Safety relief valve discharge 1

[Patrick LaPointe]

In Appendix II in B31.1, there is a sample problem to determine pressures and velocities at the outlet of safety relief valves. In one of the examples, there is a calculated value is 118 psia for the calculated pressure at the outlet of the SRV elbow. My question is how can this pressure be greater than the critical pressure ratio(approximately 0.54 for steam) multiplied by the upstream pressure which typically would be the safety relief valve set pressure. When using the equation in this Appendix II to calculate P1, we frequently get a value of P1 which is less than atmospheric pressure. This occurs when the set pressure is a "low" number. Again, physically, what does this look like?

 

[davefitz]

I think you are confusing things a bit. Acoustically choked flow , or critical flow, occurs when the velocity in the discharge nozle reaches the speed of sound based on the upstream pressure. The downstrem pressure for critical flow must then be lower than .54 times the upstream presure , but can be much loser than .54 of the upstream pressure. For example ,, when a hp bypass valve discharges into the condenser, the upstream pressure can be in the range of 2900 psi- 4000 psia( for supercritical units) yet the condenser pressure is below 2 psia , for a pressure ratio of 2000:1 !!! That is an extreme example.

The asme safety valve pressure curves are based on a 1970's paper by Bechtel engineer Laio, whihc can be found in the archive of asme tech papers.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick