The siphon is an example of a natural and extremely simple machine that we take advantage of daily.
by Joe Evans, Ph.D
November 24, 2017
A piping system illustrating siphon effectFigure 2. A piping system illustrating siphon effect
A pipeline with up and down legsFigure 3. A pipeline with up and down legs

Large siphons are used extensively throughout the world in high volume flow applications. In the U.S., many water and irrigation aqueducts use a combination of pipelines, tunnels, siphons, inverted siphons, channels, and ditches to move water many miles from its source to its point of use.

The Siphon Effect

As long as a pipeline is flowing full, the pump head required to maintain flow is equal to the head due to friction and the elevation increase between the pump discharge and the pipeline discharge. This rule holds true even if some point in between has an elevation greater than that of the pipeline discharge.

Figure 3 shows a simple piping system that illustrates this rule. The reason that the higher elevation (h2) seen between the pump and pipe discharge does not affect pump head is due to head recovery provided by the "siphon effect."

Now, h2 must be accounted for when filling the line, but once it is flowing full the down leg will cancel its additional height and the elevation seen by the pump will be h1. If the down leg is not flowing full, the head required will be that of friction plus the sum of h1 and h2. The major difference between a true siphon and the siphon effect is the latter will not maintain flow by itself.

A pipeline with several up and down legs in series. In this example, each down leg produces a siphon effect and the total head seen by the pump, once the line is filled, is equal to h1 plus the friction in the line due to flow.

Although piping systems that rise and fall can take advantage of the siphon effect, they are also vulnerable to entrained or dissolved air accumulation at their high points. If not properly vented, air pockets can reduce and, in some cases, completely prohibit flow through the system. The chance of this occurring is increased in piping systems with multiple up and down legs. If some air is trapped in these upper areas but flow is maintained, the elevation head required to maintain flow will be h1 plus the sum of the heights of the air pockets in each down leg.

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