complete f&ic slidedeck

8/9/2019 Complete F&IC SlideDeck

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Fundamentals of Instrumentation& Process Control

Interactive Training Workshop


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Copyright 2007 Control Station, Inc. All Rights Reserved

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 controller

is 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:hy do we need process control!

hat is a process!

hat is process control!

hat is open loop control!hat is closed loop control!

hat are the modes of closed loop control!

hat are the basic elements of process control!

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Motivation for Automatic Process Control

"afety #irst:people, en$ironment, e%uipment

The &rofit 'oti$e:

meeting final product specs

minimi(ing waste production

minimi(ing en$ironmental impact

minimi(ing energy use

maximi(ing o$erall production rate

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Loose Control Costs Mone

It ta)es more processing to remo$e impurities, so greatest

profit is to operate as close to the maximum impuritiesconstraint as possible without going o$er

It ta)es more material to ma)e a product thic)er, so greatestprofit is to operate as close to the minimum thic)nessconstraint as possible without going under

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"i#ht Control is More Pro$ta%le

A well controlled process has less $ariability in the measuredprocess $ariable *&+, so the process can be operated closeto the maximum profit constraint.

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"erminolo# for 'ome 'eatin# Control

-ontrol bjecti$e

'easured &rocess +ariable *&+"et &oint *"&

-ontroller utput *-

'anipulated +ariable

/isturbances */

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)hat 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 de$ice that changes

the $alue of the manipulated $ariable.The controller output (CO)is the signal from thecontroller to the final control element.

Theprocess variable (PV)is a measure of the processoutput that changes in response to changes in the

manipulated $ariable.Theset point (SP)is the $alue at which we wish tomaintain the process $ariable at.

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)hat is a Process

Aprocessis broadly defined as an operation thatuses resources to transform inputs into outputs.

It is the resource that pro$ides the energy intothe process for the transformation to occur.

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)hat is a Process

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)hat is a Process

0ach process exhibits a particular dynamic *time$arying beha$ior that go$erns thetransformation.

That is, how do changes in the resource or inputso$er time affect the transformation.

This dynamic beha$ior is determined by thephysical properties of the inputs, the resource,and the process itself.

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)hat is a Process

-an you identify some of the elements that willdetermine the dynamic properties of thisprocess!

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Process controlis the act of controlling a finalcontrol element to change the manipulated$ariable to maintain the process $ariable at adesired set point.

A corollary to our definition of process control is acontrollable process must beha$e in a predictablemanner.

#or a gi$en change in the manipulated $ariable, theprocess $ariable must respond in a predictable and

consistent manner.

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*!


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Section Assessment:Section Assessment:

Basic Terminology AssessmentBasic Terminology Assessment

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)hat is O,en Loo, Control

In open loop controlthe controller output is nota function of the process $ariable.

In open loop control we are not concerned that aparticular set point be maintained, the controlleroutput is fixed at a $alue until it is changed by an

operator.'any processes are stable in an open loop controlmode and will maintain the process $ariable at a$alue in the absence of a disturbances.

Disturbancesare uncontrolled changes in theprocess inputs or resources.

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2+


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-an you thin) of processes in which open loopcontrol is sufficient!

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)hat is Closed Loo, Control

In closed loop controlthe controller output isdetermined by difference between the process$ariable and the set point. -losed loop control isalso called feedbac) or regulatory control.

The output of a closed loop controller is a function ofthe error.

Erroris the de$iation of the process $ariable fromthe set point and is defined as0 1 "& 2 &+.

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#rom the controllers

perspecti$e theprocess encompassesthe 3T/, the steamcontrol $al$e, and

signal processingof the &+ and

- $alues.

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)hat are the Modes of Closed Loo, Control

-losed loop control can be, depending on thealgorithm that determines the controller output:

'anual

n2ff

&I/

Ad$anced &I/ *ratio, cascade, feedforward

or 'odel 4ased

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)hat are the Modes of Closed Loo, ControlManual Control

In manual controlan operator directlymanipulates the controller output to the finalcontrol element to maintain a set point.

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)hat are the Modes of Closed Loo, ControlOn-O. Control

On-Off controlpro$ides a controller output ofeither on or off in response to error.

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)hat are the Modes of Closed Loo, ControlOn-O. Control /ead%and

5pon changing the direction of the controlleroutput, deadbandis the $alue that must betra$ersed before the controller output will changeits direction again.

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)hat are the Modes of Closed Loo, ControlPI/ Control

PID controlpro$ides a controller output thatmodulates from 6 to 7668 in response to error.

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)hat are the Modes of Closed Loo, Control"ime Pro,ortion Control

ime proportion controlis a $ariant of &I/ control thatmodulates the on2off time of a final control element that onlyhas two command positions.

To achie$e the effect of &I/

control the switching fre%uency

of the de$ice is modulated in

response to error. This is

achie$ed by introducing theconcept of cycle time.

Cycle Time is the time base of the

signal the final control element will

receive from the controller. The PID

controller determines the final signalto the controller by multilying the

cycle time by the outut of the PID

algorithm.

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)hat are the Modes of Closed Loo, ControlCascade Control

Cascade controluses the output of a primary *master orouter controller to manipulate the set point of a secondary*sla$e or inner controller as if the sla$e controller were thefinal control element.

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)hat are the Modes of Closed Loo, ControlCascade Control

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0asic 1lements of Process Control

-ontrolling a process re%uires )nowledge of four

basic elements, theprocessitself, thesensorthat measures the process $alue, the finalcontrol elementthat changes the manipulated$ariable, and the controller.

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Section Assessment:Section Assessment:

Basic Process Control AssessmentBasic Process Control Assessment

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/ynamic &rocess 4eha$ior hat It Is 9 hy e -are

hat a #&/T /ynamic 'odel 3epresents

Analy(ing "tep Test &lot /ata to /etermine #&/T

/ynamic 'odel &arameters

&rocess ain, Time -onstant 9 /ead Time

;ow To -ompute Them #rom &lot /ata

;ow to 5se Them #or -ontroller /esign and Tuning

;ow to 3ecogni(e


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/namic Process 0ehavior and Controller"unin#

-onsider cruise control for a car $s a truc)how %uic)ly can each accelerate or decelerate

what is the effect of disturbances *wind, hills, etc.

-ontroller *gas flow manipulations re%uired to maintainset point $elocity in spite of disturbances *wind, hills

are different for a car and truc) because the dynamicbeha$ior of each >process> is different

/ynamic beha$ior how the measured process $ariable*&+ responds o$er time to changes in the controlleroutput *- and disturbances */

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ra,hical Modelin# of /namic Process/ata

To learn about the dynamic beha$ior of a process, weanaly(e measured process $ariable *&+ test data

&+ test data can be generated by suddenly changing thecontroller output *- signal

The - should be mo$ed far and fast enough so that the

dynamic beha$ior is clearly re$ealed as the &+ responds

The dynamic beha$ior of a process is different as operatingle$el changes *nonlinear beha$ior, so collect data atnormal operating conditions *design le$el of operation

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Copyright 2007 Control Station, Inc. All Rights Reserved 3&

Modelin# /namic Process 0ehavior

The best way to understand process data is through modeling

'odeling means fitting a first order plus dead time *#&/Tdynamic model to the process data:

where:

&+ is the measured process $ariable

- is the controller output signal

The #&/T model is simple *low order and linear so it onlyapproximates the beha$ior of real processes

d&+p ? &+ 1 @p -.*tAp,

dt


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Copyright 2007 Control Station, Inc. All Rights Reserved 3(

Modelin# /namic Process 0ehavior

hen a first order plus dead time *#&/T modelis fit to dynamic process data

The important parameters that result are:

"teady "tate &rocess ain, @p

$erall &rocess Time -onstant,

Apparent /ead Time, p

d&+p ? &+ 1 @p -.*tAp,

dt

p


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Copyright 2007 Control Station, Inc. All Rights Reserved 4+

"he OP/" Model is All Im,ortant

#&/T model parameters *@p, and p are used in

correlations to compute controller tuning $alues

"ign of @p indicates the action of the controller

*?@p re$erse actingB C@p direct acting

"i(e of indicates the maximum desirable loop sample time

*be sure sample time T 6.7 3atio of pD indicates whether model predicti$e controlsuch as a "mith predictor would show benefit

*useful if p E

'odel becomes part of the feed forward, "mith &redictor,decoupling and other model2based controllers

p

p

p

p

p


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te, "est /ata and /namic ProcessModelin#

&rocess starts at steady state in manual mode

-ontroller output *- signal is stepped to new $alue

&rocess $ariable *&+ signal must complete the response


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Copyright 2007 Control Station, Inc. All Rights Reserved 42

Process ain 78,9 from te, "est /ata

@p describes how farhow farthe measured &+ tra$els in

response to a change in the -

A step test starts and ends at steady state, so @p canbe computed directly from the plot

where &+ and - are the total change from initial tofinal steady state

A large process gain, @p, means that each - actionwill produce a large &+ response

"teady "tate -hange in the &rocess +ariable,F&+@p

"teady "tate -hange in the -ontroller .utput, F-.=


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Process ain 78,9 for ravit-/rained "an:s

-ompute &+ and - as Gfinal minus initialHsteady state $alues


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Process ain 78,9 for ravit-/rained "an:s

@p has a si(e *6.7B a sign *?6.7, and units *mD8


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Additional ;otes on Process ainMeasurin# the Process ain

&rocess 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 in$erselyproportional to the process gain that it sees

ElementControlFinalGainSensorGainProcessGainGainProcess xx=

GainProcess

1GainController

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Additional ;otes on Process ainConvertin# nits of Process ain

There is one important ca$eat in this processB thegain we ha$e calculated has units of mD8.3eal world controllers, unli)e most softwaresimulations, ha$e gain units specified as 8D8.

hen calculating the gain for a real controller thechange in &+ needs to be expressed in percent ofspan of the &+ as this is how the controllercalculates error.

SpanPV

SpanCOxCO

PV

=Gain

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Assuming the process gain of the gra$itydrained tan)s is 6.7 mD8 then, to con$ertto 8D8

Additional ;otes on Process ainConvertin# nits of Process ain

%

%0.1%100

%1000

%1.0PV

CO

%1.0 =

=m

span

spanm

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Additional ;otes on Process ain


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Process "ime Constant 7 9 from te, "est/ata

Time -onstant, , describes how fasthow fastthe measured &+responds to changes in the -

'ore specifically, how long it ta)es the &+ to reach LM.K8of its total final change *&+, starting from when it firstbegins to respond

p

p

P "i C t t 7 9 f t " t


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7 =ocate , the time where the &+ starts a

first clear response to the step change in -


p

tP!start

P "i C t t 7 9 f t " t


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K -ompute LM.K8 of the total change in &+ as:

&+LM.K1 &+inital? 6.LMK&+

p

P "i C t t 7 9 f t " t


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M

N Time -onstant, , is then:

p

t"#.$%tP!startp

t"#.$ is the time when the &+ reaches &+LM.K

"i C t t 7 9 f it / i d


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"ime Constant 7 9 for ravit-/rained"an:s

p

;ere, 1 N.7 mintP!start

"i C t t 7 9 f it / i d


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Copyright 2007 Control Station, Inc. All Rights Reserved 5!

&+LM.K1 &+inital? 6.LMK&+

1 7.O ? 6.LMK*7.6 1 K.J m


p

"i C t t 7 9 f it / i d


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Copyright 2007 Control Station, Inc. All Rights Reserved 5&


p

= t"#.$%tP!start= 1.6 inpTime -onstant,

must be positi$e and ha$e units of timep

Process /ead "ime 7,9 from te, "est


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Copyright 2007 Control Station, Inc. All Rights Reserved 5(

Process /ead-"ime 7,9 from te, "est/ata

/ead time, p, is how much delayhow much delayoccurs from the time whenthe - step is made until when the measured &+ shows afirst clear response.

p is the sum of these effects:

transportation lag, or the time it ta)es for material totra$el from one point to another

sample or instrument lag, or the time it ta)es to collect,analy(e or process a measured &+ sample

higher order processes naturally appear slow to respondand this is treated as dead time

/ead time, p, must be positi$e and ha$e units of time

/ead "ime 7,9 is the 8ill f C t l8iller of Control


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Copyright 2007 Control Station, Inc. All Rights Reserved 6+

/ead-"ime 7,9 is the 8iller of Control8iller of Control

Tight control grows increasingly difficult as p becomes large

The process time constant is the cloc) of the process. /eadtime is large or small relati$e only to

hen dead time grows such that p E , model predicti$econtrol strategies such as a "mith predictor may show benefit

#or important &+s, wor) to select, locate and maintaininstrumentation so as to a$oid unnecessary dead time in a loop

p

p

Process /ead "ime 7,9 from te, "est


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Copyright 2007 Control Station, Inc. All Rights Reserved 6*

Process /ead-"ime 7,9 from te, "est/ata

p 1 tP!start%tC&ste

/ead "ime 7,9 for ravit /rained "an:s


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/ead-"ime 7,9 for ravit /rained "an:s

p 1 tP!start%tC&ste= 0.! in

"he OP/" Model Parameters


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"he OP/" Model Parameters

&rocess ain, @p ;ow #ar &+ tra$els

Time -onstant, ;ow #ast &+ responds

/ead Time, p ;ow 'uch /elay 4efore &+ 3esponds

#or a change in -:

p

In SummaryIn Summary

'ands On )or:sho,'ands On )or:sho,


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'ands-On )or:sho,'ands-On )or:sho,

or)shop P7

0xploring /ynamics of ra$ity2/rained Tan)s

Processes have "ime


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Processes have "ime


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Processes have ;onlinear 0ehavior

The dynamic beha$ior of most real processeschanges as operating le$el changes



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Copyright 2007 Control Station, Inc. All Rights Reserved 6!


A #&/T model response is constant as operating le$el changes

"ince the #&/T model is used for controller design and tuning, aprocess should be modeled at a specific design le$el of operationQ

)hat is a ;onlinear Process


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hile linear processes may be the design goal of process

engineers, the reality is that most processes are nonlinearin nature due to nonlinearity in the final control element orthe process itself.

0xample: A heating process is nonlinear because the rateat which heat is transferred between two objects depends

on the difference in temperature between the objects.0xample: A $al$e that is linear in the middle of itsoperating range may become $ery nonlinear towards itslimits.

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)hat is a ;onlinear Process/ealin# =ith ;onlinearit > et Point ?es,onse

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)hat is a ;onlinear Process/ealin# =ith ;onlinearit

The robustnessof a controller is a measure therange of process $alues o$er which the controllerpro$ides stable operation.

The more nonlinear a process is, the lessaggressi$e you must be in your tuning approach

to maintain robustness.

!+

)hat is Process Action


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Process actionis how the process $ariable changes with

respect to a change in the controller output. &rocessaction is either direct acting or re$erse acting.

The action of a process is defined by the sign of theprocess gain. A process with a positi$e gain is said to bedirect acting. A process with a negati$e gain is said to be

re$erse acting.

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)hat is Process ActionProcess and Controller Action

The action of a process is important because it will

determine the action of the controller.

A direct acting process re%uires a re$erse actingcontroller.

-on$ersely, a re$erse acting process re%uires a direct

acting controller.

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Fundamentals of Instrumentation& Process Control

Interactive Training Workshop

Introduction to Instrumentation


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

select a particular instrument nor to familiarize you with allof the available types of instruments.

The intent of this material is to provide an introduction tocommonly measured process variables, including the basicterminology and characteristics relevant to each variablesrole in a control loop.

Detailed information and assistance on device selection is

typically available directly from the instrumentationsupplier.

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Objectives:

hat are sensors and transducers!

hat are the standard instrument signals!

hat are smart transmitters!

hat is a low pass filter!

hat instrument properties affect a process!hat is input aliasing!

hat is instrument noise!

;ow do we measure temperature!

;ow do we measure le$el!

;ow do we measure le$el!

;ow do we measure flow!

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!ou cannot control "#at $oucannot measure

'(hen you can measure what you are sea)ing about*

and e+ress it in numbers* you )now something

about it. (hen you cannot measure it* when you

cannot e+ress it in numbers* your )nowledge is of a

meagre and unsatisfactory )ind.,

- Lord Kelvin

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)hat are ensors and "ransducers


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)hat are ensors and "ransducers

Asensoris a de$ice that has a characteristic that changes

in a predictable way when exposed to the stimulus it wasdesigned to detect.

A transduceris a de$ice that con$erts one form of energyinto another.

!!

)hat are tandard Instrumentation i#nals


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

pneumatic

current loop

% to &% volt

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)hat are tandard Instrumentation i#nalsPneumatic

M to 7J psig

4efore 7OL6, pneumatic signals were used almostexclusi$ely to transmit measurement and controlinformation.

Today, it is still common to find M to 7J psig used as

the final signal to a modulating $al$e.'ost often an ID& *I to & transducer is used.

This con$erts a N2K6 mA signal *I into a pressuresignal *&.

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)hat are tandard Instrumentation i#nalsPneumatic calin#

hat would our pneumatic signal be if ourcontroller output is N68!

( ) psig3psig12OutputController%psigSignal += x

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a a e a da d s u e a o # a sCurrent Loo,

N2K6 milliamp

-urrent loops are the signal wor)horses in ourprocesses.

A /- milliamp current is transmitted through a pairof wires from a sensor to a controller or from a

controller to its final control element.-urrent loops are used because of their immunity tonoise and the distances that the signal can betransmitted.

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#Current Loo, calin#

utput "caling

"cale outputs for a one2to2one correspondence.

-ontroller output is configured for 68 to correspondto a NmA signal and 7668 to correspond to a K6mAsignal.

The final control element is calibrated so that NmAcorresponds to its 68 position or speed and K6mAcorresponds to its 7668 position or speed.

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#Current Loo, calin#

Input "caling"cale inputs for a one2to2one correspondence as well.

0xample:If we were using a pressure transducer with a re%uiredoperating range of 6 psig to 766 psig we would calibrate

the instrument such that 6 psig would correspond to NmAoutput and 766 psig would correspond to a K6mA output.

At the controller we would configure the input such thatNmA would correspond to an internal $alue of 6 psig and76mA would correspond to an internal $alue of 766 psig.

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#+ to *+


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A smart transmitter is a digital de$ice thatcon$erts the analog information from a sensorinto digital information, allowing the de$ice tosimultaneously send and recei$e information andtransmit more than a single $alue.

"mart transmitters, in general, ha$e the followingcommon features:/igital -ommunications

-onfiguration

3e23anging

"ignal -onditioning"elf2/iagnosis

&5

)hat are mart "ransmitters


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/i#ital Communications

"mart transmitters are capable of digital communications

with both a configuration de$ice and a process controller./igital communications ha$e the ad$antage of being free ofbit errors, the ability to multiple process $alues anddiagnostic information, and the ability to recei$ecommands.

"ome smart transmitters use a shared channel for analogand digital data *;art, ;oneywell or 'odbus o$er N2K6mA.thers use a dedicated communication bus *&rofibus,#oundation #ieldbus, /e$ice


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/i#ital Communications

'ost smart instruments wired tomulti2channel input cards re%uireisolated inputs for the digital

communications to wor).

&!

)hat are mart "ransmitters


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Con$#uration@ i#nal Conditionin#@ elf-/ia#nosis

Configuration

"mart transmitters can be configured with ahandheld terminal and store the configurationsettings in non2$olatile memory.

Signal Conditioning

"mart transmitters can perform noise filtering andcan pro$ide different signal characteri(ations.

Self-Diagnosis

"mart transmitters also ha$e self2diagnostic

capability and can report malfunctions that mayindicate erroneous process $alues.

&&

)hat Instrument Pro,erties A.ect a Process


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The instruments range and span.

The resolution of the measurement.

The instruments accuracy and precision.

The instruments dynamics

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?an#e and ,an

The ran'eof a sensor is the lowest and highest$alues it can measure within its specification.

Thespanof a sensor is the high end of the3ange minus the low end of the 3ange.

'atch 3ange to 0xpected -onditions

Instruments should be selected with a range thatincludes all $alues a process will normally encounter,including expected disturbances and possible failures.

(+



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Measurement ?esolution

esolutionis the smallest amount of inputsignal change that the instrument can detectreliably.

3esolution is really a function of the instrument spanand the controllers input capability.

The resolution of a 7L bit con$ersion is:

The resolution of a 7K bit con$ersion is:

The bit error.535,5

Span!nput

0"5,#

Span!nput

(*



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Accurac

ccurac$of a measurement describes how close

the measurement approaches the true $alue ofthe process $ariable.

8 error o$er a range

R x8 o$er

8 of full scale 8 of span

Absolute o$er a range

R x units o$er full scale

span

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Precision

Precisionis the reproducibility with which

repeated measurements can be made underidentical conditions.

This may be referred to as drift.

(3



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Accurac vs Precision

hy is precision preferred o$er accuracy!

(4

)hat Instrument Pro,erties A.ect a Processi i i d d i


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Instrumentation /namicsB ain and /ead "ime

The gain of an instrument is often called

sensitivit$.

The sensiti$ity of a sensor is the ratio of the outputsignal to the change in process $ariable.

The dead timeof an instrument is the time it

ta)es for an instrument to start reacting toprocess change.

(5

)hat Instrument Pro,erties A.ect a ProcessI i / i "i C


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Instrumentation /namicsB "ime Constant

As for processes, one time constant for an instrument is

the time it ta)es to pro$ide a signal that represents LM.K8of the $alue of $ariable it is measuring after a step changein the $ariable.

Instrument manufacturers may sometimes specify the risetime instead of the time constant.

3ise time is the time it ta)es for an instrument topro$ide a signal that represents 7668 of the $alue ofthe $ariable it is measuring after a step change in the$ariable.

The rise time of an instrument is e%ual to J timeconstants.

(6

)hat is In,ut Aliasin#


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Input aliasing is a phenomenon that occurs from

digital processing of a signal.

hen a signal is processed digitally it is sampledat discrete inter$als of time. If the fre%uency atwhich a signal is sampled is not fast enough the

digital representation of that signal will not becorrect.

Input aliasing is an important consideration indigital process control. &rocessor inputs that

ha$e configurable sample rates and &I/ loopupdate times must be set correctly.

(!

)hat is In,ut Aliasin#


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K ;( wa$eform sampled e$ery 6.N seconds

(&

)hat is In,ut Aliasin#C t li


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Correct am,lin# reuenc


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Correct am,lin# reuenc

K ;( wa$eform sampled e$ery 6.6KJ seconds

*++


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)hat is In,ut Aliasin#/ t i i th C t li I t l


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/eterminin# the Correct am,lin# Interval

3ules of Thumb"et the sample inter$al for an instrument at 7D76th to7DK6th of the rise time *7DK to 7DNth of the time constant.

"et the sample inter$al to 7D76th to 7DK6th of the processtime constant.

Temperature instrumentation *3T/s and thermocouples in

thermowells typically ha$e time constants of se$eralseconds or more. #or these processes sampling inter$alsof 7 second are usually sufficient.

&ressure and flow instrumentation typically ha$e timeconstants of S to 7 second. #or these processes sampling

inter$als of 6.7 second are usually sufficient.

*+2

)hat is In,ut Aliasin#/eterminin# the Correct am,lin# Interval


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/eterminin# the Correct am,lin# Interval

*+3


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(or)sho -ab:(or)sho -ab:

Inut Aliasing $Inut Aliasing $

*+4

)hat is Instrument ;oise


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*oiseis a $ariation in a measurement of a

process $ariable that does not reflect realchanges in the process $ariable.

*+5

)hat is Instrument ;oiseources


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ources


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1.ects of ;oise


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)hat is Instrument ;oiseLo= Pass ilter


109/261


Lo= Pass ilter

A lo"-pass filterallows the low fre%uency

components of a signal to pass while attenuatingthe higher fre%uency components.

Ra" #$n%iltered& Signal

'iltered Signal

*+(


110/261

)hat is Instrument ;oiseelectin# a Lo= Pass ilter


111/261


electin# a Lo= Pass ilter

4y -ut2ff #re%uency

-ut2off fre%uency is defined as fre%uency abo$ewhich the filter pro$ides 2Md4 of signal attenuation.

An attenuation of 6 d4 would mean the signal willpass with no reduction in amplitude while a largenegati$e d4 would indicate a $ery small amplituderatio.

"elect a cut2off fre%uency that is abo$e the

fre%uency of your process.

=!n$mplitue

Out$mplitue

10log20nattenuatioo&'

***



112/261



4y Time -onstant

"ome filters are configured by selecting a timeconstant for the lag response of the filter.

The relationship between the cut2off fre%uency andthe time constant of a low pass is approximately

gi$en by:

Constants(ime5

1Fre)uenc*O&&+Cut

**2



113/261



4y +alue"ome filters are selected by a $alue called alpha *,notably the deri$ati$e filter in a &I/ controller.

is an a$eraging weighting term used in controllercalculations to impart a first order lag on themeasured $ariable.

generally has $alues between 6 and 7. A filter witha 1 6 would pass the signal through unfiltered. Afilter with a 1 7 would filter e$erything allowingnothing to pass through the filter.

**3



114/261



hen a filter is specified by cut2off fre%uency, the

lower the fre%uency the greater the filteringeffect.

hen a filter is specified by time constant, thegreater the time constant the greater the filtering

effect.hen a filter is specified by , when 16 nofiltering is done, when 1 7 no signal passesthrough the fitter.

**4


115/261


(or)sho -ab:(or)sho -ab:

/oise 0iltering/oise 0iltering

**5

)hat is "em,erature


116/261


Temperature is a measure of degree of the

hotness or coldness of an object.

5nits of Temperature

Two most common temperature scales are #ahrenheit

*# and -elsius *-.

The reference points are the free(ing point and theboiling point of water.

ater free(es at MK# and boils at K7K#.

The -elsius scale uses the same reference pointsonly it defines the free(ing point of water as 6- andthe boiling point as 766-.

**6

)hat is "em,eraturenit Conversion


117/261


nit Conversion

32C5

"F +=

x

( )32+F"

5

C = x

**!

)hat "em,erature Instruments /o )e se


118/261


Temperature is one of the most common process

$ariables.

Temperature is most commonly measured by

esistance emperature Devices (D)

#ermocouples

Infrared (I) *to a lesser degree

#ermistors*may also be found embedded in somecontrol e%uipment

**&

)hat "em,erature Instruments /o )e se"hermocou,les


119/261


e ocou, es

Thermocouples are fabricated from two electrical

conductors made of two different metal alloys.;ot or sensing junction

-old or reference junction

enerate an open2circuit $oltage, called the "eebec)

$oltage that is proportional to the temperaturedifference between the sensing *hot and reference*cold junctions.

**(

)hat "em,erature Instruments /o )e se"hermocou,le Dunctions


120/261


, D

There is a misconception of how thermocouples operate.

The misconception is that the hot junction is the source ofthe output $oltage. This is wrong. The $oltage isgenerated across the length of the wire.

Another misconception is that junction $oltages aregenerated at the cold end between the special

thermocouple wire and the copper circuit. ;ence, a coldjunction temperature measurement is re%uired. Thisconcept is wrong. The cold end temperature is thereference point for measuring the temperature differenceacross the length of the thermocouple circuit.

*2+



121/261


, D

;ot junction $oltage proportional to K76#.

0xtension wire $oltage proportional to 7U#.

*2*



122/261


, D

5se of the correct extension wire is critical in

thermocouple applications.An incorrect extension will cause the temperaturedifferential across the extension leads to beintroduced as measurement error.

/oes this mean you cannot use copper terminalbloc)s for thermocouples!


123/261


,

*23

)hat "em,erature Instruments /o )e se"hermocou,le ain


124/261


,

*24

)hat "em,erature Instruments /o )e se"hermocou,le ",es


125/261

Copyright 2007 Control Station, Inc. All Rights Reserved *25

)hat "em,erature Instruments /o )e se?"/s


126/261


A resistance2temperature detector *3T/ is a temperature

sensing de$ice whose resistance increases withtemperature.

An 3T/ consists of a wire coil or deposited film of puremetal whose resistance at $arious temperatures has beendocumented.

3T/s are used when applications re%uire accuracy, long2term stability, linearity and repeatability.

766 &latinum is most common.

*26

)hat "em,erature Instruments /o )e se?"/sB Im,ortance of


127/261


is, roughly, the slope of the 3T/sresistance cur$e with respect totemperature.

A 766platinum 3T/ with a 16.66MO77 will ha$e aresistance of 7MO.77 ohms at 766-. A 766 platinum 3T/with a 16.66MUJ6 will ha$e a resistance of 7MU.J6 ohms at766-.

-ommon $alues for the temperature coefficientare:

6.66MUJ6, /I< NMVL6 "tandard *also referred to as 0uropeancur$e

6.66MO77, American "tandard

6.66MOKL, IT"2O6 "tandard

0100

0100

Rx

RR =

*2!

)hat "em,erature Instruments /o )e se?"/ Lead )ire ?esistance


128/261


The 3T/ is a resisti$e de$ice, you must dri$e a

current through the de$ice and monitor theresulting $oltage. Any resistance in the leadwires that connect your measurement system tothe 3T/ will add error to your readings.

N6 feet of 7U gauge Kconductor cable hasLresistance.#or a platinum 3T/with 1 6.66MUJ,the resistance e%uals

6.L D*6.MUJ D- 1 7.L- of error.

*2&


129/261

)hat "em,erature Instruments /o )e se?"/ elf 'eatin# 1.ect


130/261


The current used to excite an 3T/ causes the 3T/ to

internally heat, which appears as an error. "elf2heating istypically specified as the amount of power that will raisethe 3T/ temperature by 7 -, or 7 mD -.

"elf2heating can be minimi(ed by using the smallestpossible excitation current, but this occurs at the expenseof lowering the measurable $oltages and ma)ing the signal

more susceptible to noise from induced $oltages.The amount of self2heating also depends hea$ily on themedium in which the 3T/ is immersed. An 3T/ can self2heat up to 766 times higher in still air than in mo$ingwater

*3+

)hat "em,erature Instruments /o )e se"hermistors


131/261


A thermistor is similar to an 3T/ in that it is a

passi$e resistance de$ice.Thermistors are generally made of semiconductormaterials gi$ing them much different characteristics.

Thermistors do not ha$e standardi(ed electrical

properties li)e thermocouples or 3T/s.

*3*

)hat "em,erature Instruments /o )e seInfrared


132/261


bjects radiate electromagnetic energy.

The higher the temperature of an object the moreelectromagnetic radiation it emits.

This radiation occurs within the infrared portion ofthe electromagnetic spectrum.

*32

)hat "em,erature Instruments /o )e seInfrared 1mittance


133/261


The emittance of a real surface is the ratio of the

thermal radiation emitted by the surface at agi$en temperature to that of the ideal blac) bodyat the same temperature.

4y definition, a blac) body has an emittance of 7.

Another way to thin) of emissi$ity is theefficiency at which an object radiates thermalenergy.

0mittance is a decimal number that ranges

between 6 and 7 or a percentage between 68and 7668.

*33

)hat "em,erature Instruments /o )e seInfrared 1mittance


134/261


To function properly an infrared temperature

instrument must ta)e into account the emittanceof the surface being measured.

0mittance $alues can often be found in referencetables, but such tables will not ta)e into account

local conditions of the surface.A more practical way to set the emittance is tomeasure the temperature with an 3T/ orthermocouple and set the instrument emissi$ity

control so that both readings are the same.

*34

)hat "em,erature Instruments /o )e seInfrared ield of


135/261


Infrared temperature instruments are li)e an optical

system in that they ha$e a field of $iew. The field of $iewbasically defines the target si(e at a gi$en distance.

#ield of $iew may be specified as:Angle and focal range *K.M from U> to 7NW

/istance to spot si(e ratio and focal range *KJ:7 from U> to 7NW

"pot si(e at a distance *6.MK> diameter spot at U>

All of these specifications are e%ui$alent, at U inches our field of$iew will be a 6.MK> diameter spot *U>DKJ, at 7N feet our field of$iew will be L.VK> *7NW D KJ.

An infrared temperature sensor measures the a$eragetemperature of e$erything in its field of $iew. If thesurface whose temperature we are measuring does notcompletely fill the field of $iew we will get inaccurateresults.

*35


136/261


137/261


Section Assessment:Section Assessment:TemeratureTemerature

*3!

)hat is Pressure


138/261


&ressure is the ratio between a force acting on a

surface and the area of that surface.&ressure is measured in units of force di$ided byarea.

psi *pounds per s%uare inch

bar, 7 bar 1 7N.J &"I, common for pump ratingsin;K6 *inches of water, KV.LU6 in;K6 1 7 psi,common for $acuum systems and tan) le$els

mm;g *millimeters of mercury, VL6 mm;g 1 7N.Vpsi, common for $acuum systems

*3&

)hat is PressureA%solute@ au#e and /i.erential


139/261


auge pressure is defined relati$e to atmospheric

conditions.The units of gauge pressure are psig, howe$er gaugepressure is often denoted by psi as well.

Absolute pressure is defined as the pressure

relati$e to an absolute $acuum.The units of absolute pressure are psia.

/ifferential pressure uses a reference point otherthan full $acuum or atmospheric pressure.

*3(

)hat is PressureA%solute@ au#e and /i.erential


140/261

Copyright 2007 Control Station, Inc. All Rights Reserved *4+


141/261


Section Assessment:Section Assessment:PressurePressure

*4*

Common Level ensin# "echnolo#ies


142/261


&oint sensing le$el probes only sense tan) le$el at a

discrete le$el.Typically used for high2high or low2low le$el sensing to pre$ent plantpersonnel andDor process e%uipment from being exposed to harmfulconditions.

Also used in pairs in processes in which we do not particularly carewhat the exact le$el in a tan) is, only that it is between two points.

-ontinuous le$el probes sense the tan) le$el as a percentof span of the probes capabilities.

-ontinuous le$el probes are typically used where we need some typeof in$entory control, where we need to )now with some degree ofconfidence what the particular le$el in a tan) is.

-ontact and


143/261


5ltrasonic ma)es use of sound wa$es.

A transducer mounted in the top of a tan) transmits soundwa$es in bursts onto the surface of the material to bemeasured. 0choes are reflected bac) from the surface ofthe material to the transducer and the distance to thesurface is calculated from the burst2echo timing.

The )ey points in applying an ultrasonic transducer are:The speed of sound $aries with temperature.

;ea$y foam on the surface of the material interferes with the echo.

An irregular material surface can cause false echoes resulting inirregular readings.

;ea$y $apor in the air space can distort the sound wa$es resulting in

false reading.

*43

Common Level ensin# "echnolo#ies;on-ContactB ltrasonic


144/261


Common Level ensin# "echnolo#ies;on-ContactB ?adarFMicro=ave


145/261


3adar, or microwa$e le$el measurement,

operates on similar principles to ultrasonic le$elprobes but, instead of sound wa$es,electromagnetic wa$es in the 76;( range areused.

hen properly selected, radar can o$ercomemany of the limitations of ultrasonic le$el probes.

4e unaffected by temperature changes in the tan) air space.

"ee through hea$y foam to detect the true material le$el.

"ee through hea$y $apor in the tan) air space to detect truematerial le$el.

*45

Common Level ensin# "echnolo#ies;on-ContactB ?adarFMicro=ave


146/261


Common Level ensin# "echnolo#ies;on-ContactB ;uclear Level Measurement


147/261


3adiation from the source is detected on the

other side of the tan). Its strength indicates thele$el of the fluid. &oint, continuous, and interfacemeasurements can be made.As no penetration of the $essel is needed there are anumber of situations that cause nucleonic transmitters to

be considered o$er other technologies.


148/261

Copyright 2007 Control Station, Inc. All Rights Reserved *4&

Common Level ensin# "echnolo#iesContactB 'drostatic Pressure


149/261


'easurement of le$el by pressure relies on hydrostatic

principles.&ressure is a unit force o$er a unit area.

A cubic foot *7K>= x 7K> x 7K>; of water weighs LK.NVOL pounds.

The area that our cubic foot of water occupies is 7NN s%uare inches*7K>= x 7K>;, therefore our cubic foot of water exerts a force ofLK.NVOL pounds o$er 7NN s%uare inches, or 6.NMMO psig for a 7K>

water column.7 inch ;K6 1 6.6M7L psig

It would not matter how many cubic feet of water wereplaced side by side, our pressure would still be 6.NMOOpsig. ;ydrostatic pressure is only dependent on the height

of the fluid, not the area that it co$ers.

*4(



150/261


;ow would you handle measuring other fluids using

pressure!-ompare the densities.

#or instance, chocolate weighs approximately U6 poundsper cubic foot.

U6 di$ided LK.J times 6.6M7L 1 6.6N6N psig.

#or a change in le$el of one inch in a li%uid chocolate tan)the pressure measurement will change by 6.6N6N psig.

=e$el measurement by pressure re%uires a constantdensity for accurate measurements.

If the head pressure in the tan) can be other thanatmospheric, we must use a differential pressure sensor.

*5+



152/261


3# *radio fre%uency -apacitance

le$el sensors ma)e use ofelectrical characteristics of acapacitor to infer the le$el in a $essel.

As the material rises in the $essel,the capacitance changes.

The le$el transducer measuresthis change, lineari(es it and transmits the signal to theprocess control system.

A point le$el probe will loo) for a specific change incapacitance to determine whether it is on or off.

*52



153/261


3e%uires the material being measured to ha$e a high

dielectric constant.=e$el measurements are affected by changes in thedielectric of the material *moisture content.

&roper selection re%uires informing the probe $endor of thematerial to be measure, especially in applications where

you are measuring conducti$e materials or ha$e anonmetallic tan).

&oint probe sensiti$ity can be increased by welding a plateon the sensor tip to increase the capacitance, and thereforethe sensiti$ity *gain.

*53



154/261


Common Level ensin# "echnolo#iesContactB uided )ave ?adar


155/261


uided wa$e radar is similar to the non2contact radar

probes, only a rod or cable is immersed into the materialli)e an 3# capacitance probe.

The rod or cable is used to guide the microwa$e along itslength, where the rod or cable meets the material to bemeasured a wa$e reflection is generated. The transit time

of the wa$e is used to calculate le$el $ery precisely.5nli)e an 3# capacitance probe, a guided wa$e radar probecan measure extremely low dielectric material.

*55

Common Level ensin# "echnolo#iesContactB uided )ave ?adar


156/261



157/261


Section Assessment:Section Assessment:-evel-evel

*5!

)hat is lo=


158/261


#low is the motion characteristics of constrained

fluids *li%uids or gases.#luid $elocity or mass are typically not measureddirectly.

'easurements are affected by the properties of

the fluid, the flow stream, and the physicalinstallation.

*5&

actors A.ectin# lo= Measurement


159/261


The critical factors affecting flow measurement

are:Viscosit$

Fluid $pe

e$nolds *umber

Flo" Irre'ularities

*5(



160/261


/ynamic or absolute $iscosity * is a measure of

the resistance to a fluid to deformation undershear stress, or an internal property of a fluidthat offers resistance to flow.

-ommonly percei$ed as Gthic)nessH or resistance

to pouring.ater is $ery GthinH ha$ing a relati$ely low $iscosity,while molasses is $ery thic) ha$ing a relati$ely high$iscosity.

*6+



161/261


+isuali(e 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 $elocitydependent only on theamount of force applied.

The oil that is next to the sheetof glass will ha$e a $elocityclose to that of the glassB whilethe oil that is next to the stationary

surface will ha$e a $elocity near (ero.

This internal distribution of $elocities is due to the internalresistance of the fluid to shear stress forces, its $iscosity.

The $iscosity of a fluid is the ratio between the per unit

force to accelerate the plate and the distribution of the$elocities within the fluid film

*6*



162/261


0ffect of Temperature

The dynamic $iscosity of a fluid $aries with itstemperature.

In general, the $iscosity of a li%uid will decrease withincreasing temperature while the $iscosity of a gas

will increase with increasing temperature.

+iscosity measurements are therefore associatedwith a particular temperature

*62


163/261



164/261




165/261




166/261


The fluid streams in most processes can include

both high and low $iscous fluids.The $iscosity of the fluid under process conditionsmust be ta)en into account when selecting a flowinstrument for optimum performance.

*66

actors A.ectin# lo= Measurementluid ",e

i l id


167/261



168/261



169/261


The 3eynolds number is the ratio of inertial

forces to $iscous forces of fluid flow within a pipeand is used to determine whether a flow will belaminar or turbulent.

DvRe 12#=

cPiniscosit*Flui

3l-.&tinensit*Flui

&t.secinelocit*Flui

inc/esiniameterPipe

num-ere*nols/ere

==

===

v

D

Re

*6(

actors A.ectin# lo= MeasurementLaminar lo=

= i fl t l 3 ld b


170/261


=aminar flow occurs at low 3eynolds numbers,

typically ReY K666, where $iscous forces aredominant.

=aminar flow is characteri(edby layers of flow tra$eling at

different speeds with $irtuallyno mixing between layers.

The $elocity of the flow is highest in the center ofthe pipe and lowest at the walls of the pipe.

*!+

actors A.ectin# lo= Measurement"ur%ulent lo=

T b l t fl t hi h 3 ld b


171/261


Turbulent flow occurs at high 3eynolds numbers,

typically ReE N666, where inertial forces aredominant.

Turbulent flow is

characteri(ed

by irregular mo$ementof the fluid in the pipe.There are no definite layers.

The $elocity of the flow is nearly uniform through

the cross2section of the pipe.

*!*

actors A.ectin# lo= Measurement"ransitional lo=

T iti l fl t i ll t 3 ld


172/261


Transitional flow typically occurs at 3eynolds

numbers between K666 and N666.#low in this region may be laminar, it may beturbulent, or it may exhibit characteristics ofboth.

*!2

actors A.ectin# lo= Measurement?enolds ;um%er


173/261


#low instruments must beselected to accurately

measure laminar flow orturbulent flow

*!3

actors A.ectin# lo= Measurementlo= Irre#ularities


174/261


#low irregularities are changes

in the $elocity profile of the fluidflow caused by installation ofthe flow instrument.


176/261


+olumetric #low

'agnetic flowmeters&ositi$e displacement for $iscous li%uids

rifice &lates

'ass #low

-oriolis flowmeters

*!6



177/261


+olumetric flow is usually inferred.

+olumetric #low 1 #luid +elocity x Area'easured directly with positi$e displacement flowmeters.

'easured indirectly by differential pressure

+olumetric flow meters pro$ide a signal that is inunits of $olume per unit time.

gpm *gallons per minute

cfm *cubic feet per minute

lDhr *liters per hour

*!!



178/261


perate by capturing the fluid to be measured in

rotating ca$ities of a )nown $olume. The $olumeof the ca$ity and the rate of rotation will gi$e the$olumetric flow $alue.

5naffected by 3eynolds number and wor) withlaminar, turbulent, and transitional flow.

=ow $iscosity fluids will slip past the gearsdecreasing the accuracy of the flowmeter.

*!&



179/261


kB

DE

kBD

ED

vDvr

velocityAreaPipeFlow

##

#2

22

==

==

=

'agnetic flowmeters infer the

$elocity of the mo$ing fluid bymeasuring a generated$oltage.

'agnetic flowmeters arebased on #aradays law ofelectromagnetic induction 2 a

wire mo$ing though amagnetic field will generate a$oltage.

3e%uires a conducti$e fluid.

Velocit*Flui

iameterPipe

ensit*Flu4agnetic

Constant

=

=

=

=

=

v

D

B

k

kBDvE

*!(



180/261


An orifice plate is basically a

thin plate with a hole in themiddle. It is usually placed in

a pipe in which fluid flows.

In practice, the orifice plate

is installed in the pipe

between two flanges. The

orifice constricts the flow of

li%uid to produce a

differential pressure across

the plate. &ressure taps on

either side of the plate are

used to detect the difference.

*&+



181/261


rifice &late type meters only wor) well when

supplied with a fully de$eloped flow profile.This is achie$ed by a long upstream length *K6 to N6diameters, depending on 3eynolds number

rifice plates are small and cheap to install, but

impose a significant energy loss on the fluid dueto friction.

*&*

Common lo=meter "echnolo#iesMass lo=

'easured directly by measuring the inertial


182/261


'easured directly by measuring the inertial

effects of the fluids mo$ing mass.'ass flow meters pro$ide a signal that is in unitsof mass per unit time.

Typical units of mass flow are:

lbDhr *pounds per hour)gDs *)ilograms per second

*&2

Common lo=meter "echnolo#iesMass lo=B Coriolis

-oriolis flowmeters are based on the affects of the


183/261


-oriolis flowmeters are based on the affects of the

-oriolis force.hen obser$ing motion from a rotating frame thetrajectory of motion will appear to be altered by aforce arising from the rotation.

*&3



186/261


-oriolis mass flowmeters

wor) by applying a$ibrating force to a pairof cur$ed tubes throughwhich fluid passes, ineffect creating a rotatingframe of reference.

The -oriolis 0ffect of themass passing throughthe tubes creates a forceon the tubesperpendicular to boththe direction of $ibration

and the direction of flow,causing a twist in thetubes.

*&6


187/261

Common lo=meter "echnolo#iesInstallation and Cali%ration

As with any instrument installation, optimal flow


188/261


As with any instrument installation, optimal flow

meter performance and accuracy can only beachie$ed through proper installation andcalibration. 3e%uirements will $ary between typeand manufacturer by some generalconsiderations are:

The length of straight piping re%uired upstream anddownstream of the instrument. This is usually specified inpipe diameters.

The properties of the fluidDgas to be measured.

'ounting position of the flowmeter.

#lowmeters must be )ept full during operation.

*&&

Common lo=meter "echnolo#iesInstallation and Cali%ration

In general, calibration of a flowmeter re%uires:


189/261


In general, calibration of a flowmeter re%uires:

0ntering the sensor characteri(ation constants intothe flow transmitter.

3ecording the (ero flow conditions.

'any flow meters must be G(eroedH only when full.

-omparing the meter flow $alue to an independentlymeasured $alue for the flow stream.

'easuring the $olume or mass of a catch sample andcomparing the $alues.

5se the internal flowmeter totali(er $alue for

comparison.

*&(


190/261


Section Assessment:Section Assessment:0low0low

*(+

Introduction to inal Control 1lements

Objectives:Objectives:


191/261


Objectives:Objectives:

hat is a control $al$e!hat is an actuator!

hat is a positioner!

hat is -$!

hat are $al$e characteristics!

hat is $al$e deadband!hat is stiction!

hat are the types of $al$es

hat is a centrifugal pump!

hat is pump head!

hy do we use pump headand not psi!

hat is a pump cur$e!

hat is a system cur$e!

hat is the systemoperating point!

hat is a positi$edisplacement pump!

;ow does a &/ pump differfrom a centrifugal pump!

*(*

Introduction to inal Control 1lements

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 allof 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.

*(2

)hat is a Control


193/261


co t o a e s a e de ce a o

stream that recei$es commands from a controllerand manipulates the flow of a gas or fluid in oneof three ways:

Interrupt flow *S#ut-Off Service

/i$ert flow to another path in the system *DivertService

3egulate the rate of flow *#rottlin' Service

*(3

)hat is a Control


194/261


-ontrol $al$es forflow shut2off ser$iceha$e two positions.

In the open positionflow is allowed to exit

the $al$e.In the closed positionflow is bloc)ed fromexiting the $al$e.

*(4


195/261

)hat is a Control


196/261


g y

positions.The position of the $al$e determines the rate of flowallowed through the $al$e.

*(6

)hat is an Actuator

Actuators are pneumatic, electrical, or hydraulic


197/261


p , , y

de$ices that pro$ide the force and motion toopen and close a $al$e.

*(!

)hat is an Actuator

"ome actuators can place a $al$e at any position


198/261


p y p

between the on and off points.These actuators typically accepta M27J psi signal to mo$e adiaphragm, which in turn mo$esa connected $al$e stem.

A pneumatic positional actuatorwill fail into the pneumaticallyde2energi(ed position.

The interface to a control system for a positionalactuator is typically through an ID& transducer.

*(&

)hat is an Actuator

Zust as processes ha$e a time constants and dead time,l h ti t t d d d ti hi h i


199/261


$al$es ha$e a time constant and dead time, which is

largely determined by its actuator.+al$e manufacturers measure their $al$e response by aparameter called TLM, which is the time it ta)es for a$al$e to reach LM8 of its final position in response to acommand change after the dead time has passed.


200/261


-inear Actuators Pneumatic Actuators

2++

)hat is a Positioner

A $al$e positioner is an accessory to a positional


201/261


actuator that pro$ides closed loop control of the$al$es position.A positioner is mechanically lin)ed tothe $al$e stem and compares thecommand signal to the $al$e with the

actual stem position and corrects for error."ince a positioner is a feedbac) controllerit too has tuning parameters.

Tune as a cascade loop

2+*

)hat is Cv

-$ is the symbol for $al$e coefficient, a measure


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of the flow capacity of a $al$e at a set ofstandard conditions.The flow capacity of a $al$e is the amount of fluid it willpass per unit of time.

#low capacity is usually expressed in gallons per minute

*&'.

PS!inropPressure

Fluit/eo&Grait*Speci&icGPinFlo6Flui/ere

===

=

P

sgQ

P

sgQVC

2+2

)hat is Cv

A $al$e must be of sufficient si(e to pass the flow


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re%uired to satisfy the process under all possibleproduction scenarios at an acceptable pressuredrop.

2+3

)hat are


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$aries with respect to the position of its closuremember.

A closure member is the internal part of a $al$e thatmanipulates the fluid flow.

This characteristic is classified as either inherentor installed.

2+4

)hat are


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relationship between the flow rate through the$al$e and the tra$el of the closure member asthe closure member is mo$ed from the closedposition to its rated tra$el with a constantpressure drop across the $al$e.

Inherent $al$e characteristics are measured bythe $al$e manufacturer in a test stand under aspecified set of process conditions, particularly aconstant differential pressure across the $al$e.

2+5

)hat are


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are e%ual percentage, linear, and %uic) opening

2+6

)hat are


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flow coefficient *-$ to the smallest flowcoefficient *-$ of a $al$e as its closure membertra$els through its range without de$iatingbeyond specified limits for its characteristic.

A $al$e with a -+ of 766 and a rangeability of J6 will

perform within its characteristic from a -+ of K to a -+ of766.

2+!

)hat are


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is the gain of a $al$e.As gain is the

the gain of a $al$e is the slope of its characteristiccur$e, which is proportional to the -+ of the $al$e.

If you double the -+ of a $al$e, it will ha$e twicethe gain.

inputinc/ange%

outputinc/ange%

2+&

)hat are


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an o$ersi(ed $al$e will lead to large processgains.&rocesses with a large gain amplify $al$e problems*deadband and stiction and can be oscillatory or e$enunstable.

+al$es that are o$ersi(ed for a process will alsooperate close to their seat under normal processconditions.

'ost $al$es become $ery nonlinear in this operating region,or the process may saturate at a low controller outputleading to windup.

2+(

)hat are


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for e%ual increments of rated tra$el the flow characteristic *-$

will change by e%ual percentages.

0%ual percentage $al$es are the most commonly used control$al$es.

2*+

)hat are


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increments of rated tra$el the flow characteristic *-$ will change

by e%ual increments.The gain of a linear $al$e is linear and remains constant throughits full operating range.

2**

)hat are


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maximum flow to be achie$ed with minimum tra$el.

The gain of a %uic) opening $al$e is nonlinear. The gain of anopening $al$e decreases from its largest $alue near closure to itssmallest $alue at full open.

2*2

)hat are


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relationship between the flow rate through the$al$e and the tra$el of the closure member a

complete f&ic slidedeck

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