PSV Sizing

Sizing of pressure relief valve is clearly explained in API STD 520 part 1. Some formula is provided for calculation the orifice size either for vapor or liquid application, critical or non critical condition.

My friend, there is no difficult thing in sizing PSV as long as all data required is already provided. The all we need is just inputting the data in the formula or in the spreadsheet :D , then select the appropriate standard orifice size ( refer to API STD 526)

The following pictures, show the formula in API STD 520

For non critical condition, the outlet pressure has impact in sizing orifice area but for critical condition is not. In critical condition, the outlet pressure is lower than the critical flow pressure therefore there will be no impact to the sizing.

Note this, in critical condition, the flow through the nozzle will be influenced by the inlet pressure (relieving pressure). In non critical condition, the flow will depend on the differential pressure between inlet and outlet. (Relieving pressure – back pressure).
Use correct formula !

For you who looking for the sizing example, click the following link.
1. Gas/Vapor Critical and an example from Leser
2. Gas Subcritical and an example from Leser
3. Steam and an example from Leser
4. Liquid, an example from Leser

Thank you for your attention.