Liquid petroleum gas is an important fuel in many parts of the world and a major component of mineral oils; it is separated off as various mixes of the very light propane and butane fractions. Butane liquefies at -4°C and propane at-40°C. LPG, which comes as various mixtures of the two gases, must be refrigerated or pressurised for transport and storage. About 400m³ of flammable gas produces 1m³ of liquid.
But storage above ground is expensive and potentially hazardous. Steel cylinders, tanks and spheres are usually required, capable of resisting fairly high pressures of up to 1.4MPa, reached if sunlight heats them to +40°C. Mounded tanks are safer but less easy to inspect for corrosion etc. Typically, a several hundred metre wide danger zone is needed around tank farms.
Underground storage of LPG was developed as a much safer alternative to above ground systems. Above a certain size, about 50,000m³ depending on ground conditions, it is also cheaper. Maintenance and operating costs are about one sixth those of steel tanks and spheres. Loading and unloading times are quicker and evaporation loss reduced.
