Relief via a safety valve

The formula for semi-ideal gas flow through a nozzle is commonly used to obtain the flow capacity of a safety valve14 - 

The value of FB in equation (A6.4) depends on whether or hot flow is choked in the safety valve nozzle. Choking occurs if  

DECISION TREE FOR SELECTION OF METHOD FOR SINGLE-PHASE GAS FLOW CAPACITY 

CHOKED FLOW DATA FOR GASES AS A FUNCTION OF ISENTROPIC COEFFICIENT 

If choking occurs (see equation A6.5), then FB is equal to 1.0. For unchoked (subsonic) flow, FB is given by[3]  

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