What you can do to protect your facility from a combustible dust explosion
Combustible dust may not be the first thing food manufacturing plant managers think of when they come to work in the morning, but if recent history is any indicator, it probably should be. Several serious incidents in recent years involving dust explosions highlight the need for facility and EHS managers to understand and address this matter to safeguard assets and employees. The sooner they start developing a comprehensive plan to do so, the better.
Dust explosions in the news
The U.S. Chemical Safety Board (CSB), an independent agency charged with investigating industrial chemical accidents, and the Federal Occupational Safety & Health Administration (OSHA) both view dust explosions as a serious hazard. Despite a flurry of activity in recent months in Congress and at OSHA to set tougher standards, no compulsory Federal regulation yet exists - but the risk remains real.
In 2003, the CSB investigated three catastrophic dust explosions. One occurred at West Pharmaceutical Services in Kinston, NC, where plastic powder accumulated above a suspended ceiling and provided the fuel for a secondary dust explosion that killed six employees and injured 38. The second happened at CTA Acoustics in Corbin, KY, where an explosion fueled by phenolic resin powder killed seven people and injured 37. And at the Hayes-Lemmerz automobile wheel plant in Indiana, a violent aluminum powder dust explosion killed one worker and injured six others.
After investigating these explosions, the CSB undertook a larger study to determine the extent of the industrial dust explosion problem. In its investigation, the Board identified 281 fires and explosions over a 25-year period that claimed 119 lives and caused 718 injuries. Nearly 25 percent of these incidents took place in the food manufacturing industry. In its 2006 final report, the CSB made a number of recommendations to OSHA, including:
- Creation of a national Special Emphasis Program (SEP) on combustible dust hazards
- A standard designed to prevent combustible dust explosions based on applicable NFPA standards
- Revisions to MSDS to identify potential combustible dust hazards1
On February 7, 2008 a massive dust explosion occurred at the Imperial Sugar Plant in Port Wentworth, GA. The explosion, caused by a suspension of crystalline sugar, killed 14 people and left several others critically injured. The incident prompted immediate legislative action in the state of Georgia2 and a U.S. Senate hearing on July 29, 2008. At that hearing, John Bresland, chairman of the CSB, provided testimony on the hazards of combustible dust:
"Combustible dust can be a catastrophic explosion hazard at American workplaces. Since the CSB was established in 1998, three out of the four deadliest accidents we have investigated were determined to be combustible dust explosions."3
So, what is the answer? In the same report, the CSB noted that many of the incidents it investigated could have been avoided if certain consensus engineering standards had been followed. Many of these standards are already published by the National Fire Protection Association (NFPA). And OSHA last year issued a national emphasis program (NEP) on combustible dust directing their field offices to inspect facilities that generate and handle combustible dust.
For now, however, it will be up to companies and facilities managers to be proactive. The first step in fulfilling this obligation is to understand how dust explosions happen.
Science behind dust explosions
Most people think explosions only occur in the presence of highly combustible materials such as volatile chemicals or gasoline. The truth is, under the right conditions most organic substances, including wood (cellulose), polymers, metals, and many food products can explode when suspended in a dust cloud.
Explosions in general are caused by the simultaneous presence of three factors: fuel in a powder or gaseous form, such as sawdust, flour or any substance that can be burned; oxygen; and an ignition source. If all three elements are present at the same time, an explosion may follow. The only way to prevent explosions is to remove at least one of the three factors.
The ignition source is not typically part of the manufacturing process, and therefore is the variable that can most often be controlled or eliminated. Ignition sources include obvious examples like sparks or flames as well as less obvious electrostatic sources including the material itself, the equipment or even the operator.
Of all the potential ignition sources, static electricity is perhaps the most difficult to control or eliminate. Many industrial operations generate static electricity so it is always a hazard when powders are being handled. Electrostatic charges arise from the separation of two materials and accumulate when they cannot dissipate through a conductive material. All separation processes involve at least one non-conductive material, which means most material handling operations, including food manufacturing, can lead to charge generation and charge accumulation.
Although rare, dust explosions can be fatal or result in serious injury. At a minimum, they can destroy hard-to-replace equipment and disrupt daily business operations. Whether the powder is wheat flour or granular sugar, the potential for an explosion is real, and must be both recognized and avoided.
A blueprint for safety
Plant managers should carefully consider all of the options available to them to safeguard their facility. Some are relatively simple and can be achieved quickly and at little cost. Others are more involved, but also more valuable, and usually require the expertise of an outside consultant that specializes in hazard prevention.
Step 1: Practice good housekeeping
It is important to remember that combustible dust accumulations outside of process equipment provide the fuel for potentially catastrophic secondary dust explosions where ignition sources are present. Proper equipment design and compliance with electrical classification requirements are a must. Administrative controls such as work instructions, hot work permits, and good housekeeping practices can help further reduce the risk of a secondary dust explosion.
However, to truly achieve a higher level of protection against combustible dust, EHS managers should complete thorough materials testing and should consider completing a full process safety audit. Together, these measures can help identify specific areas and processes that can be improved with the ultimate goal being zero dust.
Step 2: Test materials
First and foremost, manufacturers must determine whether or not the materials they handle are combustible. While this may seem logical, it is not always clear at first. In most cases this means testing the material according to OSHA and ASTM prescribed test methods.
There are a number of different tests available, and each reveals something different about the materials being tested. Some of the most useful:
Powder explosion indices per ASTM E1226 (The 20-Liter Sphere Test):
The 20-liter sphere is used to determine the severity of combustible dust explosions. The severity indices determined are maximum explosion over-pressure (Pmax) and maximum rate of pressure rise (KST). Other critical parameters such as minimum explosible concentration (MEC) and limiting oxygen concentration (LOC) can also be measured.
Minimum Ignition Energy, ASTM E2019 (Mike 3.0):
The Mike 3.0 is a custom-designed device used to measure the Minimum Ignition Energy (MIE) necessary to ignite a dust cloud. Knowing the MIE of a material helps in identifying what sources of energy, such as hot surfaces, electrical sparks, and static electricity can ignite a dust cloud.
Auto-ignition Temperature of a Dust Cloud, ASTM E1491 (BAM Oven):
Minimum auto-ignition temperature (MAIT) of dust clouds in air can be measured in the BAM (German Federal Institute for Material Testing) oven.
Beyond this, dust explosion research and testing can be enhanced by broader process safety research and testing that takes into account operation-specific processing conditions. By monitoring these parameters, manufacturers can stay one step ahead of combustible dust.
But material testing is only one part of the equation. The other is designing safer processes.
Step 3: Design inherently safer processes
Experience has shown that proper design of powder handling and processing equipment is the best approach to preventing dust explosion hazards. Since each operation is unique, it is important that highly specific production needs be factored into the design of powder handling and processing systems, which should also comply with the latest consensus safety engineering standards.
User-oriented design can further contribute to safety by incorporating suitable feed systems, good ventilation, proper floor plans and equipment layout, and easy cleaning. It is also critical to involve employees in regular process hazard reviews where handling procedures and protection measures can be continually challenged and refined.
Step 4: Seek professional assistance
Conducting regular safety training sessions and assessments for compliance, testing materials for combustibility, and employing consensus engineering standards are not core competencies for most food manufacturing companies. These process improvements and testing protocols should be implemented in a systematic way as part of an overall safety plan.
Companies that don’t know where to begin should consult an expert. As a start, they should look for a partner with manufacturing industry experience – one that regularly participates in industry forums like NFPA and ASTM and which works closely with government-sponsored initiatives associated with dust explosion and reactive material hazard assessments. This will go a long way in helping to avoid costly citations and preventing business interruption.
Just because a facility hasn’t had a dust explosion yet, doesn’t mean it won’t experience one in the future. Changes in the production environment such as larger volumes or different materials can increase the risk of an explosion at any time. And the unpredictable nature of ignition sources, especially static electricity, can cause even a moderate build up of dust to combust, leaving managers at risk for loss in productivity, loss of physical assets or worse of all, loss of life.
For more information, contact Dean Hamel, Head of Client Services, Americas at firstname.lastname@example.org
1.CSB, Investigation Report No. 2006-H-1 Combustible Dust Hazard Study, U.S. Chemical Safety and Hazard Investigation Board, Editor. 2006. p. 118.?2.Safety Fire Commissioner, Chapter 120-3-24-0.8 Rules and Regulations For Loss Prevention Due To Combustible Dust Explosions and Fire, State of Georgia, Editor. 2008. p. 57.?3.John Bresland, Testimony of John S. Bresland Chairman and Chief Executive Officer U.S. Chemical Safety Board, in U.S. Senate Committee on Health, Education, Labor, and Pensions Subcommittee on Employment and Workplace Safety. 2008, CSB: Washington. p.14.