Abstract: In a submerged pump system for pumping liquid from a well or vessel the vertical pipe through which the liquid is pumped by a submerged impeller pump is typically fitted at its upper end with an air valve. The air valve vents air from the pipe to atmosphere upon pump start up, then closes so that only water passes through a check valve to a pipe system and subsequently allows air to re-enter the pipe when the pump stops. A special throttling valve associated with the air valve restricts the outward rate of flow of air to atmosphere to thereby reduce the rate of the rising liquid and thus the shock of this liquid on the air valve and on the check valve but allows essentially unrestricted re-entry of atmospheric air to thereby allow the vertical pipe to fill with air rapidly. One form of throttling valve includes a valve plug mounted for axial movement into and out of a vent opening and spring-biased into the opening.