---
title: "Reflux valve and air relief valve"
subject: "Water Supply Engineering"
chapter: 9
key: "semester_5/water_supply_engineering:reflux-air-relief-valves"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/reflux-air-relief-valves
site: BCT Engineering
---

# Reflux valve and air relief valve

Water Supply Engineering · Chapter 9: Valves and Fittings

- Asked in: 2081 Baisakh, 4 marks; 2074 Chaitra, 2+2 marks (asked 2 times)
- Video: [Pipe Appurtenances: Air relief valve, GATE valve, Reflux valves etc. Environmental Lecture-9](https://www.youtube.com/watch?v=FCGOh5wcYdQ) — Study Civil Engineering with Shuja Askari
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/reflux-air-relief-valves
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-9

## Question

Write about the purposes of reflux valve and air relief valve with necessary sketches.

## Short answer

A reflux valve permits flow in one direction only, preventing reverse flow that could damage pumps or cause back-siphoning. An air relief valve automatically vents trapped air at high points to avoid air-lock and admits air when the pipe is emptied to prevent vacuum.

## Answer

#### Reflux valve (non-return valve)
**Purpose:** to allow flow in one direction only and automatically prevent reverse flow.
- On the delivery pipe of a pump, so that water does not run back to the pump when it stops, which prevents damage and loss of prime.
- On pipes leaving a reservoir or tank, and in the house connection to prevent back-siphoning of dirty water.

```text
  flow ->  _____hinge
  =======|\      |=======
         | \ flap|
  =======|__\____|=======
  Flap opens by forward flow, shuts by reverse flow
```

#### Air relief valve
**Purpose:** to release the air trapped at the high points of the pipeline, and to admit air to prevent a vacuum when the pipe is emptied.
- Air in the line reduces the discharge, causes air-lock, pressure surges and vibration.

```text
       air out
        _|_
       | o | float ball
   ====|___|====  pipe crest
```
When air collects in the body, the float drops and opens the outlet; once the air has gone, the water lifts the float and closes the outlet.

## Check yourself

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

1. What is the purpose of a reflux valve?
   A. Allow flow in one direction only
   B. Relieve excess pressure
   C. Release air
   D. Regulate flow
   Answer: A. A reflux valve prevents reverse flow. (Key point: Reflux purpose)

2. Why is it fitted on a pump delivery pipe?
   A. To stop water running back to the pump when it stops
   B. To filter water
   C. To increase head
   D. To release air
   Answer: A. It prevents back flow, damage and loss of prime. (Key point: Reflux location)

3. What is an air relief valve for?
   A. Release trapped air at high points and admit air to avoid vacuum
   B. Drain the line
   C. Prevent back flow
   D. Limit pressure
   Answer: A. It removes air at crests and prevents vacuum on emptying. (Key point: Air valve purpose)

4. How does the reflux flap behave?
   A. Operated by a hand wheel
   B. Opens by reverse flow
   C. Stays open
   D. Opens by forward flow, shuts by reverse flow
   Answer: D. The hinged flap lifts for forward flow and falls shut on reversal. (Key point: Reflux working)

## Related in Water Supply Engineering

- [Cutoff valve and reflux valve](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Acutoff-reflux-valves) — Water Supply Engineering, chapter 9
- [Purpose of valves; reflux valve](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Avalves-purpose-reflux) — Water Supply Engineering, chapter 9
- [Flanged joint and reflux valve](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aflanged-joint-reflux-valve) — Water Supply Engineering, chapter 9
- [Reflux valve with sketch](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Areflux-valve) — Water Supply Engineering, chapter 9
- [Air valve: function and working](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Aair-valve) — Water Supply Engineering, chapter 9, asked 3×
- [Pressure relief valves](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Apressure-relief-valve) — Water Supply Engineering, chapter 9, asked 2×

## Related in other subjects

- [Pressure variation after sudden valve closure](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Awater-hammer-pressure-time-valve-and-midpoint) — Hydraulics, chapter 4
- [Purposes of spillway; types of gates](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aspillway-purposes-and-gate-types) — Hydropower Engineering, chapter 4
- [Why joints are necessary; expansion joint](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ajoints-why-expansion) — Building Technology, chapter 10, asked 2×