What is LPG?
The term LPG is an abbreviation for Liquefied Petroleum Gas and is produced in oil refining and the processing of natural gas liquids. LPG is pressurised and stored in liquid form in portable cylinders for a variety of purposes. Itis generally stored and distributed as a liquid and it is widely used for process and space heating, cooking and automotive propulsion.
Butane and Propane are the predominant constituents of LPG. Commercial, or fuel grade LPG mainly consists of Butane and Propane with small amounts of lighter and heavier fractions, such as Ethane and Pentane. Like other forms of energy, LPG can be hazardous if not properly handled correctly. This report covers some important aspects of LPG gas safety which the domestic consumers should know about when using LPG.
Chemical and Physical Properties of LPG
LPG sold for domestic consumption is made up of 60% by weight of propane and 40% by weight of butane. Butane (C4H10) has a boiling point of -0.5oC while that of propane (C3H8) is only -42.1oC. Commercial propane sold on its own requires an air to fuel ratio of 22.5 to 1 while commercial butane requires a ratio of 29.5 to 1 for combustion.
This partly explains the propane to butane weight ratio of 3 to 2 sold in LPG gas. Both are gases at room temperature and pressure but when compressed become liquids. LPG expands to gas by a factor of more than 250 when released into the atmosphere. Propane has a latent heat of vaporisation of 20.43 kJ/kg at 15oC while that for butane is 21.27 kJ/kg.
The expansion, which requires the addition of this latent heat of vaporisation produces a cooling effect. In warm environments like those prevailing in Zimbabwe, the ambient will provide that energy with minimal reduction in the temperature of the cylinder. In colder regions, the temperature can fall below the boiling point of butane, which results in preferential vaporisation of butane compared to propane.
The gross calorific values for propane and butane are both very high: 49.8 MJ/kg and 49.4 MJ/kg respectively. This is one of the major reasons why LPG gas is widely used for cooking and heating applications in the domestic, hotel, leisure and industrial sectors.
LPG liquid has a density of 0.536 kg/l at 15oC. Therefore, it would float on water before vaporisation. As a gas, LPG is 1.9 times denser than air. This is why it is important to ensure low level ventilation in areas where LPG is stored and used.
LPG is clear and non-toxic in both liquid and gaseous forms. LPG vapour acts like an anaesthetic in low concentration levels, but becomes an asphyxiate in the absence of adequate oxygen. When compressed to become a liquid at room temperature, LPG has a large volumetric expansion. This causes cylinders to explode when exposed to very high temperatures if they had been overfilled.
Dangers associated with LPG
The hazards commonly associated with LPG are fire and explosion. This derives from its inherent characteristic of high flammability and in extreme cases may combine with high pressure, and lead to a phenomenon known as the Boiling Liquid Expanding Vapour Explosion (BLEVE) phenomenon. This type of explosion occurs when a vessel containing a pressurised liquid is ruptured due to high temperature and pressure. Such explosions can be extremely hazardous.
As pure LPG is odourless and invisible, a distinctive garlic-like odour is usually added to warn of its presence thus enabling easier detection. Ethyl mercaptan, C2H5SH, also referred to as ethanethiol, is usually added to give it that characteristic odour to make it easier to detect. It is a colourless organic liquid that has a strong odour and is added to odourless fuel and fuel systems as a warning agent.
However, as LPG is heavier than air, (it is almost twice as dense as air) a low level leak might not be immediately detectable. LPG may leak in either gaseous or liquid form. In the case of a liquid leakage, it quickly evaporates to form a relatively large cloud of gas which will drop to the ground, owing to it being heavier than air.LPG vapours can run for long distances along the ground andcan collect in drains or basements.
Upon contact with an ignition source, it can burn or explode as shown in Figure 1. Cylinders can explode if exposed to fire. Liquid LPG has a high co-efficient of volumetric expansion which is ten times that of water. Therefore cylinders and tanks should never be completely filled but should only be filled up to 85% of the liquid level capacity.
This potential risk explains why cylinders and tanks should only be filled under the supervision of competent persons and why illegal filling is dangerous because of the risk of overfilling.
LPG can cause severe cold burns to the skin because of its rapid vaporization and consequent lowering of the temperature if it comes into contact with the skin as shown in Figure 2.
The accumulation of LPG vapour may result in the development of an oxygen-deficient atmosphere which carries a risk of asphyxiation (a condition of severely deficient supply of oxygen to the body).An additional potential hazard may arise at the point of use if ventilation is inadequate and the products of combustion are not dispersed into the atmosphere. Carbon monoxide may be produced and reach dangerous levels. LPG is non-corrosive but can dissolve lubricants, certain plastics and synthetic rubbers.
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