From the Wikipedia Rabbit Hole Day Trip — Poulaphouca, Surge Tanks and Your Washing Machine
The Blue Ridge Dam and surge tank. Public domain image by Tennessee Valley Authority on Wikimedia Commons.
If
you look to the left while crossing north over the River Liffey bridge
on the N81 at Poulaphouca, you might have noticed a large tank in the
distance on the edge of the gorge. This is a surge tank and its function is to protect the
pressure tunnel from hydraulic shock or water hammer when feed to the
turbines at the power station is turned off suddenly. Hydraulic shock
occurs when water is flowing through a conduit (pipe, tunnel etc) and
its flow is suddenly interrupted. The sudden deceleration causes a rise
in pressure which can cause damage to pipes. Have you ever noticed the
sudden thump as water turns off when your washing machine's intake valve
closes once the drum is filled? This is also a water hammer effect.
However by comparison, the pressure tunnel to the power station is over
400 m long and 4.8 m diameter. That's a huge volume of water in motion
with lots of momentum and kinetic energy and when it stops suddenly,
that energy has to go somewhere. A surge tank at the power station
allows water to flow freely up into the tank, converting kinetic energy
into potential energy, rather than as a damaging shock wave propagating
back up the pressure tunnel to the dam.
