Download Chapter 11 Flow Control

Syafruddin Hasan
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Systems
Concepts
• Automated systems
that control flow
rates or volume
employ the following
components:
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A Source
A Path
A Control Function
An Actuator
A Measuring
Instrument
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Reasons for Control
• To ensure the correct proportions of raw materials
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are combined during the manufacturing process
To ensure that ingredients are supplied at the proper
rate during the mixing and blending of the materials
To prevent a high flow rate than might cause
pressure or temperatures to become dangerous,
overspills to occur, or machines to overspeed
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Flow Units of Measurement
• Common classifications used to determine
flow instruments are:
– Volumetric flow rate - in cubic feet, gallons, or
liters per unit of time (this is an inferred
measurement)
– Mass flow rate - pounds, tons, grams, or
kilograms per unit of time
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Solid Flow Measurement
• Solid materials that are measured for mass flow rate
are typically small particles, powder, pellets, or
crushed materials
• Conveyors are usually used to transport these
materials
• Mass flow rate calculation:
WS
F=
L
F = Mass flow rate in lb/min
W = Weight of a material on a section of length
S = Conveyor speed in ft/min
L = Length of weighing platform
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Solid Flow Measurement
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Fluid Flow Measurement
• Important terms:
– Velocity - speed at which a fluid moves
through a pipe
– Density - weight per unit volume
– Viscosity - Ease of flow of a fluid
– Pipe size - Diameter of the pipe carrying
the fluid
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Reynolds’ Number
• A numerical scheme that represents the
four preceding factors on fluid flow (R
number)
V = Velocity
VDp
R=
u
R = Reynolds’ Number
D = Pipe inside diameter
p = Fluid density
u = Liquid viscosity
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Application of Reynolds’
Number
• The R-number is used to identify the type of flow
likely to occur in a process: laminar flow, turbulent
flow, or transition flow
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Fluid Flowmeter Classification
• One method of classifying flowmeters is
to divide them into the following
categories:
– Differential Pressure
– Positive Displacement
– Velocity
– Direct Reading Mass
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Differential Pressure
Flowmeter
• Most common type of flowmeter
• A restriction called an orifice is placed on the
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•
flow
An orifice plate has a specified size hole
bored through it, through which the liquid
must flow
Using Bernoulli’s Principle, the differential
pressure is measured across the restriction
and flow rates may be calculated
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Flow Restrictor Types
• Conventional
orifice plate
• Flow nozzle
restrictor
• Venturi tube
restrictor
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Rotameter
• Also known as a
variable area
flowmeter
• The float, when no
flow is present, settles
at a location in the
tube with the same
diameter as has the
tube
• As flow increases, the
float raises to a higher
level in the tube
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Positive Displacement
Methods
• Rotary instruments that mechanically
make direct measurements to the fluid
by separating the fluid into segments of
known values
• Two types of PD flowmeters are
rotary-vane and lobed impeller
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PD Flowmeters
Rotary-vane Flowmeter
Lobed impeller flowmeter
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Velocity Meters
• Velocity meters measure the velocity of fluids directly
• The most common type of velocity meter is the turbine
flowmeter
• Essentially, the turbine flowmeter is a flow-powered generator
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Electronic Sensors
• Several electronic flowmeters are available at
this time:
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The
The
The
The
The
The
The
coriolis meter
rotor flow detector
electromagnetic flow detector
thermal flowmeter
vortex flowmeter
ultrasonic flowmeter
time-of-flight meter
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• Built around a U-shaped tube that liquid flows through
• The tube is caused to vibrate by induction of fluctuating currents
• Flow through the tube causes it to twist proportionally to the flow.
The distortion of the tube is detected by magnetic sensors and the
resultant signal is conditioned and sent to control equipment
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Electromagnetic Flow
Detectors
• Electromagnetic flow
detector is a transducer that
converts the flow rate of a
conductive substance into a
voltage
• Based upon Faraday’s law of
induction
• Used for difficult and
corrosive liquids and slurries
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Thermal Flowmeters
• Liquid in the flow stream carries away
heat from the thermistor, thereby
raising its resistance, which unbalances
a bridge circuit
• Thermal flowmeters use a thermistor
inserted into the flow stream
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Figure Thermal flowmeter
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• A blunt object is placed in
the flow path of the liquid,
and as liquid is forced
around the object, vortices
are formed
• The number of vortices
formed downstream is
directly proportional to the
flow rate; the vortices
cause a change in the
pressure at the vortices’
location, which is then
measured
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Ultrasonic Flowmeters
• Using the Doppler Effect, ultrasonic
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flowmeters measure flow by analyzing the
frequency shift of induced sound waves in a
flow process. The frequency shift is directly
proportional to the flow rate of the liquid
This method is only used on liquids that have
particles present, not for clear liquids
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The Doppler ultrasonic flowmeter
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Time-of-Flight Flowmeter
• For clear liquid flow measurements using
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ultrasonic principles, the time-of-flight
flowmeter is used
It is based upon the principle that the speed
of sound transmitted in the direction of flow
will increase, and decrease if transmitted
against the flow
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Time-of-Flight flowmeter
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When selecting a flowmeter, the following issues need
to be considered:
• Is the fluid a gas or a
• What is the viscosity of
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liquid?
Is the fluid corrosive?
Is the fluid conductive
or not?
Does the fluid contain a
slurry or large solids?
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the fluid?
Will the fluid density or
viscosity change?
Is there a need for a
noninvasive approach?
What is the cost?
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