Calculating Weir Flow

[FraggyTex]

Hello. I need help calculating the amount of flow my weir can handle, so does anyone have experience with these calcs? I am attaching photos which have measurements of the weir teeth, but let me know if you need more information. Note, there is a larger 'U-shaped' notch in the middle of the overflow which is 26mm tall x 29mm wide. I used some drill bits to determine the radius of the notches. The larger notch required a 1 1/8" bit and the smaller weir teeth required a 1/4" bit. There are 75 total weir teeth plus the larger notch. Thanks!

 

[pdxmonkeyboy]

Measure the width of the gaps and multiple them by the number of gaps. Then add the width of the hole you drilled.

Plug that number as the width of your weir and flow into a contracted weir calculator.. boom. You will get the height of water flowing through the weir. Or plug in the maximum height of the water flowing through the weir and it will give you a GPH amount that will equal that water height.

Rectangular Contracted Weir Calculator

 

[ca1ore]

That larger U shaped hole is intended for the return pipe ....

There’s really no way to calculate what you are asking without specifying water column height. In other words, if you measure with the water level right at the top of the weir, you will get a lot more flow than if the water level is half an inch below the top. It may well be that the drains inside the overflow will be the restriction rather than the weir. You said nothing about those.

What is it you are trying to accomplish? Pump sizing? Something else?

 

[FraggyTex]

Measure the width of the gaps and multiple them by the number of gaps. Then add the width of the hole you drilled.

Plug that number as the width of your weir and flow into a contracted weir calculator.. boom. You will get the height of water flowing through the weir. Or plug in the maximum height of the water flowing through the weir and it will give you a GPH amount that will equal that water height.

Rectangular Contracted Weir Calculator

Thanks, PDX. By 'gap', do you mean the opening of each weir tooth? If so, the opening is 6.36mm (1/4") and there are 75 teeth. That would make 18.75" of open width. The teeth are 22mm tall, but assuming you need a small gap below the top tooth, I used 20 mm for the height (i.e. .79"). Using those numbers in the calculator, I get 40 GPM (2400 GPH). Does this sound correct? Note, this doesn't include the larger U-shaped hole, as I'm not sure what it is for...and/or if I will keep it.

 

[FraggyTex]

That larger U shaped hole is intended for the return pipe ....

There’s really no way to calculate what you are asking without specifying water column height. In other words, if you measure with the water level right at the top of the weir, you will get a lot more flow than if the water level is half an inch below the top. It may well be that the drains inside the overflow will be the restriction rather than the weir. You said nothing about those.

What is it you are trying to accomplish? Pump sizing? Something else?

Hi ca1ore. I just bought this tank (used), so I am working on the plumbing/flow design and pump selection. Also, The tank has two 6x6 glass chambers (one on each side of the overflow) for the return pipes. The tank also has 4 holes in the bottom of the tank for a closed loop system, but I don't have the details on how the original owner designed the plumbing.

 

[FraggyTex]

PS - There are 4 holes drilled inside the overflow box. 3 of the holes are 2.5" wide and one is 1.75" wide. There is one hole (1.75") in each of the 6"x6" return chambers. Here is a picture looking down into the overflow box, and you can see the return holes. Disregard the reflections. This is my first CLS tank.

 

[pdxmonkeyboy]

Thanks, PDX. By 'gap', do you mean the opening of each weir tooth? If so, the opening is 6.36mm (1/4") and there are 75 teeth. That would make 18.75" of open width. The teeth are 22mm tall, but assuming you need a small gap below the top tooth, I used 20 mm for the height (i.e. .79"). Using those numbers in the calculator, I get 40 GPM (2400 GPH). Does this sound correct? Note, this doesn't include the larger U-shaped hole, as I'm not sure what it is for...and/or if I will keep it.

That seems about right. This is why I like toothless overflows. The teeth effectively cut your weir length in half.