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Importance and 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.

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

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

TypeDescriptionRemark
Simple surge tankPlain cylindrical tank connected directly to the pipeLarge oscillations; slow damping
Restricted orifice tankSimple tank with a throttled (orifice) opening at the baseOrifice loss damps oscillations; limits rise and fall
Differential tank (Johnson)Central riser inside a larger tank; the riser has ports at the baseQuick response from the riser; storage in the outer tank; most economical
Closed (pneumatic) tankAir-tight tank with a compressed air cushionUsed when the site has no high ground
Overflow (spilling) tankTop is provided with a spillway for excess surgeLimits the maximum rise
Inclined / tunnel-type / gallery tankInclined shaft or chamber with an expansion galleryUsed where a vertical shaft is not practical
 Simple      Restricted    Differential
  | |         | |          |  |  |  |
  | |         | |          |  |__|  |
  | |         | |          |  riser |
 -+ +-       -+ +- orifice -+ ports +-
  penstock

✓ Answer checked by Claude Opus 5.5 (high effort)

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