Electrostatic dust ignition

An electrostatic dust ignition can occur when the discharge releases sufficient thermal energy within a sufficiently short period of time and small volume to ignite the suspended dust. Electrostatic ignition is complicated by the fact that there are a number of distinct ESD mechanisms important in electrostatic hazards and hazard abatement (Glor, 1988). 

See DUST EXPLOSION INCIDENTS, ELECTROSTATIC HAZARDS, IGNITION SOURCES... 

The minimum energy for ignition is measured primarily to determine whether the dust cloud could be ignited by an electrostatic spark. Ignition energies of dusts can be as low as 15 mj this quantity of energy can be supplied by an electrostatic discharge. 

The next task in considering the electrostatic hazards associated with a fluidized bed is to consider the specific conditions required for the ignition of suspended dust and powders. It is crucial to cover this subject because... 

Electrostatics enters into the problem of secondary explosions in several ways. First, an electrostatic spark may be the ignition source of the primary explosion. Second, an electrostatic spark can serve to ignite a dust cloud if particulate spews out at high velocity as a result of a sudden piping or vessel containment failure. Similar containment failures have occurred with insulating liquids and can not be ruled out for powders. For example,... 

The relative rarity of dust and powder ignitions makes them a unique sort of industrial safety threat. Because their occurrence is not routine, operating personnel eventually relax their guard, and too often this sort of behavior leads to dangerous incidents. The evidence that dust explosions are almost unknown within fluidized beds is an especially challenging problem for the safety officer, who must encourage vigilance even when no one remembers the last electrostatic incident. 

Another type of testing, that of electrostatic sensitivity, has been demonstrated in some cases to be more properly a delicate test of the ignitability of the material under localized thermal stress which correlates best to the friction sensitivity of the system under test rather than to electrostatic hazards. On the other hand, electrostatic tests done in the supposed atmosphere above a propellant mixer were reduced in absolute value by more than an order of magnitude when the ammonium perchlorate dust actually present was introduced in the test since this altered the potential path for spark discharge within the system. 

Although very unreactive in massive forms, the finely divided metal may be pyrophoric, a Japanese study of this hazard has been reported [1]. Consequent upon an explosion in a tantalum dust collector, further study of powder explosion was made. Minimum ignition energies were found to be much lower than previously supposed. An oxide coating makes it susceptible to electrostatic charge generaton and discharge [2]. See entry PYROPHORIC METALS (references 3,9)... 

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