Anyone out there can help me with this. In ASME B31.1 Appendix II there is a method for Relief Valve Discharge Piping Design. One of the equations is to determine the pressure at the vent exit P3. In certain cases P3 calculated by this equation can be less than atmospheric (14.7 Psia). From other sources I have come across, if this occurs in the pipework sizing then the exit pressure P3 must be set to atmospheric (14.7 Psia) and the process is to work backwards along the pipe to determine if the calculated pressure drop causes a problem. My problem is that ASME B31.1 does not warn the user that if the calculated value of P3 is less than atmospheric then P3 must be set to atmospheric.The example given is such that P3 is well above the atmospheric pressure hence no problem.
If I consider the other sources then by setting P3 to atmospheric, then calculate P2,V2 and check for blowback then you get a problem. On the other hand if you use the calculated value of P3 and calculate P2,V2 then blowback is not a problem. Please advise.

This kind of problem always exist if the capacity and orifice size are high and set pressure as well as back pressure of the safety valve are very low. Outlet size of the safety valve becomes large with the low back pressure. According to my experience, you can solve your problem in two ways:
1. Reduce pressure drop of the discharge pipe by shortening the length, eliminating miters or elbows and selecting discharge pipe diameter as large as possible ( however this does not work all the time due to blowback calculation ). Most of the cases we need to provide a silencer due to noise problem. Limiting pressure drop on the silencer might be helpfull.
2. Basicaly, this is a valve selection problem. Increasing the back pressure ( for example from 10% to 25% ) of the safety valve will solve the problem. Sometimes, getting two smaller valves instead of single valve may be adequate. In both cases, you need to consult your SV supplier.

I guess your calculation is on a spreadsheet. If you make a couple iterations by changing the variables ( diameter of the discharge duct,losses,the back pressure of the safety valve, capacity, valve outlet size)you can see the behaviour of the system. If you can see what makes your calculation ineffective than you can eliminate your problem easily.

I hope this helps. If you need further help, let me know the details about the system and the safety valve.

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