National Grid Up-Rates Low-Pressure System With Internal Seals

By Jeff Griffin, Senior Editor | July 2011, Vol. 66 No. 7

National Grid is an international energy delivery company that is the largest distributor of natural gas and electricity in the northeastern United States. It serves more than six million customers.

To meet its goal of providing safe, reliable service, National Grid has an ongoing program to improve and upgrade its transmission and delivery infrastructure, often using innovative construction technologies to limit disruption of street traffic and normal surface activities in busy urban areas.

For example, a recent project extended natural gas service to Boston University’s East Boiler Plant which was converted from No. 4 fuel oil.

Getting gas to the boiler plant involved several challenges. The source of the gas supply was from a 22 psig (pounds per square inch gauge) distribution main approximately 2,700 feet from the boiler plant. To get gas from the source of supply to the plant would require installation of a gas main along Commonwealth Avenue, a major city thoroughfare with typical urban infrastructure. In addition, Commonwealth Avenue crossed over the Massachusetts Turnpike and CSX railroad tracks -- the bridge span was over 400 feet.

“A cost estimate for installing a new gas main within this urban environment proved to be prohibitive,” said Edward Wencis, National Grid project manager. “To install a new gas main across the bridge alone, including a bridge redesign, would cost several hundred thousand dollars.”

This led National Grid to consider other alternatives. “Fortunately,” said Wencis, “there was an existing 24-inch cast iron, low pressure (seven-inch water column) gas main under the north sidewalk of Commonwealth Avenue. This main provided gas service to the local distribution system as well as two university buildings. There was also an existing 36-inch, mechanically joined, steel, low-pressure gas main under the Commonwealth Avenue bridge deck which was also used to support the local distribution system.”

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National Grid determined the best option was to take advantage of the existing low-pressure mains by up-rating them to the 22 psig supply pressure.

“In order to utilize these mains,” said Wencis, “a district regulator station was installed to support the local low pressure distribution system. The regulator station allowed the 24-inch cast iron and 36-inch steel mains to be converted to the 22 psig supply system. By using these mains, costs of excavating, installing and backfilling the main along challenging Commonwealth Avenue and costs associated with installing a main under the Commonwealth Avenue bridge deck were avoided.”