17539869 Surge Explained

8/3/2019 17539869 Surge Explained

1/33

LP Inlet

LPInle

tVolu

te

LP First ImpellerHP First Impeller

LP Discharge

HPInlet

HPDischarge

HP Disch. Volute


2/33

Sha

ft

LPFirs

tWhee

l

LPLas

tWhee

l

HPFirstW

heel

HPLastW

heel

ThrustD

isk

CouplingHub

BearingJ

ournal


3/33

Hub

Bore

Cover

Blade

Eye

Exit


4/33


5/33


6/33


7/33

DischargePressure

Suction

Pressure


8/33

Discharge

PressureSuction

Pressure

Normal Pressure Profile


9/33

Discharge

PressureSuction

Pressure

Surge Pressure Profile


10/33

DischargePressure

SuctionPressure

BalancePiston


11/33

P (1/2 U22)

Pressure Pulsations Associated with Surge

= gas densityU =Impeller Tip Speed

Frequency = 1/2 to 2 Hz


12/33

Incipient Surge

Stall Cells


13/33

P = 0.05 (1/2 U22)

Pressure Pulsations Associated with Incipient Surge

= gas densityU =Impeller Tip Speed

Frequency = 0.3N to 0.6N


14/33

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


15/33

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.


16/33

Compressor Performance

Equations

Hp =P

2P1( )[

k-1k( )

-1] k-1k

( )ZRT1MW

Q1 =

h Z R T1MW P1


17/33

Readily Measureable

Variables

Hp = P2P1( )[

k-1k( )

-1] k-1k

( )ZRT1MW

Q1 =h Z RT

1MW P1


18/33

Simplification of Head

Factor

=

P2

P1( )[

k-1k( )

-1

]k-1k

( )ZRT1MW

Hp

=

h Z R T1MW P1Q

1

2


19/33

h

P1

Simplification of Flow

Factor

P2P1

( )[k-1k( )

-1] k-1k

( )


20/33

The Universal Surge Curve

P2P1

( ) - 1

h

P1


21/33

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


22/33

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


23/33

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.


24/33


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.


25/33


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


26/33

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.


27/33

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


28/33

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.


29/33

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


30/33

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


31/33

Control Features


32/33

100%

0 %Closed

Open

r MAR

rPROPTM

rSUCLN

rSULIN

0

7

Proportional FunctionProportional Function

Pdischarge

Psuction

P suction

h orifice


33/33

Current Operating Point

Hover Setting

Control Setpoint

Control Line

Surge Line

Suction Flow (hx)

PressureRatio

PD/PS

Setpoint Hover FeatureSetpoint Hover Feature

Documents

17539869 Surge Explained