Function of surge tank in hydro plant

A surge tank is an open-top vertical tank or shaft placed on the water conduit between the low-pressure tunnel and the penstock, as near to the power house as possible. Its water level rises and falls as the turbine load changes.

 Reservoir   headrace tunnel    surge tank
 ~~~~~~ |______________________ |  ~~~~  |
                               |        |
                               |__    __|
                                  \ penstock
                                   \__[turbine]

Functions:

1. Protects against water hammer: when the turbine gates close suddenly (load rejection), the water in the tunnel flows into the tank and the level rises. The pressure wave is reflected at the free surface, so only the short penstock sees high pressure.
2. Supplies water on load increase: when gates open suddenly, the tank gives the extra water at once (its level falls) while the long tunnel column accelerates slowly.
3. Reduces the effective length of pipe whose water must be accelerated, i.e. reduces water starting time $T_w$, which improves governing and stability.
4. Allows the tunnel to be designed for lower pressure, saving cost.
5. Prevents negative pressure and vacuum in the conduit when the load increases.

Types: simple, restricted orifice, differential, inclined and closed (air cushion) surge tanks.