This topic is somewhat arcane, and yet we shall give it quite a lot of attention. The reason is not because siphons are necessarily any great design product to be sought after. The reason is because siphons are potentially quite dangerous and destructive, if they occur unexpectedly and are not properly controlled. (As used in this context, a siphon is a flow condition, not a particular piece of hardware.) Siphons are formed by some type of sealing action at the two ends of an open liquid circuit, as a combined consequence of the pipe geometry and flowing condition, and they most certainly can occur accidentally and totally unseen. So consider siphons a safety issue, and this section as primarily “preventative” in nature.

In this topic, we will discuss the ways siphons can form in piping systems, the necessary conditions for maintaining a stable siphon, both with and without seal legs, and especially we shall emphasize how to prevent siphons from existing. The pros and cons of letting a siphon exist are fully explained.

A siphon effect may be defined as creation of flow-induced regions of subatmospheric pressure, partial vacuums, at high spots in the liquid-carrying pipeline.