Post on 07-Jul-2018
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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%
% Inlet Capacity or Flow
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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%
% Inlet Capacity or Flow
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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%
% Inlet Capacity or Flow
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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%
% Inlet Capacity or Flow
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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%
% Inlet Capacity or Flow
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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%
% Inlet Capacity or Flow
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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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Impeller Overlap with Diffuser Impeller Overlap with Diffuser
Positive overlap
Limited
Nominal
Desired
Non Desirable
Limited
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Impeller Overlap with Diffuser Impeller Overlap with Diffuser
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%
% Inlet Capacity or Flow
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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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Internal RecycleInternal Recycle –– Labyrinth ClearanceLabyrinth Clearance
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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Internal RecycleInternal Recycle –– Labyrinth ClearanceLabyrinth Clearance
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 CPM Online SystemDR CPM Online System
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Condition MonitoringCondition Monitoring – – DR CPM Online SystemDR CPM Online System
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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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