Another Oxy Acetylene newbie question.

By dee

Date 01-18-2004 18:06

First, welcome...
I may be too casual about it but I have found even a single flashback arrestor enough to cause more problem than they're worth (not really true, but it gets the point accross) and I am quite remiss in how promptly I replace mine after they fail, and they often fail around 6 months of what some here would cosider almost no use. Fact is, I probably spend more time without one than I do with one on my gear.

If there were a safety heading available to us it would be a nice forum to open this discussion. It may get lengthy, so I hope your answers don't get hidden. Your "simple question" may excite more stimulating controversy than you expect.

I have little use for flash arrestors. Salesmen at the weld supply shop who have attained their sales quota suggest they're unnecessary, logic suggests the apparatus would not be approved if they were necessary, they're a nusance, and they often cause problems with the flame. Nevertheless, when I just HAVE to spend some money to maintian my repoir with my weld supply guys they are as good an excuse as anything else I don't really need. How's that for reckless abandon?
Seriously, I wonder what statistical data exists supporting their need or effectiveness, or OSHA regulation I've overlooked. The early training I received and the ancient books I have both neglect to mention them.
Mine is perhaps old thinking and fresh opinions might save me a trip into low earth orbit someday.

I'd say Victor knows what they're talking about and you don't need additional gadgetry. A good rule of thumb is to follow manufacturer's advice, and permit their experience and testing to put to rest all contention.

Less controversial I am sure is how to determine if your fittings require a sealant. The fittings on the gas apparatus you have do not use the threads to create the actual seal. If you think of the mechanics behind a flair or compression fitting you see how the seal is created by a tight, metal-to-metal contact between a fluted and a tapered set of rather soft parts (brass, Cu, etc.) in the fitting; they conform closely to prevent any permeation between the two surfaces. If you look closely at the various fittings on your apparatus you will see the same principal of operation. They are designed to work without sealant, and it makes sense... I'd want to shoot the moron who contrived a system by which I needed to poke around and fuss to install something switched around as regularly as hoses, tanks, or gauges. These fittings must not be ovrtightened (nor should the adjustment needle valves in your new torch) lest damage occur to the sealing surface and they seal no longer. Keep them clean, use no lubrication or sealants, and be careful of their proper alignment both before and while snugging them to service...

...by contrast, were you to remove a gauge from your regulator you will see the seal is achieved at the threads themselves. These pipe threads actually are tapered; the leading end of the pipe is a smaller diameter than the innermost (top) of the threads. It does not provide a positive seal against leakage without some help.

Of the various thread sealants (find Loctite/Permatex's site) the type I prefer LEAST is teflon tape. Little bits of it are known to enter equipment and cause trouble; some pumps I have installed warn that it's use viods warranty, and it does not seem to permit the latitude "pipe dope" type sealants permit when aligning parts (to make the gauge point forward, for example) and I've seen it's [mis]use contribute to splitting fittings. I just dislike the stuff and cuss at it whenever I wind up using it... a dap of "permatex" and I'm confident it's leakproof; some wraps of PTFE and I'm in suspense 'till testing and an occasional redo.

Regards
d

By Hooks2

Date 01-18-2004 18:52

Thanks for your response dee. The trade schools in my area don't offer any oxy/acetylene welding classes so I really appreciate all of the knowledge that experienced welders like yourself are willing to share.

The manual that comes with my torch says to check the safety valves
occaisionally but , unless there is something I missed, it doesn't explain how to test them. I would guess I could check a reverse-flow valve by blowing compressed air through it in the wrong direction to see if it passes through, but how would someone test a flashback arrestor ?

You feel that flashback arrestors are basically unnecessary but how about reverse-flow valves ? I'm still not sure that I understand the difference between a flashback arrestor and a reverse flow valve. Aren't they both one-way valves ? Wouldn't a reverse-flow valve stop a flash back ?

By dee

Date 01-19-2004 04:43

Hooks,
There are areas I know, and there are areas I think I know. I get kind of sensitive about the prospect of disseminating misinformation. I'm less than clear on details about check valves and flashback arrestors. I own no reference to them and cannot be considered any kind of authority.

I dont know if arrestors (or check valves if that be the designation) are effective, nor do I know them to be necessary. I dont have enough data to form an opinion.

Those I have purchased do not describe any service procedure. I would be cautious about use of compressed air for two reasons... excessive pressure can cause damage and risk of oil contamination can cause fire.

Fact is I use small torches most often, and I suspect the very low flow rates they demand test the low limit of the valve's design limitations, no matter by what name we call it. For me they often get in the way. No safety device is ever "necessary" per-se, but as safety goes, how does one properly calculate the risk? I understand the risk to be more an issue of damage to equipment than personal injury or explosion, and welcome any effort to correct misunderstanding if any exists.
To avoid being hypocritical, I don't condone removing them where they are an integral part of eqipment, and I think they are probably a good idea to (at least TRY) to retrofit despite the fact I am not aware of any data to show their need or efectiveness... and that describes my practice.

When a flashback occurs it's usually because regulator settings are way out of recommendations for the torch set. I've read about flashback being able to run all the way back to the regulator, and I'm aware the incredible is too often true. What I was trying to communicate is that I have been rationalizing- I think it cannot happen to me. I address flashback risk preventatively.
I prefer to think of myself as safety concious and self aware, and recognize I may be doing something dumber than I wanna be.

I too am awaiting responses. Several of us here have had hopes of seeing a new forum to cover various safety issues, and this topic fits in very well. I believe flashback was discussed recently (past 6 or 8 months?) under context of torch shut-down procedure.

Regards
d

By aircraft (**)

Date 01-20-2004 00:20

An arrester is a screen type device that will still allow reverse flow; of course a reverse flow check valve is self-explanatory. On a Victor both are needed because of the differences in pressure and how they mix the gases where Smith uses the same pressure on both lines (except for cutting and even then there is not as much difference as Victor) and they mix the gas at the tip instead of in the handle. I still use a check valves on Smith just for operator error but an arrester is not needed.

This also explains why on a Victor you turn the oxygen off first and on Smith you turn the Acetylene off first.