Flame arresters are at times mounted directly upstream of pressure / vacuum relief vents, or are connected to short runs of open vent outlet piping. These devices, known as “Vent-Line / In-Line” Deflagration Flame Arresters are designed with flanged or threaded mounting connections on both the inlet and the outlet. It is possible for a flammable vapor/air mixture to ignite within the confined space of the short outlet pipe or within the vent body.

Such an ignition is referred to as a confined deflagration, that is, a flame front moving in a confined space (e.g. constrained by pipe walls) at a speed less than the speed of sound. The Vent-Line / In-Line Deflagration Flame Arrester is designed to stop this type of flame front. Note: these arresters have very specific limitations as to where they can be properly located within a piping system.