Here in NW GA, we're expecting upwards to 6" of rain in the next 3 days. With our lake near siphon turn-on height, I wanted to get it going early to draw down some before the heavier rains hit tonight.

I capped the vent and went down to the outlet to watch it convert from drain to siphon,,,,,,,,,,,,,,nothing. No change.

It turns out that the bottom of the elbow never fills to seal off and form the siphon. The velocity is too high and the water is being ejected instead of pooling. The outlet elbow is a 45 and is extended about 12". This appears to conform with standard designs I can find on-line.

Any ideas what the common fix is to block or slow down the flow to maintain the air seal and induce a full flow siphon?

Thanks

-RFL

Ryd...if your BOW is "near" siphon start, then the dam cross over pipe still has air in it. To start your siphon in less than a high water condition, you have to cap off the air vent/siphon break,, plug the outlet, then fill the entire piping system with water, then remove the outlet plug..otherwise, you are still in simple overflow mode.

IF your pipe had some flow through it, you could plug the outlet first, wait a few minutes for overflow to fill the down pipe, then cap the vent tube and remove the outlet plug. IF the down pipe has enough weight in water and the air remaining doesn't create a suction break, the siphon may start.

The elbow at the bottom is only to prevent air from entering from the outlet when actually IN siphon mode already.

Rainman is right on the money. When I have some water flowing through but not operating at siphon, I have to remove the vent cap and close a valve at the bottom. The water then fills the pipe and I put the vent cap back on. Then I open the valve and let the water flow. For some reason if I keep the vent cap in place while trying to allow the pipe to fill it never gets all of the air out of the system and the siphon doesn't start consistently.

Last thing - be very very careful around an open vent cap or the intake side of a siphon. It may have tremendously powerful suction associated with it.

Rainman,

I agree with everything you say except the part about the reason for the outlet elbow. Once the siphon is running, you can cut off the elbow and do almost anything you want with the outlet pipe and the siphon won't stop as long as the elevation change is maintained and the velocity remains high.

The outlet elbow is there to keep the lower end sealed so there's only one place air can get into the system, the vent pipe.

My siphon design is shown in the lower graphic below. The 45 degree elbow connects to the down slope pipe and the trapped water is a very small pool. When the velocity of the water coming down the slope picks up, the trapped pool is cleared out and the seal is broken. The upper graphic shows a typical outlet with a horizontal sealing section. There's no way this seal will get "blown out" as easily as happens in my situation.

I need to fix my outlet. I can either cut out what I have and go with a new horizontal section or I can just add more pipe to what I have and create a deeper pool.

Anyone know why I shouldn't just stick on some more pipe to what I have? That's definitely the easiest fix.

-RFL

you could indeed cut off the elbow...and have the water flatten out on the bottom part of the outflow tube and allow air to run up the tube, breaking the siphon...and that is exactly what often happens if an elbow is not in place.

Unless your pool level is above the top of your siphon tube's highest point AND the vent tube is under water, a siphon will not start without manually filling the outlet pipe.

All it takes to "pulse" and break a siphon by allowing air to enter from the outlet, is a fish or turtle swimming by the inlet. Without the elbow, often a siphon will never even start and reamins as only a simple overflow pipe with gravity flow.

BTW...what is wrong with your current outlet that needs fixing?

If you cap the vent tube before letting the outlet pipe fill after being plugged, the trapped air has to be compressed. There is still air in the system unless the pond level is higher than the highest point in your crossover pipe, but it is a small enough volume that the weight of the water pulls it out when the high velocity flow of the siphon starts....the "inconsistancy" comes from the water flattening out and a large volume of trapped air causing a vacuum break.

This morning I started my siphon without the pipe totally submerged and without filling the pipe.

1) Lake level was about 2/3 of the way to the top of the 6" pipe.
2) I put a 3" PVC cap over the end of the vent pipe because the vent pipe was a couple of inches above the water's surface.
3) Siphon did not start automatically and a flat ribbon of water was shooting out of the end of the pipe and going about 3 feet.
4) Looking in the outlet I could see the elbow wasn't sealed and the flow wasn't changing from drain to siphon.
5) I placed both hands over the end of the 6" outlet pipe to slow/stop the water and keep it inside the outlet and form a water seal.
6) Once the outlet was sealed, things started happening.
7) There were several gentle surges and the volume of water increased with each one.
8) About 2 minutes later the siphon was at full flow and the torrent of water was now being shot about 10 feet downrange.

I'll shut it off and do this all again tomorrow with pictures to make it all clearer.



I agree 100%. The elbow is there to seal the outlet and allow a siphon to start. No seal..... No siphon start.



My outlet only seals when there is just a trickle of water coming down the pipe. As the volume, and velocity, increase as the lake level increases, the very small pool of sealing water is carried out of the pipe by the force of the rapidly moving water coming down the pipe. The problem is that there's not enough volume of water to resist the high energy water coming down the pipe. As soon as the pool of sealing water is gone, air enters from the bottom and no siphon can begin. When I capped off the end of the pipe with my hands, even though the pipe was not totally sealed off, I was able to keep the elbow full and the seal intact. With the intact seal and the vent pipe capped off, it only took a minute or two and the siphon was going.

In short, if I can't keep the outlet sealed and keep the air from getting into the pipe, the siphon won't start. Once the seal is restored, the flow quickly changes from drain to siphon.

I'll try to video this tomorrow and will post it on youtube.

-RFL

RFL...covering your outlet DID "fill the outlet" and there was enough velocity to draw out the remaining air from the now filled outlet side. It may be more just terminology, but there is NO seal from the elbow at the end with water. It is there for fluid/air dynamics...the upward turn of the elbow makes it impossible for air to enter as air will not "sink" into the pipe. A siphon can not just be turned on by simply covering the vent tube unless all conditions are met for a siphon...it CAN be turned off by allowing air to enter.

If your vent tube was capped, and you were able to hand cover the outlet, I would fear the outlet side of your system is not sealed air tight because the pipe would not have compressed the trapped air fully filled, if sealed.


Regardless, you got 'er going and that is what was important.