Supporting the gas industry since 1863
Sign In

The Natural Gas System

The National Transmission System

Gas is delivered to the seven reception points (called beach terminals) by gas producers operating rigs in about 100 fields on land and beneath the sea around the British Isles and from the Interconnector from continental Europe. After treatment, which includes checking the quality and adjusting the CV, it is transported through a network comprising 275,000 kilometres of iron, steel and polyethylene pipelines.

The high-pressure part of National Grid Transco's transmission system, the NTS, consists of more than 6,900 kilometres of high quality welded steel pipeline operating at pressures of up to 85 bar. The gas is pushed through the system using 24 strategically placed compressor stations. From over 140 off-take points, the NTS supplies gas to 40 power stations, a small number of large industrial consumers and the 8 Local Distribution networks that contain pipes operating at a lower pressure, which eventually supply the consumer.

The local distribution networks are:

  1. Scotland
  2. North and Yorkshire
  3. North West
  4. East of England (including East Midlands, East Anglia and Yorkshire)
  5. West Midlands
  6. Wales and South West
  7. South and South East
  8. North London

Control Centres manage the flow of the gas from the beach to the end consumer or off-takes for other GT's. It uses telemetered data from all the operational sites to monitor the system. The rate of use of gas is not constant. The National Control Centre operates and balances the high pressure NTS while the Area Control Centres are responsible for the next level down in the gas supply network - they ensure sufficient fuel is available at the right place and the right time to meet consumer needs.

Gas moves through the system at about 25 mph, which means that additional supplies cannot be delivered instantly when demand increases. If gas is needed in Newlyn in Cornwall - the most distant location west currently supplied - and the gas has been landed in Scotland, it would take around 23 hours to reach its destination. It cannot instantly arrive at the location of increased demand so the gas transportation system has to be kept constantly in balance. National Grid Transco ensures a balanced system by buying and selling gas and using stored gas to ensure that, in most normal circumstances, demand can be met. Usually, the system is balanced by gas bought on the open market through the `flexibility mechanism' set up as part of the Network Code - rules that govern the supply of gas. Also gas is held in store (the so-called `Top-up' and `Operating Margins' gas) to ensure the system is balanced and that supplies are maintained.

End of a Network

IGEM, with gas industry support, has published a guidance document (IGEM/G/1) covering the definition of the end of a network, a meter installation and installation pipework. It further defines the location of ECV's and AECV's. Diagrams explain how to avoid owners of, for example, multi-occupancy buildings, inadvertently becoming gas conveyors under legislation.

Operation, Maintenance and Safety of the Gas Transmission System

The NTS is a complex and extensive system, comprising pipeline, compressors and other equipment that is governed by the "network code". This is a set of business rules agreed by National Grid Transco and gas shippers, which enables shippers to use the network on a fair and equal basis. Shippers must sign up to the network code before they can use the system and National Grid Transco is responsible for managing the physical balance of gas in the network. Each shipper is financially responsible for the costs incurred in managing any imbalance and is required to nominate daily forecasts of demand.

The NTS is a comprehensive system. Therefore, it is usually possible to carry out necessary maintenance with little or no inconvenience to the consumer. Much of the medium and low-pressure system has been upgraded in recent years to use polyethylene pipes that have very low leakage rates.

Management of UIP Activities

IGEM has published a Standard "Adoption of Pipe Systems by a GT - Management of UIP activities" (IGEM/TD/101), which sets out the minimum requirements for a UIP to ensure all newly considered pipelines are "fit for purpose" in order to be adopted by GT's.

IGEM/TD/101 includes the following information:

  • adoption criteria and process
  • overall design (which involves sufficient information being available to enable subsequent ensurance of integrity and safety of the GT's system)
  • detailed design review for networks (ensuring the submission is compatible with the GT's existing network)
  • construction (in accordance with relevant technical standards)
  • deviation and variation procedure
  • fitness for purpose of materials and equipment.

All job specific submissions for each project undertaken should satisfy the requirements of IGEM/TD/101.

All GT's, through their Safety Case, can demonstrate that their gas networks are fit for purpose and their activities comply with the GSMR. Therefore, it is essential that GT's seek appropriate evidence that the UIPs can demonstrate compliance with all the necessary technical and legislative requirements, to satisfy the fitness for purpose criteria, prior to the construction and adoption of the assets. This objective is achieved by means of a thorough validation of the UIP, prior to work commencing and a technical audit of the work in progress.

Follow Us on:

Follow us on FacebookFollow us on TwitterFollow us on LinkedInFind us on YouTubeFind us on Flickr