UNEF to BSP adaptor - a question.

I want to make an adaptor to connect a gas torch which is designed for use with disposable gas cannisters ( 7/16 x 28 UNEF female thread) to a standard propane cylinder which has a 3/8 BSP union on the end of the hose. I can do the UNEF end OK, but I'm worried about the BSP end: here is a pic of the fitting alongside a torch which it fits:

What I need to replicate is the 'receiver' on the left. It seems that the geometry of the internal cone must be quite critical to ensure a gas tight seal with the O ring on the right. So to my question - is this a standard fitting for which there are specs somewhere? I've drawn a blank so far, perhaps because I don't know the right terms.

Thanks for any pointers, Robin

Edited By Robin Graham on 09/08/2018 22:11:38

Edited By Robin Graham on 09/08/2018 22:12:13

Assuming we're talking about the internal chamfer at the top of the item on the left it doesn't look critical to me. Provided you've got the same sort of angle (looks about 45 degrees) and you've got the same internal bore I'd have thought it would be fine. Presumably the connection is after the regulator on the propane bottle, so quite low pressure.

Andrew

I think you will find that the 3/8" BSP thread is left handed. Would a coupler such as this solve your problem ? You could silver solder it to the UNEF part.

Les.

The only thing i could find was a 3/8 flare fitting that has a cone angle of 90 deg included so maybe chuck up some scrap alloy and cut a 45 deg chamfer then use it to check the angle by putting some blue or sharpie on the seat and lap them together , if it smears most of the ink on the seat i think it will be close enough .

To the best of my knowledge the receiving taper is 60 deg included angle. I've used standard (steel) hydraulic adaptors to cobble together gas lines using this style of fitting. The old fashioned ones didn't have an O ring, they just relied on a metal to metal seal, and the male part was radiused, so I guess the angle of the socket they fitted to didn't matter much. With the new "leak proof" fittings with an O ring, make the receiving taper 60 deg then ink the O ring. You'll soon see if it is sealing on the O ring or on the metal. Don't forget to clean the O ring before passing gas through the fitting.

And Les is quite right, the flammable gas connection should normally be left hand thread. The external hexagon usually has a groove cut in it around the periphery to tell you to undo it the other way (the other right hand). Standard agricultural (hydraulic) fittings are right hand so probably no use except as a pattern for the taper.

Hope this helps

Simon

Thanks for suggestions - experiments along the lines suggested using micrometer blue (I always forget how handy the stuff can be!) reveal that the female 'cone' is in fact slightly concave, so no chance of me replicating it exactly without going to silly lengths. I think the dimensions must be quite critical - the O ring on the male part has to seal before the carrier bottoms out in the cone, but not squash too much.

It'll be far easier just to go with good old-fashioned serrated tails, pressure tubing and clips like wot we used in the Chemistry labs.

Simon - I knew that it was a LH thread, but had wondered about the groove - thanks for that nugget!

Robin.

Yes, I recall seeing a spherical face to the connection for my blowlamp to the regulator on the Propane cylinder.

Even if the faces are not an exact match (producing a long leakage path) the line contact from an inexact match would produce such pressure that the two metals deform slightly to produce a seal. And Brass is relatively soft.

(Which is what the valve/seat machining seeks to do for internal combustion engine valves)

Howard

Ditto! I leant something today already!

Neil

Propane is left hand thread done so that other gases cannot be readily interchanged.

What are we making here Robin?
One stud 28 UNEF with a 3/8"bsp the other end?
Or 28UNEF stud with a female 3/8"bsp?

The taper on the torch 'can' or could be used with olives or more in the case of high pressure air a conical stud goes in to that taper tightened by a floating nut to pull the two assemblies together.
On the right piccy looks like an o ring set below a shallow conical stud like thing, may seal on the left piccy taper at the start only or on the thin front face, just make the thread fractionally longer so it dont bottom out. Just test for leak, you will feel an o ring compress.

If you can remove or slide the female right piccy thread down to expose the o ring you can then test for fitment. You could even have the o ring seal up on a recessed shoulder allowing clearance on the bore for the conical stud.

For the avoidance of confusion:

FUEL gas (propane, acetylene etc) is left hand thread so cross connection between fuel and source of oxygen is impossible. Left hand thread isn't specific to propane.

Rgds Simon

.

I'm not arguing, Simon, just wondering: Why then is Butane not considered a FUEL gas ?

... is it just because it is not used in conjunction with Oxygen ?

MichaelG.

Edited By Michael Gilligan on 13/08/2018 00:09:17

MichaelG - you've asked the 64 thousand BTU question, and I freely admit I don't know the answer. I've got an old butane cylinder with a left hand male thread on it, but I think the modern ones have a right hand thread, or (more usually) a plug in fitting. I assume it's because Joe Public, not reliably knowing which way to turn the nut, was wrecking the cylinder valve and it was deemed the lesser hazard to use a RH thread. But that's conjecture on my part.

It is true, however, that one wouldn't normally expect to use butane and oxygen as a combination so there isn't the same propensity of filling the fuel gas bottle with oxygen. The design of an oxy-acetylene set up is all about keeping the fuel and oxygen paths separate until they meet in the copper nozzle, and one way valves so the gasses can't pass back to the other bottle which would be very exciting! One would expect to use the same set up with oxy-propane (at least for cutting) and the fittings on the bottles are compatible. Somebody is going to point out that the advice is not to pass propane through an acetylene regulator.

To return to the original point, I think you've probably hit the nail on the head - butane wouldn't be used as a fuel gas in conjunction with oxygen, if only because your local BOC (other suppliers are available) depot would refuse to supply the fittings to do so, and the OP's plan to make his own fittings isn't how the H&S tribes see this. Making your own set up begs the question of knowing enough not to create a monster.

Best rgds Simon

I think you will find that on the taper fittings that the male and female angles are slightly different to ensure a line seat type of seal which is far more reliable that trying to match in (blue) the tapers.

Regarding regulators being fuel gas specific you will find that when you buy a regulator from a supplier usually in the box are the stickers for the various differnt gasses that it works with, the usual Propane, Butane and Acetylene and some I have never even heard of!

When I was working we had a huge Chemistery department and we had to make a lot of very odd fittings to connect equipment that came from all over the world. The technician had details of over 70 types of threads and he always found one that he had never seen before!

Regards.