Surge Explained

33
LP Inlet LP Inlet Volute LP First Impeller HP First Impeller LP Discharge HP Inlet HP Disch a rge HP Disch. Volute

Transcript of Surge Explained

Page 1: Surge Explained

LP Inlet

LP Inlet Volute

LP First ImpellerHP First Impeller

LP Discharge

HP Inlet

HP Discharge

HP Disch. Volute

Page 2: Surge Explained

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

Page 3: Surge Explained

Hub

Bore

Cover

Blade

Eye

Exit

Page 4: Surge Explained
Page 5: Surge Explained
Page 6: Surge Explained
Page 7: Surge Explained

Discharge Pressure

Suction Pressure

Page 8: Surge Explained

Discharge PressureSuction

Pressure

Normal Pressure Profile

Page 9: Surge Explained

Discharge PressureSuction

Pressure

Surge Pressure Profile

Page 10: Surge Explained

Discharge Pressure

Suction Pressure

BalancePiston

Page 11: Surge Explained

P (1/2 U22)

Pressure Pulsations Associated with Surge

gas density

U =Impeller Tip Speed

Frequency = 1/2 to 2 Hz

Page 12: Surge Explained

Incipient Surge

Stall Cells

Page 13: Surge Explained

P = 0.05 (1/2 U22)

Pressure Pulsations Associated with Incipient Surge

gas density

U =Impeller Tip Speed

Frequency = 0.3N to 0.6N

Page 14: Surge Explained

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

Page 15: Surge Explained

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.

Page 16: Surge Explained

Compressor Performance Equations

Hp = P2P1

( )[k-1k( )

-1 ] k-1k( ) ZRT1

MW

Q1 = h Z R T1

MW P1

Page 17: Surge Explained

Readily Measureable Variables

Hp = P2P1

( )[k-1k( )

-1 ] k-1k( ) ZRT1

MW

Q1 = h Z R T1

MW P1

Page 18: Surge Explained

Simplification of Head Factor

=

P2

P1( )[

k-1k( )

-1 ] k-1k( ) ZRT1

MWHp

= h Z R T1

MW P1

Q12

Page 19: Surge Explained

hP1

Simplification of Flow Factor

P2

P1( )[

k-1k( )

-1 ] k-1k( )

Page 20: Surge Explained

The Universal Surge Curve

P2

P1( ) - 1

hP1

Page 21: Surge Explained

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

Page 22: Surge Explained

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

Page 23: Surge Explained

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.

Page 24: Surge Explained

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.

Page 25: Surge Explained

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

Page 26: Surge Explained

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.

Page 27: Surge Explained

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

Page 28: Surge Explained

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.

Page 29: Surge Explained

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

Page 30: Surge Explained

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

Page 31: Surge Explained

Control Features

Page 32: Surge Explained

100%

0 %Closed

Open

r MAR

rPROPTM

rSUCLN

rSULIN

0

7

Proportional FunctionProportional Function

Pdischarge

Psuction

P suction

h orifice

Page 33: Surge Explained

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