Overheating gas Cartridges - Myths and Facts
In accordance with DOT regulations, the maximum charging pressure is 150 psig at 131 F (55 C). One container out of each 25,000 produced must be pressure tested to destruction and must not burst below 285 psig. 285 psig is reached at approximately 200 F
Overheating gas Cartridges - Myths and Facts
It is quite reasonable to say that a gas cartridge (or a fuel tank, for that matter) will blow up if it gets too hot. But some people and some vendors carry this to a totally ridiculous extreme, claiming that you simply must not use any windshield around a gas stove at all. So I enquired about the safety standards required and got this answer from Coleman America. I also received similar answers from Primus. While it is written for the Powermax cartridges, I know that the DOT (US Dept of Transport) regulations apply to all small gas cartridges, with each brand having its own type-approval special conditions. However, the conditions are all rather similar.
- In accordance with DOT regulations, the maximum charging pressure is 150 psig at 131 F (55 C). One container out of each 25,000 produced must be pressure tested to destruction and must not burst below 285 psig.
- 285 psig is reached at approximately 200 F (93 C). Pressure within the container is a function of temperature only, it is immaterial whether the container is full or half full.
- The pressure capability of the valve we use exceeds the burst capability of the container.
- Burst tests have been done by an outside lab at our direction.
- Further, during the production cycle, every cylinder must be heated to 131 degrees F [55 C] without evidence of leakage, distortion, or other defect.
In this context it is worth noting several things. The first is that a workshop compressor might reach about 80 - 100 psig, which is a long way below 285 psig. So we are talking about very high pressures here. The plastics used in stoves and in the Lindal valves in the cartridges all have quite good high temperature performance, and certainly work just fine up to boiling point (100 C). I have tested this. The regulations assume you can fill up the cartridges at 55 C. The regulations require that cartridges take a pressure which corresponds to a working temperature of 93 C, and testing to monitor this is required during production. Finally, we know that you can usually put your hand on metal up to about 45 C; much above that a reflex reaction sets in to make you say 'ouch' and withdraw. Fairly obviously, the 'ouch' temperature is a long way below the 93 C mentioned above, and probably below 55 C. So we can make a simple rule here:
If you can touch the cartridge without an 'ouch', it's safe.
I normally put a windscreen right around my upright gas stove, leaving only a very small (~10 mm) gap. If I am heating two cups of water in my kettle for tea or soup, the cartridge usually gets barely warm. Actually, I want it to get a little warm, to make sure the butane is well above its boiling point and evaporating along with the propane. So I don't put a radiation shield on it. If I am going to be cooking dinner for a while I normally add a radiation shield, but in really cold weather I might not, for exactly the same reasons. What I always do is monitor the temperature of the cartridge and the behaviour of the stove all the time. This is hardly a problem as my attention is on the pot anyhow: I don't want my dinner burnt or the stove to fall over.
Isobutane cartridges - a caution
There is one place where interchangeability can be slightly hazardous, and that is with cartridges containing straight isobutane. This gas is in principle a perfectly valid alternative to the standard butane/propane mix. It comes in two different cartridge formats: one looks like a spray can, while the other looks like any other gas cartridge. Both of these cartridges have the standard Lindal valve screw thread fitting. The advantage of isobutane is that it boils midway between propane and butane (at -12C) and this means it can be very useful in the snow. However, I and several other walkers have found a small hazard with these.
The molecular weight of isobutane is pretty much the same as for ordinary butane, so it has about the same heat output. However, this gas has a lower boiling point and this means the internal gas pressure will be higher than for straight butane. (Actually, this gets a lot more complex) A higher pressure means the gas comes out of the small jet inside the burner faster - unless the jet is smaller. It also means the gas velocity could be higher coming out of the top of the burner, and with those stoves having small burner areas and large jet sizes this can present a hazard. The gas/air mix does not seem to burn fast enough (the flame propagation velocity seems to be a bit lower), and at full throttle the flame can 'lift off' a small burner. This looks startling: the base of the flame literally rises above the burner, and is extremely dangerous! The flame can go out when you turn the stove up from low power, sending unburnt gas everywhere.
The author has had success with a straight isobutane cartridge on a SnowPeak stove with a small burner by restricting the air inlets a little bit and running the stove at a low rate. The idea was to reduce the overall gas/air flow rate out of the burner holes, so that it is below the speed at which the flame can travel. However, this idea is not recommended by the author or this web site. It can cause rather long tails on the flames if the air flow is restricted too much, and this can cause sooting of your pot. Using the same isobutane cartridge on a burner with a large surface area did not seem to present any problem, which is consistent with the theory that the gas velocity is the critical element here. The larger the surface area of the burner, the lower the average gas velocity. This is one case where the manufacturers' bleating about 'use only our cartridges on our stoves' has some meaning, albeit in reverse of what they are saying.
Now, where this gets a bit strange is that I can't find these screw-valve canisters with straight isobutane any more. And yet, many of us were sure we had that problem. Were we mistaken, or were they taken off the market? It seems some (MSR) stoves do have the smaller jet suitable for the isobutane cartridge, and may also have larger burners. Conversely, putting an ordinary butane/propane cartridge on one of these MSR stoves may give a smaller gas flow and a smaller flame according to Roy Jamieson. I have not seen this problem myself, but this latter combination would be quite safe, and good for simmering stews.