Surging is an unstable phenomenon inside the plenum of the centrifugal compressor, where the flow reverses backward and again forward in cyclic order arising due to high back pressure on compressor outlet nozzle.
Surging can cause the compressor to overheat to the point at which the maximum allowable temperature of the machine is exceeded damaging the machine. It can cause damage to the thrust bearing due to rotor shifting back and forth, active to the inactive side.
This is highly undesirable and the machine has to be protected from such occurrence, and this is the reason anti-surge is very important for designing a compressor.
Surge is defined as the operating point at which the centrifugal compressor peak head capacity and minimum flow limits are reached at a given speed. If we join these points at various speeds of the compressor on Flow Vs Head plot, we shall get a straight line called the surge line and the slope of this line on Flow Vs Head plot, is called Ratio.
In normal case the compressor operates at the right side of the Surge line. During start up and emergency shutdown, the operating line moves towards surge line, increasing the ratio.
The centrifugal compressor is designed such that the operating line of the compressor always remain right side. In other words the operating line ratio should always be less than surge line ratio.
In order to ensure this a minimum flow line which recirculates the gas taking immediately from discharge of the compressor back to the suction is provided. When the operating line shifts towards surge line ie when ratio increases, the re circulation line, provided with a surge control valve, opens to supply more flow to the suction of the compressor and reducing the Head, thereby reducing the ratio.
The opening of the surge control valve, called trim and corresponding flow, is calculated depending upon the working ratio and how far it is away from surge ratio. The point at which the valve initiates opening depends upon how far it is safely away from surge ratio. The surge ratio is specific to a machine and therefore the calculation of anti surge flow at different ratio is different for machine.
The surge control line is offset to the right of the surge line by a margin; typically equal to 3- 10% of inlet volume flow at surge. The anti surge valve (ASV) is fail open solenoid valve and remains open at start up and normal stop. ASV should be sized for 1.8 to 2.2 times the maximum surge point flow .
Credits: Shakti Kumar Pandey