LONDON CAST IRON GAS MAIN
The implementation of the UK Health and Safety Executive’s national replacement programme of cast iron and ductile gas mains within 30 metres of buildings has required extensive excavation works and the disruption to services to upgrade the ageing gas networks throughout London.
Identifying one such large cast iron pipe within the densely populated district of Bethnal Green, National Grid engaged Radius Subterra to provide a cost effective rehabilitative solution that would minimise disruption to the area and maximise the existing pipe capacity.
Three sections of the main totalling 300m required renovation in Whiston Road, with an additional 140m requiring renovation in the adjoining Pritchard Road.
Whilst National Grid had previously engaged Radius Subterra for the renovation of smaller diameter mains, this 24” cast iron main was the largest attempted to date.
Furthermore, as the works were located in a densely populated area with heavy traffic and footfall, excavation was impractical and any disruption to the area or mains service needed to be minimised.
As the existing iron mains pipe was predominantly structural sound, Radius Subterra chose to resolve any corrosion and cracks or leakage issues using their Subline PF system to close-fit line the mains with a semi-structural polyethylene pipe liner.
Especially suited to large diameter pipes, the Subline PF process welds thin walled polyethylene pipes into long lengths that are then diameter reduced using a former which folds the liner pipe into compact cross section.
These are then strapped with bands that allow the liner to be easily inserted into the host pipe.
The large reduction in cross section allows for much lower winching forces, better enabling deep pipelines to be relined to seal cracks, span gaps and prevent further deterioration to the host pipe whilst also maintaining its capacity.
Once on site, the pipe was welded into continuous lengths and fed through the Subline forming unit to form the “U” shaped cross section before an automatic strapping unit located and friction welded the polyester straps at regular intervals along the pipe to keep the deformed shape.
Using the traditional slipling technique, a winch cable was connected to the front end of the liner pipe and the pipe pulled into position within the host main. As the cross sectional area had been reduced by approximately 40%, the winch load was low and an insertion rate of 1m per minute was achieved.
Once the liner was in place, reversion was carried out using cold water at a pressure of 2 bar, breaking the restraining straps to form a close-fit against the main.
The whole renovation process and recomissioning of the gas mains service was completed in a remarkably quick time and without the need for extensive excavation, and disruption to the area and traffic was kept to a minimum.
The use of thin-walled polyethylene within the Subline PF process also reduced the associated costs required for materials, whilst maximising the pipe’s existing capacity.Download: