Liquefied Petroleum Gas (LPG) is a mixture of light hydrocarbons, gaseous

at normal temperature and pressure and maintained in the liquid state by

increased pressure or lowered temperature.

The two liquefied petroleum gases in general use are 'commercial butane'

and 'commercial propane'. They exist as gases at normal ambient

temperatures but can be liquefied under moderate pressure. Propane has a

lower 'boiling point' than butane, which means it turns into gas at a lower

temperature. In order for it to become liquefied, propane is stored under

higher pressure, approximately 7 bar as opposed to about 2 bar for butane.

LPG liquid is colourless and its density as a liquid is approximately half that

of water. If LPG liquid is spilt onto water, it will float on the surface before

vapourising. One volume of liquid produces approximately 250 volumes of

gas and thus a leak of liquid poses a considerable hazard.

LPG vapour is heavier than air, butane about twice as heavy and propane

about one and a half times heavier. Because of this, LPG vapour will tend

to fall to low level and flow along cockpit floors, bilges and other low lying

places. LPG vapour may travel some distance and in poorly ventilated

places the vapour may persist for some time.

A mixture of LPG and air at a concentration of LPG between 2% and 10%

is a flammable mix. Outside of this range the mixture is either too weak or

too rich to ignite. If vapour within the flammable range is present in a

confined space an explosion will result from ignition.

There have been incidents in which escapes of LPG have been ignited

resulting in serious fires or explosions. If the escape is into confined

spaces and becomes ignited, the occupants of the craft, or persons in the

vicinity could be put at serious risk. It is also possible for vapour/air

mixtures arising from leaks or other causes to be ignited some distance

In extreme cases, if the cylinder is lying on its side or if the cylin`der valve is

faulty or if the exposure to the fire is extended, the cylinder may rupture

leading to a violent explosion.

If an LPG cylinder is involved in a fire started elsewhere on the craft, the

cylinder may be subject to direct flame impingement. After a period of

exposure to extreme heat the pressure may rise inside the cylinder to a

point where the pressure relief valve may operate releasing LPG under

pressure causing a significant jet of burning gas.

At very high concentrations, as LPG vapour replaces available oxygen

it will act initially as an anaesthetic and subsequently as an asphyxiate.

In the vapour phase LPG exists as a heavy gas, being approximately one

and a half to two times the density of air. LPG vapours are heavier than air

and will stay close to the ground seeking to enter trenches, drains and other

low areas. Dispersion of these vapours will also take longer than would be

the case with gases lighter than air. For this reason LPG should not be

stored or used in locations where any escape could concentrate in low lying

areas or enter drains, trenches or basement areas.

In its refined state, LPG is normally odourless. To detect leaks easily, using

the sense of smell, the published Standards for commercial butane and

propane call for the gas to have a characteristic odour. This is usually

achieved by the addition of a stenching agent.

The odour required has to be sufficiently strong to ensure detection, by

smell, of a leakage down to a concentration in air of one fifth of the lower

limit of flammability.

A mixture of LPG and air is flammable within certain concentrations, known as

the flammable range. This range is 1.8% to 9.0% for BUTANE and 2.2% to

10% for PROPANE. These are approximate values at atmospheric pressure.

At higher pressures, or with oxygen, the flammability limits are different.

An accidental release of LPG will be be too rich to burn until it has mixed

with air and will tend to settle in low places. On the fringe the gas cloud will

mix with air and if the resultant combustible mixture reaches an ignition

source, it will ignite. Heat generated by the burning gas causes turbulence

mixed within the flammable range. A serious hazard may then

b) petrol

Petrol evaporates easily and quickly to the atmosphere to give rise to

extremely flammable vapours. The vapours given off are three to four times

heavier than air. They sink to the lowest level of their surroundings and will

accumulate at low level in places such as pits, depressions, the hulls of

boats or in enclosed spaces such as the cabins and cockpits of boats.

For petrol vapour to ignite in air, the mixture of vapours must fall within

certain concentrations; defined as the upper and lower explosive limits. For

petrol concentrations below 1.4%, in air the mixture is too lean to ignite, and

for those above 7.6% too rich; at all concentrations between these two

limits, a mixture of petrol vapour and air will burn. Hence whenever petrol

vapours are released, although they may be too rich initially to burn, they will

always pass through the flammable range to give a mixture that is capable

of being ignited before they are eventually diluted to safe levels. How long

flammable concentrations of vapours persist and how far they travel before

being diluted to a safe level will depend on the rate of release of the

vapours and the level of ventilation acting to disperse them.

Petrol vapours will not only be released from any spilt product but, because of

the ease with which it evaporates, they are released in significant quantities

whenever petrol is handled, dispensed or transferred from one container to

another. As a consequence of the flammable limits, petrol vapour when

diluted 70 fold with air will still give rise to a flammable mixture and it can be

calculated that one litre of petrol can, on evaporation, give rise to proximately

c) diesel

Liquids do not burn - it is the vapour given off under heating which sustains

combustion. Unless a fuel is very volatile it will require heating to reach the

vapour production stage and the temperature at which this is achieved is

termed the flash point.

d) hydrogen

Colourless, odourless, highly flammable and explosive, it forms ignitable

mixtures with air over a very wide range of concentrations. It is very buoyant

(much lighter than air) and explosive mixtures are formed very quickly.

Flammable limits 4.9 to 75% by volume in air. It may be ignited by contact

with catalytic substances at temperatures considerably below its autoignition

temperature. Very easily ignited by low-energy sparks.