I need a 1" 15psi soft seal pressure relief valve.

We are building a high voltage test chamber that will be filled with sulfur hexafluoride (SF6) to prevent arching. The plan for the chamber is to pump it out to around 10^-3 torr or lower (aka higher vacuum) then to fill it with SF6 up to 1 atm (gauge pressure not absolute). My understanding is that if the pressure is kept below 15 psi above atmospheric pressure we do not have to certify it as a pressure vessel so that's the reason we need the relief valve. We bought a kunkle valve from mcmaster carr (http://www.mcmaster.com/#6017t13/=7qe10e) and when we tested it we realized it did not hold pressure and started weeping well below 15psi. So I need help in finding a soft sealing (because it has to hold vacuum) 15psi pop safety valve. It can't start weeping much before 14.7 psi.

The main reason I believe the Kunkle valve failed is that they are adjustable and 15psi is the lowest value offered so the tolerances were horrible when they would be acceptable at 150 psi. If someone can find a non adjustable valve I think that would be best.
I forgot to add I am located at Triumf in Vancouver BC, Canada so local or Canadian companies would be preferred.

What I probably should have stated earlier, is that commercial check valves are built as cheaply as possible.  One way to economize is to use fairly stiff (high K) springs, as these are typically smaller in diameter than low K springs, and much cheaper to source.  The result is a very large difference between the cracking pressure and the pressure at some higher flowrate.

Even ASME rated relief valves suffer from the same problem.

For your purposes, for a <15 psig "vessel", the ASME B&PV rules do not apply.  What you are trying to make is a precision back pressure regulator, not a relief valve per se.  Figure the worst case incoming gas flow rate, and design the valve accordingly.  Or, if the arc/electrical heating of the gas within the enclosure is a concern, include a blowout panel or burst disc, as others have proposed.

What I probably should have stated earlier, is that commercial check valves are built as cheaply as possible.  One way to economize is to use fairly stiff (high K) springs, as these are typically smaller in diameter than low K springs, and much cheaper to source.  The result is a very large difference between the cracking pressure and the pressure at some higher flowrate.

Even ASME rated relief valves suffer from the same problem.

For your purposes, for a <15 psig "vessel", the ASME B&PV rules do not apply.  What you are trying to make is a precision back pressure regulator, not a relief valve per se.  Figure the worst case incoming gas flow rate, and design the valve accordingly.  Or, if the arc/electrical heating of the gas within the enclosure is a concern, include a blowout panel or burst disc, as others have proposed.

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