Water pump (centrifugal) efficiency, does placement height mater?

[bluechipx]

I am pumping water out of a pond that is being dug with a 30 h.p. centrifugal water pump. I am lifting the water about fifteen feet and discharging it at the same level as the pump. I feel lowering the pump near the water level and pushing the water up the fifteen feet would be more efficient. I'm only interested in flow volume, not pressure. Anybody?

 

[haruspex]

I am pumping water out of a pond that is being dug with a 30 h.p. centrifugal water pump. I am lifting the water about fifteen feet and discharging it at the same level as the pump. I feel lowering the pump near the water level and pushing the water up the fifteen feet would be more efficient. I'm only interested in flow volume, not pressure. Anybody?

As far as I am aware it makes no difference until the height of the pump above the source reaches about 30 ft. Above that, a vacuum will develop, and atmospheric pressure will not be sufficient to push the water up.

 

[bluechipx]

I'm having a difficult time wrapping my head around the notion that a pump that delivers 1500 gallons per minute would continue to pump that amount right up to the point that it couldn't lift any higher at around 30 feet. Wouldn't it slowly diminish in gallons per minute as it began to reach that point? I would tend to think there must be a difference in lifting water, say 20 feet, depending on which level the pump was at, either top or bottom with total push or total vacuum draw.

 

[russ_watters]

As far as I am aware it makes no difference until the height of the pump above the source reaches about 30 ft. Above that, a vacuum will develop, and atmospheric pressure will not be sufficient to push the water up.

As long as the negative suction at the pump intake doesn't cause cavitation, that is correct.

 

[russ_watters]

I'm having a difficult time wrapping my head around the notion that a pump that delivers 1500 gallons per minute would continue to pump that amount right up to the point that it couldn't lift any higher at around 30 feet. Wouldn't it slowly diminish in gallons per minute as it began to reach that point? I would tend to think there must be a difference in lifting water, say 20 feet, depending on which level the pump was at, either top or bottom with total push or total vacuum draw.

Yes, that is correct: the pump performance follows a pressure and flow curve, so it depends on the lift height.

...but that isn't the same question you asked in your first post. For the scenario in your first post, the lift height is fixed.

 

[CWatters]

Not all water pumps can pump air very well - so it might not be able to self prime. That might be an argument for lowing the pump so it's nearer the water.

 

[OldYat47]

Keeping things very basic, in your application, the higher the total lift (distance from water level to discharge) the lower the flow rate. So in theory at a given lift (15 feet from suction to discharge) the flow rate should be the same regardless of where the pump is placed in that 15 foot range. That may or may not be the case with the particular pump you are using. In reality the design of the pump dictates the required suction pressure (or lift capacity from suction). Things like impeller design, volute design, A and B gaps, etc., all affect required suction pressure. Some pumps can lift if they are liquid full (primed) at startup, some pumps can self-prime, some pumps need flooded suction or even positive pressure at the inlet to prevent cavitation. In your application the best way to ensure maximum flow would be to put the pump close to the water level.