Control.com - Nerds in Control

I have a question to experts & professionals of control.com:

In process industry a pump running 'dry' is often a big issue (or is it in just my place of work!!). generates a lot of hue and cry. mostly there is a logic that will trip the pump if the tank/sump level goes low. My question is what will really happen if the pump will run dry in case the low level switch doesnt work. To my belief the motor should be 'happy' since there's no load now. but I know I am missing something. request to please explain me why shouldnt the pump-motor be allowed to run dry.

thank you all
Ritika

Depends on the pump and whether the pump is designed for a flooded suction or not. If not, and the pump runs dry, it can overheat and cause issues. If it is designed for a flooded suction, and doesn't have one, it could destroy the pump. If it is a turbine pump, and is designed for a flooded suction, the water hammer caused by the first big slug of fluid will have the potential for breaking turbine vanes and destroying the pump.

It is a big deal. Make sure that the check valves in the suction work right.

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I mean the pump should have its own logic. Like the motor current, I think when the load changes the motor current should be changed. So I think the motor should have function like this.

If the pump does run dry and is not designed too it can run hot, then cool and cease up.

normally short periods are ok. but i think im right in saying that the cooling comes form the water being there.

feel free to correct me if this is not your pump type

I worked for many years in mining. We never interlocked the slurry pumps on low level as that would be too disruptive. The pumps could run empty without any ill effects provided the glandwater supply was not interrupted.

Regards,
Roy

Diaphragm pumps or positive displacement pumps (piston type) do not require protection against dry run. It is inherent to design or by use of lubricating soft parts for piston.

Centrifugal pumps or positive displacement pumps (gear type) require flooded suction. During pump rotation part of the energy is converted to heat and is taken away by process fluid. When process fluid is not available, the heat is absorbed by pump internals which are:
1. Dissimilar material and has different temperature coefficient of expansion.
2. Pump internal clearances are very low to achieve high efficiency,
leading to rubbing at high speed and subsequent seizing. This mostly happens with weir ring, which has normally the lowest clearance.

Centrifugal pumps may be designed with high clearances to allow the pump run dry for certain duration but you loose efficiency. Also a less maintained pump over long period develops higher clearances allowing the pump to run dry for a while.

If reliability of level switch is a question, you may replace it with a transmitter or may adopt 2003 voting with level switch, if the pump failure is costly.

Regards.

Pumps are designed handle the fluid not air. Yes pump may run dry, but where the trapped air can go, mind it the air is compressible so there are chances of joint failures. The other part is the NPSH required to be maintained to prevent cavitation especially in case the fluid handled is hot fluids.

Hello Ritika,

It's a mechanical issue. Usually sealings are destroyed first by dry run, then water (or any other medium) goes through damaged sealings into the bearings causing their malfunction.

Regards,
Piotr Kowalski