Flow Rate calculation Help

[AZEEMM1]

I'm new here and my background is Electrical so excuse the simplicity of the approach.

I'm doing some project work in the US and have the folllowing question (all measuers are US):

I have a 50,000 gallon Water Tank that overflows in the event of a main pump failure (catastrophic-happened twice already). Flow IN is 60,000Gal/Min, cannot be stopped or slowed down otherwise we damage equipment.

I want to notch the top of the well and have it spill over into a concrete spillway and redirect the flow away frmo the buildings. The questions is how big do I make the 'hole' at the neck of the well. I only have access to one side, so I can equate an answer in square metres (feet over here!) accordingly.

Thanks in advance.

A

 

[cvg]

why are you designing this? You are an electrical engineer!

 

[AZEEMM1]

I'm working on Projects, this is going to be checked by someone else anyway! I'm just trying to get an idea for the practicality of one of many solutions put forward!

Thanks for pointing out my degree!

 

[sbush]

The diameter of the drawoff nozzle can be calculated as follows: d = 0.5 [q/hM (0.5 power)] (0.5 power)

Where:
d = nozzle diameter, in inches
hM = minimum design head above drawoff nozzle, in inches
q = flow rate, in gallons per minute

Example: Size of drawoff nozzle for a process vessel where hM is 48 inches and q is 60,000 gpm.

d = 0.5 [60,000/48 (0.5 power)] (0.5 power)
d = 46.5 inches

Notes:
1. This formula assumes that the nozzle will constitute a sharp-edged entrance.
2. This formula assumes a safety factor of four to allow for uncertainty in the resistance coefficient (K = 0.50 for sharp-edged entrance), and for liquid level variations due to agitation.


S. Bush

http://www.water-eg.com

 

[cvg]

check your numbers - did you really mean a small 50,000 gallon tank with inflow of 60,000 gpm? If that is true, you are completely filling the tank in less than 1 minute? Is this a surge tank?

Bush gave you an answer of 46 inch diameter which would give you a flow velocity of about 10 feet per second coming out your "notch". This is going to require a substantial concrete spillway to contain this large flow as well as some sort of energy dissipation

 

[AZEEMM1]

Thanks fo the help.

I ran the numbers through the following eqn and have someone double checking it for me:

A = Q / [ C x (sqrt {2xGxh)}]

C =0.62 (sharp edge concrete)
G = 32.2f/s
h = Height of notch x 0.5
Q = 133ft3/sec

and the results I have indicate I need a 13.5ft x 2ft notch to accomodate for the overflow INTO the channel and 32 ft x 1.5 ft to get the flow OUT.

Yes, the 60,000gpm is correct and yes, it does fill very quickly. It is a surge tank from a process prior to pumping to a cooling tower Header.

The solution is do-able and we have more than enough room.

If I'm missing something obvious here, any pointers would be appreciated.

Thanks

A

 

[cvg]

Assume a horizontal sharp edged weir with end contraction
Brater and King chapter 5)

weir equation Q = CLH^1.5

C values for this equation range from about 3.2 to 4.2 (see figure 5-3).

Assume a mid ranged C value of 3.7 and you get about 141 cfs. However this is assuming a horizontal approach channel. In your case, you have flow rising vertically and then turning 90 degrees to exit out the notch. This will result in an energy loss as the momentum of the fluid is lost. The result will be a higher water surface in the tank required to discharge the water through the slot. You may want to be conservative and make the notch a bit wider.

 

[AZEEMM1]

I think I have everything I need now to push this 'up'.

I will be adding some safety factors in the numbers and creating more of a slipway than I need, both INTO and OUT of the spillway.

Thanks for the help, it's greatly appreciated.