---
title: "Surge tank functions and design criteria"
subject: "Hydro Power"
chapter: 5
key: "semester_6/hydro_power:surge-tank-functions-design"
source: https://www.bctengineer.com/q/semester_6/hydro_power/surge-tank-functions-design
site: BCT Engineering
---

# Surge tank functions and design criteria

Hydro Power · Chapter 5: Components of Hydropower System

- Asked in: 2077 Chaitra, 6 marks; 2078 Chaitra, 2 marks (asked 2 times)
- Video: [Surge Tank in Hindi || What is Surge Tank || Surge Tank kya hota hai](https://www.youtube.com/watch?v=6VQUmrWxJlw) — Gear Institute Mechanical Engineering Videos and Physics by Gear Institute
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/hydro_power/surge-tank-functions-design
- Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-5

## Question

Explain about surge tank, its functions and design criteria with necessary sketch.

## Short answer

A surge tank is an open shaft at the tunnel-penstock junction that provides a free surface to absorb pressure surges. Its main functions are reducing water hammer, handling load changes, and improving governing. Design criteria include upsurge/downsurge levels, Thoma stability area with safety factor 1.5–1.8, and oscillation period avoiding resonance.

## Answer

A **surge tank** is an open vertical shaft or chamber connected to the pressure conduit (headrace tunnel) at its junction with the penstock, as near the powerhouse as possible. It provides a free water surface that absorbs pressure surges caused by sudden load changes.

```text
  reservoir          surge tank
  ~~~~~~~~~        |  ^   |  max upsurge
  |                |  :   |
  | static WL -----|--:---|-------
  |                |  v   |  min downsurge
  |  headrace tunnel (L, A)|
  |========================+
                            \  penstock
                             \
                              \==> turbine
```

#### Functions
1. **Reduces water hammer pressure:** the water hammer wave is reflected at the tank, so only the short penstock below it is designed for high water hammer; the long tunnel is protected.
2. **Load rejection:** when turbine gates close, the moving water in the tunnel rises into the tank instead of building up pressure.
3. **Load acceptance:** when gates open, the tank supplies water immediately until the tunnel flow accelerates, preventing a large pressure drop.
4. **Improves governing** of the turbine (better speed regulation).
5. Acts as a small reservoir and allows the tunnel to be lighter and cheaper.

#### Types
Simple (cylindrical), restricted orifice (throttled), differential (Johnson), one-sided/gallery or chamber type, inclined, and air-cushion surge chambers.

#### Design criteria
1. **Location:** as close as possible to the powerhouse, on sound rock, to reduce penstock length.
2. **Maximum upsurge** on full load rejection (frictionless estimate):

$$
Z_{max} = V\sqrt{\frac{L A}{g A_s}}
$$

where V = tunnel velocity, L = tunnel length, A = tunnel area, $A_s$ = tank area. The top of the tank must be above static level + upsurge + freeboard (or an overflow is provided).

3. **Minimum downsurge** on load acceptance: the bottom of the tank and the tunnel crown must remain below the lowest water level so that air does not enter the penstock.
4. **Stability (Thoma criterion):** to damp oscillations, area must exceed

$$
A_{th} = \frac{L A V^2}{2 g H_0 h_f}
$$

($H_0$ = net head, $h_f$ = tunnel head loss); a factor of safety of 1.5–1.8 is used.

5. **Period of oscillation:** $T = 2\pi\sqrt{\dfrac{L A_s}{g A}}$, should not resonate with the governor.
6. **Structural and economic:** shaft lining for rock conditions; choose the type (orifice, differential) to reduce size and cost.

## Check yourself

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

1. On full load rejection, what happens in a surge tank?
   A. Water in the tunnel rises into the tank instead of causing high pressure
   B. Pressure rises sharply in the tunnel
   C. The tunnel empties
   D. Air enters the penstock
   Answer: A. The moving water flows into the tank, absorbing the surge. (Key point: Surge tank function)

2. The frictionless maximum upsurge is:
   A. Z = V/sqrt(g)
   B. Z = L A V^2 /(2 g H h_f)
   C. Z = V sqrt(L A / (g A_s))
   D. Z = 2 pi sqrt(L A_s/(g A))
   Answer: C. Z_max = V sqrt(LA/(g A_s)), V tunnel velocity, A_s tank area. (Key point: Max upsurge formula)

3. The Thoma criterion says that for stable oscillations the tank area must be:
   A. Greater than A_th, usually with safety factor 1.5-1.8
   B. Less than A_th = L A V^2 / (2 g H0 h_f)
   C. Equal to tunnel area
   D. Independent of tunnel length
   Answer: A. Tank area above the Thoma area damps oscillations. (Key point: Thoma criterion)

4. Why check minimum downsurge on load acceptance?
   A. To increase the upsurge
   B. To size the spillway
   C. To ensure air does not enter the penstock
   D. To reduce tank period
   Answer: C. The level must stay above the tunnel crown/penstock entry so no air is drawn in. (Key point: Downsurge)

## Related in Hydro Power

- [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
- [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
- [Forebay and surge tank comparison](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Aforebay-vs-surge-tank) — Hydro Power, chapter 5, asked 2×
- [Desander and its design criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adesander-design-criteria) — Hydro Power, chapter 5, asked 4×
- [Settling basin design principle and components](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asettling-basin-principle-components) — Hydro Power, chapter 5

## Related in other subjects

- [Functions of a surge tank](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Asurge-tank-functions) — Hydraulics, chapter 4, asked 2×
- [Surge tank functions and formulas](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-tank-functions-formulas) — Hydropower Engineering, chapter 6
- [Hydraulic design of surge tank and pressure tunnel](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-tank-hydraulic-design-pressurised-tunnel) — Hydropower Engineering, chapter 6
- [Undersluice functions and design criteria](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Aundersluice-function-criteria) — Irrigation and Drainage Engineering, chapter 5, asked 2×