PRESSURE PIPES | MATERIALS
Robust plastic pipes are designed to take increasing amounts of pressure – and so must be built for the task in hand. Lou Reade reports
Taking the pressure
Most plastic pipes are able to withstand some sort of pressure, and the decision as to which material to choose for a particular application is often a delicate balance of performance, cost and environ- mental conditions. At the Plastic Pipes in Infrastructure conference in
London last month, Joaquín Lahoz Castillo, senior pipe researcher at CEIS in Spain, described three case studies involving failed PVC pressure pipes. “Longitudinal crack failure in large PVC pipes in
irrigation projects have been a recurrent topic in the last few years,” he said. “We wanted to know if this was caused by rapid crack propagation (RCP), or whether it was about pipe quality.” In the first example, PVC pipes from 2008 were found to have longitudinal fissures and cracks. There were noticeable brown and grey particles visible in the cracks. Tests showed onset tempera- tures below 180°C and a gelation degree of 61%. This fits with a ‘severe attack’ found in the DCM (dichloromethane) test. A second pipe, from 2011, showed similar
cracking. In addition, there were dark spots and white flux lines – indicating that inhomogeneous melt and degradation was the root cause of failure. A third failed pipe, dating from 2008, had
extensive white areas in the chamfered section and internal surface. The material performed very poorly in tensile stress tests. Microscopic evaluation of brittle failure revealed large embedded dark-
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grey particles. Overall, CEIS concluded that there was no evidence of RCP – and that all cases were related to pipe quality.
“DCM is still the only way to evaluate pipe
gelation level across the wall thickness and around the complete circumference of the pipe,” he said. “Pressure tests in large dimension pipes confirm the influence of impurities or badly gelated spots as stress concentrators.” A longer report on this conference, which was
organised by AMI, will appear in the next issue of Pipe & Profile Extrusion.
Oriented products One consequence of failures in pressurised PVC pipe has been to switch to tougher materials – such as biaxially oriented PVC (or PVC-O). One leading producer of PVC-O pipes – and the machinery that makes them – is Molecor. It recently doubled production capacity of PVC-O pipe in South Africa by installing a second manufacturing line at its facility there. The plant, located in the Richards Bay Industrial Development Zone (RBIDZ) in the KwaZulu-Natal region, now has a production capacity of 8,000 tonnes/year of Molecor’s TOM PVC-O pipe. It opened in 2016 with a single production line.
Molecor and its partner Sizabantu Piping Systems have already supplied thousands of
May 2018 | PIPE & PROFILE EXTRUSION 19
Main image: Molecor’s 800mm PVC-O pipe was recently used in an irrigation project in Spain
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