Sizing water tank overflows

[Taigster]

I have designed and built several dozen water tanks. Recently, I have come across a project wherein the water treatment plant has the capability of "flooding" an existing tank I've inspected. I and the Owner want to avoi adding another overflow. However, I am curious as how to accurately calculate the capacity of the existing overflow if I constrain the nappe to 2 inches. Does anybody disagree that by treating it as a sharp crested weir the calculation would be reasonably accurate? Taigon Worthen, P.E.
Epic Engineering, P.C.

 

[epbpe]

Sizing an overflow pipe is totally dependent on the size and length of pipe and fittings, available head, inlet conditions, and outlet conditions. I have found that the inlet conditions are critical to success, if you assume that it is no more than a sharp crested weir, I feel that you will be disapointed when you see the results. I have found through trial and error that a sharp inlet creates much more head loss than calculated due to a venturi effect. Providing a flared or tapered smooth inlet ring to the pipe will generally improve the inlet condition dramatically. Also, don't forget to include all losses associated with velocity head and fittings and any outlet restrictions (such as exit losses). These often overlooked friction loss points can make or break your design.