Hot water pressure testing of plastic components

Abstract

Sanitary fittings have used brass parts inside them traditionally. Development of materials has made the use of plastic parts possible in new applications. Plastic components have the advantage in quick manufacturing process, price and in lightness. Water pipeline pressure causes constant stress and can be a risk to plastic components. Ageing and degradation of physical properties, like strength, are things that limit the long-term use of plastic parts. Sanitary fittings are expected to last several years. Pressurized plastic components quality control and lifetime estimations are therefore very important. Material and part physical properties can also be simulated with computer. Physical tests are however necessity to find out quality differences between different production runs. They are also used to make accurate lifetime prediction of parts. Plastic parts were tested with both with variable pressure dynamic test and with static pressure tests. There is stress directed to plastic part when it is under pressure. Amount of this stress depends for example on pressure level used. More stress is focused on the wall of pressurized part at higher pressure level. Shape and thickness of tested part also effects to stress amount directed to part. Results achieved form various high-pressure tests can be extrapolated to low pressure. This way endurance at much lower pressure and longer time can be estimated. At the beginning most of tests were focused to finding out effects of test machine parameters and how to optimize them. Different parameters like temperature and leak rate detection were analyzed and later all tests were done with same parameters. Jigs were used to mount pressurized parts to test machine. These jigs gave support to parts and blocked all holes so that tested parts were pressure tight. Measurement accuracy of the jigs had large effect to the results. Even small dimension differences could cause large dispersion to result times at given pressure. Jigs used in the tests should be similar to real used parts as closely as possible. This way stress is correctly distributed to part and results have better relation to actual use cases. Results and deductions from tests could be said to be successful. Same type of static pressure testing is also used to estimate lifetime of plastic piping. In tests done for plastic pipes tested samples have simpler geometry and test times are much longer. In this work test times were very short, and pressure was much higher compared to values used in pipe testing. Differences in used materials and production batches were also visible in these short time tests. Breaking of samples followed typical breaking mechanism with slow crack initiation and fast crack growth phase. Some materials had large elastic deformation due to high pressure. This deformation caused leak eventually. Predicting real lifetime is still quite uncertain with short tests. This kind of short time static pressure test is more suitable for design optimization and quality control method.