Patent ID: 8171954

Claim:
A pressure-relief valve assembly for mounting on a port of a vessel, the valve assembly being able to hold a seal without leaking when the vessel is evacuated, the pressure relief valve comprising: a housing, a spring abutment surface disposed within the housing, a valve seat having a sealing surface thereon, a valve head displaceable along a valve axis, the valve head having a sealing surface thereon, a resilient valve seal disposed between the valve seat and the valve head, and a spring disposed between the spring abutment surface and the valve head for imposing a predetermined spring biasing force on the valve head to urge the valve head along the valve axis toward the valve seat, wherein the improvement comprises: both the sealing surface on the valve seat and the sealing surface on the valve head are annular planar surfaces disposed in confrontational relationship with respect to each other and oriented substantially perpendicular to the valve axis, and wherein the resilient valve seal is an annular O-ring formed of a material which when compressed by the spring biasing force between the annular planar confronting surfaces deforms to a predetermined extent into sealing engagement against the sealing surfaces on both the valve head and the valve seat such that, when the pressure of a fluid within the vessel exceeds the desired spring biasing force the valve face lifts from the valve seat to relieve pressure within the vessel, and when a vacuum condition exists within the vessel the combination of the predetermined spring biasing force and a holding force created by a pressure differential between ambient pressure and the pressure of a fluid within the vessel acts over the valve head to increase the extent of compression between the resilient valve seal and both sealing surfaces sufficient to create a vacuum-tight seal.