• Publication Date: 07/01/2020
  • Author(s):
    Osborn, Jack E.
  • Organization(s):
    Airdusco Inc.
  • Article Type: Technical Articles
  • Subjects: Dust collection and dust control

Jack Osborn | Airdusco

No matter what industry you’re in, the deadline for your facility to complete a dust hazards analysis (DHA) is drawing near. Whether you’ve never done a DHA or done the analysis only once, it’s a fairly simple process that, once completed, will last you 5 years. This article describes what a DHA is, why it’s necessary, and how to accomplish a successful DHA.

Procrastination seems to be a favorite American pastime, but when it comes to completing your facility’s dust hazards analysis (DHA), time is truly running out. If your facility handles combustible dust, such as those subject to potential fire, flash fire, or explosions or a combination of these conflagration dangers, then you have until September 7, 2020, to complete a DHA for each of your facilities. The only exception to this requirement is that the agricultural and food processing industries have until January 1, 2022, to complete their DHAs.

DHA and the NFPA

Considering all the problems associated with the coronavirus pandemic, there was the possibility that the DHA deadline would be delayed, but that’s very unlikely to happen. This is mainly due to the fact that all industries handling combustible dusts have been required to complete a DHA since September 7, 2015. Although NFPA 61: Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities added more time to complete the required DHA during the latest cycle (2020 edition), all other NFPA commodity-based standards — NFPA 484 for metals, NFPA 654 for chemicals, NFPA 664 for wood — have kept the deadline at 5 years from 2015 for completing the required DHA.

The NFPA is a nonprofit, nongovernmental organization, and therefore, it has no enforcement authority. However, NFPA 652 and subsequent commodity-based combustible dust standards are now mandated by the International Fire Code (IFC) and International Building Code (IBC), which do enforce the DHA requirement. Also, OSHA may not have any regulations about DHAs, but OSHA is able to make combustible dust compliance compulsory by other various enforcement methods and the agency has done so on multiple occasions.

The requirement for hazards analysis in NFPA standards dates back to 2002 for the wood industry, 2006 for the chemical and chemical-related industries, and 2015 for the metals industry. The food industry didn’t specifically require a DHA until NFPA 61’s 2017 edition. However, the requirement became universal for all industries working with combustible dusts starting September 7, 2015, with the publishing of NFPA 652.

A DHA is described best in NFPA 652: Standard on the Fundamentals of Combustible Dust, Chapter 7, “Dust Hazards Analysis,” and the associated annex. Basically, any new system or facility requires a full DHA, which is a guide used to improve facility safety by identifying the specific combustible dust hazards associated with a facility’s processes. After reviewing your facility and processes and identifying potential combustible dust hazards, the DHA helps you to create a plan to minimize or mitigate those risks. Completing a DHA is an ongoing requirement. Also, except for the agricultural and food businesses, this DHA requirement is retroactive and applies to all existing facilities and process systems. The agricultural and food standard, NFPA 61, limits the retroactive portion to certain types of systems and equipment, but ignoring any possible hazard source makes no sense because of the hazard’s potential for harmful personnel and property consequences.

Your DHA’s purpose

What’s the actual purpose of a DHA? First, a DHA evaluates the hazards that exist in your processes. Second, a DHA determines the best method or methods to manage those hazards. Third, this analysis creates a plan to complete the tasks or methods necessary to mitigating or eliminating those hazards. The overall DHA purpose is to determine the hazards, eliminate or mitigate the hazards, and ensure the safety of personnel and property. Because of the DHA’s purpose, completing a DHA before a system is fully designed is most logical and best practice. This allows for any adjustments to be made to the system or process. Then, once the system has been put in place and used, the DHA can be revised and confirmed as appropriate.

Personnel involved in your DHA

A critical question to ask yourself is who should be involved in the DHA. Ultimately, a facility’s owner or operator is responsible for the completion and content of the DHA and the subsequent implementation of any necessary DHA action plan based on the results. Other experts or people who assist in the DHA won’t be held responsible. Normally, conducting and completing a DHA isn’t feasible for just one person — even a knowledgeable one. An experienced facilitator is recommended to help perform the DHA. This facilitator can be someone familiar with process hazards analyses (PHAs) or the NFPA combustible dust standard requirements. Having one or more individuals who are fully experienced in the DHA process and bulk handling requirements of the processes involved is also critical. This could include processes such as dust collection, conveying, drying, and milling, among others. These individuals must also be fully up to date on the NFPA combustible dust standards and related regulatory requirements, for example IFC and IBC. Additionally, the plant manager; plant engineer; environmental, health, and safety (EHS) manager; maintenance manager; and other managerial personnel are typically involved in a DHA.

Additional part-time participants would include operators, supervisors, and other personnel directly involved in various plant operations where combustible dusts are present. Perhaps the most critical person involved would be the secretary or recorder, who documents the discussion and results using an electronic display device. Proper documentation of the process and results is critical, and these records must be maintained for the life of the process, facility, or both where applicable. There’s no need to involve regulatory agencies in your DHA as this analysis is strictly for the facility. And while the completed DHA isn’t made public, the documentation must be kept for plant reference and planning and must be available for regulatory personnel if requested.

A word of caution: Ever since NFPA 652 initiated the need for the DHA, so-called “experts” have been coming out of the proverbial woodwork. If experts are employed to assist in a DHA, verify that they’re truly experts in the selected field. Ask them for proof of this, which can range from interviews and documents to references. Some customers duped by “experts” have spent thousands of dollars on atrocious DHA results that weren’t suitable for the bottom of a birdcage.

Planning your DHA timetable

One of the most frequently asked questions regarding DHAs is how much time the analysis will require. The time it takes to complete a DHA depends upon multiple factors such as your facility’s complexity, the processes and bulk handling systems involved, and the number and type of combustible dusts involved. Some DHAs can be completed in a single day, while others have taken a week or more to finish. What’s certain is that a DHA needs to be done only once every 5 years, barring any changes that affect the initial DHA results. Often, a completed management-of-change document can suffice for changes that affect the initial DHA results, but this advice doesn’t apply to all process changes; some may require an all-new DHA to be completed. And again, a DHA does require updating every 5 years.

Preparing for your DHA

Before beginning your DHA, certain tasks should be completed to provide adequate information to properly execute the DHA. Some of these tasks are optional, depending upon multiple factors and preferences, and some are compulsory. Material testing, or obtaining the data from previous test results, of the involved combustible dusts is a highly important task that must be completed before the DHA begins. This task is referred to as “hazard identification” and is covered in Chapter 5 in all standards that are 2019 editions and later.

The following is a compressed version of the mandatory material testing requirements.

  • Test your materials for KSt, Pmax, minimum ignition energy (MIE), minimum explosion concentration (MEC), and any other explosivity characteristic — such as volumetric resistivity, ignition temperature, and others — that would influence the DHA. Having historical data available for your materials in lieu of testing is suitable in some cases.
  • If many materials are involved, testing them all may not be necessary. If the materials are handled similarly, testing those that cause the most damage in an explosion is acceptable.
  • Make sure the material sample used is truly representational of the material.
  • The sample must also represent the actual hazards and risks. Therefore, it’s recommended that the sample be a dust as opposed to larger particles as dust is more flammable and, thus, is the true hazard.
  • Document everything to make sure you can defend your testing results as you may have to defend your efforts and results in court.
  • KSt and Pmax are necessary to know for mitigation and protection determination regarding explosion vents, isolation valves, and others.
  • MIE and MEC are critical in determining the actual hazards and risk levels in processing and bulk handling operations, such as conveying, milling, and dust collection. MIE and MEC can also indicate materials that are subject to increased risk of electrostatic discharge ignition.
  • Don’t ignore other characteristics such as particle size, moisture levels, adhesive and cohesive properties, temperature, and others. Any variable that’ll influence the analysis should be considered.

While the previous tasks are compulsory (to this author), the following are additional prerequisites that’ll prove most helpful to the DHA.

  • Provide up-to-date schematic flow diagrams of the processes and systems where combustible dusts are present.
  • Provide layout drawings where feasible with building information.
  • Take pictures, videos, or both of the processes. Pictures help people remember and understand where obvious problems exist.

Additional good-to-have information includes documents associated with NFPA 652, Chapter 8, “Hazard Management: Mitigation and Prevention.” Document examples include management of change, emergency plans, operating procedures and practices, personal protective equipment (PPE) requirements, housekeeping schedules, thresholds and methods, hot work requirements, training, contractors, incident investigation as well as others.

Method of analysis

There isn’t a defined or set method to use to complete a DHA. However, there are methods that have proven to be effective, thorough, and applicable to all types of combustible compliance issues. Typically, methods used to complete a PHA will work with a DHA, but the DHA only focuses on combustible dust issues. Some people in the powder and bulk materials industry have successfully used a checklist–what-if combination for hazard determination and a risk assessment matrix for determining the current risk level and what must be done to reduce the risk to an acceptable level. A risk assessment matrix, as shown in Figures 1 and 2, is a valuable tool that’s allowed per NFPA 652, Chapter 9, “Management Systems.”

Typical risk assessment matrix example with legend. DHA dust hazards analysis.
Typical risk assessment matrix example. DHA dust hazards analysis.

The remainder of this article will provide a partial example of a typical method for a DHA using the checklist–what-if method. The key is to provide a checklist that covers the equipment and system items as thoroughly as possible, and the accompanying what-if questions should cover all the ranges of possible scenarios. Keep in mind that no checklist nor list of what-if questions can cover every possibility. The people involved in the DHA are responsible for adding items to these lists as they see fit. These lists must be flexible to cover all the foreseeable problems that are feasible, which can include some problems that aren’t even reasonable. The use of a risk assessment matrix is of great help once a hazard is determined.

Coal handling and milling system example DHA

The coal handling and milling system schematic, shown in Figure 3, represents an actual system. Only the dust collection portion, shown in magenta, will be considered since it’s universally applicable.

Coal handling and milling system schematic. DHA dust hazards analysis.

Base information. The coal’s explosivity test results showed a KSt of 155, Pmax of 8.1 bar, MEC of 70 g/m3, and MIE of 500 millijoules. However, this material is also pyrophoric under certain conditions. Additional points to note are that the system is 27 years old and has had two small events over the years. The system is located inside and is elevated with poor access, not to mention there has been no existing performance monitoring or explosion protection.

Zones and nodes. The first step in this DHA is to establish the zones and nodes. The zones represent various portions of the system or systems, and the nodes represent each individual portion of the system that should be evaluated. The dust collection system is zone 3 and includes nodes P (dust collector), Q (rotary valve), R (fan package), and S (ducting and hoods).

Checklist and what-ifs. The following are questions that were brought up for this DHA.

What is the purpose of zone 3?
Zone 3 provides for the collection, control, and disposal of the fine dust particles carried by the process airflow stream to prevent additional dust emissions within the process area.

What is the purpose of node P (dust collector)?
Node P (dust collector) separates the incoming air-dust mixture. The clean air goes through the filters, and the dust goes to the bottom where the particulates will be discharged through node Q (rotary valve) into the pneumatic transfer system. The filters are continuously cleaned using an automatic pulse-jet-type system.

Can combustible dust be suspended in air?
Yes, combustible dust can be suspended in air. Fine dust constantly exists suspended inside node P (dust collector) and especially when the filters are pulse-cleaned with compressed air (continuous and automatic). Dust levels above the MEC are nearly constantly present.

If a deflagration event was to occur inside node P (dust collector), can the heat propagate?
Yes, since there’s no explosion protection or mitigation and no isolation provided, any deflagration in the collector would propagate throughout the entire ducting system and to the multiple hood and venting connections that are dust sources. The result would be catastrophic.

Are there credible ignition sources?
Yes, credible ignition sources include embers from any of the devices directly or indirectly connected to the system, such as the mill, for example. Since the dust is pyrophoric, there’s the possibility that the dust could spontaneously heat as there are multiple ledges and other spaces in the unit where material accumulates and stays for long periods of time. This scenario is a definite and credible hazard with viable ignition sources that could have catastrophic results.

Are there plant personnel present or involved?
The dust collector is elevated, and normally, only maintenance personnel are around the equipment. Operators aren’t normally around the unit.

What methods of explosion management and mitigation exist?
None, except for irregular inspections and pressure differential measurements across the filters. No deflagration protection or other mitigating equipment or methods are provided.

Is special PPE required for personnel around node P (dust collector)?
No special PPE is required except for needing a confined space permit when entering the unit.

Are there other factors to consider?
A multitude of factors in the proper unit and system design should be taken into consideration, but these factors can’t all be covered here.

There are many checklist and what-if items that would be involved in a DHA. The previous text was only an example and only included partial checklist and what-if questions. While not a complete list, the previous questions do represent the type of questions and information that must be provided to make intelligent and viable decisions.

Risk assessment. A risk assessment was done on node P (dust collector) using the risk assessment matrix in Figure 1 as a guide. The group consensus for the coal DHA example was that the risk level was 4A, Occasional and Catastrophic, due to the combustible dust type, lack of deflagration protection, system history, and presence of credible ignition sources. To move the risk level from the red (high-risk) zone to the green (low-risk) zone, which is an acceptable level, the required action includes a dust collection system audit to ascertain the level of performance and condition; obtaining proposals for deflagration and isolation protection; and taking into account system performance monitoring methods.

This same checklist–what-if process was done for nodes Q (rotary valve), R (fan package), and S (ducting and hoods), and in applying this method, you can imagine the review of each node.

Action plan. After the zone determination, or multiple zones if need be, is completed, the next necessary step is to develop an action plan. In this coal handling and milling system DHA, the personnel at the facility realized they knew very little about the actual dust collection system, including the system’s operation, condition, design, flaws, and other system details. The personnel’s action plan included seeking expertise from outside sources to conduct an audit and make recommendations to convert this high-risk situation into an acceptable low- to very low-risk situation. A timetable was established as to when these steps would be accomplished and monthly follow-ups would occur until the situation was resolved.

The basics covered in this article are applicable to all DHAs. Although it’s common sense to do an assessment of the hazards created in a facility that handles combustible dusts, there’s been significant resistance to this concept. Considering the risks involved, ignoring or avoiding the necessity of assessing and analyzing these hazards goes against common sense. Please don’t procrastinate any longer on completing your DHA. And for those in the agricultural and food industries who have about a year and a half until the deadline, please don’t procrastinate either. The time will pass quickly, and the consequences of ignoring the requirement are significant.


For further reading

Find more information on this topic in articles listed under “Dust collection and dust control” in the article archive.

Jack Osborn (901-362-6610) is an engineer at Airdusco. He has over 45 years of experience in dust collection and bulk handling systems and is a member of all six NFPA combustible dust committees (NFPA Standards 61, 484, 652, 654, 664, and Correlating Committee).

Airdusco • Memphis, TN
901-362-6610 • www.airdusco.com

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