---
title: "Water hammer in penstock and surge tank"
subject: "Power Plant Equipment"
chapter: 1
key: "semester_7/power_plant_equipment:water-hammer-surge-tank"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/water-hammer-surge-tank
site: BCT Engineering
---

# Water hammer in penstock and surge tank

Power Plant Equipment · Chapter 1: Hydro Power Plant

- Asked in: 2071 Shrawan, 8 marks; 2072 Chaitra, 4 marks (asked 2 times)
- Video: [Why Does Water Shoot Up? The Secret of Surge Shafts](https://www.youtube.com/watch?v=N5MIO521fAs) — Hydro Vision
- 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-surge-tank
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-1

## Question

Explain the water hammer effect in penstock pipe and advantages of surge tank of hydropower plant.

## Short answer

Water hammer is a sudden pressure rise in a penstock when flow velocity changes quickly, risking pipe burst. A surge tank, placed near the powerhouse, reflects pressure waves, reducing water hammer. It also supplies water on load increase, stores water on rejection, improves speed regulation, and allows faster gate operation.

## 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.

#### Surge tank

A **surge tank** is an open-top vertical tank (or shaft) connected to the conduit just before the penstock, close to the power house. Its water level rises and falls with the load.

```text
 Reservoir     low-pressure tunnel     surge tank
 ~~~~~~~ |____________________________ | ~~~~ |
                                       |      |
                                       |__  __|
                                          \ penstock
                                           \
                                            [turbine]
```

**Advantages of a surge tank**
1. **Reduces water hammer:** the free surface reflects the pressure wave, so only the short penstock between the tank and the turbine is exposed to high pressure. The long tunnel is protected and can be thinner and cheaper.
2. **On load rejection:** when the gates close, the water coming down the tunnel enters the tank and its level rises, storing water instead of creating high pressure.
3. **On load increase:** the tank supplies the extra water immediately until the tunnel flow accelerates. So the turbine is not starved of water and the governor responds faster.
4. Reduces the effective water starting time ($T_w$), giving better speed regulation and stability.
5. Reduces pressure fluctuation in the tunnel and prevents negative pressure and collapse.
6. Allows faster gate operation and therefore better frequency control.

**Types:** simple tank, restricted orifice (throttled) tank, differential tank, closed (air-cushion) tank and inclined tank.

## Check yourself

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

1. In water hammer, what happens to the kinetic energy of the water column when the gates suddenly close?
   A. It is converted to heat in the turbine only
   B. It is converted into pressure energy producing a pressure wave
   C. It is stored in the governor
   D. It disappears with no effect
   Answer: B. The stopped water column converts kinetic energy into pressure energy, sending a wave up and down the pipe. (Key point: Cause of water hammer)

2. Which is a harmful effect of negative pressure in the penstock?
   A. Pipe collapse or cavitation
   B. Increase of turbine efficiency
   C. Reduced vibration
   D. Faster governor response
   Answer: A. Very high pressure can burst the pipe, while negative pressure can collapse it or cause cavitation. (Key point: Effects of water hammer)

3. What happens in the surge tank when the gates close on load rejection?
   A. Its level falls and it supplies extra water
   B. Water from the tunnel enters the tank and its level rises, instead of creating high pressure
   C. It empties completely and shuts the tunnel
   D. Nothing; the level stays fixed
   Answer: B. Water decelerating in the tunnel is stored in the tank, so the pressure rise is avoided in the tunnel. (Key point: Surge tank on load rejection)

4. How does the surge tank help when load suddenly increases?
   A. It reduces turbine flow
   B. It raises the penstock pressure above design
   C. It supplies extra water at once while the long tunnel column accelerates, reducing starting time Tw effect
   D. It increases the length of the pipe to be accelerated
   Answer: C. The tank supplies water immediately, so the turbine does not starve and governing is faster and more stable. (Key point: Surge tank on load increase)

## Related in Power Plant Equipment

- [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
- [Water hammer and how to minimize it](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Awater-hammer-minimize) — Power Plant Equipment, chapter 1, asked 4×
- [Steady state operation of hydropower plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Ahydropower-steady-state-operation) — 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×

## Related in other subjects

- [Surge and water hammer: 25 m³/s, 15 m surge tank](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-25cumec-2-penstocks-15m-tank) — Hydropower Engineering, chapter 6, asked 3×
- [Surge and water hammer: 45 m³/s, 20 m surge tank](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-45cumec-3-penstocks) — Hydropower Engineering, chapter 6, asked 2×
- [Surge and water hammer: 60 m³/s, 30 m surge tank](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-numerical-60cumec-3-penstocks-30m-tank) — Hydropower Engineering, chapter 6, asked 2×
- [Surge tank objectives and economic penstock diameter factors](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asurge-tank-objectives-economic-diameter) — Hydro Power, chapter 5