Articles & Case Studies

The flow must go on

Posted: Thursday 14th May 2009

When a 200 metre section of a Welsh Water main operated by United Utilities needed to be re-routed due to construction of a new dual carriageway, specialist engineering and technical services company Furmanite was called in to carry out the necessary work while the main remained on-line and under pressure.

This is a typical illustration of the value of pipeline intervention (hot tapping and line stopping) services to water utilities companies, allowing maintenance or modification work to be undertaken to water mains and pipelines operating at pressure, with no loss, leakage or disruption to service.

Hot tapping, or under pressure drilling as it is also known in the water industry, is the term applied to drilling into a pressurised line using specialist equipment and techniques to ensure the pressure and line content is safely contained, and is typically used to provide additional branches or modifications, instrumentation monitoring points, or entry points for isolation. Line stopping is used to isolate, temporarily, a section of an operating pipeline (which remains on-stream and pressurised), safely, cost-effectively, and efficiently, either stopping the flow or re-directing it through a by-pass while work is carried out.

The hot tapping process begins with the installation of a welded or mechanical fitting on the pipeline, together with an appropriately-rated full-bore valve, and a drilling machine is then used to remove a section of the existing pipe. The drilling process deploys a pilot drill to break through the pipe wall, at which point the pipeline content fills the void beneath the drill, and air is expelled through the drilling machine purge valve which is then closed to retain the pressure. The drilling process continues to complete the cut, and the cutter and coupon (the disc produced by the process, which is retained by a positive retention device fitted to the cutter pilot drill) are withdrawn into the void above the valve.

The hot tap can be vertical, horizontal or at any angle around the pipe, given that there is sufficient room to install the equipment required. Factors such as line and branch nominal size, pipe material and wall thickness as well as line content, temperature, line pressure, and operating conditions are all taken into account, and will determine the processes and equipment used.

Similarly, line stopping uses a mechanical plugging head, which is inserted into the line via a hot tap and temporary valve. The line stop equipment is installed on the temporary tapping valve (recovered once the operation is complete), which is opened to allow the line stop head to be inserted into the pipeline, rotated, and locked into position. This plugging head is fitted with a sealing element which is activated in the line under pressure, achieving a temporary seal and stopping the pipeline flow or, using a double stop and by-pass, re-directing the flow temporarily, or permanently. Once the repair, alteration or relocation is complete, the line stop head is removed and a completion plug installed, with a blind flange installed over the completion plug to complete the process. A valuable feature of this system allows the process to be reversed and the hot tap connection can be re-used to repeat the line stopping process as required in the future.

In some instances specialist equipment has been developed to meet particular challenges, one example being the folding head line stop. As its name suggests, this folds for insertion into a line through a reduced branch fitting, as opposed to the usual size-on-size fitting which, at large diameters (18 inches and over), can introduce substantial added weight considerations. Being smaller and lighter than a conventional plugging head the folding head line stop can be more readily transported to site, and can be useful in more remote conditions where access is difficult and suitable lifting equipment may not be available.

Furmanite, for example, offers capability and experience in hot tapping and line stopping at large diameters (to 72 inches), high pressure (over 100 bar), high temperatures (to 370°C) and on large scales requiring multiple simultaneous hot taps.

In the case of the Welsh Water main, re-routing was required as a result of the construction of the new A465 dual carriageway, as the main’s existing route took it underneath the new road. Disruption of the water supply to some 20,000 people in the Abergavenny region while this work was carried out was avoided when Furmanite (which offers capability and experience in pipeline intervention at diameters to 72 inches and on large scales requiring multiple simultaneous hot taps) applied its hot tapping and line stopping expertise to re-route the line while flow continued uninterrupted.

Hot taps were undertaken into the 28 inch, 7 bar water main at each end of the 200 metre section of the line, to tie-in a by-pass which was to become the new permanent route for the line. Line stops were then undertaken at each end, to redirect flow from the original line into the by-pass or new route, allowing the necessary work to be undertaken to the redundant section of original line. The whole procedure was successfully completed over ten working days, with no interruption in water supply to Welsh Water’s customers in the region.

Another typical scenario is the use of hot tapping and line stopping techniques where valves need to be repaired or removed and replaced. In one such example, a distribution system feeding a 250,000-resident region in southern England required a new isolation valve to be fitted, involving shutting down the system, but the existing valves further down the line could not be relied upon to hold the water pressure. The traditional option of shutting down and draining the system to allow the valves to be replaced would have been a lengthy process, resulting in considerable disruption to local residents. Instead, line stopping technology (in this instance requiring Furmanite to design a specialist plug to meet the non-standard cast iron pipeline dimensions) provided a temporary isolation to allow the work to be carried out between midnight and 6am, using the reservoir supply in the meantime to ensure customers would not be without water during this low-demand period.

A similar situation was encountered in another case, where replacement of four eight inch nominal bore valves would traditionally have meant several days of unacceptable disruption to supply – in this case including supply to a hotel and major conference centre facilities. Once again, hot tapping and line stopping technology was used to provide a temporary isolation at night time (during which time the hotel system was sufficient to meet the low demand) and allow the work to be carried out to replace the valves without requiring the system to be depressurised, thereby minimising the time involved and disruption to customers.

Where repairs or alterations to pipes and pipelines are needed that would once have required shutdown to a portion of line or system, these hot tapping and line stopping techniques provide a cost-effective alternative, avoiding or minimising disruption to flow while work is carried out, helping water companies and contractors provide an optimum service to consumers.

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