17539869 surge explained
TRANSCRIPT
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LP Inlet
LPInle
tVolu
te
LP First ImpellerHP First Impeller
LP Discharge
HPInlet
HPDischarge
HP Disch. Volute
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Sha
ft
LPFirs
tWhee
l
LPLas
tWhee
l
HPFirstW
heel
HPLastW
heel
ThrustD
isk
CouplingHub
BearingJ
ournal
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Hub
Bore
Cover
Blade
Eye
Exit
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DischargePressure
Suction
Pressure
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Discharge
PressureSuction
Pressure
Normal Pressure Profile
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Discharge
PressureSuction
Pressure
Surge Pressure Profile
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DischargePressure
SuctionPressure
BalancePiston
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P (1/2 U22)
Pressure Pulsations Associated with Surge
= gas densityU =Impeller Tip Speed
Frequency = 1/2 to 2 Hz
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Incipient Surge
Stall Cells
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P = 0.05 (1/2 U22)
Pressure Pulsations Associated with Incipient Surge
= gas densityU =Impeller Tip Speed
Frequency = 0.3N to 0.6N
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Opening the Recycle Valve accomplishes two thingsOpening the Recycle Valve accomplishes two things
which helps the compressor avoid surge:which helps the compressor avoid surge:
Surge Control Basics
Recirculates flow back to the compressor
inlet
Relieves resistance in compressor dischargenetwork, allowing flow to increase
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Surge Control ApplicationSurge Control Application
Accurately defining the compressor operating pointAccurately defining the compressor operating point
Locating and defining the surge limit of the compressorLocating and defining the surge limit of the compressor
Acceleration of surge due to compressor curve shape nearAcceleration of surge due to compressor curve shape near
surgesurge Interaction with other control loopsInteraction with other control loops
Special requirements for load sharing between multipleSpecial requirements for load sharing between multiple
compressorscompressors
The following factors make surge control a uniqueThe following factors make surge control a unique
and difficult control application.and difficult control application.
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Compressor Performance
Equations
Hp =P
2P1( )[
k-1k( )
-1] k-1k
( )ZRT1MW
Q1 =
h Z R T1MW P1
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Readily Measureable
Variables
Hp = P2P1( )[
k-1k( )
-1] k-1k
( )ZRT1MW
Q1 =h Z RT
1MW P1
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Simplification of Head
Factor
=
P2
P1( )[
k-1k( )
-1
]k-1k
( )ZRT1MW
Hp
=
h Z R T1MW P1Q
1
2
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h
P1
Simplification of Flow
Factor
P2P1
( )[k-1k( )
-1] k-1k
( )
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The Universal Surge Curve
P2P1
( ) - 1
h
P1
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A Surge Limit Point, in terms of & , is determinedA Surge Limit Point, in terms of & , is determined
by testing the compressor in the field or from using the predicted curvesby testing the compressor in the field or from using the predicted curves
supplied by the manufacturer. The compressor should be tested forsupplied by the manufacturer. The compressor should be tested for
at least three surge points if possible, one at minimum speed, one at maximumat least three surge points if possible, one at minimum speed, one at maximum
speed, and one at 50% of the speed range. This establishes thespeed, and one at 50% of the speed range. This establishes the
relationship of speed vs. surge limit.relationship of speed vs. surge limit.
h
P
o
s
Determining the
Surge Limit PointP
P
d
s
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If the controller is unable to prevent a compressor surge, it isIf the controller is unable to prevent a compressor surge, it isdesirable to automatically increase the surge margin so thatdesirable to automatically increase the surge margin so that
the compressor does not continue to surge. The controllerthe compressor does not continue to surge. The controller
must detect a surge, increment the margin and alarm.must detect a surge, increment the margin and alarm.
HHpp
QQ22 (ICFM)(ICFM)
NN11NN22 NN33
Surge Limit LineSurge Limit Line
Surge Control LineSurge Control Line
1122
Open Loop Response or High GainOpen Loop Response or High Gain
Auto Increase of Surge MarginAuto Increase of Surge Margin
New Surge Control LineNew Surge Control Line
Increasing Surge Margin
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InstrumentationInstrumentation
ConsiderationsConsiderations
Location: Preferred in compressor suction for simplifyingLocation: Preferred in compressor suction for simplifyingalgorithm. Commonly found in compressor dischargealgorithm. Commonly found in compressor discharge
piping and is compensated to inlet conditions. MUST bepiping and is compensated to inlet conditions. MUST be
located to measure TOTAL COMPRESSOR FLOWlocated to measure TOTAL COMPRESSOR FLOW
inside the recycle loop.inside the recycle loop.
Size: The flow measuring device and transmitter must beSize: The flow measuring device and transmitter must be
sized for maximum compressor flow. The requiredsized for maximum compressor flow. The required
pressure differential corresponding to maximum flowpressure differential corresponding to maximum flow
should be 10 WC or greater.should be 10 WC or greater.
Flow Measurement is the most important signal for proper surge control. CloseFlow Measurement is the most important signal for proper surge control. Close
attention should be given to selecting and locating the flow device and transmitter.attention should be given to selecting and locating the flow device and transmitter.
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InstrumentationInstrumentation
Considerations (cont.)Considerations (cont.) Flow Transmitter: The length of tubing between flowFlow Transmitter: The length of tubing between flow
device and the transmitter should be minimal. Thedevice and the transmitter should be minimal. The
transmitter must be reliable, repeatable, and have a speedtransmitter must be reliable, repeatable, and have a speed
of response (rise time) of 100 msec or less.of response (rise time) of 100 msec or less.
Pressure Transmitters: These transmitters should bePressure Transmitters: These transmitters should be
located as close to the compressor as possible. Forlocated as close to the compressor as possible. For
constant speed compressors with suction throttling valves,constant speed compressors with suction throttling valves,
the suction pressure must be measured downstream of thethe suction pressure must be measured downstream of the
valve.valve.
Temperature Transmitters: Location is not as critical butTemperature Transmitters: Location is not as critical but
should be located to provide correct temperature.should be located to provide correct temperature.
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InstrumentationInstrumentation
Considerations (cont.)Considerations (cont.)Flow Measurement for Load ControlFlow Measurement for Load Control
The flow measuring device for load control must be located to measure "user" flowThe flow measuring device for load control must be located to measure "user" flow
and not compressor flow. It must therefore be located outside the recycle loop.and not compressor flow. It must therefore be located outside the recycle loop.
Also, most flow control applications are "mass flow", requiring temperature andAlso, most flow control applications are "mass flow", requiring temperature andpressure readings as well as flow.pressure readings as well as flow.
SIC
FIC
FT
UIC
PT
PT
FY
TT
TT
ST
FT
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Flow MeasurementFlow Measurement
Venturi Tube- best device- most costlyVenturi Tube- best device- most costly
Orifice Plate- good characteristics-Orifice Plate- good characteristics-
unrecoverable pressure loss- less costlyunrecoverable pressure loss- less costly
Annubar - widely used, least costly, leastAnnubar - widely used, least costly, least
desirable - poor signal-to-noise ratio.desirable - poor signal-to-noise ratio.
Need to have a sufficient signal to noise ratio.Need to have a sufficient signal to noise ratio.
Accuracy of the flow coefficient is not critical. Must beAccuracy of the flow coefficient is not critical. Must be
repeatable.repeatable.
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Transmitters
Flow transmitters- Rosemount 1151 analog
and 3051 digital are most widely used for
surge control. Pressure transmitters- Acceptable to use
smarts.
Temperature transmitters- Acceptable to usesmarts.
Avoid Honeywell Smarts
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Control ValveControl Valve Typically actuated with pneumatics.Typically actuated with pneumatics.
Some axials may be hydraulic.Some axials may be hydraulic.
Size and Speed are the two most criticalSize and Speed are the two most critical
factors.factors.
Linear Valves are preferred. Others canLinear Valves are preferred. Others canbe characterized.be characterized.
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Control Valve (cont.)
Size- Valve size should be checked at all
operating conditions along the surge line
Speed: Stroking speed of 1 sec. for 6" andsmaller valves. Stroking speed of 2 sec. for
8" and larger valves. Normally requires a
volume booster in air supply
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Valve ActuationValve Actuation
SVSV
I/PI/P
OutOut
SupplySupply
InIn
1:11:1
AirAir
SupplySupply
SVSV
Piping/Tubing for pneumatic actuator and volume booster.Piping/Tubing for pneumatic actuator and volume booster.
Large CLarge Cvv Volume BoosterVolume Booster
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Control Features
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100%
0 %Closed
Open
r MAR
rPROPTM
rSUCLN
rSULIN
0
7
Proportional FunctionProportional Function
Pdischarge
Psuction
P suction
h orifice
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Current Operating Point
Hover Setting
Control Setpoint
Control Line
Surge Line
Suction Flow (hx)
PressureRatio
PD/PS
Setpoint Hover FeatureSetpoint Hover Feature