Wear and performance characteristics of some polymers in piston water pumps

Abstract

In this project, wear tests were performed on selected plastic materials to determine their suitability for constructing low cost durable piston water pumps to be used in Kenya. In the pump design, the cylinder was to be made from a plastic pipe of acceptable wear resistance, and the piston seals from a plastic material of acceptable wear rate when sliding against the chosen cylinder material. The study was a follow-up of the author's MSc project carried out between 1977 and 1981, with the same objective, where rigid Polyvinyl Chloride (PYC) was considered for use as a cylinder material. Polytetrafluoroethylene (PTFE), camphor wood soaked in linseed oil, PVC and Nylon 6-6 were tested as piston ring materials. The tests were performed in both dry and wet conditions using pm on reciprocating strip, and pin on disc test rigs. One test included sand particles of up to 2000 urn in the water. Nylon 6-6 was found much better than the other materials for sliding on PVc. It was estimated that in clean water at 20°C, a pump with a cylinder made from rigid PVC, and Nylon 6-6 piston rings could run for about 30 million cycles (20,000 working hours) before requiring the replacement of parts, while in the presence of sand particles the wear rate would be greatly increased. This estimate was not exhaustive with regard to pumping borehole water. It did not cover the effect of ground water temperature variations and compositions. The suitability of other polymers for pump construction, and the load and speed limits suitable for the end use had not been explored. Hence, the studies reported in this thesis considered the production of cylinders from pye, Polyethylene (PE) and Polypropylene (PP), because they are widely used in plastic pipe production. The candidate seal materials were Unfilled Nylon 6-6, PTFE, High Density Polyethylene (HDPE) and Nylon 6-6 filled with Molybdenum disulphide, because of their good wear resistance. The initial basic tests were performed in tap water, using a pin on cylindrical disc machine for about four months. Thereafter, prototype pumps were tested in tap water and in unfiltered borehole water on aprototype testing.The wear rates of PE, pye and PP were compared at 16°e when sliding against a .. Nylon 6-6pin. Nylon 6-6was used because of its outstanding performance in the previous tests. The order of wear resistance in this comparison was PE, pye, PP, with PP performing very poorly and pye being comparable to PE at low load/speed combinations. PE and pye were then tested, varying temperature up to the material limits which were 500e for PE and 300e for P'vC. Their wear performances were comparable at low load/speed combinations, but PE was much better in the severe wear regime. The plastic pipes available in Kenya are made from PVe. Therefore, pve was chosen to be the cylinder materia1. Since the end use was a low load/speed application, it was tribologically suitable. Further tests were only performed on PVe. The wear rates of the other pin materials when sliding on pve were compared at 20°C, and the ranking was Unfilled Nylon 6-6, Nylon 6-6 filled with Molybdenum Disulphide, HDPE and PTFE. Nylon 6-6 was therefore used in the rest of the tests where the effects of bearing area and wear track length were investigated. Finally, prototype pumps with PVC cylinders and Nylon 6-6 seals were tested for 20 million cycles in clean water and 5 million cycles in unfiltered borehole water. From the tests, it was estimated that the pumps can run in real life situations for over four years in clean water, and over one year in very turbid water, before resuming service.