How does adding a 'surge tank' to a long pipe system help control sudden big pressure changes?

A surge tank, which is a vertical chamber or reservoir connected to a pipeline, controls sudden large pressure changes by acting as a buffer that absorbs and releases fluid, thereby mitigating the effects of 'water hammer'. Water hammer is a phenomenon caused by abrupt changes in fluid velocity, such as quick valve closure or pump shutdown in a long pipe system. The fluid's inertia, its inherent resistance to changes in motion, causes significant pressure waves to propagate through the pipe.
During a sudden increase in pressure, known as a 'positive pressure surge', for example, when a valve rapidly closes, the fluid's kinetic energy (energy of motion) cannot be instantly dissipated. This energy would convert into extremely high pressure within the rigid pipe. The surge tank provides an alternative path: the excess fluid flows into the tank, causing the water level to rise. This rising movement converts the fluid's kinetic energy into potential energy (stored energy due to height) and dissipates it through friction within the tank. This process effectively dampens the pressure wave, preventing it from reaching dangerously high levels in the main pipeline.
Conversely, during a sudden drop in pressure, known as a 'negative pressure surge', such as during a pump trip or a rapid increase in downstream demand, the fluid in the pipe can momentarily slow down or even reverse, creating a suction effect. This can lead to very low pressures, potentially below the fluid's vapor pressure, causing 'cavitation' – the formation and violent collapse of vapor bubbles that can damage pipes. The surge tank prevents this by instantly supplying fluid from its reservoir back into the main pipeline. The water level in the surge tank drops, filling the void created by the pressure drop and thereby maintaining the pressure above the cavitation limit.
In essence, the surge tank decouples the fluid's large inertia in the long pipe from rapid flow changes at specific points. It provides a readily available volume to either accept excess fluid during high-pressure events or supply fluid during low-pressure events, stabilizing pressure fluctuations and protecting the pipeline and associated equipment.