This term is used for turbo compressors and represents the maximum flow condition. It is a condition which occurs in the compressor in which it operates at a very high mass flow and the flow through the compressor can’t be further increased Mach number at some part of the compressor reach to unity i.e. to sonic velocity and the flow is said to be choked. It is sometimes also referred to as ‘stonewalling’.
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 overall compressor ratio across the compressor is significantly low. 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 up to sonic condition. When the gas velocity in any of the compressor parts is about sonic velocity (MACH=1), no further gas speed increase is possible, hence resulting in stonewall (choke) conditions for compressor operation. On the compressor manufacturer performance curves, the condition can be represented by a nearly vertical drop of compressor curves, catching the idea that once choke conditions are achieved any further drop in compressor ratio will not result in further increase of compressor flow rate.
Damages due to compressor choking: Prolonged operation of a compressor at its choke point can lead to damaging the compressor parts. Compressor choking is not just damaging only to one stage of centrifugal compressors but can cause serious damage to the rotors and blades of multistage centrifugal and axial compressors.
How to prevent compressor choking: To prevent the compressor choke or stonewall it is required to enforce a certain level of flow resistance in the compressor discharge line. Anti-choke valves are usually placed to this aim on the compressor discharge downstream anti-surge loop and associated check valve. When flow resistance in compressor outlet is low and flow approaches choking conditions, the anti-choke valves close to maintain the required minimum pressure ratio across compressor casing.
