<P>Hi Experts!

<P>We are studying the selection of a optimal control strategy in order to protect centrifugal pumps with the warranty of a minimal operation flow. The idea is to use a recirculation line with a control valve and openning the last as the flow measured (installed between the pump discharge and recirculation line) required. The problem with this solution is the minimal distances required to install the flow mearurement (10 to 20 line diameters following the API 550) and the final length for the recirculation line required.
<PRE>
-------<------FIC
| |
-------VALVE------------------<---
| | |
| | |
->-----------PUMP--------FLOW-------------->
</PRE>
<P> If FLOW < minimal flow then open the VALVE to warranty minimal flow.
<BR> If FLOW >= minimal flow then close the VALVE.

<P>Could someone of you, please suggest other good or best alternative or simply justify our proposal?
In your experiences what kind of control schematic, what kind of flow measurement (primary element) and what kind of valve are using?

<P>Thanks a lot.</P>

 

Use a VFD with pressure or flow feedback. You have two advantages:
i) you dont deal with complicated mechanical construction (no by-pass valves, no actuators)
ii) decrease in energy consumption of the motor. In fact, this decrease will compansate for the value of the VFD less than 1 year.

 

Alternatively----
You can install pressure relief valves so that if the outlet valve is closed then the pressure relief valve will automatically open and maintain a minimum flow.This is practical only if the water is ciculated through a tank.(even in your case)
The main cause of such failure in a centrifugal pump is the ceramic seal will crack. if the flow is not enough, the water will heat up and when the flow resumes the thermal stress on the seal will crack it due to the sudden cooling.We also use temparature sensors at the pump to trip the pump if the water temp. exceeds the limit.
Hope this helps
Regards,
Sekar