---
title: "Air valve: function and working"
subject: "Water Supply Engineering"
chapter: 9
key: "semester_5/water_supply_engineering:air-valve"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/air-valve
site: BCT Engineering
---

# Air valve: function and working

Water Supply Engineering · Chapter 9: Valves and Fittings

- Asked in: 2072 Kartik, 4 marks; 2079 Bhadra, 4 marks; 2066 Jestha (old course) (asked 3 times)
- Video: [How to Install an Air Valve in a Water Pipeline | What Is the Function of an Air Valve in a Water...](https://www.youtube.com/watch?v=Rf_uK8HFjiY) — Civil Engineer Anand
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/air-valve
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-9

## Question

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

## 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.

## Answer

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.

```text
        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.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Where does air collect in a pipeline?
   A. At high points (crests)
   B. At the pump only
   C. At low points
   D. Uniformly
   Answer: A. Air rises to crests, reducing flow area. (Key point: Air problem)

2. What is an effect of trapped air?
   A. Lower head loss
   B. Air-lock and reduced flow
   C. Corrosion-free pipe
   D. Higher discharge
   Answer: B. Air reduces area, increases head loss, causes air-lock and vibration. (Key point: Effects of air)

3. How does the valve release air?
   A. Float drops as water level falls, opening the outlet
   B. A spring opens it at high pressure
   C. It is opened by hand
   D. Float rises, opening the outlet
   Answer: A. When air collects, the float goes down and opens the outlet. (Key point: Working)

4. Why does the valve also admit air when the pipe is emptied?
   A. To cool the water
   B. To clean the pipe
   C. To prevent vacuum and pipe collapse
   D. To increase pressure
   Answer: C. Admitting air prevents negative pressure collapse. (Key point: Vacuum)

## Related in Water Supply Engineering

- [Reflux valve and air relief valve](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Areflux-air-relief-valves) — Water Supply Engineering, chapter 9, asked 2×
- [Purpose of valves; cutoff valve](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Avalves-purpose-cutoff) — Water Supply Engineering, chapter 9

## Related in other subjects

- [Reciprocating pump working, slip and air vessel](https://www.bctengineer.com/question.md?key=semester_5%2Ffluid_machines%3Areciprocating-pump-slip-air-vessel) — Fluid Machines, chapter 4
- [Single phase motor working; effect of air gap](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Aspim-working-air-gap-effect) — Electrical Machines, chapter 7