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Fundamentals of Instrumentation& Process Control
Interactive Training Workshop
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Fundamentals of Instrumentation & Control
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Introduction to Process Control
A common misconception in process control is that it is allabout the controller that you can force a particularprocess response just by getting the right tuningparameters.In reality, the controller is just a partner. A process will
respond to a controllers commands only in the mannerwhich it can. To understand process control you mustunderstand the other partners as well: sensors, finalcontrol elements and the process itself.All of these determine what type of response the controlleris capable of extracting out of the process. It is not theother way around.
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Outline of the Course
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Introduction to Process Control
Objectives:Why do we need process control?What is a process?What is process control?
What is open loop control?What is closed loop control?What are the modes of closed loop control?What are the basic elements of process control?
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Motivation for Automatic Process Control
Safety First:people, environment, equipment
The Profit Motive:meeting final product specsminimizing waste productionminimizing environmental impactminimizing energy usemaximizing overall production rate
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Loose Control Costs Money
It takes more processing to remove impurities, so greatestprofit is to operate as close to the maximum impuritiesconstraint as possible without going over
It takes more material to make a product thicker, so greatestprofit is to operate as close to the minimum thicknessconstraint as possible without going under
Copyright 2007by Control Station, Inc.
All Rights Reserved
SPSP P V & S P ( % )
65
60
55
45
40
Time
operating constraint
m o r e p r o
f i t
poor control = large variation in PV
set point far fromprofit constraint
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Tight Control is More Profitable
A well controlled process has less variability in the measuredprocess variable (PV), so the process can be operated closeto the maximum profit constraint.
Copyright 2007 by Control Station, Inc. All Rights Reserved m o r e p r o
f i t
SPSP
Time
65
60
50
45
40
P V & S P ( % )
operating constraint
tight control = small variation in PV
set point near profit constraint
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Terminology for Home Heating Control
Control Objective
Measured Process Variable (PV)Set Point (SP)Controller Output (CO)Manipulated VariableDisturbances (D)
furnace
temperaturesensor/transmitter
set pointheat loss
(disturbance)
thermostatcontroller
valve
TTTC
controlsignal
fuel flow
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What is Process Control
Terminology:The manipulated variable (MV) is a measure of resourcebeing fed into the process, for instance how much thermalenergy.A final control element (FCE) is the device that changes
the value of the manipulated variable.The controller output (CO) is the signal from thecontroller to the final control element.The process variable (PV) is a measure of the processoutput that changes in response to changes in the
manipulated variable.The set point (SP) is the value at which we wish tomaintain the process variable at.
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What is a Process
A process is broadly defined as an operationthat uses resources to transform inputs intooutputs.It is the resource that provides the energy into
the process for the transformation to occur.
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What is a Process
Each process exhibits a particular dynamic (timevarying) behavior that governs thetransformation.
That is, how do changes in the resource or inputsover time affect the transformation.
This dynamic behavior is determined by thephysical properties of the inputs, the resource,and the process itself.
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What is a Process
Can you identify some of the elements that willdetermine the dynamic properties of thisprocess?
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What is Process Control
Process control is the act of controlling a finalcontrol element to change the manipulatedvariable to maintain the process variable at adesired set point.
A corollary to our definition of process control is acontrollable process must behave in a predictablemanner.For a given change in the manipulated variable, theprocess variable must respond in a predictable and
consistent manner.
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What is Process Control
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What is Process Control
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Section Assessment:Basic Terminology Assessment
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What is Open Loop Control
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What is Open Loop Control
Can you think of processes in which open loopcontrol is sufficient?
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What is Closed Loop Control
In closed loop control the controller output isdetermined by difference between the processvariable and the set point. Closed loop control isalso called feedback or regulatory control.
The output of a closed loop controller is a function of the error.Error is the deviation of the process variable fromthe set point and is defined asE = SP - PV.
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What is Closed Loop Control
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What is Closed Loop Control
From the controllersperspective theprocess encompassesthe RTD, the steamcontrol valve, and
signal processingof the PV andCO values.
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What are the Modes of Closed Loop Control
Closed loop control can be, depending on thealgorithm that determines the controller output:ManualOn-Off PIDAdvanced PID (ratio, cascade, feedforward)or Model Based
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What are the Modes of Closed Loop ControlManual Control
In manual control an operator directlymanipulates the controller output to the finalcontrol element to maintain a set point.
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What are the Modes of Closed Loop ControlOn-Off Control
On-Off control provides a controller output of either on or off in response to error.
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What are the Modes of Closed Loop ControlOn-Off Control Deadband
Upon changing the direction of the controlleroutput, deadband is the value that must betraversed before the controller output will changeits direction again.
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What are the Modes of Closed Loop ControlPID Control
PID control provides a controller output thatmodulates from 0 to 100% in response to error.
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What are the Modes of Closed Loop ControlTime Proportion Control
Time proportion control is a variant of PID control thatmodulates the on-off time of a final control element that onlyhas two command positions.
To achieve the effect of PIDcontrol the switching frequencyof the device is modulated inresponse to error. This is
achieved by introducing theconcept of cycle time.
Cycle Time is the time base of thesignal the final control element will
receive from the controller. The PID
controller determines the final signalto the controller by multiplying thecycle time by the output of the PID
algorithm.
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What are the Modes of Closed Loop ControlCascade Control
Cascade control uses the output of a primary (master orouter) controller to manipulate the set point of a secondary(slave or inner) controller as if the slave controller were thefinal control element.
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What are the Modes of Closed Loop ControlCascade Control
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Basic Elements of Process Control
Controlling a process requires knowledge of four
basic elements, the process itself, the sensor that measures the process value, the final control element that changes the manipulatedvariable, and the controller .
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Section Assessment:Basic Process Control Assessment
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Dynamic Process Behavior What It Is & Why We Care
What a FOPDT Dynamic Model Represents
Analyzing Step Test Plot Data to Determine FOPDTDynamic Model ParametersProcess Gain, Time Constant & Dead Time
How To Compute Them From Plot DataHow to Use Them For Controller Design and Tuning
How to Recognize Nonlinear Processes
What We Will Learn in This Section
Understanding Dynamic Process Behavior
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Dynamic Process Behavior and Controller Tuning
Consider cruise control for a car vs a truckhow quickly can each accelerate or deceleratewhat is the effect of disturbances (wind, hills, etc.)
Controller (gas flow) manipulations required to maintainset point velocity in spite of disturbances (wind, hills)are different for a car and truck because the dynamicbehavior of each "process" is different
Dynamic behavior how the measured process variable(PV) responds over time to changes in the controlleroutput (CO) and disturbances (D)
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Graphical Modeling of Dynamic Process Data
To learn about the dynamic behavior of a process, weanalyze measured process variable (PV) test data
PV test data can be generated by suddenly changing thecontroller output (CO) signal
The CO should be moved far and fast enough so that thedynamic behavior is clearly revealed as the PV responds
The dynamic behavior of a process is different as operatinglevel changes (nonlinear behavior), so collect data atnormal operating conditions (design level of operation)
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Modeling Dynamic Process Behavior
The best way to understand process data is through modeling
Modeling means fitting a first order plus dead time (FOPDT)dynamic model to the process data:
where:PV is the measured process variableCO is the controller output signal
The FOPDT model is simple (low order and linear) so it onlyapproximates the behavior of real processes
dPVp + PV = Kp CO(t p)
dt
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Modeling Dynamic Process Behavior
When a first order plus dead time (FOPDT) modelis fit to dynamic process data
The important parameters that result are:Steady State Process Gain, KpOverall Process Time Constant,Apparent Dead Time, p
dPVp + PV = Kp CO(t p)
dt
p
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The FOPDT Model is All Important
FOPDT model parameters (Kp, and p ) are used in
correlations to compute controller tuning valuesSign of Kp indicates the action of the controller
(+ Kp reverse acting; Kp direct acting)
Size of indicates the maximum desirable loop sample time(be sure sample time T 0.1 )
Ratio of p / indicates whether model predictive controlsuch as a Smith predictor would show benefit
(useful if p > )
Model becomes part of the feed forward, Smith Predictor,decoupling and other model-based controllers
p
p
p
p
p
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Step Test Data and Dynamic Process Modeling
Process starts at steady state in manual modeController output (CO) signal is stepped to new valueProcess variable (PV) signal must complete the response
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Manual Mode Step Test on Gravity Drained Tanks
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Process Gain (Kp) from Step Test Data
Kp describes how far the measured PV travels inresponse to a change in the COA step test starts and ends at steady state, so Kp canbe computed directly from the plot
where PV and CO are the total change from initial tofinal steady state
A large process gain, Kp, means that each CO actionwill produce a large PV response
Steady State Change in the Process Variable, PVKp
Steady State Change in the Controller Output, CO
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Process Gain (Kp) for Gravity-Drained Tanks
Compute PV and CO as final minus initial steady state values
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PV = (2.9 1.9) = 1.0 m
CO = (60 50) = 10 %
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Process Gain (Kp) for Gravity-Drained Tanks
Kp has a size (0.1); a sign (+0.1), and units (m/%)
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PV = 1.0 m
CO = 10 %
Kp = = = 0.1 PV 1.0 m mCO 10% %
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Additional Notes on Process GainMeasuring the Process Gain
Process gain as seen by a controller is theproduct of the gains of the sensor, the finalcontrol element and the process itself.
The gain of a controller will be inverselyproportional to the process gain that it sees
ElementControlFinalGainSensor GainProcessGainGainProcess x x
GainProcess
1GainController
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Additional Notes on Process GainConverting Units of Process Gain
There is one important caveat in this process;the gain we have calculated has units of m/%.Real world controllers, unlike most softwaresimulations, have gain units specified as %/%.When calculating the gain for a real controller thechange in PV needs to be expressed in percent of span of the PV as this is how the controllercalculates error.
Span PV SpanCO x
CO PV Gain
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Assuming the process gain of the gravitydrained tanks is 0.1 m/% then, to convertto %/%
Additional Notes on Process GainConverting Units of Process Gain
%%
0.1%100%1000
%1.0PVCO
%1.0m
span spanm
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Additional Notes on Process Gain Values for Process Gain
Process gain that the controller sees is influencedby two factors other than the process itself, thesize of the final control element and the span of the sensor.In the ideal world you would use the full span of both final control element and the sensor whichwould give a process gain of 1.0.
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Additional Notes on Process Gain Values for Process Gain
As a rule of thumb:Process gains that are greater than 1 are a result of oversized final control elements.Process gains less than 1 are a result of sensor spansthat are too wide.
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Additional Notes on Process Gain Values for Process Gain
The result of a final control element being too large (highgain) is:
The controller gain will have to be made correspondingly smaller,smaller than the controller may accept.High gains in the final control element amplify imperfections(deadband, stiction), control errors become proportionately
larger .If a sensor has too wide of a span:
You may experience problems with the quality of themeasurement.The controller gain will have to be made correspondingly largermaking the controller more jumpy and amplifying signal noise.An over spanned sensor can hide an oversized final element.
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Additional Notes on Process Gain Values for Process Gain
The general rule of thumb is theprocess gain for a self regulating
process should be between0.5 and 2.0.
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Process Time Constant ( ) from Step Test Data
Time Constant, , describes how fast the measured PVresponds to changes in the COMore specifically, how long it takes the PV to reach 63.2%of its total final change ( PV), starting from when it firstbegins to respond
p
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p
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1) Locate , the time where the PV starts afirst clear response to the step change in CO
Process Time Constant ( ) from Step Test Datap
t PVstart
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tPVstarttPVstart
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Process Time Constant ( ) from Step Test Data
2) Compute 63.2% of the total change in PV as:PV63.2 = PV inital + 0.632 PV
p
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63.2% of PV total
PV 63.2 PV total
tPVstarttPVstart
f
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Process Time Constant ( ) from Step Test Data
3)4) Time Constant, , is then:
p
t 63.2 tPVstart p t 63.2 is the time when the PV reaches PV 63.2
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P
PV 63.2
tPVstarttPVstart t63.2t63.2
( ) f d k
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Time Constant ( ) for Gravity-Drained Tanksp
Here, = 4.1 mintPVstart
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tPVstart = 4.1tPVstart = 4.1
Ti C ( ) f G i D i d T k
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PV63.2 = PV inital + 0.632 PV
= 1.9 + 0.632(1.0) = 2.5 m
Time Constant ( ) for Gravity-Drained Tanksp
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63.2% of PV total
PV = 2.563.2 PV = 1.0 m
4.1
Ti C ( ) f G i D i d T k
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Time Constant ( ) for Gravity-Drained Tanksp
= t 63.2 tPVstart = 1.6 minp Time Constant,must be positive and have units of timep
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P
4.1 5.7
PV = 2.563.2
= 5.7 4.1 = 1.6 min= 5.7 4.1 = 1.6 min P
P D d Ti ( ) f S T D
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Process Dead-Time ( p) from Step Test Data
Dead time, p, is how much delay occurs from the time whenthe CO step is made until when the measured PV shows afirst clear response.
p is the sum of these effects:
transportation lag, or the time it takes for material totravel from one point to another
sample or instrument lag, or the time it takes to collect,analyze or process a measured PV sample
higher order processes naturally appear slow to respondand this is treated as dead time
Dead time, p, must be positive and have units of time
D d Ti ( ) i h Kill f C l
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Dead-Time ( p) is the Killer of Control
Tight control grows increasingly difficult as p becomes large
The process time constant is the clock of the process. Deadtime is large or small relative only to
When dead time grows such that p > , model predictivecontrol strategies such as a Smith predictor may show benefit
For important PVs, work to select, locate and maintaininstrumentation so as to avoid unnecessary dead time in a loop
p
p
P D d Ti ( ) f St T t D t
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Process Dead-Time ( p) from Step Test Data
p = t PVstart tCOstep
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ptCOsteptCOstep
tPVstarttPVstart
D d Ti ( ) f G it D i d T k
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Dead-Time ( p) for Gravity Drained Tanks
p = t PVstart tCOstep = 0.3 min
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= 4.1tCOstep= 3.8
tCOstep= 3.8
tPVstarttPVstart
p = 4.1 3.8 = 0.3 min p = 4.1 3.8 = 0.3 min
Th FOPDT M d l P t
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The FOPDT Model Parameters
Process Gain, Kp How Far PV travels
Time Constant, How Fast PV responds
Dead Time, p How Much Delay Before PV Responds
For a change in CO:
p
In Summary
H d O W k h
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Hands-On Workshop
Workshop #1Exploring Dynamics of Gravity-Drained Tanks
Processes have Time Varying Behavior
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Processes have Time Varying Behavior
The CO to PV behavior described by an ideal FOPDT model is
constant, but real processes change every day because:surfaces foul or corrodemechanical elements like seals or bearings wearfeedstock quality varies and catalyst activity decaysenvironmental conditions like heat and humidity change
So the values of Kp, and p that best describe the dynamicbehavior of a process today may not be best tomorrow
As a result, controller performance can degrade with time andperiodic retuning may be required
p
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Processes have Nonlinear Behavior
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Processes have Nonlinear Behavior
A FOPDT model response is constant as operating level changes
Since the FOPDT model is used for controller design and tuning, aprocess should be modeled at a specific design level of operation!
2) FOPDT model response isconstant as operating level changes
1) with equal CO steps
3) but behavior of real processeschanges with operating level
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What is a Nonlinear Process
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What is a Nonlinear Process
While linear processes may be the design goal of processengineers, the reality is that most processes are nonlinearin nature due to nonlinearity in the final control element orthe process itself.
Example: A heating process is nonlinear because the rateat which heat is transferred between two objects depends
on the difference in temperature between the objects.Example: A valve that is linear in the middle of itsoperating range may become very nonlinear towards itslimits.
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What is a Nonlinear ProcessDealing with Nonlinearity Set Point Response
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What is a Nonlinear ProcessDealing with Nonlinearity
The robustness of a controller is a measure therange of process values over which the controllerprovides stable operation.The more nonlinear a process is, the lessaggressive you must be in your tuning approachto maintain robustness.
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What is Process Action
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What is Process Action
Process action is how the process variable changes withrespect to a change in the controller output. Process actionis either direct acting or reverse acting.The action of a process is defined by the sign of theprocess gain. A process with a positive gain is said to bedirect acting. A process with a negative gain is said to bereverse acting.
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Fundamentals of Instrumentation& Process Control
Interactive Training Workshop
Introduction to Instrumentation
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Introduction to Instrumentation
The intent of this chapter is neither to teach you how toselect a particular instrument nor to familiarize you with all of the available types of instruments.
The intent of this material is to provide an introduction tocommonly measured process variables, including the basic terminology and characteristics relevant to each variablesrole in a control loop.
Detailed information and assistance on device selection istypically available directly from the instrumentationsupplier.
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Introduction to Instrumentation
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Introduction to Instrumentation
Objectives:What are sensors and transducers?What are the standard instrument signals?What are smart transmitters?What is a low pass filter?
What instrument properties affect a process?What is input aliasing?What is instrument noise?How do we measure temperature?How do we measure level?
How do we measure level?How do we measure flow?
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Introduction to Instrumentation
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Introduction to Instrumentation
You cannot control what youcannot measure
When you can measure what you are speaking about,and express it in numbers, you know somethingabout it. When you cannot measure it, when youcannot express it in numbers, your knowledge is of a
meagre and unsatisfactory kind.
- L o r d K e lv i n
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What are Sensors and Transducers
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What are Sensors and Transducers
A sensor is a device that has a characteristic that changesin a predictable way when exposed to the stimulus it wasdesigned to detect.A transducer is a device that converts one form of energyinto another.
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What are Standard Instrumentation Signals
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What are Standard Instrumentation Signals
Standard instrument signals for controllers toaccept as inputs from instrumentation andoutputs to final control elements are:
pneumatic current loop0 to 10 volt
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What are Standard Instrumentation SignalsPneumatic
3 to 15 psigBefore 1960, pneumatic signals were used almostexclusively to transmit measurement and controlinformation.Today, it is still common to find 3 to 15 psig used as
the final signal to a modulating valve.Most often an I/P (I to P) transducer is used.
This converts a 4-20 mA signal (I) into a pressuresignal (P).
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What are Standard Instrumentation SignalsPneumatic Scaling
What would our pneumatic signal be if ourcontroller output is 40%?
psig3 psig12OutputController % psigSignal x
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What are Standard Instrumentation SignalsCurrent Loop
4-20 milliampCurrent loops are the signal workhorses in ourprocesses.A DC milliamp current is transmitted through a pairof wires from a sensor to a controller or from a
controller to its final control element.Current loops are used because of their immunity tonoise and the distances that the signal can betransmitted.
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What are Standard Instrumentation SignalsCurrent Loop Scaling
Output ScalingScale outputs for a one-to-one correspondence.Controller output is configured for 0% to correspondto a 4mA signal and 100% to correspond to a 20mAsignal.
The final control element is calibrated so that 4mAcorresponds to its 0% position or speed and 20mAcorresponds to its 100% position or speed.
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What are Standard Instrumentation SignalsCurrent Loop Scaling
Input ScalingScale inputs for a one-to-one correspondence as well.Example:
If we were using a pressure transducer with a requiredoperating range of 0 psig to 100 psig we would calibratethe instrument such that 0 psig would correspond to 4mA
output and 100 psig would correspond to a 20mA output.At the controller we would configure the input such that4mA would correspond to an internal value of 0 psig and10mA would correspond to an internal value of 100 psig.
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g0 to 10 Volt
0 to 10 volt is not commonly used in controlsystems because this signal is susceptible toinduced noise and the distance of the instrumentor final control element is limited due to voltagedrop.
You may find 0-10 volt signals used in controlsystems providing the speed reference tovariable speed drives.
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What are Smart Transmitters
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A smart transmitter is a digital device thatconverts the analog information from a sensorinto digital information, allowing the device tosimultaneously send and receive information andtransmit more than a single value.
Smart transmitters, in general, have thefollowing common features:Digital CommunicationsConfigurationRe-Ranging
Signal ConditioningSelf-Diagnosis
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Digital Communications
Smart transmitters are capable of digital communicationswith both a configuration device and a process controller.Digital communications have the advantage of being free of bit errors, the ability to multiple process values anddiagnostic information, and the ability to receivecommands.
Some smart transmitters use a shared channel for analogand digital data (Hart, Honeywell or Modbus over 4-20mA).Others use a dedicated communication bus (Profibus,Foundation Fieldbus, DeviceNet, Ethernet).
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Digital Communications
Most smart instruments wired tomulti-channel input cards requireisolated inputs for the digitalcommunications to work.
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Configuration, Signal Conditioning, Self-Diagnosis
ConfigurationSmart transmitters can be configured with ahandheld terminal and store the configurationsettings in non-volatile memory.
Signal ConditioningSmart transmitters can perform noise filtering andcan provide different signal characterizations.
Self-DiagnosisSmart transmitters also have self-diagnostic
capability and can report malfunctions that mayindicate erroneous process values.
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p
The instruments range and span. The resolution of the measurement.The instruments accuracy and precision. The instruments dynamics
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Range and Span
The range of a sensor is the lowest and highestvalues it can measure within its specification.The span of a sensor is the high end of theRange minus the low end of the Range.Match Range to Expected Conditions
Instruments should be selected with a range thatincludes all values a process will normally encounter,including expected disturbances and possible failures.
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Measurement Resolution
Resolution is the smallest amount of inputsignal change that the instrument can detectreliably.
Resolution is really a function of the instrument spanand the controllers input capability.
The resolution of a 16 bit conversion is:
The resolution of a 12 bit conversion is:The bit error.
535,65
SpanInput
095,4
SpanInput
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Accuracy
Accuracy of a measurement describes how closethe measurement approaches the true value of the process variable.
% error over a range x% over
% of full scale% of span
Absolute over a range x units over
full scale
span
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Precision
Precision is the reproducibility with whichrepeated measurements can be made underidentical conditions.
This may be referred to as drift.
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Accuracy vs. Precision
Why is precision preferred over accuracy?
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Instrumentation Dynamics: Gain and Dead Time
The gain of an instrument is often called sensitivity .
The sensitivity of a sensor is the ratio of the outputsignal to the change in process variable.
The dead time of an instrument is the time ittakes for an instrument to start reacting toprocess change.
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What is Input Aliasing
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Input aliasing is a phenomenon that occurs fromdigital processing of a signal.When a signal is processed digitally it is sampledat discrete intervals of time. If the frequency atwhich a signal is sampled is not fast enough thedigital representation of that signal will not becorrect.Input aliasing is an important consideration indigital process control. Processor inputs that
have configurable sample rates and PID loopupdate times must be set correctly.
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What is Input Aliasing
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Correct Sampling Frequency
Nyquist Frequency Theorem:To correctly sample a waveform it is necessary tosample at least twice as fast has the highestfrequency in the waveform.In the digital world this means sampling at 1/20th of
the period of the waveform.Period = 1/frequency
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Correct Sampling Frequency
2 Hz waveform sampled every 0.025 seconds
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Workshop Lab:
Input Aliasing 1
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Determining the Correct Sampling Interval
Rules of ThumbSet the sample interval for an instrument at 1/10th to1/20th of the rise time (1/2 to 1/4th of the time constant).Set the sample interval to 1/10th to 1/20th of the processtime constant.Temperature instrumentation (RTDs and thermocouples inthermowells) typically have time constants of severalseconds or more. For these processes sampling intervalsof 1 second are usually sufficient.Pressure and flow instrumentation typically have timeconstants of to 1 second. For these processes sampling
intervals of 0.1 second are usually sufficient.
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Determining the Correct Sampling Interval
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Workshop Lab:
Input Aliasing 2
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What is Instrument NoiseS
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Sources
Noise is generally a result of the technology usedto sense the process variable.
Electrical signals used to transmit instrumentmeasurements are susceptible to having noiseinduced from other electrical devices.
Noise can also be caused by wear and tear on themechanical elements of a sensor.
Noise may be uncontrolled, random variations inthe process itself.
Whatever the source, noise distorts themeasurement signal.
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What is Instrument NoiseEffects of Noise
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Effects of Noise
Noise reduces the accuracy and precision of process measurements. Somewhere in the noiseis the true measurement, but where?
Noise introduces more uncertainty into themeasurement.
Noise also introduces errors in control systems.To a controller, fluctuations in the processvariable due to noise are indistinguishable fromfluctuations caused by real disturbances.
Noise in a process variable will be reflected in theoutput of the controller
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What is Instrument NoiseEliminating Noise
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Eliminating Noise
The most effective means of eliminating noise isto remove the source.
Reduce electrically induced noise by following propergrounding techniques, using shielded cabling, andphysically separating the signal cabling form otherelectrical wiring.If worn mechanical elements in the sensor arecausing noise, then repair or replace the sensor.
When these steps have been taken and excessivenoise is still a problem in the process variable alow pass filter may be appropriate.
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What is Instrument NoiseLow Pass Filter
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Low Pass Filter
A low-pass filter allows the low frequencycomponents of a signal to pass while attenuatingthe higher frequency components.
Raw (unfiltered) Signal
Filtered Signal
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What is Instrument NoiseLow Pass Filter
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Low Pass Filter
A low pass filter introduces a first order lag intothe measurement.
Raw (unfiltered) Signal
Filtered Signal
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What is Instrument NoiseSelecting a Low Pass Filter
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Selecting a Low Pass Filter
By Cut-Off FrequencyCut-off frequency is defined as frequency abovewhich the filter provides -3dB of signal attenuation.
An attenuation of 0 dB would mean the signal willpass with no reduction in amplitude while a largenegative dB would indicate a very small amplituderatio.
Select a cut-off frequency that is above thefrequency of your process.
InAmplitude
OutAmplitude10log20nattenuatioof dB
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Selecting a Low Pass Filter
By Time ConstantSome filters are configured by selecting a timeconstant for the lag response of the filter.The relationship between the cut-off frequency andthe time constant of a low pass is approximately
given by:
ConstantsTime5
1 FrequencyOff -Cut
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Selecting a Low Pass Filter
By ValueSome filters are selected by a value called alpha ( ),notably the derivative filter in a PID controller.
is an averaging weighting term used in controllercalculations to impart a first order lag on themeasured variable.
generally has values between 0 and 1. A filter witha = 0 would pass the signal through unfiltered. Afilter with a = 1 would filter everything allowingnothing to pass through the filter.
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Selecting a Low Pass Filter
When a filter is specified by cut-off frequency,the lower the frequency the greater the filteringeffect.When a filter is specified by time constant, thegreater the time constant the greater the filtering
effect.When a filter is specified by , when =0 nofiltering is done, when = 1 no signal passesthrough the fitter.
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Workshop Lab:Noise Filtering
115
What is Temperature
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Temperature is a measure of degree of thehotness or coldness of an object.Units of Temperature
Two most common temperature scales areFahrenheit (F) and Celsius (C).
The reference points are the freezing point and theboiling point of water.
Water freezes at 32F and boils at 212F.The Celsius scale uses the same reference pointsonly it defines the freezing point of water as 0C andthe boiling point as 100C.
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What is TemperatureUnit Conversion
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Unit Conversion
32C5
9F
x
32-F95
C x
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What Temperature Instruments Do We Use
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Temperature is one of the most common processvariables.Temperature is most commonly measured by
Resistance Temperature Devices (RTD)Thermocouples
Infrared (IR) (to a lesser degree) Thermistors (may also be found embedded in somecontrol equipment)
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What Temperature Instruments Do We UseThermocouples
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Thermocouples
Thermocouples are fabricated from two electricalconductors made of two different metal alloys.
Hot or sensing junctionCold or reference junctionGenerate an open-circuit voltage, called the Seebeck
voltage that is proportional to the temperaturedifference between the sensing (hot) and reference(cold) junctions.
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What Temperature Instruments Do We UseThermocouple Junctions
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There is a misconception of how thermocouples operate.
The misconception is that the hot junction is the source of the output voltage. This is wrong. The voltage isgenerated across the length of the wire.Another misconception is that junction voltages aregenerated at the cold end between the special
thermocouple wire and the copper circuit. Hence, a cold junction temperature measurement is required. Thisconcept is wrong. The cold end temperature is thereference point for measuring the temperature differenceacross the length of the thermocouple circuit.
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What Temperature Instruments Do We UseThermocouple Junctions
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Hot junction voltage proportional to 210F.Extension wire voltage proportional to 18F.
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What Temperature Instruments Do We UseThermocouple Junctions
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p
Use of the correct extension wire is critical inthermocouple applications.
An incorrect extension will cause the temperaturedifferential across the extension leads to beintroduced as measurement error.
Does this mean you cannot use copper terminalblocks for thermocouples?No, it is highly unlikely there will be a temperaturedifferential across the terminal block; no error will beintroduced.
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What Temperature Instruments Do We UseThermocouple Linearization
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p
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What Temperature Instruments Do We UseThermocouple Gain
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p
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What Temperature Instruments Do We UseThermocouple Types
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p yp
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What Temperature Instruments Do We UseRTDs
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A resistance-temperature detector (RTD) is a temperature
sensing device whose resistance increases withtemperature.An RTD consists of a wire coil or deposited film of puremetal whose resistance at various temperatures has beendocumented.RTDs are used when applications require accuracy, long-term stability, linearity and repeatability.100 Platinum is most common.
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What Temperature Instruments Do We UseRTD Lead Wire Resistance
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The RTD is a resistive device, you must drive acurrent through the device and monitor theresulting voltage. Any resistance in the leadwires that connect your measurement system tothe RTD will add error to your readings.
40 feet of 18 gauge 2conductor cable has6 resistance.For a platinum RTDwith = 0.00385,the resistance equals0.6 /(0.385 /C) = 1.6C of error.
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What Temperature Instruments Do We UseRTD Lead Wire Resistance
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Error is eliminated through the use of three wireRTDs.
With three leads, two voltage measurements can bemade.When the voltages are subtracted, the result is
the voltage drop that would have occurred throughRT alone.
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What Temperature Instruments Do We UseRTD Self Heating Effect
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The current used to excite an RTD causes the RTD to
internally heat, which appears as an error. Self-heating istypically specified as the amount of power that will raisethe RTD temperature by 1 C, or 1 mW/ C.Self-heating can be minimized by using the smallestpossible excitation current, but this occurs at the expenseof lowering the measurable voltages and making the signalmore susceptible to noise from induced voltages.The amount of self-heating also depends heavily on themedium in which the RTD is immersed. An RTD can self-heat up to 100 times higher in still air than in moving water
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What Temperature Instruments Do We UseThermistors
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A thermistor is similar to an RTD in that it is apassive resistance device.
Thermistors are generally made of semiconductormaterials giving them much different characteristics.Thermistors do not have standardized electrical
properties like thermocouples or RTDs.
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What Temperature Instruments Do We UseInfrared
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Objects radiate electromagnetic energy.The higher the temperature of an object the moreelectromagnetic radiation it emits.This radiation occurs within the infrared portion of the electromagnetic spectrum.
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What Temperature Instruments Do We UseInfrared Emittance
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The emittance of a real surface is the ratio of thethermal radiation emitted by the surface at agiven temperature to that of the ideal black bodyat the same temperature.
By definition, a black body has an emittance of 1.
Another way to think of emissivity is theefficiency at which an object radiates thermalenergy.Emittance is a decimal number that ranges
between 0 and 1 or a percentage between 0%and 100%.
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What Temperature Instruments Do We UseInfrared Emittance
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To function properly an infrared temperatureinstrument must take into account the emittanceof the surface being measured.Emittance values can often be found in referencetables, but such tables will not take into account
local conditions of the surface.A more practical way to set the emittance is tomeasure the temperature with an RTD orthermocouple and set the instrument emissivity
control so that both readings are the same.
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What Temperature Instruments Do We UseInfrared Field of View
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Infrared temperature instruments are like an optical
system in that they have a field of view. The field of viewbasically defines the target size at a given distance.Field of view may be specified as:
Angle and focal range (2.3 from 8" to 14')Distance to spot size ratio and focal range (25:1 from 8" to 14')Spot size at a distance (0.32" diameter spot at 8")All of these specifications are equivalent, at 8 inches our field of view will be a 0.32" diameter spot (8"/25), at 14 feet our field of view will be 6.72" (14' / 25).
An infrared temperature sensor measures the averagetemperature of everything in its field of view. If thesurface whose temperature we are measuring does notcompletely fill the field of view we will get inaccurateresults.
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What Temperature Instruments Do We UseInfrared Spectral Response
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The range of frequencies that an infrared sensorcan measure is called its spectral response.Infrared temperature sensors do not measure theentire infrared region.
Different frequencies (wavelengths) are selected for
detection to provide certain advantages.If you wanted to use an infrared temperature sensor tomeasure the temperature of an object behind glass youwould need to select an instrument with a spectralresponse in the infrared region in which glass is"transparent.
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Section Assessment:Temperature
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What is Pressure
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Pressure is the ratio between a force acting on asurface and the area of that surface.Pressure is measured in units of force divided byarea.
psi (pounds per square inch)
bar, 1 bar = 14.5 PSI, common for pump ratingsinH20 (inches of water), 27.680 inH20 = 1 psi,common for vacuum systems and tank levelsmmHg (millimeters of mercury), 760 mmHg = 14.7psi, common for vacuum systems
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What is Pressure Absolute, Gauge and Differential
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Gauge pressure is defined relative to atmosphericconditions.
The units of gauge pressure are psig, however gaugepressure is often denoted by psi as well.
Absolute pressure is defined as the pressure
relative to an absolute vacuum.The units of absolute pressure are psia.
Differential pressure uses a reference point otherthan full vacuum or atmospheric pressure.
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What is Pressure Absolute, Gauge and Differential
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Section Assessment:Pressure
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Common Level Sensing Technologies
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Point sensing level probes only sense tank level at a
discrete level.Typically used for high-high or low-low level sensing to prevent plantpersonnel and/or process equipment from being exposed to harmfulconditions.Also used in pairs in processes in which we do not particularly carewhat the exact level in a tank is, only that it is between two points.
Continuous level probes sense the tank level as a percentof span of the probes capabilities.
Continuous level probes are typically used where we need some typeof inventory control, where we need to know with some degree of confidence what the particular level in a tank is.Contact and Non-Contact Types
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Common Level Sensing TechnologiesNon-Contact: Ultrasonic
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Ultrasonic makes use of sound waves.
A transducer mounted in the top of a tank transmits soundwaves in bursts onto the surface of the material to bemeasured. Echoes are reflected back from the surface of the material to the transducer and the distance to thesurface is calculated from the burst-echo timing.
The key points in applying an ultrasonic transducer are:The speed of sound varies with temperature.Heavy foam on the surface of the material interferes with the echo.An irregular material surface can cause false echoes resulting inirregular readings.Heavy vapor in the air space can distort the sound waves resulting infalse reading.
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Common Level Sensing TechnologiesNon-Contact: Ultrasonic
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Common Level Sensing TechnologiesNon-Contact: Radar/Microwave
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Radar, or microwave level measurement,
operates on similar principles to ultrasonic levelprobes but, instead of sound waves,electromagnetic waves in the 10GHz range areused.When properly selected, radar can overcomemany of the limitations of ultrasonic level probes.
Be unaffected by temperature changes in the tank air space.See through heavy foam to detect the true material level.See through heavy vapor in the tank air space to detect truematerial level.
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Common Level Sensing TechnologiesNon-Contact: Nuclear Level Measurement
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Radiation from the source is detected on the
other side of the tank. Its strength indicates thelevel of the fluid. Point, continuous, and interfacemeasurements can be made.As no penetration of the vessel is needed there are anumber of situations that cause nucleonic transmitters to
be considered over other technologies.Nuclear level detection has some drawbacks. One is highcost, up to four times that of other technologies. Othersare the probable requirement for licenses, approvals, andperiodic inspections; and the difficulty and expense of disposing of spent radiation materials.
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Common Level Sensing TechnologiesNon-Contact: Nuclear Level Measurement
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Common Level Sensing TechnologiesContact: Hydrostatic Pressure
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Measurement of level by pressure relies on hydrostatic
principles.Pressure is a unit force over a unit area.
A cubic foot (12"L x 12"W x 12"H) of water weighs 62.4796 pounds.The area that our cubic foot of water occupies is 144 square inches(12"L x 12"H), therefore our cubic foot of water exerts a force of 62.4796 pounds over 144 square inches, or 0.4339 psig for a 12"water column.1 inch H20 = 0.0316 psig
It would not matter how many cubic feet of water wereplaced side by side, our pressure would still be 0.4399psig. Hydrostatic pressure is only dependent on the height
of the fluid, not the area that it covers.
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Common Level Sensing TechnologiesContact: Hydrostatic Pressure
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How would you handle measuring other fluids usingpressure?
Compare the densities.For instance, chocolate weighs approximately 80 poundsper cubic foot.
80 divided 62.5 times 0.0316 = 0.0404 psig.For a change in level of one inch in a liquid chocolate tankthe pressure measurement will change by 0.0404 psig.
Level measurement by pressure requires a constant densityfor accurate measurements.If the head pressure in the tank can be other thanatmospheric, we must use a differential pressure sensor.
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Common Level Sensing TechnologiesContact: Hydrostatic Pressure
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Common Level Sensing TechnologiesContact: RF/Capacitance
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RF (radio frequency) Capacitance
level sensors make use of electrical characteristics of acapacitor to infer the level in a vessel.
As the material rises in the vessel,the capacitance changes.
The level transducer measuresthis change, linearizes it and transmits the signal to theprocess control system.A point level probe will look for a specific change incapacitance to determine whether it is on or off.
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Requires the material being measured to have a high
dielectric constant.Level measurements are affected by changes in thedielectric of the material (moisture content).
Proper selection requires informing the probe vendor of thematerial to be measure, especially in applications where
you are measuring conductive materials or have anonmetallic tank.Point probe sensitivity can be increased by welding a plateon the sensor tip to increase the capacitance, and thereforethe sensitivity (gain).
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Common Level Sensing TechnologiesContact: Guided Wave Radar
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Guided wave radar is similar to the non-contact radar
probes, only a rod or cable is immersed into the materiallike an RF capacitance probe.The rod or cable is used to guide the microwave along itslength, where the rod or cable meets the material to bemeasured a wave reflection is generated. The transit time
of the wave is used to calculate level very precisely.Unlike an RF capacitance probe, a guided wave radar probecan measure extremely low dielectric material.
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Common Level Sensing TechnologiesContact: Guided Wave Radar
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Section Assessment:Level
157
What is Flow
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Flow is the motion characteristics of constrained
fluids (liquids or gases).Fluid velocity or mass are typically not measureddirectly.Measurements are affected by the properties of
the fluid, the flow stream, and the physicalinstallation.
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Factors Affecting Flow Measurement
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The critical factors affecting flow measurement
are:Viscosity Fluid TypeReynolds Number
Flow Irregularities
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Factors Affecting Flow Measurement Viscosity
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Dynamic or absolute viscosity ( ) is a measure of
the resistance to a fluid to deformation undershear stress, or an internal property of a fluidthat offers resistance to flow.Commonly perceived as thickness or resistance
to pouring.Water is very thin having a relatively low viscosity,while molasses is very thick having a relatively highviscosity.
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Factors Affecting Flow Measurement Viscosity
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Visualize a flat sheet of glass on a film of oil on top of a flatsurface.
A parallel force appliedto the sheet of glass willaccelerate it to a final velocitydependent only on theamount of force applied.The oil that is next to the sheetof glass will have a velocity
close to that of the glass; whilethe oil that is next to the stationarysurface will have a velocity near zero.This internal distribution of velocities is due to the internalresistance of the fluid to shear stress forces, its viscosity.
The viscosity of a fluid is the ratio between the per unitforce to accelerate the plate and the distribution of thevelocities within the fluid film
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Factors Affecting Flow Measurement Viscosity
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Effect of Temperature
The dynamic viscosity of a fluid varies with itstemperature.In general, the viscosity of a liquid will decrease withincreasing temperature while the viscosity of a gaswill increase with increasing temperature.Viscosity measurements are therefore associatedwith a particular temperature
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Factors Affecting Flow Measurement Viscosity
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Viscosity is measured in units of poise or
Pascalseconds or stokes.Poise or Pascalseconds are the units of dynamicviscosity.Stokes are the units of kinematic viscosity.
Kinematic viscosity is the dynamic viscosity of a fluiddivided by the density of the fluid.
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Factors Affecting Flow Measurement Viscosity Conversion Chart
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poiseP centipoisecP PascalsecondsPas milliPascalsecmPas stokesS centistokescS
1 100 0.1 100 x 1/density x 100/density
0.01 1 .001 1 x 0.01/density x 1/density
10 1000 1 1000 x 10/density X000/density
0.01 1 0.001 1 x 0.01/density x 1/density
x density x 100 density x 0.1 density x 100 density 1 100
x 0.01 density x density x 0.001 density x density 0.01 1
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Factors Affecting Flow Measurement Viscosity of Common Fluids
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centiPoise (cP) Typical liquid Specific Gravity
1 Water 1.0
12.6 No. 4 fuel oil 0.82 - 0.95
34.6 Vegetable oil 0.91 - 0.9588 SAE 10 oil 0.88 - 0.94
352 SAE 30 oil 0.88 - 0.94
820 Glycerine 1.26
1561 SAE 50 oil 0.88 - 0.9417,640 SAE 70 oil 0.88 - 0.94
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Factors Affecting Flow Measurement Viscosity
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The fluid streams in most processes can include
both high and low viscous fluids.The viscosity of the fluid under processconditions must be taken into account whenselecting a flow instrument for optimum
performance.
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Factors Affecting Flow MeasurementFluid Type
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Newtonian Fluids
When held at a constant temperature, the viscosity of a Non-Newtonian fluid will change with relation to thesize of the shear force, or it will change over timeunder a constant shear force.Water, glycerin, liquid sugar, and corn syrup areexamples.
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Factors Affecting Flow MeasurementFluid Type
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Non-Newtonian Fluids
When held at a constant temperature, the viscosity of aNon-Newtonian fluid will change with relation to the sizeof the shear force, or it will change over time under aconstant shear force.
Shear Thickening: Peanut Butter, Starch & WaterTime Thickening: Milk, Molasses
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Factors Affecting Flow MeasurementReynolds Number
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The Reynolds number is the ratio of inertial
forces to viscous forces of fluid flow within a pipeand is used to determine whether a flow will belaminar or turbulent.
Dv Re 124
cPinviscosityFluid
3lb/ftindensityFluid
ft/secinvelocityFluid inchesindiameter Pipe
number Reynolds:Where
v D Re
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Factors Affecting Flow MeasurementLaminar Flow
f
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Laminar flow occurs at low Reynolds numbers,
typically Re < 2000, where viscous forces aredominant.Laminar flow is characterizedby layers of flow traveling at
different speeds with virtuallyno mixing between layers.The velocity of the flow is highest in the center of the pipe and lowest at the walls of the pipe.
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Factors Affecting Flow MeasurementTurbulent Flow
b l fl h h ld b
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Turbulent flow occurs at high Reynolds numbers,
typically Re > 4000, where inertial forces aredominant.Turbulent flow ischaracterized
by irregular movementof the fluid in the pipe.There are no definite layers.The velocity of the flow is nearly uniform through
the cross-section of the pipe.
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Factors Affecting Flow MeasurementTransitional Flow
T i i l fl i ll R ld
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Transitional flow typically occurs at Reynolds
numbers between 2000 and 4000.Flow in this region may be laminar, it may beturbulent, or it may exhibit characteristics of both.
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Factors Affecting Flow MeasurementReynolds Number
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Flow instruments must beselected to accurately
measure laminar flow orturbulent flow
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Factors Affecting Flow MeasurementFlow Irregularities
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Flow irregularities arechanges in thevelocity profile of thefluid flow caused byinstallation of the flowinstrument.
Normal velocityprofiles are
established byinstalling a sufficientrun of straight piping.
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Common Flowmeter Technologies
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Common Flowmeter Technologies
V l t i Fl
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Volumetric Flow
Magnetic flowmetersPositive displacement for viscous liquidsOrifice Plates
Mass FlowCoriolis flowmeters
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Common Flowmeter Technologies Volumetric Flow
V l t i fl i ll i f d
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Volumetric flow is usually inferred.
Volumetric Flow = Fluid Velocity x AreaMeasured directly with positive displacement flowmeters.Measured indirectly by differential pressure
Volumetric flow meters provide a signal that is inunits of volume per unit time.gpm (gallons per minute)cfm (cubic feet per minute)l/hr (liters per hour)
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Common Flowmeter Technologies Volumetric Flow: Positive Displacement
Oper te b c pt ring the fl id to be me s red in
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Operate by capturing the fluid to be measured in
rotating cavities of a known volume. The volumeof the cavity and the rate of rotation will give thevolumetric flow value.
Unaffected by Reynolds number and work withlaminar, turbulent, and transitional flow.
Low viscosity fluids will slip past the gearsdecreasing the accuracy of the flowmeter.
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Common Flowmeter Technologies Volumetric Flow: Magnetic
M ti fl t i f th
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kB DE
kBD E D
v Dvr
velocity Area Pipe Flow
44
42
22
Magnetic flowmeters infer the
velocity of the moving fluid bymeasuring a generatedvoltage.Magnetic flowmeters arebased on Faradays law of electromagnetic induction - a
wire moving though amagnetic field will generate avoltage.Requires a conductive fluid.
VelocityFluid
Diameter Pipe
DensityFluxMagnetic
Constant
v
D
B
k
kBDv E
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Common Flowmeter Technologies Volumetric Flow: Orifice Plates
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An orifice plate is basically
a thin plate with a hole inthe middle. It is usuallyplaced in a pipe in whichfluid flows.In practice, the orifice
plate is installed in thepipe between two flanges.The orifice constricts theflow of liquid to produce adifferential pressure across
the plate. Pressure taps oneither side of the plate areused to detect thedifference.
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Common Flowmeter Technologies Volumetric Flow: Orifice Plates
Orifice Plate type meters only work well when
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Orifice Plate type meters only work well when
supplied with a fully developed flow profile.This is achieved by a long upstream length (20 to 40diameters, depending on Reynolds number)
Orifice plates are small and cheap to install, but
impose a significant energy loss on the fluid dueto friction.
181
Common Flowmeter TechnologiesMass Flow
Measured directly by measuring the inertial
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Measured directly by measuring the inertial
effects of the fluids moving mass.Mass flow meters provide a signal that is in unitsof mass per unit time.Typical units of mass flow are:
lb/hr (pounds per hour)kg/s (kilograms per second)
182
Common Flowmeter TechnologiesMass Flow: Coriolis
Coriolis flowmeters are based on the affects of the
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Coriolis flowmeters are based on the affects of the
Coriolis force.When observing motion from a rotating frame thetrajectory of motion will appear to be altered by aforce arising from the rotation.
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Common Flowmeter TechnologiesMass Flow: Coriolis
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Common Flowmeter TechnologiesMass Flow: Coriolis
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Common Flowmeter TechnologiesMass Flow: Coriolis
Coriolis mass flowmeters
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Coriolis mass flowmeterswork by applying avibrating force to a pairof curved tubes throughwhich fluid passes, ineffect creating a rotatingframe of reference.
The Coriolis Effect of themass passing throughthe tubes creates a forceon the tubesperpendicular to both thedirection of vibration andthe direction of flow,causing a twist in thetubes.
186
Common Flowmeter TechnologiesFlowmeter Turndown
A specification commonly found with flow sensors
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A specification commonly found with flow sensors
is turndown or rangeability.Turndown is the ratio of maximum to minimum flowthat can be measured with the specified accuracy.
For example, a flow meter with a full span of 0 to100 gallons per minute with a 10:1 turndowncould measure a flow as small as 10 gallons perminute.Trying to measure a smaller flow would result invalues outside of the manufacturers statedaccuracy claims.
187
Common Flowmeter TechnologiesInstallation and Calibration
As with any instrument installation optimal flow
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As with any instrument installation, optimal flow
meter performance and accuracy can only beachieved through proper installation andcalibration. Requirements will vary between typeand manufacturer by some generalconsiderations are:
The length of straight piping required upstream anddownstream of the instrument. This is usually specified inpipe diameters.The properties of the fluid/gas to be measured.Mounting position of the flowmeter.
Flowmeters must be kept full during operation.
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Common Flowmeter TechnologiesInstallation and Calibration
In general calibration of a flowmeter requires:
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In general, calibration of a flowmeter requires:
Entering the sensor characterization constants intothe flow transmitter.Recording the zero flow conditions.
Many flow meters must be zeroed only when full. Comparing the meter flow value to an independentlymeasured value for the flow stream.
Measuring the volume or mass of a catch sample andcomparing the values.Use the internal flowmeter totalizer value for
comparison.
189
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Section Assessment:Flow
190
Introduction to Final Control Elements
Objectives:
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j
What is a control valve?What is an actuator?What is a positioner?What is C v?What are valve characteristics?What is valve deadband?What is stiction?What are the types of valves
What is a centrifugalpump?What is pump head?Why do we use pump headand not psi?What is a pump curve?
What is a system curve?What is the systemoperating point?What is a positivedisplacement pump?
How does a PD pump differfrom a centrifugal pump?
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Introduction to Final Control Elements
The intent of this chapter is neither to teach you how to
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p ysize a valve or select a pump nor to familiarize you with all of the available types of valves and pumps.The intent of this chapter is to provide an introduction tosome of the commonly used pumps and valves, includingbasic terminology and characteristics relevant to their rolein a control loop.Detailed information and assistance on device selection istypically available from your instrumentation suppliers.
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What is a Control Valve
A control valve is an inline device in a flow
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A control valve is an inline device in a flow
stream that receives commands from a controllerand manipulates the flow of a gas or fluid in oneof three ways:
Interrupt flow ( Shut-Off Service )Divert flow to another path in the system ( Divert Service )Regulate the rate of flow ( Throttling Service )
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What is a Control ValveShut-Off Service
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Control valves forflow shut-off servicehave two positions.
In the open positionflow is allowed to exitthe valve.In the closed positionflow is blocked fromexiting the valve.
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What is a Control ValveDivert Service
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Control valves for divertservice also have twopositions, however flowis never blocked.
In one position, flowis allowed from thecommon port to portA.In the other position,flow is allowed formthe common port toport B.
195
What is a Control ValveThrottling Service
Control valves for throttling service have many
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g y
positions.The position of the valve determines the rate of flowallowed through the valve.
196
What is an Actuator
Actuators are pneumatic, electrical, or hydraulic
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p , , y
devices that provide the force and motion toopen and close a valve.
197
What is an Actuator
Some actuators can place a valve at any position
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p y p
between the on and off points.These actuators typically accepta 3-15 psi signal to move adiaphragm, which in turn movesa connected valve stem.
A pneumatic positional actuatorwill fail into the pneumaticallyde-energized position.The interface to a control system for a positionalactuator is typically through an I/P transducer.
198
What is an Actuator
Just as processes have a time constants and dead time,l h i d d d i hi h i
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