Patent ID: 7069945

Claim:
A relief valve comprising: a cage having an axial bore, a first port and a second port; a moveable member disposed within the axial bore of the cage, the moveable member comprising a first end and a second end, the moveable member having an axial bore extending from the first end to the second end, the moveable member axially moveable with respect to the cage such that the moveable member can selectively restrict fluid communication between the first port and the second port; a fixed member disposed within the axial bore of the cage adjacent to the second end of the moveable member, a chamber being defined between the moveable member and the fixed member; a piston, the piston including a first stem, a second stem, and a head, the head disposed between the first and second stems, at least a portion of the first stem being disposed within the axial bore of the moveable member, the piston axially move able with respect to the fixed member between at least a first position and a second position; wherein the first stem has a shape such that in the first position, the first port is in fluid communication with the chamber with the first stem and the moveable member defining a clearance therebetween with a first area, and wherein in the second position, the first stem cooperates with the moveable member to define a first restriction therebetween with a second area, the second area being less than the first area.