Compressor Fundamentals, Performance Curves

8/18/2019 Compressor Fundamentals, Performance Curves

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Centrifugal CompressorsCentrifugal Compressors

Performance CurvesPerformance CurvesFactors that Affect CompressorFactors that Affect Compressor

PerformancePerformance


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To icsTo ics

ow a en r uga wor s nergy onvers on

Performance Curves

Operation Limits: Surge & Overload

Factors Affecting Compressor Performance

e ue – z e

Efficiency


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Return Bend Diffuser Reduces Velocity

Return Channel Im eller

Guide Vanes

Increases VelocityIncreases Static Pressure


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P4,V4,T4

P3,V1,T3

, ,

P2,V4,T2P5,V1,T5

P1,V1,T1

P3,V1,T3


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Mechanical: The “work” (energy) developed to raise aweight of 1 pound by a distance of one (1) foot. Expressed

in foot-pound (or equivalent Kgm or Nm);Gas Compressors : “ work” done by the compressor /

amount of gas. The head expressed in feet, is the height to

w c e gas cou e e


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Head ConceptHead ConceptThe height to which the gas is lifted depends on the velocity

For any given RPM, the head developed by the compressor, .

Head is depending upon:

• ompressor geome ry .e. no o s ages, mpe er ame ers• Compressor speedZ: Compressibility Factor

R: Gas Constant = 1545 / MW

Ts: Suction Temperature (°R)

r: Pressure Ratio (Pd / Ps)M:M: Polytrophic Exponent


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– –


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Fixed/Variable S eed

Surge/Overload

Effects on Performance


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Com ressor Performance CurvesCom ressor Performance Curves illustrates theoperating range and flexibility of a given compressor

110%

120%

a t i o

100% r e s s u r e

105%

90%

r e s s u r e

,

95%90%

85%

70% %

H e a

d ,

60% 90%80%

60%

120%100%

% Inlet Capacity or Flow


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om ressor Performance urvesom ressor Performance urves

,section and overall:

• nozzles such that there is no pressure drop or temp reductionbetween impellers

• overall refers to a complete compressor or compressor trainNote: a back-to-back unit with a crossover may often be considered a

- ,single section since no pressure drop or cooling is introduced between

the impellers

For single section compressors, the section curves andoverall curves are one in the same


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Design PointDesign Point is the point at which usual operation is

expec e an op mum e c ency s . s e po n a w cthe vendor certifies that performance is within the tolerance

110%

120%

a t i o

100% r e s s u r e

90%

r e s s u r e

,

70% %

H e a

d ,

60% 90%80%

60%

120%100%



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Rated PointRated Point is intersection on the 100 % speed line

corresponding to the highest flow of any operating point

110%

120%

a t i o

100% r e s s u r e

90%

r e s s u r e ,

70% %

H e a

d ,

60% 90%80%

60%

120%100%



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Stability:Stability: the percent of change in capacity between the rated (design, ,

the stability of the centrifugal compressor. Indicates the capability of thecentrifugal compressor to operate at less than design flow

110%

120%

a t i o

100% r e s s u r e

90%

r e s s u r e ,

70% %

H e a

d ,

60% 90%80%

60%

120%100%



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Turndown:Turndown: the percent of change in capacity between the rated,

pressure is measured as turndown of the centrifugal compressor. Indicatesthe capability of the centrifugal compressor to operate at less than design

110%

120%

a t i o

100% r e s s u r e

90%

r e s s u r e ,

70% %

H e a

d ,

60% 90%80%

60%

120%100%



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Rise to SurgeRise to Surge :: the percent of change in discharge pressure between.

compressor can accommodate a modest increase in discharge pressurewith a little change in flow

110%

120%

a t i o

100% r e s s u r e

90%

r e s s u r e ,

70% %

H e a

d ,

60% 90%80%

60%

120%100%



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Sur e PhenomenonSur e Phenomenon

At any given speed, there is minimum flow, below which, thecompressor canno e opera e n a s a e con on. sminimum flow value is called “surge “ point.

Surge is oscillation of the entire flow of the compressorsystem and this oscillation can be detrimental to the machine.

Compressor surge may be evidenced by the following:a) Excessive rotor vibrationb) Increasingly higher process gas temp

c) Rapid changes in axial thrustd) Sudden changes in loade) Audible sounds (if surge is severe)


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Sur eSur e – – Dama e of Com ressor InternalsDama e of Com ressor Internals

High axial displacement

Deformation due to hightemperature


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Resistance to Flow Causes

Pressure to Rise Which

Causes Flow to Decrease

Sudden Reversal of Flow

Slams Thrust Disc Against

110%

120%

a t i o

Inactive Thrust Bearing

100% r e s s u r e Pressure Builds along the

Design Curve Back

90%

r e s s u r e ,

70% %

H e a

d ,

Pressure Ratio Drops Low Enough

for Flow to Instantaneously

60% 90%80%

60%

120%100%



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The fre uenc of the sur e c cle varies inversel with the volume of the system

nozzle, the frequency will be much higher than of a systemwith a large volume in the discharge upstream of the check

valveThe hi her fre uenc of the sur e the intensit will be

lower (i.e. few cycles / minute up to more than 20 cycles /

sec)The intensity of the surge increases with gas density ,

pressure and lower temperature


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SurgeSurge - - Effects of Gas CompositionEffects of Gas CompositionBest Efficiency point

Heavy Gas (propane, propylene)

E %

Medium Gas (air, nitrogen, natural gas)

Light Gas (Hydrogen reach gases, i.e.

Surge points

Q


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SurgeSurge - - Effects of Gas CompositionEffects of Gas Composition

gas:

The low at surge is higher;

corresponding to medium gas / light gas

e r g s e o e curve urns ownwar(approaches stonewall) more rapidly

The curve is flatter in the opening stage (small


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npu gna s equ re1 - Suction Flow2 - Suction Pressure3 - Discharge Pressure

Flow ElementSuction

PressureTransmitter

PressureTransmitter FlowTransmitter FTPT

PT

Discharge

Recycle

Valve

PS

CPU

GBC

DI

COM

AI

AO

ETH

GBC

Surge ControlIn the PLCI/P


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Surge ControlSurge Control

Performance

Map

Surge Controller


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External Causes and Effects of SurgeExternal Causes and Effects of Surge

Restriction in suction or dischar e of s stem

Process changes in pressure, temperatures, or gas MW

Internal plugging of flow passages of compressor (fouling)

Inadvertent loss of speed

Instrument or control valve malfunction

pera or error

Misdistribution of load in parallel operationImproper assembly of compressor (impeller overlap)


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Restriction in Suction / DischargeRestriction in Suction / Discharge


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Parallel OperationParallel OperationTypically, for parallel operation, the flow is not split evenly and one

section or compressor handles more flow than the other, but bothsec ons are requ re o ma e e same pressure ra o

Careful analysis of the pressure ratio curves is required to insure

“ similar pressure ratio curves”• ,much more flow than of section (2)• If the total flow is reduced 10%,the compressor slows down tomaintain the same pressure ratio

• e ow o eac sec on sreduced 10% (dashed line) sincethe pressure ratio curves have aapproximately the same rise


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“ different pressure ratio curves” (section 2 pressure ratio curve isstee er than section 1

• If the total flow is reduced 10% the compressor slows down to maintainpressure ratio• - since its curve is shallower

•Section (2) reduces less than 10% (about 5% - dashed line) since its

• The two sections are nowoperating at significantly different

handling a different percentage ofthe total flow than they were at the

.

• Section (1) is nearing surge.Further reduction in flow would force section one into surge

• The difference in the curveshape results in a reduced overallrange for parallel operation


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Impeller Overlap with Diffuser Impeller Overlap with Diffuser


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Positive overlap

Limited

Nominal

Desired

Non Desirable

Limited


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It is preferable that no impeller shall have negative overlap

The negative overlap is limited to 5% of the impeller tip


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Instrument / Control System MalfunctionInstrument / Control System Malfunction


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110%

120%

a t i o

100% r e s s u r e

105%

90%

r e s s u r e ,

95%90%85%

70% %

H e a

d ,

60% 90%80%

60%

120%100%



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Choke LimitChoke LimitChoke is the maximum flow that a centrifugal compressor

. ,unable to produce any net overall pressure ratio.

curve is where the gas speeds approach Mach 1

as compress on s no onger occurr ng n e compress onchannels. This region of the curve, as it becomes almost

“ ” ,

Stonewall is usually not detrimental to the compressor, it.

design condition, the maximum volume flow can be increasedb increasin the rotational s eed


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API API 617 7617 7 thth EditionEdition – – PerformancePerformance

CurvesCurves


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Performance CurvesPerformance Curves –– Inlet GasInlet Gas

Condition EffectsCondition Effects


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Performance CurvesPerformance Curves – – Inlet GasInlet GasCondition EffectsCondition Effects


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Factors Affecting CompressorFactors Affecting Compressor

PerformancePerformance- ,

in direct proportion

Tem & Head - If the Ts increases the head for a iven ratio willincrease in direct proportion

Zave & Head - If the average compressibility increases, the head willincrease in direct proportion

N and Head - If speed increases, the head will increase in direct

Flow and Speed - If the speed increases, the flow will increase in direct

ffff


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Factors Affecting CompressorFactors Affecting Compressor

PerformancePerformance- ,

proportion to the cube of the speed. (Because flow increases

directly as speed and head increases as the square of thespeed and BHP is the product of head X mass flow)

Densit - The onl thin a com ressor im eller sees is inlet

capacity. Thus to get more capacity out of an existingcompressor it is necessary to change the density of the inletby:

• decreasing the suction temperature• increasing the suction pressure• increasing the MW of the gas


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Compressor OffCompressor Off - - Design PerformanceDesign Performance

Performance curves for axial and centrifugalcompressors are usually based on constant inlet

conditions (Ps, Ts, MW) . In actual service, thesecompressors rare y see ese ase curve con onsexactly

If the field inlet conditions deviate more then 5% fromthe curve inlet conditions then the field data can not be

data to curve conditions

o proper y eva ua e e compressor runn ng odesign), the performance parameters shall be corrected to


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Operation LimitationsOperation Limitations

Driver

Process


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Compressor Operation IssuesCompressor Operation Issues- -

EfficiencyEfficiency DropDrop

Internal recyclen- une urge on ro ys em

Leakage via by-pass valve(s) in process

Compressor operated out of “guaranteederformance envelo e”Impeller & Diaphragm erosion


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Internal RecycleInternal Recycle –– Gap at theGap at the

diaphragm / guides splitsdiaphragm / guides splits


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Internal RecycleInternal Recycle –– Gap at theGap at the

diaphragm / guides splitsdiaphragm / guides splits

b h kb h k


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Labyrinth LeakageLabyrinth Leakage

Leakage proportional to:•• Clearance••1 / (No.Laby Teeth)0.5

Eye laby leakage isapprox. 10 times

Eye LabyLeakage

pacer a y

Leakage

I l R lI l R l L b i h ClL b i h Cl


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Internal RecycleInternal Recycle –– Labyrinth ClearanceLabyrinth Clearance

Process labyrinths canbe lu ed b wetparticles in the gas flow

I l R lI l R l L b i h ClL b i h Cl


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Shaft Spacer

Impeller Cover

I t l R lI t l R l L b i th ClL b i th Cl


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Impeller Cover


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GRADE COEF. TENSILE ELONGATION SPECIFIC

EXPNSION(F)

(PSI)

Arlon CP 17 x 10 /-6 11,080 2.0 1.45or on4340

. , . .

Fluorosint500

19.4 1,100 10.0 2.32

UnUn tuned Surge Control Systemtuned Surge Control System


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UnUn-- tuned Surge Control Systemtuned Surge Control System

Recycle valve shall be calibrated at every planned S/D

• fast opening ( < 1 sec)• total travel 0-100 %; 4 – 20 mA

mec an ca s op o co nc e w c ose

Valve positioner shall match the command

FT instrument shall be calibrated at every planned S/D

– ,

FoulingFouling


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FoulingFouling

… is the deposit and the non –uniform accumulation ofdebris in the gas

Occurs due to carr over of li uids and debris from theinlet suction scrubber

compressors applications if the temperature exceeds thecritical point beyond the polymerization process occurs(235 F)

Foulin build u occurs usuall on the im eller hub andshroud. There is also a build up on the blades ( on thepressure side)

FoulingFouling


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FoulingFouling

st –

Foulin EffectsFoulin Effects Char e GasChar e Gas


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Foulin EffectsFoulin Effects – – Char e GasChar e Gas

3M7 – Eroded Sleeves

Foulin EffectsFoulin Effects


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Foulin EffectsFoulin Effects

April 25 '99 NPC Thai Foulin 9

Abrasive Scoring due to Fouling

Foulin EffectsFoulin Effects Char e Gas


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Foulin EffectsFoulin Effects – Char e Gas

3M7 - Deterioration of stage clearances

Techni ues to Prevent FoulinTechni ues to Prevent Foulin


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Techni ues to Prevent FoulinTechni ues to Prevent Foulin

Condition monitoring, both aerodynamic and

Process control

Online solvent injectionCoatings of Impellers and Diaphragms

FoulingFouling -- Condition MonitoringCondition Monitoring


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FoulingFouling Condition MonitoringCondition Monitoring(aerodynamic and mechanical parameters)(aerodynamic and mechanical parameters)

Monitor and trend the information regardingprocess conditions

• MW•

• Temperature

Vibration monitoring

• n ne sys em• Off line system


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ONLINE CONDITIONINGONLINE CONDITIONING

Condition MonitoringCondition Monitoring – – DR CPM Online SystemDR CPM Online System


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Condition MonitoringCondition Monitoring – – DR RECON Online SystemDR RECON Online System


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Compressor Fundamentals, Performance Curves

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