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What is LPG?

Liquid Petroleum Gas - LPG is a mixture of propane and butane which is a by-product of fractional distillation of crude oil. The mixture is adjusted according to the time of the year; in summer it is 30% propane / 70% butane in winter 50% propane / 50% butane or sometimes even 70% propane / 30% butane. These differences are necessary to maintain the LPG pressure.

Important reasons for using LPG are

  • It's Cheaper than petrol – need we say more?
  • LPG is an environmentally friendly fuel.
  • The exhaust pollution is lesser than with any other fuel.
  • LPG is a "waste" product which would otherwise be burnt off.
  • It has a higher octane value (LPG: approx. 100 octane, regular-grade petrol appox 95 octane and premium-grade petrol approx 98 octane)
  • LPG is easy to store as a liquid at approx. 6 bar.

LPG is the generic name for commercial propane and commercial butane. These are hydrocarbon products produced by the oil and gas industries.

The two liquefied petroleum gases which are generally available in the UK are Commercial Butane and Commercial Propane as defined in BS 4250. Commercial Propane predominantly consists of hydrocarbons containing three carbon atoms, mainly propane (C3H8). Commercial Butane predominantly consists of hydrocarbons containing four carbon atoms, mainly n- and iso - butanes (C4H10).

They have the special property of becoming liquid at atmospheric temperature if moderately compressed and reverting to gases when the pressure is sufficiently reduced.

Advantage is take of this property to transport and store these products in the liquid state, in which they are roughly 250 times as dense as they are when gases.

Butane is usually supplied to customers in cylinders, propane can be supplied in cylinders or in bulk for storage in tanks at the customers premises.

The combustion of LPG produces carbon dioxide (CO2) and water vapour, but sufficient air must be available. Inadequate appliance flueing and / or ventilation, or poor air-gas mixing (for example due to lack of servicing) can result in the production of toxic carbon monoxide.

Everyone concerned with the storage and handling of LPG should be familiar with the following characteristics and potential hazards:

(a) LPG is stored as a liquid under pressure. It is almost colourless and its weight is approximately half that of an equivalent volume of water.

(b) LPG vapour is denser than air: butane is about twice as heavy as air and propane about one and a half times as heavy as air. Consequently, the vapour may flow along the ground and into drains, sinking to the lowest level of the surroundings and be ignited at a considerable distance from the source of leakage. In still air vapour will disperse slowly.

(c) LPG can form a flammable mixture when mixed with air. The flammable range at ambient temperature and pressure extends between approximately 2 % of the vapour in air at its lower limit and approximately 10 % of the vapour in air at its upper limit. Within this range there is a risk of ignition. Outside this range any mixture is either too weak or too rich to propagate flame. However, over-rich mixtures can become hazardous when diluted with air and will also burn at the interface with air. At pressures greater than atmospheric, the upper limit of flammability is increased but this increase with pressure is not linear.

(d) Escape of even small quantities of the liquefied gas can give rise to large volumes of vapour / air mixture and thus cause considerable hazard. A suitably calibrated explosimeter may be used for testing the concentration of LPG in air.


(e) At very high concentrations in air, LPG vapour is anaesthetic and subsequently an asphyxiate by diluting or decreasing the available oxygen.

(f) Commercial LPG is normally odorised before distribution by the addition of an odorant, such as ethyl mercaptan or dimethyl sulphide, to enable detection by smell of the gas at concentrations down to one-fifth of the lower limit of flammability (i.e. approximately 0, 4 % of the gas in air). However, in certain cases where the odorant may be detrimental to a process (for example in aerosol applications) the LPG is not odorised.

(g) Escape of LPG may be noticeable other than by smell. When the liquid evaporates, the cooling effect on the surrounding air causes condensation and even freezing of water vapour in the air. This effect may show itself as frost at the point of escape and thus make it easier to detect an escape of LPG. Because the refractive index of LPG differs from air, leaks can sometimes be seen as a 'shimmering'.

(h) Owing to its rapid vaporisation and consequent lowering of temperature, LPG, particularly liquid, can cause severe frost burns if brought into contact with the skin. Personal protective equipment (e.g. hand and eye protection) should be worn if this hazard is likely to occur. A container which has held LPG and is 'empty' may still contain LPG in 'vapour form and is thus potentially dangerous. In this state the internal pressure is apmximaely atmospheric. If a valve is leaking or is left open, air can diffuse into the container forming a flammable mixture and creating a risk of explosion: alternatively, LPG can ciiffilse to the atmosphere.

Note: These properties are general characteristics of LPG, and items such as (h) should not occur in normal cylinder usage.

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