An article about this study appears in the June 2014 issue of Journal AWWA.
The use of ductile iron pipe in water systems offers utilities significant energy and cost savings compared to pipe of other materials, according to a study published in the June issue of Journal – American Water Works Association. “The larger-than-nominal inside diameter of ductile iron pipe is the reason, and our study clearly shows this,” said lead author and engineer Maury D. Gaston with AMERICAN Ductile Iron Pipe, a division of AMERICAN Cast Iron Pipe Company headquartered in Birmingham, Ala.
The study looked at energy consumption and annual operating costs of Huntsville Utilities, Huntsville, Ala., for 2012 and found a modeled annual cost savings of $666,999 based on ductile iron’s increased efficiency. These savings translate to 7,110,105 fewer kilowatt-hours to operate the system than if another pipe material was used.
“We chose Huntsville for this study because of the utility’s extensive use of gray and ductile iron pipe in its water system,” Gaston said. “Huntsville’s system is 96 percent iron pipe and totals 1,297 miles of 6- through 48-inch iron pipe.”
Key to the study’s findings is the larger-than-nominal inside diameter of ductile iron pipe. When cast iron pipe was first designed more than 100 years ago, the internal diameter of a 12-inch pipe was 12 inches and the outside diameter was 13.20 inches, due to the pipe’s wall thickness. Because of iron’s widespread usage, this outside diameter became the industry standard, and fittings, valves and other joints were designed to mate with iron pipe’s outside diameter.
With the advent of ductile iron pipe in the 1950s, a stronger, more robust material, the wall thickness of the pipe was reduced, creating a larger-than-nominal inside diameter. For example, today’s 12-inch ductile iron pipe has an inside diameter of 12.52 inches, compared with 12-inch DR 18 PVC, which has an inside diameter of 11.65 inches.
“Much has been said about the need for optimum pump efficiency to reduce utilities’ budgets,” Gaston said, “but energy savings from larger-than-nominal inside diameter pipe material has been largely overlooked. Utilities looking to reduce their energy costs should consider both pump efficiency and pipe material.”
To determine the energy savings and in turn cost savings of ductile iron pipe over other materials, AMERICAN engineers used conventional mathematical models that take into account several factors: pumping cost, head loss, velocity and flow rate.
Pumping costs are determined based on power costs, pump efficiency and head loss. Power costs vary across the country. In Huntsville, the cost is $0.09381 per kilowatt-hour. A reasonable pump efficiency is 70 percent, and a reasonable velocity in a 6-inch ductile iron line is 4 feet per second or 386 gallons per minute.
“Applying these figures to our equations, we determined the modeled pumping cost for Huntsville’s system to be $7,529,528,” Gaston said. “We then calculated the pumping cost for the system as if it were not built of iron and found the cost would be $8,196,527. That’s a savings of $666,999 per year.”
In addition to energy and cost savings, the findings point to significantly less carbon dioxide emissions from an iron pipe water system, providing an environmental incentive for utilities to specify iron. “Based on our calculations, if the Huntsville model were built of PVC, concrete or steel pipe – with less-than-nominal inside diameters than that of ductile iron pipe – it would require 7,110,105 more kilowatt-hours to operate,” Gaston said. “Regardless of the actual cost, this model shows a 9 percent savings in dollars and kilowatt-hours.”
Using a conversion calculator on the EPA’s website, this figure is the equivalent of 5.53 tons of carbon dioxide emissions, which are equivalent to:
– 1,045 passenger vehicles on the road
– 562,392 gallons of gasoline
– 66 tanker trucks of gasoline
– 690 homes’ electrical energy consumption
– 21.6 railcars of coal
– 11,666 barrels of oil
According to a 2012 State of the [Water] Industry report by Black & Veatch, a global infrastructure engineering firm, energy costs can account for 30 percent of a utility’s operating budget. “If this is true for Huntsville, our calculations mean a 3 percent annual savings in total operating expenses for the utility,” Gaston said. “Over a period of 50 years, a modest life expectancy for ductile iron pipe, this represents a present value of more than $17 million. That money could cover the cost of bonds, labor or other resources needed for expansion and other expenses.”
Utilities interested in determining the potential savings of using ductile iron in their water systems can request a complimentary analysis from AMERICAN Ductile Iron Pipe by calling 205-325-7803.
About AMERICAN: AMERICAN Cast Iron Pipe, founded in Birmingham, Ala., in 1905, manufactures ductile iron pipe, spiral-welded steel pipe, valves and hydrants for the water works industry and steel pipe for the oil and gas industry. The company’s diversified product line also includes static castings and fire suppression systems.