For buyers and sellers in the processing, handling, and packaging of dry particulates, bucket elevators are essential components in most grain handling facilities. They are also recognized as both a circulation point for combustible dust and a potential ignition source. A dust explosion starting in a bucket elevator can potentially travel to larger downstream volumes such as silos and upstream material handling systems, leading to devastating secondary explosions elsewhere in the facility.
Fortunately, there is a range of solutions available to protect both bucket elevators and associated process equipment from the hazard of dust explosions. To consider and examine the benefits of these solutions for grain producers and the powder & bulk solids industry, we discussed the advantages and benefits of this approach with Clive Nixon of Tulsa, OK-based BS&B Pressure Safety Management, a manufacturer of a range of dust explosion prevention and protection technologies.
Q: What fire and dust explosion protection standards are bucket elevators covered by?
A: In general, bucket elevator protection is covered by National Fire Protection Standards NFPA 61, NFPA 68, and NFPA 69. These standards cover methods to reduce the potential for a dust explosion (deflagration) to occur by controlling potential ignition sources, along with methods to manage a deflagration event through the use of explosion relief venting, flame-free venting, and/or suppression. All these techniques require explosion isolation if they are to prevent propagation of a deflagration beyond the bucket elevator to other parts of the process.
Q: What broad prevention and protection themes should grain producers consider in regard to dust explosions?
A: The broad themes for prevention and protection represent a choice of end points for grain producers. There are many decisions to be made in determining the safety solution. The choice of protection method is determined by the bucket elevator location and, in particular, whether it be installed inside, partially inside, or entirely outside. Venting solutions are popular for their relative ease of deployment; the method of sizing vents and the strength of the equipment varies depending on whether the grain is raw or processed. Raw grains are addressed in NFPA 61, and processed grains in NFPA 68, while NFPA 69 prevention and protection options are applicable to both raw and processed grains.
Q: What causes dust explosions at grain facilities? What equipment is typically involved, and what is the risk?
A: Grain explosions result from an ignition of dust when mixed with air during distribution, storage, or processing operations. In agricultural applications, the material might be larger, but attrition during handling can generate dust particles below 500 µm (microns), which is recognized as presenting a dust explosion hazard.
Equipment where fine airborne dust can accumulate includes mechanical conveying systems. Whenever a conveyor system is being filled or emptied, there is a potential combustible dust cloud at that point that could be ignited. All that is needed is an ignition source.
In the grain industry, equipment where dust is present is typically built for the intended use at or close to atmospheric pressure and is therefore not strong enough to withstand the high pressures associated with confined space dust explosions. In unprotected or poorly protected equipment, this can result in extensive damage and risk injury to personnel.
A small explosion event in a piece of unprotected equipment can trigger larger secondary explosions upstream or downstream. In such cases, the first explosion creates pressure waves that add turbulence and increase dust loading, followed by the large ignition source provided by the initial event.
Q: How can grain producers reduce ignition sources?
A: The first step in mitigating the risk for a dust explosion is preventing an event from occurring by reducing potential ignition sources. The main hazard areas in a bucket elevator are in the boot of the elevator and the head, which can cause an event that propagates up or down the leg casings.
Although ignition sources cannot be eliminated, they can be significantly reduced. For bucket elevators, this prevention takes the form of bearing temperature monitors, belt alignment monitoring, and belt slippage monitoring.
Preventative maintenance on equipment can also help to reduce sources of ignition.
Q: How can grain producers reduce combustible dust levels?
A: Dust extraction can reduce the dust concentration within the process equipment, reducing a potential fuel source. It is important for associated dust collection equipment to be protected.
Outside of the process, it is important to follow good housekeeping procedures since even relatively small amounts of combustible grain dust can pose a dangerous explosion hazard. According to the NFPA, 1/32 of an inch of such dust covering just 5% of the surface area of a room “presents a significant explosion hazard.”
While these measures reduce the potential for a dust explosion, the threat is not eliminated, which drives the need to implement solutions which mitigate the dust explosion once it has started within the bucket elevator. These solutions are designed to prevent explosive pressures from rupturing the bucket elevator and should include methods to prevent explosion propagation to connected process volumes upstream and downstream.
Q: What methods of explosion protection should grain producers consider?
A: The primary options for explosion protection of bucket elevators are explosion vent panels, explosion suppression, and explosion isolation. I will discuss each of these options.
Explosion Vent Panels
Venting is the most widely adopted protection mechanism because it provides an economical solution and is often perceived as a fit-and-forget solution.
During the early stages of a grain dust explosion, explosion vents open rapidly at a predetermined burst pressure, allowing the rapidly expanding combustion gases to escape to the atmosphere and limiting the pressure generated within the equipment to calculated safe limits.
Explosion panels can be applied to bucket elevators located outside or close to an outside wall where the dust explosion can be safely vented to the outside via vent ducts. These panels are mounted onto the leg casings and elevator head and open rapidly to relieve the explosion pressure of a deflagration.
For venting solutions, the method of sizing vents and the strength of the equipment is an important consideration when handling processed grain. For processed grain, NFPA 61 defers to NFPA 68, in which the required strength of the equipment is dependent upon the material explosivity index (Kst value). Bucket elevators handling raw grain are covered by NPFA 61.
The path for the explosion relief flame ball and the accessibility of the vent panels for occasional 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.
There are some applications where the boot of the bucket elevator is inside a building and/or below grade. That creates a challenge for explosion relief venting due to the release of flame and pressure into a confined space, requiring a different protection approach for the deflagration starting in that location.
Active devices such as explosion suppression equipment can help to quell a small fire before it can trigger an explosion.
Explosion suppression equipment is designed to quench an explosion in its early stages before it can create destructive pressures. In the first milliseconds of the event, the equipment signals explosion suppressors to rapidly release a flame quenching medium--such as sodium bicarbonate--into the distribution or storage equipment. This effectively stops 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.
Suppression systems can be desirable, because the speed of cleanup and refit allows for a quick return to production. With venting or flameless 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.
This method of protection is ideal for double leg bucket elevators often used in grain handling. Protection consists of explosion detection and suppression of the elevator head and boot section of the elevator, and 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 will require additional extinguishing agent injection points. Most importantly, whether applied to a double leg or single leg bucket elevator, suppression systems can readily incorporate explosion isolation. See NFPA 69.
Explosion isolation devices are vital to protect connected equipment and piping from propagation of a dust explosion that can result in a secondary event that can be more dangerous and destructive than the initial event. This includes isolating the feed point to a bucket elevator, the discharge point, and any dust extraction ducts.
Although explosion isolation is a component of explosion suppression systems, it is not an intrinsic feature of conventional venting and flame venting systems. With venting systems, a means of chemical or mechanical isolation must therefore be considered to prevent explosion propagation to interconnected process volumes such as dust collectors and silos. Use NFPA 69 compliant solutions and refer to NFPA 61-2020 section 9.7.4.
Q: What final thought do you want grain producers and distributors to consider related to seeking effective, compliant dust explosion protection?
A: For grain producers and distributors, there are many critical decisions involved in selecting dust explosion prevention and mitigation equipment given the areas at risk, the methods of protection, relevant NFPA codes, and the type of material being processed.
However, careful attention to prevention, mitigation, and isolation of bucket elevators can ensure the protection of both personnel and grain handling and storage facilities. This will also limit the potential for preventable production and business interruptions.