View
1.515
Download
6
Category
Preview:
Citation preview
LP Inlet
LP Inlet Volute
LP First ImpellerHP First Impeller
LP Discharge
HP Inlet
HP Discharge
HP Disch. Volute
Shaf
t
LP F
irst
Whee
l
LP L
ast
Whee
l
HP First Wheel
HP Last Wheel
Thru
st Disk
Coupling Hub
Bea
ring Jou
rnal
Hub
Bore
Cover
Blade
Eye
Exit
Discharge Pressure
Suction Pressure
Discharge PressureSuction
Pressure
Normal Pressure Profile
Discharge PressureSuction
Pressure
Surge Pressure Profile
Discharge Pressure
Suction Pressure
BalancePiston
P (1/2 U22)
Pressure Pulsations Associated with Surge
gas density
U =Impeller Tip Speed
Frequency = 1/2 to 2 Hz
Incipient Surge
Stall Cells
P = 0.05 (1/2 U22)
Pressure Pulsations Associated with Incipient Surge
gas density
U =Impeller Tip Speed
Frequency = 0.3N to 0.6N
Opening the Recycle Valve accomplishes two thingsOpening the Recycle Valve accomplishes two thingswhich 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 discharge network, allowing flow to increase
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 near surgeAcceleration of surge due to compressor curve shape near surge
• Interaction with other control loopsInteraction with other control loops
• Special requirements for load sharing between multipleSpecial requirements for load sharing between multiplecompressorscompressors
The following factors make surge control a uniqueThe following factors make surge control a uniqueand difficult control application.and difficult control application.
Compressor Performance Equations
Hp = P2P1
( )[k-1k( )
-1 ] k-1k( ) ZRT1
MW
Q1 = h Z R T1
MW P1
Readily Measureable Variables
Hp = P2P1
( )[k-1k( )
-1 ] k-1k( ) ZRT1
MW
Q1 = h Z R T1
MW P1
Simplification of Head Factor
=
P2
P1( )[
k-1k( )
-1 ] k-1k( ) ZRT1
MWHp
= h Z R T1
MW P1
Q12
hP1
Simplification of Flow Factor
P2
P1( )[
k-1k( )
-1 ] k-1k( )
The Universal Surge Curve
P2
P1( ) - 1
hP1
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 for supplied by the manufacturer. The compressor should be tested for
at least three surge points if possible, one at minimum speed, one at maximum at 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.
hP
o
s
Determining the Surge Limit Point
P
Pd
s
If the controller is unable to prevent a compressor surge, it is If the controller is unable to prevent a compressor surge, it is desirable to automatically increase the surge margin so that desirable to automatically increase the surge margin so that the compressor does not continue to surge. The controllerthe compressor does not continue to surge. The controllermust detect a surge, increment the margin and alarm.must detect a surge, increment the margin and alarm.
HHpp
QQ22 (ICFM) (ICFM)
NN11
NN22 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
InstrumentationInstrumentation Considerations Considerations
• Location: Preferred in compressor suction for simplifying algorithm. Commonly Location: Preferred in compressor suction for simplifying algorithm. Commonly found in compressor discharge piping and is compensated to inlet conditions. found in compressor discharge piping and is compensated to inlet conditions. MUST be located to measure TOTAL COMPRESSOR FLOW inside the recycle MUST be located to measure TOTAL COMPRESSOR FLOW inside the recycle loop.loop.
• Size: The flow measuring device and transmitter must be sized for maximum Size: The flow measuring device and transmitter must be sized for maximum compressor flow. The required pressure differential corresponding to maximum compressor flow. The required pressure differential corresponding to maximum flow should be 10” WC or greater.flow should be 10” WC or greater.
Flow Measurement is the most important signal for proper surge control. Close Flow 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.
Instrumentation Instrumentation Considerations (cont.)Considerations (cont.)
• Flow Transmitter: The length of tubing between flow device Flow Transmitter: The length of tubing between flow device and the transmitter should be minimal. The transmitter must and the transmitter should be minimal. The transmitter must be reliable, repeatable, and have a speed of response (rise be reliable, repeatable, and have a speed of response (rise time) of 100 msec or less.time) of 100 msec or less.
• Pressure Transmitters: These transmitters should be located Pressure Transmitters: These transmitters should be located as close to the compressor as possible. For constant speed as close to the compressor as possible. For constant speed compressors with suction throttling valves, the suction compressors with suction throttling valves, the suction pressure must be measured downstream of the valve.pressure must be measured downstream of the valve.
• Temperature Transmitters: Location is not as critical but Temperature Transmitters: Location is not as critical but should be located to provide correct temperature.should be located to provide correct temperature.
Instrumentation Instrumentation Considerations (cont.)Considerations (cont.)
Flow Measurement for Load ControlFlow Measurement for Load ControlThe flow measuring device for load control must be located to measure "user" flow The 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 and Also, most flow control applications are "mass flow", requiring temperature and pressure readings as well as flow.pressure readings as well as flow.
SIC
FIC
FT
UIC
PT
PT
FY
TTTT
ST
FT
Flow MeasurementFlow Measurement
• Venturi Tube- best device- most costlyVenturi Tube- best device- most costly• Orifice Plate- good characteristics- unrecoverable pressure loss- Orifice Plate- good characteristics- unrecoverable pressure loss-
less costlyless costly• Annubar - widely used, least costly, least desirable - poor signal-Annubar - widely used, least costly, least desirable - poor signal-
to-noise ratio.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 be Accuracy of the flow coefficient is not critical. Must be repeatable.repeatable.
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 use “smarts.”
• Avoid Honeywell Smarts
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 critical Size and Speed are the two most critical factors.factors.
• Linear Valves are preferred. Others can Linear Valves are preferred. Others can be characterized.be characterized.
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" and smaller valves. Stroking speed of 2 sec. for 8" and larger valves. Normally requires a volume booster in air supply
Valve ActuationValve Actuation
SVSV
I/PI/P
OutOut
SupplySupply
InIn
1:11:1
Air Air SupplySupply
SVSV
Piping/Tubing for pneumatic actuator and volume booster.Piping/Tubing for pneumatic actuator and volume booster.
Large CLarge Cvv Volume Booster Volume Booster
Control Features
100%
0 %Closed
Open
r MAR
rPROPTM
rSUCLN
rSULIN
0
7
Proportional FunctionProportional Function
Pdischarge
Psuction
P suction
h orifice
Current Operating Point
Hover Setting
Control Setpoint
Control Line
Surge Line
Suction Flow (hx)
Pre
ssur
e R
atio
PD/PS
Setpoint Hover FeatureSetpoint Hover Feature
Recommended