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.”
Design Factor and Safety Factor Explained
Essentially the revision is touted as an increase in design factor with no change to the safety factor. However, increasing the design factor effectively decreases the actual safety factor being applied.
In three AWWA pipe standards “design factor” and “safety factor” are defined clearly – design factor (DF) is the inverse of safety factor (SF):
DF=1/SF
The HDPE pipe industry is trying to revise long-established and clearly defined terms by maintaining that the two factors are independent of each other.
Expect Increased Risk of Pipeline Failure and Reduced System Design Life
The newly proposed material, PE4710, does not have any improvement in its pressure capacity compared to earlier HDPE materials: the Hydrostatic Design Basis (HDB) is identical at 1600 psi. If we divide by the 2.0 safety factor, the material’s Hydrostatic Design Stress is 800 psi. Yet the HDPE pipe industry is proposing 1000 psi, which means the safety factor is 1600/1000 = 1.6. This will increase the risk of pipe failure, shorten a pipe’s design life, and set a precedent for competing materials to seek lower safety factors.
The PVC Pipe Association recommends that water utility and consulting engineers continue with the long-established practice of using a safety factor of 2.0 in their pressure-pipe designs.
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