Centrifugal Compressor Performance characteristic curve is the curve between Polytropic Head and Inlet Volume Flow. In a characteristic curve the terms like Compressor Surge line, Anti surge line, Choke or Stone wall point are the important phenomenon to be understood. Let us see them in detail
Centrifugal Compressor Surge Line
When the inlet flow to the centrifugal compressor decreases beyond a limit an unwanted and unstable cyclic phenomenon occurs called Surge. When the compressor loses its ability to develop the pressure head due to low inlet flow, the reversal of gas flow from discharge to suction takes place. Again the compressor tries to develop the head and this cycle is repeated till the desired head is achieved. This cycle develops roaring noise and causes severe damage to compressor and even foundations. Centrifugal compressor surge line is the limit for the inlet flow below which it should not be operated.
The reasons for compressor surge might be plant trips, Power failure or Utility failure, Compressor Suction Heat exchanger leaks, Suction filter choke, Intercooler leaks, Compressor Discharge Valve closure, Molecular Weight changes due to process upsets etc
Effects of Compressor Surge
It leads to unstable flow and pressure. Surge causes damage to Seals, Bearings, Impellers and Shaft due to heavy vibration. It increases the seal clearances and shortens the compressor life. Compressor Surge lowers the output and results in low energy efficiency of the machinery. Furthermore it affects the associated plant process.
Anti Surge line
Surge control flow also varies with teh compressor speed and normally it is 10% higher than the surge flow. Centrifugal Compressor Anti surge control system is selected based on the gas which is compressed.
- It the gas is air anti surge control valve is located on the discharge line of the compressor and it is vent is routed to atmosphere.
- Wheras if the gas handled is hazardous the discharge gas is recycled to suction after cooling with the help of cooling water in a cooler.
- If the gas handled is a refrigerant like propane or Ammonia, the discharge gas is recycled to suction and to maintain the temperature the same liquid refrigerant is added in the Knock out drum
Choke or Stone wall
In a Centrifugal compressor Choke or Stone walling condition occurs when the velocity of the gas in compressor reaches sonic velocity that is velocity of sound. Stone wall or choke is a maximum flow limit in the centrifugal compressor which is an important criterion for compression system involving high molecular weight gases.