Control of Compressor

8/17/2019 Control of Compressor

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N A M E : A S H I K J A C O B O O M M E N

R O L L : 0 3

E N . N O : R 6 7 0 2 1 1 0 0 3

S A P : 5 0 0 0 1 5 1 1 1

M . T E C H P R O C E S S D E S I G N E N G I N E E R I N G

Control of Compressors


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Contents

Introduction

Types of Compressors

Polytropic Head

Compressor PerformanceProcess Symbols

Control in Turbo-Machines

Control in Reciprocating MachinesOther Control Parameters

Anti-Surge Control

Conclusion


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Introduction

Compressors are gas handling machines

Performs the function of increasing gas pressure

!one by Confinement or by "inetic energy con#ersion

Ma$or tic%et items in the capital cost of a chemical orpetroleum plant

&g ' (arge centrifugal compressor )ith a gas turbine is anin#estment of many millions


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Types of Compressors

Compressors

Positi#e!isplacement

Rotary*lo)ers

ReciprocatingCompressors

Turbo-machines

A+ial

CentrifugalCompressors


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The range of flo) capacities and discharge pressures

that different compressor design can handle


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Polytropic Head

Performance of Compressor e+pressed in terms ofPolytropic head

,here p . Polytropic &fficiency

/ . Adiabatic Inde+ Pd.!ischarge Pressure

Ps . Suction Pressure

0s . Compressibility 1actor

− −=

−

11

1

γη

γ

s

d s s

p p

p

P

P

MW

RT Z H

γ

γ η


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Compressor Performance

Compressor performance cur#esshould strictly be plots ofpolytropic head Hp againstSuction flo)

Performance cur#e of acentrifugal compressor terminates before the flo) reaches 2ero

At too lo) a flo)3 the compressor )ill 4surge5

6pper limit of Performance cur#eis 4Stone)all5


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Process Symbols


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Control in Turbo-Machines


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Introduction

The pressure dropthrough the processincreases in proportion tothe s7uare of the flo)

The e+it of the process isconstant so that )e canadd the process cur#e to

the compressorperformance cur#e

Compressor operated )here the t)o line cross


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To ad$ust the flo)3 either the process cur#e or thecompressor cur#e has to mo#e

Also has to remo#e the conditions that the discharge andthe process inlet pressures are e7ual or that the flo)s aree7ual

There are different schemes that are possible forcontrolling the flo) They are as follo)s '

!ischarge Throttling

Suction Throttling Speed Control

8ariable inlet 9uide 8anes

Recycle Manipulation


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!ischarge Throttling

Placing a control #al#e bet)een the compressorand the process

The pressure are nolonger e7ual

Process cur#e )ill mo#e

as )e change the #al#eopening


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!ue to the non-linear beha#ior of both thecompressor and the process3 the relationship bet)eenthe flo) and #al#e Pressure drop is highly non-linear

*ut since )e are e+pending energy to raise thepressure of the gas to partially reduce it again acrossthe #al#e3 the scheme is not energy efficient

Range is limited since the process cur#e approachesthe end of the compressor cur#e as )e close the #al#e3resulting in 4Surge5


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Suction Throttling

Control #al#e is relocated tothe suction of the compressor

The compressor cur#e mo#es )hen )e change the suction

pressure

More energy efficient thanSuction throttling due to

in#ol#ement of lo)erpressures

Has greater range becausesurge point reduces as suction

pressure is reduced


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It is possible for the suction pressure to fall belo)the atmospheric pressure

Any lea%s in the pipe-)or% )ould permit air to enterinto the compressor

If compressing a flammable gas3 )ill result in

e+plosion inside the machine


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Speed Control

6sed if the compressor isof #ariable speed type

*y changing the speed )e

mo#e from onecompressor cur#e toanother

This is energy efficient

Ho)e#er #ariablefre7uency dri#es :81!; forlarge electric motors arecostly


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8ariable Inlet guide #anes

6ses a set of ad$ustableguide #anes on the inletto one or more of thecompressor stages

Con#erts the inlet gasfor)ard momentum intorotational momentum

Angle of the guide #anesis ad$ustable


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Pre-rotation increases the compressor efficiencypermitting it to deli#er a greater flo) at the samedischarge pressure

Since the guide #anes are present inside the compressor3ad$usting them changes the geometry of the machineitself

Another family of compressor cur#es )ill e+ist for eachguide #ane angle3 each )ith a different surge point

Mechanically guide-#anes are more comple+ and morecostly to maintain


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Recycle Manipulation

Partially recycling gasthrough the compressor

Thereby remo#ing thecondition that flo) throughthe machine must e7ual tothe flo) through the process

Offers the greatest range

since the surge point is ne#erapproached

Ho)e#er3 costly to install

and operate


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In reality3 the system is more comple+ than dra)n

1or e+ample3 the recycle must be cooled

It is the least energy efficient approach

It can ho)e#er be used to increase the range of theother schemes described3 by recycling $ust enough toa#oid the surge


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Control in ReciprocatingMachines


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Introduction

Reciprocatingcompressor is a constant

#olume3 #ariabledischarge pressure

machine

The #olumetricinefficiency is a result of

the clearance bet)eenthe piston end and thecylinder end on thedischarge stro%e


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Schemes used for the turbo-machines can also beapplied for reciprocating machines

Suction throttling reduces the suction pressure and soless gas enters the cylinder and less gas )ill bedischarged

!ischarge throttling has little effect on the flo)

Some other methods of capacity control is listed as

follo)s ' On-Off Control

Constant Speed unloading

Speed Control


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On-Off Control

If the compressor )astes po)er )hen run continuously3 thecapacity can be controlled by starting or by stopping themotor

&+ample sho)s an air mi+ blender uses a rapid series of high-

pressure air blasts )hen the mi+er becomes fullThe high pressure air for this purpose is stored in the recie#er


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Constant Speed (oading and6nloading

The dri#er operates continuously3 at constant speed

One #aries the capacity in discrete steps by holding thesuction #al#es open on the discharge stro%e or openingclearance poc%ets in the cylinder

The most common scheme are the three and the fi#e-

step unloading techni7ues

The large number of steps sa#es the horsepo)er becauseit more closely matches the compressor output to

demand


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ThreeS!e" #$%&'()$* +),eS!e" #$%&'()$*

Capacity increment are 3 ?=> pr => ofma+imum flo)

Method is accomplished by the use of #al#e

unloading in the double acting piston Three steps include Suction3 !ischarge and

*lo)do)n Capacity increment are 3 @?> 3 ?=>3 ?> or =>

At 3 both the suction #al#es and theclearance poc%et are closed

1i#e Steps include Suction3 !ischarge3*lo)do)n 3 *ypass and Purge


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Other Control Parameters

Control of Au+iliary items on the compressorPac%age '

Oil3 )ater and gas temperature

Oil3 )ater and scrubber li7uid le#el

1uel and starting gas pressures

Control of Compressor safety Safety shutdo)n controls to be pro#ided in case of harmful

temperatures3 pressures3 speed 3 #ibration 3 engine load and li7uidle#els


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Anti-Surge Control

Ob$ecti#e is to minimi2ethe recycling )ithoutdamaging the machine

To do this3 )e need to

predict that surge isabout to occur

Comple+ity of surgea#oidance depends much

on the machine and itsduty

&+ample is as sho)n onside


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This scheme causes therecycle to open if thedischarge pressure ise+ceeding the Suctionpressure

Pressure must changesignificantly as surge isapproached

*etter for SteepCompressor Cur#es


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Anti-Surge Controller


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Conclusion

Control of compressors plays a ma$or role in achemical industry

Temperature control can also be ta%en intoconsideration


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References

4 A Real time Approach to Process Control 43 Second &ditionB ,illiamS#rce%3 !onaldPMahoney3 *rentRoung3 Dohn ,iley and Sons (td

4 Process Control a Practical Approach5BMy%e "ingB Dohn ,iley and Sons (td

4Compressor Handbook53BPaulCHanlonBMc-9ra) Hill

Publishers

“Industrial Engineer’s Handbook: Process Control” B Third&ditionB *ela9(ipta%B Chilton *oo% Company


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Control of Compressor

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