Using an out of service fire extinguisher as an air tank 4

[waskillywabbit]

I'm trying to discourage someone doing such as a "bad idea" and would like to hear your thoughts.

1. Typical red 3 gallon fire extinguisher is one that has been taken out of service due to time limits
2. Top nozzle has been replaced by a welding in a coupling
3. Mounting tabs welded onto shell
4. Operating the fire extinguisher at 100 psi and using it as an on board air system on an off-road vehicle

Thanks.



Brian

 

[SnTMan]

waskillywabbit,

Well, that's just the thing I guess, there's nothing wrong with having a air compressor in your garage. People do it all the time. All kinds of old ones everywhere. Who knows how they get maintained? It's for sure they never get inspected. I've heard people talk about making air tanks from old water heaters. It's a wonder that kind of thing doesn't happen more often.

One trend that bothers me is that as engineering tools have gotten "better", particularily for consumer products, every bit of margin is being taken out of everything to be able sell at the lowest possible price. A little extra steel can be cheap insurance.

Mike

 

[JohnBreen]

Hi Folks,

Well, this thread is realy wandering away from the original topic but I will add this:

Unclesyd is absolutely correct. It was a little known fact that "tank mounted compressors" (TMC's) were having pressure vessel failures all over the country about 25 tears ago. NASA was just getting into their "pressure vessel and pressure system recertification program" at their research centers and they were warned by an engineering consultant who had recently completed strain gage testing (and testing to destruction) of various manufacturer's TMC's. Reciprocating compressors provide a great amount of vibration sitting on top of the air receiver. The testing found that the (rather small and intermittant) fillet welds that attached the compressor platform to the air receiver were the most frequent site of crack initiation that eventually propagated through the vessel's wall and "bang". NASA had all their TMC's modified to dismount the compressor from the air receiver. The next frequent location for (vibration) fatigue induced cracks was at the location on the bottom of the air receivers where the mounting "feet" were attached by fillet welds to the "tank's" bottom - if these were bolted to a concrete floor, it took fewer cycles for the failure to occur. A trade organization had a guide developed to help the TMC manufacturers "design out" the problem. However, now you can find many TMC's on sale that are imported and have not benefited by the design guide. I guess it is just a matter of time before they are in the news again.

 

[panduru]

@JohnBreen:

The design guide you mentioned needs to be given wide publicity and the necessary awareness created in the professional circles.

 

[JohnBreen]

Agreed.

But the trade organization (which is funded by the subscriptions of the equipment manufacturers) paid the consultant to do the testing and to write the guide. The guide is consequently the intellectual property of the trade organization. Have you ever seen the prices (gasp) that EPRI wants for their reports?

When in doubt, dismount the compressor (take it off the platform that is welded to the top wall of the air receiver) and mount it separately. Alternately, examine the platform-to-vessel welds and the feet-to-vessel welds by PT or MT regularly.

 

[svanels]

I have seen the ends of a home designed airtanks been blown off, I have seen the damage caused by an ABC dry powder extinguisher, that some idiot filled with CO₂.
Luckily nobody died, but they have scars to show-off.

If the idiot wants a tank, let him by a descent airtank designed for it. There are a lot of trucks with air compressor, air brakes winches and other air operated stuff, which work close to 125 psi.

Have you ever have seen the classic video of what happens when a cylinder falls and the valve breaks off? It turns into a rocket.

 

[panduru]

@waskillywabbit:

I suppose you must show one such video to your friends. That should help.

 

[moltenmetal]

EdStainless gets my star. The issue isn't the use of the fire extinguisher so much as the welding- nobody should even attempt to make weld attachments, much less welded connections, to anything intended to contain compressed gas! That said, the 3 gallon fire extinguisher (or air receiver) falls under an exemption to our local pressure vessel code and would NOT be considered an ASME vessel.

If the off-roader had used the original threaded connection and clamped rather than welded the "vessel" to his vehicle, his risk would be greatly minimized. As Ed says, using a fire extinguisher designed for 275 psig to contain air at ~ 100 psig isn't the stupidest thing people do on a daily basis. Personally, I'd feel safer with a good quality fire extinguisher shell in my garage than I would with one of the flimsy non-branded Chinese hobbyist pancake compressors! The brand-name people are making them in China too, but they're worried about product liability a little more than the non-branded Chinese factories are, for sure, and it shows in the construction of the tanks.

Good comments about the weld attachments for mounting compressors- definitely an accident waiting to happen if done improperly, and also widely (and poorly) done. Especially on those cheap non-branded units!

I think we'll be seeing a lot of these hobbyist units blowing up in about ten to twenty years. Typical scenario would be an explosion when somebody re-starts one of these units to fill a tire after the tank has been sitting in their garage for ten years, undrained and silently rotting from the inside out...