Distribution chamber weir sizing question

[stucroft]

Hi everyone,

I'm new here but have found some posts very helpful in the past, thanks!

I have a little distribuiton problem that I'm doubting myself on as hydraulics was very far from my strong point at university!

I have been asked by my client to alter the sizes of openings in a concrete sewage distribution chamber leading to treatment tanks where the part treated sewage is trickled over a filter. These tanks need a certain m3/m2/hr to function correctly.

There are 3 openings, 2 the same width leading to 2 tanks the same size, head of water, distance away and pipe size.

The third opening is wider and leads to a larger tank, but uses the same size pipework and is approximately the same distance away as the other two tanks, with the same head of water.

During low flow situations the larger tank, "hogs the flow" and the smaller tanks do not receive enough flow to function correctly, am I correct in thinking that the widths of the openings need to be proportional to the areas of the tanks?

% of total tanks areas:
tank 1 - .291
tank 2 - .291
tank 3 - .419

Current opening widths:
T1 = 0.58m
T2 = 0.58m
T3 = 1.35m

Combined % of total area from 2 tanks - 0.581
Combined opening widths: 1.16m
Total opening width required: 1.996m

Required width of tank 3's opening: 0.835m


I understand I may be overlooking some factors but a detailed study is not warranted and there's no budget to conduct one.

Any help would be greatly appreciated, thanks in advance.. .

 

[bimr]

That is basically correct assuming the weirs are built correctly.

You also need to adjust wier lengths slightly due to wall effect. The area adjacent to the wall will have slightly less flow due to end contraction.

 

[bimr]

Here is a chart.

 

[stucroft]

thank you, most helpful