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