Surge tank functions and design criteria

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.

  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.