cylinders for gas firing, test requirements

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cylinders for gas firing, test requirements

This topic has 19 replies, 8 voices, and was last updated 6 March 2012 at 18:32 by michael howarth 1.

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27 February 2012 at 17:55 #85996
Dick Allan
Participant
@dickallan56872
Can anyone advise me on the construction and test requirements for gas cylinders for firing models. I have heard mention of 300 psi but I doubt if the tin cans in B & Q would survive that, Also does GIMRA have a lower ? 160 psi requirement?

Dick Allan
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27 February 2012 at 17:55 #22027
Dick Allan
Participant
@dickallan56872
27 February 2012 at 19:13 #86009
JasonB
Moderator
@jasonb
360psi but thats all I know

J
27 February 2012 at 20:11 #86019
Sub Mandrel
Participant
@submandrel
Looks like the tin cans in B&Q have to be ablt to withstand 10 bars or normal pressure at 50 degrees +50% if the normal pressure is >6.7 bar.

Each one has to be tested for leaks in a 5-0-degree water bath for 20 seconds.

http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1975L0324:20030605:EN:PDF

Note this is for non-refillable aerosols.

Refillable will need to be stronger as weaker ones can get damaged easily when empty.

Neil
27 February 2012 at 21:07 #86025
Russell Eberhardt
Participant
@russelleberhardt48058
If you want an adequate safety margin you will have to consider the maximum temperature the cylinder is likely to reach. The gas pressure varias considerably with temperature. See: http://www.engineeringtoolbox.com/propane-vapor-pressure-d_1020.html

Prsonally I would test to double the maximum pressure.

If it is only for your own use I don't think there is any legal requirement to comply with EU Directives or Standards.

Russell.
28 February 2012 at 20:38 #86088
Sub Mandrel
Participant
@submandrel
The euro stuff was simply by way of comparison. Propave has a VP of about 14 bar at 40 degrees and I'd extrapolate to about 17 at 50C. +50% would give 21 bar (which is roughly 300psi).

So the higher figure of 300PSI would make sense for a regulation test pressure, but only if you can be sure your installation wil stay under 50C. Note that testing too far above working pressure can cause faults, so don't overtest.

As Butane has MUCH lower vapour pressure than propane at these sorts of temperatures your B&Q aerosol doesn't need to be anywhere near as strong and will be made for 10 bar.

Neil
29 February 2012 at 08:36 #86120
Ian S C
Participant
@iansc
A good while ago there was an/ were articals in ME on testing gas canisers to destruction, some amazing pressures were reached, I think it was in the 1990s. Ian S C
29 February 2012 at 10:07 #86127
Dick Allan
Participant
@dickallan56872
Many thanks for your helpful replies. One or two points may progress this problem. Most people, including me, use the standard, widely available, mixed gas canisters which are 30% propane 70% butane. I also follow the GIMRA practice of placing the gas tanks in the tender water. This limits any rise in temperature (and protects from direct sun), aided further by the cooling of the gas tank during evaporation. I doubt if the temperature ever exceeds,say, 40 C but I will do some measurements on this. Meanwhile I would be grateful to hear from Neil how his calculations are affected by these points.

The filling /venting process uses Schraeder valves which are changed from time to time, otherwise I cannot see why there should be any difference between 'once only', non-refillable tanks and refillable ones. The tanks used by model engineers are not subject to the rough treatment dished out to plumbers' Calor gas cylinders! Yet I think it is for the latter that the rules have been devised, and hence the serious over-engineering one sees in some models.

The point of this investigation, apart from the pursuit of sensible safety measures, is that the majority of club boiler testing facilities are limited to 160-200 psi and it would be most helpful if the same equipment could be used for gas tanks. I am still hoping to hear from someone in GIMRA because I suspect their rules may be different.

Dick Allan
29 February 2012 at 11:34 #86136
Russell Eberhardt
Participant
@russelleberhardt48058
Checking a bit further it would appear that the graph given on the Engineers toolbox site is wrong. The graph given on this site would seem to verify Neil's figures: http://encyclopedia.airliquide.com/Encyclopedia.asp?GasID=53

Russell.
29 February 2012 at 21:14 #86192
Sub Mandrel
Participant
@submandrel
I'm no expert – just googled a bit.

I think that the 300psi test appears reasonable and I guess 50C is a sensible margin for safety – tenders can run dry

Neil!
5 March 2012 at 10:21 #86561
Dick Allan
Participant
@dickallan56872
Thanks for all the replies. I'm still not entirely clear about this. There must, for instance, be a capacity limit for the 'rules' or one could not fill a cigarette lighter! Also the difference between propane and butane70/propane30 mixed gas is very large – one contributor to a previous forum on this subject said at 50C the latter would reach a pressure of approx 60 PSI. At more likely working conditions it is close to 40 PSI. In use, everyone knows that the real problem is the gas tank getting too cold! One recent article on a boat installation suggested using a solid copper shunt between the boiler and the gas tank to solve this problem.

Looking at the cylinders in my local plumber's merchant it is obvious that the standards applying to propane and MAPP cylinders are totally diferent from those for butane and mixed gas cylinders. Also I,ve seen a label on a gas tank for a G1 loco saying ' tested to 160PSI' so it looks as if some dfiferent rules were being applied.

Dick
5 March 2012 at 10:49 #86567
MICHAEL WILLIAMS
Participant
@michaelwilliams41215
Hi Dick ,

Working out the test pressure is just a matter of doing a few sums based on maximum credible internal pressure and a set of design rules . I see no reason why a gas tank should be designed or approved on any different basis to a copper boiler .

More relevant I think is that gas tanks as normally made have no overpressure safety device – notionally a safety valve . Fitting a simple steam type safety valve venting to air does not however seem a good idea and could make matters worse in some cases .

It may be that what is needed is an overpressure shut off device that cuts gas supply to burner and effectively removes both heat source probably causing the overpressure and removing any ignition source for stray gas .

Regards ,

Michael Williams .
5 March 2012 at 11:21 #86572
Terryd
Participant
@terryd72465
Hi Dick,

What gas are you proposing to use as the pressures involved in propane and butane are very different at similar temperatures.

Terry
5 March 2012 at 16:01 #86594
Dick Allan
Participant
@dickallan56872
Hi Terry

mixed gas canisters from B&Q which are 70%butane and 30% propane

Hi Michael

I quite agree and it is simple to do the calculations. The problem seems to be that 1. people are not recognising the much lower vapour pressures with butane or mixed gases,2. some seem to think there is a heating problem but in practice the problem is gas tank cooling during evaporation,3.When not in use obviously there is no fire and the only way overheating can occur is by the tank being left in the sunlight or otherwise exposed to temperatures of 50C or above. This of course is precisely the same as for B&Q canisters (the cans have a warning to this effect), also the cans are more likely to be dropped or thrown about than proper installations in a loco and more likely to be left in the sun as the loco tanks are generally covered inside cabs or tenders. An overpressure switch is an interesting idea but would be a complication and expense and is the problem was exposure to direct sun it could make things worse by removing the evaporative cooling whilst leaving the heat source!

Dick
5 March 2012 at 16:32 #86599
JasonB
Moderator
@jasonb
Interesting bit on the Scotish model boats site about test requirements for tanks depending on gas mix. I suppose the 360psi would be on the high side to save a tank being filled with a different mix than it was designed for.

They also suggest not fittinga safety valve.

http://www.sfmbc.net/Fed_Boiler_test.pdf
5 March 2012 at 18:10 #86601
Dick Allan
Participant
@dickallan56872
Thanks JasonB for that. Looks like 285PSI is right for 70/30 mixed gas. I don't think there is any significant risk of filling with the wrong gas mix as 70/30 is almost universal in the shops and I believe I'm right in saying that the propane proportion decreases during use.
5 March 2012 at 18:18 #86602
michael howarth 1
Participant
@michaelhowarth1
There has been considerable discussion on this subject on the G1MRA (Gauge 1 Model Railway Association) site recently. I believe that they have decided on a 360psi tank. It will take a bit of reading but I will have a look and come back if different.

Edited By mick H on 05/03/2012 18:25:37

Edited By mick H on 05/03/2012 18:29:37
5 March 2012 at 18:51 #86606
michael howarth 1
Participant
@michaelhowarth1
Correction to above (wrong again)……G1MRA specify the use of a label on the tank stating butane only to be used and temperature of tank not to exceed 16C/60F. Tanks tested to 180psi. The subject has been done to death on the site but access requires membership.
6 March 2012 at 17:47 #86683
Dick Allan
Participant
@dickallan56872
Hi Mick

thanks for that. Do you happen to know if 70%/30% butane propane mixed gas is covered by the GIMRA rule? The canisters in the shops are the same for this mix as for pure butane

Dick
6 March 2012 at 18:32 #86686
michael howarth 1
Participant
@michaelhowarth1
Dick….as previously mentioned there has been considerable discussion on this very subject by G1MRA members, including some of the apparent anomalies and inconsistencies mentioned on this site. It seems that the "official" line is as I posted earlier, although the discussions involved some who use 70/30. The obvious advantage of 70/30 is that the problem of freezing tanks and non gasification of butane is reduced. Others site the gas tank in the boiler feed water which tends to maintain a fairly constant temperature if the bypass is open. A liquid gas feed system is also in use and this also reduces the gasification problems. I have left you a PM.

Mick

Edited By mick H on 06/03/2012 18:34:27
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