[Riley_5000]

Hello

I have a tank. The tank has a drain at the bottom which discharges to atmosphere. How can I calculate the flowrate (m3/h) coming out of this pipe as the level in the tank changes?

This is what I have so far:
1) The head in the tank as it changes
2) The fact that it's discharging to atmosphere
3) The size of the pipe (40mm I.D)

I know I need to calculate the dP through the pipe (about 3m long), but how do I do this without knowing the velocity?
Any help will be appreciated.

[sjb]

Is that a 3 metre drop? Can you use the equations of motion as a first approximation?

v_final² = v_initial² + 6g for the speed of the flow, then multiply by the cross-sectional area?

[Riley_5000]

Well, at most it's a 1 meter drop, and about a 2 meter pipe. 32mm ID pipe.

Here's what I've done so far:
For gravity flow, I guess that the friction loss in the pipe would be equal to the head in the tank (about 1 meter as mentioned). So to find the flowrate, I have to iterate to see at what flow I'll get that 1m friction loss.

Okay. So using basic friction loss through pipe calcs (considering roughness, etc), I get about 3.8m3/h or 1.31m/s at 1m head in the tank.

Using your equation (with 2g instead of six), I get 56.8m3/h . . .

I think mine is probably correct, but I'll ask you to check if my reasoning is sound . . .

Thanks very much.

[sjb]

Sounds reasonable. Maybe I shouldn't have simplified as much as I did. Try
v_final² = v_initial² + 2gs, instead (I combined the 2 and s [displacement] to get 6 initially), make sure that if you do use this equation that the units are consistent (so speed in m/s etc).

[sati]

you get exact v=2^0.5 m/s (1,41) if you use Bernoulli's equation. dp is neglectible.

[typhoon2028]

Flow rate should be a function of tank height

[Rishabh]

Hi,
so did you get the answer??