Siphon flow sizing

[Dlzook]

We are trying to design a smaller reservoir siphon to replace a 12" ID siphon that flows much higher than is desired on this site. The siphon works well and is HDPE fusion pipe and we intend to use the same design and material that currently works but in a smaller pipe size but am not sure I am calculating the projected flow right for 6" and 8" inside diameter HDPE. The inflow elevation to the high water line is 10 feet and inflow pipe length to center of dam (highest point) is 83 feet. the discharge is 28 feet below the high water line and the length of discharge from highpoint at dam center is 102 feet to the gate valve. There is then 20 feet of pipe past the gate valve for a total pipe length 205 feet. I am struggling to calculate the flow on the 6" and 8" pipe sizes. Can anyone help? Thanks

 

[cvg]

why wouldn't you just put a throttling valve on the downstream end to reduce the flow? or an orifice or some sort of nozzle that would restrict the flow.

 

[dicksewerrat]

Cut up a catch basin grate and lay it over the inlet pipe. That should cut the flow. Cheap, simple. Save the other pieces. You may need them.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[cvg]

a grate on the inlet would likely clog very quickly. theoretically there is (or should be) already some sort of trash rack installed. better to control flow at the outlet which will also provide some back pressure to keep the downstream pipe flowing full.

 

[Dlzook]

Thanks for the input. No grate on the inflow, water is clean enough and flows hard enough that we have not had problems with debris in the 12" pipe. We have tried throttling it with the gate valve at the outflow but if we get it turned down enough to meet the lower flow objective, it always loses it's prime. Based on this experience, I assumed that in order to keep the siphon flowing we had to run it close to wide open and as a result we were trying to determine what size, running wide open, would give us the flow we are looking for which would be in the area of 3-5 cfs.

 

[LittleInch]

Ok, so you have 28 feet head over a total length of 205 feet. That's a lot of head per 100 foot.

First off, in HDPE speak 12", 8" and 6" don't exist. You have at a guess 315mm, 250 and 180mm

For the 180mm pipe with say 150mm bore, you get about 80 litres per second at about 4.5 m/sec.

The 250 mm, say 210mm ID, you get about 240 litres/sec at nearly 5m/sec.

Your 315 ( 280mmID) probably gives you about 350 -400 l/sec

So 3-5 cfs (cubic feet per second?) = 80 litres/sec to 135 l/sec.

So the 180mm pipe will probably do it, especially if you choose a relatively high SDR as my graph ran out at a pressure drop of 10:100 and you have 14:100.

If you're having trouble maintaining syphon in your 315 pipe, then you're either not clearing the pipe properly once the flow starts, you're sucking in air at the entrance or there is a leak somewhere. So long as you keep your exit pipe full of liquid, reduced veleocity shouldn't make a difference, but clearly you're getting air in somewhere then you don't have the velocity to clear the air out - usually about 1- 1.5 m/sec.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[chicopee]

You could also be getting air on the discharge end of the pipe, therefor, you should keep the end of the pipe submerged in a some type of reservoir or have the discharge end be in the form of an inverted elbow.

 

[bimr]

Agree with LittleInch. All you have to do is to determine the velocity (3.6 -6.0 ft/sec) in the existing siphon. Use the same velocity to determine the flow from the smaller pipes.

For the 8-Inch HDPE, 1.9 to 3.2 CFS.
For the 6-Inch HDPE, 1.0 to 1.65 CFS.

 

[cvg]

well, that is a bit of over simplification. for similar velocities, you will have different headloss per foot in different sized pipes. this is due to the length of wetted perimeter which is not a linear relationship with pipe diameter. so in general, you will have increasing flow for larger pipes

 

[bimr]

You are correct, but for the reasonable flow velocities that are used, it should not matter that much.

A better method is to use the pressure drop across the siphon pipe, estimated at 0.3 to 0.7 psi with plastic pipe when flowing at 3-5 cfs.

For the 12-Inch HDPE 3 to 5 CFS.
For the 8-Inch HDPE, 1.8 to 2.8 CFS.
For the 6-Inch HDPE, 0.7 to 1.15 CFS.

These flow give the same pressure drop across the 205 feet of pipe.

There are online calculators that may be used to determine the headloss:

http://irrigation.wsu.edu/Content/Calculators/General/Pipeline-Pressure-Loss.php

 

[Dlzook]

Thanks for the great suggestions, very much appreciate the helpful information. we are going to test submerging the outflow and see if we can throttle this first before downsizing it. Will post result when it's been tested.