03/11/2006

Don't Let Dust Combust

A combustible dust explosion hazard may exist in a variety of industries,
including: food (e.g., candy, starch, flour, feed), plastics, wood, rubber,
furniture, textiles, pesticides, pharmaceuticals, dyes, coal, metals, and
fossil fuel power generation. Most natural and synthetic organic materials, as
well as some metals, can form combustible dust. The safety bulletin prepared by
the U.S. Occupational Safety and Health Administration contains important
information on how to assess the risk in your facility.

Elements of a Dust Explosion

Experts agree that for a dust explosion to occur, five conditions must be
present at the same time. First, the three elements needed to cause fire (fire
triangle):

  1. combustible dust (fuel)
  2. ignition source (spark or heat)
  3. oxygen in air (oxidizer)

    plus… additional elements needed for a combustible dust explosion:
  4. dispersion of dust (into the air forming a dust cloud)
  5. confinement of the dust cloud (building or ceiling)

These five elements make up the "explosion pentagon" that causes a dust
explosion. If one of the elements of the explosion pentagon is missing, a
catastrophic explosion cannot occur.

A dust cloud that is ignited within a confined or semi-confined vessel, area,
or building, burns very rapidly and may explode. This could cause fires,
additional explosions, flying debris, and the collapse of parts or all of the
building.

An initial explosion that occurs in processing equipment or in an area where
there is an additional accumulation of dust, may shake the renegade dust loose,
or damage a containment system (such as a duct or vessel). The additional dust
released into the air, if ignited, can cause one or more secondary explosions
that can be even more destructive than the first.

Reducing the risk

Facilities should conduct a dust hazard assessment to carefully identify
materials that can be combustible, processes that use, consume, or produce
combustible dusts, open areas where combustible dusts may build up, hidden
areas where combustible dusts may accumulate, ways that dust may be dispersed
in the air, and potential ignition sources.

The key factor is whether or not the specific dust from your facility is a
combustible dust hazard. Although there is currently no combustible dust hazard
class under WHMIS, there is a requirement to declare all hazards of the product
on the Material Safety Data Sheet (MSDS). Unfortunately, the dust explosion
hazard is under-recognized and often not declared on MSDSs. In CCOHS' CHEMINFO
database, the potential for a material becoming a combustible dust hazard has
been identified. As well, any reports of dust explosions involving the chemical
are included. Laboratory testing of your specific dust will help tell you if
there is a hazard at your workplace. The facility analysis must also identify
areas requiring special electrical equipment classification due to the
potential combustible dust hazard.

The following safety practices are recommended:

Dust control - the most important step towards prevention of dust explosions

  • Minimize the escape of dust from process equipment or ventilation systems.
    Use dust collection systems and filters, and try to use surfaces that minimize
    dust accumulation and are easy to clean.
  • Prevent the accumulation of dusts on surfaces. Use surfaces that minimize
    dust accumulation and are easy to clean.
    Inspect for dust residues in open and hidden areas, and clean them at regular
    intervals.
  • Use cleaning methods that do not generate dust clouds (ie. vacuum rather than
    blow or dry sweep). Only use vacuum cleaners approved for dust collection.
    Locate relief valves away from dust hazard areas.
  • Develop and implement a program for hazardous dust inspection, testing,
    housekeeping, and control program that establishes, in writing, the method and
    frequency of these steps.

Ignition Control

  • Use appropriate electrical equipment and wiring methods.
  • Keep static electricity under control; this includes bonding of equipment to
    ground.
  • Prevent smoking, open flames, and sparks, mechanical sparks and friction. Use
    separator devices to remove foreign materials capable of igniting combustibles
    from process materials.
  • Avoid contact between heated surfaces and dusts. Separate heating systems
    from dusts.
  • Ensure the proper use and type of industrial trucks, and the proper use of
    cartridge-activated tools.

More info


AplusA-online.de - Source: Canadian Centre for Occupational Health and Safety