Aim: Drain a 50 litre tank of oil using a positive displacement pump (see Diagram), so that it drains quicker than gravity alone.

The pump should not be starved at any point.

The flow rate from the 50 litre tank should be at least twice the flow rate from the 5 litre tank.

Current setup:

The 5 litre tank is present because it's used to fill the 50 litre tank (in a different configuration) and also to ensure that the pump is never starved. Once the 50 litre tank is fully drained, the pump should be able to draw oil from the 5 litre tank and then the pump is switched off by a minimum level sensor in the 5 litre tank.

All hoses are 1" inner diameter.

This is where I'm stuck:

In my current setup, will the pump draw the same volume from both tanks at the same rate?

Can I do anything to ensure that the 50 litre tank drains at a much higher rate than the 5 litre tank so I don't have to constantly top up the 5 litre tank?


If I reduce the diameter of the hose from the 5 litre tank or use a valve to restrict the flow, can I achieve a greater flow from the 50 litre tank compared to the 5 litre tank?

  • 1
    $\begingroup$ From your diagram, when the pump is off, the 5 litre tank will always empty into the 50 litre one. Why not put a sensor to check for air just above the tee feeding the pump and switch it off using that signal. $\endgroup$
    – Solar Mike
    Aug 12, 2017 at 10:24
  • $\begingroup$ Agreed. There seem to be several details missing from the question. @jcdrbm: Edit your question to write a short specification for the operation of the system. Then explain how far you've got and where you're stuck. $\endgroup$
    – Transistor
    Aug 13, 2017 at 8:19
  • $\begingroup$ @Solar Mike, could you tell me the type of sensor that would achieve this? I am concerned that any entrainment of air in the oil would cause a false reading on the sensor. I have a minimum level sensor on the 5 litre tank which will turn the pump off if the volume goes below .5 litres. $\endgroup$
    – jcdrbm
    Aug 13, 2017 at 19:14
  • $\begingroup$ then use the same type of sensor - or are you suggesting it is not reliable... Then you should consider the characteristics of the fluid and use a change of resistance or an optical sensor - you know the conditions and details of the oil you are working with. $\endgroup$
    – Solar Mike
    Aug 13, 2017 at 19:18

2 Answers 2


I think using the 5 liter tank complicates the control; I would avoid using it for the draining process.

I would recommend hooking the pump directly to the 50 liter tank. Place the pump on the ground so it gets as much suction head as possible (to reduce the risk of cavitation).

You are correct that a gear pump should not run dry, but a few seconds will not harm it (especially with oil).

One method would be to add a restriction or decrease the size of the discharge hose of that pump so that it is pumping against a pressure when at full flow (5psi or 35kPa should do). Then use a pressure switch to shut the pump down if that pressure is not reached (indicating pumping air). It will take some tuning and you will have to have a start switch that bypasses the trip.

Another method would be to turn off the pump when the pump amps go low for a second or two; indicating the pump is pumping air instead of fluid. You could use a current transducer to detect the two states and shut down the pump with a micro controller.

  • $\begingroup$ ... or put a level switch in the 50l tank to swith off when almost empty. $\endgroup$
    – mart
    Sep 13, 2017 at 9:24

A suggestion I have is that you put the 5 litre tank, or a second 5 litre tank, in the pipe between the 50 litre tank and the pump like so: extra tank

So, as soon as the 50 litre tank is empty, then the level starts to drop in the 5 litre tank and that cabe sensed to switch off the pump. The 50 litre tank is feeding via the upper pipe and the pump drawing from the lower pipe.

Whether you have the vertical space is a different question....


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