PRESSURE PIPES | PROCESSING
oil and gas gathering has accelerated for applica- tions outside the normal pressure and tempera- ture,” he said. The typical range of interest for PA12 pipe in oil and gas applications is 40-80°C, using pipes up to 10in (250mm) diameter at pressures to 350psi (24 MPa).
In the study, PA12 tensile bars (made from
Evonik Vestamid NRG 2101) were conditioned in light hydrocarbons and subjected to uniaxial stress rupture testing to measure their long-term creep strength. The study was later repeated in water. The two stress rupture lines were analysed
according to ASTM D2837. The lines were parallel, with a stress gap showing the effect of the hydro- carbon on the material. The regression line for the hydrocarbon-conditioned bars was at 5% lower stress than the water line. This showed that a fluid service factor of 0.95 should be applied to unplas- ticised PA12 pipe for use with hydrocarbons.
Old pipe Herbert Terwyen, technical manager at Basell Polyolefine in Germany, presented results of a tests on a pressure pipeline that was laid under a lake in Austria in 1971 – and is still operating. The 13km HDPE pipeline, which was laid under
Lake Ossiach. The DN355 to DN200 pipes were made from a ‘first generation’ HDPE compound. “The classification would have been close to
PE63,” said Terwyen. The pipe collects wastewater and pumps an average volume of more than 1 million m3
each
year – with a maximum pressure of 4 bar – to a nearby sewage treatment plant. The lake reaches a maximum temperature of 15°C. When it was installed, the pipe was designed to last for 50 years. This means it is now at the end of its projected lifespan. As part of a risk assessment of pressure pipes in Austrian lakes, the pipe was tested to determine its residual lifetime. A sample of DN 355 SDR 17 pipe was taken
from the pipeline and analysed for slow crack growth resistance and thermal ageing properties. Using the Arrhenius approach, it was concluded that the pipe – made from “Ziegler HDPE” – could be operated for at least another 50 years. However, Terwyen acknowledged that the test did not take other factors into account, such as excess local stresses on the concrete hoop supports. “Present-day materials such as PE100 and PE100-RC have a higher design pressure and many times the resistance to slow crack growth,” said Terwyen.
“Significant progress has also been made
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regarding additives used to protect against thermal ageing.”
Up to scratch Chris Ampfer, product engineer at US-based WL Plastics, told delegates about the ‘scratch depth’ rule that is applied to polyethylene (PE) pressure pipe – and whether it was still valid. The ‘10% scratch depth rule’ has been used by
natural gas pipeline operators since the 1970s to determine if scratched pressure pipe is acceptable for service or should be removed. It was developed for first-generation PE pipe, and has since been applied to other pressure applications such as water distribution. “PE pipe can be scratched or gouged in many ways before, during, or after it is installed,” he said. Since slow crack growth resistance (SCGR) tests
were developed in the late 1990s, research has shown that allowable scratch depth depends on SCGR, operating temperature and operating pressure. Because the SCGR of pressure pipe resins is now far higher, it has become the greatest factor in determining allowable scratch depth. He said that a new model to estimate the failure
time of scratched PE pipe – based on ligament stress, operating temperature and PENT (notch test) value – has been proposed. Among other things, the new model suggests that scratched pipe with 10,000 hour PENT has an estimated failure time at least 10 times longer than scratched pipe with 200 hour PENT. Ampfer pointed out that PENT values for PE pipe
are more than 1,000 times greater than they were in the 1970s, meaning that the 10% rule should be modified to account for increased SCGRs. “Gas distribution operators can choose to leave PE2708 or PE4710 pipe with scratch depths up to 35% in service, for pipe service temperature of
May/June 2022 | PIPE & PROFILE EXTRUSION 15
Above: Pressure pipe installed under Lake Ossiach in Austria in 1971 has been tested for longevity
IMAGE: SHUTTERSTOCK
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