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Using a propane cylinder for partable compressed air.

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Glyn Davies04/04/2019 14:59:01
106 forum posts
20 photos

I visited the US back in the 90s and saw some portable compressed air tanks for sale in Walmart. Never seen them here and often thought how useful one would be around the car and home. Anyway, I recently acquired an empty 3.9kg propane cylinder and thought I’d try it. It works a treat. When filled with compressed air at 150psi I can top up eight car tyres with ease before it empties. My question – can anyone see any safety issues with this that I’ve missed? I thoroughly purged any remaining propane from the tank before I filled it with compressed air and I see that the vapour pressure of propane is 150psi at 32 deg C, so the tank must be designed for this level of pressure. I tried unscrewing the valve on the tank so I could fill it with water and proof pressure test it to say 300psi, but the valve is too tight to unscrew.

Tim Stevens04/04/2019 15:11:33
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993 forum posts

It would be helpful if you could paint a big label saying 'Air only' - but certainly it will stand the pressure.

Cheers, Tim

Jeff Dayman04/04/2019 15:16:07
1445 forum posts
37 photos

You are guessing about the safe pressure rating of a pressure vessel that has contained flammable hydrocarbons. This is dangerous and possibly illegal. Don't do it. Hydrocarbons can continue to leach off the inside surfaces of propane tanks for a long time after they are emptied, so you can get an air/fuel mixture exiting the tank rather than just air. If the tank does not have a functioning pressure relief valve it is dangerous.

Ask yourself what would have happened to you if it had exploded during the test while you filled your 8 car tires.

Edited By Jeff Dayman on 04/04/2019 15:16:43

Andrew Tinsley04/04/2019 15:50:56
859 forum posts

Not sure how this practice can be illegal, unless the rental agreement for the cylinders specify this to be so. As for safety, people have been doing this for as long as I can remember without anything untoward happening. Just make sure you have a drain valve installed if this is for long term usage.

Andrew.

Robert Atkinson 204/04/2019 16:12:05
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167 forum posts
13 photos

As a private individual I can't see anything illegal as long as you own the cylinder, except possibly CE marking and test, but these are rarely if ever applied in practice to constructions by indidividuals. There are a couple of concerns though:
1/ if there is any propane remaining in the tank you could get a compression ignition explosion but you would have to fill it really fast.
2/ unless you are using dried air, condensate could cause corrosion and failure. Drain valve required.
3/ commonsense says fit an over pressuere device. I'd prefer one at 150PSI but the one built into the existing valve is better than nothing unless you have damaged or disabled it
4/ And this is a big one, if the worst should happen your insurance company(ies) may not pay out if they think you did not take due care or cted recklessly. This could have you losing your assets or your dependents not getting life insurance.

Robert G8RPI.

Nicholas Farr04/04/2019 16:34:28
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1769 forum posts
881 photos

Hi, as has been said, you must own the cylinder to start with. If it is not a disposable one, then it most likely is the property of whoever supplies the gas that it contained and you would be causing criminal damage if you used it in such away without the owners consent. As Robert has pointed out, liability is the next thing to consider and although it is not illegal to make such a thing, no insurance company is likely to cover you or it's use, so if it does explode and levels your shed, workshop etc. you will get no compensation, but you may get shelter via HMP if someone else gets hurt. It certainly would not be allowed in any commercial work place.

Regards Nick.

Brian Sweeting04/04/2019 16:41:12
340 forum posts
1 photos

Your cylinder should have the test pressure stamped on it somewhere.

It's probably a low pressure cylinder so the test pressure would have been around 480 - 500 psig.

Nick Clarke 304/04/2019 17:17:49
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199 forum posts
3 photos

As a student 40+ years ago I paid my way with a number of jobs including labouring at backstreet garages/car workshops.

One used an empty propane bottle for just this purpose and both partners are still with us, still working and each with the right number of arms and legs.

As to legality - well it was at the iffy end of the motor trade so make your own mind up.

SillyOldDuffer04/04/2019 17:22:20
3996 forum posts
810 photos

Agree with Robert and Jeff, proceed with caution!

The difficulty is what we don't know the characteristics of a Propane Cylinder when used outside specification. For example, filled as intended the cylinder's metalwork doesn't have to cope with corrosion. Compressed air is very different. We don't know how well, or badly, the cylinder will resist a combination of water vapour and compressed oxygen. Plain mild-steel would be vulnerable. How long before it perforates or a seam let's go? Dunno.

It's probably not illegal to fill a propane cylinder with compressed air in the sense a specific regulation forbids it. (It's possible - consult a legal expert specialising in pressure vessels!) But in the event something goes wrong there are plenty of laws that can be applied particularly if someone else is hurt or suffers property damage. Endangerment, Recklessness and Manslaughter are all crimes in most administrations. And the embarrassment! The best defence to a charge of reckless endangerment is to prove lack of capacity, ie you're so daft you can't tell right from wrong. Your brother-in-law will tease you forever, and SWMBO could close the workshop.

Despite sounding like a Health and Safety zealot, I like to be pragmatic in my workshop. I do a bit of research, assess the risk and have some counter-measures ready if it goes wrong. In this case, the internet tells me Propane Bottles are rated between 100 and 200psi (puny compared with a Carbon Dioxide cylinder). I notice Glyn's 150psi is high for a low end propane bottle. However, I'd be happy to fill one to up to 100psi a few times as a temporary measure. What I wouldn't do is incorporate a propane cylinder into a permanent installation and expect it to stay safe for long.

Not tried to calculate the amount of energy in a 3.9kg capacity cylinder at 150psi but it's relatively low compared with a bomb. The bang could deafen and splinters could blind. Worst case is probably the valve coming off allowing the cylinder to accelerate over several metres before hitting your head. Clamping the cylinder down inside a metal screen and wearing eye and ear protection would be simple enough.

Roberts point about insurance is a good one too. Check the policy, like as not this sort of adventure isn't covered.

Dave

Vic04/04/2019 17:25:15
1992 forum posts
10 photos

Its probably safer to use this then. smiley

**LINK**

Andrew Tinsley04/04/2019 18:13:01
859 forum posts

Well now, doesn't that just look like a propane cylinder in disguise!

Jeff Dayman04/04/2019 18:34:57
1445 forum posts
37 photos

It may look like a fuel cylinder, but I see a welded-on cert tag and a PRV as well as a gauge. Welds look substantial too. Most importantly it is designed and made and rated by the manufacturer as a compressed air tank.

Rockingdodge04/04/2019 18:40:20
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77 forum posts
10 photos

I question the argument re corrosion when compressor tanks use the same/ similar metals and I suspect that propane tanks are of a thicker gauge than the air tank sat in your workshop. How many of those have been tested during their life and how many of you religiously drain your tanks on at least a weekly basis?

Propane tanks are tested to approx 400 psig on a 10 to 15 year cycle and they are not bought they are hired and should be returned after the need has been fulfilled.

Edited By Rockingdodge on 04/04/2019 18:42:33

martin perman04/04/2019 18:58:06
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1519 forum posts
64 photos

When I need more volume of compressed air I use a truck brake tank, bought new, which carries a plate with requied info stamped on it, when used it is always connected to the compressor and I dont use it as a stand alone.

Martin P

SillyOldDuffer04/04/2019 19:03:44
3996 forum posts
810 photos
Posted by Rockingdodge on 04/04/2019 18:40:20:

I question the argument re corrosion when compressor tanks use the same/ similar metals and I suspect that propane tanks are of a thicker gauge than the air tank sat in your workshop. ...

Maybe, but my point is we don't know any of those things. Might be the same metal, might not. Might be made of thicker gauge metal than an air-tank, or not. Not unlikely that because Propane is almost inert the cylinders are bare inside whilst a compressor tank is plated or painted because wet oxygen eats steel.

Given that pressure vessels can be dangerous, I'd prefer to assume the worst. To find out for sure, we would need to find an expert or test a Propane Bottle to destruction and examine the innards.

Dave

vintage engineer04/04/2019 19:44:25
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89 forum posts

I have numerous Calor Bottles and I still have the invoices which prove I purchased them.

Andrew Tinsley04/04/2019 20:01:09
859 forum posts

Well now, doesn't that just look like a propane cylinder in disguise!

Peter G. Shaw04/04/2019 20:15:55
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946 forum posts
39 photos

Readers may be interested in ME p486 dated 20 Oct 1989 where John Chappell reports on an old Non-Refillable Propane bottle tested to destruction both hydrostatically and under steam pressure. (Ok, I know the grammar is wrong, but that's what I've copied off my home-made database: I assume he tested two bottles.)

The report was followed up in ME p114,19.01.90; ME p424, 06.04.90; 18.01.102 with letters from Tubal Cain, Peter Spenlove-Spenlove and P.R. Perkins.

From memory, ie I haven't actually looked up the references, I think the bottle was buried and the tester monitoring from some distance away.

Peter G. Shaw

Simon Williams 304/04/2019 21:08:02
365 forum posts
64 photos

Uh OH!

PED (Pressure Equipment Directive) doesn't differentiate between home use and commercial use, on the assumption that if anything does go bang it's not going to be choosy about who it kills. So use of a LPG cylinder outside its design parameters (wrong gas for a start) is straightaway negligence for a starter.

The other thing I haven't noticed in the above discussion is any consideration of the statutory requirement for a pressure vessel (exceeding the exclusion clause minimum volume x pressure, which this does) to be tested annually. No test, no insurance.

Rgds Simon

bricky04/04/2019 21:19:24
356 forum posts
47 photos

Peter,I remember reading of this test in an old magazine and if I remember correctly it was a small hand held gas torch and I was amazed at the pressure reached before it exploded.I can,t remember the pressure.

Frank

I

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