DTPH56-06-T-0004 Plastic Pipe Failure, Risk, and Threat Analysis

Reports, publications, papers, and databases were reviewed to better define risks and threats to plastic gas distribution piping. Failure modes were described for plastic PE piping with the most significant being slow crack growth (SCG). Short-term mechanical tests such as tensile, quick burst, melt index, and density tests did not show a correlation with a material's susceptibility to SCG failure. The bend-back test was able to visually identify 1971 low-ductile inner wall materials. PENT test failure times were reported for materials manufactured during the period1972-1985. The PENT test did not show correlations with the material's susceptibility to SCG failure for these materials. Life expectancy was determined to be a key measure of the susceptibility of PE gas pipe materials to SCG field failures. Long-term hydrostatic stress-rupture data combined with the Rate Process Method or with the Bi-Directional Shift Functions predicted the remaining life expectancy of several PE materials at 60�F average field temperature under varying loading conditions. Data showed rock impingement loads and pipe squeeze-offs can result in the greatest reduction in remaining life expectancy. Lower operating field temperatures and pressures significantly increased the predicted remaining life expectancy of PE materials. Fifty-five PE pipe samples that failed in field service were examined in the laboratory to identify the root cause of the failures. Eight of the samples underwent in-depth analysis, which included density and melts index tests and differential scanning calorimetry, infrared spectroscopy, and microscopic examination of the fracture surfaces. The samples were combined with another set of additional data resulting in 45 material, 36 procedural, 12 quality control, and 7 miscellaneous failures. A separate categorization method attributed a total of 321 failures to their respective pipe/component, with most occurring at joints. RCP in large diameter PE materials was investigated through laboratory testing. Critical pressure was determined for 6 pipe materials. The critical temperature was determined for 3 materials.