Unit Extra PDC

7/27/2019 Unit Extra PDC

1/261

Fundamentals of Instrumentation& Process Control

Interactive Training Workshop


2/261

Copyright 2007 Control Station, Inc. All Rights Reserved

Fundamentals of Instrumentation & Control

2


3/261


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.

3


4/261


Outline of the Course

4


5/261


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?

5


6/261


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

6


7/261


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

7


8/261


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

8


9/261


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

Copyright 2007 by Control Station, Inc. All Rights Reserved

9


10/261


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.

10


11/261


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.

11


12/261


13/261


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.

13


14/261


What is a Process

Can you identify some of the elements that willdetermine the dynamic properties of thisprocess?

14


15/261



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.

15


16/261



16


17/261



17


18/261


Section Assessment:Basic Terminology Assessment

18


19/261


20/261


What is Open Loop Control

20


21/261


What is Open Loop Control

Can you think of processes in which open loopcontrol is sufficient?

21


22/261


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.

22


23/261



23


24/261



From the controllersperspective theprocess encompassesthe RTD, the steamcontrol valve, and

signal processingof the PV andCO values.

24


25/261


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

25


26/261


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.

26


27/261


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.

27


28/261


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.

28


29/261


What are the Modes of Closed Loop ControlPID Control

PID control provides a controller output thatmodulates from 0 to 100% in response to error.

29


30/261


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.

30


31/261


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.

31


32/261


What are the Modes of Closed Loop ControlCascade Control

32


33/261


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 .

33


34/261


Section Assessment:Basic Process Control Assessment

34


35/261


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

35


36/261


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)

36


37/261


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)

37


38/261

Copyright 2007 Control Station, Inc. All Rights Reserved 38

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


39/261


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


40/261


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


41/261


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

Copyright 2007 by Control Station, Inc. All Rights ReservedCopyright 2007 by Control Station, Inc. All Rights Reserved

Manual Mode Step Test on Gravity Drained Tanks


42/261


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


43/261


Process Gain (Kp) for Gravity-Drained Tanks

Compute PV and CO as final minus initial steady state values


PV = (2.9 1.9) = 1.0 m

CO = (60 50) = 10 %


44/261


Process Gain (Kp) for Gravity-Drained Tanks

Kp has a size (0.1); a sign (+0.1), and units (m/%)


PV = 1.0 m

CO = 10 %

Kp = = = 0.1 PV 1.0 m mCO 10% %


45/261


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

45


46/261


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

46


47/261


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

47


48/261


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.

48


49/261



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.

49


50/261



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.

50


51/261



The general rule of thumb is theprocess gain for a self regulating

process should be between0.5 and 2.0.

51


52/261


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


p


53/261


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


tPVstarttPVstart


54/261



2) Compute 63.2% of the total change in PV as:PV63.2 = PV inital + 0.632 PV

p


63.2% of PV total

PV 63.2 PV total

tPVstarttPVstart

f


55/261



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


P

PV 63.2

tPVstarttPVstart t63.2t63.2

( ) f d k


56/261


Time Constant ( ) for Gravity-Drained Tanksp

Here, = 4.1 mintPVstart


tPVstart = 4.1tPVstart = 4.1

Ti C ( ) f G i D i d T k


57/261


PV63.2 = PV inital + 0.632 PV

= 1.9 + 0.632(1.0) = 2.5 m



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


58/261



= t 63.2 tPVstart = 1.6 minp Time Constant,must be positive and have units of timep


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


59/261


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


60/261


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


61/261


Process Dead-Time ( p) from Step Test Data

p = t PVstart tCOstep


ptCOsteptCOstep

tPVstarttPVstart

D d Ti ( ) f G it D i d T k


62/261


Dead-Time ( p) for Gravity Drained Tanks

p = t PVstart tCOstep = 0.3 min


= 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


63/261


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


64/261


Hands-On Workshop

Workshop #1Exploring Dynamics of Gravity-Drained Tanks

Processes have Time Varying Behavior


65/261


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


66/261

Processes have Nonlinear Behavior


67/261


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

Copyright 2007 by Control Station, Inc. All Rights Reserved

What is a Nonlinear Process


68/261



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.

68



69/261


What is a Nonlinear ProcessDealing with Nonlinearity Set Point Response

69



70/261


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.

70

What is Process Action


71/261


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.

71


72/261


73/261

Fundamentals of Instrumentation& Process Control

Interactive Training Workshop

Introduction to Instrumentation


74/261



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.

74



75/261



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?

75



76/261



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

76

What are Sensors and Transducers


77/261


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.

77

What are Standard Instrumentation Signals


78/261



Standard instrument signals for controllers toaccept as inputs from instrumentation andoutputs to final control elements are:

pneumatic current loop0 to 10 volt

78



79/261


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).

79



80/261


What are Standard Instrumentation SignalsPneumatic Scaling

What would our pneumatic signal be if ourcontroller output is 40%?

psig3 psig12OutputController % psigSignal x

80



81/261


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.

81



82/261


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.

82



83/261


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.

83



84/261


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.

84

What are Smart Transmitters


85/261


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

85



86/261


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).

86



87/261


Digital Communications

Most smart instruments wired tomulti-channel input cards requireisolated inputs for the digitalcommunications to work.

87



88/261


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.

88

What Instrument Properties Affect a Process


89/261


p

The instruments range and span. The resolution of the measurement.The instruments accuracy and precision. The instruments dynamics

89



90/261


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.

90



91/261


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

91



92/261


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

92



93/261


Precision

Precision is the reproducibility with whichrepeated measurements can be made underidentical conditions.

This may be referred to as drift.

93



94/261


Accuracy vs. Precision

Why is precision preferred over accuracy?

94



95/261


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.

95


96/261

What is Input Aliasing


97/261


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.

97


98/261

What is Input Aliasing


99/261


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

99

What is Input Aliasingl


100/261


Correct Sampling Frequency

2 Hz waveform sampled every 0.025 seconds

100


101/261


Workshop Lab:

Input Aliasing 1

101

What is Input AliasingD i i h C S li I l


102/261


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.

102

What is Input AliasingD i i h C S li I l


103/261


Determining the Correct Sampling Interval

103


104/261


Workshop Lab:

Input Aliasing 2

104


105/261

What is Instrument NoiseS


106/261


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.

106

What is Instrument NoiseEffects of Noise


107/261


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

107

What is Instrument NoiseEliminating Noise


108/261


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.

108

What is Instrument NoiseLow Pass Filter


109/261


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

109

What is Instrument NoiseLow Pass Filter


110/261


Low Pass Filter

A low pass filter introduces a first order lag intothe measurement.

Raw (unfiltered) Signal

Filtered Signal

110

What is Instrument NoiseSelecting a Low Pass Filter


111/261


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

111



112/261



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

112



113/261



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.

113



114/261



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.

114


115/261


Workshop Lab:Noise Filtering

115

What is Temperature


116/261


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.

116

What is TemperatureUnit Conversion


117/261


Unit Conversion

32C5

9F

x

32-F95

C x

117

What Temperature Instruments Do We Use


118/261


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)

118

What Temperature Instruments Do We UseThermocouples


119/261


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.

119

What Temperature Instruments Do We UseThermocouple Junctions


120/261


p

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.

120



121/261


p

Hot junction voltage proportional to 210F.Extension wire voltage proportional to 18F.

121



122/261


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.

122

What Temperature Instruments Do We UseThermocouple Linearization


123/261


p

123

What Temperature Instruments Do We UseThermocouple Gain


124/261


p

124

What Temperature Instruments Do We UseThermocouple Types


125/261


p yp

125

What Temperature Instruments Do We UseRTDs


126/261


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.

126


127/261

What Temperature Instruments Do We UseRTD Lead Wire Resistance


128/261


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.

128

What Temperature Instruments Do We UseRTD Lead Wire Resistance


129/261


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.

129

What Temperature Instruments Do We UseRTD Self Heating Effect


130/261


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

130

What Temperature Instruments Do We UseThermistors


131/261


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.

131

What Temperature Instruments Do We UseInfrared


132/261


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.

132

What Temperature Instruments Do We UseInfrared Emittance


133/261


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%.

133

What Temperature Instruments Do We UseInfrared Emittance


134/261


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.

134

What Temperature Instruments Do We UseInfrared Field of View


135/261


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.

135

What Temperature Instruments Do We UseInfrared Spectral Response


136/261


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.

136


137/261


Section Assessment:Temperature

137

What is Pressure


138/261


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

138

What is Pressure Absolute, Gauge and Differential


139/261


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.

139

What is Pressure Absolute, Gauge and Differential


140/261



141/261


Section Assessment:Pressure

141

Common Level Sensing Technologies


142/261


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

142

Common Level Sensing TechnologiesNon-Contact: Ultrasonic


143/261


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.

143

Common Level Sensing TechnologiesNon-Contact: Ultrasonic


144/261


Common Level Sensing TechnologiesNon-Contact: Radar/Microwave


145/261


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.

145

Common Level Sensing TechnologiesNon-Contact: Radar/Microwave


146/261


Common Level Sensing TechnologiesNon-Contact: Nuclear Level Measurement


147/261


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.

147

Common Level Sensing TechnologiesNon-Contact: Nuclear Level Measurement


148/261


Common Level Sensing TechnologiesContact: Hydrostatic Pressure


149/261


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.

149



150/261


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.

150



151/261


Common Level Sensing TechnologiesContact: RF/Capacitance


152/261


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.

152



153/261


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).

153



154/261


Common Level Sensing TechnologiesContact: Guided Wave Radar


155/261


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.

155

Common Level Sensing TechnologiesContact: Guided Wave Radar


156/261



157/261


Section Assessment:Level

157

What is Flow


158/261


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.

158

Factors Affecting Flow Measurement


159/261


The critical factors affecting flow measurement

are:Viscosity Fluid TypeReynolds Number

Flow Irregularities

159

Factors Affecting Flow Measurement Viscosity


160/261


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.

160

l fl h f l f l f l f fl



161/261


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

161



162/261


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

162



163/261


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.

163

Factors Affecting Flow Measurement Viscosity Conversion Chart


164/261


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

164

Factors Affecting Flow Measurement Viscosity of Common Fluids


165/261


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

165



166/261


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.

166

Factors Affecting Flow MeasurementFluid Type


167/261


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.

167

Factors Affecting Flow MeasurementFluid Type


168/261


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

168

Factors Affecting Flow MeasurementReynolds Number


169/261


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

169

Factors Affecting Flow MeasurementLaminar Flow

f


170/261


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.

170

Factors Affecting Flow MeasurementTurbulent Flow

b l fl h h ld b


171/261


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.

171

Factors Affecting Flow MeasurementTransitional Flow

T i i l fl i ll R ld


172/261


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.

172

Factors Affecting Flow MeasurementReynolds Number


173/261


Flow instruments must beselected to accurately

measure laminar flow orturbulent flow

173

Factors Affecting Flow MeasurementFlow Irregularities


174/261


Flow irregularities arechanges in thevelocity profile of thefluid flow caused byinstallation of the flowinstrument.

Normal velocityprofiles are

established byinstalling a sufficientrun of straight piping.

174

Common Flowmeter Technologies


175/261


Common Flowmeter Technologies

V l t i Fl


176/261


Volumetric Flow

Magnetic flowmetersPositive displacement for viscous liquidsOrifice Plates

Mass FlowCoriolis flowmeters

176

Common Flowmeter Technologies Volumetric Flow

V l t i fl i ll i f d


177/261


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)

177

Common Flowmeter Technologies Volumetric Flow: Positive Displacement

Oper te b c pt ring the fl id to be me s red in


178/261


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.

178

Common Flowmeter Technologies Volumetric Flow: Magnetic

M ti fl t i f th


179/261


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

179

Common Flowmeter Technologies Volumetric Flow: Orifice Plates


180/261


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.

180

Common Flowmeter Technologies Volumetric Flow: Orifice Plates

Orifice Plate type meters only work well when
http://www.answers.com/main/ntquery?method=4&dsname=Wikipedia+Images&dekey=OrificePlate.JPG&gwp=8


181/261


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


182/261


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


183/261


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.

183



184/261




185/261



Coriolis mass flowmeters


186/261


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


187/261


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


188/261


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.

188

Common Flowmeter TechnologiesInstallation and Calibration

In general calibration of a flowmeter requires:


189/261


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


190/261


Section Assessment:Flow

190

Introduction to Final Control Elements

Objectives:


191/261


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?

191

Introduction to Final Control Elements

The intent of this chapter is neither to teach you how to


192/261


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.

192

What is a Control Valve

A control valve is an inline device in a flow


193/261


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 )

193

What is a Control ValveShut-Off Service


194/261


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.

194

What is a Control ValveDivert Service


195/261


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


196/261


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


197/261


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


198/261


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

Date post:	02-Apr-2018
Category:	Documents
Upload:	nirmal-kumar-pandey
View:	218 times
Download:	0 times

Unit Extra PDC

Documents