[ELI5] Why does pumping air wreck water pumps?


Saw it on a package for a water pump that it shouldn’t be run dry for more than 1 second. If air is going through it, which requires less energy to move than water, then it should be no big deal, right?

In: 8

I’m thinking it overheats without water to cool it?

Pressure. Air compresses at a different rate than water, so when a pump designed for water gets an air bubble in the line or pump that pressure differential can damage internal working of the pump.

What sort of water pump?

If it has a mechanical seal it will fail without a lubricant, this case the water.

If it uses a packing gland, it can over heat and fail prematurely.

As the shaft spins, it needs to prevent water running along the shaft and out to grade. This is done by having 2 faces float extremely close to each other with a lubricant between them and a force keeping them closed (typically a spring or bellows) , ie. A mechanical seal.


The shaft spins in a felt like packing thats pressed around the shaft and into the pump casing. This slows the leakage rate to a negligible amount.

Both these scenarios require a lubricant to prevent failure. Water acts as a lubricant and a cooling medium.

Pumps intended for water often require it to keep their seals and/or bearings cool and lubricated. If there’s no water, the seal just heats up until it melts or otherwise degrades. Water is quite effective at carrying that heat away.

Pumps exist that can pump either air or water, like peristaltic pumps, but when you need to move a lot of water or air it’s more efficient to design the pump specifically for that.

The seals aren’t designed to run dry and will fail if the pump is run with with no water being pumped. They usually fail by overheating as the water is used as a coolant and lubricant.