Liquefied petroleum gas
Liquefied petroleum gas (also called LPG, LP Gas, or autogas) is a mixture of hydrocarbon gases used as a fuel in heating appliances and vehicles, and increasingly replacing chlorofluorocarbons as an aerosol propellant and a refrigerant to reduce damage to the ozone layer.
Varieties of LPG bought and sold include mixes that are primarily propane, mixes that are primarily butane, and the more common, mixes including both propane (60%) and butane (40%), depending on the seasonin winter more propane, in summer more butane. Propylene and butylenes are usually also present in small concentration.  A powerful odorant, ethanethiol, is added so that leaks can be detected easily. The international standard is EN 589.
LPG is manufactured during the refining of crude oil, or extracted from oil or gas streams as they emerge from the ground.
At normal temperatures and pressures, LPG will evaporate. Because of this, LPG is supplied in pressurised steel bottles. In order to allow for thermal expansion of the contained liquid, these bottles are not filled completely; typically, they are filled to between 80% and 85% of their capacity.  The ratio between the volumes of the vaporised gas and the liquefied gas varies depending on composition, pressure and temperature, but is typically around 250:1.  The pressure at which LPG becomes liquid, called its vapor pressure, likewise varies depending on composition and temperature; for example, it is approximately 220 kilopascals (2.2 bar) for pure butane at 20 C (68 F), and approximately 2.2 megapascals (22 bar) for pure propane at 55 C (131 F).  LPG is heavier than air, and thus will flow along floors and tend to settle in low spots, such as basements.  This can cause ignition or suffocation hazards if not dealt with.

# Production
LPG is synthesised by refining petroleum or 'wet' natural gas; it was first produced in 1910 by Dr. Walter Snelling, and the first commercial products appeared in 1912. It currently provides about 3% of the energy consumed.

# Usage in vehicles
When LPG is used to fuel internal combustion engines, it is often referred to as autogas. In some countries, it has been used since the 1940s as an alternative fuel for spark ignition engines. More recently, it has also been used in diesel engines. Its advantage is that it is non-toxic, non-corrosive and free of tetra-ethyl lead or any additives and has a high octane rating (108 RON). It burns more cleanly than petrol or diesel and is especially free of the particulates from the latter.
There are two downsides. Firstly it has a lower energy density than either petrol or diesel, so the equivalent fuel consumption is higher, but since many governments impose less tax, it is still usually more cost effective. Secondly, some designs of internal combustion engine require the lubrication of petrol or diesel with lead or lead substitute, and LPG's lack thereof can damage valves or shorten their life. Engines designed for unleaded fuel, equipped with hardened valve seats, are suitable for use with LPG without added upper cylinder lubrication.

# Refrigerant
In highly purified form, various blends of the LPG constituents propane and iso-butane are used to make hydrocarbon refrigerants. Hydrocarbons are more energy efficient, run at the same or lower pressure and are generally cheaper than HFC 134a.  However, a major concern relating to the use of LPG hydrocarbons in HVAC systems is that the hydrofluorocarbon HFC 134a does not present a significant flammability hazard according to the American organization ASHRAE's applicable Standard 34 and addenda.  The flammability of LPG hydrocarbons restrict their use to specially designed systems where the risk of ignition is mitigated especially considering leaks and accidents.

# LPG as cooking fuel
According to the 2001 Census of India, 17.5% of Indian households or 33.6 million Indian households used LPG as cooking fuel in 2001. 76.64% of such households were from urban India making up 48% of urban Indian households as compared to a usage of 5.7% only in rural Indian households. LPG is subsidised by the government. Increase in LPG prices has been a politically sensitive matter in India as it potentially affects the urban middle class voting pattern.
LPG was once a popular cooking fuel in Hong Kong; however, the continued expansion of town gas to buildings has reduced LPG usage to less than 24% of residential units.
LPG is the most common cooking fuel in Brazilian urban areas, being used in virtually all households. Poor families receive a government grant ("Vale Gas") used exclusively for the acquisition of LPG.

# LPG and SNG
LPG has a higher calorific value (94 MJ/m3 equivalent to 26.1kWh/m3) than natural gas (methane) (38 MJ/m3 equivalent to 10.6kWh/m3), which means that LPG can not simply be substituted for natural gas. In order to allow the use of the same burner controls and to provide for similar combustion characteristics, LPG can be mixed with air to produce a synthetic natural gas (SNG) that can be easily substituted. LPG/air mixing ratios average 60/40, though this is widely variable based on the gases making up the LPG. The method for determining the mixing ratios is by calculating the Wobbe index of the mix. Gases having the same Wobbe index are held to be interchangeable.
LPG-based SNG is used in emergency backup systems for many public, industrial, and military installations, and many utilities use LPG peak shaving plants in times of high demand to make up shortages in natural gas supplied to their distributions systems. LPG-SNG installations are also used during initial gas system introductions, when the distribution infrastructure is in place before gas supplies can be connected. Developing markets in India and China (among others) use LPG-SNG systems to build up customer bases prior to expanding existing natural gas systems...

# Fire risk and mitigation
LPG containers that are subjected to fire of sufficient duration and intensity can undergo a boiling liquid expanding vapour explosion (BLEVE). Due to the destructive nature of LPG explosions, the substance is classified as a dangerous good. This is typically a concern for large refineries and petrochemical plants that maintain very large containers. The remedy is to equip such containers with a measure to provide a fire-resistance rating. If the containers are cylindrical and horizontal, they are referred to as "cigars" or "bullets", whereas circular ones are "spheres". Large, spherical LPG containers may have up to a 15 cm steel wall thickness. Ordinarily, they are equipped with an approved pressure relief valve on the top, in the centre. One of the main dangers is that accidental spills of hydrocarbons may ignite and heat an LPG container, which increases its temperature and pressure, following the basic gas laws. The relief valve on the top is designed to vent off excess pressure in order to prevent the rupture of the tank itself. Given a fire of sufficient duration and intensity, the pressure being generated by the boiling and expanding gas can exceed the ability of the valve to vent the excess. When that occurs, an overexposed tank may rupture violently, launching pieces at high velocity, while the released products can ignite as well, potentially causing catastrophic damage to anything nearby, including other tanks. In the case of "cigars", a midway rupture may send two "rockets" going off each way, with plenty of fuel in each to propel each segment at high speed until the fuel is spent.
Mitigation measures include separating LPG tanks from potential sources of fire. In the case of rail transport, for instance, LPG tanks can be staggered, so that other goods are put in between them. This is not always done, but it does represent a low-cost remedy to the problem. LPG rail cars are easy to spot from the relief valves on top, typically with railings all around.
In the case of new LPG containers, one may simply bury them, only leaving valves and armatures exposed, for easy maintenance. Great care must be taken there though, as mechanical damage can occur to the primers, which can result in hazardous corrosion of the containers. For the buried container, only the exposed parts need to be treated with approved fireproofing materials, such as intumescent and or endothermic coatings, or even fireproofing plasters. The rest are amply protected by soil. Speciality removable covers exist for easy access to the dials and components that must be accessed for proper maintenance and operation of the equipment.
LPG containers are subject to significant motion due to expansion, contraction, filling and emptying; even with very thick steel walls. This operational motion makes the burial option less attractive in the long run because it is difficult to detect mechanical damage to the outer waterproofing of the vessel through soil. A small stone scraping back and forth across the epoxy-painted hull can jeopardise the waterproofing and be the cause for corrosion.
Whilst one may calculate and justify on paper the use of inorganic plasters to cover entire spheres, it can be difficult to keep plasters operable for extended periods of time. Major errors have also been made in the past in this field, as the presumption was that the steel substrate would be adequately protected from rusting through the use of alkaline plasters. The alkalinity in such plasters is due to the presence of cement stone. This alkalinity, however, does not typically have a permanent character, which means that waterproofing with high quality epoxy primers is very important. Also, exterior waterproofing of the plaster is required by some fireproofing plaster vendors, as reduced alkalinity in exposed plasters can have a deleterious effect on the cement stone, which binds the plaster in the first place. By contrast, the intumescent and endothermic coatings are usually epoxy based to begin with, meaning that corrosion of the substrate is no problem whatsoever.
Fireproofing, not unlike all passive fire protection products, is subject to stringent Listing and approval use and compliance. The problem with this is though, that exterior structures of this nature are not subject to the building code or the fire code, meaning that one still sees the majority of LPG containers without any fireproofing at all, as there are often no local regulations, let alone any Authority Having Jurisdiction, apart from an insurance inspector, to force owners to use the proper mitigation methods. Insurance companies are also in a competitive quandary, where such items are concerned, as they compete not only on the basis of rates, but also on the strictness of the demands by their inspectors. LPG vessel fireproofing tests are varied. The only realistic exposure offered is done at the Braunschweig test facility of "BAM" Berlin. BAM's procedure is to expose a small LPG container to the hydrocarbon test curve and to quantify the results. North American methods are based on UL1709. While UL1709 uses the correct time/temperature curve for testing, it is limited to testing steel columns (not even beams), whereas BAM actually exposes a real LPG container that has been fireproofed. No matter the fireproofing method one uses, it is very important to pay close attention to listing and approval use and compliance and to be sure that the product one chooses has undergone product certification, whereby the original test included the environmental exposures that the product will be exposed to during operations. Particularly with organic products, such as the endothermic and intumescent ones, one must closely review the ageing criteria and be able to quantify how long the product is expected to be operable for. This is where UL1709 "shines". Anything that can withstand the full battery of environmental exposures prior to the actual fire test, is a very tough product indeed. The idea is to rule out conditions that may render the product inoperable before it is ever exposed to a fire. By using products that have received the appropriate environmental tests FIRST, and the fire expose afterwards, using the very same test sample with all the applicable exposures, one can then demonstrate due diligence, but not otherwise. Likewise, the DIBt ageing qualifications for intumescents have proven to be very reliable. With close attention to the bounding and coverage of ageing and environmental exposures, it is absolutely possible to buy a lot of time for firefighting measures to relieve the LPG containers of the energy exposure from accidental fires and thus reduce the likelihood of a BLEVE to the maximum possible extent.