Here is a write up I did a little while back on a bean animal. I use the same overflow so there should be 3 holes in the back of the outer box, allowing for this type of plumbing.
In simple terms, 3 pipes.
1. First pipe, main syphon. Its a pipe with an elbow pointed down. Its the lowest of the 3 pipes, not by a whole lot though. There needs to be a valve in this line (most use a gate valve for this to be able to fine tune)
2. Second pipe, gravity feed pipe. It too has an elbow pointed down. This elbow has a hole drilled in the top and a piece of hose in the hole that comes out of the top, loops back around and points back down into the tank. The end of that hose is fastened so it is a little above the normal water level in the tank but below the emergency drain pipe.
3. Emergency drain, pipe facing up, well below the tank overflowing point.
How it works:
Under normal operating conditions the main syphon handles most of the water draining. There should be no bubbles coming out of this pipe. It should be a full syphon ( think of a water change hose. You put one end of a hose in the tank, suck on the other end, water flows with no air, the second air is intoduced water stops syphon is broke. The second pipe is just gravity feed. The water level rises above this pipe and just falls down it there is air in this pipe. The third pipe does nothing at this point. Just sits there dry.
Ok so your return pump goes off for some reason. Water level in the tank drops down to where all 3 pipes are dry and your sump fills up ( hopefully you accounted for this and it doesn't overflow). When you turn your pump back on, water flows and starts filling the Dt. Water rises and gets to the main syphon, but at this point its not a syphon yet, its just gravity fed because there is air in the line and its sucking air until the water rises more. It starts gurgling and makes a racket, the water keeps rising until it reaches the gravity feed pipe. Now this pipe starts gurgling and making a racket. Meanwhile the syphon pipe is still clearing air. The water continues to rise until it covers the tube that you have coming out of the top of the gravity drain. Now what you have is 2 pipes trying to become full syphons. About this time your full syphon has cleared all the air and is truly a full syphon, with no bubbles coming out of the end and is pulling a lot of water. The water level in the tank actually starts dropping because you basically have 2 syphons going. It drops until the hose from the gravity drain gets above the water level and air is introduced into the second pipe thus creating the gravity drain again and everything levels out. Now if not enough air (or too much)is introduced into the gravity feed pipe you will hear gurgling. This is where the valve in the syphon line comes in. You open or close this valve until the noise goes relatively silent. Also having a valve after the return pump is handy to adjust the flow going back into the tank.
The emergency drain is only there in case 1 of the other pipes gets clogged. If the full syphon gets clogged, water level rises, plugging up the airline hose , turning the gravity feed into a full syphon and the emergency drain becomes the gravity feed.
The ends of the pipes in the sump are pretty critical. You want them to be below the water level of the normal sump level to eliminate splashing noise, but not so deep that they cant clear the air out to reach full syphon. The deeper they go the longer it takes to get to full syphon and sometimes it cant do it so it remains noisy. Most people keep the emergency drain pipe well above the sump water level so when it is flowing it is loud and it alerts you that there is a problem. Hope this helps
