Can anyone help me calculate the GPH of a specific length of overflow. I am in the process of designing my new tank and need to know how I can calculate the water flow over this "fence". The reason I ask is that I plan on having a rather large overflow and having two intakes for recirculating into the main tank come out of the overflow. I just need a formula that will determine the max water flow because I know the length of the overflow. This will aid me in determining the size of the recirculating pumps. I know that typically the slots are 1/4" wide and the the space between slots is 1/4" wide. Also, I guess it depends on the height of the slots. Just need a physics whiz to help me! Thanks Russ
overflow flow calculation
- Thread starter Russ1
- Start date
I think your thinking too much
Your water will only overflow as fast as you pump the water into the tank.
If you use a 300 GPH pump then your overflow will be 300GPH. I run an 800 GPH pump with an overflow box (or fence) of about 6 Inches wide, with no hint of trouble
Your water will only overflow as fast as you pump the water into the tank.
If you use a 300 GPH pump then your overflow will be 300GPH. I run an 800 GPH pump with an overflow box (or fence) of about 6 Inches wide, with no hint of trouble
I think he wants to know so he builds it large enough for a specific flow rate he already has in mind. It would suck if he wanted 3000 gph and he only built one two foot long overflow. It would overflow into the floor.
Also it will help you guesstimate where your water level will end up.
On my Acrylic tank with 1.25 inch thick back panel the slots are .5 inches wide and 1.5 inches tall. For every one foot of run it flows ~1000 gph max. I would plan on running it at 1/2 to 3/4 capicity for buffer and to keep your water off of your tank top supports.
The key is area of the slot openings but you will need to figure this out. Larger slots = better flow but things like fish will be able to pass easier. I have 4 feet of overflow slots and I run around 3000 to 3500 Gph over them. If I run both ampmasters wide open it can handle the flow but the water sits on the upper supports.
Hope that helps. Sorry I could not give you a formula but my dimentions can give you a place to start. Your tank manufacturer may be able to also give you some insight into the design.
Jeff
Also it will help you guesstimate where your water level will end up.
On my Acrylic tank with 1.25 inch thick back panel the slots are .5 inches wide and 1.5 inches tall. For every one foot of run it flows ~1000 gph max. I would plan on running it at 1/2 to 3/4 capicity for buffer and to keep your water off of your tank top supports.
The key is area of the slot openings but you will need to figure this out. Larger slots = better flow but things like fish will be able to pass easier. I have 4 feet of overflow slots and I run around 3000 to 3500 Gph over them. If I run both ampmasters wide open it can handle the flow but the water sits on the upper supports.
Hope that helps. Sorry I could not give you a formula but my dimentions can give you a place to start. Your tank manufacturer may be able to also give you some insight into the design.
Jeff
Jeff,
You're exactly right! I want to build this thing to handle large volumes of water flow and possibly pull water for recirculating out of the overflow. To do this, the fence needs to have a huge capacity. If 1 foot is ~1000 gph with a .5 slot, then a .25 slot would be approximatley 1/2 of that. I am considering an eight foot tank with an overflow across the entire back of tank. That would be 4000 gph with .25 slots or 8000 gph with .5 slots. What do you think? Thanks for the input. Russ
You're exactly right! I want to build this thing to handle large volumes of water flow and possibly pull water for recirculating out of the overflow. To do this, the fence needs to have a huge capacity. If 1 foot is ~1000 gph with a .5 slot, then a .25 slot would be approximatley 1/2 of that. I am considering an eight foot tank with an overflow across the entire back of tank. That would be 4000 gph with .25 slots or 8000 gph with .5 slots. What do you think? Thanks for the input. Russ
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The height of the slots matters too. If you need more, you can drill holes further down to increase the flow. The other limiting factor will be the size of the bulkheads (holes drilled in the bottom of the tank) and any elbows that you use to get from the overflow to the sump. Durso mods decreases the flow some also, IME.
Ernie
The height of the slots matters too. If you need more, you can drill holes further down to increase the flow. The other limiting factor will be the size of the bulkheads (holes drilled in the bottom of the tank) and any elbows that you use to get from the overflow to the sump. Durso mods decreases the flow some also, IME.
Ernie
In my experience though, with my overflows, the rate limeting step is the overflow fence. My 4 1.5 inch bulkheads in the bottom of my overflows are more than enough to handle the max flow my overflow can deliver to them.
In other words, my fence gives out first. Yes if you can put the overflow all the way across then you should have around 8000 roughly at max capacity. Remember that is using the dimentions of my slots. yours may be slightly different. Also keep in mind you don't want to run right up to capacity. You will have no reserve in case of some of the overflow fence getting pluged with a dead fish or algae.
Also in reality, you probably will not be able to use all of your 96 inch length. You may be a few inches short.
As esmithiii said. If you need more flow capicity then make your slits taller. My slits are 1.5 inches tall. You could go with 2 inch tall ones and that would give you more area to work with. You could also put them closer together and have more slits per foot and that would work also. Just don't compromise the strength of your tank.
Remember what ever you choose will ultimatly affect your water height once running and the amount of water that falls into your sump when the power is off.
Jeff
In other words, my fence gives out first. Yes if you can put the overflow all the way across then you should have around 8000 roughly at max capacity. Remember that is using the dimentions of my slots. yours may be slightly different. Also keep in mind you don't want to run right up to capacity. You will have no reserve in case of some of the overflow fence getting pluged with a dead fish or algae.
Also in reality, you probably will not be able to use all of your 96 inch length. You may be a few inches short.
As esmithiii said. If you need more flow capicity then make your slits taller. My slits are 1.5 inches tall. You could go with 2 inch tall ones and that would give you more area to work with. You could also put them closer together and have more slits per foot and that would work also. Just don't compromise the strength of your tank.
Remember what ever you choose will ultimatly affect your water height once running and the amount of water that falls into your sump when the power is off.
Jeff
Gravity will pull objects downward at about 3 feet per second. but what size outlet is in the bottom of our sump lets say 1 inch. a 1 inch cube of water will fall 3 feet in a second so that would be approxiamatley 36 cubic inches of water
a Cubic inch of water is about .00433 gallons so 36 x .00433 would be about .156 Gallons per second falling through a 1 inch hole or about 9 gallons a minute or 561 Gallons per hour.
So you can see that overflow does not really have to be that big to support the flow you are talking about. If you are going to be running your overflow the entire length of the tank. I have calculated a flow rate of over 50 thousand gallons per hour if the height of your overflow holes or your fence height were 1 inch high.
Now that is just the total capacity of water that can overflow based on gravity before the tank would start to fill over the top of your overflow and possibly spill onto the floor.
But you have to also have a draw or outlet of equal ratio to your expected GPH
Using a little math. If I wanted to pull 1100 Gallons per hour I would need a 2 inch hole in the bottom of my sump ( 2 Cubic inches x .00433 gallons ) x 36 inches of fall rate of gravity then times that by 3600 seconds in an hour I would get 1122 gallons per hour will flow through the 2 inch hole using gravity alone. Of course the material of pipe and any obstructions would cause some restrictions but I dont think in this case you will be pumping any more water than that.
I hope this helps
a Cubic inch of water is about .00433 gallons so 36 x .00433 would be about .156 Gallons per second falling through a 1 inch hole or about 9 gallons a minute or 561 Gallons per hour.
So you can see that overflow does not really have to be that big to support the flow you are talking about. If you are going to be running your overflow the entire length of the tank. I have calculated a flow rate of over 50 thousand gallons per hour if the height of your overflow holes or your fence height were 1 inch high.
Now that is just the total capacity of water that can overflow based on gravity before the tank would start to fill over the top of your overflow and possibly spill onto the floor.
But you have to also have a draw or outlet of equal ratio to your expected GPH
Using a little math. If I wanted to pull 1100 Gallons per hour I would need a 2 inch hole in the bottom of my sump ( 2 Cubic inches x .00433 gallons ) x 36 inches of fall rate of gravity then times that by 3600 seconds in an hour I would get 1122 gallons per hour will flow through the 2 inch hole using gravity alone. Of course the material of pipe and any obstructions would cause some restrictions but I dont think in this case you will be pumping any more water than that.
I hope this helps
Hi Guys, good post.
Maybe one of you can answer my question.
Tank 72x20x24 150g
overflow box on short 20" end 12x5
I want to flow 1500gph
I want it quiet with Durso's standpipe
Q1.Is 1.5" drain with Durso's standpipe enough to handle this flow rate or do I need to go to 2" drain
Q2.The fence slots .25x1.5" every .25 or .50x1.5' every .50 or what
Maybe one of you can answer my question.
Tank 72x20x24 150g
overflow box on short 20" end 12x5
I want to flow 1500gph
I want it quiet with Durso's standpipe
Q1.Is 1.5" drain with Durso's standpipe enough to handle this flow rate or do I need to go to 2" drain
Q2.The fence slots .25x1.5" every .25 or .50x1.5' every .50 or what
You can use the following equation to determine flow over a sharp weir
Q = (2/3)Cb(sq.rt 2g)H^3/2
Q = flow (cfs)
C = constant (use 0.62)
b = length of weir (ft)
H = height of water flowing over the weir
g = 32.2 ft/sec^2
so say you had a 2 foot long weir and the water flowing over it was 1/2" high, the approximate flow rate would be:
= (2/3)x(0.62)(2)(sq.rt.2x32.2)((.5/12)^3/2) = 0.056 cfs
0.056 cfs x 3600sec/hr x 7.48 gal/c.f. = 1518 gph
keep in mind that this is approximation. The "C" factor is a correction for non-uniform velocity distribution. It varies based on site specific conditions, but this equation should give you an idea of how much length of weir you would need.
OK, I'm not a geek, I just play one on tv.
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Q = (2/3)Cb(sq.rt 2g)H^3/2
Q = flow (cfs)
C = constant (use 0.62)
b = length of weir (ft)
H = height of water flowing over the weir
g = 32.2 ft/sec^2
so say you had a 2 foot long weir and the water flowing over it was 1/2" high, the approximate flow rate would be:
= (2/3)x(0.62)(2)(sq.rt.2x32.2)((.5/12)^3/2) = 0.056 cfs
0.056 cfs x 3600sec/hr x 7.48 gal/c.f. = 1518 gph
keep in mind that this is approximation. The "C" factor is a correction for non-uniform velocity distribution. It varies based on site specific conditions, but this equation should give you an idea of how much length of weir you would need.
OK, I'm not a geek, I just play one on tv.
_________________
Futurama Forum
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