I will try and get some photos or a sketch at some point
It depends on how that single standpipe is being used (with regards to the risk involved). If the standpipe is at open flow (not a siphon) then it is nowhere near it's capacity. In other words there is a lot of headroom before an overflow condition occurs. At some higher rate of flow or partial blockage, the open flow will form a siphon. This will increase the velocity and flow in the pipe dramaticaly. The added velocity may even partially clear the blockage.
If that single standpipe is at a siphon, then it will fall into 1 of 3 states. In the steady state, the siphon will dynamicaly adjust to a range of head pressures. Once the head increases past the upper threshold, an overflow condition will occur (this btw is where we want to run the siphon assisted overflow). Simple "hole in a bucket" equations can be used to simulate what happens here. If the head decreases below the minimum threshold, the siphon breaks and the system flushes. This cycle will keep repeating. Note that this is only a problem with U-tubes. A standpipe will self correct if the siphon breaks.
You are correct in that the U-Tube restriction is similar to using a ball valve to create a restriction. There are some difference in the system however. The U-Tube siphon can be broken by air and will not recvover. The standpipe siphon can be broken by air, but will self revover. The U-tube will possibly break siphon at a low flow from the return pump if it's capacity is LARGER than that of the return pump. In other words it will suck the water out of the tank faster than it is being replaced. In the standpipe situation this only serves to make gurgling noises as the standpipe siphons and flushes in a repetative cycle.
It should be clear that the basic problem with the U-Tube setup is the possibility of a siphon collapse and the possibility of an overflow due to a change in the pump parameters or the u-tube cross section (blockage due to algea, a snail etc).
So even though both the U-Tube and a single standpipe are "similar" they are worlds apart in terms of safety.
The ideal operating rate for a U-tube would be somewhere right in the middle of the siphons dynamic range. The problem becomes determining exactly what that range is. IE, how much flow will cause a flood, and how little flow will cause a siphon break. The return pump and the height of the water in the sump will change as the siphon changes the level in the display tank.
Remember there is a very dynamic relationship between sump water level and pump rate that directly corrolate to the head pressure on the siphon. The surface area of the display and return compartment are usually very different. This means that a small change in pump output can cause a LARGE change in the sump water level but not a lof of change in the head pushing on the siphon from the display.
I guess I could put it another way. I WOULD NEVER trust a siphon type of overflow without an emergency drain of some kind. You are trusting a balancing act that you have very little control over.
I never said it "would not work" I am saying that it is not the best idea, not matter how many people use it
It is a flood waiting to happen. I would never be able to sleep knowing that I had to rely on a u-tube to keep my floors dry.
Using multiple u-tubes can add some redundancy to the setup, but can also make adjustment more complicated and they still leave the basic issue of what happens when the siphons fail. Then there is the old "return compartment will not hold enough to flood my tank" trick. Though this works... it severly limits evaporation capacity and puts the return pump in jeapordy of "run dry" failures.
Just some food for thought. There is no right or wrong way to plumb a tank... There are however ways that are much better than others.
I will be changing it out soon.
I'm not sure what part of my quote you are refering to. Are you saying that you have a ball valve that has frozen after only a year? #1. That why I say use true unions so the valve can be removed and cleaned. #2. Sound like you are precipitating calcium carbonate at a high rate (oversaturation, improper balance/pH).
