# Need help in flow rate

1. Jun 12, 2009

### makingapond

need help in flow rate....

hi, i am just new here... hope anybody could help me here....

i am making a pond just like my username says.....

my problem is, my pond is around 2000L and has an outlet of 2inches at the bottom corner of the pond. how do i compute the flow rate, coz i wanna know how many times an hour my water circulate. it would have been easier to use an electric pump coz it would say it on its box, example 200L per hour so it would be 1:10 circulation per hour since my pond is 2000L.... suggestions or formulas would be greatly appreciated thanks...

2. Jun 12, 2009

### minger

Re: need help in flow rate....

As a quick first iteration you could take the hydrostatic pressure at that point $$p=\rho g h$$. Using Bernoulli's equation, you can get a quick estimation of exit velocity.
$$\left[\frac{p}{\rho} + \frac{V^2}{2} + gz\right]_{1} = \left[\frac{p}{\rho} + \frac{V^2}{2} + gz\right]_{2}$$
Assume that outlet pressure is zero, and inlet velocity is zero.

From this, you could Google pipe losses for an estimate of pressure losses, which then affect the velocities (iterative process). It might not have decimal places of precision, but it will get you in the ballpark.

3. Jun 12, 2009

### makingapond

Re: need help in flow rate....

am kinda lost with the variables could you tell me what rho is and p and g and zeta are?

4. Jun 15, 2009

### minger

Re: need help in flow rate....

rho is density, p is pressure, z is height

5. Jun 15, 2009

### makingapond

Re: need help in flow rate....

minger, the idea of my problem is that i will know how many times the water circulates back to the pond if the drain was flowed back into it. too many variables and am kinda lost so am thinking ill just fill the pond, as i am draining it i will get the time it is drained. Or i could mark the pond and drain some up to 5 mins and then compute how much is drained in an hour???? which do you think is better coz the 1st one you can loose pressure if you drain it all. right???

6. Jun 15, 2009

### minger

Re: need help in flow rate....

If you can get the time it takes to drain the pond, then that would be perfect. That's better than any analytical guess you can make.

7. Jun 16, 2009

### makingapond

Re: need help in flow rate....

if i drain it all i would loose pressure if the pond is, letsay 3/4 empty already. what if i mark the pond then time it, after 5 mins, i stop draining then mark it and compute for the volume it drained so i have and idea how much the rate is... what do you think???

8. Jun 16, 2009

### minger

Re: need help in flow rate....

As long as the amount drained is significantly larger than uncertainties such as waves, volume estimatation, etc.