Storage
In transporting gas from the production field to the end user, the demand fluctuates throughout the day, known as “diurnal swing”. The production field exports gas at a steady rate, but it is used variably throughout the day by customers – more at breakfast and evening, and less during the day and overnight. To compensate for this, the gas is stored when demand is low, so that it can be used when demand is high. This is all controlled by a central team, and to help them they have various methods to achieve this.
As gas is transported along a pipeline, its pressure falls due to friction in the pipe, and also as gas is taken off at various points. To compensate, compressor stations are built at strategic points, which lift the pressure back up. This helps transport larger amounts than would otherwise be possible. At times of low demand, this pressure can be raised, and then allowed to fall gradually during high demand, when the flow is greater. This is call “linepack storage”, and is utilised mainly within the higher pressure pipelines used to transmit gas throughout the country.
At the Local Network distribution areas, other types of storage are utilised. The gas can be stored at high pressure in “bullets”, which are long cylindrical pipes, either below or above ground level, ranging in diameter from about 1m to 4 m. This high pressure gas can then be released in to the local area network at times of high demand, so that gas supplies are secure. Another type of gas storage, much older and dating back to the days when gas was made from coal in most towns and cities, is called the Gas Holder, sometimes referred to as gasometers. These are still visible in a number of town centres, and are usually filled overnight, and the gas slowly released during the daytime. The amount of gas stored can be seen, as they rise and fall as they are filled and emptied. This is a form of low pressure storage, and the gas is stored at about the same pressure as it is when it arrives at the customer's house.
Another novel type of storage, particularly in the North of England Network, is the use of salt cavities to store gas underground. When brine is extracted from deep boreholes, it leaves behind a large cavity in the ground. This can be adapted to store gas at a high pressure, and is useful to help with the demand at peak times.
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