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

# Pressure relief valves

Water Supply Engineering · Chapter 9: Valves and Fittings

- Asked in: 2069 Chaitra, 1+3 marks; 2066 Bhadra (old course) (asked 2 times)
- Video: [Pressure Relief Valves (Types and Working Principle)](https://www.youtube.com/watch?v=n3xcv1BKbrc) — AMJ Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/water_supply_engineering/pressure-relief-valve
- Whole chapter: https://www.bctengineer.com/md/semester_5/water_supply_engineering/chapter-9

## Question

Why are pressure relief valves necessary? Describe with a neat sketch.

## Short answer

Pressure relief valves protect pipelines from water hammer and excessive pressure by automatically opening at a set limit to discharge water. A spring-loaded valve lifts off its seat when pressure exceeds the setting, then closes when pressure drops, preventing bursts.

## Answer

#### Why pressure relief valves are necessary
Sudden closure of valves, the stopping or starting of pumps, or any rapid change in flow produces **water hammer** (a high-pressure wave), and pressure can also become too high in low-lying parts or in pumping mains. High pressure may burst the pipes, joints and fittings, or damage pumps. A pressure relief (safety) valve opens automatically when the pressure goes above a set limit, discharges the excess water and protects the pipeline. It is also used on the delivery side of pumps and where water hammer is expected.

#### Construction and working
```text
          adjusting screw
              |
           [spring]
              |
        ___ [plug] ___
   =====|__ valve seat __|===== pipe
               |
          discharge pipe -> drain
```
- A spring-loaded (or dead-weight) valve sits on a seat on a branch connected to the pipeline.
- The spring is adjusted to the maximum allowed pressure.
- When the pressure in the pipe exceeds this value, it lifts the valve off its seat and water escapes through the discharge pipe, so that the pressure goes down.
- When the pressure falls below the set value, the spring closes the valve again.
- A relief valve is placed near the pump delivery or at the points of high pressure; the discharge is led to a drain.

## Check yourself

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

1. What causes water hammer?
   A. Low flow
   B. Sudden valve closure or pump stop/start
   C. Cold water
   D. Slow valve closing
   Answer: B. Sudden flow changes create a high-pressure wave. (Key point: Water hammer)

2. When does a pressure relief valve open?
   A. When pressure exceeds a set limit
   B. When the pipe is empty
   C. When pressure is zero
   D. At all times
   Answer: A. It lifts automatically above its set pressure and discharges water. (Key point: Valve action)

3. What does the spring adjustment set?
   A. Minimum pressure
   B. Flow direction
   C. Maximum allowed pressure
   D. Pipe diameter
   Answer: C. The spring is set to the maximum permitted pressure. (Key point: Spring)

4. What happens when pressure falls below the set value?
   A. Pipe drains
   B. Valve stays open
   C. Spring closes the valve again
   D. Valve bursts
   Answer: C. The spring reseats the valve. (Key point: Closing)

## 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×
- [Break pressure tank: need and construction](https://www.bctengineer.com/question.md?key=semester_5%2Fwater_supply_engineering%3Abreak-pressure-tank) — Water Supply Engineering, chapter 9, asked 9×

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