I've posted this on a couple of campervan forums but we all know STW is the best and can answer anything 🙂

I have a vapour take off underslung gas tank on my campervan. Its new, but I filled up for the first time last week and am having issues with the oven and hob flaring slightly and then the gas pressure dropping off over about 1 minute until it goes out.

Autogas is generally accepted to be about 90:10 Propane:Butane in the UK, I think my symptoms are the classic 'Butane doesn't work in the cold'.

Cars use liquid take-off tanks and use a heated evaporator (or something like that) to create vapour for the engine, so they are not affected by Butanes dislike to the cold.

So, could it be that the Butane makes up a larger percentage of the autogas during the colder months as demand drops off and it becomes cheaper than Propane?

I've used less than 10% of the tanks contents so it must be nearly all Butane if this is my problem. The only thing that is strange is the pressure appears to increase momentarily to more than the regulator should allow, so maybe the regulator is faulty.

Anyone?

I've just googled ratio's and it seems that up to 50% butane is acceptable. Whereas the engine will run fine on that I think a regulator may have problems dealing with the varying pressures.

I would imagine that Autogas will have a relatively tight composition definition in the same way that petrol does with octane numbers so composition variations are likely to be small.

Autogas is generally accepted to be about 90:10 Propane:Butane in the UK, I think my symptoms are the classic 'Butane doesn't work in the cold'

I've used less than 10% of the tanks contents so it must be nearly all Butane if this is my problem..

It doesn't really work like that I'm afraid. As I'm not at work I don't have access to the software that would actually let me do any calculations but I don't think that the temperature will be having as big an effect as you think. It certainly won't be the case that it will be causing the pressure in the tank to fall as this is set by the temperature within the tank. Provided there is a reasonable amount of propane (about 30%) in the mix the temperatures that we generally see in the UK are unlikely to be low enough to cause the pressure to fall low enough that the gas cannot burn.

I'd check to see if your regulator is suitable for use the autogas and if it is see if it is faulty. Other than that another issue may be that you are trying to burn a gas that the burners aren't designed to deal with. If your burners are set to deal with pure butane then the wobbe index (no I'm not making that name up) of the relatively propane rich gas may be outwith the burner specification.

Thanks for the replies. I hadn't found any figures for the mix but I know in Europe the autogas is generally made up with a much higher percentage of butane, which does cause problems when people fill up a motorhome tank at the end of the summer, then try and use it back here in the winter.#

I don't think that the temperature will be having as big an effect as you think. It certainly won't be the case that it will be causing the pressure in the tank to fall as this is set by the temperature within the tank. Provided there is a reasonable amount of propane (about 30%) in the mix the temperatures that we generally see in the UK are unlikely to be low enough to cause the pressure to fall low enough that the gas cannot burn.

The tank is under the van, so temperature in the tank will be the same as the outside temperature. Butane has difficulty vapourising (boiling) as you get towards 0 celsius, and won't at all by -0.5c. The Propane would burn off first until I'm left with a tank of useless Butane. The pressure would drop off nothing, but my gauge will still read 70% as its a float level gauge, not pressure.

The reg and cooker are both suitable for Autogas, most new appliances are as Butane and Propane both operate at 30mb now 🙂

Another forum has been pretty unanimous that the Autogas in the UK is almost all Propane so looks like the gas might not be the problem, someone did suggest I might have moisture in the tank. This could be the cause as the tank is brand new, could be freezing in the regulator which could explain the excessive pressure and flaring.

Don't have the same problem as my LPG tank is internal (under a bench seat, 70 lt tank looking like a bomb) but the original external one (just for the cooker and heating) had a heater for the regulator to prevent freezing and exactly the problems you are experiencing.
Assuming they knew what they are doing fitting it, maybe a faulty heater regulator that is not, ermm, heating?

The Propane would burn off first until I'm left with a tank of useless Butane. The pressure would drop off nothing, but my gauge will still read 70% as its a float level gauge, not pressure.

Nope it doesn't work like that. The vapour evolved from a propane/butane mixture will not be pure propane, it will be a mixture of butane and propane. It will contain a higher proportion of propane than is in the liquid but it will still not be pure propane. If I were at work I'd be able to give you a precise composition of the gas that exists in equilibrium with a 70/30 propane butane liquid, but it's the weekend.

I must admit that i didn't consider that there could be water in the tank. Rather than getting bogged down in the thermodynamics of phase equilibrium I'd say this, or some such similar contaminant, is a far more likely cause of you problems.