Safety narrowest flow cross section

Despite the complex flow phenomena, the technical rules and standards give simple nozzle equations without application limits for the sizing of safety devices, and the valve manufacturers report only two discharge coefficients as constant values for all gases/vapors and liquids in their catalogs. Physically, this is only possible if the flows in the valves behave similarly. The location of the sound velocity must be essentially the same, and the narrowest flow cross section in the valve may not vary significantly (Reynolds and Mach number similarity). 

Fluid dynamic critical pressure and temperature in the throat area of the nozzle and the narrowest flow cross section of the safety valve ... 

The velocity in the narrowest cross section in a safety valve may reach the sound velocity at maximum. Simultaneously, the pressure decreases to its minimum value, the fluid dynamic critical pressure, and the mass flow rate is maximized. 

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