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# Safety relief valves outlet pressure

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## Safety relief valves outlet pressure

(OP)
When calculating outlet pressure at the end of a vent pipe using the calculating procedures outlined in B31.1 Appendix II, what is the significance of a calculated outlet pressure of 9.0 psia? How does this physically occur that the pressure at the end of the vent pipe is sub-atmospheric? The example in B31.1 is for superheated steam.

### RE: Safety relief valves outlet pressure

Cool. What pages are this 9 psia example on?

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Safety relief valves outlet pressure

(OP)

Latexman, the 9 psia is a number that I calculated for an application recently. Try running the Appendix II calculation for the following:
Safety relief valve set pressure = 45.0 psig (59.7 psia)
T = 350 F
W = 85,175 lbs per hour of slightly superheated steam.
SRV is an 8" inlet by 10" outlet T orifice
h = approximately 1208 BTU/lb.
I calculated 11.36 psia
If your calculation yields approximately the same as mine, then what is the physical significance of a sub-atmospheric pressure at P1? How can that happen?

### RE: Safety relief valves outlet pressure

II-2.3.1.2 speaks of not blowing steam back (at the PSV, I guess) where the elbow outlet inserts into the vent pipe inlet. They overlap a bit. I take that to mean steam must not "blow out" of this open air junction. That requires the pressure at the end of the elbow to be subatmospheric. Thus, air is induced at the inlet to the vent pipe.

Make sense now? The momentum balance between the induced air and high speed, super-heated steam results in a higher pressure to push the combined streams out the exit at the top.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Safety relief valves outlet pressure

(OP)
Sort of. What happens when there is no vent pipe but rather just a safety relief valve and an elbow. The P1 calculation noted above is still 11.36 psia; sub-atmospheric so what is going on? Is there a shock wave with a step change in pressure between the SRV disc and the outlet?

### RE: Safety relief valves outlet pressure

The equation of II-2.2.1 (A.1) to determine P1, gives the critical pressure at the elbow outlet. As it is less than the atmospheric pressure, the real value of P1 will be 14.7 psia. The meaning is that the elbow has capacity to discharge a flow rate greater than the flow rate of the valve of W = 85,175 lb/h.
casflo

### RE: Safety relief valves outlet pressure

The critical pressure ratio (0.53-ish) at the SRV flow nozzle is most likely exceeded by a good bit (14.7+ /59.7), so the exit of the SRV flow nozzle is Mach 1. There's your shock.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

### RE: Safety relief valves outlet pressure

(OP)
Thank you Casflo and Latexman

Pat

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