Going Beyond Dust Hazard Analysis

Options to address the risk of combustible dust explosions for NFPA 61 compliance

Published: Thursday, January 26, 2023 - 13:02

For owners and operators in the agricultural and food-processing industries, Jan. 1, 2022, was the deadline for completing a dust hazard analysis (DHA) for existing facilities in accordance with Chapter 7 of the National Fire Protection Association’s Standard 61 (2020) for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities.

NFPA 61 and other related NFPA standards detail essential strategies and procedures for protecting people, processes, and property from the hazards presented by fires and dust explosions in facilities handling, processing, and storing bulk grains such as corn, wheat, oats, barley, sunflower seeds, soybeans, their byproducts, and other agriculture-related combustible dusts. All new processes and facilities handling and generating combustible dusts are now required to perform a DHA.

Appendix F of NFPA 61 (2020) provides a comprehensive checklist that serves as a blueprint for generating a DHA. This checklist includes evaluating the dust explosion protection for process equipment that can often be at risk, such as bucket elevators, conveyors, grinders, silos, and systems for spray-drying and dust collection.

Recognizing the risk of noncompliance along with the potential liability, grain and food processors have completed their mandated DHA and are determining what the next steps are to comply with NFPA 61 as a component of their local jurisdictional safety requirements. This includes making decisions relating to the explosion protection of process equipment, which can represent a considerable financial investment requiring careful consideration.

After preparing the DHA document, the job isn’t finished. The DHA team must then develop an action plan for identifying hazards and prepare a prioritized action-item list with specific tasks, assigned parties, target time frames, and required resources. This serves as both a road map for hazard reduction and a working document demonstrating reasonable effort toward managing risk. Recommendations for addressing deficiencies in managing combustible dust hazards must be documented and addressed in a timely fashion acceptable to the authority having jurisdiction.

“The DHA is only the starting point for bringing a facility and process into compliance with the relevant NFPA standards,” says Clive Nixon, sales manager at Tulsa, Oklahoma-based BS&B Pressure Safety Management, a manufacturer of a broad range of dust-explosion prevention and protection technologies. “The next step is implementation. Before doing so, owner/operators should consider some key questions.”

Nixon advises owners and operators of facilities to ask these questions:
• Was the DHA completed by a person or team having credible experience in the facility and its process equipment as well as knowledge of applying the relevant NFPA standards?
• Are there any areas of the DHA that are missing or incomplete?
• Have there been any relevant changes to the facility or process that merit an update to the DHA?
• Are all components of the DHA clear with specific action items, or are clarifications needed?
• Do stakeholders know how to implement the action items arising from the DHA?
• What are the protection options to achieve compliance?

Nixon recommends reviewing or updating the DHA with a safety expert if anything is missing or unclear, because the risk and potential liability of insufficient compliance and protection can be costly if a combustible dust explosion were to occur in a facility.

“There is a range of solutions to provide cost-effective protection against the combustible dust hazard, but each facility’s choice will vary depending on their specific needs” says Nixon. “The technology selection should not only consider factors such as the nature of the dust hazard and its characteristics, but also equipment location, strength, operating pressures, temperatures, and process interconnections. 

“Owners and operators are best served by working with an expert supplier with access to a complete range of safety solutions for a particular application,” he says. “They should be aware that a supplier having only one particular solution may be biased in their recommendations.”

According to Nixon, the primary options for explosion protection and prevention in grain and food processing facilities broadly include explosion venting, explosion suppression, and explosion isolation.

Explosion relief venting

Venting is the most widely adopted protection mechanism because it frequently provides a convenient and economical solution. While it is often perceived as a fit-and-forget solution, it does require regular inspection per NFPA 68. Venting is often the most practical solution for equipment located outside or near to an outside wall where there is a clear path for the projection of a dust explosion fireball to a safe area where it will not endanger people or damage equipment or nearby structures.

During the early stages of a dust explosion, explosion relief vents open rapidly at a predetermined burst of pressure. This allows the rapidly expanding combustion gases and dust/air mixture to escape to the atmosphere and limit the pressure generated within the equipment to calculated safe limits. Most agricultural dust materials would develop a pressure in excess of 100 psi in a fraction of a second if the process enclosure was sufficiently strong.  

As an example, explosion panels can be applied to bucket elevators outside or close to an outside wall where the dust explosion can be safely vented to the outside via short vent ducts. These vents are mounted onto the leg casing(s) and elevator head, and open rapidly to relieve the explosion pressure of a rapidly burning dust, known as a deflagration.

For venting solutions, the method of sizing vents and determining the strength of the equipment is an important consideration. Sizing vents for bucket elevators that handle raw grain is covered by NPFA 61. For processed grain, NFPA 61 defers to NFPA 68, in which the required strength of the equipment is dictated by the material explosivity index (Kst value). The vent-area calculation basis isn’t the same for raw grain and processed grain, so vent quantity and position typically differ for the two forms of material.

The path for the explosion relief flame ball and the accessibility of the vent panels for occasional inspection and maintenance are important considerations when selecting this method of protection. These are covered in NFPA 61 (2020) Section 9.3.14, and NFPA 68 (2018) Section 8.8.

Flamefree vents

According to Nixon, issues can occur when the boot of a bucket elevator or other food processing equipment is situated inside a building or below grade. That creates a challenge for explosion relief venting due to flames and pressure releasing into a confined space. Those situations require different protection approaches, such as flamefree venting and suppression.

Where equipment is located inside, or where people or combustible material are present, conventional vents, which will release a fireball, can be replaced by flamefree vents. These are designed to diffuse the pressure wave and eliminate the flame that would normally be projected by a vented explosion. For applications outside of the grain, feed, and food industries, consideration should be given to the potential for toxic byproducts being generated inside a building when these devices are activated.

For all applications, a safety zone should be established around the flamefree vent that will emit hot gases when venting a deflagration. Additionally, the size of the room in which the protected equipment is located must be factored in to avoid pressurizing small rooms.

Flamefree vents consist of a conventional vent mounted in front of a housing that incorporates a stainless-steel mesh to extract the deflagration’s heat while allowing the expanding gases to discharge safely. This mesh arrests the flame front and acts, at least partially, to filter the release of dust and soot. The stainless-steel mesh represents a restriction to flow, and an allowance is made for this by assigning a vent efficiency to ensure that the correct size of vent is selected. The mesh is configured to partially absorb pressure and capture dust and soot particles to a varying degree that determines the device’s efficiency. Vent efficiency typically ranges between 50 percent and 95 percent, which means that a flamefree vent-area calculation is going to be larger than a conventional vent for the same application. Great attention to this point is required whenever replacing a conventional vent with a flamefree vent is considered.

The weight and vent efficiency of flamefree vents are considered when selecting them for applications such as protecting bucket elevators. The relevant codes are NFPA 61 (2020) Section 9.3.14, and NFPA 68 (2018) Section 8.8.

Explosion suppression

Explosion suppression systems are designed to suppress a deflagration in its initial stage before destructive pressure can be generated—in contrast to explosion venting, which allows the deflagration to proceed to completion while exposing the equipment to combustion temperatures.  

Explosion suppression equipment is designed to respond in milliseconds to the signal generated by pressure or flame detectors monitoring the process. Suppressors rapidly discharge a flame-quenching agent, such as sodium bicarbonate, into the protected equipment volumes. This effectively halts the explosion in its infancy and results in a reduced explosion pressure that is safe for the protected equipment.

“Think of explosion suppression as a fire extinguisher that triggers automatically but at about 1,000 times the speed,” says Nixon.

Suppression systems can be desirable because the speed of cleanup and refit allows for a quick return to production. With venting or flamefree venting, the explosion fully develops in the process equipment, requiring cleanup, mitigation of fire-related damage, and other consequences that take time to get the process back into operation.

The suppression method is often applied to double-leg bucket elevators used in grain handling. Protection consists of explosion detection and suppression of the elevator head and boot section, as well as explosion isolation of the leg casings, feed and discharge points, and dust extraction points.

Suppression solutions for single-leg bucket elevators is also practical in grain applications, although the open internal volume between boot and head will require additional extinguishing-agent injection points. Most important, whether applied to a double leg or single leg bucket elevator, suppression systems are recommended to include explosion isolation to prevent an explosion occurring to connected equipment volumes such as silos.

Explosion isolation

Explosion isolation is a vital component in any explosion protection strategy. A dust explosion can result in secondary events in connected equipment that can be more destructive than the initial event. Isolation should be considered for all process interconnections such as inlet ducts, discharge ducts, and dust extraction points.

Although explosion isolation is a component of explosion suppression systems, it’s not an intrinsic feature of explosion venting systems. When explosion vents are selected for explosion protection, a means of chemical or mechanical isolation also must be considered to prevent explosion propagation to interconnected process volumes via inlet ducts, outlet ducts, conveyors, and dust extraction points. See NFPA 69 for compliant solutions and refer to NFPA 61 (2020) Section 9.7.4.

While it can be tempting for the owner or operator to consider applying partial protection to a process system, care must be taken to manage the risk both from and to connected equipment. A piece of unprotected equipment can be the Achilles heel in a facility protection plan.

For grain and food processors, there are many critical decisions involved beyond the DHA when selecting dust explosion prevention, protection, and mitigation equipment. The areas at risk, methods of protection, relevant NFPA codes, other jurisdictional requirements, and the type of material processed must all be considered.

However, careful attention to dust explosion mitigation and isolation in processing equipment can ensure the protection of both facility personnel and infrastructure. This will also limit the potential for preventable production and business interruptions.

For more information, contact BS&B Safety Systems.

About The Author

Del Williams

Del Williams is a technical writer based in Torrance, California. He writes about business, technology, health, and educational issues, and has a master’s degree in English from California State University-Dominguez Hills.