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The canisters that I have purchased so far have included a warning not to let the canister get above 120 degrees Fahrenheit. It looks like the inside of car can get well above that, hot enough to bake cookies.
I would prefer not to leave on a backpacking trip with an extra canister left in my vehicle and come back to find that it had exploded. What are some things that I can do to prevent that from happening?
The background
It took me a moment to find it, but an example of a car in which this happened can be seen here. If the fuel in the canister becomes sufficiently warm the pressure can rupture the canister. Usually the bottom everts first, popping outwards, although I have heard of instances where this was immediately followed by it coming apart, so this should not be considered any form of early warning measure. At this point you have a few ounces of liquid butane that's rather far above its boiling point and suddenly under very little pressure, so it almost instantaneously transitions to gas and vapor.
Such an event is called a BLEVE (boiling liquid expanding vapor explosion). Fire does not need be involved; in the referenced image above there are no signs of fire damage. If you are particularly unlucky, friction or sparks can ignite the cloud of vapor, possibly resulting in a fuel-air explosion.
How to avoid it
Don't park your car in the sun, or don't keep fuel canisters in it. However, on some trips this might be unavoidable. I suggest considering the following:
I kept a spare in my trunk for two years in the southeastern US and I never had a problem (YMMV). This, like most things in life, is not perfectly safe. So you should decide if you really need to store this in your car. If you do, things you should consider:
By far your largest risk is from puncture. Rupture due to heat appears to be something that happens after "prolonged exposure to fire or intense heat". Even if the canister ruptures, it isn't at all likely to turn into a fireball without an ignition source.
https://pubchem.ncbi.nlm.nih.gov/compound/isobutane
MSDS: https://www.mesagas.com/images/Isobutane.pdf
All that being said, most recommendations are to store at less than 125 degrees Fahrenheit. So don't go parking your car in directly sunlight in the summer.
Lot's more interesting reading on canister failures here.
Keeping it low down against the bottom of the car and insulated is best. The spare wheel well is ideal (and used when you get a can of tyre foam instead of a wheel; they have similar limits) or those compartments you sometimes get under the boot floor. These places are much cooler than the rest of the car, especially with the parcel shelf closed. If they aren't an option, under the front seats with insulating material on top is probably your best bet.
Also do what you can to keep the car cooler - reflective windscreen covers etc.
Don't store them in the car. Hide them in the engine compartment or under the car. I don't even store butane in my house.
Hopefully they have a pressure relief valve that they don't actually explode but I still would not store them in the vehicle. Combined with an ignition source you have a very nasty if not fatal fire. It is technically not an explosion but it goes boom and is very hot.
Think how many meals you can cook and getting that much heat once.
If you really need to store fuel in the car then consider liquid fuel.
A leak and ignition would be very bad if not fatal.
Nelson man died after gas from leaking butane cylinder exploded in his home
Butane is odorless so unless they mark it with a scent you would no even know it leaked.
I see this got a down vote and don't want to argue with you but leaving a butane cylinder in the car is a bad idea.
