HOB overflow

[atavus]

I've been fighting with my HOB overflow and I've got a question. The basic problem I have is even though it's rated for 600GPH, it wont do more than a couple hundred (currently 215). I've got a 1" drain that's never more than barely covered with water so that shouldn't be an issue however when I crank up the flow, I notice the tank level rises much more than the outside overflow box. So, I think the siphon can't keep up.

Doing some math, to get 600 GPH out of this I need about a 2" difference (1.9").

My question is, for people that have HOB overflows w/tubes if you:
1) turn the return off and let it settle down, measure the water level in the inside overflow box
2) turn the pumps back on and measure the water level again
3) subtract the two

What do you get and what do you think the flow through it is? Also, do you ever have bubbles trapped in the tube?

If my math is right and you have a single 1 1/8" siphon tube then:
100 GPH = .05"
200 GPH = .21"
300 GPH = .48"
400 GPH = .86"
500 GPH = 1.35"
600 GPH = 1.94"
1000 GPH > 5"

 

[Adam Black]

I've been fighting with my HOB overflow and I've got a question. The basic problem I have is even though it's rated for 600GPH, it wont do more than a couple hundred (currently 215). I've got a 1" drain that's never more than barely covered with water so that shouldn't be an issue however when I crank up the flow, I notice the tank level rises much more than the outside overflow box. So, I think the siphon can't keep up.

Doing some math, to get 600 GPH out of this I need about a 2" difference (1.9").

My question is, for people that have HOB overflows w/tubes if you:
1) turn the return off and let it settle down, measure the water level in the inside overflow box
2) turn the pumps back on and measure the water level again
3) subtract the two

What do you get and what do you think the flow through it is? Also, do you ever have bubbles trapped in the tube?

If my math is right and you have a single 1 1/8" siphon tube then:
100 GPH = .05"
200 GPH = .21"
300 GPH = .48"
400 GPH = .86"
500 GPH = 1.35"
600 GPH = 1.94"
1000 GPH > 5"

Get another U tube! Or by mine.
http://www.reeffrontiers.com/forums/showthread.php?t=39366

 

[atavus]

Thats just trading math for fighting with bubbles trapped in the tube I prefer math.

 

[returnofsid]

Ummmm if you have bubble traps in your U tubes, you either aren't getting enough flow through it or too many air bubbles in your tank. I have a U tube overflow that originally had 1 u tube. I had a 600 GPH return pump and everything was fine. However, I wanted a little more flow through my refugium/sump so upgraded to a larger pump. My return pump was putting out about 900 GPH after discounting for 2 feet of rise. My single U tube didn't quite keep up so I added a second u tube and everything's just fine. I do have to remove an air bubble from the utube every now and then but not often at all. There's more to take into consideration besides just diameter of utube when it comes to how much flow your overflow box will handle. Do you have a durso pipe or any type of stand pipe to limit noise? If so, is there a hole in the top of it? This will greatly effect flow. A standard U tube will handle 600 GPH, so somewhere either your math is flawed or there's something else you need to include in your calculation. Also, wouldn't your math need to take into consideration the size and shape of overflow box? You suggest others with overflow boxes follow your experiment, but if their overflow boxes are shaped different, your measurements wouldn't equate to the same amount of water.

 

[Adam Black]

I've been fighting with my HOB overflow and I've got a question. The basic problem I have is even though it's rated for 600GPH, it wont do more than a couple hundred (currently 215). I've got a 1" drain that's never more than barely covered with water so that shouldn't be an issue however when I crank up the flow, I notice the tank level rises much more than the outside overflow box. So, I think the siphon can't keep up.

Doing some math, to get 600 GPH out of this I need about a 2" difference (1.9").

My question is, for people that have HOB overflows w/tubes if you:
1) turn the return off and let it settle down, measure the water level in the inside overflow box
2) turn the pumps back on and measure the water level again
3) subtract the two

What do you get and what do you think the flow through it is? Also, do you ever have bubbles trapped in the tube?

If my math is right and you have a single 1 1/8" siphon tube then:
100 GPH = .05"
200 GPH = .21"
300 GPH = .48"
400 GPH = .86"
500 GPH = 1.35"
600 GPH = 1.94"
1000 GPH > 5"

What return pump do you have?

 

[atavus]

A standard U tube will handle 600 GPH, so somewhere either your math is flawed or there's something else you need to include in your calculation.

Any siphon tube will do any amount of water*, the controlling factor is the difference in water levels. If the water level in the inner box is equal to that of the outer box, you won't have any flow. Since the water level in the outer box is controlled by a weir separating the tube side from the drain side (or by a stand pipe set some height above the drain) it's effectively fixed. But, as you add water to the tank the inner level rises and that causes a siphon.

The velocity of the water is:
v = sqrt(2gh)

v*a gives you the rate of water transferring, h is the difference in water levels and g is the gravitational constant.

Now, if you have 900GPH through 2 tubes, you should have a ~1" drop. Measure the water level in your inner box, turn the pumps off let it sit for a bit then measure it again.

* there's some limits around edge cases where the pressure drops low enough in the fluid to release dissolved gasses.

 

[atavus]

Also, wouldn't your math need to take into consideration the size and shape of overflow box? You suggest others with overflow boxes follow your experiment, but if their overflow boxes are shaped different, your measurements wouldn't equate to the same amount of water.

Sorry, I missed this. In some ways yes, but in others no. There's a lot of factors that control the water flow. The shape controls water flow from the tank into the inner overflow box. You need to have sufficient linear inches to allow the water to flow in, however if you don't have a big enough perimeter, that means the water level in your tank will rise and just cause a larger cross section of water. The tube size, shape and water level difference between the boxes controls the flow into the outer box and the return pipe controls into the sump. The max water flow will be the most restrictive one of these, in my case that's the siphon action.

 

[returnofsid]

This could be due to more than just U tube diameter though. Is your drain line bent at all? Is the end under water and up against glass or skimmer or live rock? These things could restrict flow. If you have a stand pipe, most have very small holes drilled in the top, the size of this hole will influence your syphon as well. The more air,(larger the hole) the less water flow. In fact, that hole is a very good way to make small adjustments to your flow. I drill a hole in the top of my durso stand larger than I need. Then tape if off with scotch tape, using a toothpick, poke a hole in the scotch tape. You can make the hole slightly larger by pushing the toothpick further into the tape. This will effect your flow rate. The added benefit of the durso stand is to limit noise. If you're getting air in your U tube often, you need to figure out why. If you have a 600 GPH return pump, realize that once the water is pumped from your sump to your display tank, you aren't getting an actual 600 GPH. Your u tube can handle 600 GPH and if your tank level is rising then there's something else limiting your drain flow, besides your U tube. I recently had a snail get stuck in my drain hose....not only decreasing drain flow but increasing my display tank level until it flooded...lol. A smaller snail would have probably only decreased flow rate.

One other thing to check....your Utube is longer on one end than the other. Are you sure you have the correct end in the correct side of your overflow box?

 

[atavus]

This could be due to more than just U tube diameter though. Is your drain line bent at all? Is the end under water and up against glass or skimmer or live rock? These things could restrict flow.

The drain line isn't backing up at all, therefore it's not the limit.

If you're getting air in your U tube often, you need to figure out why. If you have a 600 GPH return pump, realize that once the water is pumped from your sump to your display tank, you aren't getting an actual 600 GPH.

The air bubbles are due to insufficient flow. The flow isn't high enough because I don't have a large enough hight difference to siphon water quickly enough.

The pump is rated at 780GPH. It's teed of with a ball valve (should have used a gate valve, these suck to change) and has way more than half the flow going right back into the sump. I can't find the chart, but IIRC 6' head height on this pump pushes >650GPH and I'm only going up 4'.

Your u tube can handle 600 GPH and if your tank level is rising then there's something else limiting your drain flow, besides your U tube.

I realize your trying to help, so don't get me wrong I do appreciate it. However, I've been messing with this for a while. I've checked many things and done a fair bit of reading. Now, Bernoulli was smarter than me so I'm going to trust his equations.

I've checked everything else, and while you're certain I'm wrong, I'm reasonably certain I'm right. I'm looking for people to actually look and tell me I'm wrong, then I'll look for another reason. Or they'll tell me I'm right, but off by X.

If I can get an answer from someone to my original question, that'd be great. If not, I'll just trust my math and cut the weir on the overflow. Maybe I'm right and it works, maybe I waste 30 minutes.

I can give you a lifetime, money back guarantee in writing that you've got a water level change in the inner overflow box in your tank that's proportional to the square of the water flow you're putting through. If you don't believe me, then measure it, please!

 

[atavus]

I never got a reply, so I just went ahead and trimmed the baffle. Unfortunately, between the length of the U-tube, the depth of the inner overflow box and my paranoia* I could only drop the level of water down another inch. Still, it worked out mostly like I had expected. I didn't take the height of the water as it overflows the baffle in the outer box, which raises the water level in the outer box about a 1/4". But, it's allowed me to redirect quite a bit more of the water into the tank. I haven't measured the water level difference, but I'm guessing it's about 450 GPH going through the tank now, which is quite an improvement.

* I wanted at least 1/2" of water over both ends of the U-tube. So when I bump the U-tube, I don't break the siphon.

 

[Haole]

Hey atavus
I’m trying to visualize this without a brand name as to what your overflow looks like. I take it that maybe the inner box isn’t adjustable in order for you to try different water levels within your display tank.

Here are a couple more things to take into consideration. Is the inner box smooth at the top or does it have grated slits where the water from the tank flows through them into the inner box? If it has slits then maybe widening the openings would allow more water to enter the inner box. Also, when determining your amount of flow, don’t forget to factor in the amount of head loss that is produced from each 90 degree bend that you’re using on your return line. Each 90 degree fitting used creates 1 ft. of head.

 

[atavus]

Ok, the full story:

The overflow is a no-name bought at Reef Mystique in Lake Stevens.

The inner box is about 4" tall, with slits at the top going about 2" down the side. The box is adjustable but as it came it would be basically impossible to have the water level internal box below that of the tank. I'll explain this in a minute.

The outer box is much taller, about 8". The outer box has two sections, one to hold water and a second that holds a 1" bulkhead (which I've measured and compared to 1" PVC I have). There is a weir in the middle separating the two. The drain pipe was a simple solid section of PVC, no holes.

First, the water level was set by the middle weir. This is set about an inch (just a guess) below the top of the glass of the aquarium. Since the water coming from the siphon can't drop below this level, the tank was always filled no more than 1" from the surface.

This means if the water level in the tank raises 1", it spills onto the floor. That's 2.8G in my tank. The return section of my sump is about 5G, so I need a gap of close to 2" to prevent overflows in the case of a lost siphon or stuck drain.

So, first I cut off another inch from weir, but this made the stand pipe taller than the weir, so I cut it down as well. This lowered the water level in the tank sufficiently. But I couldn't get any reasonable amount of flow through the overflow box. I checked that the pipe would handle it by pumping lots of water directly into it, it did just fine. However, that connected to the tank sucked the sump dry and got dangerously close to overflowing the tank by the time I pulled the plug.

Since the drain was doing it fine and the water level in the inner box matched that of the tank (implying the inner box was getting plenty of water), I assumed it was the siphon and did some reading. The velocity of water exiting a siphon is v=sqrt(2*g*h). g is 9.8m/s/s and h is the difference in water levels. The flow rate is the cross section of the tube multiplied by the velocity (A*V), so I needed h=1.9"

That's when I made this thread, but didn't get a reply and went ahead and cut the weir and drilled holes in the stand pipe. This lowered the water level another 1". With the pump off, the U-tube sticks about 1/2-3/4" below the water line in the two boxes and thats a good 1.5" below the water line in the tank.

When I turn the pump on, the water level in the inner box raises to slightly below the slits and the water level in the tank rises 1/8"-1/4" above the bottom of the slits to get enough water flow into the inner box. If I wanted to, I could cut these wider and reduce that last bit, but I don't mind it.

Any air bubble in the u-tube get blown out easily (so far at least), and I've got at least 3x the water flow. Still not a full 600GPH but that's due to me not wanting to here it flush and not having the room in the drain section of the outer box for a durso.

The pump is a Rainbow Lifegard Quiet One 3000. Some flex PVC connects the pump to a T. One side connects to a ball valve (really should have done a gate valve here, but Lowes didn't have non-bronze ones and I didn't realize what a pain it is) that feeds back into the sump. Up goes to 2 90-degree angles into the tank. That's about 3' up, so say 5' of head which puts this pump at somewhere around 500GPH. But, this isn't particularly relevant since my issue was too much flow, not a shortage of flow.

The return line is a 3' flex tube going straight down. Okay, diagonally down.

Perhaps if I was more willing to overflow my tank and get the water line closer to the surface, the overflow would work. But I'm not and it didn't. But, with some changes it does and I'm happy now.