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Air valve: function and working

Short answer

An air relief valve automatically releases trapped air at pipeline crests to prevent air-lock and head loss, and admits air during emptying to avoid vacuum. A float drops when air accumulates, opening the outlet; rising water lifts the float to close it.

What is the function of an air (relief) valve? Describe its working with a neat sketch.

An air (relief) valve is an automatic valve fitted on a pipeline to release the air that collects in the pipe and to admit air when the pipe is emptied, so that the line works properly.

Function

  • Air accumulates at the high points (crests) of the pipeline; it reduces the flow area, increases the head loss, causes air-lock and may make the line vibrate or burst.
  • The valve lets the trapped air out automatically while the pipe is being filled or when it is working.
  • It also admits air when the pipe is emptied or when negative pressure forms, preventing a vacuum and collapse of the pipe.
        outlet (air)
          |  _|_
          | | o |<- float ball
     _____|_|___|__
    |  body        |
  ==|  water ~~~~~ |==  pipe (crest)
    |______________|

Working

  1. The body has a small outlet at the top and a float (hollow ball) below it.
  2. When air collects, the water level in the valve body falls and the float goes down, opening the outlet and releasing the air.
  3. When all the air has gone, water rises and lifts the float, which presses against the seat and closes the outlet.
  4. This is repeated automatically whenever air accumulates again.

It is placed at the summit of the pipe, with an isolating sluice valve below it for maintenance.

✓ Answer checked by Claude Opus 5.5 (high effort)

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