Specialist intervention techniques maintain the flow

Hot tapping and line stopping can be used to undertake repairs and modifications online, with no disruption to flow or service. Mike Tucker, head of sales at Furmanite International, reviews the technology

A 200m section of a Welsh Water main, operated by United Utilities, needed to be rerouted due to construction of a new dual carriageway. Specialist technical services company Furmanite was called in to do the necessary work while the main remained online and under pressure.

The example is one typical illustration of the use of pipeline intervention (hot tapping and line stopping) technologies by water and utilities companies, to enable 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 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, on the other hand, 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 redirecting it through a by-pass while work is carried out.

Briefly, 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. 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disc produced by the process, which is retained by a positive retention device fitted to the cutter pilot drill) are withdrawn into the void above the valve.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, redirecting 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 reused 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. This, as its name suggests, folds to enable it to be inserted into a line through a reduced branch fitting, as opposed to the usual size-on-size fitting which, at large diameters (457mm 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 it can be useful in more remote conditions where access is difficult and suitable lifting equipment may not be available.

Pipeline intervention techniques are not new in themselves, but expertise is expanding all the time as boundaries are pushed.

Furmanite, for example, following its acquisition of Flowserve GSG including Ipsco last year, is now able to offer an extended capability and experience in hot tapping and line stopping at large diameters (to 1.83m), high pressure (more than 100 bar), high temperatures (to 370°C) and on large scales requiring multiple simultaneous hot taps.

In the case of the Welsh Water main, rerouting 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's hot tapping and line stopping technology was applied to reroute the line while flow continued uninterrupted.

Hot taps were undertaken into the 711mm, 7 bar water main at each end of the 200m 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 done to the redundant section of original line.

The whole procedure was completed over ten working days, with no interruption in water supply to Welsh Water's customers in the region. Another typical scenario sees the use of hot tapping and line stopping techniques deployed where valves need to be repaired or removed and replaced.

One such instance saw the use of line stopping techniques to stop flow temporarily, enabling work to be carried out to remove and replace a defective valve.

In this case, the 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 203mm nominal bore valves would traditionally have mean 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 it allowed the work to be carried out to replace the valves without requiring the system to be depressurised. This minimised the time involved and disruption to customers. In these and many other similar examples, the aim is to avoid disruption of service to customers, while still carrying out vital maintenance.

Where historically it was common practice to shut down a portion of line or system to do repairs and modifications, these hot tapping and line stopping technologies provide a cost-effective alternative. They allow water mains and pipelines to remain on-stream during maintenance, retro-fitting, alterations, and emergencies, enabling optimum service to customers to be maintained, and often representing significant cost savings.

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