---
title: "Functions of a surge tank"
subject: "Hydraulics"
chapter: 4
key: "semester_4/hydraulics:surge-tank-functions"
source: https://www.bctengineer.com/q/semester_4/hydraulics/surge-tank-functions
site: BCT Engineering
---

# Functions of a surge tank

Hydraulics · Chapter 4: Unsteady flow in pipes

- Asked in: 2079 Chaitra, 2 marks; 2078 Chaitra, 2 marks (asked 2 times)
- 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-functions
- Whole chapter: https://www.bctengineer.com/md/semester_4/hydraulics/chapter-4

## Question

What are the functions (primary and secondary purposes) of a surge tank?

## Short answer

Primary: reflects water hammer pressure waves, protecting most of the conduit; stores or delivers water to accommodate abrupt flow changes. Secondary: improves turbine speed regulation by immediate water supply, lowers pressure rise in long pipes allowing lighter design, and provides a nearly constant-head reservoir during transients.

## Answer

A surge tank is an open (or air-cushioned) storage chamber connected to a pressure conduit (penstock or tunnel) near its downstream end, to control the pressure changes caused by sudden changes of flow.

**Primary purposes**
1. **Reduce water hammer:** the pressure wave is reflected at the tank, so only the short length of pipe between the tank and the valve or turbine is subject to water hammer, instead of the whole conduit up to the reservoir.
2. **Supply and absorb flow:** when the turbine load increases, the tank supplies the extra water immediately; when the load is rejected, it takes in the surplus water, so the long upstream conduit does not have to accelerate or decelerate rapidly.

**Secondary purposes**
- Improves the speed regulation of the turbine, because water for the changed load is available at once.
- Reduces the pressure rise in the long tunnel or penstock, which allows a lighter pipe design.
- Acts as a reservoir of water for the first moments after a change in demand, so the conduit upstream stays under a nearly steady head.

## Check yourself

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

1. Where is a surge tank connected to the conduit?
   A. At the upstream intake, away from the turbine
   B. Near the downstream end of the pressure conduit, close to the valve or turbine
   C. Only at the tailrace
   D. At the mid-point of the penstock always
   Answer: B. It is placed near the downstream end so it can shorten the length of conduit exposed to water hammer. (Key point: Where a surge tank is placed)

2. How does a surge tank reduce water hammer?
   A. It reflects the pressure wave, so only the short length between tank and valve sees water hammer
   B. It increases the wave celerity in the pipe
   C. It makes the valve close more quickly
   D. It removes all water from the penstock
   Answer: A. The wave is reflected at the tank, so the full conduit up to the reservoir is not involved. (Key point: Reducing water hammer)

3. What does the surge tank do when the turbine load is increased or rejected?
   A. Supplies extra water on load increase, and takes up surplus water on load rejection
   B. Absorbs water on load increase and supplies it on load rejection
   C. Does nothing to the flow in either case
   D. Stops the flow in the upstream conduit instantly
   Answer: A. This lets the long upstream conduit change its velocity gradually instead of rapidly. (Key point: Supply and absorb flow)

## Related in Hydraulics

- [Importance and types of surge tank](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Asurge-tank-importance-and-types) — Hydraulics, chapter 4

## Related in other subjects

- [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 functions and formulas](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asurge-tank-functions-formulas) — Hydropower Engineering, chapter 6
- [Primary versus secondary geological structures](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aprimary-versus-secondary-geological-structures) — Engineering Geology I, chapter 5, asked 4×
- [Primary and secondary structures; strike and dip](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aprimary-and-secondary-structures-strike) — Engineering Geology I, chapter 5, asked 2×