process feature | Standards and testing
Testing for corrosion At AMI’s Polymer Testing & Analysis conference last year, Karin Jacobson of Swerea – a grouping of Swedish research institutes – told delegates about the impor- tance of testing polymers in corrosive environments. While pipes can undergo ageing due to the influence
of trace amounts of chemicals in drinking water, for example, she cited cases of pipes used to transfer aggressive fluids such as chlorine gas and strong acids. She cited the example of a corroded chlorinated PVC
(CPVC) pipe that had been exposed to chlorine – which showed a brittle outer layer and discoloured brown region underneath. The pipe was being taken out of service after more than eight years. The brittleness was caused by exposure to hot, wet
Many methods exists to fix plastic piping that has failed in service
stress cracking. Most failures began in pockets of cement at the joints – and spread into the CPVC fitting or pipe. Further analysis showed system-wide contami- nation with polyol ester (POE) compressor lubricant oil – due to a leak in the heat exchanger – which had caused the cement to fail. The answer to the problem was to replace all the pipe – and ensure that it was compatible with POE oil, as there is always potential for leakage in such a system. In a second example, a multi-storey hotel began
experiencing leaks in its CPVC fire suppression system shortly after initial pressurisation. Most were slow leaks at joints. Initial testing focused on non-destructive examination of the joints, using ultrasonic pulsed-echo mapping.
Many of the fittings were found to have incomplete bonding, due to pipes not being rotated a quarter turn (as per standard procedure). Further testing looked at pipe ductility – and also
inspected fittings for installation defects. This revealed that most pipes were still ductile, but there were installation defects in every fitting that was inspected. These defects included: cement drips running down pipes; unbevelled pipe ends; pipe ends with unsquared cuts; and short insertion. As a result, a major portion of the system was examined using a borescope – which showed that 25% of all fittings were installed with excess cement. The whole system could be replaced at once, but another option is to increase system monitoring, so that failing fittings can be quickly identified and fixed. “Compared to the $30m+ cost of total replacement of
the CPVC piping, we suspect the owners will choose the increased monitoring option,” Priddy told delegates. Statistical modelling and extrapolation predicted that, by 2018, more than 95% of the defective fittings would have failed – and been replaced.
40 PIPE & PROFILE EXTRUSION | April 2017
chlorine gas and brine solution. Dye staining helped to identify the depth of the
underlying discoloured region. When tested, its glass transition temperature was found to be lower than that in unaffected regions – and was probably cause by dehydrochlorination of the polymer, she said. Although higher impact grades of CPVC had similar
corrosion resistance to standard grades, the processing conditions of the CPVC did cause a difference – and could be 1.5 times more resistant, she said. A similar study looked at the effect of concentrated sulphuric acid on PVDF pipe. The pipe had been used at a Swedish steel pickling plant, and was exposed to other acids such as hydrofluoric acid. The pipes had survived for 30 years without leaks. They were all in good condition, though showed some discoloration – and some socket joints used for sulphuric acid showed signs of cracking. Swerea researchers developed a special indicator
technique to study acid diffusion into the plastic, and found that the main cause of the cracking was when changing from 96% to 98% acid concentration.
Cracking test Scientists at Chevron Phillips in the USA have reported their findings on a study comparing two variants of a rapid crack propagation (RCP) test. RCP is a specific mode of pipe failure that occurs
under certain specific conditions (and combinations) of pipe size, pressure and temperature. It requires a specific triggering event. Although RCP in PE pipe is still quite rare, pipe makers want to understand it better – so that it can be eliminated completely. The two variants being assessed are the small-scale
steady state (S4) RCP test, and the full-scale (FS) RCP test. The FS test acts as the primary or reference method for characterising RCP performance. Both methods were used to test a range of its pipe
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