What is surge tank?

When the valve in a hydroelectric power plant is suddenly completely closed, because of its small inertia the water in the penstock stops almost at once. The water in the pipeline, with large inertia retards slowly. The difference in flows between pipeline and penstock causes a rise in the water level in the surge tank. The water level rises above the static level of the reservoir water, producing a counter-pressure so that water in the pipeline flows towards the reservoir and the level of water in the surge tank drops. In the absence of damping, oscillation would continue indefinitely with the same amplitude.

The flow into the surge tank and water level in the tank at any time during the oscillation depends on the dimension of the pipeline and tank and on the type of valve movement.