Other OSHA Issues Not Easily Understood

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1 Other OSHA Issues Not Easily Understood 399 Combustible Dust Explosion Pentagon Based on the Combustible Dust Explosion Pentagon, five elements are all necessary for a combustible dust explosion to occur. These elements include a combustible dust, oxygen, dispersion method, confinement and an ignition source. Keep in mind that all of these conditions must be in the same place and at the same time for deflagration to occur. Consequently, that is why deflagrations are rare events but with devastating consequences. Element 1. Combustible Dust Any combustible material (and some materials normally considered noncombustible) can burn rapidly when in a finely divided form. If such a dust is suspended in air in the right concentration, it can become explosive. Materials that may form combustible dust include agricultural products, agricultural dusts, carbonaceous dusts, chemical dust, plastic dust and metal dust. The dust includes starch, powdered milk, sugar, flour, cocoa powder, malted hops, grain dust, coke, pine soot, coal, wood charcoal, sulfur, resins, aluminum, magnesium, zinc and bronze. Particle size is important and rarely are all of the particles the same size. The finer the particle the more surface area exposed and makes it more explosible. The smaller the particle, the faster it will combust. Since you do not have the ability to size your material, use ordinary table salt or granulated sugar to gauge the potential size that would be capable of supporting a deflagration. Particle shape is also important. Just because flakes or fibers of the same material as your dust do NOT pass through the same sieve, does not mean that they are NOT a deflagration hazard. Particle size distribution does NOT stay the same throughout a process. Usually particle size gets smaller as the particulate is processed. Fugitive dust accumulations generally are the finest (most hazardous) fraction of the process material. Generally the finest fraction of the accumulated fugitive dust settles highest in the compartment of building. Element 2. Oxygen The Oxygen content in the air is necessary to support an explosion. However, any oxidizer, such as nitrous oxide, bleach, peroxide compounds, ozone, etc. may also support an explosion. Element 3. Dispersion Dust needs to be dispersed in the air to achieve a concentration at which it can explode. Suspension is controlled by particle size and shape. Smaller particles are more easily suspended. When solid particles are propelled upward into the air or poured through the air from above, the particles separate and become surrounded by air. This maximizes the surface available for combustion and maximizes the rate at which combustion can occur. The particles eventually fall out of suspension with the smaller particles falling more slowly than larger ones. Suspension of a dust in air requires the investment of energy (mechanical work) to lift the particulate into the air. Some sources of suspension are moving air, pneumatic conveying, mechanical conveying, pouring, acoustic impulses, other deflagrations, mechanical impact and vibrations. When particles are suspended, a concentration gradient will develop where concentration varies continuously from high concentration to low concentration. The explosible range is quite narrow, approximately g/m 3 on the lean side to 2 3 kg/m 3 on the rich side. There is a minimum concentration that must exist before a flame front will propagate. This concentration depends on particle size and chemical composition. The concentration may be measured in grams/cubic meter or ounces/cubic foot. Smaller particles provide

2 /2012 OSHA Handbook more energy and a lower minimum explosible concentration. Concentrations for a combustible dust explosion far exceed concentrations considered a health hazard by an Industrial Hygienist. A Rule of Thumb that can be used to determine if an ignitable dust concentration exists is that you can not see a 25 watt light bulb six feet away. Element 4. Confinement Buildings, process equipment, ducting, piping and dust collection equipment are types of confinement. Combustible dust that is confined may achieve concentrations sufficient to explode easier. Element 5: Ignition Source Ignition occurs when sufficient energy per unit of time and volume is applied to a deflagrable particulate suspension. Energy per unit of mass is measured as temperature. When the temperature of the suspension is increased to the auto-ignition temperature, combustion begins. Usually measured as the Minimum Ignition Energy (MIE). Ignition sources can vary from electrical to mechanical. When ordinary electrical wiring, lighting and devices are covered in dust, they may provide sufficient heat to start a fire or explosion. Another source of ignition is static charge. You probably experienced walking across a carpet and getting shocked when you touched the doorknob. You generated mj which was sufficient to ignite a fire or explosion. Believe it or not, moving, conveying and pneumatically transporting material can generate static charge. Grain filling has been measured to show it may generate mj. Mechanical work on materials or particulates may generate combustible particles and also serve as sources of ignition. Such operations include grinding, cutting, milling, shaping and sanding. Other sources of ignition include frictional heat from conveying equipment from parts such as bearings, conveyors themselves, fans and tramp iron. Moisture content in the dust reduces both the violence of the deflagration and ignition sensitivity of the material. The role of moisture is complex. The evaporating water acts as a heat sink. It also acts as an inerting material in the atmosphere. Water increases inter-particle cohesion and prevents dispersion. Equipment & Operations You ask yourselves what equipment and operations can be a deflagration hazard. One such example of a piece of equipment is blenders and mixers. A source of ignition may be generated by rubbing solids or through the rubbing of internal parts. Electrostatic charge may also be generated by the movement of solids. Dust formation will occur from the moving particulates inside of the equipment. You have containment, ignition source, dispersion, combustible dust and oxygen from the air. Dryers are another potential deflagration hazard. The two main types are Direct-Heat dryers and Indirect-Heat dryers. Direct-Heat dryers provide heat using heated air or gas directly. Indirect-Heat dryers transfer their heat by conduction such as steam jacketed dryers. Again, you have containment, ignition sources, dispersion, combustible dust and air. Dust Collectors (air/material separator) are probably the most well know deflagration hazards. Fabric filter or cartridge type bag houses provide a contained atmosphere with ignitable fine dust and high turbulence. Moving air is used to pick-up and transport a particulate through ducts. When the conveyance air stream passes into the dust collector, bag-house, cyclone or other air/material separator the dust falls out of suspension and the concentration increases to levels above the MEC! After operating for a while the filter bags of a dust collec-

3 Other OSHA Issues Not Easily Understood 401 tor become caked with dust. Accumulated dust reduces air flow and conveyance efficiency. Most dust collectors have an automatic bag cleaner to shake or blow the dust down to a lower bin. Again, this increases levels of combustible dust above the MEC! If burning material is introduced into the dust collector, a deflagration can result from the operation of the bag or filter element cleaning cycle. Many bag houses have a history of fires over the years. Ignition sources range from electrostatic spark discharges to hot glowing particles from the work area. A dust collector, by its very operation, maintains a cloud of finely divided particles suspended in air. If a source of ignition initiates the combustion of the dust cloud, the collector casing could experience a violent rupture. When dust particles are known to be combustible, precautions for an explosion must be taken and suitable protection provided to reduce the risk of personal injury. Bucket elevators by their nature also maintain a cloud of suspended dust in the enclosure. Sources of ignition that occur because of lack of maintenance include belts that slip, bad bearings, static build up, etc. Inside bucket elevators must be provided with deflagration venting ducted to the outside or otherwise be properly protected against explosion. Elevators must be basically dust tight and noncombustible. Inlet and discharge hoppers are to be accessible for cleaning and inspection. Pneumatic conveying systems are used in various industries to transport flour, plastics, etc. Downstream equipment has a high rate of risk for fire and explosion. You have containment, combustible dust, air (oxygen) and dispersion. Ignition is the only thing missing. Static electricity may be generated from particle to particle contact or from particle to duct wall contact. Heated particles created during grinding or drying may be carried into the pneumatic conveying system and fanned to a glow by high gas velocity. Tramp metal in the pneumatic system may cause frictional heating. Charged powder may leak from joints into the air and electrostatic sparking can occur resulting in an explosion. Size reduction systems include such machines as: hammer mill, crusher and ball mill. You have containment, combustible dust, air and dispersion. Ignition is the only thing missing. The size reduction equipment itself is regarded as a possible ignition source because of friction and hot surfaces arising from grinding. Possible tramp metal may enter into the equipment and create friction. Lastly, if the feed rate for material is too slow, it can generate heat and increase the possibility of fire/explosion hazard. Silos and hoppers shall be located outside the buildings with some exceptions. The silos are not to be connected by venting to allow ignition of nearby silos. Air cannons are not to be used to break bridges in silos and created dispersion. Detection of smoldering fires in silos and hoppers may be achieved with methane and carbon monoxide detectors. Pressure containment, inerting, and suppression systems provide the best protection against explosions. However, venting is the most widely used protection against explosions. National Electrical Code Electricity of course is a source of ignition. What you have to determine is whether your facility needs any special type of electrical distribution system for combustible dust. This section should help you figure out if you have a problem or not. The National Electrical Code (NEC) establishes the criteria for electrical safety for Hazardous Locations in Articles 500 through 504. The sole concern of the NEC is preventing the electrical service from serving as an ignition source or electrical shock hazard. The NEC classifies areas containing combustible dusts as Class 2 Hazardous Locations. Under the NEC, however, there is no such thing as a Class 2 Dust. Classification is performed in accordance with NFPA 70, National Electrical Code, Article 500. Again, the underlying concern of the NEC is electrical safety.

4 /2012 OSHA Handbook Class II Hazardous Occupancies contains combustible dusts. A Division 1 area is where dust is suspended in the air under normal operating conditions. Also where production system upset or equipment failure occurs and produces dust suspension and an ignition source. Lastly, combustible dusts that are electrically conductive (Group E), such as metal, in hazardous quantities may only be present in Division 1 locations. A Division 2 area is where ignitable dust suspensions do not normally exist outside process equipment or where dust accumulations are not normally sufficient to cause electrical equipment to overheat. The following factors determine the extent of Class II locations: Combustible material involved Bulk density of the material Particle sizes of the material Density of the particles Process or storage pressure Size of the leak opening Quantity of the release Dust collection system Housekeeping Presence of any flammable or combustible gas Class II locations are those that are hazardous because of the presence of combustible dust. The following are Class II locations where the combustible dust atmospheres are present: Group E. Atmospheres containing combustible metal dusts, including aluminum, magnesium, and their commercial alloys, and other combustible dusts whose particle size, abrasiveness, and conductivity present similar hazards in the use of electrical equipment. Group F. Atmospheres containing combustible carbonaceous dusts that have more than 8 percent total entrapped volatiles (see ASTM D 3175, Standard Test Method for Volatile Matter in the Analysis Sample of Coal and Coke, for coal and coke dusts) or that have been sensitized by other materials so that they present an explosion hazard. Coal, carbon black, charcoal, and coke dusts are examples of carbonaceous dusts. Group G. Atmospheres containing other combustible dusts, including flour, grain, wood flour, plastic and chemicals. All electrical equipment must be listed for use in the occupancy based upon the Class, Division and Group classification. When all electrical equipment in the occupancy is listed for use in that occupancy, the electrical system is not deemed to be a likely igniter. All electrical equipment shall be installed per NFPA 70, National Electrical Code. Hazardous areas shall be classified per NFPA 70, Article 500, NFPA 499 and All parts of process equipment shall be bonded and grounded. All motors and bearings shall be external to the process stream Explosion Prevention and Deflagration Techniques Explosion management can be handled in two ways, Prevention and Mitigation. Prevention means not letting the situation get to the level of deflagration. Mitigation is handling the energy after deflagration has occurred. Prevention includes Construction, Dust Control, Housekeeping, Oxidant concentration reduction, Bonding and Grounding, Elimination of Ignition, Fire Prevention Plan and Training. Damage created by combustible dust may be controlled by construction of the work place. Operations that are especially susceptible to fire or deflagration should be isolated by

5 Other OSHA Issues Not Easily Understood 403 outside or in another building. Separation by distance with in same room and segregation by use of a barrier are other methods to reduce damage. Dust control can be handled by utilizing a good ventilation design for process equipment and cleaning up of fugitive dust. When it comes to housekeeping, construction of the building interior is important. Interior wall surfaces shall be smooth. Wall penetrations need to be sealed dust-tight. Ledges shall be designed to not hold dust. Inaccessible spaces shall be sealed off. All interior space shall be cleanable. Rooms containing dust explosion hazards shall be vented per NFPA 68. Process equipment shall be designed by a qualified person such as licensed P.E. Where dust concentration exceeds MEC such as a bag house, explosion prevention or deflagration relief venting is required. Isolation between vessels is also required. Ventilation systems shall be steel or other conductive material with smooth bends and changes in diameter with 10o maximum taper transitions. Lastly, combustible particulate solids should not pass through fans. Wet Scrubber/Collectors draw air into the collector and is forced to churn through a torturous path, through a partially submerged baffle. Dust is separated by making contact with water in this section. This reduces the possibility of ignition. Housekeeping means facilities shall be maintained as dust free as possible. Accumulations of dust not only occur on the floor on overhead beams, rafters, cable trays, etc., and above ceiling tile. For example, a 1/8 inch thick layer of dust, once disturbed, can easily form a dangerous cloud that could explode. However, the amount of dust accumulation necessary to cause an explosive concentration can vary greatly. This is because there are so many variables the particle size of the dust, the method of dispersion, ventilation system modes, air currents, physical barriers and the volume of the area in which the dust cloud exists or may exist. As a result, simple rules of thumb regarding accumulation such as writing in the dust can be subjective. Some believe, if you can write your name in the dust there is probably sufficient dust present to blow the building away. The hazard analysis should be tailored to the specific circumstances in each facility and the full range of variables affecting the hazard. The majority of the property damage and personnel injury is due to the fugitive dust accumulations within the building or process compartment. Control, limitation of and elimination of accumulated fugitive dust is CRITICAL and the single most important criterion for a safe workplace. One method of cleaning is blow-down. However, ALL electrical power and processes have to be shut-down. Oxidant concentration reduction is another means of preventing deflagration from occurring. It is sometimes possible to maintain the air around combustible dust particles outside the combustible range through oxidant concentration reduction, fuel enrichment or both. There are detailed engineering requirements listed in Chapter 6 of NFPA 69. Some difficulties include the life safety hazard of utilizing inert gases to reduce oxygen concentrations to below oxygen deficient levels. Metals are problematic in that some un-oxidized combustible metal in inert gas can become pyrophoric and burst into flame when exposed to oxygen in air. In addition, the limiting oxygen concentration (LOC) for some metals is very low, far lower than the LOC of many dusts, approximately 8 to 14%. Normal air is 20.9% oxygen and we are glad to have it. Another method is to eliminate ignition sources including but not limited to: heat sources, friction including mechanical friction and frictional sparks, electrical sparks and static electricity. Control of heat sources can be handled a number of ways. If the material is subjected to heat as part of the normal process (e.g. during drying), the temperature should be maintained

6 /2012 OSHA Handbook below the self heating temperature for solids. Prevent the overloading of processing plant (grinders, conveyors, etc.). Internal buildup will not dissipate the heat from the material and therefore increase operating temperature above normal. Consider the installation of overload protection devices on drive motors. Isolate or shield hot surfaces. Prevent or remove dust accumulations on hot surfaces. Use approved electrical equipment with the correct temperature rating. Mechanical equipment and components could be a source of ignition due to friction. Mechanical impacts may produce small flying fragments of hot/burning material or a pair of hot spots where impacting bodies touch. Prevent foreign material from entering the system when such foreign material presents an ignition hazard. Consider the use of screens, electromagnets, pneumatic separators, etc. Floor sweepings should not be returned to any machine. Mechanical friction may also be caused by objects rubbing against each other and producing hot surfaces. Prevent overheating due to misalignment, loose objects, belt-slip/rubbing etc. by regular inspection and maintenance of plant. Impact sparks can occur when, for example, operators use, drop, or otherwise strike metal equipment with metal tools or objects. Minimize the likelihood of impact sparks through selection of proper tools, using techniques to prevent dropping tools e.g. wrist straps and operator training. Thermitereaction can result because of impacts involving aluminum and rust. Hot work operations should be controlled by a hot work permit system in accordance with NFPA 51B, Standard for Fire Prevention During Welding, Cutting and Other Hot Work. Formation of dust clouds should be prevented, and dust deposits should be removed. A gas/vapor detector should be used to ensure flammable vapors/gases are not present. Static electricity is another ignition source that needs to be controlled. Electrostatic charges are usually generated when any two materials make and then break contact. The build up of the charge on electrically isolated conductors and/or on insulating materials, can give rise to electrostatic discharges. Depending on the energy of the discharge, a combustible dust atmosphere can be ignited. Grounding all metal parts to earth should reduce the possibility any discharge. Fire Prevention is another method to reduce the possibility of a deflagration. Establish an emergency preparedness plan. Employees need to be trained to recognize and prevent hazards associated with combustible dust. They also need to know how to take preventative action, and/or how to alert management to a hazard that needs attention. Maintenance is probably the most important method to control potential deflagration. Performing vibration monitoring, heat recording, etc. makes for a good Preventive Maintenance Program. This program should keep bearing, belts, vibration fatigue, etc. from generating frictional heat, tramp metal, leaks, etc. throughout a process.