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Study on parameters affecting durability of polyethylene pipes for hot water application

 

Hirofumi Kyutoku, Kazuhisa Igawa, Hidekazu Honma, Sakiko Fukunishi, Hiroyuki Nishimura

Papers # 2018 Las-Vegas

There are mainly two grades of polyethylene pipes for hot water supply and heating. The excellent molecular structure polyethylene resins having resistance to stress crack growth are used because hot water is flowed for a long time in both polyethylene pipes. Furthermore, various additives are mixed to prevent the thermal degradation of polyethylene pipes at the elevated temperature. In this study, the stress rupture test and the continuous hot water circulation test were conducted using extruded polyethylene pipes mixed two kinds of polysilane and a nucleating agent. As a test result, the time to failure of polyethylene pipes mixed two kinds of polysilane was longer than that of a polyethylene pipe without additives. The time to failure of polyethylene pipes mixed a nucleating agent was almost similar to that of a polyethylene pipe without additives. According to precise observation at the failure part, it was found that there was a small degraded colored layer at the inner surface of polyethylene pipes mixed two kinds of polysilane compared with polyethylene pipes without additives. In addition, it was found that the generation and the growth of degraded colored layer and small cracks was delayed due to diffusion of polysilane to the inner surface and to protection of adhesion of metal oxides such as copper oxide and iron oxide

Hot water distribution pipes connecting a gas cogeneration system with the water heater and terminal appliances are expected to high durability at present. There are mainly two lines for the hot water distribution system. A single pipe is used for water and hot water supply, and a double pipe for circulation of hot water is used for such as central heating, floor heating, and bath water heating.

The hot water circulation test with partial replacement of fresh water which is reflected to the actual operating condition is a useful test method. The hot water circulation test using metal ion containing aqueous solution for central heating is also suitable as the accelerated evaluation test for polyethylene pipes. The pressurized hot water circulation test using chlorine aqueous solution for water and hot water supply is the accelerated evaluation test. It was clarified that parameters such as temperature, pressure, and concentration of aqueous solution of copper ion and/or chlorine ion affected durability of polyethylene pipes for hot water application.

Various additives such as two kinds of polysilane and a nucleating agent as well as antioxidants were mixed to prevent the thermal degradation of polyethylene pipes at the elevated temperature. As a test result, the time to failure of polyethylene pipes mixed two kind of polysilane was longer than that of a polyethylene pipe without additives. The time to failure of polyethylene pipes mixed a nucleating agent was almost similar to that of a polyethylene pipe without additives.

Related keywords : polyethylene pipe, additives, polysilane, hot water circulation test, degradation, actual operating condition.
Please note that the whole article content is available on PPCA website only


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