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.
2) 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.
3) 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.
4) 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.
5) 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.