What is surge tank in hydro power plant?
A surge tank is a vertical open tank, provided just upstream of the penstock of the hydroelectric plant for eliminating water hammer action. It controls the sudden pressure change in the pipeline.
Surge tank diagram
Location of surge tank in hydro power plant
A surge tank is located upstream of the penstock of the hydroelectric power plant. It is generally placed very close to the power station. It is placed just above the ground. The surge tank is placed near the power house to reduce the length of the penstock pipe.
Function of surge tank in hydroelectric power plant
- To reduce the effect of water hammering effect. (the main function of a surge tank is to intercept and dampen the high-pressure wave and not to allow them in low-pressure systems i.e. power tunnels)
- To store water when the load is decreased (quick storage device)
- Temporary supply of water when the load on the turbine is suddenly increased (starting up the is phase).
Operation system of the surge tank
Under normal operating conditions, the flow through the pipeline is uniform and the pressure gradient is normal. The level of water in the surge tank is lower than that in the supply reservoir by an amount equal to frictional head loss. If the load on the turbine is suddenly decreased, the governing mechanism acts to decrease the flow accordingly.
However, the rate of flow in the main pipeline cannot drop to the required quantity of supply. Water is able to flow into the surge tank. This flow causes the level in the surge tank to rise and that decelerates the flow from the supply reservoir.
The usual water hammer phenomenon is limited to the short pipe length between the surge tank and the power plant.
When the power demand is suddenly increased, the valve is opened more and the surge tank supplies additional flow until the water in the supply main is accelerated.
The water level in the surge tank is lowered; the difference of head along is increased thereby accelerating the flow until the flow equals that required by the turbines. In the absence of a surge tank, the drop in pressure at the turbines could be excessive at the time of sudden demand.
Surge tank types
- Simple cylindrical surge tank
- Restricted orifice type
- Differential surge tank
a. Simple cylindrical surge tank
The simple surge tank is of uniform cross-section (cylindrical in shape) and opens to the atmosphere, acting as a reservoir.
The water flows into and out of the tank without appreciable loss of head. It is directly connected to the penstock and has an unrestricted opening into it and must be of adequate size so that it does not overflow.
b. Restricted orifice type
The orifice surge tank has a restricted opening at the base of the tank. The restriction increases the head loss and consequently, there is a large acceleration and deceleration of water in the penstock. At small and rapid load changes, the orifice surge tank is not very effective in speed regulation.
c. Differential surge tank
A differential surge tank is connected at its bottom to the supply pipe through a small vertical pipe called a riser.
The riser is provided with a number of ports at its bottom and it reaches the height of the surge tank, When there is a decrease in load on the turbine, the water rises fast in the riser and that provides a nuíck retarding effect.
Similarly when the load increases, the water flows out first from the riser and that provides enough water for the turbine. The differential surge tank is rapid in its action and has the additional advantages of having low-pressure rise and surge of limited magnitude.
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