weir calculation

[mickeysreef <*))))<]

so im back at it again with my weir calculations. i found 2 calculators online to calculate the volume of water that can pass through a weir. the first calculator was simplistic and easy to use. gave me a calculation of 1gpm for 1" x 1/4" weir. the second calculator was a bit more complicated and im pretty sure i didnt use it right, it gave me 25 gpm for the same weir.

the first one does seem more likely out of the two, however it is possible that they are both wrong. any engineers or knowledgable folk out there with water flow experience?

 

[Timfish]

I think the first time I had an aquarium with an overflow was back in '91. Can't say I've ever tried or worried about the actual flow rate for a weir of a given size though. But if I was to guess, the 25 GPM seems fine. Some of the pertinent variables would be acceptable water depth over the weir, does the weir have teeth (comb) and if so what are the dimensions and spacing of the teeth?

 

[mickeysreef <*))))<]

sorry i am not being clear. my question is how much flow can i get out of a 1" x 1/4" hole
would 4 1"x1/4" holes be equal to 1" pvc pipe?

for instance, i read that a 1" pvc pipe at gravity can get 960-1100gph. im trying to match my drains with the weir.

 

[Timfish]

Well, it's been probably a decade since I've bothered to figure out the cross section of a 1" pipe. Besides compareing cross sections you'll need to figure the resistance of gravity pulling water through 4 holes vs a pipe. Keep in mind you'll have algae and biofilms build up on the openings and with one dimension only 1/4" it will be easy to clog. With a typical weir on an overflow if the teeth or comb gets clogged water will run over the overflow without overflowing the tank so take that into consideration in your design.

 

[ca1ore]

sorry i am not being clear. my question is how much flow can i get out of a 1" x 1/4" hole
would 4 1"x1/4" holes be equal to 1" pvc pipe?

for instance, i read that a 1" pvc pipe at gravity can get 960-1100gph. im trying to match my drains with the weir.

Just brain-balling it, 4 @ 1/4”*1” slots probably have more area than a 1” PVC pipe, though that will depend somewhat on the radius of the slot corners. Keep in mind that slots correlate better with an open channel drain. 1,000 gph through a 1” pipe seems high to me and only achievable on a siphon. I would think 60 gph per slot makes more sense than 1,500 gph per slot LOL. I have always sized weirs based on 25-30 gph per slot.

The other thing to keep in mind is that to effect good surface skimming the water level in the tank should be below the top edge of the weir slots, which will significantly reduce flow per slot.

 

[reddogf5]

Flow over a weir is completely different than flow through a pipe.
Flow over a weir is determined by the height of water over it and the area of the opening. Or conversely, the height of water over the weir is determined by its area and how much flow you are shoving though it.
Flow through a pipe is determined by the head driving it and the losses along the way. There is a lot of existing data to calculate this.
You can't determine how much flow you will get through a pipe just by knowing its area, you need to know all of the loses along the way, and the head driving it.