surge tank

(engineering) A standpipe or storage reservoir at the downstream end of a closed aqueduct or feeder pipe, as for a water wheel, to absorb sudden rises of pressure and to furnish water quickly during a drop in pressure. Also known as surge drum. An open tank to which the top of a surge pipe is connected so as to avoid loss of water during a pressure surge.
(mining engineering) In pumping of ore pulps, a relatively small tank which maintains a steady loading of the pump.

In a water-supply system, a reserve tank which supplies some of the water when there is a sudden drop in pressure, helping to maintain a more uniform flow.