright 2016
ELO 4.3
Operator Generic Fundamentals
119
Flow Meter Failure Modes
ELO 4.4 – State the typical failure modes for head flow meters including
the effects of vapor on a flow instrument.
• Leakage is a common problem with head flow meters
• Erosion or blockage of orifice plates
• Vapors/gases in liquid systems
• Loss of density compensation
© Copyright 2016
ELO 4.4
Operator Generic Fundamentals
120
Flow Meter 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
The orifice size will increase due to
the erosion. This results in a lower
D/P for the same flows.
4.
Loss of density
compensation
input
Indicated flow
less 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 indicated mass
flow rate will be lower.
© Copyright 2016
ELO 4.4
Operator Generic Fundamentals
121
Flow Meter Failure Modes
Condition
Indication
Discussion
5.
Steam pressure Indicated flow
input fails low
less than actual
Apparent density has decreased, less
mass is sensed passing the flow
detector.
6.
Steam pressure Indicated flow
input fails high
more than
actual
Apparent density has increased, more
mass is sensed passing the flow
detector.
7.
Vapor in a liquid Erratic, unstable As vapor goes through the measuring
flow indication
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.
© Copyright 2016
ELO 4.4
Operator Generic Fundamentals
122
Flow Meter Failure Modes
Knowledge Check
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
Correct answer is D.
© Copyright 2016
ELO 4.4
Operator Generic Fundamentals
123
Flow Meter Failure Modes
Knowledge Check
Refer to the drawing below of a pipe elbow used for flow measurement
in a cooling water system. 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
Correct answer is A.
© Copyright 2016
ELO 4.4
Operator Generic Fundamentals
124
Environmental Effects On Flow Detection
ELO 4.5 – Describe the environmental conditions which can affect the
accuracy and reliability of flow sensing instrumentation.
Fluid Density
• Effect of density is most important when flow sensing instrumentation
is measuring gas flows, such as steam
• Density of gas directly affected by temperature and pressure
– Any changes in either of these parameters has direct effect on
measured flow
© Copyright 2016
ELO 4.5
Operator Generic Fundamentals
125
Environmental Effects On Flow Detection
Ambient Temperature
• Ambient temperature variations will affect accuracy and reliability of
flow sensing instrumentation
– Directly affect resistance of components in instrumentation
circuitry
– Affects calibration of electric/electronic equipment
• Effects reduced by design of circuitry and by maintaining flow sensing
instrumentation in proper environment
© Copyright 2016
ELO 4.5
Operator Generic Fundamentals
126
Environmental Effects On Flow Detection
Humidity
• Humidity will affect most electrical equipment, especially electronic
equipment
– High humidity causes moisture to collect on equipment
‒ Can cause short circuits, grounds, and corrosion, which, in turn,
may damage components
• Effects due to humidity are controlled by maintaining equipment in
proper environment
© Copyright 2016
ELO 4.5
Operator Generic Fundamentals
127
Position Detectors Overview
TLO 5 – Describe the operation of position detectors and conditions
which effect their accuracy and reliability.
5.1 Describe the following switch position indicators to include basic
construction and theory of operation:
a. Limit switches
b. Reed switches
c. Coil stacks
5.2 Describe the following variable output position indicators to include
basic construction and theory of operation:
a. Potentiometer
b. Linear variable differential transformers (LVDT)
© Copyright 2016
TLO 5
Operator Generic Fundamentals
128
Enabling Learning Objectives for TLO 5
5.3 Describe the environmental conditions that can affect the accuracy
and reliability of position indication equipment.
5.4 Describe the failure modes for the following position detectors:
a. Reed switch
b. Limit switch
c.
Potentiometer
d. LVDT
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TLO 5
Operator Generic Fundamentals
129
Switch Position Indicators
ELO 5.1 – Describe the following switch position indicators to include
basic construction and theory of operation: limit switches, reed switches,
and coil stacks.
• Mechanical limit switches provide valve open and shut indications
• Reed switches can provide intermediate valve position(s)
• Reed switches or Coil stacks also used for control rod position
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ELO 5.1
Operator Generic Fundamentals
130
Limit Switches
• A limit switch is a mechanical
device used to determine
physical position of equipment
– Limit switch gives on/off
output that corresponds to
valve position
• Electric circuit contacts usually
provide for dual indication
– Any position other than full
open or full closed
Figure: Limit Switches
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ELO 5.1
Operator Generic Fundamentals
131
Reed Switches
• Extension used with reed switch
is permanent magnet
– As magnet approaches, the
reed switch closes
– When magnet moves away,
reed switch opens
• On/off indicator is similar to
mechanical limit switch
• Incremental positions can be
measured
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Figure: Reed Switch - Valve Position
ELO 5.1
Operator Generic Fundamentals
132
Reed Switch
• Used for position indication of
control rods
• Permanent magnet installed on
control rod drive shaft
– attracts moveable contact
arm of each reed switch as
drive passes by
– Closes contacts as rod
withdrawn (S1, then S2,
etc.)
– Shorts out resistors
Figure: Reed Switch – Control Rod Position
– Current increases as rod is
withdrawn
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ELO 5.1
Operator Generic Fundamentals
133
Coil Stacks
• Coils wired in sets on outside of
control rod drive housing
• with rod on bottom
– coil impedance is balanced
– all detector sets send a 0.0V
signal
• As rod drive shaft passes through
Coil A, impedance increases
– Current decreases in
ammeter A
• As rod withdrawal continues
– Current in ammeter A stays
the same
– Current in ammeter B
decreases
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ELO 5.1
Figure: Coil Stacks
Operator Generic Fundamentals
134
Limit Switches
Knowledge Check
Reed switches are being used in an electrical measuring circuit to monitor the
position of a control rod in a reactor. The reed switches are mounted in 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.
Correct answer is D.
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ELO 5.1
Operator Generic Fundamentals
135
Potentiometers and LVDTs
ELO 5.2 – Describe the following variable output position indicators to
include basic construction and theory of operation: Potentiometer, Linear
Variable Differential Transformers (LVDT).
• Several applications require something other than full OPEN or full
CLOSED indication
– Turbine governor control valves, for example
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ELO 5.2
Operator Generic Fundamentals
136
Potentiometer
• Provides an accurate indication
of position throughout travel of
valve
• Extension is physically attached
to variable resistor
– As extension moves up or
down
o resistance changes,
changing current flow
– Amount of current is
proportional to valve
position
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Figure: Potentiometer
ELO 5.2
Operator Generic Fundamentals
137
Linear Variable Differential Transformers
• Similar to potentiometer, but no
physical connection
• Valve position indication
provided by:
– Coupling primary to
secondary windings of a
transformer
• Secondary voltage ranges from
– -10 vdc to + 10 vdc
o Full closed to full open
• Valve position shown in
– Percent open
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Figure: Linear Variable Differential Transformer
ELO 5.2
Operator Generic Fundamentals
138
Variable Output Detectors
Knowledge Check
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
Correct answer is C.
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ELO 5.2
Operator Generic Fundamentals
139
Environmental Effects On Position
Detection
ELO 5.3 – Describe the environmental conditions that can affect the
accuracy and reliability of position indication equipment.
Ambient Temperature
• Variations in ambient temperature can directly affect resistance of
components, and therefore, indications
• Effects are reduced by design of circuitry and by maintaining position
indication instrumentation in proper environment
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ELO 5.3
Operator Generic Fundamentals
140
Environmental Effects On Position
Detection
Humidity
• High humidity causes moisture to collect on equipment
• Moisture can cause short circuits, grounds, and corrosion, which, in
turn, may damage components
• Effects due to humidity are controlled by maintaining equipment in
proper environment
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ELO 5.3
Operator Generic Fundamentals
141
Environmental Effects
Knowledge Check
Variations in ______________ can directly affect the ____________ of
components in the instrumentation circuitry.
A. ambient temperature; voltage
B. voltage; resistance
C. voltage; temperature
D. ambient temperature; resistance
Correct answer is D.
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ELO 5.3
Operator Generic Fundamentals
142
Position Indication Failure Mode
ELO 5.4 – Describe the failure modes for the following position detectors:
Reed switch, Limit switch, Potentiometer, Linear variable differential
transformers.
• Limit switch failures are normally mechanical in nature
• If proper indication or control function is not achieved, limit switch is
probably faulty
• In case of failure, local position indication should be used to verify
equipment position
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ELO 5.4
Operator Generic Fundamentals
143
Reed Switch Failures
• Failures normally limited to reed switch which is stuck open or stuck
shut
– If reed switch is stuck shut, indication (open or closed) will be
continuously illuminated
– If reed switch stuck open, position indication for that switch
remains extinguished regardless of valve position
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ELO 5.4
Operator Generic Fundamentals
144
Potentiometer Failures
• Normally electrical in nature
• Electrical short or open will
cause indication to fail at one
extreme or other
• Increase or decrease in
potentiometer resistance leads
to erratic valve position
indication
Figure: Potentiometer
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ELO 5.4
Operator Generic Fundamentals
145
Linear Variable Differential Transformer
Failures
• LVDTs are extremely reliable
• Failures limited to rare electrical
faults or loss of power
– An open primary winding will
cause indication to fail to
zero volts
o Normally corresponds to
mid-position
– Failure of either secondary
winding
o cause output to indicate
either full open or full
closed
– regardless of actual
valve position
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ELO 5.4
Figure: Linear Variable Differential Transformer
Operator Generic Fundamentals
146
NRC KA to ELO Tie
KA #
KA Statement
RO SRO
ELO
K1.01 Characteristics of venturis and orifices
2.2
2.4 4.1, 4.2
K1.02 Temperature/density compensation requirements
2.7
2.9
4.3
K1.03 Effects of gas or steam on liquid flow rate indications (erroneous reading)
2.7
2.9
4.4
K1.04 Modes of failure
2.7
2.7
4.4
K1.05 Explain the operation of a flow D/P cell type flow detector
2.6
2.8 4.1, 4.2
K1.06 Temperature/pressure compensation requirements
2.5
2.6
4.3
K1.07 Theory and operation of level detectors
2.5
2.6
3.2
K1.08 Effects of operating environment (pressure and temperature)
2.8
3.1
3.5
K1.09 Modes of failure
Theory and operation of pressure detectors (bourdon tubes, diaphragms, bellows,
K1.10 forced balance, and variable capacitance)
2.9
3.0
3.6
2.3
2.5
2.2
K1.11 Effects of operating environment (pressure, temperature)
2.7
3.0
2.3
K1.12 Modes of failure
2.8
2.9
2.3
K1.13 Theory and operation of T/C, RTD, thermostats
2.6
2.8 1.2, 1.8
K1.14 Failure modes of T/C and RTD
2.8
2.9 1.5, 1.6
K1.15 Failure modes of reed switches, LVDT, limit switches, and potentiometers
2.3
2.4
K1.16 Applications of reed switches, magnets, LVDT, potentiometers, and limit switches
2.3
2.7 5.1, 5.2
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5.4
Operator Generic Fundamentals