When LPG is stored in a gas cylinder bottle, it is under pressure. The pressure range for LPG (Propane) is from 153 kpa (25 P5IG) at 0°c to 1795 kpa (258 P5IG) at 54°c. The pressure inside a gas cylinder bottle or container is dependent upon the temperature of its surroundings. The higher the temperature, the higher the pressure of LPG within the gas bottle.
Pressure is the force produced by a particular amount of gas or liquid in a confined space or container. So, you can see now that LPG can exists as both liquid and vapour (gas) within the cylinders.
But that doesn't answer the question - have it?
LPG changes from liquid to gaseous state so quick because every time you turn on the gas regulator and open the appliance being fuelled, LPG in your gas cylinder starts to boil. If you could see through the steel, you would also notice that it looks just like water boiling. Perhaps, you could go purchase the composite transparent gas cylinder bottle to see how this changes occur.
As with water, the big difference is that it happens at -42°c or -44°f. The more heat around its surroundings, the more rapidly it boils. Liquid LPG changing to gas vapour is called VAPORIZATION. To boil, the liquid LPG draws heat from the steel walls of the gas cylinder, which in turn, get heat from the ambient air. Liquid LPG then boils and turns back into gas vapour when you release some of the pressure in the bottle by turning on the appliance being fuelled.
This vapourates at a faster rate, for the above illustration. Vaporization also makes the gas cylinder or tank feel colder when you are actually using the gas. The LPG gas vapour is held on the top of the bottle, and the liquid LPG at the bottom. Almost all of the uses of LPG involves the use of the gas vapour, not the liquified gas.
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Basic LPG