Surge tanks are normally designed to accept main drain water by gravity flow using a modulating (float operated) valve to hold back main drain flow when gutter water is flowing into the tank.  Many instances occur when the filtering rate for the pool cannot be maintained on gravity flow.  An example would be an increase in pool turnover.  The Mer~Made inline surge tank allows the main drain flow to be improved by using pump suction.  A modulating valve is mounted ahead of a "T" which opens into the tank.  A high tank water level, caused by gutter water flow, throttles main drain water giving preference to the gutter water in the tank.