I have questions about a well pump application I am working on. It consists of a well that with a submersible pump that fills a reservoir tank. As far as I know, the tank is not pressurized. Here are some values associated with the system.

  • Static well level = 39 ft
  • Tank Discharge level = 28 ft
  • Well drawdown = 67 ft

A couple of questions I have are listed below.

Am I correct in assuming that the static head requirement for the system curve is 67 ft?

Drawdown is a function of flow, so it's considered dynamic head. I keep on getting tripped up on the difference in suction vessel and discharge vessel pressure, though. The well is open to the atmosphere so there is no suction "vessel" pressure, but the tank is closed and so I keep on thinking there might be discharge vessel pressure, but if the water is releasing into open space and then dropping to fill the tank, wouldn't this also be atmospheric pressure, unless the tank is pressurized? Appreciate the assistance.

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    $\begingroup$ Please provide a diagram so we know how those distances "stack up" . $\endgroup$ Jan 10, 2019 at 18:47

2 Answers 2


In the absence of a digram, I assume the well drawdown is 67 ft. And if the tank discharge level is at 28 then the total head for pump should be 95 ft if the tank is at grade level.

As for the tank head if it's closed then as it gets filled with water its air volume shrinks so the pressure increases and depending on the ratio of volume of air lost to water you need to add the increased pressure of trapped air to the head required for the pump.

For that you need to stablish a relationship between the q of pump WRT the volume of the tank. Regardless of where is the pump outlet in the tank, above water level or below. If you do that then you need to modify the well drawdown, because the pump will be pumping less water due to more head it needs to overcome.

However the numbers that you have in you question are suggesting that the tank is configured to have a constant water height, may be a floater switch device.

If you edit your question and add a diagram, it will clarify the situation.


My interpretation is the maximum free water surface is limited to EL 39.0, as there is continued feeding to the tank, the pump will be started when the water reaches EL 28.0 and the pump will be stopped when the steady water surface is at EL -28.0.

In this scenario, there is a free board of 11', the actual drawndown varies depending on the rate of feeding and the rate of pumping, the maximum drawndown capacity is 67'.


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