LP gas tank construction

Hello. I am thinking of making a butane/propane tank for a small steam plant I have made?

I have a number of steam boilers in the past and am familiar with the design and stressing of them as well as the silver soldering etc.

I know that the safety factor generally accepted in copper boilers is 8 with a uts of 25000 psi and various additional factors for temperature, lap joints etc.

Now to my question. Can anybody give me the acceptable safety factor used in the design of miniature gas tanks as I cannot believe that the safety factor is as much as 8 after having looked at the thickness of the material in the tanks of some of the garden railway locos in our club.

I am quite happy to do the stress calcs before making the tank but would appreciate advice regarding the safety factor.

Regards.

Brian.

The safety factor copper boilers is often 10 ! Then you look at a can of butane, just a tin can ! Butane has a low vapour pressure ! Just comments. Noel

I suppose once you start drilling holes in the barrel for bushes there is a need for something a bit more substantial than a tin can. Though for the few inches of copper tube needed for a tank it won't cost much to be cautious and allow a decent thickness. They also need a higher test pressure, about 330psi if I remember rightly.

The vapour pressure of liquid butane at 20C is about 2.5 bar (37psi) so a test pressure of 330 psi is indeed a safety factor of around 9-10. However, the VP rises rapidly with temperature. It's about 5 bar (75psi) at 50C and 10 bar (150 psi) at 80C so you lose your safety factor rapidly if the tank gets hot - it's down to a factor of 2 at 80C.

I have just realised that I have been making a silly mistake in that I have been applying the safety factor to the proof test pressure and not the max working pressure.

The pressure regulation document published by the UK boiler test group stipulates that all miniature gas tanks shall be designed to be able to use 60/40 butane/propane and must not be heated to more than 60deg C.

At this temp the tank pressure will be 219 psi, so if the tank is designed for this pressure a 2in dia copper tube of .080in wall thickness will have a safety factor of 9 which seems much more sensible.

Cheers. Brian.

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Hi br.

Your tank dimensions give a safety factor of around 7 which sounds pretty sensible to me.

Cheers

Brian.

A few considerations:

1. As the metal is stressed in a way that causes fatigue failure I'd be inclined to overrate the safety factor generously.
2. Unlike a steam boiler, the contents aren't hot and corrosive, meaning welded steel could be used rather than Copper. Stronger.
3. The energy potential of a full gas cylinder is much higher than that of a boiler of the same size. The boiler only stores thermal energy, whereas gas contains a lot of chemical energy. Although the gas container giving way won't cause much damage in itself, a spark will cause a fire with risk of a full blown explosion if the gas/air mix is right in a confined space. There's more energy in a small camping gas cartridge than a hand grenade:

The design is more about preventing leaks than resisting a relatively low pressure. The metal could be exremely thin provided careful attention was paid to joints and any other feature likely to develop fatigue cracks..However, to keep it simple I'd be inclined to over-engineer it: so what if the safety factor is massively high?

Dave

Edited By SillyOldDuffer on 07/01/2022 21:35:48

But the end plates will have to either be thicker, or have stays, or if you're keen on metal bashing be 'torispherical', otherwise known as a domed end

Hi. Yes domed ends and a stay is not a problem if we are looking at say .064 or .08 thick material, it was my original thought of having to flange .16 thick 2in diameter ends that felt was a bit extreme.

Cheers Brian

You should also find designs and advice around the Gauge 1 loco and model boat fraternity.

Thanks.

Brian

Aerosol cans that contain butane as a propellant when dropped into an open fire incinerator will go off like a hand grenade, propelling the remnants about 50 feet in the air, yes we tried it and you definitely need to retreat to a safe distance. Dave W

G1MRA publish the code adopted by themselves and a number of other organisations on the public part of their website. The code does not cover a butane/propane mix above 60/40 and requires an initial hydraulic test at 400 psi even if the tank is marked "butane only" (the maximum mix for which the tank is designed must be indelibly marked on the tank ).

G1MRA website

Brian G

Edited By Brian G on 10/01/2022 18:00:40

Another factor to think about is that the tank must never be filled right up, the liquid needs some gas volume to allow for changes of temperature.

Another consideraton is low temperature. The liquid gas will cool was it evaporates. Many materials, including many steels become brittle at low temperatures.