Pump rainwater into the bottom or top of a tank? Which has the lower head?

[saxman2u]

Hi,
I have a rainwater collection system where we have to pump water to the main cistern during a rain event. The system works great the way I have designed it and the rainwater is awesome to use in the house and water the plants and grass. The length of pipe I pump the water is almost 300 feet and elevation is 50 feet to the top our property where the main cistern sits. The main cistern is 15 feet high and I currently pump into the top of the tank, so elevation head is 65 feet of head. I use a 2" SCH40 PVC pipe to pump the water.

I am making some pluming improvements (adding an additional pump line and increase pipe diameter)and was wondering, if I pump into the bottom the tank, does it make that big of a difference if the tank is full of 14 feet of water? I did not know if I could subtract 15 feet of elevation head since I am pumping into the bottom of the tank, but then again, there is 14 feet of water above the bottom of the tank and the inlet that I am pumping into. These tank is 12 feet in diameter.

I currently pump around 85 gallons a minute. Here is the pump curve for the pump I am using.
pentair challenger 3 hp pump curve - Google Search

Thanks for your help.

-S

 

[Chestermiller]

The Bernoulli equation predicts that the pressure the pump would have to exert would be the same in both cases. But this doesn't include the frictional flow resistance, which would involve 15 extra feet of pipe. What would be the frictional pressure drop for flowing 85 gpm of water through 300 ft of schedule 40 pvc pipe, and would an additional 15 ft of piping make a significant difference?

See the following reference for pipe roughness factors: https://www.pipeflow.com/pipe-pressure-drop-calculations/pipe-roughness

See also: https://www.engineeringtoolbox.com/pvc-pipes-friction-loss-d_802.html

 

[saxman2u]

Thanks for the reply. To answer your question, no, 15 feet or additional friction loss is negligent.

It seems that the only situation where this would be somewhat beneficial to pump into the bottom of the tank is if the tank were empty. But, since the tank is filling with water, and according to your answer and Bernoulli's principle, as the tank fills, the elevation head increases as water in the tank increases.

Thanks again for your post!

-S

 

[gmax137]

if you develop a leak in the pipe from the pump, it won't drain the tank if the pipe goes to the top of the tank.

 

[Tom.G]

And to avoid siphon action, have the fill pipe above the top of the tank edge.