Gas Distribution is the supply of gas to homes and industry through mains and services mostly located under roads footpaths and grass verges.

Gas Mains

Gas mains in the united Kingdom fall into the following types:

1. Intermediate pressure mains operating between 2 and 7 bar and constructed from either steel or polyethylene pipe.

2.Medium pressure mains operating between 75mbar and 2bar andconstructed from either steel. polyethylene, cast iron or ductile iron pipe.

3.Low pressure mains operating at approximately 30mbar and up to pressuresof 75mbar and constructed of polyethylene, cast iron or ductile iron pipe.

Gas Services

Gas services are smaller diameter pipes, usually up to 63mm diameterwhich are connected to low,medium or intermediate pressure mains andtake gas into homes. commercial or industrial buildings. A gas serviceregulator is connected to the end of the service pipe before the gas meter to reduce the gas pressure, usually to 21 mbar, to supply gas appliancesinstalled in the premises.

Laying gas mains and services in the highway

Laying gas mains and services in roads and footpaths is controlled byStatutory Regulations called the New Roads and Street Works Act (NURSWA). Companies laying mains inthe public highway have to give notice toLocal Authorities of their intention to excavate in a road or footpath. They must also provide estimates of the length of time a construction project is going to take to complete. Where projects run over the estimated completion time, Local Authorities are able to levy penalties against offending companies.
This is known as Section 74.

Gas mains replacement

There are approximately 275,000km of gas mains in the United Kingdom. Approximately 115,000km are made from cast iron or ductile iron dating back 40 years and insome cases
are over 100years old. The Health and Safety Executive announced in September 2001 that Transco should replace 91,000km of the cast Iron and ductile iron mains system over a 30 year period. Gas mains replacement on this scale will need more people with technical skills for planning and construction. Employees of companies laying gas mains and services must be properly trained and have attained an appropriate Gas Network Operations (GNO) qualification as required by the Gas and Water Industry National Training Board (GWINTO).

Gas on new housing sites

Gas mains and services on new housing sites must be sized so that they can deliver gas to homes at a pressure of approximately 25mbar in the quantities needed to supply all the gas appliances installed in the house. The design of gas mains and services involves a system called Network Analysis. Many of the gas mains and services installed on new housing sites are owned operated and maintained by Licensed Gas Transporters other than Transco. Licensed Gas Transporters must demonstrate through their Safety Case that
their gas networks are fit for purpose and comply with the Health and Safety Executive Gas Safety(Management Regulation). Licensed Gas Transporters must ensure that organisations contracted to lay gas mains and services undertake the work to industry standards before themains and services are adopted into their ownership.
The Gas Industry Registration Scheme (GIRS) ensures that Companies undertaking gas mains and service work are competent. The GIRS scheme is operated by Lloyds Register. A list of Companies known as Utility Infrastructure Providers (UIP's) who are accredited to lay gas mains and services can be found on the Lloyds Register web site

Need a gas service connection

Home owners, except those on new building sites should contact their Gas Supplieror Gas Transporter, in most cases National Grid Transco if they need a gas serviceconnected to their homes. The gas Transporters (GT) license conditions obliges the GT to make connections to any property which lies within 23 metres of a gas main and consumes less than 2,196,000 kW per annum. The GT must provide the connection to the gas main and the first 10 metres of pipework locatedoff the home owners premises free of charge. Full details on how to get connected can be obtained by contacting The Office of Gas and Electricity Markets ofgem.

Gas storage

The gas demand for home and industry varies throughout the day and is usually at a peak around evening tea time. This change in gas demand is called diurnal swing. Gas is stored in Gas Holders at times of low demand to be used when the gas demand increases. Low pressure gas holders were constructed before the 1950's and some were constructed as early as the late 19th century. Gas holders are usually of the water sealed type where the structure of the gas holder moves up and down in a tank of water like a telescope, with water forming the seals between each section of the holder. Each section of the gas holder is called a lift. There are also a small number of waterless gas holders
where a piston moves inside a steel cylinder with gas stored below the piston. Gas is stored in gas holders at low pressures of approximately 15mbar and has to beboosted from the holder into the gas distribution system. No new low pressure gas holders have been constructed for many years. Existing holders have to be inspected and maintained to ensure they continue to operate correctly. They will be demolished when they reach the end of their economic life.

Pressure reduction stations

The intermediate pressure and medium pressure gas distribution system is supplied from the high pressure gas transmission system through pressure reduction stations. Pressure reduction stations usually called gas governors reduce the gas pressure from the intermediate and medium pressure mains into the low pressure distribution system. Pressure reduction stations are designed to ensure that the pressure in a gas main or gas service pipe does not exceed the maximum design pressure. Pressure reduction stations must also be designed so that the gas supply will only fail in exceptional circumstances. If the pressure in the gas distribution system fails then this could result in a dangerous situation arising, unless the supply to all gas appliances is isloated before gas is brought back on stream. Gas mains, services and pressure control equipment are all constructed to
high standards which have been developed by experts in the UK gas industry over many years.