Plastic Pipes Conference Association # 2006 Washington DC
Zhou, Chang
High density polyethylene (HDPE) pressure pipe may be scratched during installation. The scratches reduce the wall thickness and therefore increase the dimension ratio (SDR pipe). 1” SDR 11 pipe samples extruded from bimodal PE100/PE4710 and unimodal PE80/PE3408 pipe materials were used to investigate the effect of scratches on the hydrostatic performances. The 60o V-shaped notches along the pipe longitudinal direction were introduced to mimic the installation scratches. The percentage of notch depth over the minimum wall thickness was controlled at 5%, 10% and 15%. Hydrostatic test on the un-scratched and scratched pipe samples were conducted at 90ºC and 80ºC. The scratch sensitivity, SS, was established to study the impact of scratches on the failure times of the scratched pipes. SS was defined as the failure time of the scratched pipe divided by the failure time of un-scratched pipe at the same stress and temperature. The SS values were determined for these two benchmark pipe samples using the hydrostatic data at 90ºC and 80ºC. As the hoop stress increases, the SS values increase. The 3parameter Rate Process Method (RPM) was used to extrapolate the hydrostatic data at the elevated temperatures to the room temperatures. Up to 15% scratches, the bimodal PE100/PE4710 pipe was projected to last at least 100 years at 10 MPa/20ºC for PE100 and at least 25 years at 11 MPa (1600 psi)/ 23ºC for PE4710. The unimodal PE80/PE3408 pipe was projected to last at least 100 years at 8 MPa/20ºC for PE80 and at least 10 years at 11 MPa (1600 psi)/23ºC for PE3408 at 15% scratches. At the maximum operation pressure (MOP), both pipes were projected to last thousands of years according to this analysis methodology.
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