Dissecting a DHA

Combustible dust fire and explosion expert Jason Krbec discusses the critical role dust hazard analyses play in incident prevention.
By Patrick C. Miller | July 20, 2018

It’s not difficult to understand how fires and explosions occur in pellet mills. Combustible dust in enclosed areas—process equipment, ducts and pipes—can be ignited by a spark, a hot surface or a flame. When mixed with the oxygen in the air, dust becomes the fuel for fires, flash fires and explosions. The dispersion of a dust cloud generated after an initial explosion can lead to even larger fires or bigger explosions.

Most pellet mill operators are aware of the potential hazard of fires and explosions from dust generated by their processes. But what’s the best way to prevent them from occurring? The first step, says Jason Krbec, sales engineering manager for CV Technology, is a dust hazard analysis (DHA).
Krbec notes that in 2020, facilities handling combustible dust will be required to conduct a DHA to meet National Fire Protection Association standards, which also require the analysis to be brought up to date at least every five years.

A registered professional engineer in the state of Florida, Krbec is an active member in the American Society of Mechanical Engineers, the American Society of Safety Engineers and the NFPA. He serves on several NFPA technical committees related to combustible dust. He has experience and knowledge in a variety of mitigation technologies, including explosion venting, flameless venting, mechanical isolation valves and chemical suppression equipment.

CV Technology, a Jupiter, Florida, company that specializes in explosion and fire prevention solutions for industries manufacturing and handling dry bulk material, was founded with the goal of providing a passive means of protection against combustible dust explosions. Its engineering, manufacturing and system integration capabilities can tailor specific safety solutions to conditions within an operation that can put employees and equipment at risk.

At CV Technology, Krbec is involved in the development of mitigation solutions, research and development projects, dust hazard analysis, full-scale explosion testing, dust testing and prevention solutions. He spoke to Pellet Mill Magazine on what pellet mill operators should know about conducting a DHA.

What is a DHA, and what does it entail?

Krbec: DHA stands for dust hazard analysis, and it is a relatively new requirement in the National Fire Protection Association combustible dust standards. A DHA is a review of a facility to identify and evaluate the fire flash fire, and explosion hazards associated with combustible dust. The basic goals of a DHA are to evaluate the hazard, determine what safeguards exist, and identify where safeguards are needed. This requirement is also retroactive to all processes handling combustible dust.

What special challenges do wood pellet plants pose when it comes to understanding and managing dust-related risks at their facilities?

Krbec: Combustible dust poses both a fire and potential explosion risk in wood pellet plants. Both hazards need to be considered, since the material being handled is ultimately making a fuel.
Typical processes in wood pellet plants also lend themselves to potential sources of ignition.

Mechanical conveying equipment can often produce sparks or overheated elements. Milling is always an area of potential ignition source generation. The pellet mills or presses heat up the pellets, and can create issues in the dust extraction or cooler systems. Wood pellets themselves may even be at risk for self-heating or spontaneous combustion, depending on how they are stored. Silo fires have become an expected event that almost every wood pellet plant has had to deal with during operation. The goal of total elimination of fires and explosions is not very practical in a wood pellet plant. Implementing mitigation tactics for fires and explosions is a realistic goal for a wood pellet facility. The DHA is the first step in that process.

Under what circumstances should an outside consultant or expert be brought in for a DHA? What type of qualifications and experience should this person have? How much should a producer expect to spend?

Krbec: The person leading or performing the DHA should be qualified. Generally, that means a level of familiarity with pellet milling processes and some expertise with the hazards of combustible dust. A DHA is generally conducted by a team, as it may take more than one person to achieve this level of knowledge. Team members typically include engineers, operators, maintenance staff and safety personnel. Still, in some cases, an outside expert or consultant should be involved.  Experts are able to provide experience with combustible dust hazards, and typically have conducted multiple DHAs. Consultants can also provide a level of practicality to the process, which is of great benefit. A consultant with DHA experience, knowledge of the NFPA standards, and an understanding of bulk material handling systems is recommended. DHA consultant costs are the same as any other engineering consulting services. Expect the consultant to need one to three days on-site, and another seven to 10 days to complete the report. Additional resources exist that can help a producer understand the DHA process. The Center for Chemical Process Safety has published a book on how to conduct DHAs. NFPA 652 Standard on the Fundamentals of Combustible Dust has an example DHA in the annex of the document.

At what interval do you recommend that pellet mill operators have DHAs conducted?

Krbec: All facilities handling combustible dust should have a DHA completed by September 7, 2020, in accordance with the latest NFPA 652 Standard. NFPA 652 requires a DHA to be reviewed and updated every five years. However, you may need to update a DHA more often depending on your operation. If your process or facility undergoes any modifications, you should review and update your DHA to reflect the changes. An example would be replacing a piece of process equipment like a dust collector. This change may not require the entire DHA to be updated, but you will need to update the section of your DHA that covered the replaced dust collector. A good management of change (MOC) process in a facility is a good start to prompt when a DHA should be reviewed.

What do you consider the least understood aspect of conducting a DHA?

Krbec: A DHA does not always have to take the form of a prescriptive, traditional DHA. In some cases, facilities can decide to go with risk-based approach. A risk-based DHA involves incorporating a severity of the consequence and the frequency of an event aspect to the analysis. A risk-based DHA might be used in conjunction with or as an alternative to a traditional DHA. The challenge with using a risk-based DHA in a wood pellet process is that the risk level is already documented to be higher in these types of operations than other types of processes handling combustible dust. Explosion and fires have occurred in almost every type of process equipment in a wood pellet plant. After all, these facilities are producing a fuel product.

In terms of dust hazards, what are some of the most common hazardous scenarios you see at pellet plants that producers tend to be unaware of? What are the most serious overlooked issues that lead to fires or explosions?

Krbec: Bucket elevators are an often overlooked piece of process equipment in wood pellet plants. Elevators can create ignitions through overheated bearings, misaligned or slipping belts, and sparks generated from friction or impacts. Bucket elevators themselves should be equipped with explosion mitigation systems. Additionally, the discharge and dust aspiration lines of the elevator should be considered for spark detection monitoring to prevent fires and explosions in other areas of the process.

Explosion propagation is the other hazard often overlooked or misunderstood. Many plants will point to explosion vents on a dust collector and think they are properly protected. When you have a deflagration inside a piece of equipment, the flame front and pressure wave can travel through pipelines to interconnected equipment or building compartments. This event is commonly referred to as a secondary explosion. Explosion propagation can be the difference between a localized event and a catastrophic event in a facility. Explosion isolation is a mitigation strategy that can prevent a secondary explosion. Examples of isolation devices include fast-acting mechanical explosion isolation valves or chemical isolation barriers.

What are the most important types of safeguards (equipment or systems) that you most often recommend for pellet mill operations?

Krbec: Wood pellet plants are somewhat unique in that a variety of different safeguards are needed. The fire and ignition hazards are generally addressed using spark detection and extinguishing (usually water) systems that have become commonplace. Explosion venting has become a common mitigation technology seen in wood pellet plants. A growing number of wood pellet plants have equipment located indoors. As a result, safeguards like flameless vents, which can be used indoors, are more common. A flameless vent is an explosion vent with a flame arrestor that allows the vent to be exhausted indoors. Passive safeguards like vents and flameless vents tend to work best in wood pellet operations. These types of safeguards require little or no maintenance and have a longer service life in environment of a pellet mill. Chemical suppression systems are another safeguard used in facilities when venting is less practical. Bearing temperature and belt slippage monitors are very important safeguards that should be included on any type of mechanical conveyor.

What should a finished DHA look like, and what are a producer’s next steps when one is completed? How can pellet mill operators ensure a DHAs long-term effectiveness?

Krbec: A DHA should be your roadmap to providing a safe operating facility. Smaller facilities may only have a document that is a few pages long. A larger pellet operation may have a full report that covers every processing area. The DHA may result in action items like adding safeguards, increasing housekeeping activities, or even changing a process to make it inherently safer. In some cases, your DHA may reaffirm that you have all of the safeguards in place, and now you have them documented. The effectiveness of the DHA really comes down to whether training and hazard awareness systems are implemented to make everyone in the facility understand potential hazards posed by combustible dust. 

Author: Patrick C. Miller
Staff Writer, Pellet Mill Magazine