Sizing of High-Pressure Safety Valves for Gas Service

Sizing of High-Pressure Safety Valves for Gas Service 

The predominant numbers of safety valves are used at set pressures below 5 MPa (50 bar). For gas service, they are sized according to EN-ISO 4126-7 [1] or national 

With the sizing coefficient Cg for a nozzle with a throat area equal to the seat area of the safety valve, the mass flow rate is defined in a dimensionless form  

Industrial Pressure Ap ications Processes, Equipment and Sajety, First Edition. Edited by Rudolf Eggers. 2012 Wiley-VCH Verlag GmbH Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH Co. KGaA. 

Equations (15.1) and (15.2) are definitions that are valid for both ideal gas and real gas flow. 

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In practice, often an ideal behavior of gases is assumed at moderate pressures when sizing a safety valve for gas service. Real gas behavior is only assumed at a very high pressure, for example, at a pressure of more than 100 bar. In general, the real gas behavior is rather determined from the proximity of the thermodynamic critical point. With the reduced thermodynamic pressure and the reduced thermodynamic temperature, the deviation from ideal behavior can be described much better than with the absolute values of pressure and temperature. If the reduced pressure and the reduced temperatures at the entrance of the nozzle exceed p/pc > 0.5 or T/Tc > 0.9, the deviations from the ideal behavior are usually no longer tolerable. 

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