Do you guys know how many civil engineers have screwed up the sizing on pipeworks, culverts, sewers and storm drains because the do not understand the controlling mechanisms of pressure flow versus gravity flow? Many flooding problem are caused by the failure to understand the mechanics of the transitional flow.
A siphon is a form of pressure flow. A siphon is pressurized by potential energy in the form of water head instead of a tank or pump. A siphon is a form of pressurized flow where a negative pressure is generated and must be maintained by having a completely closed system in the vacuum portions of the system.
The laws of pressurized flow and gravity flow are two different animals. The transition between forms of flow are even more difficult to understand.
I will freely admit that I did not read very word of every post to get to this point but I will insert my two cents.
First, We all understand why siphons via normal hangon siphon overflow boxes stink. I will not even go there.
Second, Using any siphon for outfall in a tank is very dangerous and not a good idea. The only advantage is the increased flow (generally relatively small) it results in for equivilent sized piping as compared to a gravity system. The problem is twofold.
First, when your system loses power there is the possibility that the siphon will put too much water in your sump causing it to overflow. This is the same problem as with a normal hang on siphon overflow box. This problem can be overcome by having an extra large unused sump volume or trying to keep the top of the siphon close to the normal waterline so that the siphon is broken quickly. This is very difficult to do and normally results in the gravity/siphon surging (read more noise)previously discussed in the thread . This is not the real problem, however.
The real problem is that when your pumps come back on line, there is the high probability of overflowing your main tank. This is due to the fact that a siphon requires more "work" or potential energy to start than to maintain. This means that the water level in the tank must be higher to get the siphon started than to maintain it. This is not a small amount of head either relative to normal tank operating heights and freeboard. Typically, you see the gravity/siphon surging cycle for a while before full siphon occurs. This happens because the water height increases such that it is sufficient to partially start the siphon at the entrance. When the siphon partially begins it will begin to suck more water out than is available as extra potential energy. At that moment the air introduced will cause gravity flow conditions to occur and the cycle starts again. In the mean time you have caused your main tank to overflow at some point. Now you can overcome this problem by limiting your sump size or keeping the height of the water in the main tank so low normally that you can pump the entire sump contents into it without overflowing your tank.
These are generally not practical solutions.
There is a reason everybody uses gravity flow and a "durso", or valve at the discharge, or other means to limit noise. All of these mechanisms essentially try to increase the friction of, or headloss through, the gravity pipe work such that the flow through the system behind the bulkhead is limited such that it is close to the capacity of the bulkhead. This eliminates the ability of falling water to form small vacuum areas (or partial siphons) in various portions of the pipe which creates the noise we hate.
As to head on a bulkhead/orifice. In gravity flow systems, the head (water height) above the bulkhead regulates the flow. The orifice opening acts as a miniature pressure system. Of course if the bulkhead is not completely submerged it is a weir not an orifice. Water height still controls flow.
In a fully pressurized siphon system, the head has no significant bearing on the flow through the bulhead opening relative to the other controling factors. If the pressurized system is not a siphon, the flow has everything to do with the head and almost nothing to do with the opening.
Well that should muddy up the waters.
(I'm serious, I know you can do it.)
