The OPW 201 Emergency Vent is designed to prevent an aboveground storage tank from becoming over-pressurized by providing high-capacity venting in the event of a fire or blockage of the OPW 23, 113, 523V, or 623V. The 201 is a weighted, mushroom-style emergency vent. When the AST builds pressure, the weighted cast iron lid is forced up off its seat to relieve the pressure. When pressure is relieved, the lid lowers and is automatically reset. The appropriate 201 Series vent is determined by the emergency venting capacity requirements of the AST and the type of connection at the tank's emergency vent opening. The OPW 201 Emergency Vent is available in 4", 6" and 8" openings with female NPT, male NPT or flanged connections (8") to allow for easy installation in new or existing aboveground tanks. Various lid pressure settings determine the initial venting of the valve.