---
title: "Function of surge tank in hydro plant"
subject: "Power Plant Equipment"
chapter: 1
key: "semester_7/power_plant_equipment:surge-tank-function"
source: https://www.bctengineer.com/q/semester_7/power_plant_equipment/surge-tank-function
site: BCT Engineering
---

# Function of surge tank in hydro plant

Power Plant Equipment · Chapter 1: Hydro Power Plant

- Asked in: 2082 Baisakh, 5 marks
- 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/surge-tank-function
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_plant_equipment/chapter-1

## Question

Explain the function of surge tank in a hydro power plant.

## Short answer

A surge tank is an open-top shaft placed between tunnel and penstock that reflects pressure waves during load rejection to protect the penstock, supplies immediate water when gates open suddenly, and reduces the effective length of water column to improve governing and stability.

## Answer

A **surge tank** is an open-top vertical tank or shaft placed on the water conduit between the low-pressure tunnel and the penstock, as near to the power house as possible. Its water level rises and falls as the turbine load changes.

```text
 Reservoir   headrace tunnel    surge tank
 ~~~~~~ |______________________ |  ~~~~  |
                               |        |
                               |__    __|
                                  \ penstock
                                   \__[turbine]
```

**Functions:**
1. **Protects against water hammer:** when the turbine gates close suddenly (load rejection), the water in the tunnel flows into the tank and the level rises. The pressure wave is reflected at the free surface, so only the short penstock sees high pressure.
2. **Supplies water on load increase:** when gates open suddenly, the tank gives the extra water at once (its level falls) while the long tunnel column accelerates slowly.
3. **Reduces the effective length** of pipe whose water must be accelerated, i.e. reduces water starting time $T_w$, which improves governing and stability.
4. Allows the tunnel to be designed for lower pressure, saving cost.
5. Prevents negative pressure and vacuum in the conduit when the load increases.

**Types:** simple, restricted orifice, differential, inclined and closed (air cushion) surge tanks.

## Check yourself

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

1. Where is a surge tank placed?
   A. At the reservoir intake far from the power house
   B. Inside the draft tube
   C. On the water conduit between the low-pressure tunnel and penstock, close to the power house
   D. At the tailrace outlet
   Answer: C. Placing it near the power house shortens the pipe exposed to hammer. (Key point: Location of surge tank)

2. Why does the surge tank protect the tunnel from water hammer?
   A. Its free surface reflects the pressure wave, so only the short penstock sees high pressure
   B. It absorbs the wave by heating the water
   C. It increases the pipe wall thickness
   D. It slows down the turbine speed
   Answer: A. The wave is reflected at the free surface, confining high pressure to the short penstock. (Key point: Water hammer protection)

3. What does the surge tank do to the water starting time Tw?
   A. Increases it
   B. Reduces it by reducing the effective length of water to be accelerated
   C. Makes it infinite
   D. Has no effect
   Answer: B. A shorter column to accelerate means smaller Tw, which improves governing and stability. (Key point: Water starting time)

## 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 reactor in power system](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Areactor-function) — Power Plant Equipment, chapter 4, asked 3×
- [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
- [Advantages of combined power plant](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Acombined-advantages) — Power Plant Equipment, chapter 8

## Related in other subjects

- [Forebay function and surge tank types](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aforebay-function-surge-tank-types) — Hydro Power, chapter 5
- [Pumped storage plant: power, turbine, setting, surge](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apumped-storage-turbine-surge-numerical) — Hydropower Engineering, chapter 7
- [Surge tank functions and design criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asurge-tank-functions-design) — Hydro Power, chapter 5, 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