EXPLODING DUST

Your plant has a good safety and maintenance plan. Floors are spotless. Equipment has the proper lockout tags. Employees get regular safety training. But if you are not inspecting for and removing accumulated fugitive combustible metal dust, your facility is at risk of explosions and fires that could kill workers.

HOW DUST IS CREATED

Normal operations engaged in by fabricating and metalworking plants inherently create metal dust. These include grinding, polishing, cutting, stamping, and transporting metal materials. Combustible dust is defined by the National Fire Protection Association (NFPA) as any finely divided solid material 420 microns or less in diameter that presents a fire or explosion hazard when dispersed and ignited in air. A dust accumulation only 1/32 in thick, covering 5 percent of a room's surface, can be dangerous according to NFPA 654.

The smaller the particles are, the greater their combustibility because they have more surface area. Dust particles easily become airborne, settling on surfaces, in crevices, and on equipment. If there are sufficient small particles of combustible dust suspended in air and an ignition source is present, a dust explosion can occur.

NO SPECIFIC OSHA STANDARD

Currently the Occupational Safety and Health Administration (OSHA) has no specific standards for combustible dust anywhere other than grain elevators. Prior to adoption of the grain dust standard in 1988, explosions causing fatalities were common there, but since then, fatalities have decreased considerably. OSHA uses the general duty clause for enforcement at other dust hazard sites.

NFPA has developed a general dust standard (NFPA 654), as well as a specific standard for metal dust (NFPA 484). Although these standards have been adopted in some state and local fire and building codes, "that hasn't been effective at achieving the level of protection that should be expected," noted NFPA Division Manager Guy R. Colonna, P.E. Congress is currently considering requiring OSHA to adopt combustible dust standards, but the likelihood of passage in 2008 is small.

NFPA 484 stresses minimizing the generation and release of dust, improving containment of dust through good housekeeping practices, frequent cleanups, designing equipment and surfaces for easy cleaning and reduced tendency for fugitive dust accumulation, and control of ignition sources. A critical failing common to most explosion incidents documented by the Chemical Safety Board (CSB) was a lack of awareness that a combustible dust hazard existed, Colonna noted.

Material Safety Data Sheets, which are commonly used to provide hazard characteristic information, often say nothing about combustible dust potential. Plant operators must understand the specific dust characteristics and combustibility of the various metals they work with and this information can be found in NFPA 484. Metal dusts are more dangerous than other kinds of dust because they ignite at lower temperatures and burn hotter.

Using water can make a metal dust fire worse, depending on the type of metal. "If you apply water in the wrong fashion, water can instantly cause a violent reaction and can make it burn hotter and explode," explains Colonna. Inert substances like sand, dry chemical, or argon may work or the fire may have to burn itself out while surrounding areas are protected. Each metal may behave differently; NFPA 484 has a chart of extinguishing agents. Water, foam, halogen extinguishing agents, and carbon dioxide should never be used on metal dust fires.

HAYES-LEMMERZ

Of the 281 dust explosions investigated by CSB which caused 120 fatalities and 700 injuries from 1980 to 2005, 9 percent are represented by fabricated metal products facilities. The most notable was at the Hayes-Lemmerz International plant (Huntington, IN) where cast aluminum and aluminum alloy automotive wheels are made. Scrap metal was chopped into chips and recycled in a melt furnace. Dust from transporting and drying these chips went into a dust collector.

On October 29, 2003, a dust explosion occurred, spreading through the building's ducts and causing an intense fire. One worker was killed and several others seriously injured. Although the cause is still under investigation, one theory is that the explosion started in the dust collector due to inadequate ventilation and cleaning, and because it was too near the scrap processing area. Secondary explosions from fugitive dust essentially destroyed the building.

HOW TO RECOGNIZE AND CONTROL DUST

Because metal dust creation is inevitable, aggressive collection at the point of generation is essential. "You need to have a capture velocity of 100 to 150 ft per minute and then pneumatically convey that dust to a dust collector," says fire protection engineer John Cholin, PE, PSFPE, MEE of J.M. Cholin Consultants, Inc. (Oakland, NJ).

He recommends using a wet collector because it captures the metal fines and places them into a liquid slurry that can't be ignited. Dry collectors, if used, must be located outside and designed according to NFPA 484, but still pose a serious explosion hazard.

"If you can write your name in the dust, you can blow the plant away," stresses Cholin. If metal fines are not fully collected, only Class 2 Division 1 electrical equipment, which costs more than 20 times as much as ordinary electrical boxes, must be used. "If you get a little metal dust inside the electrical equipment, the little bits of metal line up in a row due to the electromagnetic interaction between the circuit conductors and the dust particles," he adds. This makes a conductive path which can lead to catastrophic failures and ignitions.

Just as importantly, fugitive metal dust must be removed. "The finest, most hazardous dusts end up highest in the space," notes Cholin. "You must have a cleaning program that goes from the roof or ceiling membrane all the way down to the floor. Dust will accumulate on all the ledges and support beams and all of that dust becomes available for ignition whenever there is an initiating event."

Typically, a small explosion shakes the building, which releases hidden dust, and the fire ignites secondary explosions. "In every explosion that I've investigated, the employees were injured or killed not by the initial explosion, but by secondary explosions that occurred from fugitive dust," Cholin says.

He has a simple technique for inspecting for metal dust, which is typically gray in color. Look at the tops of pipes, electrical conduits, and ducts. If they are not the same color as the bottoms, they are dusty. Use binoculars to inspect high places where there is little air movement and dust can settle. Inspection frequency depends on how fast dust accumulates.

Initially, inspect weekly and clean monthly. Once no dust is found, inspect monthly, cleaning as needed. Operational changes like adding new equipment or processing new materials can increase dust production. After any such change, inspections should increase until results are known.

Some companies use stick-on labels to monitor dust. These labels have the words "clean enough" printed in 1-in-high letters. They are attached to surfaces around the plant and a record of their locations is kept. Any employee can inspect for dust using a checklist and map of these locations. If the label cannot be read, the area needs cleaning.

Metal dusts are more difficult to clean up than other dusts because they are electrically conductive. "All cleanup should be done with conductive, non-sparking scoops and soft brooms," states Colonna. A bonded and grounded vacuum cleaning system can be used on a limited basis when removing quantities too small for broom and scoop or located in high spaces. Compressed air and water should not be used.

Following NFPA dust standards is all that is necessary to prevent metal dust explosions, Cholin said. Plant operators can learn more by attending an NFPA dust explosion seminar.

Jean Feingold is a technical writer for Fabricating & Metalworking magazine.

J.M. Cholin Consultants, Inc.