pump ragging vs speed/flow

[panelman]

We need to limit the flow through a raw sewage centrifugal pump, basically the flow needs to remain constant with suction heads ranging from 1 to 6 m. My solution was to insert flow meter and then use a VSD to slow the pumps down to limit flow. Client has concerns about pump ragging if they are slowed down so wants valve to throttle the pumps which can then run at full speed.

Question: is pump ragging (or lack of) related to rotational speed or flow?

All comments welcome

Cheers

Mike

 

[pumpingwell]

Hi All

To answer the question: Is ragging linked to speed or flow?

My opinion: Too small a flow may lead to ragging of the pump. In low flow or low speed in VSD operation, the pump ragging risk increases mainly due to complicated pump internal flow patterns, i.e. recirculation.

Many pumps cannot be operated below 50 to 60 % of BEP flow without incurring internal recirculation. During recirculation, the internal flow is unstable and stall, backflow, eddy-type circulation and turbulence all could take place.

Too small a flow may also lead to excessive pump wear from sand or other abrasive matter remaining in the pump. The running clearances of the pump could increase rapidly due to excessive wear. This increases leakage across the clearance from discharge to suction disturbs the flow pattern and incurs internal recirculation.

No pump is perfect. From a recent investigation, I found "internal recirculation " shall be the main reason of the rapid mechanical damages (Lots of stringy materials strangled around the upper and lower mechanical seals. Premature seal failure, severe wear of the suction cone and serious erosion of the impeller back plate were found in the consequent maintenance service.) and performance deterioration of a Hidrostal sewage pump. See thread 407-45276 "NPSHr (H2O vs Flowing Fluid) and Calculations" for more info of the investigation.

pumpingwell