Centrifugal pump performance.

[TicklyT]

I'm trying to get an idea of the performance to expect from an American designed centrifugal water pump adapted by the manufacturers for use in the UK.
Obviously, the motor has been specified for the European market, but the motor shaft speed is lower (1425 RPM Vs 1725 RPM).

Would you expect the reduction in output to be directly proportional to the difference, or proportional to the difference of the squares, or even the cubes?

 

[Onetap]

I'm trying to get an idea of the performance to expect from an American designed centrifugal water pump adapted by the manufacturers for use in the UK.
Obviously, the motor has been specified for the European market, but the motor shaft speed is lower (1425 RPM Vs 1725 RPM).

Would you expect the reduction in output to be directly proportional to the difference, or proportional to the difference of the squares, or even the cubes?

http://www.contractorsunlimited.co.uk/toolbox/affinity-laws.shtml

The flow rate is directly proportional to the speed, so should be 1425/1725 of the flow for any given head. The flow rate is meaningless unless you have the associated head/pressure.

The head (pressure) is proportional to the square of the speeds.

H2 = (1425/1725)^2 x H1.

You'd calculate a few such points, plot them on the pump curve and draw a revised pump curve for the reduced speed.

 

[Agile]

There are too many variables for us to tell you anything useful if you dont tell us the application parameters!

It can also made a difference if its direct coupled or belt driven!

If its for a launderette for example, the main difference will be a lower head. Its been traditional to supply WMCs at about 2 Bar but 1.5 is still fine.

If its a pool filter/heating circulation then the head loss is very small and the flow rate is important but thats not a critical application.

Tony

 

[TicklyT]

Thanks Onetap and Tony. I should have said it is a 1 HP pool pump, pumping through a sand filter. The delivery drops off very quickly once the pressure drop across the filter reaches about 0.6 Bar instead of the expected 0.9 - 1 Bar given by the manufacturers. That's in the (1425/1725) ^2 = 0.682.... ballpark suggested by Onetap.

Unfortunately the pump is direct drive, so not much chance of playing with the drive ratio.

Maybe I should get a 3,000-odd mile long extension lead, and plug it in on the right continent.

 

[Agile]

You could get a frequency converter but I dont see thats its too important for that application!

Why do you think its so important?

Whats about thinner sand?

A direct drive is far more efficient!

Tony

 

[TicklyT]

There's nothing too crucial, just working out an appropriate maintenance regimen to keep the filter in good condition.

The makers tabulate pressure drops across the filter Vs flow rate, and the filter life Vs flow rate, but their figures assume a higher maximum pump output pressure.

Now at least I can account for the differences between the claimed and the observed performance of the pump.

Fortunately the system is quite healthily over-specced for the particular application, so it looks like it will still do the job required at the lower maximum output pressure I'm getting.

 

[Agile]

By the way its costing you £2.50 a day if you run it 24/7.

For most filter applications you only need it timed for about two hours in the morning before usage times.

Tony