Conical Weir 1

[zaskarle2000]

I'm working on the rehabilitation of an elevated storage tank. There is a 6-foot diameter conical overflow that necks down to an 18-inch overflow pipe and then down to the ground.

How do I calculate the maximum flow over a circular weir? My maximum head above the lip is 6-inches.

I was planning to calculate it as a broad crested weir.

Q=(2/3)(C1)(b)(SQRT(2g))H^(3/2)
C1 = 0.5 to 0.57

After the water goes over the weir it drops 150-feet by gravity, straight down, so I think my weir length is my limiting factor.

Any thoughts?
Thanks.
Chuck

 

[cvg]

it's a sharp crested weir, not broad crested and I think this is the equation:
Q=CLH^(3/2)
C=3.2 to 4.6
Brater and King, (5-34) and table 5-2

 

[katmar]

In a circular weir (assuming inward flow) the flow path narrows as the liquid flows over the edge. This restricting effect is more pronounced in small diameter pipes and may not be relevant in your situation with a 6' weir.

For inward flow over a circular weir Lowenstein (Chem Eng., 61, 224 (Oct 1954)) gave the formula
W = 187.2 x L x (H^1.4)
where
W is flow rate lb/sec
L is length of wetter perimeter of the crest of the weir, ft
H is height of head above the crest, ft

I have seen simlar installations where the angle of the cone is quite shallow, and in that case your assumption of a broad crested weir may be better than a sharp crest. I would calculate the flows for the various different assumptions - i.e. straigh sharp weir, straight broad weir and circular weir and see what range I got.

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