Restricted orifice vs simple cylindrical surge tank

In a simple cylindrical surge tank the tank is open to the tunnel through a full-size connection, so the tank level responds freely and the oscillations decay slowly, with high upsurge and deep downsurge. In a restricted orifice tank, the connection to the tunnel is narrowed by an orifice of small area.

Why the restricted orifice type is more efficient:

• Damping: when the water flows in or out of the tank, the orifice creates an extra head loss $h_o=\dfrac{Q^2}{2gC_d^2A_o^2}$. This opposes the motion, so the oscillation dies out sooner and the amplitude of the maximum upsurge and minimum downsurge is smaller.
• Quick action: on a load change, the orifice head loss at once builds a head difference between the tunnel and the tank, so the tunnel flow is retarded sooner and less water has to be stored or supplied. The tank can therefore be smaller (less volume and cost) for the same permissible surge.
• Better stability of the governed system, thanks to the greater damping.

A too-small orifice produces a high water-hammer pressure in the tunnel (it behaves almost like a closed valve), so the area is selected as a compromise (typically about 0.25-0.5 of the tunnel area, giving a head loss of the order of the surge height).