Dry Running a Motor

Some pumps rely on water for cooling and /or lubrication of bearings.

If all else fails...read the instructions...

otherwise don't do it just to be safe.

If you already have run it even briefly...send £2000 in used notes to the following adress and we won't tell anyone.

The pumped medium is used for cooling the pump and in some instances lubricating support bearings [eg vertical submersible pumps]. If the pump is run dry, no cooling would take place and the clock begings to tick as to whether the bearings seize up first or the impeller / efficiency ring clearance "evaporates" thus seizing the impeller onto the efficiency ring. This seizing effect is quicker if the pump is shut in i.e. discharge valve closed as the water boils.

Some types of positive displacement pumps woulkd themselves out because of the lack of lubrication on the sliding surfaces.

what shall be effect on current taken by the motor in this case.whether motor will get overload or not.

The power requirements of a pump is relative to the product of the volume of the water (mass) and the pressure head. A pump running dry or with a closed valve will therefore only use power to overcome internal friction and warming the water.

In most cases this will be small in comparison with the motor.

The moment a bearing or other part goes the power usage will increase drastically and may cause damage to the motor

It is much cheaper to install low level switches.

Is there any way to sense dry running without the use of level switches ?

Pressure switches can be used.

Current detectors in the motor supply circuit. When the current drops away, the pump is doing no work. Assume that it has run dry, and trip off.

We live in Central India and we have a chronic problem of dry running of submersible pumpsets.

We have tried tripping the starters running the pumpsets by sensing the drop in supply current when the pumsets run dry. But we have found that they do not work with all types of pumpsets. For radial flow pumpsets, the drop in current is significant and can be utilised, but for axial flow and mixed flow pumpsets there is no measurable change in current when they run dry.

What is the reason ? Is there any other 'fool-proof' method to sense dry running ?

Hi,

Try using or installing a flow switch instead. This is different from a pressure switch. For new model submersible pumps nowadays, there is already included a low level float switch that is normally set to stop the pump when the water level goes down below the pump body.

It is also possible that the tripping device you installed is for overload protection, which will not cutoff the switch if there is a low current. Unless if the tripping device is an electronics type that can sense over and under current in the load system.

that depends on the type of pump..what have you got?

Dry running a pump for more than a few minutes will eventually burn out the motor.

Hello DVader1000 - I can't agree with that. If the pump is running dry the motor draws less power and has no problem. Even on a submersible, I'd expect the loss of motor cooling by the fluid to be more than made up by reduction in heat dissipated.

The pump suffers as seals are likely to be damaged. Some special pumps have no seal, but a clearance round the shaft, with fluid leakage/recirculation, so these are OK, but are not common.

Also as others have mentioned some pumps use the pumped fluid for cooling bearings.

Codey

"Also as others have mentioned some pumps use the pumped fluid for cooling bearings."

It's lubricating....the bearings, that causes the problems, if the fluid is the lubricant, and you run it dry it will seize.

Seize does matter!

Hello Del - what sort of bearings are these? Do you mean plain (sleeve) bearings? I can't think of another type that could be lubricated by pumped fluid (unless it's oil being pumped)

Codey

Yes I'm thinking plain bearings.

I think I've seen brass...

I believe an ordinary central heating pump has steel on steel (or is one ceramic?), I took one apart a while back...they don't like running dry.

#11 was me of course...a light hearted comment & it was my brother's boat!

Maybe I'm mixing my bearings with my seals to some extent?

Del

No, you're right, I hadn't thought about central heating pumps. Grundfos data says bearings (upper and lower) ceramic, shaft ends ceramic. It also says seal - plastic but I don't think it's a rotating seal as the pump has a flooded rotor.

And I'm sure the bearings wouldn't like to run dry.

Codey

Hmmm... More likely the seals and bearings in the pump casing?

If you have a centrifugal pump and it is run with no water or liquid, it will still continue to rotate (the motor) but you will surely damage the shaft seals. The electric motor is provided with a cooling fan at one end of the casing. The shaft seals are made of ceramic rings or hard steel that is pushed to one stationary ring by a spring. Running without liquid will wear out these two sliding rings quickly.

If you have a positive type gear pumps, lobe pumps, screw pumps or piston pumps, and these are run without liquid, the pumps eventually seize or stop rotating due to heat generation. If the electric motor is provided with an overload protection, this should stop or disconnect the power source before it burns out.

The prop shaft on my boat runs through a rubber output bearing which is designed to run wet.

If I take my boat out of the water it doesn't work at all, I feel this demonstrates the problems with dry running!

Depends on the pump.

A peristaltic pump will run dry indefinitely.

Try it over long periods with other types of pump and there is a risk of cooking the seals.

Yes,true. A diaphragm pump will also run dry indefinitely. Peristaltic pumps are normally with very small capacity and found mostly in the lab. They use tubing that is squeezed by two or more rotating rollers. Diaphragm pumps are run either by compressed air or a pulsating action created by a cam or solenoid coil. Sand Piper pump or a Wilden pump are examples of a very good compressed air diaphragm pump, while a Prominent pump is pulse pump run by a solenoid coil.