Most facilities that are handling combustible dusts are now familiar with the term “Dust Hazard Analysis” or “DHA” and understand that it comes from a standard issued by the National Fire Protection Association (NFPA), nfpa.org. The most referred to standard is NFPA 652 – Standard on the Fundamentals of Combustible Dust. From NFPA 652, the key requirements for conducting a DHA are:
- Existing facilities must complete DHAs for all processes by September 7, 2020 (or January 1, 2022 for Food and Agricultural Facilities under NFPA 61);
- DHAs must be performed or led by a qualified person;
- DHAs must be reviewed and updated every 5 years;
- For new processes or processes undergoing modification, a DHA must be done as part of the project.
As part of the DHA, all process equipment and areas where combustible dusts are generated and handled need to be identified. The purpose of this article is to identify examples of where hazardous atmospheres may be generated and potential ignition sources in those areas.
Hazardous combustible dust atmospheres may exist in the following pieces of equipment (but are not limited to):
- Bins, tanks, and silos;
- Hammermills, pulverizers, and grinders;
- Dust Collectors;
- Conveyors, screw augers, and bucket elevators; and,
- Sifters, screens, and classifiers.
In addition to the equipment processing combustible powders, there is the potential for a hazardous combustible dust atmosphere in rooms and buildings housing this equipment. A hazardous atmosphere could be present in these locations from open processes, from malfunctions and leaks within closed equipment, or from the potential of dust accumulated on overhead surfaces due to inadequate housekeeping. NFPA 499 – Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas is a good resource to help determine location ratings based on example processes.
Once all areas and pieces of equipment where combustible dusts are handled or generated have been determined, we need to then identify all credible ignition sources that could occur in each area or piece of equipment that could pose potential fire, deflagration, and explosion hazards. Ignition sources will generally fall into one of the following categories:
- Open Flames;
- Mechanical Sparks;
- Electrostatic Discharge;
- Hot Surfaces;
Open flames are a powerful ignition source that can ignite any combustible dust. This can be fires external to the process or a result of a fire or deflagration in connected process equipment. Open flames can also result from smoking or performing hot work in hazardous areas. Open flame hazards can be controlled by implementing a good hot work permit program, designated smoking areas away from hazardous locations, installing adequate fire protection systems, and installing fire and explosion isolation systems between connected equipment.
Improperly rated electrical equipment in hazardous combustible dust atmospheres can ignite dust clouds and accumulations. Dust has the potential to enter improperly rated electrical enclosures and come into contact with live electrical components, which could ignite the dust and start a fire. High surface temperature of electrical equipment can also ignite dust accumulations that have settled on top of the fixtures. Because of this, it is important to use properly rated electrical equipment in rooms or locations where hazardous combustible dust atmospheres exist. Electrical fixtures should be installed in accordance with Article 502 of NFPA 70 – National Electric Code
Mechanical sparks are ignition sources that are generated from mechanical action on a solid material such as grinding, impact or friction. Examples include sparks being generated from metal-to-metal contact due to misaligned pieces of equipment or from tramp metal that has entered the system striking metal surfaces. These ignition sources can cause sparks or hot surfaces to ignite a combustible dust cloud inside the piece of equipment. Mechanical sparks can also generate hot particles or embers that could be carried downstream to connected equipment. One way of controlling mechanical sparks is by reducing the velocity of mechanical equipment where alignment may be a concern, by installing bearing, vibration, and/or temperature monitors on equipment such as hammermills or bucket elevators, and by implementing a robust Preventative Maintenance (PM) program to reduce the likelihood of introducing tramp metal due to malfunctions.
Electrostatic discharges can occur when a spark discharges between two electrically conductive objects. Cone discharges can also occur between a charged heap of non-conductive powder and the wall of the container, typically in large silos or bins. Propagating brush discharges can occur in long conveyance lines between highly charged non-conductive surfaces and a non-conductive part or section. It is very important to ensure that electrostatic hazards are controlled. Most static hazards can be controlled through proper bonding and grounding. NFPA 77 – Recommended Practice on Static Electricity is a good resource for bonding and grounding techniques. Explosion protection systems should be installed on bins or silos where cone discharges are possible. Pneumatic conveyance lines should be constructed of conductive or static-dissipative ducting, and all flexible connections should be properly bonded.
Hot surfaces can result from high surface temperature electrical equipment or motors, from high-temperature pieces of equipment such as dryers, ovens, heaters, etc., or from equipment bearings that have not been properly inspected and greased. To control hot surfaces, it is important to use only properly rated electrical equipment and motors in accordance with Article 502 of NFPA 70 – National Electric Code. Good housekeeping practices should ensure that dust is not allowed to accumulate on hot pieces of equipment such as dryers or ovens and that dust is not able to blanket motors, insulating them and causing them to overheat. Bearing, vibration, and/or temperature monitors and a good PM program can also be implemented to ensure that bearings do not overheat.
Self-ignition results from thermal decomposition of deposits inside certain pieces of equipment such as cyclones. This can be controlled by performing routine inspection and cleaning schedules inside the cyclone.
Once we have identified all credible ignition sources for a room or piece of equipment, the DHA will aid in determining specific fire and deflagration scenarios that can occur and evaluate the severity and risk of a fire or explosion. Current safeguards in place are considered and determined whether additional safeguards or ignition controls should be implemented to reduce the risks of a fire or explosion.
CTI has decades of experience in identifying and reducing hazards from combustible dust and in completing DHAs for new and existing processes. CTI is a principal member of both the NFPA 61 Technical Committee for Agricultural Dust and the NFPA 664 Technical Committee for Wood and Cellulosic Materials Processing and has worked with several of the other committees. We offer the following, and many more, consulting services for dust:
- Dust Hazard Analysis;
- Hazardous (Classified) Location Determination;
- NFPA and OSHA Compliance Review;
- Explosion Protection Design and Selection;
- Dust Sampling Plan and Execution
- Dust Explosibility Testing
Please feel free and contact us to discuss the above-mentioned services or if you have any questions regarding the dust hazard present at your facility.