System Pressure Rating & Residual Life # 2004 Milan
The recent Department of Transportation Advisory Bulletin, “Notification of the Susceptibility to Premature Brittle-Like Cracking of Older Plastic Pipe” cited lower ductile inner wall Aldyl “A” and Century polyethylene (PE) pipe as two materials that were known to be susceptible to brittle-like cracking or slit failures. Many miles of these materials are still in service in the United States. The key question is – what is the projected life of these PE pipes still in the ground? The Rate Process Method (RPM) is used to project performance (lifetime) of PE materials at their in-ground temperatures and pressures based on both internal pressure as the primary load and secondary loads such as rock impingement and squeeze-off. This paper will provide a discussion on the Rate Process Method. RPM involves exhuming samples of pipe from service and subjecting them to laboratory elevated temperature sustained pressure testing that results in slit failures. These slit or SCG (slow crack growth) failures are the long-term failure mode for PE materials. A three-coefficient equation is then used to project performance at actual service temperatures and pressures. RPM testing is conducted not only with pressure as the primary load, but is also used to investigate secondary loads such as rock impingement, deflection, bending and squeeze-off. RPM testing was conducted on exhumed Aldyl “A” PE pipe. The RPM projected performance is correlated with actual field experience from two gas utilities for control pipe (primary load only), indented pipe (to simulate rock impingement) and squeezed pipe. This paper also correlates control, indented and squeezed RPM projections for exhumed Century PE pipe with its corresponding field experience from another gas utility.