gph overflow calculations

[mickeysreef <*))))<]

ive been looking to buy an overflow for my tank, but i havent gotten past the gph calculation the sellers are using. so i wont mention any names here but i have emailed a couple of manufacturers about the gph calculation without any response whatsoever to my question. what seems to be a constant in their advertising is that the wider the wier, the more gph they claim.

i'm a little confused because i always thought that drain gph was calculated by size of the drain pipe(s). so imagine you have a 36" wier going across your tank but only a 3/4" drain pipe. i think the drain pipe will limit you where the wier may not.

i have a 75 gallon reef ready tank with a corner wier. now that corner wier isnt that long but i have a tremendous amount of flow from my sump really only limited by the size of my drain pipe(s).

do i really need a 36 or 48 inch wier if my drain pipe(s) are 1.5 or whatever - anyway thres two holes drilled through the tank into the external box which remain constant no matter the size of the actual wier. for me the wier is more of a water on the floor overflow function than a gph calculation.

i could be wrong but it doesnt make sense to me,

 

[JoshH]

Realistically they both play a part in the GPH rating of an overflow, Yes if you have a super long weir and a 3/4" drain you're limited by the drain size. But if you have a 12" weir and a 2" drain line, well a 12" weir won't be able to handle the flow at max drain capacity and as you mention, you'll end up with water on the floor. The GPH rating of these overflows take both into effect and give you a number that the overflow should operate silently at and a number that is the absolute max the overflow (both drain and weir) can handle.

Not all overflows have dual bulkheads and the bulkhead sizes between the outer and inner box will vary depending on the size of the overflow as well

 

[mickeysreef <*))))<]

yeah i think you are right re: 2 inch drains and 6 inch wier, there will probably be some gurgle and splashing but i think there is more middle ground than is being marketed.

and im in a bit of disbelief over the numbers. i believe that flow can be a lot higher than is being let on. without dragging brands into it. i'd guess that drain style and drain size has more to do than wier size. but i suspect wier size in some of the kits is exaggerated. and i dont know why, i believe people would rather have a smaller wier. particularly when you only have one or two holes leading to the drain.

 

[JoshH]

What makes you feel that flow could be a lot higher? Honestly I feel a lot of the overflows are wildly exaggerated flow wise and most can't even do what they say they can.

Larger weirs are actually preferred, not because of flow atall but because of the idea that longer weirs promote more surface skimming of the DT. This helps pull unwanted things into the overflow to be removed via the sump.

 

[Seanb1]

Everything depends on your drain pipe diameter and what type drain you have, does it have a gate valve ?
I seen a graph on here somewhere that had flow rates for pipes.
Try a search and go from there.

 

[Seanb1]

Im kinda in the consensus that less turnover is better relying on power heads for flow rate.
More time for skimmers and ats to be in contact with water before its shot back into the display.

 

[mickeysreef <*))))<]

What makes you feel that flow could be a lot higher? Honestly I feel a lot of the overflows are wildly exaggerated flow wise and most can't even do what they say they can.

Larger weirs are actually preferred, not because of flow atall but because of the idea that longer weirs promote more surface skimming of the DT. This helps pull unwanted things into the overflow to be removed via the sump.

ok so in my 75, i have the sump return high enough so that there is gurgling in my sump. so it is a corner wier and you can visibly see the skim of the water traveling from all corners of the tank to the drain. if i feed without rinsing the food and turning off the return, i can see the oils travel right across the surface into the drain. so its only a corner wier, maybe 8 inches or so. thats why i really believe the wier size is inconsequential (not to absurdity) and gph is really measured by drain style and pipe size.

and i believe that is why my questions to the 2 or 3 manufacturers i sent were not answered.

Everything depends on your drain pipe diameter and what type drain you have, does it have a gate valve ?
I seen a graph on here somewhere that had flow rates for pipes.
Try a search and go from there.

im with you on that.

Im kinda in the consensus that less turnover is better relying on power heads for flow rate.
More time for skimmers and ats to be in contact with water before its shot back into the display.

i used to believe that, i do for refugiums, but not so much for skimmers anymore. i could be wrong though.

 

[theMeat]

You’re onto it. Most times the size of the pipe/pipes is the limiting factor, but depending on gph weir can be a factor as well

 

[NanoDJS]

all that matters is your flow from the pump has to be lower than the rating under gravity of your drain pipe size , some popular examples are 1" pvc draining under gravity pressure will flow max of 960 gph without any 90's going vertically. each 90 would add 2' of restriction , this is why so many overflows use 1" as long as your flow is below 960 gph from your supply nozzles your will have correct flow , a 3/4 pvc line under pressure from a pump (20-100) psi can flow upto 2200 gph alot of people buy a crazy return pump and mess up this ratio brutally. or have dual supply and single return (weir and overflow) which are not sized correctly, this applies to all styles of overflow especially the ones that use a siphon effect the weir size is irrelavant if the drain pipe is sized correctly to the supply, this is how they used to do it no weir just a basket strainer on the drain pipe.

 

[theMeat]

Have seen a weir being the limiting factor many times.
Why do you think it can’t be, magic?

 

[NanoDJS]

Have seen a weir being the limiting factor many times.
Why do you think it can’t be, magic?

please elaborate , I can take an 800 overflow box and add another drain and increase its flow to 1600gph with out making any changes to the weir , so I am curious as to what you mean. if it has even 1" of open grate it will flow 800 gph , 2" of openings and it will flow 2200 gph , the flow is dictated by the supply rate, its basically the rate which it overflows the tank, as long as your drain capacity is above your supply rate your golden.

 

[theMeat]

Depends on the size of the opening, and how many there are. Weir can be a limiting factor, it’s that simple.
On my 220 I have a 16” reef dynamics ghost overflow. It’s rated for like 1600gph if my memory serves me. Yet if I ask it to do more then about 800 the water level in tank rises, while the emergency drain remains unused. This is because of the weir, not the size of the plumbing. Because the water level in boxes hardly changes, and doesn’t get hi enough for emergency drain to come into play. The water is simply damming up in tank by the weir

 

[NanoDJS]

Its rated for 2500 gph and has 2 1.5" and 1 1.5" emergency drain....but You cant even get half that rating out of it you claim..... it has 2 3/4" bulkheads to connect thought the tank right.( around 1200 gph max on horizontal) That is the bottle neck it could only get around 800 through the tank to the overflow,then on the collection side each U bend is like 9' of restriction that is a horribly engineered box and the company is touting numbers that cannot ever be, due to science ... you cannot reduce the size of piping on drainage at one point then increase it without massive changes to flow characteristics the smaller pipe used will dictate flow rate .

 

[NanoDJS]

If they had 2 1.5 bulkheads through the tank , it would do what they claim, not sure what the sadface is for . This is simple stuff.

 

[JoshH]

The sad face is because theres no face palm emoji and I'm not in an argumentative mood

@theMeat mind gluing shut your weir except for 1" worth of teeth and see what happens? I'll buy you some extra towels I promise

 

[theMeat]

Its rated for 2500 gph and has 2 1.5" and 1 1.5" emergency drain....but You cant even get half that rating out of it you claim..... it has 2 3/4" bulkheads to connect thought the tank right.( around 1200 gph max on horizontal) That is the bottle neck it could only get around 800 through the tank to the overflow,then on the collection side each U bend is like 9' of restriction that is a horribly engineered box and the company is touting numbers that cannot ever be, due to science ... you cannot reduce the size of piping on drainage at one point then increase it without massive changes to flow characteristics the smaller pipe used will dictate flow rate .

I have an earlier version that has 3 one inch pipes coming down to sump from outside box.
converted a friends 200dd, that had 2 corner overflows, into a herbie style. Had the same weir limiting issue on the stock corner overflow. Had to modify the weir.
Not sure why you’re arguing. You should be happy you now have a better understanding

 

[NanoDJS]

I am not arguing anything , simply stating the science of fluid dynamics that will never change. If they dont design the overflow correctly then yes it will be the limiting factor. One would hope that people selling them have a basic apprentice level understanding of plumbing, apparently this is not the case. your bulkhead holes you cut were 35mm correct ? to attach the tank side weir to the drain side collector. this means they are 3/4 " . My understanding of these things is how I make my money , I am a Mechanical Contractor. I also have been reefing for 30 years and run 5 systems as we speak. and never had any overflow issues , unlike you for example.

 

[theMeat]

I am not arguing anything , simply stating the science of fluid dynamics that will never change. If they dont design the overflow correctly then yes it will be the limiting factor. One would hope that people selling them have a basic apprentice level understanding of plumbing, apparently this is not the case. your bulkhead holes you cut were 35mm correct ? to attach the tank side weir to the drain side collector. this means they are 3/4 " . My understanding of these things is how I make my money , I am a Mechanical Contractor. I also have been reefing for 30 years and run 5 systems as we speak. and never had any overflow issues , unlike you for example.

Do not wish to go back and forth and in the process hi jack op’s thread. I speak from experience. If you don’t want to believe me that’s cool. At least google it, maybe you will believe others. In the meantime I’d hope you’d not want to hand out false info to fellow members

 

[ca1ore]

To me, it’s something of a progression through the variables. If the drains inside the external box are limited to 1,000 gph, doesn’t matter how large the through bulkheads are or how long the weir is .... you’re stuck with 1,000 max. If your external box drains can handle 3,000 gph, then the next thing to look at is whether the through bulkheads can move that volume of water. If the answer is yes, then the final thing to look at is whether the weir is practically big enough to move that volume without making a racket or driving the water level in the tank too high. Even a 6” weir can move 3,000 gallons, it just won’t do it quietly or without building sizable head above the weir edges.

My current overflow is a 36” modular marine model ‘rated’ at 3,600 gph. Although I don’t run it anywhere close to that volume, I did test it when first received and satisfied myself that it could run that volume. But, drain down on power loss is much higher at the maximum volume and it was noisier.

 

[theMeat]

There’s no way a 6” weir is moving 3000gph. According to this a 20” weir, at 2400 gph , will raise water level by 3/4 “

http://irrigation.wsu.edu/Content/Calculators/Water-Measurements/Rectangular-Contracted-Weir.php