Wednesday, 25 April 2007 - 3:30 PM
: Aluminum-Dust Explosion
An explosion occurred in equipment that was being used to pulverize
aluminum-alloy ingots, to form a powder with small particle size.
In this process, air was recycled through a grinder, cyclone product
separator, and blower, with a side stream to extract undesired
dust to a dust collector. However, the concentration of dust in
the circulating air stream was above the Minimum Explosible Concentration
in most of the recycle system. Ignition of this dust/air dispersion
occurred as a result of an exothermic oxidation of accumulated
combustible dust in the return-air ductwork, accelerated by the
relatively high temperature of the circulating air. Because the
system was not protected against internal explosion, an access
door on the grinder was forcibly ejected, and the door struck
the LPG cylinder on a nearby forklift truck. Escaping LPG vapors
– ignited by the dust fireball from the open grinder door
– resulted in a flash fire that caused fatal burn injuries
to the forklift driver.
This powder-producing operation initially was a batch process,
and it had operated without incident for many years. When a decision
was made to convert the batch process to a continuous process,
some hazardous changes were introduced that were not recognized.
These changes included (1) a much higher rate of dust generation,
(2) high concentrations of very fine dust in the recycle stream,
and (3) accumulation of heat in the recycle stream. In addition,
the installer of the recycle system did not have experience with
combustible powders, and no explosion prevention or protection
devices or systems were provided.
Among the lessons learned from this incident were: (1) every
significant change in a process should be subjected to a Management
of Change analysis; (2) producers of hazardous materials (and
particularly small-company producers) need to follow Codes and
Standards that apply to their processes; (3) code-enforcing authorities
should become more familiar with Codes and Standards that apply
to dust-explosion hazards; and (4) dust-explosion characteristics
need to be included in Material Safety Data Sheets.