Pond Boss Forums POND CONSTRUCTION Building a dam Siphon system control

I have a hydrology related question. Can
someone give me a rough estimate of the
pressure (psi) that exists on a closed
valve plate of a valve at the discharge
end of a 12" siphon pipe. The input end
of the pipe extends into the water at a
22.5 degree angle for 20' (about 7' below
the water level at the intake of the pipe).
I would like the capability of shutting the
pipe off prior to the drought months of summer.
The pond has 70' of emergency spillway and
could probably survive heavy rain without the
pipe.

Squeaky, the pressure at the output depends upon how much vertical height difference there is between the water level in the pond and the valve in the pipe outlet. If you are putting the valve at the end of the outlet pipe then you need to know the vertical height distance from the outlet end of pipe to pond surface. Once you figure that distance, and remember it is vertical distance, doesn't matter what angles the pipes are at, then you can just multiply the vertical height distance by 0.45 and you will get the pressure on the valve in pounds per square inch. This is called the hydraulic head pressure, it's called this because what you are really measuring is the weight per sqaure inch of the water column on top of the valve. Remember also that it doesn't matter how much the inlet pipe goes up before it turns downward on the other side of dam. When the pipe is full in syphon mode the weight of the water column in the inlet pipe going up to the highest point tries to pull the water back to pond. The weight of water from the highest point in pipe back down the outlet side to the same height as the water level in the pond will pull the water away from pond. Since these two heights are equal they will pull equally against each other and therefore balance each other out. Thus the only thing that matters is the height difference between pond surface and the outlet which contains the water that pulls the water out of pond. So once you figure this pressure it not only represents the positive pressure on the outlet valve but also the negative suction pressure at the inlet. They are equal exept for whatever friction is in the pipe which will be very small for your large pipe.

Thanks bz for your input - it makes sense
to me. I'm guessing the vertical distance
from suface water to the pipe discharge
end below the dam is about 130". This
would be a little less than 60 psi on
the valve. This pretty much eliminates
using an irrigation valve since their
pressure rating is somewhat lower. Also,
I'm curious if the constant pull
of this force would loosen the connections
on the backside pipe sections.

I think I should have used vertical pipe
length in feet rather than inches. Because,
the vertical weight(lbs) component of water
in a 12" pipe 11' below the surface would be
~ 11 * diameter(in) squared * .34 , and,dividing this by the valve plate area in
(sq inches) gives a value a little less than
5 psi. This is close to your .45 * vertical
pipe length.

Correct, you should have used feet not inches. As to your question about the pull of the water being hard on the down stream pipe. I'm no piping expert but just considering a force balance on the whole system I think the water on the outlet pipe will put a slight stretching force into the pipe but only due to the friction of water running in the pipe. So I think this would be very small. The water is pulled out of the pond by a negative pressure caused by the weight of the water in the outlet pipe thus creating a suction in the pipe. So the major force on the pipe is a vacuum inside rather than any pull on the joints. If the pipe is sticking out from the dam in to the air with no support I think the weight of the pipe itself and the water in it will be harder on the pipe than the pull of the water. So if the pipe is supported well you should have no problems. By the way, another slick way to turn a syphon on and off is to have a vent pipe installed at the highest point of the syphon with a valve on it. You can either set this up so it drains or syphons when needed automatically or depending on your system geometry you can use the vent pipe valve to simply shut the syphon off when you open the valve and let air in. There are some posts on this forum somewhere that explain how to make all this work.

Yes, thanks for the input, I can stop
the siphon effect, but, it is the continued
draining of water through the pipe after
the siphon stops that I would like to control.
The water will continue draining until the pond
surface level drops below the bottom of the
highest point of the pipe. I'd actually like
to have the capability of holding the surface
level just above the pipe (about a foot higher
than where it would drain to).

Squeeky,

With a siphon, you can keep the pond at any level you wish. All you have to do is raise the height of the pipe that goes through the dam to the maximum level you would ever want the water level to be. Set the vent tube to the desired water level, and you're good to go.

I'm confused, raising the pipe that goes
thru the dam would be quit a task. There
is 5' of dam (freeboard) above the pipe
and 14' across at the crest. That would
be a lot of excavation. And, each 20' pipe
section weighs 250lbs.

You are right. You don't want to do that. You've got a tough question there. It sounds like you need a valve at the outlet of the siphon.

yep no moving pipe once set. Jersey beat me to it. How about a valve on the discharge end? This can be shutoff and the lake level will raise to as high as you want it. Not sure how much pressure the valve can handle. Have fun when you get ready to open it,