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The Health and Safety Executive (HSE) is warning occupiers of sites...
The Health and Safety Executive (HSE) is warning occupiers of sites

and local authorities with their own gas mains that medium pressure

ductile iron (MPDI) pipes need to be replaced.

This follows enforcement action requiring Transco to carry out an

agreed accelerated programme of replacement of all MPDI gas mains

located within 30 metres of buildings. This work is due for

completion by the end of 2002.

The risk of failure of MPDI mains located close to buildings is such

that they should be replaced on all networks as soon as reasonably

practicable. As a result HSE is to begin a programme of work to

ensure non-Transco MPDI networks are also replaced as soon as


HSE has recently been notified of a small number of sites where such

networks may be in place and are contacting the owners of these

premises. HSE is also contacting the owners of other sites such as

hospitals, universities, prisons, Ministry of Defence premises as

well as some industrial and commercial premises to eliminate any

other potential problem areas.

Dan Mitchell, head of land division, HSE Hazardous Installations

Directorate, said:

'Presently we don't know how many of these networks exist. Although

Transco has brought a number to our attention, there may be many

more. Occupiers of large sites and local authorities need to

establish who own the pipes, determine what material they are made of

and, where appropriate, implement a suitable programme of


'The problem with ductile iron is that it can fail unpredictably. In

view of this and the difficulties with accurately assessing its

condition, mains close to property need to be replaced as soon as

reasonably practicable. As Transco's programme is drawing to a close,

HSE is now turning its attention to other organisations who may own

MPDI networks.

'This is an important issue and inspectors will be taking a firm

enforcement line to ensure occupiers take the necessary action


While HSE is currently focusing on MPDI networks, occupiers and local

authorities should be aware that there is also an aging population of

cast iron and low pressure ductile iron networks which should be

considered for replacement.


1. Cast iron mains can fail unpredictably particularly through

fracture if exposed to excessive loading, such as heavy traffic or

ground movement. Ductile iron pipe was introduced in the 1970s in

response to problems with cast iron. It has greater strength and

flexibility compared with cast iron. It was later found, however,

that ductile iron could fail unpredictably through corrosion. When

this happens, there is a risk that the gas will track into adjacent

properties, where it may subsequently be ignited. Ductile and cast

iron have now been superseded by polyethylene for all low and medium

pressure mains.

2. Transco has already replaced all MP cast iron mains below 12'

diameter within 20 metres of property.

3. 'Medium pressure' means a pressure above 75 millibar.

4. An Improvement Notice was served on Transco in September 2000 to

confirm an agreement reached that all 'at risk' MPDI mains should be

replaced by the end of 2002, a total length of 2,300 km.

5. An Improvement Notice is a statutory notice requiring specified

improvements to be made within an agreed time scale. A failure to

comply is a prosecutable offence under the Health and Safety at Work

etc. Act 1974 (the HSW Act).

6. Recipients have the right to appeal against an Improvement Notice

under Section 24(2) of the HSW Act, which states: 'A person on whom a

notice is served may within such period from the date of its service

as may be prescribed appeal to an Employment Tribunal; and on such an

appeal the tribunal may either cancel or affirm the notice and, if it

affirms it, may do so either in its original form or with such

modifications as the tribunal may in the circumstances think fit.'

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