Dusts produced when manufacturing and processing food products create significant challenges. Dust particles often become airborne, which can threaten employee health and cause combustible dust incidents. Food dust particles 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, and flavoring additives.
Manufacturers must comply with Occupational Safety and Health Administration (OSHA) regulations to protect their employees from exposure to airborne dusts, as well as National Fire Protection Association (NFPA) standards to provide a safe working environment. In addition, food processors must follow regulations from the USDA and FDA, which has begun implementing the Food Safety Modernization Act (FSMA).
Manufacturers need to control the dusts generated in food and beverage facilities that can:
- Cause serious harm to human health and negatively impact the environment;
- Cross-contaminate and proliferate the spread of pathogens and allergens; and
- Become combustible and cause devastating explosions that harm workers, damage machinery, and destroy buildings and corporate reputations.
OSHA regulations govern employers whose processes generate dust, and will issue citations and fines for lack of compliance. Food industry employers are required to protect workers from exposure, and each food or beverage manufacturing application will have its own unique set of process conditions. Under OSHA, companies must control dust emissions into the indoor workplace atmosphere to comply with legal limits set for a particular material. If no legal limits are applicable, then the company is required to define in writing, implement, and measure its own environmental safety plan.
Traveling dust in a food processing plant can result in allergen exposure or a pathogen outbreak from the spread of microorganisms. Preventing cross-contamination requires effectively cleaning equipment and processing suites—collecting and removing all contaminants before they become widely dispersed. Collecting, controlling, and filtering pathogens and allergens minimize the spread of harmful contaminants and keep them from returning to the processing environment.
Regulations Governing Occupational Exposure and Cross-Contamination
OSHA. OSHA 1910 is a broad, general standard that covers most industries. It is a comprehensive and complex standard with 20 subsections. The only food industry sector that has its own separate standard is agriculture, which is covered by OSHA 1928.
A list of key parts of OSHA 1910 that are important to the food processing industry and require dust control include:
- 1910.22: Walking-Working Surfaces;
- 1910.134: Personal Protective Equipment;
- 1910.263: Bakery Equipment;
- 1910.272: Grain Handling Facilities; and
- 1910:307: Hazardous (Classified) Locations.
In addition to the above standards relevant to the food processing industry, OSHA has issued a Safety and Health Information Bulletin titled Occupational Exposure to Flavoring Substances: Health Effects and Hazard Control.
The FDA’s FSMA requires food processing facilities to implement measures to ensure contamination hazards will be minimized or prevented. These include controls for processes, food allergens, sanitation, and supply chain, as well as having a recall plan. Food processors must also include and document actions to:
- Identify and correct a problem implementing a preventive control;
- Reduce the likelihood the problem will recur;
- Evaluate affected foods for safety; and
- Prevent those food products from entering commerce if they cannot ensure that the affected food is not adulterated.
FSMA gives the FDA the authority to suspend a food facility’s registration if there is a “reasonable probability” the food product in question will cause serious adverse health consequences or death to humans or animals. A suspended license means the food produced in that facility can no longer be sold.
The mission of the USDA Food Safety and Inspection Service (FSIS) is to protect public health by ensuring the safety of meat, poultry, and processed egg products. As a public health regulatory agency, FSIS investigates reports of foodborne illness associated with FSIS-regulated products. If a foodborne illness investigation determines that a food product contains a pathogen or is otherwise harmful to human health, FSIS could recommend a recall of that item. Other possible agency actions—like initiating criminal, civil, or administrative action—depend on the evidence collected and how strongly human illness is linked to the FSIS-regulated product.
Controlling Exposure to Dust
The best way to reduce hazardous dust exposure and cross-contamination is to install dust collection systems with high-efficiency primary and secondary cartridge-style filters. Primary filter media should be selected for each application based on the dust particle size, flow characteristics, quantity, and distribution. If the primary filtration system does not use a HEPA filter, it is recommended that a secondary HEPA filter be used downstream. Secondary filters prevent hazardous dusts from discharging to the atmosphere and can be configured to prevent return air ducting contamination and the associated costs of cleaning hazardous dust leakage.
A wide, uniformly pleated filter allows the collected dust to release from the filter, keeping the resistance lower through the filter for a longer time. When pleats of the filter media are tightly packed, the reverse-pulse cleaning system of the dust collector will not eject the dust that has settled in between the pleats; tightly packed pleats increase the resistance of the air through the filters and diminishes airflow.
There are two basic categories of media commonly used in pleated cartridge filters. The choice is usually driven by dust type, operating temperatures, and level of moisture in the process.
- Nonwoven cellulosic blend media is the most economical choice for dry dust collection applications at operating temperatures up to 160 degrees Fahrenheit (71 degrees Celsius).
- Synthetic polyester media or polyester-silicon blend is a lightweight, washable media that can handle dry applications with maximum operating temperatures ranging from 180 degrees Fahrenheit (82 degrees Celsius) up to 265 degrees Fahrenheit (129 degrees Celsius). These filters are washable and can recover from a moisture excursion, but they are not intended for wet applications.
Standard and nanotechnology filter media treated with a flame retardant are recommended for applications considered a fire risk. Conductive or antistatic filters may be used where conveyed dusts generate static charges that require dissipation. Cartridge filters with antistatic media can also be used in explosive dust applications, making it possible to conform to NFPA requirements and lessen the risk of ignition sources due to static electricity charges.
High-efficiency dust collection systems also use self-cleaning mechanisms that regularly pulse dust off the filters, allowing units to run longer between filter change-outs. When a layer of nanofibers is applied on top of the base filter media, it promotes surface loading of fine dust and prevents the dust from penetrating deeply into the filter’s base media. This translates into better dust release during cleaning cycles and lower pressure drop readings through the life of the filter.
Combustible Dust Explosions
A dust explosion occurs when a confined and concentrated combustible dust cloud meets an ignition source. Many solid food and beverage ingredients produce explosive dusts including sugar, starch, flour, spices, tea, grain, and proteins. Good housekeeping and installing a well-designed dust collection system can prevent airborne dust from building up in the work environment, on electrical equipment, and on other areas where dust can accumulate, such as false ceilings.
These measures help negate the risk of a primary or secondary explosion. The primary explosion is the first point where an explosion occurs and is usually an isolated incident. A secondary explosion occurs when the primary explosion pressure disturbs the dust collected in the areas mentioned above, creating a far more extensive and potentially deadly explosion.
Regulations Governing Combustible Dust
In the U.S. there are three key entities involved in combustible dust issues, each with its own area of responsibility.
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. Exceptions would be where the authority having jurisdiction, such as Factory Mutual or local fire marshals, specifies an alternative safety approach, which might be even more stringent.
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.
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, sifting through evidence to determine root causes and then publishing findings and recommendations.
Relevant NFPA Standards
In trying to sort through the list of combustible dust standards, a good starting point is NFPA 652, the Standard on the Fundamentals of Combustible Dust. This covers the requirements for managing combustible dust fires and explosions across industries, processes, and dust types. The owner or operator of any facility where combustible dust exists is responsible for conducting a dust hazard analysis to identify hazards, create a plan for managing hazards, and provide training for anyone affected by 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, covers explosion protection of dust collectors when venting is not possible.
Mitigating Combustible Dust
In food manufacturing, it is critical to know the explosive potential of the dusts, gases, and dust/gas mixtures emitted during processing. NFPA states that a hazard analysis is needed to assess the risks and determine the required level of fire and explosion protection from combustible dust. The analysis can be conducted internally or by an independent consultant, but either way the authority having jurisdiction will ultimately review and approve the findings.
The first step in a hazard analysis is determining whether a facility’s dust is explosive. NFPA classifies dusts according to their explosibility, that is, 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. (See Combustible Dust Properties list.)
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 approach 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. 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.
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.
A dust collection equipment supplier will need the Kst and Pmax values to correctly size explosion venting or suppression systems. Failure to provide this information will increase 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.
Dust Collectors and Explosion Protection
Combustible dust explosions are a risk in many areas of a food processing plant, but one of the most common locations is the dust collection system itself. There are many types of devices and systems used to comply with NFPA standards for the explosion protection of dust collection systems, but they fall into two general categories: passive and active.
Passive devices include the following.
Explosion venting. Designed to be the “weak” link of the dust collector vessel, an explosion vent opens when predetermined pressures are reached inside the 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, thereby reducing the safety hazard.
Flameless venting. Designed to install over a standard explosion vent, a flameless vent extinguishes the flame front exiting the vented area, not allowing it to exit the device. This allows conventional venting to be accomplished indoors where it could otherwise endanger personnel and/or ignite secondary explosions. A safe area around the flameless vent still needs to be established due to the release of pressure and dust/gases.
Passive float valve. Designed to be installed in the outlet ducting of a dust collection system, this valve utilizes a mechanical barrier to isolate pressure and flame fronts caused by the explosion from propagating further through the ducting. The mechanical barrier reacts within milliseconds and is closed by the pressure of the explosion.
Backdraft damper. A mechanical backdraft damper is positioned in the inlet ducting. It utilizes a mechanical barrier that is held open by the process air and is slammed shut by the pressure forces of the explosion. When closed, this barrier isolates pressure and flame fronts from being able to propagate further up the process stream.
Flame front diverters. These devices divert the flame front to atmosphere and away from the downstream piping. Typically, these devices are used between two different vessels equipped with their own explosion protection systems. The flame front diverter is used to eliminate “flame jet ignition” between the two vessels that could overpower the protection systems installed.
Active devices include the following.
Chemical isolation. This system 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. This system detects an explosion hazard within milliseconds and releases a chemical agent to extinguish the flame before an explosion can occur.
Fast-acting valve. This valve creates a mechanical barrier within the ducting that effectively isolates pressure and flame fronts from either direction, preventing them from propagating further through the process.
Effectively controlling the dusts generated in food manufacturing facilities is an essential, life-saving legal obligation. Dust can cause serious harm to employee health, reduce product quality, and cause devastating explosions that can hurt or kill workers and bring irreparable damage to a food processing operation. A high-efficiency dust collector designed specifically for your facility is an accepted and proven engineering control that will filter hazardous contaminants to make indoor environments safer.
Steil, pharmaceutical market manager at Camfil Air Pollution Control, holds a Bachelor of Science degree in safety and industrial hygiene from the Indiana University of Pennsylvania. Reach him at David.Steil@camfil.com.