Occasional and Recurring Surge Design Considerations for HDPE Pipe

The current generation of High Density Polyethylene (HDPE) pipe materials, classified as PE4710 by U.S. standards and PE100 by ISO standards, has greatly improved toughness characteristics over previous generation materials. Most noteworthy is an increased resistance to slow crack growth and fatigue. In the U.S., PE4710, PE3608, and PE3408 (previous generation material) pipes are used in water mains and forcemain sewers. These pipelines daily experience numerous pressure surges above their standard pumping pressure. For pipes operated at their rated pumping pressure, AWWA C901 and C906 allow "recurring" pressure surges equal to 0.5 times the pipe's Pressure Class (PC) and "occasional" surges to equal the pipe's Pressure Class. Thus, the surge allowance results in a total pressure (combined pumping pressure plus surge pressure) equal to 1.5 x PC during recurring surges and 2.0 x PC during occasional surges. While all three materials have the same long-term hydrostatic design basis of 1600 psi, a higher design factor for PE4710 pipe allows it to operate at a pressure 25% higher than either PE3408 or PE3608 pipe. This paper addresses the question, "Does the higher surge pressure resulting from operating PE4710 pipe at a higher pressure compromise PE4710 pipe's performance?". This paper gives an overview of design procedures for water and forcemain sewers given in current standards. It discusses considerations for a reasonable safety factor against over-pressurization during transient surge and shows that this factor is met by PE4710 material when designed in accordance with AWWA and ASTM standards. Aside from the pressurization itself, repetitive surge may lead to fatigue. This paper references studies wherein PE4710 material has a fatigue life in the millions of cycles when operated at its pressure rating and subject to recurring surge equal to 0.5 x PC.