Using a propane cylinder for partable compressed air.

[VicParticipant @vic]

Posted by Paul Kemp on 04/04/2019 21:46:51:

For those that do have they checked their home insurance to see if it covers pressurised air systems within the home?

Paul.

There’s no way my wife would let me use a compressor in the home given the noise that it makes! Not that I use any air tools in the house anyway, they’re all in the shed with the compressor.

[not done it yetParticipant @notdoneityet]

Why bother to re-purpose a gas cylinder when there are hundreds of compressor tanks out there, sitting under dead compressors? I could buy compressors, from tiny to several KW, for very little money every fortnight during the summer months. Most sell for chicken feed prices because the compressors are likely (or certainly) dodgy.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Glyn Davies on 04/04/2019 21:33:38:

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.

Not sure about the UK where bottles are usually rented, but in the USA small propane bottles must be recertified 12 years after manufacture and ever 5 years thereafter.

I don't think the fact the bottle once contained propane is any kind of risk. The age of the bottle is! Would you trust a parachute made in 1975? And so is the risk of the bottle failing due to corrosion – see recent thread about broken calipers.

Considered as an escaping spring, I think compressed air is rather more vicious than propane. Propane's expansion rate is limited by the liquid's need to absorb external energy in order to boil. Air has a much lower boiling point and the release will be correspondingly faster.

Dave

[Anthony KnightsParticipant @anthonyknights16741]

If you don't fancy making a pressure vessel you can always make a stove

**LINK**

[HopperParticipant @hopper]

Using a barbecue gas bottle as an air tank was such common practice when I lived in the USA some years ago that you could buy a kit that replaced the gas valve with a schrader valve and nitto-type fitting. So you could pump the tank up at the gas station compressor and then use it at home to inflate tires etc. Or out camping to inflate innertubes for river floating etc.

You cuold get the kits to convert Freon bottles too.

I imagine cheap compressors have put an end to all that.

Edited By Hopper on 05/04/2019 09:21:20

[Simon Williams 3Participant @simonwilliams3]

Paul Kemp makes a perfectly valid point in that he is correct, the PED doesn't proscribe the inspection and test intervals for a pressure vessel, it is to be determined on the basis of a risk assessment done by a competent person. My point was that a regular disciplined regimen is written in law.

The fact that many of us don't comply doesn't make it right.

And no, a casual user cannot be the "Competent Person" (which phrase has a specific meaning) making the risk assessment.

HTH Simon

[not done it yetParticipant @notdoneityet]

Definition (or perhaps an explanation) of a competent person, copied from the HSE website.

(Quote) “A competent person is someone who has sufficient training and experience or knowledge and other qualities that allow them to assist you properly. The level of competence required will depend on the complexity of the situation and the particular help you need.” (0Unquote)

I would think that the first requirement would be the ability to make a comprehensive risk assessment of the intended modification.

Like fitting right handed connections, so the item cannot be used for flammable gas containment (just as a starter).

[Ian S CParticipant @iansc]

In use on a compressor you could mount the gas cylinder as I have done with an ex RNZAF low pressure oxygen bottle(probably from a Hudson or Ventura bomber), with the inlet at the bottom end(outlet and gauge at the top), tank mounted vertically, and fed from the compressor via a water trap, any water that gets by the trap on the way in, drains back to the trap after switch off.

Hopper, with the shrader valve, if you were out and about in a vehicle the tank could be refilled using a 12V portable compressor, some of those will go to near 150 psi. Another power source (an Australian invention I think), A chainsaw attached to a car alternator in place of the chain.

Ian S C

Edited By Ian S C on 05/04/2019 11:35:16

[A SmithParticipant @asmith78105]

When testing a diving air cylinder with air, with the cylinder in a pit, the test adaptor failed and was expelled at high speed, retained only by the long hose used to connect the air supply. The adaptor hit the leg of a very robust cast iron bench and cracked the leg. The remains of the adaptor looked like it had been through a hydraulic press. I was about 30 or so yards away and thought that a bomb had exploded. The individual conducting the test wasn't great on thread forms and had found an adaptor that "fitted".

I pressure test things as part of my professional life (40 years on from the above) and I would not consider re-purposing a gas cylinder for industrial use. At home? I would only consider it if I could hydrostatically test it to at least twice the working pressure. Darwin rules OK.

[Steve WithnellParticipant @stevewithnell34426]

As a practical question, just how much useful air can you store? Gas bottles are filled with say propane in its liquid state which vaporises as the pressure is released from the bottle. This allows for a huge volume of gas to be available from quite a small cylinder. Compressed air – the receiver on my 1.5HP compressor doesn't store enough enough for any useful purpose (even running my Victoria or spraying it, the pump is forever kicking in). Amateur mistake, I should have bought a bigger one…

Steve

[Paul KempParticipant @paulkemp46892]

Posted by A Smith on 05/04/2019 12:41:40:

When testing a diving air cylinder with air, with the cylinder in a pit, the test adaptor failed and was expelled at high speed, retained only by the long hose used to connect the air supply. The adaptor hit the leg of a very robust cast iron bench and cracked the leg. The remains of the adaptor looked like it had been through a hydraulic press. I was about 30 or so yards away and thought that a bomb had exploded. The individual conducting the test wasn't great on thread forms and had found an adaptor that "fitted".

I pressure test things as part of my professional life (40 years on from the above) and I would not consider re-purposing a gas cylinder for industrial use. At home? I would only consider it if I could hydrostatically test it to at least twice the working pressure. Darwin rules OK.

Why would you pressure test a diving cylinder with air????? No self respecting or competent establishment would do that and clearly from the comment re the fitting thread form the operative was not competent to test a balloon! There is a world of difference between a diving cylinder and the average home compressor tank something like a factor of 15 on pressure.

Paul.

[Paul KempParticipant @paulkemp46892]

Posted by Simon Williams 3 on 05/04/2019 09:23:02:

Paul Kemp makes a perfectly valid point in that he is correct, the PED doesn't proscribe the inspection and test intervals for a pressure vessel, it is to be determined on the basis of a risk assessment done by a competent person. My point was that a regular disciplined regimen is written in law.

The fact that many of us don't comply doesn't make it right.

And no, a casual user cannot be the "Competent Person" (which phrase has a specific meaning) making the risk assessment.

HTH Simon

I don't think I mentioned a casual user? However at home I could easily be defined as a casual user. At work I am a member of a professional body with formal engineering qualifications and more years experience than I care to count. Does that make me an incompetent casual user at home? Self certification is never a good idea, independent inspection and certification is always better. Strange though that the PED does not preclude 'self certification' don't you think? Funny old world.

Paul.

[SillyOldDufferModerator @sillyoldduffer]

Posted by Steve Withnell on 05/04/2019 14:03:35:

As a practical question, just how much useful air can you store? …

Steve

As air doesn't liquefy under pressure, Boyles Law applies:

Volume(atmospheric) = Pressure(compressed) * Volume(container) / Pressure(atmospheric)

In SI (metric) units, using Glyn's 9 litre cylinder full of compressed air at 30bar as an example:

Va = 30bar * 9 litres / 1bar, which is 270 litres, or about 9.5 cubic feet of air.

Hope this is right, SoD-Brand maths is highly error prone…

Dave

[SillyOldDufferModerator @sillyoldduffer]

Further to how much energy is stored in Glyn's cylinder, the sums are complicated, but I found this calculator.

Assuming I put the numbers in correctly and understood the answer, expanding 270 litres of compressed air will release nearly 54kJ of stored energy.

The conversion efficiency will be low, but assuming 10%, 5.4kJ might be available to do damage. This is a fair wallop – the muzzle energy of a .303 rifle bullet is less, about 3.5kJ.

I wonder how far a cricket ball would fly if it was fired out of a drain-pipe to which Glyn's bottle was connected via a quick release valve. The energy calculation suggests the experiment would be illegal – it's well over the limit that defines a firearm in British Law.

A test of the calculation would be to compare it with the energy needed to compress the gas in the first place. The energy input is the size of the compressor's motor in watts multiplied by the number of seconds it's has to be run to get 30bar in the bottle. I'd guess compression is also wasteful, about 10% efficient.

Usual warning about my maths, happy to told it's all wrong!

Dave

[JasonBModerator @jasonb]

Dave, why are you using 30bar in your calculations when Glyn has said he only pressurised the container to 150psi?

[SillyOldDufferModerator @sillyoldduffer]

First the reasonable excuse, honest guv. Glyn said 'It is marked TP 33.2 bar (481psi), SP 30.9 bar (448psi) and WC 9.2l. ' I think SP is Static Pressure, ie the maximum pressure in the bottle when gas isn't being pumped in or released. So I deliberately used the maximum pressure the bottle can take. All lies!

Real reason, it's a cock-up. I meant to used Glyn's actual pressures, 10 bar rather than 30. Perhaps someone more able should take over!

Dave

[not done it yetParticipant @notdoneityet]

At 30 Bar, it might be likely to kill/injure anyone adjacent to the firing point. First risk assess the strength of your drain pipe!

[the artfull-codgerParticipant @theartfull-codger]

Having watched the various posts with intrest I might ad my little bit, I borrowed my brothers home made gas bottle storage tank,much the same as glyn's & I must say it's very usefull & works a treat,he also uses it to blow tyres up, when away from the compressor,I used it with a blow gun in the foundry for blowing loose sand away as the compressors are not worth starting up especially @ 2-3 psi.he filled it full of water for a few days then fitted it up with fittings,[I'm going to make my own] my large teclamit compressor only gets used for sand blasting everything else I use my home made gas bottle tank with a green godess twin cylinder air brake pump & 2.5 hp motor & the usual fittings/safety valve, regulator/moisture seperator & drain valve,it's chiseled every bit of plaster from our 17c cottage sprayed numerous cars,& scores of everyday workshop jobs,as for dangerous hydrocarbons leaching from the walls,& exploding etc all I can say is I'm still here & i built it nearly 50 yrs ago,the only time it failed was when the oil pump got a bit of dirt in the non return valve & 1 big end melted I then made a jig & re-white metalled it & bored it on the faceplate & blued & scraped it in 30 yrs ago & its still going strong.

Graham.

[bill ellisParticipant @billellis45274]

If anyone is interested and is in the East Anglia region I have a BA (breathing app) cylinder which I used as an auxiliary storage device before I got a larger compressor, free to a good home. Good for 3000 psi but obviously out of certification. Fair bit heavier than an empty propane bottle.

[JonParticipant @jon]

Nearly bought one Bill two years ago,very heavy leave in car whilst being topped up to 300bar (4500 psi).

I do my own pressure testing on high pressure cylinders i make and normally go for 1 1/2 times the operating pressure. No law for fixed cylinders, no test but easily would pass.

Worst of the lot are the US and Candians, no sense of danger. Quite a few have blown like drilling and tapping in to a Luxfer cylinder where its around 1/8" thick and tapping 1/8"BSP! Thats only a max of 3 threads if any good and out they strip at 232bar.
Likewise others using CO2 containers where it freezes around 110bar, operating much less to avoid this. They fill with oxygen, argon you name it to 3000 psi + and write it off as a bad day when it explodes.
As kept saying its not you i care about, you have an obligation to all others around whether in car or in actual use.

[Colin HeseltineParticipant @colinheseltine48622]

Dive cylinders are rated at either 232 bar or 300 bar and have to be tested on a 2.5 year basis. This test includes a visual test after having removed the valve, test of the valve and hydrostatic test of cylinder assuming it has passed the visual and valve tests.. There are plenty of cylinders, 3 ltr, 10 ltr, 12 ltr and 15 ltr available on Ebay. You could purchase one of these and for your own peace of mind have it tested at local dive shop. If it fails the test it legally has to be destroyed. If it passes it is stamped with the date it passed. Most vendors will list the tank as being in-date or out of date. In date will cost more.

Colin

[VicParticipant @vic]

It depends how you use a Dive Cylinder. I used mine for Airgun filling so they only need to be tested every five years.

”Air tanks fitted with a ‘surface use only’ valve, where an integral gauge is fitted as part of the valve assembly, need only be tested every five years – the test being a full, hydrostatic one “

**LINK**

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