Valve with variable secondary orifice

In a valve of the type having an annular valve seat fixed in a valve body, the body providing a fluid path through the seat between an inlet and an outlet, and a movable valve head engageable with the seat and movable away from the seat in response to urging of fluid pressure above a predetermined amount of said inlet, the combination with said body of

• nozzle means of generally tubular configuration in said fluid path, communicating interiorly with said inlet and having an end portion defining said seat and an annular outer edge portion external to the seat, and
• a valve head having an central surface formed to close on the seat, an annular intermediate surface extending radially outwardly from the central surface, and a shroud surface extending radially outwardly from the intermediate surface and having a portion forming with said outer edge portion a variable annular secondary orifice, said surfaces forming with said nozzle means a huddle chamber communicating interiorly with the seat, the secondary orifice communicating between the huddle chamber and the outlet and comprising a restriction in said fluid path, the shroud surface being curved to vary the cross sectional area of said restriction from a minimum valve smaller than that of any restriction within the huddle chamber external to said seat with said central surface in a closed position on the seat to a maximum valve with said central surface at a predetermined maximum spaced position from the seat, the rate of change of said cross sectional area as a function of the spacing between said central surface and the seat increasing from said closed position toward said maximum spaced position, and the cross sectional area of said restriction being smaller than that defined between said central surface and the seat at positions of said central surface above an initial value upon movement from said closed position between said closed and maximum spaced positions.