The wear of rigid PVC in relation to water pump designs

Abstract

This study arose from the need to know the wear characteristics of rigid PVC because it was proposed that the material should be used for making piston water pump cylinders. It was important to establish its wear properties, and also to find a material with an acceptable wear resistance, when sliding against PVC; so that such a material could be used for making the pump seals.

The wear rates of rigid PVC, Wood, PTFE and Nylon 6-6, when sliding on PVC were measured in screening tests, in order to• find out the material with the least wear rate. The screening tests were performed on a wear test rig, designed and built to simulate the sliding of a pump seal, on a pump cylinder, submerged in water. The simulation was done by clamping a PVC strip representing a section of a pump cylinder, onto a platform, and moving the specimen representing the seal, along the strip with a reciprocating motion. From the screening tests, Nylon 6-6 was found to be the best material for sliding on PVC, followed by Wood, PTFE, and PVC, in the order of wear resistance.

More detailed studies were then carried out, with PVC and Nylon 6-6 when sliding on PVC, in dry conditions and in the presence of water. All the tests were carried out at room temperatures, approximately 20°C.

From the detailed tests, it was found that in dry conditions, there were critical limits below which the sliding pairs had low wear rates, and above which they had very rapid wear rates. The critical limit in each case depended on the length of the wear track, the load W applied to the sliding pair, and the number of passes the seal specimen made over the PVC strip per unit time, denoted by N. For a chosen wear track length, the limiting load and the number of passes per unit time were found to be related by the equation. W = Wo – kN Where Wo and k are constants. It was also noticed that the critical limits are largely affected by the rate of heat generation and dissipation, at the sliding interface.

From the tests performed under water, it was found that the presence of water prevented the occurrence of the very rapid wear rates, which were observed above the critical limits in dry conditions. The wear rates under water free from grit were much smaller than those measured in dry conditions; but if the water was contaminated with grit, the wear rate was greatly increased. For example9 for a Nylon specimen sliding on PVC under water, the presence of grit increased the wear rate by a factor of approximately 2000.

The experimental results were used to estimate the life expectancy of a water pump, constructed with a PVC cylinder and Nylon seal so It was calculated that if the pump is used to pump water free from grit, it could work for at least 20,000 running hours, before requiring the change of seals, from a well approximately 35 meters deep_ This study has therefore shown that in regard to wear rates, a satisfactory pump Could be constructed. However, grit in the water may be a major problem.