Splitter Box and 3 Weirs - Uneven Flow

Hmmm, feel free to shoot at me folks, but I'd say this sort of problem is solved by making empirical adjustments, not by formuae...After all isn't it the 'mathematical approach' that says your 'identical' weirs will give the same flow in the first place?

Del

I believe you right, The line forms behind me, bring your own gun.

But before I start, what is it removing, BOD's? I've work with some liquids that actually thicken when it's being worked (non-newtonian) or temperature raises.

"BANG"

missed that dam cat, where did he go?

phoenix911

I agree; ditch those weirs and install three adjustable gates, then figure out where they need to be set to equalize flow. Besides, with the gates the plant operators will bless your name every time they have to take a clarifier offline for cleaning or service. :)

Could you use a Flout 3 way flow divider instead of the weir box? All the Flouts will sink at the same time dividing the flow very evenly, every time. What is the flow rate into the splitter box?

I would tend to agree with the others that you are probably not going to solve this using an out of the box equation. The issue is most likely the momentum of the fluid as you mentioned in your original post. Have you considered treating weir #1 and #3 as sidespill and weir #2 as perpendicular to the flow path.

For sidespill weirs, from my modeling experience:

Q=2/3*Cd*B*(2g)^0.5*(H)^1.5

Q= flow (cfs)

Cd = coefficient of discharge (usually taken to be 0.457)

B = length of weir (ft)

g = gravitational constant (32.18 ft/sec^2)

H = Head above weir (ft)

This takes into account the momentum carrying the flow past the weir. This gives a flow over the sidespill weir of approximately 75% of the perpendicular weir. This of course will vary depending on the velocity of approach to the weir.

For the perpendicular weir you can continue to use the Francis equation or similar

QW = C (L- 0.1 N HW) HW1.5

QW= flow (cfs)

C = coefficient of Discharge (usually a value of 3 to 3.33 is used)

L = length of weir

N = number of contractions

HW = Head above weir

Again, this will not give as accurate an answer as CFD modeling or possibly some other methods, but will give an idea. Hope this helped.

@Del the cat I agree with making empirical adjustments. the formulae would serve as a basis for making adjustment.

@phoenix911 Secondary clarifier for removing solids. [Wastewater treatment plant]

@Dave M. Weirs already been installed (splitter box and weirs were not my design). City is having budgetary problems so it will be hard to convince change order.

@Sparkchaser Flow rate to mixed splitter box = 225MGD max

@d_m_rosenberg Thanks for pointing the way. i don't think there's a cheap way to solve this.

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By the way, can you make out the text in the pictures I've attached? I have a hard time just looking at it. If requested, I'll try to upload it again when I get home b/c the company has internet SmartFilter which blocks many legitimate work-relevant sites.

On a side note from design perspective, the easy way to make a flow splitter box is to have a pipe that terminates into the bottom center of a square box and create a baffle inside the square box with a hole cutout in the center serving as an energy dissipation baffle. So the water exiting the hole will have the same velocity and overspill evenly to the 4 side weirs.

to see better view of mixed splitter box see download links below.

plan view PDF
http://www.plunder.com/-download-w2WzUv71Y0ONiQhS2PZQ3w.htm

section view PDF
http://www.plunder.com/-download-ceJc4Guntk2Na_zqUPMYGw.htm