New 48-Inch Valves Help Prevent Water Shortages

A leak in a 48-inch main that supplies 90 percent of the water to Quincy, Mass., illustrated to engineers of the Massachusetts Water Resources Authority (MWRA) that additional valves were required. Without valves to isolate critical sections of the main, a future problem could create a crippling water shortage.

The engineers began planning the installation of two new 48-inch valves. Because the line provides between 16 and 23 mgd to the city of Quincy, it could not be contained with existing valving. Summer peak flow requirements prohibited reducing the water supply and dictated that the work be done during the winter with its associated period of reduced demand.

The line’s pipe and fittings are steel while most of the joints are made with couplings. Only some of the couplings were restrained with welded strap, and some of those straps have failed over the years. Lack of thrust blocking, failure of restraint devices, line pressures of 105-110 PSI and poor soil characteristics all contributed to movement that was straining the line’s couplings.

Hydra-Stop Inc. of Blue Island, Ill., helped in the development and engineering of thrust restraint designs and provided special fittings for that purpose. In late 1997, the line stop fittings were installed and pressure tested. The MWRA reactivated a normally closed storage tank and increased the flow in two 24-inch lines so that they provided two thirds of Quincy’s water rather than the 10 percent normally supplied. In mid-January 1998, the actual linestopping procedure was ready to commence.

An isolation valve was mounted on each of the two sleeves and the pipe was tapped at both locations. A linestopping unit was placed on each valve. When MWRA crews were ready to begin the valve installation work, they opened valves and set the line stops into the main, stopping the flow of water and isolating the section of pipe between them.

Water volume in this section was drained down and crews at each location worked to cut out a section of pipe and replace it with a 48-inch valve assembly, which was then enclosed in a vault. During the line stop shutdown, MWRA personnel were able to inspect fittings that could not have been exposed while the line was under pressure.

When this work was completed, the line stops were removed, placing the line back into service. Specially designed completion plugs were placed in the sleeves to seal off line pressure, and the isolation valves were removed. The sealed sleeves with protective blind flanges remain as access points should they be required at some point in the future.

The use of Hydra-Stop’s technology allowed the extent of the shutdown area to be greatly reduced. The locations of the valve installations were isolated by Hydra-Stop line stops, putting only that section of the line temporarily out of service. This enabled the MWRA to maintain service to the City of Quincy and perform the valve installations simultaneously.