Sonic choking occurs in a pipe system when the gas attempts to accelerate to a velocity faster than the local sonic velocity at some point in a system. Being unable to accelerate any further, the gas reaches its maximum velocity at sonic velocity. Changes to the downstream conditions, such as lowering the pressure, produce no additional flow. In many pipe applications sonic choking dominates the system behavior.

Predicting the flow rates under such sonic choking conditions can be very difficult depending on where the choking occurs. AFT xStream is designed to identify sonic choking and predict the system behavior under such conditions. In addition, xStream can approximate moving shock waves in the system.