Critical Flow

The critical flow which also referred to as sonic flow, choked flow, or Mach 1 is the limitation point of a compressible fluid flow through an orifice. The critical flow can occur on a relief valve orifice or a choke. A choke is a condition where piping goes from a small branch into a larger header, where pipe size increases, or at the vent tip. The maximum flow occurs at sonic velocity, which exists as long as the pressure drop through the orifice is greater than the critical pressure drop given by:

For hydrocarbon service which has critical pressure ratio at 0.5, the relief valve will decrease its capacity if the backpressure is greater than 50% of set pressure.

As long as the pressure ratio exceeds the critical-pressure ratio, the throughput will vary with the inlet pressure and be independent of outlet pressure. For example, a relief valve set at 100 psi will have the same gas flow through it as long as the back-pressure is less than approximately 50 psi.

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