A wet or dry device (or assembly of devices) in a fuel gas line designed to perform the following three functions: (a) Prevent backflow of oxygen into the fuel gas supply system; (b) Prevent the passage of flame into the fuel gas supply system (flashback); (c) Prevent the development of a fuel gas–oxygen mixture at sufficient pressure so that its ignition would achieve combustion pressures that could cause failure to perform functions (a) and (b). This device is given a diagram symbol, PF.