Centrifugal Pump pulling against a PD pump 1

[Fudge30]

I have been asked to look at a piece of plant - a tank full of water with an exit at the bottom leading to a t-piece. Each branch of the 'T' leads to a pump, which will run independant of each other from 0-20 m^3/h. The question I was asked was 'would there be any problems here'. My immediate response was no. However, I've now been told that one of the pumps will be a positive displacement pump - probably a gear pump, and the other will be a centrifugal pump. Im now starting to worry that if the PD pump is working hard it could cause problems for the centrifugal pump - particularly when it is only on a low duty. Worst case, I could be looking at cavitation.

Anyone know of a calculation I can use to calculate some operational limits, or has anyone worked on a similar problem???

 

[hydromech]

Hey...

Both pumps have positive suction so there shouldn't be an issue...unless the pipe is too small!

You need to know the outlet pressure and the performance curve for each pump and the size of the inlet tube.

What do you mean "working hard".

The centrifugal pump will outperform the PD pump at low pressure but they will swap as the pressure rises. The flow from the centrifugal pump will drop as the pressure rises.

The pressure rise in the PD pump will not effect the centrifugal pumps performance.

You will have a problem if either of the pumps is driven by a variable speed drive. It may be starved of water or it may starve the other pump as the speed increases, causing cavitation in either case.

Can you provide any more information?

Adrian

 

[mk2000]

As hydromech mentioned look at the pipe sizing and make sure the centrifugal pump does not suck all the water leaving the PD pump running dry.