What is LPG?

Liquefied Petroleum Gas (LPG) is a term used for hydrocarbon gases. It is produced either as a by-product when refining crude oil or direct from the North Seas oil or gas wells.

The two most common LPG gases are known as Commercial Propane and Commercial Butane as defined in BS 4250.

Commercial Butane is predominately stored in blue cylinders up to 15kg and generally used for leisure applications and mobile heaters.

Commercial Propane is predominately stored in red cylinders and bulk storage vessels and especially used for heating, cooking and numerous commercial & industrial applications.

LPG has one key characteristic that distinguishes it from Natural Gas. Under modest pressure LPG gas vapour becomes a liquid. This makes it easy to be stored and transported in specially constructed vessels and cylinders.

The combustion of LPG produces Carbon Dioxide (CO²) and water vapour therefore sufficient air must be available for appliances to burn efficiently. Inadequate appliance flueing and ventilation can result in the production of toxic Carbon Monoxide (CO).

Everyone concerned with the storage and handling of LPG should be familiar with the key characteristics and potential hazards.

Behaviour

LPG exists as a gas at normal atmospheric pressures & temperatures, but may be liquefied by the application of moderate pressure. If the pressure is released the liquid will revert back to vapour.

Colour

LPG as a liquid is colourless and as a vapour, cannot be seen.

Smell

Pure LPG has no distinctive smell so for safety reasons a stenching agent is added during production to give a pungent, unpleasant smell and so aid detection by the human nose at very low levels.

Toxicity

LPG is non-toxic but at very high concentrations in air, LPG vapour acts as an anaesthetic and subsequently an asphyxiate by diluting or decreasing the available oxygen.

Flammability

When LPG is mixed with air, a highly flammable mixture is produced. The flammability range is between 2% to 11% by volume of gas to air. Outside this range any mixture is wither to weak or rich to propagate a flame.

Vapourisation

One volume of liquid will produce approximately 250 volume of gas vapour.

Vapour Density

LPG vapour is heavier than air. Any escape will find its way to the lowest level where it can remain and form a flammable mixture. Therefore LPG vessels must be sited away from drains and appliances must not be sited in basements or cellars.

Liquid Density

LPG liquid is lighter than water and therefore floats on top of it in a similar way to oil and petrol. Therefore LPG vessels must be sited away from drains and gullies.

Vapour Pressure

The pressure LPG exerts on a vessel varies with temperature. The higher the temperature of the liquid the higher the vapour pressure, conversely the lower the temperature the lower the pressure.

This means LPG vessels must be protected from heat sources and protective safety distances imposed on the siting and storage of LPG.

Commercial Propane has a vapour pressure of approximately 7bar (100psi) at 15°C (Similar to the pressure found in a lorry tyre).

Commercial Butane has a vapour pressure of approximately 2bar (30psi) at 15°C (Similar to the pressure found in a car tyre).

Because of these characteristics Commercial Butane can be used indoors and Commercial Propane must only be used outdoors.

Expansion

When LPG is heated it expands very rapidly. In order to allow for expansion LPG cylinders and tanks are only filled by volume to 87%.

Boiling Point

The boiling point is the temperature below which LPG will not vaporise to form gas vapour.

Boiling point of Commercial Propane is approximately - 42°C
Boiling point of Commercial Butane is approximately - 2°C

Therefore, Commercial Butane can be affected by cold weather resulting in poor pressure and should not be used outdoors in winter months.

Commercial Propane is not adversely affected by cold weather in the UK and is an ideal fuel source for heating, cooking and industrial applications.

Searching

LPG in both its liquefied and gaseous state has a very low viscosity and will flow very easily like water, petrol etc. This means they will flow with ease and penetrate any break or weakness in the installation. Therefore, special jointing compounds must be used for LPG installations.

Chemical Reaction

LPG is aggressive to certain non-metallic material like natural rubber and many plastics; therefore equipment and hoses must be suitable for LPG.

Calorific Value

The Calorific Value of a fuel is described as “The amount of heat released when a know quantity of fuel is burned”.

Commercial Propane = 95 MJ / m³
Commercial Butane = 121 MJ / m³
Natural Gas = 38 MJ / m³

Because LPG appliances release more heat than Natural Gas, it is important that any gas appliances fuelled by LPG are designed and manufactured for that purpose.

Therefore, Natural Gas appliances cannot be used with LPG and vice versa for LPG appliances with Natural Gas.

Air / Gas Ratio

LPG vapour requires a higher ratio of air / gas to burn correctly.

Fuel / Air Mix

Commercial Propane = 23:1
Commercial Butane = 30:1
Natural Gas = 9.6:1

Therefore, it is important that appliances fuelled by LPG are provided with adequate ventilation and serviced regularly to ensure that they burn efficiently.

Persons who are involved with the storage and handling of LPG should have adequate information, training and supervision.