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Pump Efficiency and Model Equations

What is it?

The efficiency of the rope pump is a measure of how much water you get compared to the work you do. So a person using the same energy and time will get more water from a more efficient pump.

A set of equations have been developed and tested that predict the behaviour of the rope pump. These equations confirm practical experience of what the main inefficiencies (or wastes of energy) of a rope pump are. More importantly the equations allow an easy way of testing and suggesting the best specifications for the pump. 

The equations have been put into this spread-sheet for easy use and plotted for easy analysis. The derivation of the equations can be found in the extract from the report on ‘Rope-pump System Modelling Using Alternative Power Combinations’ by Beattie, Parker, Read & WilliamsThe equations and spread-sheet can also be used to size a motor for the pump telling you the required RPM, power output and running torque. The specified motor can either be powered by dieselsolar (PV) panels or wind turbine

To test the efficiency of a rope pump you should turn the pump until the water is sitting (not rising not falling) at a constant level just below the level of the spout. This will tell you the following:

  • The velocity (speed) at which the rope is moving is known as the "slip velocity", this is the same velocity the water is normally lost back down the pipe. 
  • The energy you are putting is the nearly the same as the energy you are normally wasting whilst pumping, the main wastes of energy are:

1. Friction over the guide

a) The tightness of the rope has the biggest effect on pump efficiency as well as the pump life.

i) The below picture demonstrates the correct tightness of the rope.

(1) If the rope is too tight it will be too hard to turn and the rope and guide will rub and cut each other

(2) If it is too loose the rope will slip and wear fast

b) The guide should be smooth and have as large a diameter as possible to reduce friction. See the Guide page for more information on different guide designs

2. Friction losses at the bearings

a) Friction losses at the bearings (or bushings) have the second biggest effect on pump efficiency. If they are not aligned or oiled properly they will be too hard to turn and will wear quickly. See 'The Rope Pump Concept, RWSN' for more information on different bearing designs

3. Water flowing back down the pump

a) The gap between the piston (or washer) and the pipe (rising main) has the second biggest effect on pump efficiency. The below graph shows how big an effect the gap between the piston and pipe has on the flow rate and efficiency.

See the Pistons page for more details on the best ways of making pistons.

b) The internal diameter of PVC pipes vary a lot, this is especially true between different manufacturers but it is even true of pipes from the same manufacturer. You should always check how big the gap is between the pistons and the pipe

c) If the pistons are too small they will allow too much water to flow back down the pipe

d) If the pistons are too large for the pipe they will rub and be hard to pull

e) The faster you turn rope the less important the slip or lost flow back down the pipe is relative to the water delivered

i) However, the above graph shows that increasing the speed of the rope is only useful up to a point, if the pistons move too fast they create too much turbulent drag. The graph shows the best rope velocity is around 1.2m/s.

f) The smaller the space between each piston and the next, the less pressure there is pushing the water back down the pipe.

i) However, the above graph shows that reducing the distance between the pistons is only useful up to a point, if there are too many pistons on the rope they both take up too much space and more importantly they create too much turbulent drag on the rope. The graph shows the best distance between pistons is around 1m.

4. Turbulent drag of the water in the pipe on the pistons. If you turn the rope faster this drag will increase exponentially.

a) Turbulent drag has the fourth biggest effect on efficiency, as the pistons move through the pipe the pistons create a wasteful drag on the rope that ultimately loses energy to the swirling water. You can see this turbulence in the below picture

b) As the above graph shows, if you turn the rope too slowly then the amount of water delivered at the top will be too little compared to the water that is flowing back down the pipe, resulting in an inefficient pump. As detailed above the best rope speed is around 1.2m/s