Fluids question - Pump flowrate ofdue to elevation head

[nlaham]

Hey guys, I've been strugling with a concept recently and was hoping someone familiar with fluid dynamics could answer.

I am trying to calculate the flowrate of a flow circuit with a given pump. I have the pump performance curve (flow vs. head) of the pump and I know all the given pipe lengths. I am able to calculate the friction head loss from the piping and other valves/components, but here is the concept I struggle with.

I am using a computer simulation to verify my results, and when I change the elevation of the pipes, I don't see a change in flowrate. Now my initial response was, yes the flow does goes up, but since it's a circuit, it returns back to the source at the same elevation. So the net elevation change was 0 over the circuit.

I just want to make sure I'm saying this right. So does this mean that if I were to pump the fluid up the side of a building, it wouldn't matter how high the building is, I would get the same flowrate if it was 2 stories, or 10 stories? (This is of course assuming the pump has the required head to get up the building)

Another way to say it is, as I approach the maximum head of pump going higher and higher up the building, the flow rate remains constant, and then once that max head is reached, the pump wouldn't be able to get over the top and the flow would go from X to 0 gpm??

Here is my conclusion (please correct me if I'm wrong): This would imply that in a closed loop circuit as long as the net elvation change is zero, other elevation changes within the circuit do not affect the head of the system, and therefore they don't affect the flowrate. As long as the elevation change is not higher than what the pump can handle, otherwise the flow would never have a return path, and the flow would go to 0 due to the added head of elevation without return from gravity.

It just seemed odd to me that even in a recirculation line, the height does not affect the flowrate. Can anyone explain this better to me or point out my mistakes.

Thanks,
Nick L

 

[russ_watters]

Your understanding is correct.

 

[nlaham]

Well that was easy. Thanks.

Follow up question- Does the power required to operate the pump decrease as the head in the system increases? If head is added to the system, the pump will consquently run at slower flowrates. When this happens I am seeing that the power to operate becomes less. My first inclination was that if the flowrate decreases, but the head is increased, the pump still has to work just as hard, but maybe I'm wrong about that.

I'm not sure if that makes sense.