Compressor Choke or Stonewall

Compressor choke is an abnormal operating condition for centrifugal compressor. Choking of

centrifugal compressor occurs when the compressor is operating at low discharge pressure and very

high flowrates. These high flowrates at compressor choke point are actually the maximum that the

compressor can push through. Any further decrease in the outlet resistance will not lead to increase

in compressor output. This operating condition is also known as stonewalling of a centrifugal

compressor.

Figure 1 - compressor map showing the choke line which represents the maximum flow limit for a

compressor

How does a compressor choke?

Stonewall or choke point for a centrifugal compressor occurs when the resistance to flow in the

compressor discharge line drops significantly below the normal levels. Due to low resistance,

compressor discharge sees very low back pressure. As suggested by the compressor maps for a

fixed RPM value, compressor output increases as the backpressure at compressor discharge drops

down. This leads to increased gas velocity in the centrifugal compressor. The increase in gas

velocity can occur until it can reach its maximum at sonic velocity. When the gas velocity in any of

the compressor parts reaches close to sonic velocity (MACH1), this is said to be the choke point or

stonewall for compressor operation. The gas velocity and gas flow rate cannot go beyond this value

at the choke point.

Damages due to compressor chokingProlonged operation of a compressor at its choke point can lead to damaging the compressor parts.

Compressor choking is not particularly damaging to single-stage centrifugal compressors but can

cause serious damage to the rotors and blades of multistage centrifugal and axial compressors.

How to prevent compressor chokingTo prevent the compressor choke or stonewall from happening it is needed to maintain a certain

level of flow resistance in the compressor outlet line. Anti-choke valves can be used for this purpose

in the compressor outlet line. Anti-choke valves close to restrict the flow to keep compressor from

stonewalling. When flow resistance in compressor outlet falls and flow begins to increase, the anti-

choke valves close to develop resistance to the increasing flow.