reasons that may cause a flash back.
Hot refractory causes a flash back
If the refractory lining a burner or furnace is hot enough to bring the hydrocarbon/air mixture to the auto ignition temperature, rapid combustion will start. Heat transfer from the hot surface depends on gas velocity and turbulence, explaining why “swirl”, type burners often seems more stable than more linear types.
Flame causes a flash back
Flames from pilot burners are the typical means of initiating combustion of a hydrocarbon/air mixture. Nozzle mix burners (where the fuel mixes with the combustion air within the furnace) have zones that are too lean or too rich for combustion, so the pilot flame must be positioned to heat a volume of well mixed gas. Large pilot flames can overcome poor positioning of the pilot tip.
Sparks cause a flash back
Sparks are used to ignite pilot burners and also main burners in some cases. Occasionally small sparks (static electricity) are capable of initiating combustion, but the extra energy in a large spark helps insure light off. Undesired sparks, such as those resulting from debris moving through steel ducting or fans, can initiate combustion and require careful design to avoid.
Dust causes a flash back
Dust or very fine debris are generated while welding or cutting and this dust when in contact with the nozzle causes a blockage resulting a loud sound “puut” and the flame due to the pressurization results in the flame to fold back into the nozzle.