For owners and operators in the agricultural and food processing industries, January 1, 2022, was the deadline for completion of a dust hazard analysis (DHA) for existing facilities in accordance with Chapter 7 of National Fire Protection Association (NFPA) 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 the protection of 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, and soybeans, their by-products, as well as other agricultural 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.
Having completed their mandated DHA, many grain and food processors are working to determine the next steps 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.
Following preparation of the DHA document, the job is not finished. The DHA team must then develop an action plan for the identified hazards and prepare a prioritized action item list with specific tasks, assigned parties, target timeframes 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. The next step is implementation. Before doing so, owner/operators should consider some key questions,” 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.
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 with knowledge on the application of the relevant NFPA standards?
- Are any areas of the DHA 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 we know how to implement the action items arising from the DHA?
- What are our protection options to achieve compliance?
Nixon recommends reviewing or updating the DHA with a safety expert if anything is missing or unclear, since the risk and potential liability of insufficient compliance and protection can be very costly if a combustible dust explosion were to occur in a facility.
“There are a range of solutions to provide cost-effective protection against a combustible dust hazard, but each facility’s choice will vary depending on its specific needs. 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,” says Nixon. “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. 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 venting
Explosion 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, venting does require regular inspection per NFPA 68. It 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 pressure. This allows the rapidly expanding combustion gases and dust/air mixture to escape to the atmosphere and limits 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 in a sufficiently strong process enclosure.
As an example, explosion vents can be applied to bucket elevators located outside, or close to an outside wall where the dust explosion can be safely vented outside via short vent ducts. These vents are panels mounted onto the leg casing(s) and elevator head that open rapidly to relieve the explosion pressure of rapidly burning dust, known as a deflagration.
For venting solutions, the method of sizing vents and the strength of the equipment are important considerations. The sizing of vents for bucket elevators handling 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 is not the same for both raw and processed grain, with the arising vent quantity and position typically different for each form 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.
Some applications, such as where the boot of a bucket elevator or other food processing equipment is located inside a building or below grade, create a challenge for explosion venting due to the release of flame and pressure into a confined space. These applications require different protection approaches, such as flame-free venting and suppression.
Flame-free vents 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 of toxic byproducts being generated during activation of these devices when located inside a building. For all applications, a safety zone shall be established around the flame-free vent, which will emit hot gases for a short time when venting a deflagration. Additionally, the size of the room in which the protected equipment is located must be considered to avoid pressurizing small rooms when venting the expanding gases from a deflagration.
Flame-free vents consist of a conventional vent mounted in front of a housing with a stainless-steel mesh that extracts the heat of the deflagration while allowing the explosively expanding gases to be discharged safely. This mesh arrests the flame front and at least partially filters the release of dust and soot. Because the stainless-steel mesh partially restricts flow of the expanding gases, flame-free vents are assigned efficiency ratings to ensure correct sizing. The degree to which the vent’s mesh is configured to absorb the pressure wave of the expanding gases and capture dust and soot particles determines the device’s efficiency. Vent efficiency typically ranges between 50% and 95%, which means that a flame-free vent area calculation is going to be larger than a conventional vent for the same application. Great care on this point is required when considering replacing a conventional vent with a flame-free vent.
The weight and efficiency of flame-free vents are considered when selecting them for applications such as the protection of 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. This is in contrast to explosion venting, which allows the deflagration to proceed to completion and exposes the equipment to combustion temperatures.
Explosion suppression equipment is designed to respond in milliseconds to a deflagration. Pressure or flame detectors monitoring the process generate a signal that triggers explosion suppressors to rapidly discharge a flame quenching agent, such as sodium bicarbonate, into the protected equipment volumes. This effectively halts the explosion in its infancy and reduces the explosion pressure to a level 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 conventional or flame-free venting, the explosion fully develops in the process equipment, requiring cleanup, mitigation of fire-related damage, and other consequences that take time before the process can be placed back into operation.
The suppression method of protection 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 sections, as well as explosion isolation of the leg casings, feed and discharge points, and dust extraction points.
Suppression of single-leg bucket elevators is also practical in grain applications, although the open internal volume between boot and head requires additional extinguishing agent injection points. Most importantly, whether applying suppression to a double-leg or single-leg bucket elevator, explosion isolation is recommended to prevent an explosion from propagating to connected equipment volumes such as silos.
Explosion isolation
Explosion isolation is a vital component to any explosion protection strategy. The propagation of a dust explosion can result in secondary events in connected equipment, which 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 is not an intrinsic feature of explosion venting systems. When explosion vents are selected for explosion protection, a means of chemical or mechanical isolation 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/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 to a facility protection plan.
For grain and food processors, dust explosion protection involves many critical decisions beyond the DHA. Selecting dust explosion prevention, protection, and mitigation equipment involves careful consideration of the areas at risk, the methods of protection, relevant NFPA codes, other jurisdictional requirements and the type of material being processed. However, careful attention to dust explosion mitigation and isolation in processing equipment can ensure that both facility personnel and infrastructure are protected. This will also limit the potential for preventable production and business interruptions.
Del Williams is a technical writer based in Torrance, California. He writes about health, business, technology, and educational issues and has an MA in English from C.S.U. Dominguez Hills.
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