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caps for acetylene cylinders with coarse threads?

metalmagpie

Titanium
Joined
May 22, 2006
Location
Seattle
I have an acetylene cylinder, about 150 cf, that came to me with no cap. I figured oh well, I can pick one up somewhere. But it seems this cylinder has oddball threads: 3.5-8, not the usual 3.5-11 threads. I found caps for sale at MSC, but now I'm curious. Anyone know any history about the threads used on cylinders where the cylinder caps thread on?

metalmagpie
 
Around here, I would just wait until I exchange the tank. They will charge you something like $15 for not having the cap on the "turn-in". The one you take home will have a cap on it.
 
i turned in a set of oxy/acet cylinders a couple months back sans caps, they got me for
5 bucks each to replace the caps.

not a bad price for having something that takes care of the issue, the replacement cylinder comes with a cap so any oddball thread is their problem.

apparently it really isn't a problem for them as they seem to have caps to fit everything that needs one.

bob g
 
Are caps a leagle requirment over there? There not here, only toxic - special gasses ever have them here.
 
Caps are required to legally transport any high pressure cylinder or any flammable gas cylinder above a certain capacity.

Small acetylene cylinders, like an MC or B size, don't have cap provisions. Nor do cylinders like carbon dioxide and other similar liquefied gases at relatively low pressure.
 
The cap thread is usually specified by the original owner of the tank. For example. The local Airgas store where I used to buy gas had bought up several local small suppliers. Thus the tanks were integrated into the system. One of the old suppliers oxy tanks had course threads & the rest did not. So I had to buy & carry one each of the yellow safety caps for my service truck. That was the reason they gave me, there seems to be no standard.
 
I found what I was looking for - a source. There is a local business that does most of the hydrotesting for the local business community. They of course see tanks that fail, so the guys who make bells buy scrap tanks there. They have lots of old caps. As it happens I have a WWII-era 100-lb propane cylinder which is heavily made and galvanized so it has no rust. Plus it has threads for a cap - its previous owner thought it was a huge acetylene cylinder. I use cylinders like that for the propane burners in meat smokers, so I decided I'm going to go get it hydrotested. They'll just throw in the cap.

I know I could just swap it without a cap. Around here we have 3 main welding supply stores: Airgas, Praxair and Central Welding. The one with the cheapest price on acetylene (Praxair) also is the one who charges the most for missing caps. Sigh.

Some of the people posting on this thread seem to be under the impression that there are many random threads for cylinder caps. Not true. High pressure cylinders use 3-1/8" caps, and they come in 3.5-7 or 3-11 threads. Fuel gas cylinders use 3-1/2" caps, and they come in 3.5-8 or 3.5-11 threads. So for each size there are two threads. Not many, not random, just coarse or fine.

metalmagpie
 
Are caps a leagle requirment over there? There not here, only toxic - special gasses ever have them here.


Yes, I believe that they are. I've not seen a tank without the cap except when in use. Years ago I saw a large GN2 (nitrogen) cylinder fall over when I was working in aerospace. The brass neck broke and the tank went about 50 feet and buried itself in a hill side nearby. It was exciting. LOL.

I have spare cylinders and I store them chained up with their caps on.
 
I have been told by 2 different gas suppliers that the course thread bottles are all AIRCO bottles all of which are rental bottles. So you probably will not be able to get that oddball refilled.

Unless they were lying to me which is at least as likely as not.
 








 
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