why is lpg filled in cylinders under pressure


LPG – Liquefied petroleum gas LPG is the abbreviation or short form for
liquefied petroleum gas. Like all fossil fuels, it is a non-renewable source of energy. It
is extracted from crude oil and natural gas. The main composition of LPG are hydrocarbons
containing three or four carbon atoms. The normal components of LPG thus, are propane (C
). Small concentrations of other hydrocarbons may also be present. Depending on the source of the LPG and how it has been produced, components other than hydrocarbons may also be present. LPG is a gas at atmospheric pressure and normal ambient temperatures, but it can be liquefied when moderate pressure is applied or when the temperature is sufficiently reduced. It can be easily condensed, packaged, stored and utilized, which makes it an ideal energy source for a wide range of applications. Normally, the gas is stored in liquid form under pressure in a steel container, cylinder or tank. The pressure inside the container will depend on the type of LPG (commercial butane or commercial propane) and the outside temperature. When you start using LPG, some of the pressure in the container is released. Some of the liquid LPG then boils to produce vapour. Heat is needed to convert the liquid to vapour (known as the latent heat of vaporization). As the liquid boils, it draws the heat energy from its surroundings. This explains why containers feel cold to touch and why, if there is a heavy off-take, water or ice may appear on the container. When you stop using LPG, the pressure will return to the equilibrium value for the surrounding temperature. The pressure of the LPG in the container varies with the surrounding temperature. It is also much higher than is needed by the appliances that use it; it needs to be controlled to ensure a steady supply at constant pressure. This is done by a regulator, which limits the pressure to suit the appliance that is being fuelled. It is a colourless and odourless gas to which foul-smelling mercaptan is added so that leak can be easily detected.


LPG is highly inflammable and must therefore be stored away from sources of ignition and in a well-ventilated area, so that any leak can disperse safely. Another reason why care should be taken during storage is that LPG vapour is heavier than air, so any leakage will sink to the ground and accumulate in low lying areas and may be difficult to disperse. LPG expands rapidly when its temperature rises. So whenever a container is filled, sufficient space is left to allow for such expansion. LPG will cause natural rubber and some plastics to deteriorate. This is why only hoses and other equipment specifically designed for LPG should be used. Although LPG is non-toxic, its abuse (like that of solvents) is highly dangerous. LPG should always be treated with respect and kept away from children whenever possible. Liquid petroleum gases were discovered in 1912 when Dr. Walter Snelling, an American scientist, realized that these gases could be changed into liquids and stored under moderate pressure. From 1912 and 1920, LP-gas uses were developed. The first LPG cook stove was made in 1912, and the first LPG -fueled car was developed in 1913. The LPG industry began sometime shortly before World War I. At that time, a problem in the natural gas distribution process popped up. Gradually facilities were built to cool and compress natural gas, and to separate the gases that could be turned into liquids (including propane and butane). LPG was sold commercially by 1920. LPG Measurement – How is LPG Measured? The distinctive properties of LPG mean that it is measured in a number of ways. Vapour Pressure – kilopascals (kPa), Bar or PSI A pressure-temperature chart also shows temperature effects. When LPG is measured by weight, the units of measure are kilograms, pounds, tons or tonnes (metric tonnes). volume is typically measured in litres or gallons. When LPG is vapour (gas), the volumetric units of measure are cubic metres (m ). LPG energy content is expressed in Megajoules (MJ), Gigajoules (GJ) or Petajoules (PJ), with the unit of measure dependent on the quantities discussed.

BTUs and Therms are also units of measure for energy. The units of measure for pressure are kilopascals (kPa), Bar or pounds per square inch (psi). A common way of measuring and selling LPG is by weight, in kilograms. There are and the gas is sold based on the contents of a full bottle. For example, LPG for homes is frequently delivered in. BBQ gas is typically supplied in 8. 5kg gas bottles. These numbers represent the weight of the gas inside the bottle, excluding the tare weight of the gas bottle. Larger quantities of LPG are measured in tonnes (1,000kg). In the US, the measurements would be in pounds and US tons (2,000 pounds). For example, a typical BBQ gas bottle in the US contains 20 pounds of LPG. LPG is a liquid when it is under pressure. When hose or LPG dispenser, for Autogas, the unit of measure is typically in litres. As the specific gravity of LPG propane is about half that of water, there are roughly 2 litres of LPG per kilogram. The exact weight to liquid conversion depends on the, relative to the propane and/or content. It is also temperature dependent. The temperature of the LPG must be known to calculate measurement by volume. LPG liquid meters have an automatic temperature correction process. See the table below for propane conversion values. Once again, the US measurement is different, this time being in US gallons. One gallon equals about 3. 8L. When it is not under pressure, LPG exists in its gaseous or vapour form. As with other gases, the volume of LPG is measured in cubic meters (m ). In the US, they use cubic foot (ft ) as the unit of measure. One ft. The energy value or calorific value of LPG is frequently expressed in. Larger quantities are expressed in Gigajoules (GJ). Petajoules (PJ) are used when huge quantities are being discussed, as in national consumption or production. In the US they use BTU. BTU, or British Thermal Unit, is another measure of energy based on the Imperial System.

One BTU is the energy required to raise the temperature of 1 pound of liquid water by 1 F, at sea level. 1MJ equals 948BTU, so to get an equivalent number of Megajoules, just divide the BTU rating by 948. As previously mentioned, when LPG is stored in a gas bottle, it is under pressure. The term pressure refers to the average force per unit of area that the gas exerts on the inside walls of the gas bottle. The pressure is measured in kilopascals (kPa). Bar is yet another unit of measure for pressure. 1 Bar = 100 kPa, so it is metric based but not an SI unit of measure. LPG vapour pressure can vary greatly based on temperature. The level of fill in the gas bottle comes into play when the LPG is being used, as it affects the rate of vapourisation. As LPG is a liquefied gas, the vapour pressure inside the cylinder will remain the same from full until the last of the liquid LPG is vapourised. Then the pressure will fall quickly as the last of the LPG vapour is used, as well. The use of pressure as a measurement in LPG use is very limited. A steady pressure only shows that liquid LPG is inside the cylinder, but not how much liquid is left. In the US, vapour pressure is measured in pounds per square inch (psi). Psi is defined as 1 pound of force applied per square inch. As a comparison, 1 psi = 6. 89476 kPa. How you measure LPG relates to the context in which you are interested. It also depends upon the country in which you live. Most small LPG cylinders are sold by weight (kg or pounds) and most large tanks are filled and invoiced using litres (or gallons). Comments, questions or feedback? This email address is being protected from spambots. You need javascript enabled to view it. The information in this article is derived from various sources and is believed to be correct at the time of publication. However, the information may not be error free and may not be applicable in all circumstances.

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