Production and distribution
Production and importation
Gas comes from offshore fields in the North and Irish Seas. It is also brought over from Ireland, Belgium and the Netherlands via three interconnector pipes and imported in the form of liquefied natural gas (LNG). A small amount is produced onshore, too. Gas producers, LNG importers and interconnector operators bring the gas onshore to reception terminals and LNG importation terminals.
Hydraulic fracturing, commonly known as fracking, is the process of injecting pressurised fluids into drill holes in a bid to create cracks in the deep rock to access natural gas and petroleum. Once the fracture is made in the rock, a solid material, such as sand, is used to keep it open.
The main hazard is uncontrolled release of hydrocarbon gas due to a failure in the well structure, which may then reach a source of ignition, leading to a fire or explosion. Refer to the Health and Safety Executive website for further information.
The national grid is the sole owner and operator of the gas transmission infrastructure in the UK. Gas producers supply gas to the National Transmission System (NTS) through reception terminals. Gas from the importation terminals is injected into the NTS after quality checks. Also, gas that has been held in storage can be reintroduced into the system. Compressor stations keep the gas flowing through the system.
In the UK, gas leaves the NTS and enters the distribution networks at high pressure. It is transported through a number of reducing pressure tiers until it is finally delivered to consumers. Gas is owned by regional gas suppliers; however, the national grid is responsible for this gas while it is being transported.
Gas leaves the NTS at 49 points across the UK. An odour is added for safety and the gas is then transported through the distribution networks for final delivery to consumers. The majority of the UK's gas is distributed by:
- National Grid Gas - North West, Midlands, East of England and North London
- Northern Gas Networks - North East and North Yorkshire
- SGN - Southern England, South East England and Scotland
- Wales & West Utilities - Wales and the West Country
Refer to the National Grid website for a map showing the gas distribution network.
Whichever gas supplier they choose, gas is delivered to most consumers' premises through a pipe belonging to the local distribution network. Distribution networks deliver gas to around 10.8 million consumers.
A compressor station (or pumping station) is a facility (or engine) that helps transport natural gas from one location to another. Natural gas needs to be constantly pressurised at intervals of 40 to 100 miles while being transported through a gas pipeline. A compressor station compresses the natural gas (increasing its pressure), thereby providing energy to move the gas through the pipeline.
Pipeline companies install compressor stations along a pipeline route. The size of the station and the number of compressors (pumps) varies, based on:
- Diameter of the pipe
- Volume of gas to be moved
- Number of gas wells in the vicinity
- Number of elevation changes
The basic components of a compressor station are turbines, motors and engines. Some of the specialist equipment and terms found in them are:
- Liquid separators - As the pipeline enters the compressor station the natural gas passes through scrubbers, strainers or filter separators. These vessels are designed to remove any free liquids or dirt particles from the gas before it enters the compressors. Although the pipeline is carrying 'dry gas', some water and hydrocarbon liquids may condense out of the gas stream as the gas cools and moves through the pipeline. Any liquids that may be produced are collected and stored for sale or disposal. A piping system directs the gas from the separators to the gas compressor for compression.
- Prime movers - Three commonly used types of engines drive the compressors. They are known as 'prime movers':
- Turbine/centrifugal compressor - This type of compression unit uses a natural gas-fired turbine to turn a centrifugal compressor. The centrifugal compressor is similar to a large fan inside a case, which pumps the gas as the fan turns. A small portion of natural gas from the pipeline is burned to power the turbine.
- Electric motor/centrifugal compressor - In this type of compressor unit, the centrifugal compressor is driven by a high-voltage electric motor. A highly reliable source of electrical power must be available near the station.
- Reciprocating engine/reciprocating compressor - These large piston engines resemble automobile engines, but are much larger. Sometimes referred to as 'recips', these engines are fuelled by natural gas from the pipeline. Reciprocating pistons, located in cylinder cases on the side of the unit, compress the natural gas. The compressor pistons and the power pistons are connected to a common crankshaft. The advantage of reciprocating compressors is that the volume of gas pushed through the pipeline can be adjusted incrementally to meet small changes in customer demand.
Hazards (for further information refer to National Operational Guidance: Utilities and fuel and National Operational Guidance: Industry)
- Electricity (high and low voltage)
- High-pressure gas supplies
- Flammable/explosive atmospheres
- Confined space
References and further reading