Liquefied Petroleum Gas is an important source of energy for millions of people around the world. LPG consists mainly of gases at atmospheric temperature and pressure (propane and butane), which when subjected to modest pressure or refrigeration can liquefy. This makes it possible to transport and store as liquid in pressurized cylinders and containers, which must be safely and carefully handled.
Liquefied Petroleum Gas (LPG) can be simply described as hydrocarbons that exist as vapours under ambient conditions of temperature and pressure, but can be liquefied by the application of moderate pressure.
When gas is liquefied, the volume occupied by the vapour considerably decreases, thus the liquid formed requires less storage space. The material is therefore stored and distributed in the liquid phase in pressurized containers and systems and is finally allowed to return to the vapour phase at the point of eventual utilization.
Sources of LPG:
LPG as the name suggests, consists mainly of a mixture of hydrocarbons (Propane and Butane) with a little proportion of unsaturates (Propylene and Butylene). These hydrocarbons and unsaturates (LPG) can be produced through two main sources which are:
Wet Natural Gas, which consists entirely of saturated hydrocarbons (Propane and Butane) and can be found in oil or gas fields, being removed as condensable products from natural gas and also extracts from crude oil during the stabilization process applied in order to reduce the vapour pressure prior to shipment;
Refining process to remove impurities like moisture and sulphur compounds (hydrogen sulphide and mercaptans), which may lead to clogging of valves and corrosion. However, due to the odourless nature of LPG, ethyl mercaptan which has a unique odour, is added to alert the user of LPG incase a leak takes place.
The product of crude oil refining fall into three main categories:
The permanent gases, Methane and Ethane which remain gaseous regardless of pressure, unless refrigerated.
Hydrocarbons having five or more carbon atoms per molecule. They are liquid or solid at atmospheric temperature and pressure and account for most of the crude oil refined.
Propane, Butane and Isobutane, together with Propylene, Butylene and Isobutylene have three or four carbon atoms per molecule. All have the special property of becoming liquid at atmospheric temperature if moderately compressed and reverting to gases when the pressure is sufficiently reduced.
Properties of Liquefied Petroleum Gas (LPG)
Vapour pressure is a measure of the volatility of the gas and where vapour exists in conjunction with the liquid phase is referred to as the ‘saturation vapour pressure’. At the boiling point it is equal to atmospheric pressure and increases as the temperature rises to the critical. Propane with its lower boiling point thus exerts a greater vapour pressure under identical conditions than butane.
Knowledge of the vapour pressure of a gas is thus essential in order to be able to specify the design conditions for the pressurized system. It is also required to enable the gas offtake rates by natural vapourization to be calculated.
In practical terms, systems are often specifically designed to be suitable for either butane or propane which thus precludes a butane system from being used for propane, but enables the propane system to be classed as dual purpose.
Boiling Points of LPG
The constituent gases found in a commercial LPG mixture all have very low boiling points and will thus normally exist in the vapour phase, under atmospheric conditions, unless they have been liquefied or refrigerated.
Where the gases are held at a temperature at or below their boiling point, the vapour pressure will be equal to or less than atmospheric. This property has led to the development of large scale storage at marine terminals where the product is held in refrigerated form in what is essentially a non-pressurized system.
Above ambient temperature, the gases exert an increasing vapour pressure, thus increasing the pressure required for liquefaction. This pressure continues to increase until the critical temperature is reached, (96.67oC for propane; 152.03oC for n-butane), above which temperature of the gases cease to exist in the liquid phase even if further pressure is applied.
Latent Heat of LPG
The latent heat of a liquid product is the quantity of heat absorbed to enable vapourization to occur.
In the event of liquid LPG being allowed to vapourize naturally, the latent heat required is taken from itself and its immediate surroundings at the same time, causing a drop in temperature. The process is known as auto-refrigeration.
Very low temperatures can be achieved with propane under such conditions; therefore in order to avoid operators receiving severe cold burns, protective clothing is required.
Specific Volume, Relative Density
LPGs exist as heavy gases approximately 1.5 – 2.0 times the density of air in vapour phase. They reduce in volume considerably on liquefaction (ratio of gas volume to liquid volume at 15.6oC/1016mbar is 233 for butane and 274 for propane) to exist as a clear liquid which is approximately half the weight of water (Propane 0.50 – 0.51, Butane 0.57 – 0.58).
It can thus be seen that LPG vapours heavier than air will tend to cling to the ground seeking to enter trenches, drains and other low areas, which could make it take considerably longer time to disperse.
Leaking liquid phase LPG will rapidly expand to around 250 times its own volume, therefore creating a greater risk than would occur with a similar sized vapour leakage.
Where temperature conditions permit the existence of free liquid from a leakage, the product will float on any water present. This normally occurs with butane in freezing conditions and a typical scenario would occur during firefighting operations.
Coefficient of Cubical Expansion of Liquid
Liquid phase LPG expands considerably when its temperature increases. The coefficients of cubical expansion at 15oC are approximately 0.0016 per oC for propane and 0.0011 per oC for butane. These values are around 4 times the equivalent for fuel oil, 10 times that for water and 100 times that for steel.
This high rate of expansion has to be taken into consideration when specifying the maximum quantity of LPG permitted to be filled into any pressure vessel, ie the filling ratio defined by codes of practice for different specification of LPGs under different ambient conditions.
Because the filling ratio precautions taken to prevent the hydraulic filling of storage systems cannot be extended to the connecting liquid phase pipework, these parts of the system are protected by the provision of small hydrostatic relief valves situated in all areas where the liquid LPG can be trapped between closed valves.
Limits of Flammability
Gaseous fuels will only burn when mixed with air in proportions which lie between two well defined limits, known as the lower and upper limits of flammability. The lower limit being the smallest quantity of combustible gas which, when mixed with a given quantity of air (or O2) will support self-propagating flame.
A leak of 1m3 of liquefied propane will produce 274m3 of propane vapour, which will cause immediate entrainment of air and progressive dilution of the concentration.
When the upper limit of 10% is reached, the propane/air mixture becomes flammable (ie when the propane entrains, 274 – 10 = 2740m3 of air). The mixture only becomes flammable when the lower limit of 2% is reached (ie when the propane entrains 274 – 50 = 13,700m3) of air.
Therefore, should a leak of propane occur, the propane/air mixture will be flammable and hence extremely dangerous until it has been diluted with more than 13,700m3 air per m3 of propane leakage.
The following are the limits of flammability of LPG and some other fuels:
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