Limits of the Standard Valve Sizing Procedure

This testing of safety valves at a fixed lift presumes that the valves will open at least to the nominal lift in case of an emergency relief 

Overall, the sizing method according to EN-ISO 4126-7 is based on very simple equations mainly for ideal gases. In the literature, the validity of this sizing method was shown by numerical calculations up to pressures of about 10 MPa (100 bar) [3,4]. No limits of application are formulated in the standards for high-pressure valves. In addition, the discharge coefficients for safety valves are generally measured at pressures between 0.1 and 5 MPa - the application limit of current test facilities amounts to about 25 MPa (250 bar). The question arises whether the application of the current standard beyond a pressure of more than 10 MPa is allowed. 

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). 

It is expected that the mass flow rate through a safety valve and possibly also the discharge coefficients for gases/vapors are influenced by an entire series of parameters (gas type, valve geometry, and property data) that are presently not fully accounted for either in the nozzle model per EN-ISO 4126-7 or in the testing procedure. 

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