Saddle siphon working principle

The basic principle under which saddle siphon works/functions is summarized briefly as follows.

  1. When the water starts to rise in the reservoir above the Normal water level (NWL), then water starts to spill from the siphon spillway.
  2. When the water rise just above the depriming hood, the duct of the siphon is locked y the water from both the end.
  3. In order to increase the rate of priming of the siphon spillway, bay priming arrangement is made.
  4. The flowing water from the crest of the spillway will suck the entrapped air inside the duct.
  5. This will cause the gradual increase of vacuum pressure (sucking pressure) inside the duct.
  6. The negative pressure will suck more water from the reservoir and finally the duct will be running full of the water.
  7. The siphon is in the full operation now.

NOTE: The time period between the instant the water just starts to spill from the crest to the instance that siphon runs in full is called priming.

When the water level of the reservoir decreases and reaches just to the inlet of the depriming hood, air will be passed to the throat section and negative pressure developed will be released and siphon will stop running. If the deprimer is not provided then water level will decrease just to the upper level of the inlet pipe which will cause the loss of water.