A venturi is a completely different principle and I was not making any such comparison. When a convergent nozzle is coupled with a divergent nozzle to increase the velocity of a fluid, it creates a low pressure area that can be used for suction. The venturi principle has nothing to do with this.
Sorry but this makes no sense first you say a bigger pipe can take more. Then you say the larger emergency drain can't handle what you are feeding into the tank.
In Herbie's system, he uses the large bulkhead as the "normal" drain, and the smaller bulkhead that was the return line as the "emergency" drain. That is what I did. So for now on there is the normal and emergency. I never said the "larger" emergency.
I increased the size of my return lines and my drain lines. The only restriction in my drain is the bulkhead fitting itself. My stand pipes, drain lines, and return piping are all bigger. I can return much more from my 1262 than my overflow grate will flow. It backs up into my tank. So I throttle it. I have not done a test to find out if my emergency drain will handle all the flow, because I don't feel it is likely my normal will ever be plugged 100%.
This is possible. What (I think) is happening is that smaller pipe gets covered with water it creates a siphon a siphon can flow more water that a pipe open to the air.
There is NO siphon. It simply drains from gravity. A full pipe will flow more water than one half full of air. Period. a 3/4 inch stand pipe for my emergency will flow so much full. Siphon has nothing to do with anything. Either the pipe is full or it isn't. There is no lift. There is not suction.
A brief note
When water enters a siphon, it is accelerated by gravity (32ft/sec^2). It enters the pipe with zero speed. One second later it exits at 32 ft/sec. Now something has to enter the pipe. If air is available it is much easier to move so it is sucked in (vortex) and an equilibrium is reached. If no air is available water will be sucked in at 32 ft/sec and it accelerates to 64 ft/sec. The limiting factor to how much water you can get down the pipe is resistance from the pipe (smoothness of the walls, bends, etc.), viscosity of the liquid, and a whole bunch of other variables.
For a siphon sure. If it was in true free fall. But we don't have a siphon. Water will fall at the rate of gravitational acceleration. But the flow is restricted by the gate valve. So it is not in free fall. Only the head pressure above the valve, and the opening in the valve is driving the flow. It is not in free fall. It is in free fall if you start a siphon and let it fall to the floor. 4 feet.
So back to the discussion, once the main drain is covered with water it sucks water in rather than air. I think that the problem maybe that the emergency over flow is too high and can not get covered with water to create the siphon and flow more water.
There is no suction. It does not suck, it drains. The gate valve has 3 foot of head pressure on it and water flows. A siphon does indeed flow more water than a drain. And it flows more proportional the distance the water on the other side of the bend falls. Herbie is a drain. It is not a siphon.
How far below the water line is the emergency drain? As a test, is it possible to pull out of the emergency drain standpipe (I am assuming a RR with overflow box and floor bulkheads)? What I expect to happen would be that it will drain the water. Then it will start to create a full siphon. The siphon will drain the water and allow air in to the pipe (breaking the siphon). The overflow will fill up again and create a full siphon and drain. This should repeat. If not let me know "“ you really want the emergency drain to handle the full flow (as unlikely as it is).
You are making this much more complicated than it needs to be. It is NOT a siphon. The drain will drain the capacity of the size of pipe. It will not flow a certain amount, then flow more, then drain, then fill up again, then drain. It will drain to the capacity of the line PERIOD. If there is enough flow to fill the line then it will remain full. If it can't flow enough, it will back up. As it backs up, head pressure will increase. Then you do not have free fall, you have head pressure driving more flow. You will reach a equilibrium at some point that head pressure will flow the equal of return. A 1" line will flow more water at 30 psi, than one at .5 psi. Whether or not that equilibrium is reached before it overflows the tank is TBD.
. Also keep the drain return simple to a few bends.
