Gas-tight fittings

[Cursitor Doom]

Gentlemen,

There's a system of joining under-pressure gas/air pipes which involves no PTFE tape nor sealing compound nor gasket. The sealing principle relies solely on a mushroom-shaped dome seating up against an inverse, mushroom-shaped dome. They're pressed together by a screw collar which generally requires very little torque to effect a complete seal. Most commonly you see these on large industrial gas cylinders where they're coupled to pressure gauges. Now, there's a much smaller version which uses the exact same principle and commonly uses a 1/4" BSP collar to effect the seal. Can some erudite individual kindly inform me of the correct nomenclature for this kind of union, please? Thanks.

[Richard]

[newshound]

We are all, I think, familiar with the "gas cylinder" connector but I have never seen the smaller sort.

The metal to metal seal principle is exactly the same as found in compression fittings using olives.

I can't help with terminology other to say that I would call them a type of compression fitting.

The "quick release" fittings on air lines and gas cooker bayonets actually rely on an O-ring seal: a different principle altogether.

[The Natural Philosopher]

used to couple brake pipes in a car

dont think it has a special name. i've always called them flares

[PeterC]

Used in refigeration, where contamination can be very bad. Just one big potential disadvantage: forget the backnut and you've had it! I did that on 1 1/8" pipe and had to put in another bit by swaging (also common on refigeration).

[Cursitor Doom]

Thanks, guys, but you're all wide of the mark on this one. Possibly my description was crap. Here's the actual coupling I was after and it doesn't rely on any of the principles y'all thought it did:

[Cursitor Doom]

Still don't know the name for it, though! :-(

[newshound]

Neither do I, but it *does* rely on exactly the same principles as used on gas cylinders, like I said.

[The Natural Philosopher]

same principle as a brake line tho. Just done in brass

[SH]

The connector found on gas cylinder connectors is called POL :-)

Very common on Acetylene, Oxygen, Propane, Butane, Hydrogen, nitrogen wtc.

Be aware that there are left hand and right hand thread versions simply to avoid mixing up regulators and gas bottles so that flammable gases and oxidisers do not meet..... :-)

[SH]

Think the OP is looking for this?

[SH]

As a side point without knowing the OP's exact project, I would be very wary of connecting any form of pipework to a gas cylinder directly without a regulator......

The cylinder's internal gas pressure when supplied new can be as high as

200 Bar which is about 3000 psi.

[newshound]

One internet source suggests that POL is "put on left" (so only the flammable gas version).

Not sure if I believe that. POL is also given as Petroleum, Oils, and Lubricants and while it is a suitable coupling, that does not sound quite right either.

Someone here must know!

[Cursitor Doom]

I don't think you have the same kind in mind, Newsy. The ones I'm attemtping (very badly) to describe are infinitely re-usable unlike anything requiring olives or any other system which typically involves deforming metal or rubber to get the seal. There's no deformation here.

[newshound]

Indeed, though many camping stoves using butane manage without a regulator, as does my plumbing blowtorch on to a 1.5kg propane cylinder.

So no problem for LPG, only an issue for permanent gases (also carbon dioxide, acetylene).

[Cursitor Doom]

Oh, I see what you're driving at here now. Very very similar, but not

*quite* the same!! Copper deforms readily to get the seal, the brass in this type doesn't deform at all, nor is it required to.

[Cursitor Doom]

Yup, SH is on the right lines. BTW, this will be used on the low pressure side of a reg with typically no more than about 12psi involved. What's the bleedin' name for it???

[Robin]

is POL what you have in mind?

[Robin]

D'oh. The name had been nagging at me. But how the f*ck did I not read what SH wrote etc . Sorry.

[newshound]

First a bit of basic physics. Whenever there is *any* load at a contact, there is *some* deformation. Maybe not much, but some. Initially, it is elastic.

One problem is that I am a tribologist, and you are not. You are quite right that O rings are (more or less) infinitely re-useable because they seal by elastic deformation. Olives have a more limited life (especially the cylindrical soft copper type like Simplifix, less so for the hard brass ones used in plumbing). But there is some (not very much) plastic deformation in the contacts in a plumbing fitting, and in "gas cylinder" ones. The fact that these have, in practice, a pretty long life in not very benign use is a tribute to the optimisation that has been done in the design. You need to do the sums, but it turns out to be not easy to obtain a purely elastic contact between metal components under any useful load. This means very smooth surfaces and very hard metals. Gears and ball bearings approximate to this, but you still get plastic deformation at the grinding groove level, especially without lubricant.