For over 90 years, AAF has been a leader in providing dust collection systems that have become a critical component of a facility’s compliance with combustible dust applications. This document is meant to be a guide for end users to use as a reference on how AAF handles typical combustible dust applications and to increase awareness for the necessary precautions needed in these applications.
To continue our leadership in the air pollution control industry, AAF International will work with the customer to supply explosion and fire protection systems that are compliant with the latest NFPA and OSHA standards for dust collectors.
AAF offers explosion and fire protection systems based on the particulars of each application for the utmost safety and protection. Combustible dusts generally present both fire and explosion risks so it may help to consider the management of these risks separately. The following flow chart is a guideline for selecting the proper explosion protection system needed for dust collectors. AAF engineers are always available to assist you in determining the most suitable explosion/fire protection approach for your customer’s application.
Fire Management Strategy
By controlling or eliminating one of the key components of a fire (oxygen, ignition source, and fuel), the risk of a fire in a dust collector decreases dramatically. AAF has many years of experience on best practices to control these items.
When a combustible dust in an enclosure is added to a fire safety situation, it then escalates to an explosion hazard. In a dust collection system, the combustible dust is the fuel and the ignition source could be a spark created from many possible sources. When a filter is pulsed clean, a dust cloud of fine particles are dispersed in a small confined area within the dust collector. If an ignition source comes in contact with the dust cloud, an explosion can occur.
With many years of experience, AAF can offer best practice solutions for fire and explosion risk management strategies.
Fire Protection Systems
Most manufacturing processes can create fine dust particles that can be flammable. Fire management is traditionally focused on three elements necessary for a fire- oxygen, fuel, and heat. It is important to focus on all of these elements, instead of trying to restrict just one. AAF offers a wide range of equipment to help mitigate fire hazards, that can range from equipment used on the ductwork, to equipment used on and after the dust collector.
Explosion vents are the most economical and commonly used solution for explosion protection.
AAF uses low burst pressure, stainless steel membrane style explosion vents, fixed over an opening on the structure of the dust collector. The explosion vents should face a safe area to prevent the risk of injury to personnel and/or secondary explosions. Substantial clearance must be provided to surrounding equipment or facilities. Consult with your appropriate local governing authority regarding the safe release of materials into the atmosphere. Unlike other manufacturers, AAF selects and sizes the explosion vents by the specific Kst and Pmax values for each particular application, and designs the explosion vents and collector to meet NFPA 68.
Collectors with explosion vents will normally require suitable dust discharge devices to isolate the hopper to comply with NFPA 654. Such dust discharge devices include rotary valves, double dump valves, or screw conveyors. Contact an AAF Product Manager for assistance.
Explosion Suppression Systems
Explosion suppression systems per NFPA 69 will be applied to dust collectors where explosion vents are not an appropriate solution (see Flow Chart). In a matter of milliseconds, the explosion suppression system can detect the buildup of pressure for a potential explosion and discharge a suppressant into the enclosed space, extinguishing the deflagration before destructive pressures are created. Typically, the collectors need to withstand a higher pressure (Pred) than normal operation.
Explosion Isolation Systems
NFPA 654 requires deflagration isolation to be considered to stop flame propagation between interconnected vessels. AAF highly recommends high speed isolation valves or chemical suppressors on the inlet and outlet ducts when an explosion vent or explosion suppression system is installed on the dust collector. If the customer recirculates the air back into the plant, NFPA 654 and the OSHA directive require a provision to be installed on the exhaust duct to prevent the transmission of an explosion or fire back into the building. This provision can be an isolation valve or an abort gate depending on the location of collector and duct work.
Below is a general list of equipment that AAF can provide to ensure safety for the end user on their dust collection equipment:
- Explosion Vent with Burst Sensor
- Fire Suppression, Sprinkler System, and Smoke Detection
- Rotary Valve Discharge
- Chemical Isolation Device
- Automatic Fast Acting Abort Gate
- Chemical Suppression System
- Spark Detection & Extinguishing System
- Explosion Isolation Valve
- Actuated Blast Gate
- Spark Traps & Abatement
Safety and Protection
Dust Control Measures
- The dust-containing systems (ducts, dust collectors, and filter housings) should be designed in a manner that fugitive dusts are not allowed to accumulate in the work area (always under negative pressure).
- The facility has a housekeeping program with regular cleaning frequencies established for floors and horizontal surfaces, such as ducts, pipes, hoods, ledges, and beams, to minimize dust accumulations within operating areas of the facility.
- The working surfaces are designed in a manner to minimize dust accumulation and facilitate cleaning.
- Air changes per hour (if needed) should be determined per ventilation requirements set by ASHRAE Standard 62.1
- Control Rooms should be under positive pressure to keep dust from entering
Safety and Protection Measures
- The facility has an emergency action plan.
- Dust collectors are not located inside of buildings. (some exceptions- see flowchart)
- Dust collectors have explosion relief venting distributed over the exterior wall of buildings and enclosures. (see flowchart)
- Ductwork designed to operate under same pressure as dust collector to avoid failure during operation and explosion
- Explosion venting is directed to a safe location.
- The facility has isolation devices to prevent deflagration propagation between pieces of equipment connected by ductwork.
- The dust collector systems have spark detection and explosion/deflagration suppression systems.
- Emergency exit routes are maintained properly.
- Ductwork is checked routinely for buildup
Ignition Control Measures
- Electrically powered cleaning devices such as vacuum cleaners, and electrical equipment are approved for the hazard classification for Class II locations.
- The facility has an ignition control program, such as grounding and bonding and other methods, for dissipating any electrostatic charge that could be generated while transporting the dust through the ductwork.
- Areas where smoking is prohibited are posted with “No Smoking” signs.
- Duct systems, dust collectors, and dust-producing machinery are bonded and grounded to minimize accumulation of static electrical charge.
- The facility selects and uses industrial trucks that are approved for the combustible dust locations.
- Emergency exit routes are maintained properly.
- The facility has separator devices, such as spark traps or water sprays, to remove foreign materials capable of igniting combustible dusts.
- Material Safety and Data Sheet (MSDS) are available for materials which could become combustible dust under normal operations to employees.
- Employees are trained on the explosion hazards of combustible dusts.
- Employees follow proper procedure for discarding the material per OSHA and EPA requirements.
Information was taken from “Combustible Dust - Does your company or firm process any of these products or materials in powdered form?” OSHA, 2008. Please reference this poster for more information. All dusts should be tested if concerns are present.
AAF Combustible Dust Statement
The National Fire Protection Association (NFPA) has issued standards and guidelines related to prevention and mitigation of effects of fire and explosions. In addition, the Occupational Safety and Health Administration (OSHA) enforce regulations related to workplace health and safety that relate to NFPA standards and guidelines.
As a manufacturer of dust collectors, AAF will recommend to end users best practices to comply with current NFPA and OSHA regulations. However, compliance with applicable codes and standards remains the responsibility of the end user. Among other considerations, the current NFPA standards require end users whose processes involve potentially combustible materials to have a current Hazard Analysis, which can serve as the foundation for the end user’s hazard mitigation strategies.
Upon request, AAF can assist end users to incorporate AAF Products into a comprehensive fire and/or explosion mitigation strategy. To provide this support, AAF needs complete and accurate information on all potentially combustible contaminants and the fire and/or explosion mitigation strategies that the end user intends to pursue. If the customer has their dust tested, they should use a sample that most resembles the dust at the inlet of the dust collector (not swept up from floor or a bulk volume sample).
AAF can assume the Kst and Pmax based on the dust name from the customer. The value will be selected from NFPA listings or other reliable resources. Actual Kst and Pmax may vary depending on the dust particle size, dust source and ingredient, etc. Customer MUST verify and agree to use the assumed information for the application. AAF will not assume responsibility for any damage caused by unverified Kst and Pmax values.