Dust Up

During the past year, a number of news items have been published regarding the high risk of explosion in seemingly harmless circumstances that involve dust. In response, the Occupational Safety and Health Administration (OSHA), through a national  campaign, has made it clear that facilities dealing with dust hazards should take a proactive approach to avoid the ignition of dust clouds.

Dust-related fires and explosions are a realistic threat to the workplace. They can occur when five different factors—oxygen, an ignition source (heat, an electrical spark or a spark from metal to metal contact by machinery), fuel (combustible or conductive dust), dispersion of the dust (layers or clouds) and confinement of the dust (to create sufficient concentration to be ignitable)—are present.

These five factors are referred to as the “Dust Explosion Pentagon.” If any one of these factors is removed or is missing, an explosion cannot occur. However, while the risk of explosion is eliminated by removing a side of the Dust Explosion Pentagon, the risk of fire remains when dust collects on surfaces that may become hot. The subsequent fire may then create the Dust Explosion Pentagon and lead to an explosion.

NOTES OF PRECAUTION
Requirements to protect against arcing and sparking electrical components are well-known and accepted. Throughout the years there has been an increasing awareness about the dangers caused by the buildup of static electricity, which is another dust ignition source. However, even today, few codes and regulations address the dangers of surface temperatures. This is especially true for the protection of powered industrial equipment.

If housekeeping is not managed to minimize dust within a facility, most powered industrial equipment can create significant dust clouds as they move loose material or travel throughout facilities during normal operations. Combine this dust cloud development with the large number of ignition sources within this type of equipment, and you can find plenty of reasons to be concerned.

With powered industrial equipment, special care should be given to limit surface temperatures, especially for internal combustion- (IC-) powered equipment. IC-powered equipment can be compared to a vacuum cleaner. This equipment sucks up all the dust from the ground and the atmosphere and deposits it inside the engine compartment and radiators and onto unprotected manifolds, exhaust, catalytic converters. Allowing dust to collect on components that can reach temperatures up to 600 degrees Fahrenheit significantly increases the risk of fire and explosions.

If dust is allowed to build up inside the radiator, it may result in a reduction of airflow and radiator cooling performance. This, in turn, may lead to an increase in engine running and surface temperatures. Dust may collect in open rotating parts, where rubbing of the component against the dust may increase the risk of ignition by friction.
For powered industrial equipment many factors must be considered to determine the appropriate level of equipment ignition source protection and surface temperature limitation requirements, including:

• The type of dust material;

• Whether the dust is conductive or combustible;

• The cloud ignition temperature;

• The 5-millimeter, or 0.25-inch, layer auto ignition temperature; and

• The minimum spark energy required to ignite dust cloud.

SOURCES OF IGNITION
Depending on the equipment’s power source, the following dust ignition sources must be addressed on powered industrial vehicles used in dust hazardous areas.

Elimination or protection of electrical equipment:

• Arcs or sparks may arise from unprotected electrical equipment, such as starters, motors and batteries.

• The vehicle should be supplied with electrical equipment suitable for use in explosion hazardous areas, depending on the level of protection required.

• Care should be taken to ensure that vulnerable items, such as lights, are durable enough to withstand normal and arduous wear and tear.

• Electrical equipment may be eliminated using mechanical alternatives, such as mechanical fuel, temperature and pressure gauges or starters operated by spring recoil, hydraulic fluid or compressed air.

• Units may be fitted with electric starters and wired to a plug-and-socket connection and used with a starter battery that is stored and used in a safe area of the facility.

Control of surface temperature:

• The surface temperature of the engine, exhaust, electrical equipment, brakes and any other “hot-spots” on the vehicle should be kept below the lowest ignition temperature of any flammable materials likely to be encountered, as provided by the supplier.

•  The exhaust manifold and associated piping may be cooled using a water jacket or water-cooled heat exchanger, by passing the exhaust gases through a water bath or by air cooling.

• Air cooling may be provided for hot-spots such as the friction surfaces of brakes and clutches and the outer casings of electrical equipment. In extreme cases, oil-filled or labyrinth-type brake enclosures may be required.
• Thermal sensors may also be used to detect overheating, trigger an audible or visual alarm and to provide automatic shutdown of the vehicle.

• The use of insulation cladding as protection from hot surfaces is not recommended as it may be damaged  or penetrated by flammable dust or fibers.

NO GUIDANCE
Unfortunately codes, regulations, construction specifications and testing procedures that would ensure safe operation of powered industrial equipment in explosion hazardous dust areas have never been developed in the United States. Regardless of this fact, guidelines from OSHA and the National Fire Protection Association (NFPTA), Quincy, Mass., endorse and sometimes enforce the use of equipment that, according to Underwriters Laboratories has not been tested, certified or found suitable for use in these dust hazardous areas. This contradiction may leave equipment users and operators “in the dust” when it comes to equipment fire safety.