Why is it advised that compressed gas cylinders must be kept upright and not in lying position? Avoid rolling or leaking? Regards, P

Any horizontal cylinder subject to cap damage = a torpedo. Part of the reason cylinders should be secured by chains etc.

Not had a lot to do with compressed gases recently, but as a SCUBA instructor we were taught that the metal at the base of the cylinder/bottle was thicker than the side's and that the compressed gas was rarely totally water free this reduced the risk from corrosion. Brian

As it's Friday I'm going to act as devil's advocate. Yes, if the regulator shears off, or the valves shear off, the whole thing takes off like a rocket (0-34 mph in 1/10th of a second was quoted in the 60s). There is a good 'Mythbusters' video at However, in my experience, the commonest reason that a regulator or valve shears off is where someone drops the cylinder from the upright position onto the ground (I have personally witnessed such an event), so you COULD argue that it is safer if it is already on the ground! It has nowhere further to fall. Tiptoes quietly away.....

This is what I have been told in the past but cannot guarantee it is correct. LPG bottles have a tube that goes from the valve to the bottom of the bottle which draws the liquified gas from the bottom of the bottle. When the liquified petroleum gas contacts the atmosphere at the valve or burner it turns from its liquified form to gas which can then be ignited. (The petroleum gas in gaseous form is about 230 - 260 times the volume of the liquified gas which is partly why it causes such a big explosive 'cloud' after leaking to air). If the bottle is laid on its side the liquified gas may not be drawn up the tube and the bottle may appear empty, or worse, cause a flame to go out leaving the potential for gas to leave the bottle in an uncontrolled way. Some LPG bottles (eg for forklift trucks) are purpose made to go on their side and the internal tube is shaped in such a way to draw the liquified gas from the side instead of the bottom of the bottle. These bottles should have an indicator on the bottle showing which way up the bottle should lie. Acetylene bottles are different to LPG bottles and if they are on their side they should be put in the upright position for a while (hours) so that the dissolved acetylene gas can be drawn off from the top - bit like taking the creamy head off a pint of a well known make of beer/stout. If the acetylene bottle is on its side the formation of the 'creamy head' or clean acetylene gas won't happen. The previous comments about the safeguarding of valves etc is also correct. Incidentally the TV programme 'How's it made' recently showed how gas bottles are made from one circular disc of metal and tested - fascinating. (I'll now get back to sewing on my elbow patches).

Thanks Chas, very informative. I'd loose the elbow patches though, I think they went out the same time black board rubbers did and kids stopped calling their teachers Sir.

achrn....You may well be right although I understand the liquified gas turns to gaseous gas (if that's the right way to put it) as it reaches atmospheric pressure and not within the bottle itself. It is that very difference in pressure that turns it from a liquid state to gas state. I dare say someone with a greater knowledge in gas bottles than I will come along with a definitive answer to the original question.

No, it's sitting at an equilibrium point in which the pressure is such that both the gas and the liquid phase are stable at the temperature in question. What then happens if you draw off some gas is that the pressure in the cylinder drops, so then the liquid propane (or whatever) is not stable at that pressure and temperature. So some of the liquid propane turns to gas. But (as noted previously) the volume of gas propane is much greater than the volume of the same quantity of liquid propane, so that increases the pressure. So the liquid propane in the cylinder boils off until it restores the pressure in the cylinder to the pressure at which the liquid phase is stable, and the boiling off stops. That is to say, it is the reduction of pressure that drives it, but it immediately equilibrates in the cylinder. It's the drop from 300psi to 299.999psi in the cylinder that drives the process, not the drop from 300psi in the cylinder to 15psi in the atmosphere. The boiling process definitely happens in the cylinder - not in the burner. But this is all a continuous process - the pressure would actually only drop detectably if your rate of extraction was sufficient that your boiling off process was limited by the rate at which heat energy could get into the cylinder. That doesn't much happen with propane, but it does happen with butane - which is why if you go camping in winter it's better to take red gas cylinders than blue ones.

Friday, September 30, 2011 2:32:49 PM, I think. Personally, I find knowing how and why make the process of deciding what much easier. In general, it's easier to make the right decision if you understand what underpins the decision, rather than trying to blindly remember a load of rules within any defined causation, and then extrapolate. Likewise, I find people comply with instruction more reliably if they know why they get the instruction. Clearly you need to tailor the level of detail required - for example, talking to a site labourer about concrete you'd say waterproof PPE is required because it can give you chemical burns, but you wouldn't embark on discussing the pH or actual chemistry (though with a graduate engineer you might).

Achrn I don't disagree with you but where was LPG mentioned in the posting - are we saying that the reasons for storing co2 cylinders or compressed air cylinders upright are the same as for LPG? so when we answer a question lets answer a question and not invent our own!

Achryn Thank you for that I had not realised that co2 could liquefy in a cylinder at what I consider low pressure so you have taught me something. Certainly compressed air in the cylinders I was using at over 200 bar I do not believe liquefied. Clearly your answer now makes sense but it would have been more helpful to provide this information for all gases and not just to specify LPG in your answer. Clearly then the reasons for standing a cylinder upright is many fold.

I didn't specify LGP in my answer. I provided on example of a gas exhibiting that type behaviour. I stated it to be an example, using the wording "propane (or whatever)". If you're going to lambast people for lack of precision, it might be worth checking the precision of your lambasting. Air doesn't liguify, no (or rather, if we're worrying about precision, the major components of breathable air do not liquify at the pressures and temperatures customarily adopted in breathable air cylinders). The air in breathing cylinders remains gaseous, which is why you can get a measure of how much time you have left by looking at the pressure gauge - the volume of air in teh cylinder being the volume of the cylinder times the pressure in atmospheres (to a pretty good first approximation). That doesn't work for CO2, for example (FOR EXAMPLE - JUST AN EXAMPLE OF ONE GAS EXHIBITING SUCH BEHAVIOUR) - the pressure in a 100% full cylinder and a cylinder that's 5% full is the same, being the pressure at which both liquid and gas states are stable at the ambient temperature.