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Avoiding Combustible Dust Explosions

May 6, 2010

By Niels H. Pedersen, Dantherm Filtration

In Mississippi, a rubber fabricating plant suffered a dust explosion that killed five and injured 11. Wood dust in a Pennsylvania particleboard plant exploded killing three and injuring 10. In North Carolina, plastic powder used at a pharmaceutical plant caused an explosion that killed six and injured 38. A series of wheat dust explosions in a grain storage facility in Kansas killed seven. At a Michigan electrical power generation facility, six people were killed and 14 injured by an explosion of coal dust. At a wheel manufacturing plant in Indiana, explosions of accumulated aluminum dust severely burned three employees–one fatally. And in 2008, a series of dust explosions and fire at a sugar plant in Georgia killed 14 and injured 36.

On average, two to three dust explosions occur in U.S. factories every day

The list could go on and would include many different types of combustible dust present in a wide range of industries. According to OSHA and the U.S. Department of Labor, since 1980 there have been at least 350 combustible dust explosions in the United States, killing more than 130 workers and injuring almost 800. In many accidents, employers and employees were unaware that a hazard even existed.

What is Combustible Dust?
Combustible dusts are fine particles that present an explosion hazard when suspended in air under certain conditions. OSHA defines combustible dust as a solid material composed of distinct particles or pieces, regardless of size, shape, or chemical composition, that presents a fire or deflagration hazard (the process that produces the explosion) when suspended in air or some other oxidizing medium over a range of concentrations. Types of dusts include, but are not limited to: metal dust, wood dust, plastic or rubber dust, biosolids, coal dust, organic dust (such as flour, sugar, paper, soap, and dried blood), and dusts from certain textiles.

Result of an explosion developing through connected air material separators

The Risk
OSHA estimates at least 30,000 U.S. facilities and hundreds of types of processes may be at risk for a combustible dust incident. Those industries include food, grain, tobacco, plastics, wood, paper, pulp, rubber, furniture, textiles, pesticides, pharmaceuticals, dyes, coal, metals, and more. By recent estimates, on average two to three dust explosions occur in various manufacturing facilities in the United States every day. In addition to the potential loss of life and injury, a single incident can cost millions in damage, large fines from OSHA, and shut a facility down for months—if not permanently.

Explosions can happen in any manufacturing process or facility where combustible dust is produced—in the air, stored, or accumulated. The National Fire Protection Association (NFPA) currently states that dust accumulations of as little as 1/32 of an inch (approximately the same thickness as a paper clip) are sufficient to create a dust deflagration when dispersed and exposed to an ignition source. Employee safety is threatened by the initial explosion, ensuing fires, additional explosions, flying debris, and collapsing building components. Recognizing unsafe conditions and then doing something about them can break the chain of events that could lead to a dust explosion and its catastrophic results.

Evaluating Your Risk
Although the industries and materials differed, accident investigators found similar conditions that resulted in massive and tragic dust explosions. They all met the five conditions necessary for a dust explosion to occur. Known as the “Dust Explosion Pentagon”, these conditions include the presence of: combustible fuel (dust); ignition source (heat); oxygen in the air (oxidizer); dispersion of dust particles in sufficient concentration; and usually confinement of the dust cloud. Your facility may have these same conditions. Many manufacturers remain unaware they are at high risk. They are also unaware that solutions exist to avoid combustible dust explosions before they can occur.

Performing a thorough risk evaluation (also known as a hazard assessment) at your facility is essential to identify and eliminate factors that could contribute to an explosion. This is one of the mandatory requirements set forth in the NFPA standards for the handling of combustible dusts, which are now being enforced by OSHA and local authorities (more on this later in this article). Your dust collection equipment supplier should have the expertise to offer hazard analysis and assessment among their services.

The First Steps in Addressing the Risk
Your first step would be to  determine whether your dust is potentially explosive. According to OSHA, the vast majority of natural and synthetic organic  materials, as well as some metals, can form combustible dusts. A dust control expert can help you determine if the dust produced in your facility can be explosive.

The obvious place for a dust explosion to begin is where dust has accumulated. Therefore, the primary method for preventing a dust explosion is by reducing or eliminating the amount of fuel (dust) with proper and sufficient dust collection and a mandatory housekeeping program. Dust  removal can be accomplished by proper ventilation, using the correct dust collection/extraction system, and manual collection and removal where automated cleaning systems cannot reach.

The Current Official Steps  to Take
The ultimate governing body for explosion protection is the AHJ (Authority Having Jurisdiction). For the most part, local and state governments have adopted standards identical to the Federal OSHA standards. This does not prevent the local AHJ from adopting even stricter standards.

Specific guidance measures to prevent explosions can be found in OSHA’s SHIB Combustible Dust in Industry: Preventing and Mitigating the Effects of Fire and Explosions (available at www.osha.gov), which lists measures to control dusts, eliminate ignition sources, and limit the effects of explosions to minimize injuries.

According to OSHA, initial preventative steps are to contain combustible dust to areas that are properly designed and located, with ignition sources controlled or eliminated. Equipment such as ductwork, dust collectors, and processing equipment should be designed in a manner to prevent leaks and minimize the escape of dust into work areas. Any dust that settles on work surfaces must routinely be removed. Areas or equipment potentially subject to explosions, including the dust collection/extraction system, should also be designed to vent explosive pressure in a safe manner, or be provided with proper suppression, explosion prevention systems, or an oxygen-deficient atmosphere.

The NFPA and  Combustible Dusts
OSHA’s first line of enforcement is the General Duty Clause, Section 5(a)(1). It can be very broadly interpreted and requires employers to take measures to control the dangers of specific hazards to protect employees in the workplace.

The United States government gave OSHA authority to enforce NFPA standards. They also tasked OSHA to make its own laws in the future to prevent combustible dust explosions in U.S. manufacturing facilities. NFPA publishes a great number of standards relating to combustible dusts (available for viewing at www.nfpa.org). The main standards are: NFPA 654 – Standard for the Prevention of Fire and Dust Explosions for the Manufacturing, Processing and Handling of Combustible Particulate Solids (a revised, more comprehensive NFPA 654 will be issued in June of this year); NFPA 69—dealing with explosion/deflagration prevention; and NFPA 68—relating to vent of deflagrations to force that hazard to a safe area.

ATEX – Testing and Certification
As OSHA is gathering information to determine what it will develop for future laws relating to combustible dust, one thing being looked at is Europe’s closest equivalent to NFPA standards—ATEX. ATEX stands for ATmosphere EXplosive. It is the law in the European Union for dealing with combustible dust and potentially explosive concentrations of gas, vapor, or mist in the air.

There are two main ATEX directives: ATEX 1999/92/EC, which concerns the safety and health of workers potentially at risk from exposure to explosive atmospheres; and ATEX 94/9/EC, which concerns the testing and certification of equipment and protective systems used in potentially explosive atmospheres. In the countries of the European Union, only ATEX-certified equipment is allowed in locations where there is potential for combustible dust explosions. Although there is currently no testing  certification required in the United States, American manufacturers are seeing the benefits of equipment that has been explosion tested and has achieved ATEX certification. For equipment manufacturers, in addition to providing “tested” equipment to their U.S. customers, they are also able to sell into the European market and to U.S.-based European companies that require their equipment to meet the more stringent European standards.

While ATEX is currently only required in Europe, it could well become the International standard (including the United States, as ISO 9000 did years ago). Contact your dust collection  experts to learn more.

Resources: www.osha.gov; www.nfpa.org

Niels Pedersen has more than 30 years experience working in the dust collection/extraction industry. Dantherm Filtration (formerly DISA Systems) designs, manufactures, and services dust, fume and mist collection systems for a wide range of industries all over the world.  For more information, visit  www.danthermfiltration.us.