U.S. Department of Labor
Process: Housekeeping Safety
Areas of Concern – Dust and Dirt Removal
Dusts are a common byproduct of shipyard activities and when given the proper conditions, can result in a fire, flash fire, or explosion. Dusts produced from sandblasting activities, if allowed to accumulate, can ignite and burn when airborne. Also, metal dusts that result from cutting and grinding work can be particularly energetic, burning at high temperatures and at a quick rate. When finely divided solid materials dispersed or suspended in air result in a fire, flash fire, or explosion, this is called a combustible incident. The U.S. Chemical Safety and Hazard Investigation Board (CSB) identified 281 combustible dust incidents between 1980 and 2005 that led to the deaths of 119 workers and injured 718 others. Such incidents occur depending on which of the following conditions (or elements) are present at the same time and location.
For a fire to occur, elements 1 through 3, described below, must exist simultaneously. The term fire triangle is commonly used to describe these three elements.
- Fuel, consisting of small dust particles (finely divided). Some examples of dusts found in shipyards that are combustible are paint particles, iron, hemp, epoxy, aluminum, and zinc coatings.
- Oxygen or another oxidizing agent is present in substantial quantity.
- Heat or ignition source with enough energy to ignite the dust (above the minimum ignition energy). Potential ignition sources may include temporary electric equipment, welding and other hot work.
A fourth element, which is dispersion, must exist in addition to fuel, oxygen, and heat for a flash fire or deflagration (a type of flash fire with a strong pressure wave) to occur.
- Dispersion of dust particles, suspending them in the air, can result from shipyard processes that include pneumatic air, welding gas and pressure scarfing.
Flash fires are much more dangerous to workers than an ordinary fire because it spreads too quickly to outrun. Workers can sustain burns and other injuries immediately or while attempting to escape. Depending on whether the flash fire spreads, damage can range from minimal to severe. Workers can sustain severe injuries even if property damage is minimal.
The fifth and final element needed for an explosion to occur is confinement.
- Confinement of accumulated and suspended dust particles sufficient to create sudden and dramatic pressure effects. This may consist of any enclosure—equipment, ductwork, dust collectors, compartments, or spaces. Explosions are extremely fast and can result in flying shrapnel and collapsing structural members over a large area. Workers often sustain burns or traumatic injuries in explosion incidents.
Together, these five elements (fuel, heat, oxygen, dispersion, and confinement) make up the Explosion Pentagon (shown to right).
Poor housekeeping practices, where various types of dust are allowed to accumulate, are a major cause of combustible incidents. Whenever work processes produce dust, good housekeeping is extremely important in reducing the accumulation to a safe level. Below are tips for addressing combustible dust buildup:
- Ensure dust-handling systems (such as exhaust ducts, dust collectors, vessels, and processing equipment) are designed to prevent fugitive dust in the work area (i.e., there is no leakage from the equipment).
- Use electrical grounding and bonding for dust systems. In some cases, inert atmospheres should be used. Dry powders can build up static electricity charges when subjected to the friction of transfer and mixing operations.
- Use a vacuum cleaner that is listed for use in Class II hazardous locations, or use a fixed-pipe system with a remotely located exhaust and dust collector to clean areas where dust may fall.
- Perform regular cleaning on horizontal surfaces, floors, decks, walls, and bulkheads, including equipment, ducts, pipes, hoods, ledges, beams, stair rails, and above suspended ceilings and other concealed surfaces. At a minimum, this should be performed at a frequency sufficient to prevent dust accumulations of 1/32 inch or greater.
- Ensure that dust control equipment, such as local exhaust ventilation and material transport systems for handling dust and dirt, contains either explosion relief vents or an explosion suppression system, or indicates an oxygen-deficient environment.
- Allow dust layers to accumulate to hazardous levels. The NFPA identifies dust accumulations of as little as 1/32 of an inch to be hazardous.
- Use compressed air or steam to blow down surfaces unless there is no other practical alternative. If compressed air or steam must be used, it is vital to first ensure that potential ignition sources in the vicinity are eliminated and apply the air or steam only under low pressure to avoid disbursing clouds of dust to other areas.
For more information on combustible dust see Combustible Dust.