An LPG cylinder is a pressure vessel made from 2 to 3mm mild steel plate and of welded construction. The surface of a cylinder in direct contact with the Liquefied Petroleum Gas is known as the wetted area.

A ‘Full’ cylinder is not full of liquid, space is left for expansion of the liquid and a supply of compressed vapour in the space above the liquid level.

The percentage fill varies with the size of cylinder, but normally has a maximum fill of 80 to 87%.

When the valve is opened, LPG is pushed out of the vessel by the pressure and, as a result, the pressure above the liquid is reduced.

The liquid takes in heat from the cylinder and the outside atmosphere and begins to boil, giving off vapour.

As the conversion of liquid to gas continues, the liquid level slowly falls. When the valve is closed again the pressure inside will rebuild up to the vapour pressure of the liquid, irrespective of how much liquid is left in the container.

LPG is liquefied petroleum gas and is:

Obtained during the processing of crude oil, or direct from the North Sea.

Colourless and odourless. An odourising agent is added before distribution to give its characteristic smell.

Half as heavy as water when in liquid form. It will float on water before vapourising.

Easily liquefied by pressure, taking up only around 1/250th of its gaseous volume. This means that a large amount of LPG can be stored in a small place.

Either commercial butane or commercial propane. These are similar in use but propane has a boiling point and hence a higher storage pressure. Commercial propane is predominantly an outdoor fuel.

At ambient temperature (i.e. 15 degrees Celcius), propane and butane bottles contain the following pressures:

Commercial BUTANE
2BAR (28psig)


Commercial PROPANE
7BAR(100psig)

LPG:

LPG is approximately twice as heavy as air when in gas form and will tend to sink to the lowest possible level and may accumulate in cellars, pits, drains etc.

LPG in liquid form can cause severe cold burns to the skin owing to its rapid vapourisation.

Vapourisation can cool equipment so that it may be cold enough to cause cold burns.

LPG forms a flammable mixture with air in concentrations of between 2% and 10%.

It can, therefore, be a fire and explosion hazard if stored or used incorrectly.

Vapour/air mixtures arising from leakages may be ignited some distance from the point of escape and the flame can travel back to the source of the leak.

At very high concentrations vapour can have an anaesthetic effect and subsequently become an asphyxiant by diluting the available oxygen.

A cylinder that has contained LPG is normally empty but may still contain LPG vapour and be potentially dangerous. Therefore treat all LPG cylinders as if they were full.