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It is important to not increase the relief valve setting on a tank above that for which it was designed. At 120 deg F, butane will develop about 55 psig. Propane, however, will develop 225 psig or more. The boiling point of most liquids is dependent on the pressure it is under. That is why pressure cookers work, raising the pressure raises the temperature the water boils at so the food is cooked at a higher temperature, and up in the mountains, with lower pressure, it can be hard to boil an egg because the water boils at a lower temperature. Pure propane will boil at 120 deg F at 255 psig, and will boil at 0 deg F at about 24 psig. At standard atmospheric pressure (15 psig) it will boil as low as -40 deg F. So a propane tank will continue to produce gas down to below -30 and below. Commercial propane is a mixture of propane, butane, ethane, and other gases but is mostly propane. After extended refilling and use, liquids like butane will tend to collect in the bottom because they don't boil as quickly as the propane does. Ethane boils more quickly and won't make a liquid above 80 psig regardless of the pressure. We replaced the level gauge on my 500 gallon propane tank a few years back and found 20 gallons or more liquid in the bottom that did not want to boil off with the tank open. I am saying all this to say when a tank gets very low the presence of material in the tank other than propane will cause the gas coming out to act differnet than propane. So if you have an old tank and it has not been drained in a long time, it may not produce propane until it is dry.
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