BLUF: Air trapped in a pump housing can prevent it from being primed even if a solid column of water is supplied to the pump.

It is not uncommon for the water storage tank to run low and the water pump begin sucking in air along with the water. Symptoms can vary from the faucets spitting an air/water mixture to the pump loosing its prime and water ceasing to flow altogether.  At some point the pump will no longer be able to maintain water pressure and it will run continuously.  Some pumps are capable of running dry without damage but others will over heat and sustain permanent damage like the big AC centrifugal pump in the photo.

This system is composed of an AC water pump and a DC water pump plumbed together into a common supply manifold.  The AC pump is the primary and it is set to maintain a 50-60 psi system pressure.  The DC pump is set to come on if the pressure drops below 25 psi and shut off at 30 psi which happens when the boat is disconnected for shore power or the generator.

This particular system demonstrated a rather perplexing symptom recently. While using the AC supply pump the tank ran low and air was entrained with the water.   The problem was quickly remedied by refilling the tank with no further issue until a week later.  The boat was at anchor with the AC power turned off.  All was well until a faucet was opened dropping the water pressure to 25 psi engaging the DC pump. The user noticed that the flow was meager but after completing the task she closed the faucet and went about her business without further thought. In the mean time the DC pump was discovered to be running continuously and the pressure gauge read 10 psi. For some reason it would supply a small flow of water but would not bring the system pressure up enough to trigger the shut off switch.

Now that’s a head scratcher but I kinda spoiled the mystery by hinting to the source of the problem. Possibly the DC pump has lost its prime. But normally when this happens the pump will not deliver any water at all.

I confirmed that there was a solid column of water supplied to the DC pump inlet by disconnecting the inlet line and watching water flow immediately flow by gravity out of the line. Hum, normally when a pump needs to be primed I find no water at the inlet. So I then turned to the outlet side of the pump.  I loosened the hose clamp, energized the pump motor, and wiggled the hose to break the seal. A very small amount of water squirted out at first but within a couple of seconds the pump motor loaded up and a good bit of water began spraying about as you can see in the photo. I quickly tightened the connection to stop the flow and within a few more seconds the water pressure came up and the pump switched itself off. Problem fixed. But why?

As a mechanical engineer I have spent my entire career studying and applying the discipline but I am still intrigued to see the principles played out in real life systems. The pump inlet is well below the fresh water tank so a solid column of water was available to flow into the pump. It should have displaced any air that may have been in the line. But what about air that may have been trapped within the cavities of the diaphragm pump?  Is it possible that enough air could be trapped in the chamber such that the air could not be compressed to the 30 psi shutoff pressure by the diaphragm’s displacement but not enough air to totally prevent water flow?

This video published on YouTube by MarineMechanik demonstrates how a diaphragm pump works:

The governing principle is that water is incompressible therefore when the diaphragm moves it does not compress the water into a smaller space, it expels the water past the spring-loaded discharge check valve and out of the pump casing.  Air is compressible meaning if air is present in the housing the diaphragm will squeeze the air into a smaller space raising its pressure directly proportional to the relative before and after size of the air space.   If the discharge spring is strong then the pressure against the value face may not overcome the spring and nothing flows out of the pump housing.  The diaphragm simply compresses the air only to allow it to re-expand again.

In this case the pump outlet was exposed to about 10 feet of water head pressure (4.3 psi) due to the difference in elevation between the pump in the bilge and the faucet. I can imagine that as the pump diaphragm attempted to expel water from the pump cavity it first had to compress the trapped air to at least 4.3 psi.  It must have been able to achieve this because when the problem was discovered the pressure gauge read 10 psi and a small amount of water was flowing out of the faucet. So the amount of air in the pump was such that the displacement of the diaphragm could raise the pressure to 10 psi but not up to the 30 psi shut off pressure.  And the flow was so low that the air would not be swept out along with the water.

Boating Lessons Learned: When I loosened the outlet pipe connection I removed the head pressure, the pump was then able to expel a lot of water and sweep out the air.

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