is there a way to determine the water pressure at the bottom of a siphon if the siphon tube is full of water.?

Question

this is for a project at work. we would like to irragate a planting below an earthen dam using water from behind the dam. the surface of the water in the lake is 30 verticle feet below the rim of the dam and the planting area is 150 verticle feet below the rim of the dam on the other side. we have access to 6 inch draft hose and were wondering what pressure would be present at the planting site if we could get the siphon started? also the planting site is appx. 700 feet away horizontaly from the rim of the dam. thanks for any input. cameron

Answer 1

Bernouli's equation is the relationship that will describe the pressure for a flowing system.

http://www.lerc.nasa.gov/WWW/K-12/airplane/bern.html
http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html

In this case, you can probably ignore the more difficult terms of the equation (like the pressure drop of the flowing water in the hose due to friction on the side of the hose)

Unless friction loss is large, your pressure is primarily given by the mgh term of the equation.

That is very simple and it gives the maximum possible pressure, even if flow is blocked.

The pressure is equal to the weight of the column of water the height of the difference between the top of the above water resevoir and the end of the hose. It doesn't matter if it goes up and then down and for this term it doesn't matter how far it goes side-to-side.

If I read your problem right, that would be 120 feet in this case.

The weight of 120 feet of water can be expressed as pressure in many different units (one of them is feet of water - 120)

It would be about 52 psig.

The pressure loss from this amount due to friction would depend on the size of the hose and how fast the water flowed (based on nozzle or restriction usually)

The other problem you will have is with that siphon going 30 feet above the lake level. That is very close to the theoretical maximum siphon lift (about 33 feet). You will have a very deep vacuum at the top of the siphon so you will have to have a hose that can take it. If you could find a slightly lower place over the dam it will be a LOT easier to get started.

To start the siphon you will need to COMPLETELY fill the hose. You might try putting a restriction valve on the crop end and pumping into the hose from the lake end (perhaps with a submersible pump so it will be easier to start up). You will have to bleed the air slowly out of a very restricted valve on the down end. You will have to get all the air out and the hose full of water. Then you can remove the pump (without letting any air in the hose), open the crop end valve and the siphon might begin.

I'm sure there are other ways to fill the hose. I think your biggest problem will be getting the siphon started with a 30 foot lift.

good luck

Answer 2

The net height is 120 feet. A really rough estimate is that you will have about 60 psi at the planting area. To get the siphon started, I suggest a submersible pump on the supply end if you can get one, because getting the first 30 ft will be very hard unless you are close to sea level. The siphon might not continue to work with that much of a lift to get over. If the water has a lot of dissolved air, it might just foam up in the lift tube.

Added: I'm not sure what you mean by a 6inch draft hose. If it is 6 inches in diameter, you will have a hard time getting that to work. Multiple, smaller diameter tubes would be better to keep the water from just running down the tube and letting air back up to the top. (you are already going to barely get the water to the top of the dam, if at all, and a large hose would just act like a channel for the low flow over the top to fall down the tube, and you would need a huge pump to fill a 6 in dia hose.

Also, the hose will have to stand up to the vacuum, and smaller diameter black PVC pipe would be rigid enough to hold it's shape. Remember, with a siphon, the pressure pushing the water through the hose is atmospheric (14.x PSI or so at sea level.), and by the time the water is lifted to the top of the dam, there isn't much pressure left. None if you are much above sea level.

Answer 3

Yeah, good luck with that one.