Combustible Dust Series: Part 3 – What Are the Mitigation Methods for Combustible Dust?

In Part 2 of this series, we learned that NFPA 652, Standard on the Fundamentals of Combustible Dust, provides basic principles and requirements regarding identifying and managing flash fire and explosion hazards due to combustible dust. NFPA 652 requires owners to conduct a Dust Hazard Analysis (DHA) where combustible dusts are present to identify hazards and implement the necessary safeguards. After reviewing the basics of a DHA in Part 2, we’ll now focus on the types of corrective actions that can be implemented once gaps in safeguards or processes have been identified. The National Fire Protection Association (NFPA) and OSHA are the best sources for information pending the completion of a consolidated standard. 

How Can Building Design Minimize Combustible Dust Hazards?

Chapters 8 and 9 of the 2019 edition of NFPA 652, Standard on the Fundamentals of Combustible Dust, provides instructions on hazard management, including methods to mitigate and prevent combustible dust flash fire and explosion hazards. Section 9.2 pertains to building design and provides guidance on protection features, including:

• Dust deflagration venting: You may recall from Part 2 that rapid combustion of dust, known as deflagration, becomes an explosion when confined to a piece of equipment, a vessel, room, or building. Buildings or building compartments containing a dust deflagration hazard must have venting through an outside wall or roof to relieve pressure.
• Access to interior surfaces where dust accumulation can occur: These areas must be designed to facilitate cleaning to minimize dust accumulations. Inaccessible areas must be sealed to prevent dust accumulation but remain accessible for periodic inspection to determine if cleaning is needed.
• Separation of hazard areas: In Part 1 of this series, we learned that initial explosions can set off a chain reaction of subsequent explosions due to accumulated dust in other areas becoming airborne from the initial explosion and also igniting. The remainder of Section 9.2 discusses three methods – segregation, separation, and detachment – to minimize damage from a fire or explosion.

What Equipment Design Protection Methods Are Specified?

Section 9.3 of NFPA 652 specifies that all components of enclosed systems that handle combustible particulate solids must be designed to prevent the escape of dust. Dust should be controlled and contained at its source, and if this is not possible, dust collection methods must be employed. Specific requirements for several types of industrial dust collection systems used in pneumatic conveying systems are spelled out in this section, including dust collection systems and centralized vacuum cleaning systems.

Requirements for air-material separators (AMS), air-moving devices (AMD) – such as fans and blowers, and duct systems are also specified. NFPA 91, Standard for Exhaust Systems for Air Conveying of Vapors, Gases, Mists, and Particulate Solids, is referenced as the governing standard for AMDs and duct systems.

Section 9.3.9 specifies requirements for bulk storage enclosures such as bins, tanks, hoppers, and silos. Generally, fixed bulk storage enclosures should be located outside of buildings if an explosion hazard exists. Interior surfaces must be designed to facilitate cleaning and minimize combustible dust accumulation.

The remainder of NFPA 652 Chapter 9 specifies equipment design criteria for other industrial systems, including pressure protection systems, material feeding devices, enclosed conveyors, and process equipment, such as mixers, blenders, and dryers.

What Types of Ignition Sources Must be Controlled?

Section 9.4 of NFPA 652 examines the various potential ignition sources that must be controlled in an environment containing combustible dust, including (but not limited to):

• Hot surfaces must be at least 112° (50° C) below the dust’s ignition temperature.
• Bearings must be monitored for overheating.
• Confirm, document, and comply with Article 500 of the National Electric Code (NEC) for proper classification regarding electrical installations in hazardous locations.
• To reduce the risk of static electricity, particulate handling equipment must be conductive, with some exception. Furthermore, personnel involved in filling or emptying particulate containers must be grounded during these operations.
• Activities that release or lift combustible dust cannot be conducted within 35 feet (11 m) of an open flame.
• Industrial trucks must be approved for the electrical classification of the area and used in accordance with NFPA 505.

What Measures Must be Taken to Control Dust?

Dust control measures are covered in section 9.6 of NFPA 652, which begins by requiring continuous suction or other methods to control combustible dust emissions from normal operations. Other methods to control dust include liquid dust suppression methods and fans to limit dust accumulation in elevated areas difficult to access for housekeeping. Additional housekeeping methods are covered in section 8.4, including:

• Vacuum cleaning methods: Contains information on portable vacuum cleaners and special requirements for Class II hazardous locations. Also, reference NFPA 484 when metal particles, dusts, or powders are being cleaned.
• Sweeping, shoveling, scooping, and brush cleaning are permitted.
• Water washdown cleaning methods are permitted, but again, reference NFPA 464 when cleaning metal-containing dust or powder.
• Compressed air blowdown methods are acceptable once the methods above have been performed and additional precautions are followed.

It’s important to also comply with other applicable NFPA standards, such as NFPA 654, NFPA 61, NFPA 484, NFPA 664, and NFPA 655, containing additional dust control requirements for specific industries such as manufacturing, agriculture, food processing, combustible metals, wood processing, and others.

Finally, OSHA recommends cleaning dust at regular intervals, using cleaning methods that do not generate dust clouds. Additionally, companies should develop and implement a hazardous dust inspection, testing, housekeeping, and control program. This program should be in writing with established frequency and methods.

Stay tuned for Part 4 of this series, where we’ll examine the implementation of management systems, such as procedures and training to address combustible dust hazards, as well as the use of PPE and flame resistant garments as a last line of defense to protect workers from catastrophic injury.