Pipe Issues Blog

This technical paper addresses claims made by the Ductile Iron Pipe Research Association (DIPRA) suggesting that ductile iron (DI) pipe has better hydraulic performance over the lifetime of the pipe compared to PVC. DIPRA maintains that DI’s larger inside diameters (ID) offset PVC’s better flow characteristics. To support this contention, DIPRA’s brochures and on-line calculator use inaccurate hydraulic assumptions and product information, resulting in incorrect conclusions. When industry-accepted data are used, however, PVC pipe is shown to be more efficient, cost effective and sustainable than DI pipe.

For example, DIPRA compares 24-inch Pressure Class (PC) 200 DI pipe to PC 235 PVC pipe. The use of a higher PC for PVC means that PVC is penalized by having a thicker wall, which results in a smaller inside diameter (ID) and higher friction loss. Moreover, the DI pipe selected has the largest ID (thinnest walled) available but is not commonly used in design and construction. An unbiased analysis would use PC 200 for both materials.

Another questionable assumption is that the flow coefficient (the Hazen-Williams coefficient or “C” factor) for DI pipe remains constant over time (140 for DIPRA’s example). This is contrary to accepted hydraulic theory and pump design which acknowledge a deteriorating flow coefficient over time for pipe materials that use cement-mortar lining. This is especially noteworthy because DIPRA’s own website shows a decline in “C” values over time for several of the cities referenced in its literature. A more accurate analysis would include an overall decline in the “C” factor for DI pipe as it ages.

These incorrect assumptions are addressed in the document, “Hydraulic Analysis: Pumping Costs for PVC and Ductile Iron Pipe.” The results from this analysis are quite different from DIPRA’s — the revised analysis shows that PVC pipe has superior hydraulics and lower pumping costs than DI pipe.

Click here to read the technical paper.