LL Level trip on a Water Tank

[Ahmedmabrouk]

Dear All,

I am a junior Engineer and currently working for a small job for delivering water to my plant.

A well water tank is used to pump water to ISBL.

I want to put a LL level switch on the tank to close delivery the pump.

Keepin in mind that NPSH required for the pump is 5 m

Friction loss in piping is 2 m , water is pumped @ 25 C ( VP head is 0.328 m ), then Pump cavitation is not expected due to the atm pressue exerted on the tank ( 10.37 m).

Now LL level switch is not related to NPSH. How can I proceed?

Best Regards,
Ahmed

 

[TD2K]

I'm not following you. What does the system look like? I assume you have some sort of controls to start and stop the well pump based the level in the tank and then you have another pump that pumps water to the plant.

Is the LL level switch designed to stop the pump (the pump sending water to the plant) in order to avoid running the pump dry? You just stop the pump if you reach the low level on the tank.

 

[LittleInch]

Ahmed,

To get good responses you need to supply gold information. See the questions below and respond and you may get it.. A drawing or sketch is always very useful.

You're delivering water. How much water?
A tank doesn't pump water, a pump does. How big is it?,
what is the inlet pipe diameter?
What is ISBL?

What I think you want to know is what height above the bottom do you set the trip level.

I can only assume from your post that the pump is level with or below the bottom of the tank.

I assume your pump is a centrifugal pump.

Most pumps like this don't like pumping air or a mixture of air and water.

Hence your LL limit is the level above the inlet that prevents air getting into the inlet pipe. This depends on your flow rate, nozzle size, tank entry (straight in or with an elbow pointing down). If it's a standard nozzle, start with at least 1.5 times the nozzle diameter above the top of the nozzle.

If you tell us the information requested in this post you may get more responses.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[racookpe1978]

Friction loss is 2 meter?

And why do you claim the tank "head" is 10.376 meter, but claim that head is due to atmospheric pressure? Idf the tank is 10.37 meters high, then what is the actual water level inside that 10 meter tall tank when you consider it "empty" and when you consider it "full"? What elevation is the pump inlet centerline?
The parameters for this (homework ?) problem make no sense.

 

[silebi66]

Hi Ahmedmabrouk

The Low low level alarm for the tank would be alarmed at a level below the pump stop set point, not at the set point. The low low level alarm set point for the pump would be set about 150mm~300mm above the the pump suction nozzle's elevation to avoid the pump cavitation.

Hope it is helpful to you.