Preventing Combustible-Dust Fires and Explosions

Dec. 6, 2010
A recent report on controlling static hazards in combustible dust atmospheres reveals that from 1980-2005 there were 281 explosions in U.S. operations, resulting in 199 fatalities and 781 injuries.

It has been two years since the devastating Imperial Sugar refinery explosion, which cost 14 employees their lives in an accident that the U.S. Chemical Safety Board — an independent federal agency that investigates industrial chemical accidents — was “entirely preventable.” The explosion at the plant near Savannah, GA, was under investigation for over 19 months. It was the second most-deadly industrial explosion in the U.S. in the last decade, and the worst dust explosion that the CSB had ever investigated.

A study by the U.S. Chemical Safety Board analyzed sectors with recorded incidents of combustible dust fires and explosions from 1980-2005.

According to the chairman of the CSB, John Bresland, poor equipment, poor maintenance, and poor housekeeping were the causes of the accident. “If the dust was not allowed to build up, this terrible accident would not have happened,” Breslin stated.

Whether the accumulation results from sugar refining, as at the Savannah plant; or from more recognizable particulates, such as combustible resin in a 1999 case Jahn Foundry Corp. in Springfield, MA, controlling static hazards in combustible dust atmospheres is very often a simple matter of diligence.

Newson Gale has been supplying hazardous-location static grounding protection for over 25 years. Recently, the company published a report on controlling static hazards in combustible dust atmospheres. It cites a CSB finding that from 1980-2005 there were 281 explosions, resulting in 199 fatalities and 781 injuries in U.S. operations. Similar studies in the U.K. and Germany found 303 explosions over a nine-year period, and 426 over 20 years, respectively.

The ignition of a combustible dust cloud is contributed to by several factors: 1) a dispersed dust cloud-oxygen mixture above minimum explosion concentration (MEC); 2) physical containment of the dust cloud that will lead to rapid pressure build-up, causing deflagrations out of process equipment; and 3) a heat source with enough energy to ignite the combustible atmosphere.

Incendive electrostatic sparks usually result from the lack of a thorough and detailed risk assessment, unintended changes to equipment during routine maintenance, and unsafe operator working practices. To prevent explosions, operators should risk-assess their processes and equipment to ensure any potential sources of ignition are identified and managed correctly. According to an U.S. Occupational Health and Safety Administration bulletin, facilities should identify the following factors in order to assess their potential for dust explosions:
• Materials that can be combustible when finely divided;
• Processes which use, consume, or produce combustible dusts;
• Open areas where combustible dusts may build up;
• Hidden areas where combustible dusts may accumulate;
• Means by which dust may be dispersed in the air; and
• Potential ignition sources.

The primary factor for assessing these hazards is whether the dust is in fact combustible. Different dusts of the same chemical material will have different ignitability and explosibility characteristics, depending upon many variables (particle size, shape, moisture content). These variables can change while the material is passing through process equipment.

Industrial settings may contain high-energy ignition sources, such as welding torches or heating sources. Test methods for dust ignition and explosion characteristics from ASTM International, and standards from the National Fire Prevention Association (NFPA 654-Standard for the Prevention of Fire and Dust Explosions from Manufacturing, Processing, and Handling of Combustible Particulate Solids) should be researched and considered during these assessments, too.

It is critical to consider multiple locations in any assessment. An obvious site for a dust explosion is where it is concentrated, such as dust collecting equipment, or where dust can settle. The amount of accumulation, particle size, method of distribution, ventilation system modes, air currents, physical barriers, and the volume of the area in which the dust cloud exists or may exist also should be considered.

After hazards have been assessed and hazardous locations are identified, dust control, ignition control, damage control, training, and management must be addressed.

Dust control can be handled by applying the NFPA 654 standard mentioned earlier. Some of the steps include:
• Minimize the escape of dust from process equipment or ventilation systems;
• Use dust collection systems and filters;
• Provide access to all hidden areas to permit inspection;
• Clean dust and inspect for dust at regular intervals; and
• Use cleaning methods that do not ge erate dust clouds if ignition sources are present;
• Develop and implement a hazardous dust inspection, testing, housekeeping, and control program — preferably in writing, with established frequency and methods.

The OSHA ventilation standard (29 CFR 1910.94) contains requirements for certain types of operations.

Controlling ignition is also covered by the NPFA 654 standard, and several recommendations are made in this area as well, including:
• Using appropriate electrical equipment and wiring methods;
• Controlling static electricity including bonding of equipment to ground;
• Controlling smoke, open flames, and sparks;
• Controlling mechanical sparks and friction;
• Separating heated surfaces and heating systems from dusts; and,
• Adequately maintaining all equipment.

Newson Gale’s product line of static-grounding products and monitors and associated printed publications are recommended to help companies formulate their best practices for controlling static hazards in combustible dust atmospheres.

If such an explosion does happen, proper damage-control steps can minimize the danger and damage from the explosion. The NPFA 654 standard suggests:
• Separating and segregating the hazard;
• Deflagration venting of a building, room, or area;
• Pressure-relief venting for equipment;
• Provision of spark/ember detection and extinguishing systems;
• Explosion protection systems;
• Sprinkler systems; and,
• Using other specialized suppression systems.

Finally, the training of employees and management is where it really all begins. If the workers closest to the source of the hazard are trained to recognize and prevent hazards associated with combustible dust in the plant, they can be instrumental to recognizing unsafe conditions, taking preventative action, and/or alerting management. A qualified team of managers should be responsible for conducting a facility analysis prior to the introduction of a hazard and developing a prevention and protection scheme tailored to the operation. Training also should include identifying how workers and managers can encourage reporting of unsafe practices and facilitate abatement actions.

Individaual Air Sampling

CASELLA USA’S Apex line of personal air sampling pumps feature two intrinsically-safe versions, the Apex Standard I.S. and Apex Pro I.S. Both are fully compliant to ATEX 100 (94/9/EC0 and UL 913 ANSI standards and are approved to undertake personal and area sampling for dust, fumes, gases, and vapors in hazardous, classified areas where potentially explosive atmospheres may be present. When used with a low-flow adapter assembly, the pumps have flow rates between 5-200 ml/min. The Apex I.S. pumps employ digital control technology, for long-term flow stability with an accuracy rate less than ±3% of set-point. The Apex Pro programmable version can be configured via software with scheduled on/off sampling regimes, allowing the unit to be left onsite to automatically sample when required.