How to make recirculation line from discharge to suction line of centrifugal pump? 1

[Zabarieq]

Dear Seniors,

I am new to pump installation work and this time I face situation that it seems I have to put recirculation line from discharge to suction line of the pump.

I will use centrifugal volute pump, with flow of 265 LPM at 7.7 Bar (BEP), 2.5" suction and 2" discharge port. The media is water at ambient temperature. This pump will serves 10 point with identical flow (26.5 lpm each). The operation option may go to 10 point at the same time at max, and 1 or 2 point at the same time at minimum. This situation will make the operating flow to be 10 or 20% of the BEP. Based on the internet source I read, it is said that if the operating flow is below 50% of the BEP, then we need to put recirculation line from discharge line to suction line, but I don't know how to do it.

Some source said that I will need suction tank to receive excessive flow, but the other source said that I can directly put the excessive flow back to the pipeline before the suction of the pump. I prefer to choose the second source since I don't have space to put suction tank.

So please kindly advise what should I do?
Can I make recirculation line from discharge to suction line?
What component should I put on the recirculation line?

Thanks,
Zab

 

[LittleInch]

I'm sure the vendor makes other single stage pumps which are more suitable.

Try advising the user his chosen pump is too big and explain how much money he can save if he chooses a different one, especially if you include 5 or 10 years worth of electricity costs.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[moltenmetal]

I'm not for one second advocating that you select a pump grossly too large for the job! I haven't opened the OP's pump curve to look at it so can't comment on that. But if you have 10 users, all of which need a particular head to operate, and any number of which can be on or off at a given time, it stands to reason that the pump will be overzied massively when only one or two users are operating. That's the nature of the service. With a centrifugal, you can't fix that particular problem with a VFD or a different design of impeller etc. But you CAN operate small pumps this way, and have them last forever, if you satisfy their minimum flow requirement at all times. All we're talking about is how best to do that.

Yes, the pump curve is flat at the left hand side, and yes you are walking a fine dividing line when using a backpressure regulator in that case. People usually solve this problem by setting the BPR setpoint low enough to sop up all the variations in head (or they select a pump capable of generating sufficient head), thereby forcing the flow through the BPR quite a bit closer to the BEP even when no downstream users are demanding. That's obviously a situation which wastes energy, because the recirculated flow is much higher than needed. If the downstream users aren't sensitive to riding the pump curve (i.e. if a forward pressure regulator is installed upstream of the users), an orifice or globe valve to satisfy the minimum flow requirement is the better choice, as I said in my previous post.

Still, the only time an automated recirculation valve is required is if the minimum flow is a large fraction of the BEP flow. That's true of big pumps- it can be 1/3 or even more of the BEP flow- but not true of small pumps. For a small pump, not only is the minimum flow a small fraction of the BEP flow, it's also a small flow in absolute terms and hence not a big energy consumer. You donj't worry about that energy any more than you do about the energy required to pump seal fluid through a double mechanical seal etc.- it's an efficiency loss, but one necessary to keep the pump happy.