VSD Pumps performance Curves

[Sawsan311]

Dear All,

I have noticed that various pump vendors provide their performance curve for VSD pumps based on fixing the rated head and flow at THE MAXIMUM SPEED and without considering any further speed curve for further modulation beyond the rated speed.

If we compare the case to centrifugal compressors, the rated guaranteed point can be at 90% speed while compressor is capable of absorbing the variations in system's head and flow with the variable speed ramping up to 105% speed.

Can I understand why pumps tend to be designed differently and is there a possibility to specify the vendor to provide a higher speed range within the selected impeller design of the pump and the selected gearbox.

Regards,

 

[pierreick]

Sawsan 311,

Consider this document to support your query .

https://www.gsengr.com/uploaded_fil...ive Recommendations For Centrifugal Pumps.pdf

Good luck
Pierre

 

[Sawsan311]

Thank you Pierre

I understand that the major concern in running the pumps towards the lower speed range reside in:

- effect on pump efficiency for systems with high static head.
- effect on motor efficiency as explained in page 6 due to the slip in the torque even if the pump efficiency remains constant
- effect on the cooling system which is relying on the fluid rotation dependent on the pump rotation, so motor slow down will induce risks of motor overheating specially during start-up as well as the risk of clogging of any debris material as a result of sedimentation. this concern is eliminated when we have an external independent cooling system as explained above.

However, looking at pages 5 and 6 I can see some confusion:
page 6 is stating that torque remain constant if the voltage is proportional to the frequency. However, the figure in page 6 is showing that the effect on the torque and motor efficiency is less for the voltage x f than for when the voltage x f^2. In fact, the figure is contradicting with the torque equation in page 5 showing that the torque indeed changes when the speed and hence frequency is reduced through the VFD drive.

can you please clarify based on your comparison and interpretation of pages 5 and 6?
regards,