The OPW 199ASV Anti-Siphon Valve is designed to help prevent siphoning of an aboveground storage tank should a leak or break occur in the fuel supply line. The 199ASV installs on the top of an aboveground storage tank with a suction rod on the inlet and the fuel supply line (to a remote pump) on the outlet of the valve. A poppet in the valve remains normally closed, isolating the tank from siphoning. When the remote pump is activated, it overcomes a factory-adjusted spring set to a predetermined hydrostatic head pressure and allows flow of fuel out of the AST. An internal pressure relief valve helps absorb thermal expansion in the fuel supply line. The 199ASV is available in female NPT threads with four different factory-adjusted hydrostatic head pressure settings. 1 1/2 in. NPT threads.