API instrumentation unit 3

Unit Three of Instrumentation for Operators explaina the pure poses and operation of automatic controllers and of control modes used in process control Instruments, In Section 1 you will

INSTRUMENTATION FOR OPERATORS

Unit 3 Controllers and Control Modes

Section 1

Controllers

PILOT SIX — INSTRUMENTATION FOR OPERATORS UNIT THREE — CONTROLLERS AND CONTROL MODES Section |

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Why We Need Controllers

Bow a Controller Works

The Control Loop an

„ Mĩ The Throttling Range of the Controlley, , 21

Review and jÄNHHUARYY co bu non nu co ko con từiđuix8falk ae 25

Bate Agtan «cee eves ceases

How the Feedback Biiows Can Affect Rute Action , Section 3

kLK / 19 he 6 Introduction

Problems with Controller Settings

How Control on One Process Affects ‘Another Proceas

How Reset and Rate Action Affect Control

Working with the Control Mode

Who Should Adjust Controllers

Unit Three of Instrumentation for Operators explaina the pure poses and operation of automatic controllers and of control modes used in process control Instruments,

In Section 1 you will learn how controllers work, the nature

of w vontrol loop, and the factors in system response, You

will learn about offset and ogeillation, on-off controllera and

proportional controllers, the throttling range of the valve and of the controller, and how the proportional band affects control,

In Section 2 you will Jearn how changes in proceaa load affect proportional controllers and how changes {n process load can

he compensated for by use of feedback bellows and by reset

and rate setion

In Section 8 you will learn how to work with controllers and

control modes and how to identify controller problems

INSTRUCTIONS

This is a programed learning course

Programed learning gives information in @ series of steps

called frames Each frame gives some information and asks

you to make use of it,

Here is how it works First, cover the response column at the

right with a maak,

Read this frame and use the information it gives to ll in the

Move the mask down to uncover the word at the right of the

frame, HỶ you have filled the blank with that word or a word

that means the same, you are ready lo go ahead to the next

frame,

The drawing of a micrometer provides information that will

help you fill in the next blanka

MEASURED

CAP

Seven major parts are shown In the drawing, but only

object to be measured,

small

unvil; spindle

The next frame calla for a choice, Cirele or underline the ap-

propriate word,

Of the two parts that contact the object, unly the (anvil,

apindle) moves

A program is a series of frames that work like the ones you

have just done:

Read the frame,

Use the information to fill in the blanks or make a cholee

Move the mask down and check the response column

Go on to the next frame,

Remember to cover the response column with a mask before

you begin cach page

spindle

SECTION |

CONTROLLERS

Why We Need Controllers

1 Process variables are controlled by opening and clos-

NE eet

8, Let's gay that you want a flow rate of 100 gallons per

minute for a process

The control _ 2 is set to allow just 1H

gallons of Hquld per minute into the process

4 Tf the flow rate never changes from 100 gallona per

t Let’s aay that there is a change in the pressure acting

in the liquid which supplles the process,

6 The flow rate to Lhe proven:

——— À atayn at 100 gallona per minute

If procead varlables could be held perfectly constant,

there would be no need for a controller,

Is this likely ta happen in a typicu) installation?

You

Na

How a Controller Works

12 The controller Is det to meet the tarwet for the proves

13,

1

1ã,

1ô,

variable buing controlled

The controller muat adjuat the in keep

the process variables on target

The target far the controller ia called the set point

Let’s say a temperature meseurement is on target

The controller freads/doea not read) al the aek point

If a temperature is not un lurgel, the temperature in-

dleater does not match the

of the controller,

A controller goes into action when:

A the process Ig off target

— BB, the process ia on target

When the process strays from the set point on any

process variable, (all‘some) controllers affecting thut

variable should react

17, The drawing represents a simple temperature control

18 His sense of — _ is the Lemperature indicator,

19 Hia ~ apens ov cligea the valve

20, His touch gives him information He already “knows”

what the aet point ahould he,

To find cut whether the process is on target, he

si what he foola to the sel point,

21 The controller first receives information from a

device, and then —_— —.1L tụ the desired sel, point

22, If the process meseurement and the vet puint are the

same, the operator:

—— A, makes an adjustment,

— _ B, dues nothing,

23, But if the measurement and the eet point are different,

he adjuxls the vonbrol

24, Let's aay he closes the control valve to cool the process,

How can he tell if the procesa has cooled?

Hia sense of -_ Indlenten thìm,

operator touch hand

compares

measuring, or sensing; compares

valve

touch

35

26,

Mĩ,

28

For every adjustment the controller makes, the meas-

uring device must feed information back to indicate

what has happened,

In the previous example, thy operntur peta feedback

information from hia é

These are the steps nueded to control a process:

Slep 1: The process variable ia measured

Step 2: The measurement is - - to the

Step 6: This feedback “tells” the -

what has happened,

If the feedback doea not match the set point on a mod-

ern controller;

— _ A the controller stopa working,

= B, the controller keeps working until the set

point is reached,

Put an A on the drawing to chow where measuring is

being done

Put a A to show where comparing is being done

Put a C to show where yulve positioning is being done

Put a T to show where feedback is given

A MEASURE

The operator gets a measurement und feedback from;

——— A his sense of touch

BR a reading on the thermometer dial

The controller is Lhe - as

The valve ia adjusted (hy hands automatically},

This is an automatic control situation

The process temperature ix meusured at (ABYC),

AL B, the process measurement is _—— —la

the aet point

The Control Loop

H3, This diagram shows a complete control loop

CONTROLLER

COMPARISON ) MEASUREMENT

POSITION

CONTROL:

A control lonp consists of all the elements needed to

the movement of a valve,

84, A control loop as shown above!

_—_— A ja clreular and closed,

——B is open-ended

86 First, the provess measuring element sends a signal

tw the controller,

The controller is the —— ——— teteetor

36 At the same time, the alroady-determined =

— ix fed into the controller (error detector)

87, The error detector makes & — _— between

thia signal from the process and the set point signal

98 Lf there is a difference between the measured signal

and the set point:

A nothing happens

Tt an error signal ig sent to the positioner

of the control valve,

39, The positioner adjusts the setting,

40, The control valve changes the process

Information about this change is picked up by the

instrament and fed {nto the controller,

comparison

valve

measuring

teadback

It takes time for the perta of the control loop to detect

a proces change, make a valve adjustment, and get

process feedback

The leas time it takea, the (greater/Ieseer) the effi-

ceney of the control loop,

‘The inatrumenta should react aa fast as it ia practical

for them to react when the _ changes,

Sometimes there is a time lag in the inatrumenta

The greater the distance the signals must he sent, the

it tales to receive them

This means that the insteuments will not react imme-

diately tan in process

Sometimes there is a time lag between instrument

changes and procesa changed,

PROCESS

This control valve is (close to/tae away from) the

proves

A change in the control valve powition would affect

the process (immediately,after some time lag)

far away from

after some time lag

49 Once an adjustment has been made, it takes time for

the proccss to change,

15 MINUTES

With the amount of heat being applied to the bouker,

an increase in temperature from 40°F to 212°F re»

quires 16 minutes,

Turning the heat up under this beaker (will/will not) will not

couse it to reach 212°F ingtantly

50, The temperature of the water (changes ‘does not

change) immediately with on increase in heat, dues not change

51 A temperature change in # process;

— _A, will always œeeur lwwmediately with

contral valve change

——— B may take time to ocenr, no matter haw B

much the control valve is changed

52, The lime il dakes for Seth the inatrumenta and the

process to change ia called the ayatem reaponed

The aystem responde Indieates how _ Lhe Taat

control Jovp is moving

63 Which of the following events occur In a system re-

sponse?

Ver No

54, Which of the following occur in the aystem reaponde?

— _ A Instrument response

——_-B Process response

—— ©, Beth A and B G

10

Offset and Oscillation

BK, Offset oceura when the process mensurement differs

from the set point

Instrument fA‘B) shows offset,

56, Recording instruments can he uaed to ahow procesd

When the process lemperatnve is ar the sect point, the

pen and the pointer are at the — — point,

58, When the pen moves away from the pointer, an

51 When the process measurement keeps bouncing above

and below the set point, the process js oscillating

The process iy at the set point in

There is an nffset recorded in:

—==Ñ&

loth B and É, The process is oscillating in| ———H—_

an instrument to position and adjust the control

62, An on-off controller ix one which keeps the control

valve either fully open or fully — — —

68, ‘This drawing represents the control loop for a simple

on-vff controller,

ELECTRICAL SET POINT CONTACTS

64 The bourdun tube is connected to a puluter

When the pointer movea hack and forth, it opena and

6h ‘The position of the bourdon tube acta as the (process!

G66 The set point is indicuted by the position of one of

68, Which of these parts is the error detector?

———— A Bourdon tube

HK Pointer and electrical contacts - h

C, Solengid coil

48, When the hourton lube senses a pressure change, the

70 When the contwtia are elosed, current flows to the

solenoid

71 When this solenoid is energized, it positions the

72 The pointer, by either opening or cloning oleetricn] con-

tacts, acly like (au oneulf swilth/an electrical Írang-

13

78 Let'a aay the set point for pressure is 20 PSIG,

INPUT TO SENSOR

SOLENOID (DN-DF F)

The electrical contacts (open/close) if Lhe pointer falls

below 20 PSIG

74 This activates the solenoid, whieh tn turn ——

the control valve,

7h, Tf pressure rised agai, the clectriva) contacts open,

and the eontrul valve is by the solennid

76, The pointer acts ua the error detector, but ft can only

give two error alemile; valve - , or valve

977, Sinee these are the windy two settings, this kind of con

troller is catled aun - controller,

7K, Identify the parts uf this on-off control loop

The controller (error detector) in the HW

Comparing the set polnt to the measured signal ia done

Problems with an On-Off Controller

79 The moat serious drawback with an on-off controller

{s that {t can poaltion a valve in only = =

An on-off controller changes the valve setting (the game

amounl/differently) fur different provess changes,

16

the same amount

An on-off controller treats a large offset (the same us,

differently from} a small offset

On-off controllers (are.‘are not) sensitive to differences

in the amount of offset,

Even if the amount of offset is tiny, the valve

all the way or — —Rll the way,

Which graph repredenta flow rate being controlled by

86 The on-off control loop (isis not) able to matntain

4 smooth, even fluw rate,

87 A graph of flow controlled with an on-off controler

lookg like thìa,

\VWALVE 0PEN SET non |

—H ia nmaath and nteady,

H8 (Im«nf rontrollers (allsw-da net allow) the process to

The Throttling Range of the Valve

$1 On-off controllers cannot give exact control of 1 process

hecausé they react (the same way to,'differently for}

large and amuall deviations from the set point,

13 For more exact control, the controller must respond

to the process change in proportion ta the amount of

change,

Large changes should canse the controller ta make

=—— chines in the valve setting, and eraall

changes should cause valve changes,

17

allow

opening} closing cannot give

the same way to

largo small

and fully closed

A proportional controller must be able to rend (two

different signalsa range of signals) to the control

valve,

A controller that sends a range of signals to the con-

trol valve is called a _. _ controller

Here is a valve vperuted by an air motor, The Ukrol-

tling range of this valve is 8 to 16 PSIG

AIR PRESSURE

When the yalve is in the position shown in the drawing,

the air pressure on the piston is (8 PS1G/more than

% PSIG)

This valve does not open fully until the pressure on

the piston reaches PSIG

This valve is upen hullway when the peessuce on the

piston is — P§Iũ

Thrụe lu 1ð P§IG ¡is the —_—_———— range 0Ï this

valve,

The controller's signals to the contro] valve must match

The controller moves the valye proportionally to com-

pensute tur different rites and amounts of process

throttling range

change, or variation

101 This contro! valve operates through a range of aettings,

kiíff

The valve must be shut: completely when the liquid level

192 Ten fept ía (ahave'belaw) the net paint, above

163, The set point is at 9 feet,

The valve (nermita some flow: does not permit ffuW) permits some flow

when the liquid level is ot $ feet,

104 The liguid level has fallen te B feet

19

105, The drawing shows a proportional control leap,

This system sends (pneumatiyelevtrical) signals,

106, Procead flow rate is being moasured by a

meter,

107, The flow meter aenda a preaaure signal ta the (meaa-

urement.set point) bellowa in the controller,

08, There ure two bellows in this controller

One does the process measuring; the other glvea the

108, The flapper pivoted between the two bellows acta aa

the error detector,

As long as process pressure and set point pressure stay

the aame, the error detector (keeps changing, does not

change) position,

110 Let's say pressure in the process (decreases

The error detector (muvos,'doos not move),

111 As the flapper movea away from the nozzle, preaaure

in the pneumatic system connected to the sir motor

112 This change in pneumatic pressure acta as the — _

signal,

118 The pneumatic signal operates the air mator

The alr motor positions the

20

The Throttling Range of the Contratier

Tha throttling range of a control valve is a range of

(pneumatic sytem pressures /process measurements},

There is a range of procead meaaurements within which

the controller must keep the process

This range of process measurements is the throttling

range of the (controller/control valve)

The throttling range uf a cuntroller is sometimes called

the proportional band,

Which one of these valued could represent a propor-

tlonal band?

— 8 gallons per second

—— 68°F to 88°F

—- 100 PSIG

The range of process measurements between the con-

trol valve being completely open or closed is called the

This pressure gage and controller are regulating proc-

ea prossure Identify the following points

proportional band; controller throttling range; valve

111

120,

Set point is found on;

_ proportional controllers anty

—— all controllers,

Throttling range is found on:

proportional controllers only,

on-off controllers only

Here are the dials of two controllers which have the

same set point

OPEN mm CLOSED OPEN

_ CLOSED

On controller A, the preaaure must inereaue to

PSIG to completely close the vontrol valve and decrease

ta —- PSIG ta completely open the contro! valve

th controler B, pressure must increase to only —

PSIG to completely close the control valve and decrease

to only — _PSIG to completely open the control

valve,

Tt takes 4 change of 6 PSIG from the sect point to eom-

pletely open or close the valve on controller A,

It takes a change af only (PSIG from the set

point to completely open or close the valve on con-

126

126i,

124,

128

Let'a aay proceas pressure changes 4 PSIG

SET POINT SET POINT PSIG

OPEN CLOSED OPEN CLOSED

Controller (A’B) would make the biggest change in

the valve setting,

Controller (A;H) reacts more sensitively to a change

in process pressure,

» There are both advantages and disadvantages ta make

ing a controller more sensitive

One advantage of the more yenaltive controller Is that

it can keep the process closer to the

Hat prepertional conteroflers can be tov sensilive

Making the proportional band narrower means that

for every process change, the controller mover the

control valve (more,‘less)

Let's way that a proportional ¢ontroller fully opens a

valve for u change of 1 PSIG from the sut point,

FULLY OPENS FULLY OPENS

A change of B PSIG get? (the samen diferent) vo-

sponse av a change of | PSIG,

23

yet point

more

the same

180 Hf thí nrareas canstanlly chunges by maore than Ì

PSIG, the controller will be treating all the changes

above | PSIG [the same,‘differently),

TẠI Also, within its throttling range, the valve may be

moved ton far lor the process change,

Graph GASB) representa the valve movement of an

oversensitive proportional controller

152, The proportional hand can be made 20 marrow that the

controller acts almost like an - - —

controller,

138, Oversensitive controllera may canuae the pneess to

184 These grapha ahow walve setting changea daring a

Graph (A,B) represents valye movement made by a

controller with the widest proportional band

136, ‘The proportional band can be made so wide that it

takes » very large process change to affect the valve

Setting

Assume that the controller is adjuated to a very wide

proportional band satting, and the valve setting haa

not changed for 10 minutes

Has the process remained steady during this time?

——%

—Na _ Can't tell trom the information given

Review and Summary

186, A controller goes into action only when there ia a

proces

187, Here is what happens in a tontrol luợp;

A The proceaa variahle is ——

B This measurement of procean is compared to the

in the controller,

OG If there is a difference, the controller'a output

changes, and the control : setting ia

changed

D The measuring device continues to send proceaa in-

Thia information is valled process

E If the last valve change did not eorreet the upaet,

the vontroller makes a further ———

138, Which graph shawa offaet?

189 Which graph shows oscillation?

Section 2

Proportional Controllers

with Rate and Reset Action

SECTION 2

PROPORTIONAL CONTROLLERS WITH RÀTE

AND RESET ACTION

Process Load

1 The load on a process is like the load on an engine

This engine is driving an empty truck at 50 MPH,

‘The load on the engine is the:

road surface,

= size of the engine

empty truek empty truck

2, As long ag tho truck stays emply and is driven on a

level rund at 50 MPH, the toad (changea/doea not

8, What happens now thal lhe weight of the truck haa

The apeed of the truck ——— unless more decreases

fuel is fed into the engine

4 Putting more weight on the truck is the same os in-

creasing the _ un the engine, if speed load

stuys the same,

27

Let's say thal we want the truck to move faeter,

More ia fed to the engine

- Demanding more speed from the truck (ínereasen

decreases} the load on the engine

The total weight to bo moved and the speed at which

it must be moved is the ——— øn the engine

As long as the weight moved by the engine stays the

same, and the speed at which the truck ia driven on a

The throttle will have to make an adjustmuut to change

the _ af the truck,

What will happen if we du not adjust the throttle again

after passing the curve?

A, The truck will reeurn to 60 MPH

B The truck will continue going 40 MPT

ven a lemporary udjustment for a curve changes the

a on thy engine

After the truck ie returned to 50 MPH, the load on the

truck has (changed returned to the same level},

Which of the following con cause a load change?

The weight on the Lruck is reduced (Yeu,Nu}

The truck muat be ilriven up a hill at the same apeed

(Yes/No]

The truck slows down [or a curve (Yes/No)

Tn the truck, we demand vutpul from the engine,

Do we demand output from an industrial process?

(Yes/No)

28

fuel inereases load load

speed

load returned to the same level Yes

Yea

Yes

Yes

15, There is a - — 0u each process load

16, The load on a process acta like the load on an engine

KEROSINE DISTILLATE

BARRELS PER HOUR

barrels of kerosine per hour

17, The amount of material and the speed at whieh it must

be produced iz the _tmm_on this process load

18 Reducing thia output to 100 harrela nn hour, under

the sume conditions, is the same as reducing the

19, Let's say that the preaaure of liquid entering this proc-

ess drops, The process must be adjusted to maintain

production of 500 barrels an hour,

$s

=

su” 500 : 500 BARRELS BARRELS PER HOUR PER HOUR

Thia (is,'is not) the same as an inerease in load, is

ah, The drawing showa three different valve settings,

Per each valve aetting there:

_— are eeveral How rates,

~ ois one flow rate,

We want to open a valve so that the flow rate equals

a specified set point,

Under one load condition, how many valve positions

will give us the desired set point?

Several positions

One position

Proportional controllers are aligned for one load eon-

dition

With these controllurs, there (are several valye pori-

tions/ia only one valve position) that will give the set

point as long as the loud does not change

Sometimes there is a hill ov “eurve” in the process, or

slight variutions in load, that the controller must adjust

for,

The eontroller muat move th ——

But, after the adjustment, if the load has not changed,

the cuntrul valve should (move to a new,/go back to

the old) set point position,

26 Look at the pressure gage in this pneumatic controller,

When thia controller ia at set point, there ia juet one

output _ setting to the contro] valve,

Each flapper position produces (one,’seyeral) pressure

setting (8),

Under ono load emmdition, in this controller;

A there ia just one flapper position which

will maintain the process at the set point

A there are several valve positions

B, there ia only one valve position

—— B, there are several flapper positions which

will maintain the process at the set point

If the fuppur muvos, the control valve (moves/does

not move),

To change the control valve the right amount to keep

the process atuble at ita desired set point, the output

tu the control valve must become bal-

anced to the need of the process, and the

must he returned to its set point position,