Aaron,
Have you ever placed a bucket of water say on a counter or bench, and siphoned the water out of it with a hose or tubing ?
You know, same way some of us siphon out the water from our tanks when we do a water change.
Stick the hose into the tank, suck on it to pull the air out, the water starts to fill the hose, then place the end you were sucking the air out into bucket, and the water starts flowing from the tank to the bucket. That is known as a siphon.
Water moving from a higher place to a lower place, through a hose or tube.
It's possible because the atmospheric pressure is pusing down on the surface of the tank water. In a retangular tank, the water is contained on five sides by the glass ( bottom and four sides ). Gravity keeps the water in the tank, but this atmospheric pressure is pushing down on this water as well. Atmospheric pressure is like 14.7 pounds per square inch, at sea level.
Anyway, when you place a siphon hose in the tank and evacuate the air in the hose, you'd think that you are sucking the water up into the hose. Actually you are removing the air in the hose, creating a low pressure area within the hose, less pressure than the atmospheric pressure. The atmospheric pressure acting upon the tankwater is in effect, "pushing" the water into the hose, up and over the side of the tank, and it flows down to the bucket.
Placing the hose deeper in the tank will allow a faster flowrate of water to siphon to the bucket - because now you've added some additional pressure ( tank water head pressure ). Raise the end of the hose in the tank, the water will still continue to siphon, but at a little less flowrate.
Pull the end of the hose out of the tank, and you let air back into the hose, the pressure in the hose is equal to the atmospheric pressure again, and you BREAK THE SIPHON.
No difference of pressure = no flow.
You must always have a difference of pressure for most anything to flow from one place to another. It's called differential pressure - how clever
Same type of thing occurs when you are pumping water up to the tank with a return pump. You've got the sump with water in it ( like the bucket ).
You've got the hose.
You've got the return spraybar or jet IN THE TANK creating a continuously filled hose, connecting the sump and tank together, as long as the pump is running.
As long as the return hose is in the tankwater ( say the open end of the hose is sticking down into the tank 1 inch ) and this hose leads down to a lower level ( the return pump/sump ) then if the POWER GOES OUT, or you turn off the pump - you have the conditions for a siphon to start on it's own.
No need to suck the air out of the hose - it's already out of there.
So once again the atmospheric pressure does it's thing, it starts pushing the water through the spraybar in the opposite direction it was flowing from the pump, and the water in the tank starts to drain back down to the sump.
How much of it drains back down idepends on how far the return line is positioned in the tank. In our example here, we had it down in the tank that 1 inch. The water in the tank will keep draining back to the sump ( siphoning ) until the water level in the tank drops the 1 inch. Once it does .... the open end of the return hose in the tank is exposed to the air, it vents, and allows any remaining water that was in hose to fall ( gravity kicks in here ) down to the sump.
On the other hand, if you had the return spray fitting located deep in the tank, let's say all the way to the bottom of the tank, and you turned off the power to the pump ..... the water in the tank would drain ALL THE WAY DOWN to the where this spray fitting was located, again until the open end of the hose is exposed to air . You could easily empty most of the tank of water to the sump.
Of course your sump is normally not the same size of the tank, so it would have overfilled way before the tank water level even got close to the bottom, and all that water would end up on your floor. I don't need to explain what that means, let's just say that is a really bad thing to happen.
So what people do to prevent this from happening, is to keep the return spraybar up near the surface of the tankwater. Or if they want to position it a little deeper in the tank, they drill a small hole in the returnline, just underneath the normal water level in the tank. Under normal conditions, everything is cool. Water is pumping up to the tank, most of it is shooting out the spraybar, a little bit is shooting out this drilled hole ( but you don't see either because both are underwater ).
When pump loses power, the natural siphon begins as it is expected to, water starts draining back to the sump, until the water level in the tank reaches this little drilled hole.
Once this happend, air rushes in the hose, it vents, atmospheric pressure no longer is pushing the water through the hose anymore, the remaining water in the hose falls back to the sump and nothing overflows.
That's what can and does happen all the time when you use a sump, return pump, return line, and overflow set up in our hobby.
The siphon effect is possible due to a combination of atmospheric pressure, differential pressure and gravity.
Knowing HOW it works in not all that importatant - but knowing that it DOES happen and what to do to handle it is the key to keeping the system running smoothly, and keeping all the water where it's supposed to be.
In your tank, in your sump, in your hoses and drain lines ............ and not on the floor.
