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Technical Features

Is Your Facility Combustible Dust Compliant?

US Chemical Safety Board (CSB) Sugar_Plant_Explosion_CDC.jpg
The 14 deaths at Imperial Sugar could have been avoided with proper dust collection, conveyor and bucket elevator monitoring and venting, and housekeeping.
Combustible dust compliance is the best insurance against a deflagration event and its consequences.

In our world of information overload there is no lack of “buzz” words. To those of us who work in industries that are concerned with the hazards and risks of fire, flashfire, and explosions, the latest “buzz” words are “combustible dust compliance.” At first glance, this appears to be self-defining, but in practice it is not necessarily a simple goal.

What exactly is combustible dust compliance? There is no accepted definition offered by the National Fire Protection Association (NFPA). So, due to my experience as a participating member of all six combustible dust committees (61, 484, 652, 654, 664, and Correlating), the following definition fore Combustible Dust Compliance (CDC) is offered for our use: A process of determining the current hazards and associated risks inherent in the handling of combustible dust, and determining the best method or methods required to eliminate or mitigate these same hazards and associated risks.

Not exactly something easy to remember. However, fundamentally, CDC involves determining the hazards and risks you currently have and then how to mitigate or eliminate them.

General Requirements for Combustible Dust Compliance (CDC)

There are CDC requirements that are generally applicable to any facility where combustible dusts are handled and/or processed. This information is found in the first eight chapters of NFPA 652 (Standard on the Fundamentals of Combustible Dust-2019), and in the commodity-specific combustible dust standards (NFPA 61, 484, 654, and 664).

As previously stated, for a facility to be Combustible Dust Compliant, it is necessary to comply with the general requirements of these first eight chapters. It is not necessary for the reader to be intimately familiar with the information in these chapters as a general summary can be provided which will provide a satisfactory foundation for further discussion.

First, it needs to be emphasized that the responsibility for CDC is to be borne by the owner/operator of the facility. There is no equivocating of this responsibility. Even if outside assistance is used, such as subject matter experts (SME), the actual responsibility still lies on the owner/operator.

Next, it is the responsibility of the owner/operator to determine what the explosivity characteristics are of their materials. This usually means combustible dust testing of their materials to determine the true hazards and risks involved with the handling and processing of these materials. It does not mean you have to test all your materials as in consideration of explosion mitigation, it is best to use the “worst-case” material explosivity values. In rare cases, historical data can be used regarding very well-known materials such as some grains, chemicals, and raw food materials such as corn starch. However, be careful when using this approach as your material may not be the same as what is listed in the historical data.

Knowing what explosivity characteristics to test for is also important. Too often only Kst and Pmax are tested. This is only of significant value in the determination of deflagration protection and often does not represent the true hazards and risks involved. A low Kst and Pmax does not necessarily mean the material is a low hazard or risk. A low Kst and Pmax material can be a significantly more dangerous material than a high Kst or Pmax material – depending upon such factors as Minimum Ignition Energy (MIE), Minimum Explosive Concentration (MEC), ignition temperature, and volumetric resistivity.

MIE, MEC, and other factors are usually of more importance to understanding your facility’s hazards and risks in handling and processing combustible materials than Kst and Pmax. The former characteristics define the hazards and risks of everyday handling and processing. Low MIE and low MEC combustible materials/dusts inherently have higher associated risks than high MIE and high MEC materials.

Keep in mind that other material characteristics--such as particle size--are important. If you are handling and processing combustible materials that are normally below 200 microns (or less), then your hazards and risks are generally more significant than if most of your material is over 1,000 microns.

Determining the explosivity characteristics of your materials is only part of what is the most critical Combustible Dust Compliance requirement – the Dust Hazards Analysis or DHA. Presently, all existing facilities should have completed their DHA by September 2020 (except the agri and food industries that have until January 1, 2022). Unfortunately, many facilities have not completed this critical step in Combustible Dust Compliance.

Consider this question: Are you willing to gamble the lives of your employees and yourself, and the property of your facility by not diligently completing the required DHA? It would appear immensely logical to assume no one would deliberately accept this risk. Does it not make sense to understand and know how to handle your potentially hazardous materials and to determine the best methods for reducing associated hazards and risks to an acceptable level?

That, simply considered, is the purpose of a DHA. It involves gathering a group of people expert in the operations, processes, and handling of your combustible materials and in the methods to mitigate and/or minimize any hazards or risks involved. This is done once for any existing facility (if management of change is properly followed) and reviewed every five years.

Any new system that is anticipated must be evaluated by a DHA before it is purchased and installed. This will avoid many costly after-the-fact problems that often occur with new systems. All systems, whether new or existing, must have a DHA.

Too often a task such as this is done and then quickly forgotten. The results of the DHA and the CDC actions that are designated, must be completed on a timely basis (often based upon risk level). Ignoring the results can lead to severe problems with OSHA, fire marshals, etc. State regulations often include compliance with the International Building and Fire Codes, which now mandate compliance with NFPA combustible dust standards.

Chapter 8 of the standards is about management systems or general activities of a facility that affect the safety of personnel and property. This includes, but is not limited to, housekeeping, management-of-change (MOC), emergency planning, operating procedures, training, hot work, PPE, and document retention. Facilities that apply and follow these management systems are Combustible Dust Compliant. Those that ignore them are not.

Specific Requirements for Combustible Dust Compliance

The list of specific requirements for CDC is extensive and often dependent upon which commodity-specific (e.g. food, wood, metal, chemical, etc.) NFPA standard is involved. There are fundamental requirements presented in the 652 standard, but many facilities have variations in equipment types, use, and materials which requires further consideration. Also, given the limitations on the length of this article and the complexity of the subject only common systems and/or equipment items will be considered.

Buildings must consider the equipment and material hazards and risks contained within that building. Venting of walls, separation, segregation, and detachment are various methods used to provide mitigation and/or elimination of hazards. For example, locating a wood planning operation in a separate area will assist in isolating the high risks associated with such a process, as would milling operations, etc.

Dust collection systems represent one of the most recognized hazardous and higher risk systems where combustible dust are involved. The most important single step in assuring a CDC dust collection system is proper design using the balanced-by-design method. Use of slide or “blast” gates to attempt to modify the flow in a dust collection system results only in non-compliance. All dust collection systems must ensure, at all times during normal operation, that sufficient air flow is provided to keep all the ducts clean of accumulations. This cannot be done with slide or “blast” gates. Proper hood design that follows the three C’s of containment, capture, and control is required. Also, any change in the addition or subtraction of dust sources requires a complete review of the design to assure the system will function properly.

Centralized vacuum cleaning systems have different requirements for proper design and use. They are not the same as dust collection or pneumatic conveying but share the requirement to operate with sufficient velocity (i.e. air volume) in the piping or ducts to not allow accumulations. Too often these systems are improperly designed by using manifolding for multiple, simultaneous operators, while ignoring the fact that most systems use one or two operators at most. Manifolding into larger diameter lines results in insufficient velocity and results in plugging. Designing to assure this does not occur is a CDC requirement.

When properly designed, installed, and operated, pneumatic conveying or transfer systems represent one of the most proven and safest methods of handling combustible materials. Whether dilute phase, semi-dense phase, dense phase, etc., these systems can be used to safely convey even very high hazard/risk materials. The key, again, is proper design. Only truly qualified persons and/or companies should be used.

For full Combustible Dust Compliance of dust collection, vacuum cleaning, and pneumatic transfer systems, it is also necessary to consider the main equipment items (i.e. dust collector, filter receiver, fan package, blower, etc.) and the hazards they represent. This includes ducting, piping, and other portions of the equipment system.

Fully enclosed bucket elevators represent one of the highest hazards and risk types of equipment when handling and conveying combustible materials. Deflagration event records confirm that bucket elevator bearings and belt-and-pulley alignment represent consistent ignition source risks. As a result, for CDC monitoring of the head and tail section bearings, belt alignment, and pulley alignment are required. Additionally, since monitoring does not eliminate the possibility of a deflagration/explosion event, it is necessary to protect the bucket elevator against those hazards and to provide isolation to prevent propagation to connected additional combustible dust sources such as bins or silos. Dust collection is also strongly recommended for CDC as a method to help mitigate the hazards involved.

Screw conveyors, drag conveyors, and enclosed belt conveyors do not typically represent higher hazard or risk bulk material handling equipment. With proper dust collection, monitoring of bearings, maintenance, and housekeeping CDC can be achieved. Care must be taken to consider the development of ignition sources for creation of embers that would be carried by these devices.

Mixers and blenders are often ignored as sources of ignition and resulting combustible dust hazards. For CDC it is important to assure the incoming material has been thoroughly “cleaned” of unwanted hazardous materials such as metals, rocks, or other foreign material that can lead to upset conditions and creation of an ignition source. The process of filling the mixer/blender with material must also be considered for CDC. The filling process often creates clouds of combustible dusts (e.g. air displacement and bag dumping as examples) inside and outside of the equipment. For CDC, control of these emissions is required.

Milling operations are often higher frequency sources of ignition. A common example is milling processes that involve hammermills. CDC requires assuring that foreign material is prevented from entering the mill and monitoring of its performance parameters (to indicate an upset condition). Often these devices are capable of withstanding significant Pmax levels but there is no consideration of the possibility of propagation of the explosion event to the equipment before and after the mill. CDC requires mitigation of propagation due to an event in the mill.

Storage bins, silos, day-bins, etc., represent a complicated subject for CDC. It is incorrectly assumed that NFPA requires deflagration protection for all bins, silos, etc. This is not true as one of the purposes of the DHA is to determine which of the facility’s storage vessels represent a viable or credible hazard that requires mitigation. This requires an honest assessment of the hazards and risks to determine if methods can be used to assure the hazard is mitigated and/or eliminated to the level that deflagration protection is not required to achieve CDC.

Material feeding devices (e.g. screw feeders, vibrating feeders, etc.) and screeners represent low-hazard and low-risk devices. With proper maintenance and operational use of this type of equipment no special CDC mitigation is required. However, these devices should be part of the DHA to determine if there are unusual conditions that do require mitigation.

Dryers, inherently, represent significant combustible dust hazards and an obvious ignition source. There are a wide range of dryer types and only through a properly done DHA can it be determined the type and severity of the hazards and risks involved. All dryers require monitoring, and many require deflagration protection. This represents the minimum for CDC. Often the commodity-specific NFPA standard can provide additional information to assure your dryer operation achieves CDC.

Combustible dust compliance at a minimum requires owner/operator responsibility, knowing your material’s explosivity characteristics, and performing a thorough DHA for existing and new equipment systems and processes. Additionally, there are certain common types of equipment and equipment systems that require specific CDC considerations. Combustible dust compliance is the best insurance against a deflagration event and its consequences.

Jack Osborn, senior project engineer, Airdusco Engineering and Design Services LLC. Osborn has 45+ years experience in the concept, design, project management, start-up, and operations/maintenance for an extensive range of mechanical equipment systems, including dust collection, ventilation, bulk handling mechanical systems (pneumatic and mechanical), storage, weighing, mixing, batching, etc. He is also a member of all six NFPA committees on combustible dusts (61, 484, 652, 654, 664, and correlating). For more information, call 901-362-6610 or visit www.airdusco.com.

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