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Comparison between stress rupture test and hot water circulation test considering actual operating condition of PE pipes for hot water application

 

Takehiro Fujii, Kazuhisa Igawa, Hidekazu Honma, Kazushi Yamada, Hiroyuki Nishimura

Papers # 2016 Berlin

The hydrostatic stress rupture test has been conventionally conducted as a good method of the lifetime evaluation. The hot water circulation test which is reflected to the actual operating condition is also useful method. The correlations and differences between the two tests were examined. Compared with the hydrostatic stress rupture test, it was found that the antioxidant in the resin was eluded into the circulating water and was consumed rapidly in the hot water circulation test. The lifetime of the pipe became shorter in the hot water circulation test. In addition, it was also found that when using the brass fittings, copper ions solved from the joint accelerated degradation of a resin, a crack initiation at the pipe inner surface and a slow crack growth through the pipe thickness due to the hoop stress. The hot water circulation test using metal ion containing aqueous solution is suitable as the accelerated evaluation test for plastic pipes.

Plastic pipes have been used for the residential hot water supply and space heating in Japan. The long-term performance of polyethylene pipes was evaluated under the copper irons solution as well as at elevated temperature. The concentration of copper ions increased partially and this brought about rapid degradation due to copper activation when using a brass joint in the both hydrostatic stress rupture test and hot water circulation test. This was because copper ions were solved from the joint. Although there were differences in degradation process depending on the test method, it is found that the plastic joint is effective to extend the lifetime of the whole hot water pipe system. In the hot water circulation test, the pipes tend to last for only a short period because antioxidants in the resin were eluted into the hot water and was consumed quickly. In the actual operating condition, oxygen in circulated hot water is continuously forced to diffuse towards the outer surface due to the internal pressure. Therefore, it is considered that the hydrostatic stress rupture test is not sufficient as the evaluation method that reflects the actual operating condition.

Related keywords : polyethylene pipe, hydrostatic stress rupture test, hot water circulation test, degradation, post-crystal, actual operating condition.
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