Thermal combustion units in chemical, pharmaceutical. and petrochemical plants use burner systems for thermal waste disposal to reduce emissions. The advantage of premixed burner units is related primarily to greater stability of flame, the reduction of NOx emissions, and control and technical reasons. Unfortunately, the production of premixed flames requires the highly flammable fuel and oxidant gases to be mixed in confined chambers before they are burned. To form the flame.. the mixed gases must pass through the burner exit ports where combustion takes place. Under certain conditions.. the flame can burn back through the burner exit ports and ignite the potentially explosive gas-air mixture in the piping system. Of course, burner system designers have the expertise to design suitable burners that prevent flashback by flow-control measures. Flame arresters can be used as a secondary measure to increase plant safety. Flame arresters are autonomous safety devices that protect workers, environment, and plant against deflagration and detonation caused by flashback from a combustion process while allowing vapor flow. To maximize the effectiveness of flame arresters, attention should be given to proper selection, application, and maintenance of the device. For this, it is most important to know tinder which conditions the arrester was tested and have the capability to decide,which test standard is the most effective for the specific application. This article focuses on the application of in-line deflagration vs. in-line detonation arresters and provides design criteria based on live field testing on when to use which arrester type. (c) 2006 American Institute of Chemical Engineers.