Residual Quality of First Generation PE Gas and Water Pipes

Session 2B
2:00

Frans Scholten, Kiwa Gas Technology

More than 150 pipe segments taken from pipelines that were installed in Germany in 1975-1976 (32mm gas pipes operated at 80 or 800 mbars) and in 1967-1968 (40mm water pipes operated at 7 bars) have been excavated in 2007 and tested for residual quality. The pipes were characterised regarding internal water pressure resistance, frozen-in stresses, ethyl branch content, antioxidant content and resistance to oxidation. PENT test results of excavated Dutch first generation HDPE gas pipes were added.

Two different HDPE resin populations have been discovered using DSC Fractionation (DSC-SIS). The same two resins were found among the excavated Dutch gas pipes.

Internal water pressure tests during up to 13,800 hours show that the residual quality of both HDPE pipe populations is still remarkably good. The times to failure in internal water pressure tests at 20, 60 and 80 °C are not lower and sometimes even higher than regression curves published in the nineteen seventies. Between 1400 and 4300 hours at 20 °C the gas and water pipes go through an early ductile-brittle transition, however without change in slope.

Using ISO 9080 a Lower Prediction Limit (“MRS value”) for extended operation during additional 50 years was predicted. For the gas pipes an “MRS value” of 5.3 MPa was found and for the water pipes, having been operated at higher pressure, 3.9 MPa.

The internal water pressure data were also plotted using Barton & Cherry’s alternative model, which assumes a log (time) versus linear stress correlation. All data follows this model as well. The consequences for the predicted residual lifetime are discussed.

Although only low antioxidant concentrations were detected in some pipes, no oxidation effects were noted. The residual “ambient temperature thermo-oxidative lifetime” was also predicted by extrapolation from exposure tests at higher temperatures.

The good test results indicate that pipeline owners can probably keep using these first generation PE pipe materials in the coming decades, provided point loading effects and joint failures are absent.

Scholten F.L. and Wolters M., Kiwa Gas Technology, Apeldoorn, Netherland
Wenzel M. and Wuest J., SKZ, Würzburg, Germany