Centrifugal Pump: Suction & Discharge Line Bypass

[ismat143]

Please refer the drawing and let me know what happens if the pump suction and discharge line bypass together.

Thanks.

 

[SteamKing]

What does 'pump suction and discharge line bypass together' mean? Are you talking about isolating the pump from the suction and discharge simultaneously?

 

[ismat143]

Thanks for reply,

To reduce pressure/flow from the pump, it is ok to circulate/pass water from discharge line(@ 7-8 bar pressure) to suction line?

 

[Travis_King]

This happens pretty frequently, though it's a terrible waste of power.

If your pump is discharging at too high a pressure you've got a few options:

1) VFD. Pro: It's very good at running your pump on different curves, and they save on operating costs (as you waste less energy by pumping a lot of water that you aren't using). Con: More expensive than some mechanical workarounds depending on the size of the pump. They're pretty darn cheap for an 18.5 kW motor though.

2) Bypass with control valve would be the second option if flow control is required in the recirculating line. An orifice plate would be best if you are just sizing the line for minimum continuous flow requirement. This line should go back to the tank, not the suction line.

Piping back to the suction line causes the heat added to the water during the pumping process to be recirculated and will eventually cause overheating if the volume of recirculated water is high enough in volume compared to the tank water. You are also introducing a lot of turbulence into the suction line which is typically undesirable (unless you are designing for that purpose, like in specially designed froth pumping systems).

If you can't do a VFD, then I'd suggest installing the recirc line back to the tank, rather than to the suction side of the pump, closed loop pumping in CF pumps isn't good for them and wastes energy. Is the pump oversized, or is this for intermittent use?

 

[Q_Goest]

Note also that you are putting energy into the fluid so if you recirculate for an extended period of time, that energy builds up as thermal energy in the liquid and temperature rises. The temperature the liquid rises to can be determined by calculating the heat transfer out of the fluid, through convective heat transfer for example. The work in will equal heat transfer out under steady state. So if you do this, expect the water to heat up over time. You may need additional cooling for the recirculated water if there isn't enough heat transfer.