Relief valve with overturn surge control for storage tank

A tank valve apparatus including a sealed piston assembly for regulating movement of a spring biased poppet, the poppet being in cooperative engagement with the sealed piston assembly and a valve seat of an associated valve opening, the valve opening being in communication with ambient atmosphere and pressure within the tank so that the poppet can be activated by a predetermined pressure developed within the tank to overcome the spring bias and vent the pressure to ambient atmosphere. The piston assembly includes a piston head and a flow path through the piston head for flow of material therethrough within a closed piston chamber which encloses the piston head. A snubber valve assembly is included which is carried by the piston head and contained within the closed piston chamber. The snubber valve assembly includes a shut-off disc for momentarily closing the flow path of the piston head during initial overturn of the tank thereby momentarily preventing movement of the piston head within the chamber which in turn restricts movement of the poppet and prevents venting of any surge pressure built up within the tank during the overturn. The snubber valve assembly also includes a member for moving the shut-off disc to open the flow path after a predetermined time period during which any surge pressure is dissipated within the tank in order to permit movement of the piston assembly and the poppet and venting of the valve.