Bulkhead ?

[i<3fish]

In overflow boxes, the water stops flowing when the pump shuts off. Since the main tanks water level lowers. Well if you have a builkhead in the middle of the back of your tank, how will the water stop flowing in a power outage?
Since the bulkhead is under so much water, it would never really stop flowing if your pump shut off. So it would flood your sump. Unlike an overflow box.

 

[scsinet]

You never want a bulkhead in the center or bottom for this very reason. Normally, you have them at the top and drill siphon breaks in the plumbing to introduce air in a backflow situation.
If this is a return (inflow), you'll need to use a check valve. Buy a good one... because I can assure you through a very bad experience that cheap ones can and do fail, plus they put a lot of back pressure on your pump.
If it's a drain, you can install a "T" fitting on the back of the bulkhead outside the tank. Run one pipe down to the sump normally, and run the other end up to about 12" above the water level in the tank and leave the top open as a "vent." That will stop a siphon as well. It's the same thing they do in houses to keep the sewers draining well.

 

[i<3fish]

Can you just have a bulkhead be your inflow also? Just a big hole, or can you add a nozzle or something like that to the inside of the bulkhead? Also, I would be using it for downflow and upflow probably (unless it is better to not have a bulkhead as upflow). What size bulkhead would you reccomend for a 180g tank? Also, I have seen people put the standpipe right where the water comes out. So how would this be?

 

[turningtim]

How are planning on getting the water out of the tank? Do you have a Reef ready, HOB OFs or are building OFs? JMO but the returns should be high in the corners of the tank. No need for a check valve or anti-siphon if you calc enough room in the sump for backwash. OF should be in the center of the tank then divided to go to refuge skimmer and both meet in the middle for return. You don't want to just drain water from the tank through pipes. You want to surface skim the water from the top of the tank. This is what OF boxes are for.
HTH
Tim

 

[i<3fish]

I didnt know you needed to surface skim it. If you do, then I am just going to buy an oferflow instead of drill my tank.

 

[turningtim]

I would say that you do need to skim the water. Thats where the proteins/waste are that need to be removed by the skimmer. Also if you place the drains down low you will depend on a check valve to stop the water flowing in to the sump. Its a gamble!
You can still drill the tank! Just build a OF box in the tank and place to 2-1" BHs out the back. There are modified durso standpipes that can be used to make it quiet.
JMO
Tim

 

[iambailey]

If your tank isnt drilled (and you dont want to drill) You need a HOB overflow.... also, even if you drill your tank, you are essentialy installing the overflow from the outside to the inside.. so in a power outage, and water level for the overflow once it gets low enough will not flow down into the sump anymore

 

[i<3fish]

Yeah, I think I am just going to get a HOB overflow. Then have my inlet hang over the back. Then I can have a nozzle. Its just alot simpler that way.

 

[scsinet]

Sorry, I wasn't thinking straight. My idea would never work because you aren't dealing with a siphon, you are dealing with what you said, the weight of the water simply pushing down. Since it doesn't sound like you have put that center bulkhead in yet, don't.
If you are willing to drill your tank, a built in overflow is far simpler than a HOB overflow. They are quieter and more efficient, and don't carry any risk of failure.
I assumed you had already drilled your tank in the center. If you have not, then do what Tom said, locate your drains at the top, because you absolutely need to skim your surface. If you don't, you need to put a powerhead or something up there to break up the surface of the water because crud in the water will accumumulate at the surface in an icky skin that will inhibit gas exchange.
If you want to really do this cheap, you can drill the bottom of your tank and run an internal standpipe up from your bulkhead to the level of your water. The water in the tank will drain out until it reaches the top edge of the standpipe, and you'll have a poor man's overflow, although there could be a risk of a fish getting caught in it, so you'll want to use a strainer or something.
And yes, you can have a nozzle on your inflow bulkheads. Look for a product called "Loc Line" that most online equipment retailers carry. If you use a bulkhead that is threaded on the tank side, you can get the right fittings to screw the Loc Line directly into it.
Your diagram shows that your INFLOWS (returns, from the pump, not TO) are in the middle. This could work with a check valve, but check valves aren't reliable. Instead, your inflows should either be "U" tubes that are ready made or loc-line as mentioned above, but the nozzles will definitely extend several inches below the water line, so unless you have a really big sump to handle a bunch of excess water, you'll need siphon breaks.