Thursday, April 14, 2011
City plans to replace cast iron pipe
By SUE WATSON
The city received a plan from the Holly Springs Utility Department to replace 32 miles of cast iron gas pipe in the central part of downtown at a price tag of about $6.5 million.
Engineers Larry Britt and Mike Bridges presented their study of the total system and how they recommend to go about replacing the cast iron pipe, much laid in the central part of the city beginning in about 1947 and on through the 1960s. Bridges said carbon steel pipe was used from then until the 1980s when plastic gaslines were introduced. Plastic pipe holds up better and does not corrode, he said.
Replacement of cast iron pipe is now mandated by new federal regulations.
“Cast iron is located in the older parts of town and around the square and neighborhoods adjacent to that,” Bridges said.
New Public Service Commission codes are pressuring utilities to retrofit natural gas systems to modern-day conditions, he said. The cast iron pipes in downtown are about 50 to 60 years old – have met their natural life expectancy.
“It’s still good pipe but needs to be replaced,” Bridges said, to reduce loss of gas, reduce maintenance of the lines. Since much of the cast iron pipe is under sidewalks, parking lots, and streets, excavation of the old lines would be expensive.
Bridges proposes to bore in replacement pipe. They would divide the city into quadrants and work one quad at a time – taking five years to complete the work. New documents which act as a kind of “birth certificate” for the replaced pipe would be required by law for all new operations.
Bridges said cast iron is more brittle and during extreme cold weather when the ground shifts, the pipe cracks. This causes leakage of product, which over a 30-year period could cost the utility $1.8 million, he said. All-in-all the replacement will save the city about $2.8 million over 30 years, Bridges said.
Five regulators that step down the pressures in the city through cast iron pipes would be phased out to equalize the pressures across the system. Regulators on Falconer, Chesterman and North Memphis would be eliminated. Some systems that have operated at 25 pounds per square inch would be upgraded to 50 pounds in a step-wise process to assure safety of the change-over.
Holly Springs owns about 104 miles of natural gas pipe over the total system, according to Bridges. About 1,300 customers are on the cast iron system and will be transferred over to plastic pipe. There are over 2,000 customers in all.
New plastic pipe will be inserted into old steel pipe, when practical. In other cases a special plow will be used to bore the pipe into the ground, disrupting as little of the soil as possible. Gas meter replacement is also part of the plan. As the meters age, they respond poorer to the gas flow and 5-10 percent of the gas flowing into a building is not metered. Accurate metering of gas will also save the utility money, he said.
He said 43 percent of the cost of the construction would be recovered in time due to stopping the loss of product.
Utilities manager Don Hollingsworth said the utility and consultants have a proposal of how to pay for construction.
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