A surge tank is a small reservoir or tank in which the water level rises or falls to reduce the pressure swing at the turbine inlet which are transmitted in a fully closed circuit. In a summarized way the surge tank in hydro power plant serves the following purposes.
- The main purpose to reduce the distance between surface and turbine thereby reducing the water hamming effect on penstock and also to protect the upstream tunnel from the high pressure rises.
Water hamming effect: it is defined as the change in pressure rapidly above or below normal pressure caused by sudden change in water flow through the pipe according to the turbine power demand.
- It serves as a supply tank to the turbine when the water in the pipe is accelerating during increased load conditions and as a storage tank when the water is decelerating during load rejections or reduced load conditions.
Types of surge tanks used in hydro power plants:
The following are the general types of surge tanks:
- Simple surge tank
Inclined surge tank
Gallery type surge tank
Restricted orifice surge tank
Differential surge tank
Simple surge tank:
Simple surge tank is a simple vertical pipe which is connected to penstock which supplies the water to the turbine generator. The surge tanks are built in high enough so that the water cannot overflow even under full load condition on the prime mover. Some cases the surge tanks can be over flowed to reduce the high pressure but this is not economical to use. Under some circumstances where there is no suitable space to construct high tank, the high of the tank should be increases with some support to obtain the required pressure at the inlet of the turbine.
Inclined surge tank
When the tank is inclined to the horizontal its effective water surface increases and therefore lesser height surge tank is required of the same diameter. If the high tank is used the diameter can be reduced for the same purpose. So the occupied space is less when inclined surge tank is used. But this type of tank is more costly than the normal surge tank. It is rarely used tank, mostly used when the topological conditions are in favor.
Gallery type (Expansion chamber type) surge tank:
This type of surge tank has an expansion tank at top and expansion gallery at the bottom which reduces the extreme pressure surges at the inlet of the turbine. In erecting this type of tank the care should be taken to the upper expansion chamber up above the dam level and lower gallery tank below the dam level.
Restricted orifice surge tank:
This is also called as throttled surge tank. This tank contains a throttle or orifice before the surge tank. The main objective of this orifice is to provide a friction loss when the water is flowing to or from the tank. When the load demand on the turbine is reduced, the water enters through the surge tank orifice which provides a friction to the water and hence the pressure surge of the water reduces. The size of the orifice can be designed according to the retarding head that it has to build in conduit. Normally the size is calculated by the amount of rise of the water in the surge tank when the gate valve is closed.
Differential surge tank:
A differential surge tank has a riser with a small hole at its lower end through which the water enters when the surge rises. In this way the surge pressure can be minimised at the prime mover inlet.