Dust is a well-known hazard around tissue mills. Controlling it is an around-the-clock effort.
At the 75th annual Pulp & Paper Safety Association (PPSA) meeting held in St. Petersburg, FL, in 2018, Mike Snyder, DEKRA Insight, presented a paper: “Conducting dust hazard analysis (DHA) using NFPA 652 (2019): Practical tips and approaches.”
He said that combustible dust fire explosion needs five conditions he described as a “pentagon”: confinement, ignition source, fuel, suspension, and an oxidant. He noted that often the secondary or tertiary dust explosions cause the most damage.
Secondary explosions pose additional risks. A primary explosion can disturb an existing dust layer creating a cloud. The blast wave can create a secondary explosion.
Speaking about the pentagon, Snyder said that if confinement is deleted, then a flash fire may occur. Delete suspension and a regular fire can start. By removing any other leg, there will be no explosion or fire.
He noted that any particle of less than 500 microns is combustible.
Snyder went on to describe the basis of safety for preventing and mitigating hazard management. Mills should avoid flammable atmospheres, eliminate ignition sources and provide against the consequences of ignition.
Managing combustible dust fire and explosion hazards requires a detailed knowledge of combustible material properties, process equipment, operating conditions, maintenance practices, and existing controls. These are generally collected and analyzed through a DHA, using NFPA 652 (National Fire Protection Association).
As a framework for dust hazard management, NFPA 652 states that the owner/operator of a facility with potentially combustible dust shall be responsible for:
• Determining combustibility and explosibility hazards of material;
• Conducting a DHA—identifying and assessing fire, flash fire, and explosion hazards;
• Managing identified fire, flash fire, and explosion hazards;
• Establishing safety management systems.
Snyder also discussed the methodology and evaluation of a DHA. He provided some excellent tips for controlling combustible dust atmospheres.
Equipment should be maintained and operated in a manner that minimizes the escape of dust. Continuous local exhaust ventilation should be provided for processes where combustible dust is liberated in normal operation so as to minimize its escape. Collectors should be installed for this dust. Regular cleaning frequencies should be established for floors and horizontal surfaces such as ducts, pipes, ledges, and beams to minimize dust accumulation within operating areas of the facility (1/32 in., 0.8 mm).
Dust collectors play an important role in collecting and removing fugitive dust and controlling accumulation. There are some key design requirements to consider. There must be adequate air transport velocity in the ductwork to prevent settling and accumulation as well as proper design and direction of explosion venting. Collectors are typically installed outdoors. Specific design features are required to recirculate air. Never store dust in a hopper.
DEKRA does have a safety guide, which is a strategic guide to the characterization and understanding of handling dusts and powders safely.
For those interested in more about safety and the PPSA, the 76th Annual Safety and Health Meeting will be held June 23-26 at the Marriott Riverwalk Hotel in San Antonio, TX. Along with a number of sessions devoted to various occupational health and safety issues such as safety leadership strategies, OSHA update, contractor safety, and employee engagement, there will also be a New Technology Showcase and vendor exposition with the latest in safety equipment and services on display.
The meeting includes the annual awards ceremony where the PPSA’s most prestigious prize, the Executive Eagle, will be bestowed, along with numerous other honors.
For more information, contact: [email protected] or ppsaconference.org.