9 replies to this topic

[onizuka_eiji]

Hi all,

 

I think this is a rather dumb question but I just could not find sufficient resources (information and time) to satisfy myself properly.

 

In the case of fixed speed centrifugal pumps, does the electric power consumption at the motor end change based on discharge flow at the pump?

 

My own thinking is that for a fix speed pump, the motor power and thus shaft power delivered to the fluid is fixed. Thus, for a given fixed speed pump, having a lower flow rate will result in greater head and thus total hydraulic power delivered to the fluid is conserved. i.e. for fixed speed pumps, the motor power consumption remains constant regardless of the actual fluid flow.

 

Is the idea above correct?

 

Once again, sorry for asking such a noob question here and thanks in advance for the advice!

 

 

[fallah]

Hi,

 

Your thinking isn't correct...

 

Power consumption in a centrifugal pump with fixed speed will normally be increased with flow increment. Indeed, if it's possible the pump would operate at end of curve condition, the relevant motor should be sized accordingly...

Edited by fallah, 03 February 2015 - 04:11 AM.

[gegio1960]

 Detailed-Pump-Performance-Characteristic-Curve.jpg   16.98KB   14 downloads

[shan]

The power marked on a motor name plate is the motor max power capacity.  The actual motor power consumption depends the equipment that the motor drives.  If a motor drives a pump, the motor power consumption will be determined by the pump pressure head, flow rate and efficiency.  The flow rate of pump driven by a fix speed motor is constant.  You may adjust the discharge rate by a recycle valve, which does not change the motor power consumption because there are no change of pressure head, flow rate, and efficiency through the pump.

[onizuka_eiji]

Thanks to all for the replies.

 

I think I might not have been clear in my description, let me try and ask my question better.

 

I was thinking of pumping systems in which the flow at the pump discharge is adjusted via valves (be it manual valves or control valves), so that the discharge flow follows the pump curve (e.g. the one posted by gegio1960). If such adjustments are made by the operator, does the motor change its electrical power consumption accordingly? The pump curve posted by gegio1960 shows that the power line increases as the flow increases, this implies that the power consumption of the motor increases until it reaches its maximum power capacity?

 

Once again, appreciate the efforts taken in answering my doubt.

 

Cheers..

[fallah]

Hi,

 

Yes, the motor changes its power consumption according to the flow adjustment...

[shan]

You are unable to adjust flow rate/discharge pressure simply via a discharge line valve for a pump driven by a fixed speed motor.  This is why recycle lines and variable speed drivers are in application.

 

Then, fixed speed motor, fixed pump flow rate, fixed discharge pressure, and fixed power consumption.  Period.

Edited by shan, 04 February 2015 - 11:14 AM.

[fallah]

System curve could be changed via partial opening/closing of a discharge line valve that, in turn, will lead to the change in  flow rate/discharge pressure of the pump, i.e. changing the pump's working point all with a fixed speed motor...Then a centrifugal pump curve is generated by the pump's working points can be created through the change(s) in system characteristics such as variation in discharge line valve's opening while the relevant motor is working with a fixed speed...

[shan]

Please see the attachment for pump rate vs. rpm curves.  You may move your operating point along the pump curve to match the modified discharge line system with the condition of variable speed driver.

Attached Files

•  aalborg_pump_flow_rates_TPV1.jpg   28.72KB   9 downloads

[onizuka_eiji]

Thank you all for the answers!

 

My conclusions are as follows:

 

1. The hydraulic power required to pump different flowrates of fluid changes as the flowrate varies. That is why the power curve on the pump characteristics curve tends upwards as flow increase.

 

2. While a recirculation line at pump discharge allows for different discharge flows to be achieved, the same may also be done by adjusting pump discharge valve (for systems without pump recirculation - unlikely I know, but I have seen these before) or via a variable speed motor. 

 

Cheers.