---
title: "Importance and types of surge tank"
subject: "Hydraulics"
chapter: 4
key: "semester_4/hydraulics:surge-tank-importance-and-types"
source: https://www.bctengineer.com/q/semester_4/hydraulics/surge-tank-importance-and-types
site: BCT Engineering
---

# Importance and types of surge tank

Hydraulics · Chapter 4: Unsteady flow in pipes

- Asked in: 2070 Bhadra, 1.5+1.5 marks
- Video: [Surge tank|Surge tank and its types- in hydroelectric power plant](https://www.youtube.com/watch?v=WI-719pGuy4) — Fetch Solution
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/hydraulics/surge-tank-importance-and-types
- Whole chapter: https://www.bctengineer.com/md/semester_4/hydraulics/chapter-4

## Question

Explain the importance of surge tank. Describe the types of surge tank.

## Short answer

A surge tank protects the penstock from water hammer by reflecting pressure waves, supplies water during load increase, and dampens oscillations. Types include simple, restricted orifice, differential, closed pneumatic, overflow, and inclined tanks.

## Answer

#### Surge tank
A surge tank is a vertical open standpipe or chamber connected to the penstock or pressure tunnel near the powerhouse (as close to the turbine as possible). It provides a free water surface near the valve or turbine.

#### Importance
1. **Controls water hammer.** When the turbine gates close suddenly, the water column cannot stop at once, so it flows into the surge tank. The pressure wave is reflected at the tank instead of travelling back along the long tunnel, so the long conduit is protected from high pressure.
2. **Supplies water when the load increases.** When the gates open, the tank supplies water quickly until the water in the long tunnel has accelerated. This avoids a large pressure drop.
3. **Reduces pressure fluctuations** at the turbine, giving better speed regulation by the governor.
4. **Cheaper design.** The tunnel can be made lighter because it is not designed for water hammer pressure; only the short penstock beyond the tank is.
5. **Damps the surges.** By storing and releasing water, the tank lets the mass oscillations die out (they are damped by friction and by the tank area).

#### Types of surge tank
| Type | Description | Remark |
|---|---|---|
| Simple surge tank | Plain cylindrical tank connected directly to the pipe | Large oscillations; slow damping |
| Restricted orifice tank | Simple tank with a throttled (orifice) opening at the base | Orifice loss damps oscillations; limits rise and fall |
| Differential tank (Johnson) | Central riser inside a larger tank; the riser has ports at the base | Quick response from the riser; storage in the outer tank; most economical |
| Closed (pneumatic) tank | Air-tight tank with a compressed air cushion | Used when the site has no high ground |
| Overflow (spilling) tank | Top is provided with a spillway for excess surge | Limits the maximum rise |
| Inclined / tunnel-type / gallery tank | Inclined shaft or chamber with an expansion gallery | Used where a vertical shaft is not practical |

```text
 Simple      Restricted    Differential
  | |         | |          |  |  |  |
  | |         | |          |  |__|  |
  | |         | |          |  riser |
 -+ +-       -+ +- orifice -+ ports +-
  penstock
```

## Check yourself

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

1. The main purpose of a surge tank is to
   A. Reflect pressure waves near the turbine so the long conduit is protected from water hammer
   B. Reduce pipe diameter
   C. Store sediment
   D. Increase turbine speed
   Answer: A. The free surface reflects the wave, protecting the long tunnel from high pressure. (Key point: Main purpose)

2. When turbine gates open for more load, the surge tank
   A. Closes the penstock
   B. Takes water from the tunnel
   C. Supplies water quickly until the tunnel water accelerates
   D. Stops all flow
   Answer: C. It feeds the extra demand and avoids a large pressure drop. (Key point: Load increase)

3. The differential (Johnson) tank is characterised by
   A. An inclined shaft
   B. A central riser with ports inside a larger tank
   C. A spillway at the top
   D. An air cushion
   Answer: B. The riser responds quickly while the outer tank gives storage. (Key point: Differential tank)

4. What does a throttled orifice at the base of a simple tank do?
   A. Increases the speed of oscillation
   B. Raises the surge
   C. Removes the need for a tank
   D. Damps oscillations through orifice head loss
   Answer: D. Orifice losses dissipate energy and limit rise and fall. (Key point: Restricted orifice)

## Related in Hydraulics

- [Functions of a surge tank](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Asurge-tank-functions) — Hydraulics, chapter 4, 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 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×
- [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×
- [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×