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

• saxman2u
In summary, the conversation discusses a rainwater collection system and the design of the pump used to pump water into the main cistern. The speaker is making some plumbing improvements and is wondering if pumping into the bottom of the tank would make a significant difference. The Bernoulli equation and frictional flow resistance are mentioned, and it is determined that pumping into the bottom of the tank would not make a significant difference. The conversation also mentions the potential for leaks and siphon action in the system.
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

-S

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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

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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.

-S

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.

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

## 1. What does it mean to "pump rainwater into the bottom or top of a tank"?

When pumping rainwater into a tank, you have the option to either pump the water into the bottom of the tank or the top of the tank. This refers to the location within the tank where the water is being pumped in.

## 2. Why would someone want to pump rainwater into the bottom of a tank?

Pumping rainwater into the bottom of a tank allows for better mixing of the water and helps to prevent stagnant water from accumulating at the top of the tank. This can also help to reduce the growth of bacteria and other organisms in the water.

## 3. What are the benefits of pumping rainwater into the top of a tank?

Pumping rainwater into the top of a tank allows for a more natural flow of water, similar to how rain falls onto the surface of the tank. This can help to prevent any disturbance to the sediment at the bottom of the tank and can also help to oxygenate the water.

## 4. Which method of pumping rainwater has the lower head?

Pumping rainwater into the bottom of a tank typically has the lower head, as the water does not need to travel as far to reach the bottom of the tank. This can help to reduce the amount of energy and power needed to pump the water into the tank.

## 5. Are there any factors to consider when deciding whether to pump rainwater into the bottom or top of a tank?

Yes, there are a few factors to consider such as the size and shape of the tank, the type of pump being used, and the intended use of the water. It is important to consult with a professional to determine the best method for your specific situation.

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