Recirculation - Centrifugal Pump

[EngMecBra]

Hi everybody

I have a pump (manufacturing yet) that in its alternative condition shall handle only 90 m3/h, but its minimum continuous stable flow is 110 m3/h.

Shall I consider a recirculation of 20 m3/h, right?

But my question is: can I consider a recirculation to suction line or must I consider a recirculation to suction tank?

I'm asking this because the suction tank has a high elevation, then I need to increase diameter rotor to meet this alternative condition.

If you need any information to answer me plase let me know.

Regards

 

[LittleInch]

For the duty you describe, i.e. part flow circulation, a return to the suction line is Ok. if the whole min flow was re-circualted, a return to the tank is advised to avoid heating up the fluid too much.

Yes 20m3 is all you need to recirculate.

Look up ARV - automatic re-circualtion valves on this site and you'll get some good information.

I don't understand why you would need to increase the rotor diameter though- the inlet pressure at the pump plus discharge should be more than inlet pressure, unless you have some big losses in the suction line and only a small differential head pump...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[EngMecBra]

Littleinch,

Considering 340 m3/h (rated flow), the head rated is 24m.

But considering recirculation to suction tank, due to high elevation of this tank, the head is 30m, it's the reason I need to increase rotor diameter.

 

[LittleInch]

Is that total head from the pump or the differential? What is the inlet head / pressure at your minimum flowrate (110m3/hr) - friction will reduce considerably compared to full flow and also the pump differential head would rise a bit. I'm just being curious here and trying to show that you have a choice without changing the pump.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[EngMecBra]

The informed heads are both differential.

Pressure loss at suction considering flow 340 m3/h is only 1m, then I won't reduce enough head even considering losses in suction at minimum flow.

My client have suggested this solution with recirculation line to suction tank in discharge line (with control valve), but I think that the solution with automatic re-circualtion valve will be cheaper, right? ARC valve could be cheaper than control valve, and we will have less piping too.

 

[LittleInch]

I've never bought an arv, but given that they are simple mechanical valves they should be cheaper than an instrumented system.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[JJPellin]

An ARV may be a good option. We have number of these running in boiler feed water service with good results. You could use an orificed spill-back and leave it open all the time. This will waste some energy since you will be spilling back even when the outgoing flow is high enough to satisfy the minimum flow. You could add an instrumented spill-back. But, as LittleInch noted, this can be expensive.

There is no need to have a larger impeller when spill-back to the supply tank, even if the tank is at a higher elevation. The pump differential head will always be added to suction head. You could easily spill-back to the supply tank with the installed impeller as long as the piping losses are not outrageously high.


Johnny Pellin

 

[EngMecBra]

I think my problem with this recirculation is because I've considered minimum tank level elevation 99.500 in the pump calculation, but the recirculation line will reach the tank at its roof elevation 113.000. Tank bottom is elevation 98.000.

 

[LittleInch]

I still think at minimum flow you will generate more than 113m of head compared to your datum point. Any way that is not required so long as you have some flow coming from the tank at your min flow.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way