---
title: "Purpose of valves; reflux valve"
subject: "Water Supply Engineering"
chapter: 9
key: "semester_5/water_supply_engineering:valves-purpose-reflux"
source: https://www.bctengineer.com/q/semester_5/water_supply_engineering/valves-purpose-reflux
site: BCT Engineering
---

# Purpose of valves; reflux valve

Water Supply Engineering · Chapter 9: Valves and Fittings

- Asked in: 2073 Chaitra, 4 marks
- 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/valves-purpose-reflux
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-9

## Question

What are the purposes of valves in a pipeline? With a neat sketch, describe a reflux valve.

## Short answer

Valves isolate, regulate, prevent backflow, release air, relieve pressure, and drain lines. A reflux (non-return) valve allows one-way flow: a hinged flap opens by forward flow and closes by weight and back pressure when flow stops or reverses, preventing backflow.

## Answer

#### Purposes of valves in a pipeline
Valves are fitted in a pipeline to:
1. start, stop or isolate the flow (for repair, cleaning, extension of the line),
2. regulate the discharge and the pressure,
3. prevent back flow (reflux valve),
4. release trapped air (air valve) or relieve excess pressure (safety valve),
5. drain the line (washout or scour valve).

#### Reflux valve
A reflux valve (also called non-return or check valve) allows water to flow in one direction only and closes automatically when the flow tries to reverse.

```text
   flow ->   _______hinge
   =========|\       |=========
            | \ flap |   (flap lifts for
   =========|__\_____|=========  forward flow,
        valve seat           falls shut on reversal)
```
- **Construction:** a cast-iron body with a flap (door or disc) hinged at the top, which rests on a seat. Forward flow pushes the flap open; when flow stops or reverses, the weight of the flap and the back pressure press it onto the seat.
- **Uses:** on the delivery side of a pump, to prevent back flow when the pump stops and to protect the pump from water hammer; on pipes from a reservoir that must not be drained back; at the foot of a suction pipe (foot valve); on rising mains and at service connections to avoid back siphonage.

## Check yourself

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

1. Which is NOT a purpose of valves in a pipeline?
   A. Isolate flow for repair
   B. Drain the line
   C. Increase pipe strength
   D. Regulate discharge
   Answer: C. Valves control, isolate, drain, release air and prevent back flow; they do not strengthen pipes. (Key point: Valve purposes)

2. What does a reflux valve do?
   A. Drains the line
   B. Allows flow in one direction only
   C. Regulates flow
   D. Releases air
   Answer: B. It automatically blocks reverse flow. (Key point: Reflux function)

3. How does the flap close?
   A. By screw spindle
   B. By float
   C. By hand wheel
   D. By weight and back pressure on reversal
   Answer: D. Reverse flow and weight seat the flap. (Key point: Reflux construction)

## Related in Water Supply Engineering

- [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
- [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×
- [Reflux valve with sketch](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Areflux-valve) — Water Supply Engineering, chapter 9
- [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
- [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

## Related in other subjects

- [Water hammer in 20 m steel pipeline with valve](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Awater-hammer-20m-pipeline-pressure-time-curves) — Hydraulics, chapter 4
- [Valve timing diagram of four-stroke engine](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Avalve-timing-diagram-four-stroke) — Internal Combustion Engines, chapter 4