Safety valve capacity for two-phase relief

Research is currently being earned out for DIERS and in Europe to measure safety valve discharge coefficients for, two-phase flow, including high viscosity systems. Note that any two-phase discharge coefficient is the ratio of measured flow to flow calculated using a particular two-phase flow model. Discharge coefficients should therefore only be used with the flow model for which-they were derived. 

Note that it is also worthwhile to find the actual nozzle flow area for a particular valve, since there is some variation in this between different manufacturers, even for standard nozzle sizes. The standard nozzle sizes (rather than the actual sizes) for safety valves are defined in references 19 and 22. 

Safety valve instability must be avoided. The rapid cycling of the valve from open to closed (chattering) can destroy the valve. Resonance could, in certain circumstances, lead to fracture of the associated piping. Furthermore, the capacity of a safety valve during cycling will be considerably less than if it remained fully open, and is very likely to be insufficient to prevent overpressurisation of the upstream vessel. 

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