---
title: "Water hammer and how to minimize it"
subject: "Power Plant Equipment"
chapter: 1
key: "semester_7/power_plant_equipment:water-hammer-minimize"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/water-hammer-minimize
site: BCT Engineering
---

# Water hammer and how to minimize it

Power Plant Equipment · Chapter 1: Hydro Power Plant

- Asked in: 2076 Chaitra, 5 marks; 2079 Bhadra, 5 marks; 2071 Chaitra, 3 marks; 2080 Bhadra, 4 marks (asked 4 times)
- Video: [What is Water Hammer?](https://www.youtube.com/watch?v=xoLmVFAFjn4) — Practical Engineering
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_plant_equipment/water-hammer-minimize
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-1

## Question

What do you mean by water hammer in context of hydro power plant? How it can be minimized?

## Short answer

Water hammer is a sudden pressure rise in a penstock caused by rapid velocity change, like quick gate closure. It is minimized by a surge tank, slow gate closure, relief valves, jet deflectors, and a strong penstock with air valves.

## Answer

**Water hammer** is the sudden rise (or fall) of pressure in a penstock when the velocity of water is changed quickly, for example when the turbine gates close quickly after a load rejection. The moving column of water is suddenly stopped; its kinetic energy is converted into pressure energy, and a pressure wave travels up and down the pipe with a hammering effect.

**Magnitude (Joukowsky relation):** for instantaneous or rapid closure ($t_c \le 2L/a$),
$$
\Delta p = \rho\, a\, \Delta v \quad\text{or}\quad \Delta H = \frac{a\,\Delta v}{g}
$$
where $a$ = speed of the pressure wave in the pipe (about 1000–1400 m/s), $\Delta v$ = change in velocity, $L$ = pipe length. For slow (gradual) closure ($t_c > 2L/a$) the rise is smaller. Rigid-column theory gives $\Delta p \approx \rho L \Delta v / t_c$; Michaud's formula, which allows for the reflected pressure wave, gives about $2\rho L \Delta v / t_c$.

```text
 Reservoir
 ~~~~~~~ |
         |\  penstock (length L)
         | \
         |  \  pressure wave
         |   \  <---->
         |    \______[gate]--turbine
                      closes
```

**Effects:** very high pressure can burst the penstock and damage valves and the turbine casing; negative pressure on opening can collapse the pipe or cause cavitation; it causes vibration and noise; and it makes speed governing more difficult.

#### Methods to minimize water hammer
1. **Surge tank** near the power house: gives a free water surface close to the turbine, so the length of pipe in which water hammer occurs is reduced and the pressure wave is reflected from the tank.
2. **Slow closure of gates:** make the closing time longer than $2L/a$ (governor relief/time setting).
3. **Relief valve / pressure relief valve** on the penstock or casing, which opens when the pressure rises and lets water escape.
4. **Jet deflector (Pelton turbines):** the jet is deflected away from the buckets at once, while the needle closes slowly.
5. **Bypass / synchronous bypass valve**, which opens as the gate closes so that the penstock flow does not stop suddenly.
6. **Strong penstock** designed for the expected surge pressure, and air valves to prevent vacuum collapse.
7. Keep the penstock as short as possible and the water velocity low (larger diameter).

## Check yourself

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

1. What causes water hammer in a penstock?
   A. Slow opening of the reservoir intake
   B. Air leaking into the draft tube
   C. Sudden change of water velocity, e.g. gates closing fast after load rejection
   D. A steady rise in reservoir level
   Answer: C. When the moving water column is suddenly stopped, its kinetic energy becomes pressure energy and a pressure wave travels along the pipe. (Key point: Definition of water hammer)

2. For rapid closure (t_c <= 2L/a), the pressure rise is given by:
   A. Delta p = rho a Delta v
   B. Delta p = rho g L
   C. Delta p = rho L Delta v / a
   D. Delta p = Delta v / (rho a)
   Answer: A. The Joukowsky relation gives Delta p = rho a Delta v, i.e. Delta H = a Delta v / g, for closure faster than 2L/a. (Key point: Joukowsky relation)

3. Which closing time of the gate reduces water hammer?
   A. Shorter than 2L/a
   B. Equal to zero
   C. Longer than 2L/a
   D. Independent of 2L/a
   Answer: C. Closing in more than 2L/a lets the reflected wave return before closure ends, so the pressure rise is smaller. (Key point: Slow closure)

4. How does a surge tank near the power house reduce water hammer?
   A. It increases the wave speed in the pipe
   B. It gives a free water surface that reflects the wave, shortening the pipe exposed to hammer
   C. It increases the velocity of water in the penstock
   D. It closes the gate faster
   Answer: B. The free surface reflects the pressure wave, so only the short penstock between tank and turbine sees the high pressure. (Key point: Surge tank and relief devices)

## Related in Power Plant Equipment

- [Water hammer in penstock and surge tank](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Awater-hammer-surge-tank) — Power Plant Equipment, chapter 1, asked 2×
- [Operation of pumped storage hydro plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Apump-storage-operation) — Power Plant Equipment, chapter 1, asked 2×
- [Function of surge tank in hydro plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Asurge-tank-function) — Power Plant Equipment, chapter 1
- [Combined plant heating feed water with exhaust](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-feedwater-layout) — Power Plant Equipment, chapter 8, asked 2×
- [Layout of diesel power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Adiesel-plant-layout) — Power Plant Equipment, chapter 5, asked 3×
- [Reheat gas turbine power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Agt-reheat-plant) — Power Plant Equipment, chapter 6

## Related in other subjects

- [Water hammer in 250 m penstock](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Awater-hammer-250m-penstock) — Hydro Power, chapter 1
- [Water hammer in 200 mm pipe](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Awater-hammer-200mm-pipe) — Hydro Power, chapter 1
- [Water hammer in elastic cast-iron pipe](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Awater-hammer-cast-iron-pipe) — Hydro Power, chapter 1
- [Water hammer effects and gradual closure](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Awater-hammer-gradual-closure) — Hydro Power, chapter 1