Gas
Transmission is the supply of gas at high pressure along
steel pipelines. On the UK mainland pressures are
generally between 7bar and 100bar. Transmission
pipelines deliver gas from gas fields to Local
Distribution Networks which supply gas
at much lower pressures to our Homes
and Industry. As gas flows
along a pipeline it loses pressure due to the friction of the gas
against the walls of the pipeline.
Compressor Stations installed
at strategic points along the pipeline recompress
the gas back up to
pressure. The gas used by homes and industry varies throughout the day
and is usually at a peak at evening tea time, this is known as 'diurnal
swing', however gas flows into a pipeline from a gas field at a fairly
constant rate.Gas has to be
stored in pipelines during periods of of low demand to maintain supplies
when
the demand is higher. This is achieved by allowing pipelines to increase
in
pressure at times of low demand and reduce when the demand is higher.
This is known as 'line pack'.
Pressure Reduction Stations are installed
at the ends of the Transmission system to reduce the gas to a lower pressure
and deliver the volumes of gas needed to supply Local
Distribution Networks. Pressure reduction stations are designed
to ensure that the pressure in the pipeline they are supplying does
not exceed the design pressure of the pipeline and that a failure to supply
could only happen in exceptional circumstances. Transmission pipelines
must be designed to contain gas at high pressure and be positioned so
that they do not present a risk to the community in the unlikely event
of failure. Transmission pipelines must be designed to meet the requirements
of the Pipeline Safety Regulations.
Pipeline operators must also ensure that they comply with the Pressure
Systems Safety Regulations.
Much of the high pressure Transmission system on the UK mainland is owned
by National Grid Transco, There are a number of pipelines owned by companies
supplying gas to large plant such as power stations. Transco
will normally only allow connections to its national transmission system
for gas loads in excess of 58,600,00 kWh per annum (2milllion therms.p.a.)
Any company who is intending to operate a Transmission pipeline must satisfy
the requirements of the Gas Safety Management Regulations by submitting
a Safety Case to the Health a nd Safety
Executive. They must construct, operate
and maintain the pipeline in accordance with internationally recognised
Standards.
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