What Is Deflagration?

The combustion wave propagating at subsonic speed is called deflagration. ^[1] is the phenomenon that the combustible mixture accumulated in the furnace boring instantly and simultaneously burns, so that the pressure on the flue gas side of the furnace suddenly rises. In severe cases, the pressure caused by deflagration can exceed the allowable value of the design structure and cause damage to the water-cooled wall, rigid beam, furnace roof and furnace wall. There must be three conditions for the creation of deflagration (that is, the three elements of deflagration), and one of them is indispensable. The first is the accumulation of fuel and combustion air; the second is that the fuel and air mixture has reached the concentration of deflagration; the third is that there is sufficient ignition energy. How to avoid the accumulation of fuel and combustion air during the start-up, operation and shutdown of the boiler is the key to preventing the detonation of the furnace.

Deflagration is a combustion wave

There must be three conditions for the creation of deflagration (that is, the three elements of deflagration), and one of them is indispensable.

The first is the accumulation of fuel and combustion air;

The second is that the fuel and air mixture has reached the concentration of deflagration;

The third is to have enough ignition energy.

How to avoid the accumulation of fuel and combustion air during the start-up, operation and shutdown of the boiler is the key to preventing the detonation of the furnace. Because deflagration occurs at an instant, and the flame spreads very fast, reaching hundreds of meters to several kilometers per second, the spherical shape of the flame propagates to the four sides, burning embers within a few percent to a few tenths of a second, which is equal to the fuel at the same time When it is ignited, the volume of the flue gas suddenly increases, so the resistance of the flue gas is very large, so it has no time to leak out and explode. ^[1]

It is required that flame propagation is restricted to the initial stage (such as during subsonic flow), and explosion-proof pipeline flame arresters can be used. Explosion-proof pipeline flame arresters can extinguish the flame before the deflagration-detonation transition. The flame arrester uses a characteristic called Maximum Test Safety Gap (MESG), and tests performed in accordance with test standards show that this gap is the largest gap where the flame will not spread. The flame arrester shall be designed and manufactured with a unit smaller than the maximum test safety clearance. As the flame leading edge passes through each unit, heat is transferred between the flame leading edge and the unit wall. The heat transfer through the boundary layer to the cell wall will cool the combustion gas until it goes out. For current guidelines on deflagration and detonation fire arrester testing, refer to the standard ISO16852. The US Coast Guard (USCG) standard only covers detonation fire arrester testing. ^[1]

Of course, the vent is also one of the risk control measures in the pipeline, which is used to release pressure and flame into the atmosphere. The arrangement of vents should be less than the expected explosion spread distance. Special attention needs to be paid to the location of the vent according to the released flame and pressure. The vent will reduce the pressure effect, but will not prevent flames from continuing through the vent. The key is to determine the safe release position of the flame.