Technical Blog

More than 50,000 North American water and sewer utilities use cost-effective, corrosion-proof PVC pipe today and it has been in service on the continent for more than 70 years.

This document compares two pipe materials for response to “occasional surge,” a term that might not be well understood. So that everyone is on the same page, AWWA standards define “occasional surge” as: Occasional (emergency or transient) surge pressure: Surge pressures caused by emergency operations, usually the result of malfunction (such as power failure, sudden valve closure, or system component failure).

Back in 2007, the AWWA C900 standard for PVC pipe was revised. The most important change was the reduction of the standard’s safety factor from 2.5 to 2.0. A significant factor in the decision was more than 40 years of use of PVC pipe in ASTM pressure pipe applications with a safety factor of 2.0.

There has been confusion recently in the plastic pipe industry regarding the terms “design factor” (DF) and “safety factor” (SF). Mathematically, the relationship is simple: DF is the inverse of SF. At least three AWWA standards state explicitly in the definition for DF that design factor is “the inverse of the safety factor.”

Conservative safety factors are key to the durability and cost-effectiveness of today’s infrastructure. Utility engineers have helped to ensure the longevity and high performance of North America’s piping networks by adhering to a minimum safety factor of 2.0 for their water system designs. Unfortunately, the polyethylene (HDPE) pipe industry is proposing a risky revision to the AWWA C906 standard that will reduce the safety factor on pressure class to 1.6. The change is based on material properties unrelated to pressure capacity and for which there is no precedence or valid scientific basis. Moreover, discussion of this issue has been muddled by the HDPE pipe industry’s inclusion of unrelated arguments about improvements in the “design factor.”

I sometimes hear the statement that PVC pressure pipe requires “special backfill," while ductile iron does not. This is a mind-set that is not based on engineering principles.

It is common practice to pressure test installed pipelines to ensure that the pipe materials and installation are satisfactory. In fact, it makes sense that every installed pipeline be pressure tested to ensure a leak-free system.

SHARE The PVC pipe industry sometimes makes use of technical abbreviations that may not be fully understood by utility and consulting engineers. “DR” is a case in point.

Some utilities are hesitant to use PVC pipe in deep-burial applications. This reluctance is based more on misconception than on engineering principles, since PVC pipe has been used for many years at depths in excess of 50 feet.

There has been discussion in the piping industry about the ability of PVC pipe to withstand internal vacuum caused by pumping from fire hydrants. In fact, some designers mistakenly contend that only thicker-walled PVC pipe (DR18 or less) can resist fire-flow requirements.