In my installation, I have a pump. This pump is intended to transfer water to other installations. The suction of the pump is piped to a water tank. The tank is equipped with a level transmitter. This transmitter measures the level of water in the tank and sends a 4-20mA signal to a PLC. I want to protect the pump against cavitations when the level of water in the tank is very low. Thus in the ladder logic, the pump should stop when the level gets very low.

I want to know, how can I calculate this Minimum level set point to trip the pump safely (without cavitations)?

Thanks

 

The pump has a minimum suction head. That is, the minimum pressure at the suction inlet. Unless it is designed (like a sump pump) to pull against a negative suction head, you can set your low level alarm contact at some point above the minimum suction head.

If you have no way of figuring out the minimum suction head, or you have a negative suction pump, figure out how fast the pump stops, and determine the level of fluid that will draw down in that time, and set the alarm contact above that level.

And if all else fails, in a tank of any decent size, 1 foot of level above the pump suction will probably work.

Walt Boyes
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Awful hard to calculate unless you know the level that the pump cavitates at, which can be detemined by experiment or guessed at or deduced if say, the pump feeds from a pipe fitting that needs to stay covered. But, I sincerely doubt it could be calculated sight unseen. Once you know that, you just guardband by the accuracy of your transmitter plus whatever you are comfortable with and you have it.

Regards.

 

bob,
This is not something that can be calculated reliably (there are tank geometries and flow profiles which have to be considered).
The easiest way to determine this level is to observe the tank level and the water surface. When the water surface begins to vortex record the tank level and add 1 m or 1 yard to this value and use that for the pump shut off level.

Dennis