Engineering Economy: Pumping Costs and Equivalent Pipelines
Engineers must consider pumping costs to appreciate the energy savings with Ductile iron pipe. Pumping costs are proportional to the quantity of water pumped as well as the head loss. Head loss calculation using the Hazen-Williams equation is related to the flow velocity, flow coefficient (C- Factor), and Actual inside diameter. Check DIPRA’s brochure on Hydraulic Analysis of Ductile Iron Pipe.
The hydraulic characteristics of various pipe materials differ based on their internal diameter and their surface smoothness. Historically, most substitute pipe materials (such as PVC and Asbestos-Cement) were introduced to the market by matching the outside diameter of existing infrastructure made from Gray iron pipes. However, substitute pipe materials have different mechanical properties, which directly influence their wall thickness, thus varying their inside diameter.
With the invention of a stronger and more durable Ductile iron pipe in the 1950s, the wall thickness of Ductile iron pipe was reduced compared to gray iron, thereby increasing its inside diameter. In fact, Ductile iron pipe has an internal diameter that is larger than the nominal diameter. On the other hand, most substitute pipe materials (i.e., PVC, HDPE, PCCP, steel) have an inside diameter equal to — or in some cases, even less than the nominal pipe size.
Pumping Costs Through Ductile Iron Pipe vs Equivalent Pipelines
With the invention of a stronger and more durable Ductile iron pipe in the 1950s, the wall thickness of Ductile iron pipe was reduced compared to Gray iron, thereby increasing its inside diameter. In fact, Ductile iron pipe has an internal diameter that is larger than the nominal diameter. On the other hand, most substitute pipe materials (i.e., PVC, HDPE, PCCP, Steel) have an inside diameter equal to - or in some cases, even less than the nominal pipe size.
In 1922, the first cement-lining was field applied to Gray iron pipes in Charleston, SC. In 2018, DIPRA evaluated the flow test of the 97-Year-old Cement-Mortar Lined Iron Pipe. The historic test was captured in a video, available here. DIPRA obtained definitive field data through five tests that confirm its recommendation of a C-factor of 140. These encouraging results indicate that the lining is doing its job, protecting the inside of the pipe, while maintaining the smooth inside surface of the pipe after 97-years of service. Modern Ductile iron pipes are lined with cement-mortar lining as per ANSI/AWWA C104/A21.4 Standard, by either the centrifugal process or by the projection method, thus ensuring the quality and longevity of cement lining for decades of efficient service.
Consequently, with a larger-than-nominal inside diameter, and a smooth inside surface, American-made cement-mortar lined Ductile iron pipe presents energy savings to utilities, unequaled in the waterworks industry.