Overflow capacity calcs

[neilp2006]

Hi all

Putting together my 125, and 40b sump, and was doing some calculations regarding overflow capacity in case of return pump power loss and backsiphoning etc.

I understand the rational of drilling a hole in the return plumbing, about a half inch below the water line to prevent excessive backsiphoning into the sump. However, in running the numbers, and thinking through the location of the hole, isn't it the depth of the overflow teeth (or smooth weir, if so inclined) that ultimately sets the amount of water that drains back to the sump? On my 125, if I have 3/4 inch water level above bottom of teeth, that's 4.2 gallons heading back into the sump via the overflow box in the 'forward direction'

If I put my syphon break hole 1/2 inch below the surface - that's 2.8 gallons backsiphoning through the pump. But the other 1.4 still goes into the overflow box.

If your hole is above this level, sure, once air gets in, it'll break siphon going backwards through the pump, but water (1.4 gallons in this case, for a total of 4.2) is still going to go into the overflow box and into the sump in the 'forward direction'.

Doesn't this mean then, if your returning outlet is above the lower level of your weir, the hole is pointless? Since the outlet itself will suck air before the weir finishes draining water into the overflow box?

The hole really only comes into play if your outlet is below the weir/teeth, and even then, only really is useful if it is below the level of the weir, since any water above that is heading in the normal direction anyway (over the weir and into overflow box).

It seems to me that, Essentially, I should calculate overflow capacity as a function of weir depth and tank dimensions, and as long as my return outlet is above my weir/ teeth, the hole and backsiphoning volume are irrelevant.

Or am I going wrong somewhere?

Thanks fir any insight!


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[Meshmez]

I think what you are saying is true, however, generally speaking, peoples return will be below the teeth on the overflow.

0.75" of water above the teeth of your weir, sounds high to me. Even if that is accurate, is your return going to be higher than that?

 

[neilp2006]

To be clear, I'm talking about the amount of reserve space to keep in my sump to contain the water that settles in a pump off powerloss situation.


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[neilp2006]

I think what you are saying is true, however, generally speaking, peoples return will be below the teeth on the overflow.

0.75" of water above the teeth of your weir, sounds high to me. Even if that is accurate, is your return going to be higher than that?

That's a good point. In probably most situations, the return WILL be below the weir- since it'll need submerged more than 0.5-0.75 inches just not to introduce bubbles.

That's what I was missing! Thanks


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[Meshmez]

That's a good point. In probably most situations, the return WILL be below the weir- since it'll need submerged more than 0.5-0.75 inches just not to introduce bubbles.

That's what I was missing! Thanks


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:thumbsup:no problem

that being said, I would calculate and account for the full return siphon amount, without the hole. holes tend to clog. im not saying not to put the hole in, but dont rely on that to not flood your sump.

 

[neilp2006]

I think what you are saying is true, however, generally speaking, peoples return will be below the teeth on the overflow.

0.75" of water above the teeth of your weir, sounds high to me. Even if that is accurate, is your return going to be higher than that?

0.75 " above the bottom of the teeth. My teeth are an inch deep so about 3/4 of the way up the teeth. Not 0.75 above the top of the teeth

Thanks


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[Meshmez]

0.75 " above the bottom of the teeth. My teeth are an inch deep so about 3/4 of the way up the teeth. Not 0.75 above the top of the teeth

Thanks


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ya i understand, still seems high, but then again to be honest i havent paid much attention to mine. i picture it being more like 0.25"-0.50" at most, but again i havent paid much attention.

 

[neilp2006]

:thumbsup:no problem

that being said, I would calculate and account for the full return siphon amount, without the hole. holes tend to clog. im not saying not to put the hole in, but dont rely on that to not flood your sump.

Sure, sounds prudent. If I put it 2inches deep, that'll be approx 11.5 gallons of reserve capacity needed. Ouch. I have it designed for 13 as reserve.

To ease the math, since my sump is the same width, and half the length, I just double the inches of water for the same volume. So I'll need 4" of reserve height in my sump to account fir the 2 inch depth of the return outlet. As designed, I have that covered.


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