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

# Reflux valve with sketch

Water Supply Engineering · Chapter 9: Valves and Fittings

- Asked in: 2080 Bhadra, 4 marks
- Video: [Check /Reflux Valve/ Non Returning Valves(NRV)#watertank #civilengineering #valve #jalnigam](https://www.youtube.com/watch?v=wTEldnugO6E) — Sikandar Civil Craft
- 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-valve
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-9

## Question

Why is a reflux valve necessary? Describe it with a neat sketch.

## Short answer

Without a reflux valve, backflow would damage the pump, empty the rising main, cause water hammer, and allow pollution. A reflux (non-return) valve is a cast-iron body with a hinged flap that opens for forward flow and closes automatically when flow reverses, preventing backflow.

## Answer

#### Why a reflux valve is necessary
Without a reflux valve, water flows back when pumping stops or the pressure fails. This makes the pump run backwards (damage), empties the rising main and loses prime, creates water hammer, and allows polluted water to flow back into the supply. A reflux valve prevents these by closing automatically on reverse flow.

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. What happens without a reflux valve when pumping stops?
   A. Pressure rises permanently
   B. Water flows back, pump runs backwards and prime may be lost
   C. Pump speeds up forward
   D. Nothing
   Answer: B. Back flow can damage the pump, empty the main and cause water hammer. (Key point: Need)

2. What is the main moving part?
   A. A wedge gate
   B. A float ball
   C. A hinged flap resting on a seat
   D. A spring plug
   Answer: C. A flap hinged at the top rests on the seat. (Key point: Construction)

3. What closes the flap on reversal?
   A. Hand wheel
   B. Float
   C. Flap weight and back pressure
   D. Key
   Answer: C. Weight and back pressure press it onto the seat. (Key point: Closure)

4. Where else is it used?
   A. Only on sewers
   B. Foot of a suction pipe
   C. Only at the source
   D. At tap outlets only
   Answer: B. A foot valve is a reflux valve at the foot of suction pipe. (Key point: Foot valve)

## 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×
- [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