Abstract: An apparatus for controlling fluid discharge through an outlet of a conduit comprises a clamshell-type gate including first and second opposed gate members pivotally mounted to the exterior of the conduit. The gate members are oppositely rotatable through an arc of approximately 90.degree. between an open position and a closed position just downstream of the outlet. In the closed position, the gate members are closed together along a lip seal at the center of the conduit. A collar seal, mounted to the conduit, is urged against the gate members to block peripheral fluid discharge from the outlet. When the gate is fully or partially open, a portion of the outlet extends downstream of the lip seal to minimize "fan out" of the discharge. In the fully open position, the gate members are open symmetrically beyond the diameter of the outlet for unrestricted fluid discharge through the outlet. The gate members are pivotally attached to trunnion members at each side of the conduit through support arm plates.