Water Detection at Pump Intake

I think what you are looking for is called a "Pumpsaver".

Thanks for your comment.

A conductivity or capacitance switch could be used to signal the presence or absence of water in the pipe and to interlock the pump in the event there's no water.
There are two components, the electrode probe that goes into the pipe,

and an electronic controller with a relay output:

The pipe must be grounded to be the other electrode in the sensing circuit.

The ones I'm familiar with are designed for level applications, where the electrode probe is inserted from the top or side of the tank. As you can see from the dimensions , the probe is several inches (55mm) long. The probe can be inserted into a Tee with a nipple and coupling to as a stand-off so that the probe can sense the water but not obstruct the flow path.

Some good information here...

https://www.rainbrothers.com/post/2016/02/08/how-to-prevent-your-pump-from-burning-up

I think I would rather have a booster pump on the ground that would correct the problem instead of just adapting to it...or possibly a larger water tank as an alternative...In any case I would demand a constant supply of water...

I am assuming that the pump (on the third floor) is located somewhere near the tanks which, as you have stated, then feed the house.

I also assume that the main water supply (which at times does not have sufficient pressure to reach that third floor) comes into the house at ground level.

As pumps generally have a higher maximum head (the height they can push the water) than they do suction lift (the depth from which they can suck the water, would it not be a better proposition to relocate the pump down to a lower level (preferably ground level) so that it does not suffer this problem?

This may be a simple cut and shut operation to achieve, assuming of course (I do a lot of that), that the water supply can keep the pump flooded even at ground level

As well as a water presence sensor you need a water pressure sensor. If there is water but at a very low pressure, when the pump starts it will possible suck the feed line dry but more likely it will cause cavitation damaging the impellers. Seal damage you have to replace the seals and possibly the motor if the pump seizes, impeller damage you have to replace the pump.

There is a non-intrusive thing called a phase angle detector, which measures the peak of the voltage waveform, the peak of the current waveform, and infers the angle between two.

• If the peaks are close together, then the pump is doing work, which classes as "water present".
• If the peaks start to get spaced out then the pump is not doing any work, which classes as "water absent". The pump can then be stopped through external controls.

Will that do?

_{[Used successfully on a water treatment plant many years ago.]}

Thanks for your comment and advices.

As suggested by several, a low pressure switch on the suction seems to be the best solution, inexpensive & reliable. This one allows pumping with 5 psi or more on the suction, installs in a small threaded hole, and costs $22 from this source.

My first thought was a conductive type level sensor, but this idea is much better.

https://www.freshwatersystems.com/products/low-pressure-shut-off-switch-1-8-male-npt

Hi, I agree that is the simplest way, but the pressure shut off is high, 5 psi means 3 meters of column of water. To much for my case.

Thanks for your interest.

Gabriel

You can’t get 5 psi from your water supply on the 3rd floor? Maybe you would be better off with a vented head tank, filled with a float valve or valve run by a level switch. The fact that you have water in the suction pipe won’t prevent you from operating the pump with inlet pressure low enough to damage the pump due to cavitation, perhaps more damage than running dry. Interlock the pump based on level, that way you know you have some atmospheric pressure for NPSHA. Home Depot sells these, similar equipment used in industrial apps for sewers & dirty sumps. Highly reliable in my 30 yrs of application in industrial locations.

Great idea, far less costly than a conductivity switch.

5 PSI = ~3.5m water column

The pressure switch would have to mounted in the vertical supply line ~3.5m below the top of the suction inlet to the pump to "prove" a columm of water 3.5m above the pressure switch.

The only concern is that when flow starts, a venturi effect of the flow across the sensing will is likely to reduce the pressure and the switch could trip, unless the pressure reduction is in the switch deadband. I know of no formula for calculating pressure drop across a dead headed orifice port. Does anyone else?

The calculation is the same as for calculating the pressure drop for flow through the run of a tee fitting. Since there is no flow in the tee branch, the pressure there will be slightly less than at the entry of the tee, but the velocity will be relatively low, designed for economic operation. Unlikely to cause even a 1/2 psi drop for normal 1” flow applications.

Do you have a swimming pool at ground level where you reside?

If your upstairs water does not need to be potable, then a swimming pool could be made to function as a stand-by water supply...

An electronic device is not low budget compared to a mechanical pressure switch.

You should get the minimum suction pressure for your pump from its manufacturer. Then you can find a suitable switch.

From what you write & size of pump; it may not have a starter with overload protection. There is a risk the pump will start & the pressure loss in suction pipe resets suction pressure switch - causing a cycle of starts, which are high stress (heating) for motor. The simple precaution is a timer which, when the pump stops, does not allow it to start again for about 10 minutes. This may be no problem -compared to your observation of service running times & stationary times.

I guess there is now a level switch controlling the pump, with enough differential to avoid frequent short runs of motor.