Over the past year, a number of articles have been published regarding the high risk of explosion in seemingly harmless dust applications. The backlash that resulted from an increasing number of dust-related incidents has been felt throughout those industries. Through a national emphasis campaign, OSHA has made it very clear that facilities dealing with dust hazards should take a more proactive approach to avoid ignition of dust clouds.
Dust-related fires and explosions are a realistic threat to the workplace, and they may occur when five different factors are present: oxygen; an ignition source (heat, an electrical spark or a spark from metal-to-metal contact, i.e., 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). These five factors are referred to as the “dust explosion pentagon.”
If any one of these factors is removed or missing, an explosion cannot occur. However, while the risk of explosion is eliminated by removing a leg of the pentagon, the risk of fire still remains when dust is allowed to build up on surfaces which may become hot. The subsequent fire may then create the dust explosion pentagon and lead to an explosion.
Requirements to protect against arcing and sparking electrical components are well known and accepted. Over 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, very few codes and regulations pay attention to 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 the facility, most powered industrial equipment operated in dust hazardous facilities 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 will 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) equipment. IC-powered equipment can be compared to a vacuum cleaner. They suck up all the dust from the ground and the atmosphere and deposit it inside the engine compartment and onto unprotected manifolds, exhaust, catalytic converters and inside radiators. Allowing dust to build up on components that can reach temperatures up to 600 °F significantly increases the risk of a fire and explosions as a result of smoldering and dust combustion.
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 the 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.
Areas of Concern
For powered industrial equipment, a number of factors must be considered to determine the appropriate level of equipment ignition-source protection and surface-temperature limitation requirements:
The type of dust material;
Is the dust conductive or combustible?;
The cloud ignition temperature (Cloud AIT - °C/°F);
The 5-millimeter or ¼-inch layer auto ignition temperature (Layer AIT - °C/°F);
The minimum spark energy required to ignite a dust cloud (mJ).
Depending on the type of power source within the equipment, the following dust ignition sources must be addressed on powered industrial vehicles used in dust hazardous areas.
Arcs or sparks may arise from unprotected electrical equipment, such as starters, motors, batteries and instrumentation. The vehicle should be supplied with electrical equipment suitable for use in explosion hazardous areas, depending on the level of protection required. The temperature classification of the equipment should be checked to ensure it is appropriate for the range of dust and fibers likely to be encountered.
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 by the use of mechanical alternatives, such as mechanical fuel, temperature and pressure gauges or starters operated by spring recoil, hydraulic fluid or compressed air.
Equipment may be fitted with electric starters and wired to a plug-and-socket connection for use with a starter battery that is stored and used in a safe area.
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. The supplier will provide details of the maximum surface temperature that may be achieved by the truck in normal operation.
The exhaust manifold and associated piping may be cooled by using a water jacket or water-cooled heat exchanger, by passing the exhaust gases through a water bath or by air cooling. Water levels should be monitored regularly and maintained at the recommended level.
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 (for example, a truck required for heavy work cycles in an area where materials with a low ignition temperature are present), oil-filled or labyrinth-type brake enclosures may be required.
Thermal sensors may also be used to detect overheating. These may be used to trigger audible or visual alarms and to provide automatic shutdown of the vehicle/engine.
The use of insulation cladding to provide protection from hot surfaces is not recommended because it is liable to be damaged and may be penetrated by flammable dust or fibers.
Non-electrical items can cause electrostatic or frictional ignition by rubbing or impact. Therefore, adequate clearance is needed between fixed and rotating parts, with allowance made for engine movement during normal operation. Engine-cooling fans should be made from non-metallic material, such as plastic.
The use of aluminum, magnesium, titanium and light alloys is generally minimized because of the risk of sparks from a thermite reaction (which may occur on impact between light metals and rust). If such components are used, they must be shielded from impact by an enclosure. Surface coatings incorporating these materials are normally considered unsuitable.
The risks of sparks from static build-up are minimized by the use of electrically conducted materials for drive belts, tires and other components.
The load contact points of the equipment may cause frictional sparks when they contact a metal drum or other object or are allowed to impact on concrete floors. They are often fitted within brass or stainless steel sleeves.
Spark arresters are also used to prevent any spark from the exhaust system reaching the outside atmosphere. Both inlet and exhaust systems are designed to withstand internal explosions.
Ingestion of flammable dust may result in reduced engine performance and increased carbon emissions from the exhaust. Installation of a multi-stage dust filter is recommended. The filter should be cleaned on a regular basis.
Unfortunately, codes, regulations, construction specifications and testing procedures to ensure safe operation of powered industrial equipment in explosion hazardous dust areas have never been developed in the United States. Regardless, OSHA and NFPA guidelines endorse and sometimes enforce the use of equipment types, which according to UL, have 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.
Robert Zuiderveld is general manager at Pyroban Corp., a provider of safety and environmental solutions.