The potential for a combustible dust explosion is a reality in food and beverage manufacturing operations — even within a dust collection system itself. Dust particles that become airborne vary in size, and some are so fine they are not visible to the naked eye. Common food dust hazards include cereal ingredients, spices, feed and raw grain agricultural products, egg shell dust, flour, corn starch, sugar, tea and flavoring additives.
A dust explosion is caused by a concentrated, confined combustible dust cloud coming into contact with an ignition source. Diligent housekeeping and installation a dust collection system that is properly designed for your operation can significantly reduce airborne dust in the work environment and help to mitigate the risk of a primary or secondary explosion. A primary explosion is the first point where an explosion occurs, and is often an isolated incident. A secondary explosion occurs when the primary explosion pressure disturbs dust that has collected in the workplace, resulting in a much more extensive explosion.
Combustible Dust Watchdogs
Three organizations in the United States are involved in combustible dust issues, each with its own area of responsibility:
- The National Fire Protection Association (NFPA) sets safety standards regarding combustible dust, amending and updating them on a regular basis. Most insurance agencies and local fire codes state that NFPA standards shall be followed as code. Important NFPA standards include:
- NFPA 652, the Standard on the Fundamentals of Combustible Dust, covers the requirements for managing combustible dust fires and explosions across industries, processes and dust types. It requires the owner or operator of any facility where combustible dust exists to conduct a dust hazard analysis, develop a plan for managing the hazards and provide training for personnel potentially affected by the hazards.
- NFPA 654, the Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids, is an all-encompassing standard on how to design a safe dust collection system.
- NFPA 61, the Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities, covers facilities engaged in dry agricultural bulk materials or manufacturing and handling starch.
- NFPA 68, the Standard on Explosion Protection by Deflagration Venting, focuses on explosion venting on devices and systems that vent combustion gases and pressures resulting from a deflagration within an enclosure.
- NFPA 69, the Standard on Explosion Prevention Systems, also covers explosion protection of dust collectors when venting is not possible.
- The Occupational Safety and Health Administration (OSHA), together with local authorities, enforces the standards published by NFPA. OSHA's Combustible Dust National Emphasis Program (NEP) outlines policies and procedures for inspecting workplaces that create or handle combustible dusts.
- The U.S. Chemical Safety Board (CSB) is an independent federal agency responsible for investigating industrial chemical accidents. The CSB conducts thorough investigations of combustible dust explosions to determine root causes and publishes findings and recommendations.
Dust Collectors and Explosion Protection
Combustible dust explosions are a risk in many areas of a food manufacturing plant, but one of the most common locations is the dust collection system itself. Fortunately, there are many types of devices and systems that help to protect dust collection systems from explosions and keep them in compliance with NFPA standards. These devices fall into two general categories: passive and active.
Passive devices include explosion vents, flameless vents, passive float valves, backdraft dampers and flame front diverters:
- An explosion vent opens when predetermined pressures are reached inside the dust collector, allowing the excess pressure and flame front to exit to a safe area. It is designed to minimize damage to the collector and prevent it from blowing up in the event of a deflagration.
- A flameless vent is installed over a standard explosion vent in order to extinguish the flame front exiting the vented area, preventing it from leaving the device. Flameless vents enable venting indoors, where it otherwise could endanger employees and/or trigger secondary explosions. It's necessary to establish an area per the manufacturer’s specifications around the flameless vent for safe release of pressure, dust and gases.
- A passive float valve is installed in the outlet ducting of a dust collection system. This valve uses a mechanical barrier to isolate pressure and flame fronts caused by an explosion, and to keep them from propagating further through the ducting. The pressure of the explosion closes the mechanical barrier, which reacts within milliseconds.
- A backdraft damper is placed in the inlet ducting. It uses a mechanical barrier that is held open by the process air and is slammed shut by the pressure forces of the explosion. When this barrier is closed, it isolates pressure and flame fronts from spreading further up the process stream.
- A flame front diverter redirects the flame front to the atmosphere and away from the downstream piping. These devices typically are used between two different vessels equipped with their own explosion protection systems.
Active devices include chemical isolation, chemical suppression and fast-acting valves:
- Chemical isolation creates a chemical barrier that suppresses the explosion within the ducting, eliminates the propagation of flame and minimizes pressure increase within connected process equipment.
- Chemical suppression detects an explosion hazard inside the dust collector and within milliseconds and releases a chemical agent to extinguish the flame before an explosion can occur.
- A fast-acting mechanical valve creates a physical barrier within the ducting that isolates pressure and flame fronts from either direction, preventing them from propagating further through the process.
Combustible Dust Hazard Analysis
NFPA 652 and 654 require a combustible dust hazard analysis (DHA) to assess risk and determine the necessary fire and explosion protection. You can conduct the analysis internally or use an independent consultant. Ultimately, the authority having jurisdiction will review the findings and grant approval. This DHA must be completed by the fall of 2020 and demonstrate reasonable progress toward completion of the DHA in each of the years approaching the deadline.
The first step in a hazard analysis is determining whether your dust is explosive. Data values indicate the amount of pressure an explosion can generate and how fast it can travel. NFPA classifies dusts according to their explosibility in terms of their Kst values.
Kst is the normalized maximum rate of explosion pressure rise, measured in bar m/s. A bar is a metric unit of pressure, which is slightly less than the average atmospheric pressure on earth at sea level. In addition to Kst, it is important to know other combustible dust properties such as Pmax (the maximum explosion pressure of a dust cloud, measured in bar) and Pred (the maximum pressure developed in a vented enclosure during a vented deflagration). These can be determined using ASTM E 1226-10, Standard Test Method for Explosibility of Dust Clouds.
NFPA Class ST1 dusts are rated below 200 Kst, Class ST2 dusts range from 200 to 300 Kst, and Class ST3 dusts are rated above 300 Kst. As a rule of thumb, when dusts reach 600 Kst, they are so explosive that wet collection methods are recommended. However, any dust above 0 Kst is considered to be explosive, and the majority of dusts fall into this category. For example, sugar has a relatively low Kst, but it fueled a tragic explosion in 2008 at the Imperial Sugar refinery in Georgia that killed 14 people. If OSHA determines that even a very low Kst dust is present in a facility with no explosion protection in place, a citation will result, per OSHA's NEP policy.
Your dust collection equipment supplier will need the Kst and Pmax values at a minimum in order to correctly size explosion venting or suppression systems. Failure to provide this information will increase your costs, since the supplier will have to use worst-case estimates of the Kst and Pmax values or may even refuse to provide the equipment.
Because combustible dust issues are complex and incidents can be devastating, it is important to use an experienced independent professional engineer that works in conjunction with the supplier to help you design and install your dust collection system. That way you can be certain that your food or beverage manufacturing facility complies with NFPA requirements and those of your local fire marshal and insurance carriers.
David Steil is a Market Manager at Camfil Air Pollution Control.