A gas holder (commonly known as a gasometer, sometimes also gas bell, though that term applies to the gas holding envelope alone) is a large container where natural gas or town gas is stored near atmospheric pressure at ambient temperatures. The volume of the container follows the quantity of stored gas, with pressure coming from the weight of a movable cap. Typical volumes for large gasholders are about 50,000 cubic metres, with 60 metre diameter structures. Gasholders tend to be used nowadays for balancing purposes (making sure gas pipes can be operated within a safe range of pressures) rather than for actually storing gas for later use.
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Gas more recently was stored in large underground reservoirs such as salt caverns. In modern times however line-packing is the preferred method.
Throughout the 1960s and 1970s it was thought that gasholders could be replaced with high pressure bullets. However, regulations brought in meant that all new bullets must be built several miles out of towns and cities and the security of storing large amounts of high pressure natural gas above ground made them unpopular with local people and councils. Bullets are gradually being decommissioned. It is also possible to store natural gas in liquid form and this is widely practiced throughout the world.
Gasholders hold a large advantage over other methods of storage. They are the only storage method which keeps the gas at district pressure (the pressure required in local gas mains). Once the District Low Pressure Switch falls, and the booster fans come on, the gas in these holders can be at homes, being used, in a very short space of time. Gas is stored in the holder throughout the day, when little gas is being used. At about 5 p.m. there is a great demand for gas and the holder will come down, supplying the district.
There are two basic types of gasholder, rigid waterless and telescoping. Rigid waterless gas holders were a very early design which showed no sign of expansion or contraction. There are modern versions of the waterless gas holder, e.g., oil-sealed, grease-sealed and "dry seal" (membrane) types.[1]
Telescoping holders fall into two subcategories. The earlier of the telescoping variety were column guided variations and were built in Victorian times. To guide the telescoping walls, or "lifts", they have an external fixed frame, visible at a fixed height at all times. Spiral guided gasholders were built in the UK up until 1983. These have no frame and each lift is guided by the one below, rotating as it goes up as dictated by helical runners.
Both telescoping types use the manometric property of water to provide a seal. The whole tank floats in a circular or annular water reservoir, held up by the roughly constant pressure of a varying volume of gas, the pressure determined by the weight of the structure, and the water providing the seal for the gas within the moving walls. Besides storing the gas, the tank's design serves to establish the pressure of the gas system. With telescoping (multiple lift) tanks, the innermost tank has a ~1ft wide by 2ft high lip around the outside of the bottom edge, called a cup, which picks up water as it rises above the reservoir water level. This immediately engages a downward lip on the inner rim of the next outer lift, called a grip, and as this grip sinks into the cup, it preserves the water seal as the inner tank continues to rise until the grip grounds on the cup, whereupon further injection of gas will start to raise that lift as well. Holders were built with as many as four lifts. [2]
Gasholders are often a major part of the skylines of low-rise British cities, due to their large distinctive shape and central location. The pollution associated with gasworks and gas storage makes the land difficult to reclaim for other purposes, but some gasholders, notably in Vienna, have been converted into other uses such as living space and a shopping mall and historical archives for the city. Many sites however were never used for the production of 'town gas', therefore the land contamination is relatively low.
Most British cities will have several gasholders. London, Birmingham, Manchester, Sheffield, Leeds, Newcastle and Glasgow (which has the largest gasometers in the UK[3]) are noted for having many gasholders. Some of these gasholders have become listed buildings.
A gasworks in South Lotts, Dublin, Ireland was converted into apartments.[4]
In the past, holder stations would have an operator living on site controlling their movement. However with the process control systems now used on these sites, such an operator is obsolete. The tallest gasometer in Europe is 117 metres (384 ft) tall and is located in Oberhausen.[5]
Gasometers are comparatively rare in the United States. The most notable of these were erected in St. Louis by the Laclede Gas Light Company in the early 20th century. These Gasometers remained in use until the early first decade of the 21st century when the last one was decommissioned and abandoned in place. The most recently used gasometer in the United States is on the southeast side of Indianapolis but it is to be demolished along with the Citizens Energy Group coke plant. Another pair of holders at the Newtown Holder Station, in Elmhurst, Queens, in New York City, was a popular landmark for traffic reporters until the holders were demolished in 1996.
The term gasometer was originally coined by William Murdoch, the inventor of gas lighting, in the early 19th century. Despite the objections of his associates that his so-called "gazometer" was not a meter but a container, the name was retained and came into general use. The word is also used to describe a gas meter (a meter for measuring the amount of gas flowing through a particular pipe). The term "gasometer" is discouraged for use in technical circles where the term "gasholder" is preferred.
A dry-seal gasholder can be designed to have a gross (geometric) volume ranging from 200 to 165,000 m3 (7,100 to 5,800,000 cu ft), whilst having a working pressure range between 15 and 150 millibars (1.5 and 15 kPa). The dry-seal gasholder is finished with an anti-corrosive treatment to counteract local climatic conditions and also any chemical attack from the stored medium. This anti-corrosive treatment is fully compatible with the sealing membrane and also the environment.
The dry seal gasholder has four major elements — the foundation; the main tank; the piston; the sealing membrane. Each of these elements can be divided into various sub-elements and associated accessories.
A concrete and hardcore base designed to withstand the weight of the steel gasholder structure constructed upon it and to withstand dynamic climatic conditions acting upon the gasholder etc.
The main tank is designed to accommodate the design requirements laid down by the customer and climatic conditions There are three main sub-elements to the tank:
The gasholder piston moves up and down the inside of the shell as gas enters and exits the gasholder. The weight of the piston (less the weight of the level weights) produces the pressure at which the gasholder will operate. The piston is designed to apply an equally distributed weight to ensure that the piston remains level at all times. The piston made up of the following sub-elements:
The seal of the gasholder is designed to operate in the conditions specified by the client and to suit the stored medium. The seal rolls from the shell to the abutment surface of the piston and vice versa providing the piston with a frictionless self-centering facility. During depressurisation the seal also provides a gas tight facility that protects the holder from vacuum damage by blocking the gas outlet nozzle. During commissioning of the gasholder the sealing membrane is set into an operating condition. This setting must be carried out every time the gasholder is depressurised, otherwise known as "popping" the seal.