Need 15psi soft seal pressure relief valve

[stballard]

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.

Thanks,
Steve

 

[stballard]

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.

 

[curtis2004]

stballard,

What model of Kunkle valve have you got?
I haven't dealt this kind of issue, and have question:
Do you need to supply PRV for vacuum too? I though PRV works only for excessive pressure.

regards,

Curtis

 

[vpl]

stballard

You may want to ask this over in forum408. That's where the valve folks "hang out."

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[stballard]

The pressure relief valve only needs to pop at 15psi but it does need to be able to hold a vacuum. If the relief valve used a hard seal like bronze it would leak when put under vacuum.

 

[GenB]

how much vacuum, I use some soft seat o low pressure,
snd can go up to 13.5 psi w/o popping.
genblr

 

[moltenmetal]

Put a NON-FRAGMENTING rupture disc under it, or eliminate it entirely with a rupture disc. Even a soft seat relief valve will leak inward under vacuum.

If you do put a disc under the valve, make SURE you put a proper means of leak detection in there between the disc and relief valve inlet. There are numerous options for what to do there, and which one you choose depends on how frequently it will be attended and how probable leakage through the rupture disc is.

 

[GenB]

"It can't start weeping much before 14.7 psi."
too close for comfort,
even soft seat valves will start weeping at 14 psi and pop at 15.- what's your vacc7um pressure?
genble

 

[stballard]

If it starts weeping at 14psi that is absolutely fine. the one I have weeps at 11 psi (if I'm lucky) and when pressurized and left overnight it doesn't hold any pressure (and yes I leak checked everything with a helium leak detector, it's not my fittings). The vacuum pressure will be around 10^-3 torr which I stated in my original post. We don't want to use a burst disk because we want a usable pressure of around 14 psi (no higher than 14.7 psi which is about 1 atm) and we don't want to lose all of our expensive SF6 if we accidentally go a bit over.

I am just looking for a reliable 1" soft seal pop safety valve that will hold a pressure of around 14psi and pop at around 15psi. It's great that your valves work well but it's pretty useless to me if you don't tell me what kind or send me a link.

 

[moltenmetal]

stballard: such an animal DOES NOT EXIST, so don't get mad at us for not pointing you to it!

A relief valve typically simmers (i.e. starts relieving) within 90% of its set pressure unless it is a very special design- in your case that's 13.5 psig. I would imagine a low pressure soft seated valve will probably simmer even lower than 90% of its set pressure. As to inward leakage against vacuum, they're going to leak.

Either be satisfied with slightly lower pressure or trip over the 15 psig magic ASME limit- it's your choice. You can't have your cake and eat it too!

 

[btrueblood]

I can design you a valve that would do as you ask, but getting one valve built to that design would cost quite a bit. I'm sure you could do an equally adequate design using the Parker O-ring handbook, a Lee (or similar) spring catalog, a spreadsheet, and the CAD software (or pencil and paper) of your choice. The trick is getting a low spring constant and keeping that spring from buckling.

 

[vpl]

At the risk of being obnoxious, may I again recommend talking to the valve folks (including some valve manufacturers) who frequent the valve forum (forum408).

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[stballard]

I was trying to follow the forum policies as outline in your signature. I posted the topic in that forum with a link to this one and it promptly got deleted. I think I'll try and restart the thread over there this time.

 

[GenB]

steam equipment Co. in Los Angeles,
talk to Alex stocks soft seat valves.
you can buy one really cheap and test.
But why not Get an ASME cert. vessel,
how big is the unit that you want to get around a certification.
if small it should not be that expensive.,
ask around.
Ican not -post any commercials but, seems easy.!!
genblr

 

[btrueblood]

vpl,

He did, posted a link back here. It's what he is supposed to do, according to the posting guidelines.

 

[btrueblood]

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.

 

[btrueblood]

Ok, and one last comment. Sorry for the disjointed posts, but I haven't had my 2nd cup of coffee this AM.

One reason you are struggling is that you probably bought too small a check valve. If you want a closer limit between your flowing and cracking pressure, purchase a check valve that is too large by about a factor of 10, i.e. if you were using a 1/4" pipe size valve, go buy a 1.25" or 1.5" check valve, and you should see the two pressures move closer together. I could give you reasons for this, but it's pretty simple math.

 

[stballard]

I definitely did not buy a too small check valve. The designer and head engineer spec'd a 1" NPT pressure relief valve. The whole valve is about 6" tall, way overkill but he could not be convinced otherwise. The flow is ridiculously high at 178 SCFM, far more than we need.

The problem is it doesn't hold any pressure. When we left the valve pressurized overnight it had 0 psi of pressure. If it held even 11psi that would be acceptable although not ideal.

I did design a very simple valve using an O ring and a spring but it would cost far too much make it and the head engineer wouldn't even accept it because it's not certified. The valve we bought cost $155. For a custom valve (even the very simple one I designed) the machining would take at least 2 hours billed at $89/hour which would be more expensive without even taking material cost into account.

 

[vpl]

Here's the link to the thread in the valve forum: thread408-275599

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[btrueblood]

"The problem is it doesn't hold any pressure. "

What material is the seal? Is it an o-ring, and if so, can you source a butyl rubber replacement. Butyl is much less permeable than the (probably) nitrile rubber that the seal is made from. Also, check if any dirt/debris is on the seal. Finally, get some silicone grease and smear on the seal. Last - are you SURE the valve, and not some other part of your system, is the leak path?