Does such a relief valve exist? 1

[ShaggyPE]

I am searching for a relief valve for an argon and/or helium laboratory aparatus. We have an expensive piece of equipment in the circuit that we do not want to over pressurize. The pressure that we have determined to be the maximum is .03 psig. Have any of you guys used a relief valve (preferably a pop safety) or rupture disk in this low range.

I appreciate any info
Thanks,

Dustin Biber
Optivus Technology Inc.

 

[iainuts]

Might be easier just to have a verticle tube with a thin rubber sheet over the end and a light weight on it which gives you the same affect. A 'weight actuated relief valve' if you will.

 

[pennpoint]

Shaggy18VW

You might want to look at these sites.

http://www.halky-roberts.com

http://www.agmcontainer.com

High precision, low pressure valves.

I have used products from both manufactures with very good results.

Good Luck

pennpoint

 

[pennpoint]

Shaggy18VW, et al.
Sorry I miss-typed my first link. It should have been:

http://www.halkey-roberts.com

Regards
pennpoint

 

[jte]

Shaggy18VW-

0.03 psi is about 1 inch of water if I did the conversion right. For a laboratory setup, would it make sense to bend some glass tubing such that it holds 1 inch of water? Hook it up to your apparatus and if the pressure exceeds the 1 inch of water it simply blows the water out of the tube. Just keep an eye on the water level and refill it when it evaporates. We have some similar devices here (though industrial sized!) which blow at 1.8 psig.

jt

 

[CHD01]

Relief valves are out of the question. Rupture discs will not be that accurate for such a low pressure. A weighted vent is at its lower limit for such a low set pressure. The use of a static liquid head is the best approach.

The more you learn, the less you are certain of.

 

[quark]

Excellent suggestion by CHD01 as usual. Get the star.

Regards,

 

[sailoday28]

Have you considered what is causing the overpressure. Could a pressure/or temperature sensing device interact to prevent the overpressure?

 

[ShaggyPE]

Thank you all for the suggestions. Unfortunately my design requirement is not quite as "laboratory" as would allow a liquid device. This initial design would allow for such a device, however when we complete our testing of the equipment we need to move to an industrial application. When the final system is put into place, the relief valve will be on rotating machinery. In other words, any liquid will drain out at some point of operation. Also, we can't have the constant maintenance issue.
I like the idea of a sensing device interacting to prevent the overpressure. I currently require a pressure transducer and temperature detector (RTD) in the system to measure the T & P in the equipment. I suppose it is possible to have a shut off valve that will be told to close when the pressure reaches .03 psi. I like this idea for the industrial application. I think it may require a bit of software that may not be available in time for the test. If this is the case, I can probably use the liquid methods mentioned in earlier posts.
Does this seem plausible?
Thanks again

 

[ShaggyPE]

After talking with some collegues, I will probably use a solenoid valve actuated by a pressure switch.

 

[pennpoint]

Shaggy18VW
A pressure switch with an actuation point so light (.03psi) will probably not handle the inrush current of a solenoid. A low inrush coil relay may be called for in between. The switch action will not be a snap-action type for sure therefore low current capabilities only.
Be aware of the hysteresis of the pressure switch as this will result in a sloppy solenoid response.
You might also concider a solid state type pressure switch from Omega.

Regards
pennpoint

 

[ShaggyPE]

The Omega Low Differential Pressure Switch SPDT relay output is actually what I am looking at (PSW-141-HR). It seems it like it will do the trick.

 

[CHD01]

I don't believe you understand what the static liquid head device consists of, it is similar to the plumbing beneath your kitechen sink with what some call a pee-trap that is always full of liquid that cannot drain. It can evaporate, but that can be rectified by installing a liquid level sensor with a liquid make up auto valve.

The more you learn, the less you are certain of.

 

[ShaggyPE]

What prevents it from draining if it is on rotating machinery? If I turn my sink upside-down, any liquid left in the trap will come out the drain. Is a static liquid head device something off the shelf, or will I have to design my own. Who are some sources? Thanks for the info.

 

[CHD01]

Yes, normally the engineer designs it incorporating it into the pipe system. But it is always placed in a fixed position, it should never rotate. I don't understand why this protective device cannot be installed on the inlet or outlet piping to the rotating element.

My experience is that when we get into design situations like this we are trying to make somthing work that is not the best solution. Usually, in this situation I try to look outside the box and find a much different way to do it.

The more you learn, the less you are certain of.

 

[sailoday28]

Can the device be subjected to negative pressure? If so, couldnn't this possibly cause the "loop seal" pee trap to have a reverse flow or drain the seal flow into the device?

 

[ShaggyPE]

I may have mislead you when I went along with the terminology "liquid" in my last post. I am using argon at a low flow rate, about 450 sccm. Will this nullify the "static liquid head" device?

 

[CHD01]

No, a static liquid head device can be used; but the liquid used must be compatible with Argon and your process. Potential for a vaccuum is always a concern; and is another reason for a liquid makeup control system.

The more you learn, the less you are certain of.

 

[ShaggyPE]

Where can I find info on designing such a device. When I do a google search for "static liquid head" I get too much irrevelant information. Thanks again

 

[CHD01]

This is basic engineering. Refer to your fluid dynamics textbook. The height of liquid head required is simply equal to the desired set pressure in psi divided by 0.4335 psi/ft

INWC = (12xPset)/(0.4335)

So for 0.03 psi set point

Required INWC = (0.03/0.4335)*12 = 0.83 inches of water.

For fluids other than water adjust required static liquid head in proportion to the liquid density used. Note this is independent of the type of gas used and the diameter of the pipe or tubing used. Normal liquid level in hte U-tube loop will be 1/2 that required, so that the liquid seal is broken at the required pressure.

One last caution, I still think you should be trying to find an entirely different way to do this, the height of liquid head needed is so small I would be concerned about equipment vibrations causing occasional releases.



The more you learn, the less you are certain of.