Using a propane cylinder for partable compressed air.

[Glyn DaviesParticipant @glyndavies49417]

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.

[Glyn DaviesParticipant @glyndavies49417]

[Tim StevensParticipant @timstevens64731]

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

Cheers, Tim

[Jeff DaymanParticipant @jeffdayman43397]

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 TinsleyParticipant @andrewtinsley63637]

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 2Participant @robertatkinson2]

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 FarrParticipant @nicholasfarr14254]

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 Sweeting 2Participant @briansweeting2]

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 3Participant @nickclarke3]

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.

[SillyOldDufferModerator @sillyoldduffer]

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

[VicParticipant @vic]

Its probably safer to use this then.

**LINK**

[Andrew TinsleyParticipant @andrewtinsley63637]

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

[Jeff DaymanParticipant @jeffdayman43397]

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.

[Roger ClarkParticipant @rogerclark]

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 perman 1Participant @martinperman1]

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

[SillyOldDufferModerator @sillyoldduffer]

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 engineerParticipant @vintageengineer]

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

[Peter G. ShawParticipant @peterg-shaw75338]

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 3Participant @simonwilliams3]

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

[brickyParticipant @bricky]

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

[Glyn DaviesParticipant @glyndavies49417]

Many thanks to all those that troubled to reply!

I’ve just been examining the cylinder for any information marked on it. It is dated 1978 and a plumber gave it to me; there is no gas company mark on it, so I’m going to class it as mine!

It is marked TP 33.2 bar (481psi), SP 30.9 bar (448psi) and WC 9.2l. I assume SP is the Safe Pressure? The fact that propane molecules seem to embed themselves in the internal surface of the cylinder and then eke out over time did worry me a bit (the air coming out does smell a bit of propane, or whatever they add to propane to make it smell) and I see that the lower flammability limit for propane is just over 2% by volume at 150psi. That means that there needs to be 184cc of propane in the 9.2 litre cylinder when I fill it for there to be a combustible mixture and I just can’t see that much propane detaching itself from the steel interior. On the matter of condensation, I wonder if all the free water in the air will have condensed when it was first compressed by my shop compressor; I inverted the propane tank and opened its valve and no water came out.

I fully understand the views of those replies that urge caution (or cessation!), but having seen the test pressure value and lower flammability level of propane at high pressure, I’m fairly reassured that it won’t go bang. I think I’d be very wary of pressurising the 50 litre receiver of my Clarke compressor to 480psi, particularly after having regularly drained very rusty water from its drain tap over the years.

[AJWParticipant @ajw]

My first compressor was home made using an ex gas cylinder.

Cut a 4 inch hole in the end and welded on a heavy ring to take a bolted on flange for all the fittings.

Welded a frame on top to take large motor and pump and a frame underneath for wheels.

Hydraulic tested it to 300psi and used it for years – did me proud!

Alan

[VicParticipant @vic]

Posted by Jeff Dayman on 04/04/2019 15:16:07:

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 this is correct then there are potentially already 8 tyres out there somewhere with a possibly explosive fuel/air mixture in them?!

The bottom line is why do this though when the proper equipment is commonly available? It’s not cheap but it’s hardly expensive either and it looks well made and has the proper valves and gauge fitted.

[Paul KempParticipant @paulkemp46892]

Posted by Simon Williams 3 on 04/04/2019 21:08:02:

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

Last time I read the PED it didn't specifically require an annual test? I believe it requires a pressure system in operation to have a written scheme of examination defining the system and its components drawn up by a competent person with a frequency of examination appropriate to its use. In fact the definition of competent person I believe can include an employee and doesn't necessarily need to be independent. Appropriate for its use meaning a pressure cooker manufactured and sold for home use will never see a repeat test where a high volume and pressure commercial system would. The distinction on commercial or home use is blurred because to manufacture something for sale (destination may well be a home) it has to satisfy the design criteria of the PED under one of the classes which determines how rigorous the testing and supervision process is. If you are manufacturing something for your own use outside of a commercial setting with no intent of putting it on the market there is no requirement as far as I was aware to comply with it?

Returning to the annual test point at a previous place of work post 2002 our air receivers had an annual visual inspection and functional exam by the insurance surveyor, a full hydrostatic pressure test at wp + FOS was only carried out every ten years. This is the same regime applied to full size steam boilers.

How many of the righteous folk on here have any inspection certificate for their machine mart compressor since purchase? For those that do have they checked their home insurance to see if it covers pressurised air systems within the home?

Paul.

[VicParticipant @vic]

I’m still unclear who actually owns the smaller sizes of cylinders. I was under the impression they remained the property of the company. It says here you should have a certificate if you own it.

**LINK**

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