Flow is considered to be choked when the Mach number reaches a value of 1.  When a subsonic flow becomes choked, the flow rate has reached its maximum possible value - it cannot be increased even if the downstream pressure is lowered.

Choking conditions in PIPE-FLO Compressible are indicated on the FLO-Sheet by pipelines that are colored cyan (a bright greenish blue color).  In the List Window, the word "Choked" appears in the status field of pipelines or devices where the Mach number at the outlet is 1 or close to 1.  If choking occurs prior to the pipeline or device outlet, the word "Invalid" appears in the status field, indicating that the program cannot converge to a valid solution.

It is usually desirable to avoid choking conditions in a system.  Choking most often occurs in the following situations:

• There is a large change in area (either at a node or in a reduced seat valve).
• A system demand has been set too high.
• An outlet pressure has been set too low for the inlet conditions.
• A pressure control or a flow control has been set to a value that is impossible to achieve.
  

If choking is occurring in your system, you should review the pipeline sizes and reduced seat valve installations.  If it is not possible to change the system configuration in any way, the system boundary conditions (set pressures and demands) and control settings should be reviewed and possibly modified.