What is it?
The pistons (or washers) are what push the water up to the top of the pipe. They have a large affect on the energy wasted from the pump. They fit closely inside the pipe, with between a 0.2mm to 1mm clearance between the pipe and the piston all the way around. The Pump Efficiency and Model Equations page explains how important it is that the pistons fit as closely to the internal diameter of the pipe as possible. However, they should not rub on the pipe and this, therefore, limits how close they can fit to the pipe. Pipes from an individual pipe manufacturer usually vary by about 1mm. Practically the piston should be sized at 0.5mm less than the smallest measured internal diameter from at least 10 measured pipes. The pipes should always be bought from a single manufacturer as the internal diameter of pipes from different manufacturers can vary by up to 3mm!
If the pistons are moulded, the choice of plastic should be HDPE, this is based on the below experiences. It should also be remembered that an additional allowance should be made for shrinkage, this is 2.5% for virgin HDPE grains and 3% for recycled HDPE plastic.
- The use of PVC for the pistons is not recommended, apart from the poisonous chlorine gas which it releases if burnt, the produced pistons would be too flexible and wear relatively quickly.
- Nylon is relatively expensive and unavailable.
- Polypropylene and polyethylene have a fusion temperature of 165 °C and 134 ˚C respectively, making polyethylene considerably easier to work.
- Polyethylene is also much more easily obtained than polypropylene; it is not only a more readily available raw material but is also available from recyclable materials such as beer crates.
Used Ideas
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