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71

Saudi Aramco EDP 2004
Compressors 71

A Model of Surge
Surge Definition
Surge is self-oscillations of pressure and flow, including a flow
reversal. The surge flow reversal is the only point of the curve
when pressure and flow drop simultaneously.

Characteristic Curve of a Typical Centrifugal Compressor
– B to C in 20 to 50 mSec
– Cycle B to B at 0.33 to 3 Hertz

Page 11

81

Saudi Aramco EDP 2004
Compressors 81

Compressor

Inlet
Gas

FT PT PT

UIC

FT I/P FV

Recycle

∆Pc

Discharge
Gas

Simple Antisurge System

Flow measured in suction
(∆Po)
Suction and discharge
pressure transmitters for
pressure differential
calculation
Control strategy is
proportional plus integral
control to manipulate
recycle valve
Increase flow through
compressor and reduce
discharge resistance when
required to prevent surge

In a simple antisurge system, flow and pressure measurements on the suction and
pressure measurements on the discharge are sent to the antisurge controller, UIC.
The UIC controls the recycle valve which is normally closed. If surge is
approached, the UIC opens the recycle valve.

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