Functions of a surge tank

A surge tank is an open (or air-cushioned) storage chamber connected to a pressure conduit (penstock or tunnel) near its downstream end, to control the pressure changes caused by sudden changes of flow.

Primary purposes

1. Reduce water hammer: the pressure wave is reflected at the tank, so only the short length of pipe between the tank and the valve or turbine is subject to water hammer, instead of the whole conduit up to the reservoir.
2. Supply and absorb flow: when the turbine load increases, the tank supplies the extra water immediately; when the load is rejected, it takes in the surplus water, so the long upstream conduit does not have to accelerate or decelerate rapidly.

Secondary purposes

• Improves the speed regulation of the turbine, because water for the changed load is available at once.
• Reduces the pressure rise in the long tunnel or penstock, which allows a lighter pipe design.
• Acts as a reservoir of water for the first moments after a change in demand, so the conduit upstream stays under a nearly steady head.