Over Flow From Tank

Banu,

Please try at least to make at least a weak attempt by yourself to investigate your issues.

There is a wonderful tool called "GOOGLE" that, when used by the competent, can answer many questions.

Froude, Banu, Froude...

You need to calculate the discharge (flow) value of the orifice, weir or overflow pipe. The discharge must be in excess of the pumping delivery flow to the tank, which you gave as 300 Liters per minute.

This is "Hydraulics 101". Were you sleeping in class again when the Professor discussed this?

Why not trim the top of the tank to the desired full level?

Then the question of overflow sizing is then solved.

As mentioned above you have to size an orifice to meet atleast pump flow. But flow to happen through orifice, water head required above that. So you can hole little below the desired level and let water go to your desired level and for that head, size the hole to meet pump flow

"... [then simply] size the hole to meet pump flow" The size of this single overfill relief hole is the question. It is inferred that the smallest size is desired. A horizontal slot will be better than a round hole... I guess. There are many things undefined, but homework questions usually are not well defined.

Why are you answering your own question?

Is there any problem with making the hole larger than necessary, say 4" (100mm)?

install a float level valve (mechanical) or a flow level switch (electrical) to shut off pump.

Using a float or switch (to de-energize the pumps) level control would be ideal, but many codes like AWWA require an overflow pipe/outlet on storage tanks.....it makes the overflow issue sort of "idiot-proof" in the event of a power outage or disruption, etc..

All of the above comments are very nice, but the do not address Banu's original question about tank overflows...... How are they to be sized ?

What does an overflow do anyway ?...Is this flow "self venting flow ?

Froude, banu, Froude !

(William Froude an English engineer, did the original theoretical work..)

C'mon banu, you can do it !....

Here are some hints...... GOOGLE and Froude !!!

I strongly suggest that the OP obtain a copy of Brater & King's Hydraulic textbook and let him figure it out all by himself. there are free ebook downloads of this text available on the web, if you look hard enough.......try a Google search.

A lot depends on the orifice configuration (re shape, hence applicable coefficients) and depth of head, in addition to the diameter of the orifice.