A wet or dry device (or assembly of devices) in a fuel gas line designed to perform the following three functions: (1) prevent backflow of oxygen into the fuel gas supply system; (2) prevent the passage of flame into the fuel gas supply system (flashback); (3) 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 (1) and (2). This device is given the diagram symbol PF.