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P22P Case Studies

Typical pump curves for a 300kW centrifugal water pump after about 15 years operation

Efficiency is about 10% down on manufacturer's data
Efficiency is about 10% down on manufacturer's data.

The pump head has dropped off accordingly
The pump head has dropped off accordingly.

Power requirement is higher
Power requirement is higher.

The pump is run most cost-effectively (lowest kWh/kl) at the highest flow rate
The pump is run most cost-effectively (lowest kWh/kl) at the highest flow rate.

Identification of operational problems

The first two graphs illustrate the effect of discharge cavitation in a 120kW centrifugal water pump

This pump was one of four in a pump station that were cavitating, to various degrees
This pump was one of four in a pump station that were cavitating, to various degrees.

With the onset of cavitation, the head dropped off and the differential temperature rose by about 20 mK
With the onset of cavitation, the head dropped off and the differential temperature rose by about 20 mK.

This 170 kW mixed-flow raw water pump had a defective discharge valve
This 170 kW mixed-flow raw water pump had a defective discharge valve.

System analysis

A portable unit was being used to test an 800kW raw water pump. Five other pumps were being operated in parallel with the pump under test. These pumps were switched off one by one, and then on again one by one, to give a total of 11 tests. One continuous data log was obtained, with data points obtained every 10 seconds over a period of about 2 hours.

System tests, Head vs Time
System tests, Flow Rate vs Time

The results show that the common manifold on the outlet of the six pumps run in parallel is providing significant back-pressure. The pump under test provided about 30% more flow rate for the same energy consumption when it was operated on its own, compared with the parallel operation with 5 other pumps. Consideration could be given to re-design of the outlet pipework to reduce energy costs.