(126e) In line flame arrester application limits and matrix concept for process plant safety from flash back of thermal combustion units

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 to greater stability of flame, the reduction of NOx emissions, control and technical reasons etc. Unfortunately the production of premixed flames requires the highly explosive fuel and oxidant gases to be mixed in confined chambers before they are burned. To form the flame, the mixed combustion 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 explosive gas air mixture in the piping system. Of course burner system designers have the expertise to design suitable burners which prevent flashback by flow controls measures (figure 1). Flame arresters can be used as a secondary measure to increase plant safety. Flame arresters are safety devices which protect man, environment and plant against deflagration and detonation caused by flash back from a combustion process whilst 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 under which conditions the arrester was tested. This paper will focus on the application of in line deflagration versus in line detonation arresters and provide design criteria based on live field testing on when to use which arrester type. One of the difficult questions to answer is when to use in line deflagration arresters which have much lower pressure drops, are less expensive and facilitate reduced maintenance cycles. Testing has been carried out on a typical burner system with different piping configurations. It has been shown that for most piping set ups the conservative approach of in line deflagration testing with a closed ignition vessel provides a sufficient safety buffer for most systems. This is due to the acceleration of unburned gases through the burned gases. It is important to install the in line deflagration arrester within the tested L/D ration of the ignition source. In addition a safety matrix concept that is part of the operational instructions of the TRbF 20 (German Tanks Safety Standard) will be introduced. In this concept three independent measures are required for waste gas zone 0 as shown in figure 1 if a permanent operational ignition source is present.