April 2010 Vol. 237 No. 4

TechNotes

New Flowstopping System For Large-Diameter, Low-Pressure Gas Mains

A new system for gas-free working on large-diameter, low-pressure natural gas mains using semi-supported flowstop bags was approved by National Grid Gas in the United Kingdom. The technique provides an easy-to-use and cost-effective alternative to squeeze-off on polyethylene (PE) mains and Iris Stop on metallic mains for maintenance, repair and modification of distribution networks while preserving continuity of supply.

Developed by ALH Systems, the Bagtube system is suitable for PE mains of standard diameters from 355-630 mm and metallic mains of 14-24 inches diameter operating at pressures up to 75 mbar. In February, field trials were undertaken to extend the use on metallic low-pressure mains of up to 36 inches diameter.

The new Bagtube system was approved by National Grid Gas following tests at Germanischer Lloyd’s (GL) facility at Royal Air Force Spadeadam, UK, and field trials. NGG Standards Manager Laurence McGurk commented, “The new Bagtube system fills a gap for flow stopping 630-mm diameter low-pressure PE mains. It is more cost-effective than squeezing-off because it requires a smaller excavation. There is no sterilization of the pipe as with squeeze-off applications, no damage to the pipe wall and no axial stresses are induced on the pipe. So, re-entry can be repeated at the same location, should this be necessary. The system significantly reduces excavation requirements, thereby reducing the probability of damage to other buried apparatus. It also reduces landfill waste and imported materials and therefore meets our requirements for sustainability to the environment.”

The Spadeadam tests employed a specially built rig to simulate flowstopping operations on a 36-m long section of 630-mm PE pipe. The trials assessed the strength of the bags, the effectiveness of the seal and the coefficients of slip against the pipe wall. Various dynamic load conditions were created, using four compressors capable of delivering air at volumes of 2,000 cubic feet per minute. Bags were inserted and inflated 20 times at flow velocities up to 5 m per second at 75 mbar pressure.

The results of the Spadeadam tests have been confirmed in field trials carried out by the National Grid Gas Pipeline Maintenance Center in London and Manchester. Furthermore, the large-diameter Bagtube system was evaluated by Scotia Gas Networks (SGN). The network operator conducted factory tests on PE pipe with Radius Systems and Flowstop Services, and 40 field trials on both metallic and PE mains.

The SGN trials were conducted on a mixture of operations in Scotland and southern and southeast England at line pressures of up to 40 mbar. According to Paul Pearson, engineering policy manager, “The large-diameter flowstopping system offers significant potential savings on PE and iron pipe, compared with squeeze-off and Iris Stop. We are still collating the results, but the feedback from operators has been universally very positive. They are gaining confidence every time and are increasingly keen to use the system.”

The new Bagtube range uses a semi-supported inflatable bag, originally developed by ALH Systems as a gas-free secondary bag in the Iris Stop system. Already approved by National Grid Gas, the modified bag (designated E20A) incorporates an extended neck and a backbone, which has eliminated the risk of “twisted bladder.”

In order to minimize equipment quantities, the range has been covered in three sets: 14-18 inch, 18-24 inch and 24-36 inch. The bags are inserted through the pipe wall through, respectively, 4-, 5- and 6-inch diameter BSP holes, using the 10-24 inch Iris Stop valve. Relaxation of the D/4 rule to D/3.2 was permitted following a National Grid investigation.

The E20A bags have a bladder construction using double-layer “virgin” rubber, with a cover made from high tenacity unproofed nylon (all conforming to GIS/E20 specification). Bags of 24-inch diameter have been tested to destruction in a special maximum-size pipe (25.5-inch diameter) at ALH’s factory. The lowest burst pressure was 54 psi, providing a bag safety factor of eight times over the current National Grid standard for Iris Stop operations (5 psi). The support post has been calculated to withstand six times the maximum load, on the basis of the straightforward bending moment, when used as an Iris Stop secondary bag.

The new flowstopping system will be increasingly valuable in the UK as the length of large- diameter PE mains employed in gas distribution networks continues to grow. Andrew Lucas, Managing Director ALH Systems explained, “At present, the maximum size PE pipe that can be squeezed-off is 500 mm, and above 355 mm there can be difficulties with crack propagation and joint stress. The same position can only be squeezed-off once, whereas bagging operations can be repeated using the same access tee.”

The Bagtube method provides gas-free flowstopping with the entire procedure carried out through an arrangement of valves. On PE mains, bag insertion is via a DN150-fused branch saddle manufactured by Radius Systems. Radius has developed a permanent plug for the tee, so that the entry valve does not have to be left in place after the operation is completed. According to Derek Muckle, Technical Director of Radius Systems, “Recovering the valves makes the technique even more attractive. It not only saves costs but, by lowering the height of the installation, it also reduces the risk of subsequent third-party damage.”