Gas success in the city
Replacing gas mains in a pedestrianised area of Glasgow city centre was never going to be an easy task. But the dead/live insertion method proved the answer.
Replacing large diameter cast iron gas mains in city centres always poses enormous challenges for utility companies. Maintaining supply to businesses and homes, limiting the number and extent of excavations and keeping traffic and pedestrian disruption to a minimum are top of the list.
One hundred and twenty five metres is not a particularly long project for live gas mains insertion, but when the project is in a pedestrianised area just off Ingram Street right in the heart of Glasgow, speed, efficiency and minimum disruption were essential in this high-profile location.
Scotland Gas Networks and its contractor, Forward Construction Group, elected to carry out the project using the dead/live insertion method because of the large number of commercial properties – including restaurants, shops and hotels – being fed by the 15″ cast iron pipe. This allowed them to renew the old main by pushing lengths of 315mm PE into the cast iron pipe with just two excavations open at either end of the inserted section.
Most importantly, customers were able to remain on gas throughout the insertion process. Large diameter live mains insertion differs from the usual process for pipe diameters up to 180mm as smaller diameter PE is delivered in coils and can be gassed up prior to insertion, with a ‘live’ head on the leading end of the PE allowing a two-way flow of gas into the annular space.
With larger diameters, which are only available as straight lengths of 6, 12 or 18m, the PE is blanked off with a PE domed End Cap to avoid the need for squeeze-offs when the lengths are fused. The PE is inserted through a special Lyontech gland box produced by Steve Vick International.
In Glasgow, Forward Construction Group carried out the job at night to minimise disruption to the public. Each length of PE was inserted and butt fused to the next length until the exit excavation at the junction with Ingram Street was reached.
A rider was installed to maintain a two-way supply of gas to the annular space, preserving supply to customers then a flow-stopping operation was carried out by a Steve Vick International Special Contract Services engineer, using a ‘Foam restraining bag’ Foambag to seal off the annular space at the exit end.
The old main was then broken out and access gained to the inserted PE which was tested before being connected to the existing network at the entry excavation. Finally, an Endseal was fitted to the cut-out end of the main to seal off the annular space. Forward Construction Group used a Steve Vick International Pipe Pushing machine to speed up the insertion of the 315mm pipe into the 15″ cast iron. This hydraulically operated machine is capable of pushing in PE pipe up to 355mm in diameter at speeds of up to 4m per minute.
Colin Skinner, team manager with Scotland Gas Networks, says: “We chose to carry out this section of insertion by the dead/live method because of the large number of commercial services being fed by the cast iron. The contractors worked overnight in order to keep disruption to a minimum so we started at 10pm and had completed the insertion by 6am.
“It took longer than it might have done because we had to keep stopping to fuse the lengths of PE together. Using the live insertion method was a success and we can now schedule connection of the services over the coming weeks.”
This is one of five similar renewal projects being carried out by Scotland Gas Networks in Glasgow city centre.
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