A mechanical seal system configured to inhibit the emission of process gas from a rotating machine. An annular plunger is axially slidable between a inboard closed position and an outboard open position such that when the plunger is in the closed position, the plunger inhibits the flow of the sealing fluid between the inlet passageway to the seal interface, and when the plunger is in the closed position, the plunger enables the flow of the sealing fluid between the inlet passageway and the seal interface. A biasing mechanism and solenoid provide opening and closing forces to the plunger. Normally-open and normally-closed configurations are provided.