If the flow is conveyed through the pressure headrace tunnel, then a tank is
provided near by the powerhouse at the entrance of the penstock pipes for the
release of any water hammer development due to sudden closer of the valve. This
tank is called as surge tank. Water hammer thus developed has the high surge
pressure due to the sudden momentum change of flow by the closer of valve and
may cause the burst of the penstock pipe, if the surge is not countered by any
means.  Either we have to increase the thickness of the penstock pipe or we
could provide the open place to release the surge developed. The surge thus
developed at the penstock pipe is compensated by the surge tank through the
oscillation of its water surface. This oscillation comes to rest by the
frictional resistance. In short the surge tank works to absorb the shock wave
developed from water hammer.

Beside this function, surge tank also serves as the storage tank. Stored water
could be utilized when plant has to switch the increased load and similarly
receive the flow when there is decrease in load.

Types of surge tank and their excellence;

  1. Simple surge tank:


It is very simple in design and efficiency is comparatively not much.

  1. Restricted-Orifice type:


In this type of surge tank the surge dampening rate is high due to the
constricted flow (minimize about 20-30% of surge amplitude as simple surge
tank). Here the orifice design is very much complex.

  1. Differential surge tank:


It is featured with double frictional contact surface, inside overflow of water
through the riser (quick rise in water and overflow), several orifice flow takes
place inside the tank. Its construction is very complicated however, offers very
efficient surge dampening. Its diameter is about 30% smaller than that of simple
surge tank. It functions quick supply of water to turbine during the start
through the riser pipe as compared than other surge tanks types.

  1. Expanded chamber:

Here the surge dampening occurs due to the vortex formation as a result of
irregular shape. It is featured to have much frictional contact surface even in
same height as that of simple surge tank.

  1. Overflow type:

In this type of surge tank the surge pressure is released with the overflow
water as there will be not much occurrence of down surge. To function it
perfectly its location area should have favorable topographical condition for
the overflow of water.