HYDRAULIC ANALYSIS: Pumping Costs for PVC and Ductile Iron Pipe

This paper critically examines the hydraulic performance claims made by the Ductile Iron Pipe Research Association (DIPRA) regarding ductile iron (DI) pipes, specifically concerning energy efficiency, cost-effectiveness, and sustainability in comparison to polyvinyl chloride (PVC) pipes. DIPRA suggests that DI pipes, with their larger inside diameter (ID), lower pumping costs when compared to PVC. However, this analysis reveals that DIPRA's calculations and materials contain misleading data and unfounded assumptions, resulting in skewed comparisons that do not reflect the real-world performance of these materials. Key Areas of Analysis 1. Inside Diameter Comparison DIPRA’s comparisons often pair DI pipes with larger ID and lower pressure classes against higher-pressure class PVC pipes, leading to inaccurate efficiency claims. An equivalent pressure class (PC) should be considered in such comparisons. For example, DIPRA compares DI PC200 with PVC PC235, which inherently has a smaller ID due to its higher pressure rating. When comparing equivalent classes, such as DI PC200 with PVC PC200, the ID difference promoted by DIPRA diminishes. Additionally, DI pipes require a corrosion allowance, as around 75% of the U.S. has corrosive soils. Corrosion concerns lead to specifying thicker walls, which can reduce the ID advantage DIPRA highlights. PVC, being non-corrosive, does not need this allowance, so its wall thickness remains purely based on pressure requirements. 2. Flow Characteristics Over Time PVC pipes begin with a Hazen-Williams “C” factor of 155-165, potentially stabilizing at 150 over time. DIPRA’s assumptions use a “C” value of 150 for PVC, while they apply a constant “C” of 140 for DI pipes. However, research shows DI’s “C” factor initially starts at 140 but declines over time, which DIPRA’s materials overlook. This decline in flow efficiency is supported by pump station design principles, which account for a reduced flow coefficient to maintain pipeline capacity. The DI industry’s production of “double thickness” cement-mortar-lined pipes underscores that the linings deteriorate with time, further impacting DI pipes' efficiency. Overall, by incorporating corrosion factors, accurate pressure classes, and realistic flow characteristics, PVC proves to be more efficient, durable, and sustainable for water transmission in various settings, contrasting sharply with the claims made by DIPRA. For Details: www.uni-bell.org/Portals/0/ResourceFile/hydraulic-analysis-pumping-costs-for-pvc-and-ductile-iron-pipe.pdf