Safely Convey Combustible Dusts
Unload & Convey Dusts...
Safely Unload and Convey Combustible Dusts Abhi Bhargav Bhargava a REMBE Inc.
Dust explosions are a major hazard of unloading and conveying operations. Protect your facility from explosions with venting panels, ﬂameless venting systems, and chemical suppression systems.
any chemical process industries (CPI) faciliin a way that prevents and/or mitigates combustible dust ties receive raw materials in bulk quantities in explosions. National Fire Protection Association (NFPA) (NFPA) trucks or railcars. Depending on the distance to Standard 652 (1), (1), Section 9.5, provides guidance on training the downstream equipment and the nature of the product, and hazard awareness. the material can be unloaded via open conveyors, such as NFPA Standard 69 (2) denes (2) denes various prevention belt conveyors, conveyors, or enclosed enclosed conveyors, conveyors, such as drag-cha drag-chain, in, methods, such as deagration prevention by reducing the screw, screw, and pneumatic conveyors. concentration of oxidant or combustible material, and For example, raw food ingredients like sugar and our pre-deag pre-deagratio ration n detecti detection on and control control of ignition ignition source sources. s. are almost always transported in enclosed conveyors to These measures can eliminate the requirement for explosion prevent prevent foreign foreign materials materials from entering entering the the product product stream. stream. protection protection in certain certain systems, systems, as well as reduce reduce downti downtime me Cleanability Cleanability and 100% discharge of product are also caused by deagrations in a protected system. In cases important in food applications, which makes pneumatic pneumatic conveying conveying system systemss a popular popular choice choice,, Bucket Elevator Bucket Associated especially over shorter distances. Products Elevator Head Dust Collector like grain and coal are typically transferred by mechanical conveyors, such as screw conveyors and drag-chain conveyors. The unloading process (Figure 1) is particularly critical because any sparks or ignition Screening Truck sources introduced at this point may travel to and Metering Unloading Equipment the entire downstream process. To help prevent explosions, plant personnel should be trained to recognize combustible dust explosion risks and methods to mitigate those risks. Product delivMechanical ery protocols for truck drivers and unloading Unloading Storage Silos Conveyors supervisors should be established — even for (Screw Conveyors) Receiving protected protected systems systems — to to prevent prevent false triggering triggering Bin Bucket (i.e., inadvertent over-pressuring, which may Elevator Boot cause explosion vents to rupture or chemical suppressant to be released, etc.). And, convey1. Many facilities receive feedstock in bulk quantities by truck and use conveyors p Figure 1. Many to transfer material to do wnstream equipment. ing processes must be designed and operated
Copyright © 2018 American Institute of Chemical Engineers (AIChE)
where prevention methods will not sufce, explosion protecprotection systems allow for safer operation and reduce the probability of deagration events in downstream equipment.
Explosion prevention methods The combustible dust explosion pentagon (Figure 2) is a useful graphic for understanding the risks associated with a particula particularr process. process. The penta pentagon gon identi identies es the ve eleme elements nts necessary to create an explosion: combustible product (fuel), dispersion of dust, connement of a dust cloud, ignition source, and oxygen. The risk of combustible dust explosions can be eliminated if any one (or more) of the pentagon elements can be entirely eliminated. Proper operating procedures and equipment conguration can reduce or prevent ignition hazards that might otherwise spread through the conveying system and create a combustible dust explosion risk. For example, allowing trucks to cool down before starting the unloading process can reduce the ignition hazards from hot brakes, hot exhaust pipes, etc. All unloading equipment, including trucks, should be properly grounded, and a grounding monitoring system should be installed to protect against static discharges. Magnets, screens, and sifters should be installed in the unloading process to remove foreign materials, another common ignition source. Conveyors have additional potential ignition risks, which can be redu reduced ced by limi limitin ting g the the conv conveyo eyor’s r’s speed, speed, select selecting ing approappro priate priate materi materials als,, and and using using a safet safety-c y-comp omplia liant nt desi design gn (1). However, it is not always practical (or, in some cases, even possible) to assuredly eliminate one of the ve eleele ments of the explosion pentagon by mere prevention means, which is critical to a safe process without explosion protection. In most processes, explosion protection systems must be implem implemente ented d to maint maintain ain proces processs efcienc efciency y and safety safety in a cost-effective manner.
Explosion protection systems Conveyors differ in their design, so their protection requirements will vary vary.. Open conveyor belts are considered to be the least hazard-prone, since the conveyed material does not typically create a dust cloud and is not in direct Combustible Dust (Fuel)
2. A pentagon t Figure 2. A Dispersion
Ignition Source (Flames, Sparks, Friction, Heat, etc.)
Oxygen in Air
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depicts the ﬁve elements needed to create an explosion: combustible dust (fuel), dispersion of dust, conﬁnement of a dust cloud, ignition source (ﬂames, sparks, friction, heat, etc.), and oxygen.
contact with hot surfaces. Although an open belt conveyor does not allow an explosion to propagate because it lacks an enclosure to conne a dust cloud, it can still allow a smoldering re to propagate and cause a deagration in the downstream equipment. Thus, explosion isolation is required at the outlet of the conveyor. Housekeeping measures must be in place for open belt conveyors because they are prone to spillage, which can lead to secondary dust explosions. Secondary dust explosions occur after an initial explosion stirs up dust that has accumulated on surfaces near the conveyor. When the accumulated dust becomes airborne, it also ignites. It is important to note that secondary explosions are much more severe because because of their higher higher initial initial pressures pressures and and availab availability ility of thermal energy. energy. An often-cited example of a secondary dust explosion is the Imperial Sugar Co. incident that occurred on Feb. 7, 2008, in Georgia, which caused 14 fatalities and injured 36 people (3). Explosion Explosion venting venting device devices. s. Enclosed Enclosed conveying systems typically require explosion venting devices such as explosion venting panels or ameless venting systems, depending on several factors, including: • material characteristics, e.g., particle e.g., particle size, moisture level, dusting propensity • conveyor conveyor type and and speed, e.g., metal-to-metal e.g., metal-to-metal contact, internal bearings • interconnected plant equipment • zoning zoning,, e.g., electrical e.g., electrical hazard classications such as Class 2, Div. 1, etc. • ignition risk assessment (dust hazard analysis) [Editor’s note: For a discussion of dust hazard analysis, see Murphy, Murphy, M. R., R. , CEP, Apr CEP, Apr.. 2016, pp. 28–32.] Explosion venting panels (Figure 3) are among the most cost-effective ways of relieving pressure during an explosion. These panels are designed to open at a set pressure, which takes into consideration the dust properties as well as the cross-sectional area, total length, and strength of the associated conveyor. conveyor. The panels act as the weak point in the system to relieve the pressure and prevent structural damage to the equipment. If the explosion ames and pressure cannot be directed toward a safe area, a ameless venting or chemical suppression system must be used. Flameless venting systems are a cost-effective, passive solution consisting of a vent panel surrounded by a metal mesh (Figure 4). The mesh allows for pressure relief while capturing the heat and ames from the explosion event. Advantages of ameless venting systems include minimal maintenance requirements requirements and low cost of ownership. Explosion venting panels are most commonly used for outdoor applications, whereas ameless vents are used for both indoor indoor and outdoor outdoor applica applications tions.. Even with explosion venting devices, ames and prespresCopyright © 2018 American Institute of Chemical Engineers (AIChE)
t Figure 3. Explosion venting panels, such as these (indicated by the yellow ovals) on a bucket elevator, relieve pressure to prevent structural damage to equipment.
4. A ﬂameless u Figure 4. A venting system, such as this one installed on a bucket elevator, consists of a vent panel surrounded by a metal mesh that allows for pressure relief while capturing the heat and ﬂames of an explosion.
Chemical suppression systems require regular maintenance and inspections by authorized personnel, which can increase the cost of ownership of such systems. On the other hand, passive protection devices, such as explosion venting panels panels and ameless ameless ventin venting g systems, systems, requi require re minima minimall main main tenance, and inspections can be performed by trained plant personnel. personnel. In additio addition, n, replace replacement ment costs for an explosion explosion panel panel are signicant signicantly ly lower lower than than those those of active active systems. systems.
Protecting downstream equipment sure can propagate to upstream and downstream equipment. Therefore, explosion isolation may also be required. Explosion Explosion isolat isolation: ion: Materi Material al chokes. chokes. The The mass of bulk solids or powders contained in an enclosed mechanical conveyor or rotary airlock can provide a tortuous path through which gas and ames have to pass, thus isolating an exploexplo sion. This mass of product is referred to as a material choke. Therefore, under certain conditions, enclosed conveyors can be used used for explosion explosion isolat isolation. ion. NFP NFPA A 69 (2), (2), Section 12.2.4, covers the use of material chokes for the isolation of solids processing processing equipment. equipment. It is important to note that the reliability of a material choke varies based on several factors. For a screw conveyor, the orientation and speed of the conveyor, the owability and burning index (BI) of the product, and other design constraints must be taken into account. It is also important to note that material chokes are only suitable for dusty products. Any such isolation methods, as well as any explosion protection systems in general, must be approved by the authority responsible for enforcing any applicable codes or standards. In other situations, such as interconnected enclosures or conveyors handling chunkier products, rotary airlocks must be used as material chokes. Where material chokes are not possible, chemical suppression must be used to provide isolation. Explosion Explosion isolat isolation: ion: Chemic Chemical al suppre suppression. ssion. Chemical suppression systems employ pressure detectors or infrared sensors to identify ames and explosions. When an exploexplosion occurs, within a few milliseconds, the suppression system triggers the release of a chemical agent that extinguishes the explosion.
Copyright © 2018 American Institute of Chemical Engineers (AIChE)
Many facilities use the conveyors discussed previously to discharge product from a truck in bulk. Product is then transferred into bucket elevators (Figures 3 and 4), one of the most common ignition sources. Bucket Bucket elevat elevators. ors. Reference Reference 4 describes work by Kauffman that emphasized the essential role played by bucket elevators in 14 carefully investigated grain dust explosions in the U.S. Five of the explosions originated in the bucket elevator. elevator. In six other accidents, bucket elevators amplied and propagated the explosion, although the combustion process process did not origin originate ate there. there. In only only three three of the 14 14 cases cases was a bucket elevator not involved. Explosions often originate in bucket elevators because conveying can create an explosible dust cloud inside the housing. The conned dust cloud can easily be ignited to cause deagration. Bearings are a major source of heat, and if the bearings are not mounted externally, externally, temperature monitoring is required. If there is no system to ensure proper belt alignment, a belt alignment monitor is also required. To To protect the equipment in the event of an explosion, the head and boot section of the bucket elevator (Figure 1) can be protected protected with explos explosion ion venting venting panels, panels, amele ameless ss venting venting systems, or chemical suppression systems. Properly protected bucket elevators must be isolated from connected equipment, such as an associated dust collector, silo, trough auger, etc. Aspiration Aspiration lines. lines. Aspiration lines for bucket elevators and associated dust collectors must be taken into consideration as well. Inlet isolation ap valves va lves (Figure 5, next page) (2, Section 12.2.3) provide 12.2.3) provide only one-way isolation in the
5. (Top) Under t Figure 5. (Top) normal conditions, the ﬂap of this one-way inlet isolation device is open to allow ﬂow into the protected process equipment. (Bottom) In the event of an explosion in the process equipment, the ﬂap closes to prevent the ﬂames and pressure from propagating.
direction opposite ow to the dust collector, so any aspiraaspira tion lines must be individually isolated by means of a pinch valve (2, Section 22.214.171.124) or 126.96.36.199) or another suitable means that provides provides two-way two-way isolation. isolation. Dust collec collectors. tors. A A dust collector can be protected by standard means such as venting panels, ameless venting, or a chemical suppression system. For new installations, it is prudent to select an outdoor location where an explosion can be vented safely with an explosion panel. If venting by means means of an explosi explosion on panel panel is not feasibl feasible, e, a deect deector or (Figure 6), ameless venting, or chemical suppression must be conside considered. red. The dust dust collec collector tor must must then then be properly properly isoisolated by means of an inlet isolation ap valve, pinch valve, or chemical suppression. Trough augers. A trough auger contains material only in the lower portion of the trough; the upper cross-section contains no product, thereby creating an enclosed space in which a dust cloud can form. Trough augers located indoors require ameless venting systems or chemical suppression. Ductwork. Ductwork. Another Another means of explosion isolation that is not often considered involves venting of the ductwork. For large-diameter large-diameter ducts (more than 40 in.) that connect two enclosures, such as a cyclone and a dust collector, traditional isolation methods are typically not cost-effective. In these cases, the connected enclosures as well as the inter-
6. Explosions t Figure 6. Explosions should be vented to an outdoor location. If that cannot be done safely, a deﬂector can be installed to direct the ﬂame and pressure upward.
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Literature Cited 1.
National Fire Protection Association, “Standard on the Fundamentals of Combustible Dust,” NFPA 652, NFPA, Quincy, MA (2016).
2. Nation National al Fire Prote Protecti ction on Assoc Associati iation on, “Standard on Explosion Prevention Systems,” NFPA 69, NFPA, Quincy, MA (2014). 3. Vorde orderb rbru rueg egge gen, n, J., “Imperial Sugar Renery ComCom bustible Dust Explosion Investigation,” Investigation,” presented presented at the AIChE Spring Meeting and Global Congress on Process Safety, www.aiche.org/academy/videos/conference presentations/imperial-sugar-renery-co presentations/imperial-sugar-renery-combustible-dustmbustible-dustexplosion-investigation, explosion-investigation, San Antonio, TX (Mar. 2010). 4. Eckh Eckhof off, f, R. K., K., “Dust Explosions in the Process Industries,” 3rd ed., Gulf Professional Publishing, Houston, TX (2003). 5. Nation National al Fire Prote Protecti ction on Assoc Associati iation on, “Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Particulate Solids,” NFPA 654, Appendix E — Deagration Propa gation Isolation Methods, NFPA, Quincy, MA (2017). Additional Resources Resources Murphy, M. R., “Making Sense of Combustible-Dust Hazard Analysis,” Chemical Engineering Progress, 112 (4), pp. 28–32 (Apr. 2016). Perry, J., et al., “Addressing Combustible Dust Hazards,” Chemical Engineering Progress, 107 (5), pp. 36–41 (May 2011).
connecting ductwork can be vented with panels. Under this protection protection arrangeme arrangement, nt, an explosion explosion will will travel travel to either either enclosure, but the pressure will be relieved throughout the process, process, includi including ng the ductwork ductwork (5).
Closing thoughts The likelihood of combustible dust explosions can be reduced by using preventive means such as lowering the conveying speeds, reducing the concentration of dust below the minimum explosible concentration (MEC), blanketing with inert gas, etc. But it is important to remember that all of these methods reduce process efciency or increase the operational costs of the process (or both). The explosion protection protection systems systems discussed discussed here here improve improve process process efefciency and can signicantly reduce the hazardous effects of human errors. CEP ABHI BHARGA BHARGAVA VA is is a sales engineer with REMBE Inc. and spends much of his time on the road, working with clients’ processes to identify combustible dust risks and comprehensive solutions. He has previous experience in dry bulk processing, and he holds an MS in mechanical engineering from North Carolina State Univ.
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