CONFERENCE REVIEW | PLASTIC PIPES IN INFRASTRUCTURE


The research encompassed: the selection of


material and samples; tests on rings – both with and without longitudinal cracks; and conclusions. In all, five samples – of differing ages, filler content and size – were chosen and tested, by techniques including fractography and finite element analysis.


Big barrier Josef Dobrowsky, managing director of pipe die specialist Conextru, explained the importance of barrier pipes. “You need the barrier to protect the substance – either inside or outside the pipe – from permea- tion of potentially toxic substances,” he said. Examples might be a fuel transport pipe running


Below: CEIS in Spain has investigated failure in PVC irrigation pipes


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. This fits with a ‘slight attack’ in the DCM test. A third failed pipe, dating from 2008, had


extensive white areas in the chamfered section and internal surface, in addition to longitudinal cracks. The material performed very poorly in tensile stress tests. Microscopic evaluation of brittle failure revealed large embedded dark-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.” However, he said that DCM (and DSC) testing will not pick up everything – and may miss defects such as inhomogeneous melt, degassing micro- bubbles and degraded black spots. Further lab testing could confirm a relationship between gelation and pressure test failure, he added.


Hammer effect Lucien Laiarinandrasana, senior researcher at Mines Paristech in France, explained the effects of ageing and repetitive water hammer on the failure of PVC pipes. The motivation for the study was to find reasons behind the premature failure of pipe – which is meant to last for 50 years, but is prone to leak, he said.


30 PIPE & PROFILE EXTRUSION | June 2018


through across land that must not become con- taminated – or a drinking water pipe running across an area polluted by hydrocarbons. “Barrier pipe for water transportation is very


important, as it often runs across contaminated soil,” he said. The best option is to add a barrier layer – such


as EVOH, PVDC or aluminium – to a PE pipe, he told delegates.


Using an aluminium pipe can create problems –


not least an increased cost – so the preferred option is to use multi-layer plastic pipes. There are two main ways to do this: create a multi-layer pipe in one operation, through a single extrusion head; or to make a monolayer pipe, then pass it through a ‘coating’ crosshead. The second option is the easiest way to convert existing monolayer lines, he said. Some advantages of the single-step process include: it uses conventional downstream proces- es; existing lines can be converted by modifying the existing die head; and the process is straight- forward.


Advantages of the two-step process include: adhesion is adjustable by temperature – which is important if the layer needs to be peeled; die crossheads have a wide diameter range; and printing on the outer surface is possible. However, Dobrowsky added that some disadvantages include: pre-heating of the pipe is needed for good adhesion; the extrusion line is longer; and, downstream operations are more complicated.


CLICK ON THE LINKS FOR MORE INFORMATION: � www.kiwa.com � www.pccl.at/en � www.impact-solutions.co.uk � www.ceis.es � www.mines-paristech.fr � www.conextru.eu


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