Calculate the critical flow velocity and safe discharge volume of gas pipelines

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I am currently working on a project related to the discharge from safety valves. It concerns the situation where a level control valve between the high-pressure side and the low-pressure side fails, allowing gas from the high-pressure side to enter the low-pressure side. The amount of discharge associated with this scenario has not been clearly defined in any standards or manuals I’ve reviewed: SY/T10043 provides some conceptual information on such conditions, while HG 20570.2 includes a calculation formula, but I still need to verify whether it is correct. Under normal operating conditions, this control valve allows liquid to flow through it, while gas flows through it in case of a fault. But I remember that there is a critical flow velocity for gases; some people say it’s called the speed of sound in Aspen, I’m not sure if that’s correct. And how is this value calculated? ； Additionally, could we consider using the critical pressure ratio for small orifices? Since control valves generally have a reduced diameter, the critical pressure ratio can be determined using the ratio of the valve’s orifice diameter to the pipe’s inner diameter, along with the adiabatic index. Given the pressures before and after the valve, it is possible to determine whether the gas meets the criteria for flow restriction, thereby allowing the flow rate to be calculated back