Pneumatic burst valve - what's it's real name?

[loply]

Hi folks,

I'm building a box which will contain a fluid and then be pressurised by my compressor.

It only needs a few PSI and I will regulate it down, but there's a danger of accidentally charging it highly.

The box is welded and this would be dangerous so I want to install a little brass fitting which has a disc which ruptures if the pressure exceeds a certain amount, as a safety thing.

I believe these are called burst valves and thought they were really common, but for the life of me can't find them on ebay and I now think I have the wrong name for them!

Would appreciate any pointers.

Cheers,
Rich

[Chazz]

Do you mean 'Relief Valve' like used on hot water tanks?

Cheers,
Chazz

[Davo J]

Try pressure relief valve. For only a few PSI you could make one easy enough.
You will need one that will let the air escape quicker than it will fill from the compressor to be safe.

Dave

[daz]

I think you are looking for a fusible plug? We use them on refrigeration circuit instead of a safety valve. Normally quarter or three eigth bsp brass fitting with a lead core soldered into the middle. Can't remember seeing one as low pressure as that though. If that doesn't come up with an answer on google check for pressure safety valve.
Daz

[daz]

Forgot to say that the size you will need will be 1.6 times your working pressure and make sure your welds on the tank have full penetration. If this kit is on your premises youy might also need an insurance inspection to keep your home insurance ok

Daz

[75Plus]

Google "Rupture Disc" then separate the mechanical from the medical.

Joe

[Stilldrillin]

Would a toy steam engine safety valve do the job?

http://www.mamodparts.com/spares-Safety_Valve.asp

Usually rated at 15psi.

David D

[David Jupp]

Forgot to say that the size you will need will be 1.6 times your working pressure ...

1.6 times working pressure seems very high for a relief device.  I'd expect the test pressure for the container to be something like that.

The relief device should be sized (discharge area) such that the container can't be overpressured (typically no more than 10% above max working pressure) - make sure it is big enough, remembering that there is less driving pressure to get the air out than you compressor could have available to force it in (unless fitted on inlet pipe).

Depending on the rupture disc style, there may be the potential for fragments to be ejected on operation.  Consider where these might fly to.

[John Rudd]

You could incorporate a bursting disc  into a small pipe section such that if it ruptures then the air is dissipated to a safe location...

The bd need be no more complicated than a disc with score lines across the diameter to create the weak point, mount the bd between two flanges connected to your piping arrangement....the disc doesnt have to be made from metal! You could work out somewhere near the force required to rupture based on the surface are of the disc and the desired rupture pressure...
On other thing, the diameter of the relief line needs to be bigger than the supply...

[daz]

 :doh:You are right of course, I was getting a bit mixed up. Pressure test is 1.6 times max working pressure, safety valve should be around 10-15 percent higher than normal working pressure. It's been a while since I had to do anything like this.

Daz

[loply]

Hi folks,

Thanks for the useful replies.

The kind I am looking for is the kind I have seen in the past. They are very simple non-moving parts, it is a screw-in thing that resembles a greese nipple, but it has a disc approx 3-5mm in diameter which is either copper or a tin-foil looking substance. I imagine they are about 50p each and I like the idea of them because, unlike some mechanical relief valves, I don't see how it could fail to work.

I thought these things were as common as muck, but maybe I'm wrong.

Having had a good look around all I can seem to find are mechanical valves. I'm sure these are pretty reliable but I would the type above! Just need to locate a supplier.

Ultimately I guess it won't make a lot of difference though.

Rich

[David Jupp]

Will cost much more than 50p - They are critical safety devices, hence PED approved (in the EU), a surprising amount of technology goes into ensuring they always work at the right pressure.

[John Rudd]

So let's just recap a little here...maybe we are over complicationg things..

You have a welded box, has the box been subject to any ndt to ensure it is safe to use as a pressure 'container'?
So there is one safety feature if it is depending on what it is fabricated from of course..
Secondly you are supply compressed air from a compressor? So here we have a machine capable of reaching lets say for arguments sake 100 psi...if the pressure switch were to fail, then the backup is a relief valve...So potentially your box could reach a pressure which is essentially relying on an RV...
If the liquid were to be discharged due to a loss of containment, what are consequences? This is the ultimate....unless there's somethnig I've missed...

[David Jupp]

If the box is totally filled with liquid, then the hazard due to stored energy is not great - if there is a head space of compressed air (say if air is used to drive the liquid out) then there is much more stored energy involved (to drive parts of the box towards you at high speed if things go wrong).

You should also consider mal operation (e.g. could it be pressurised without any liquid inside?).

I don't mean to be unduly alarmist...

[fixerup]

Hi Rich,
Nest time you are in a coffee supply shop, have a look at there stove top expresso coffee machine, they have a small over pressure nipple like you describe.
Cheers!
Phil

[daz]

Youre right David, the pressure test is 1.6 time the max working pressure, the safety valve should be around 10-15% hugher than the normal worming pressure.
What you could also do to alay any fears or explsion is to fit a very simple pressure switch to the tank, you can buy them pre-set and adjustable. That way the pressure switch will interrupt the compressor at the correct pressure, if that fails then the safety valve will pop. I have NEVER seen a pressure switch fail in the open position so all should be well. I have been working as a refrigeration engineer for quite a few years before the job I am in now and also used to repair compressors. The set-up I just mentioned were used all the time on both systems and always passed the insurance examination. Oh, and some of the system I have worked on run at silly pressures, sometimes 30 or 40 times what you are proposing.
If you need any more info regarding pressure switches etc feel free to contact me.

cheers
daz

[loply]

If the box is totally filled with liquid, then the hazard due to stored energy is not great - if there is a head space of compressed air (say if air is used to drive the liquid out) then there is much more stored energy involved (to drive parts of the box towards you at high speed if things go wrong).

You should also consider mal operation (e.g. could it be pressurised without any liquid inside?).

I don't mean to be unduly alarmist...

This is precisely what I'm wary of. The box will have a regulator where the air enters it, but clearly it could be accidentally tampered with or could fail in some way or through some other mistake the thing could end up at 100psi with or without liquid in it. Shouldn't happen, but you never know!

I thought a burst valve nipple type thing permanently threaded into the box somewhere would be the best do-it-and-forget solution. The regulator could be removed or fail etc, whereas the burst valve would be more or less foolproof.

I just want a solution which I needn't ever worry or think about. Will keep trying to track down somewhere to buy em!

[Lew_Merrick_PE]

You are basically making a "burst plug" for you system.  I use them with fair regularity in gas generators to insure that the gas inside the chamber has reached a specific pressure before being passed along to the rest of the system.  In this application, they are called "burst shim."  The real question is, what exhaust flow rate would you need for safety?  That establishes the size of the hole (orifice) you need.

I normally pay to determine exactly what the ultimate tensile and shear strengths are of any material being used for this type of application -- and then only use part of the sheet I had tested for my burst shims.  Book values for these ratings will either by typical or minimum -- and there is a world of difference and variation between a book value and what you actually get!  Book values will often leave you ±15% or more from your desired end.

Given your pressure and the fact that your ullage for the system is compressed air, it would be a lot simpler to get an off-the-shelf pressure relief valve and calibrate it with a ±1% pressure gauge.  Just be sure to evaluate the Cv of the relief valve against your total required flow for the worst case scenario.  Also, remember that the SCFM listed at whatever the rated pressure is represents the volume of air at atmospheric pressure and not the operating pressure (and remember that the values for figuring this should be represented in PSIA -- PSI-Absolute -- rather than PSIG -- rather than PSI Gauge)!

[naffsharpe (Nathan)]

Hi, We used to use a burst disc fitting on pressure wash pumps and also on small pressure accumulators. For most pressure washers the pressure would be way above your requirements but I do recall that they were available in quite low burst pressures. We used to source ours through Hayley Bearings ( UK ) trading as Bearing Man. This was however some time ago. The fitting itself was just a Wade type bsp/cone compression fitting with the burst disc replacing the compression "olive", disc material was normally plastic of some kind with heavy score marks to allow it to rupture. You could easily make your own and test it on your air line.   

Nathan.

[Pete.]

The reason pressure washers use bursting discs is because they are positive-displacement pumps and the disc is there to protect the pump hardware should there be a blockage in the gun. A relief valve is better for a compressed-air system.

[Fergus OMore]

I assume that you are Newcastle- that is 'on Tyne'
Why not try Shiremoor Compressors in Front Street, Seghill?

It's a long time since I dabbled in compressors but they were very helpful then.

[loply]

Hi guys,

Thanks for the replies.

Fergus, I had no idea it was there, will remember in future though.

In the end I made a burst valve from two parts on the lathe and a tin foil insert. I tested different sizes till I found one that consistently splits at about 80psi.

To replace it you just unscrew the top cap, cut out a new bit of tinfoil and screw em back together again!