Pump or System Problem?

Summer arrives, and it is time to forget about pumps. You go on holiday, but just when you think you have gotten away from the pump world, a friend asks you to look at his cabin’s water system, and so it begins.

Tom isn’t getting any water from the lake to his cabin.  He understands that a centrifugal pump is just an impeller inside an enclosure; if the impeller is rotating and you can get water in, chances are you will get water out. The problem is he feels he has water in, the pump is rotating, but nothing is coming out. I tell Tom, as I have with many others, that 95% of the time pump problems are actually system problems, so let’s look at your system first. Tom assures me that it is a broken pump but indulges me as we go to look at the system.

It is based around a small horizontal pump that is electrically driven by an integral ½ HP motor. The suction side consists of an HDPE pipe dropping about 2 ft to the lake surface, then out into the lake, with a strainer/foot valve at the very end. On the discharge side, a transducer is mounted just above the pump, and an HDPE pipe runs up about 8 ft into the single-story cabin. A small fill port on the pump outlet is used for priming, as it cannot self-prime.

While reviewing the situation, Tom tells me that if he pours water into the priming port just before opening a tap in the cabin, the system works, but an hour later he has to re-prime to obtain water flow. Based on this, I advise Tom that the first thing to check would be the foot valve, as it is probably the culprit. I explain that water is probably leaking back into the lake through the faulty valve, leaving the impeller spinning in air and unable to create any pumping pressure. In the pouring rain, I leave him to fix the valve and hope that resolves his issue.

Two days later, Tom is back with the same problem, so we take a boat out to look at the suction pipe. A good portion of the pipe is floating, but the end is weighted down. A new, shiny strainer sits on the lake bottom. Tom has changed out the old strainer. Before proceeding any further, I attach a float to lift the strainer 2 ft from the bottom of the lake and attach weights to submerge the floating line. This should avoid air locks in the suction pipe, keep boat props away from the pipe, and prevent solids from entering the system and/or jamming the foot valve.

Back on shore, we fill the pump through the priming port and go to lunch. After lunch, sure enough, the water level has dropped out of sight, so we disconnect the suction pipe, bend it upward and top it up with water. To our surprise, the level does not drop in the pipe, so Tom asks where the priming water is going and why the water in the cabin does not flow back to keep the pump full of liquid.

The pump in question was very similar to the illustration to the right and I thought Tom needed a quick lesson on the two common pump systems used to feed cabins before we went any further. Here is a quick review.

Electronic pressure transducer systems eliminate the need for a water tank. Instead, an inline pressure sensor monitors the water pressure downstream from the pump. When a tap is opened in the cabin the pressure in the system drops and the transducers electronics switch on the pump. When the tap is closed the pressure in the pipe climbs and the transducers electronics switch off the pump.

This type of system normally utilizes an integral check valve located within the transducer assembly. This check valve eliminates back pressure on the pump and allows the priming port to be opened while the cabin pipes stay charged with water.

Note: If the pump is not equipped with a self priming feature, the system relies on a good foot valve and no leaks in the system to function.

Pressure tank systems have a tank installed in the discharge line. Inside the tank there is a rubber bladder that can be pre-charged with pressurized compressed air (usually  around 20 psi).

When the pump starts, with all taps in the cabin closed, the pump starts to fill the tank compressing the bladder/air. As the back pressure created in the pump reaches a preset level a small diaphragm switch attached to the pump shuts the pump off. When a tap is opened in the cabin the pressure in the tank’s air bladder  forces water into the cabin. As the bladder expands, back pressure on the pump drops and the diaphragm switch attached to the pump turns on the pump and it maintains pressure until the tap is closed and the pressure tank is again charged with water.

Note:  This system must have a tight sealing foot valve or the system will bleed pressure and repeatedly cycle on and off.

After the quick pump lesson Tom understood that he had a transducer based system and that the integral check valve was why the water within the cabin was not backflowing and keeping the pump primed.

Knowing that the suction pipe was not leaking and understanding why the pump was not getting resupplied with water from the cabin left us with only one open question: where was the water in the pump going?  To answer this, I got down on my hands and knees in the pump pit with a mirror and found traces of water on the bottom of the pump casing. This meant that either the casing had a small hole in it, the back plate gasket had failed, or the mechanical seal in the pump had failed.

At this point Tom told me that a small rusty 15-year-old water pump was not worth any more trouble and he would just replace it.

In summary. When the pump was properly primed, the cabin received water. When the taps were closed for more than an hour, water drained out through the damaged pump, lowering the water level in the casing to a point where the pump could not develop enough pressure to open the check valve in the transducer.

At the end of the day we sat and had a beer. Tom couldn’t resist telling me that his pumping problem turned out to be a pump problem not a system problem and maybe my 95% rule was overstated.  I disagreed and replied “nope, Tom as always you are just part of the oddball 5% ”

Cheers

RJ