Combustible Dust

Transcript

Objectives • OSHA - Enforcement • Define the terminology • Recognize a Dust Explosion hazard • Learn hazard management methods Enforcement • OSHA Established an NEP for Combustible Dust • Started the Standard Process – Holding Stakeholder Meetings – Gathering Information – Proposed Standard – Not sure when this will be done (No specifics at this time) 1 Types of Industries Involved in Dust Incidents Types of Dust Involved in incidents Definitions and Terminology • Combustible Dust • Deflagration • Minimum Ignition Energy (MIE) • Kst 2 Combustible Particulate Solids Chunks Chips Flakes Dusts Fibers Fines “Combustible Dusts” are a sub-category of “Combustible Particulate Solids” Combustible Dust • “A combustible particulate solid that presents a fire or deflagration hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape.” [NFPA 654-2006] • Dusts are just a small fraction of combustible particulate solid. • This definition does NOT encompass all of the hazardous particulates! Combustible Dust A particle that will pass a #40 Sieve A fiber particle 420  1.3 mm 12  3 Particle Size of Common Materials Deflagration • Propagation of a combustion zone at a velocity that is less than the speed of sound in the unreacted medium. • This action releases large quantities of heat very rapidly. • This in turn produces large pressures to develop that can lead to structural failure and cause employee injuries. Deflagration Dust cloud 4 Deflagration Deflagration Deflagration 5 Deflagration Minimum Ignition Energy • MIE is the minimum energy, discharged into the dust cloud by the test apparatus and sufficient to cause flame propagation. The MIE is measured in units of joules (J) or millijoules (mJ). Minimum Ignition Energy Increasing particle size increases the MIE, decreasing the particle size decreases the energy necessary to initiate deflagration. 6 Minimum Ignition Energy MIE varies with concentration. MIE is reported at the concentration that deflagrates with the lowest MIE. Kst • Volume-Normalized Rate of Pressure Increase. • KSt is also referred to as the “Deflagration Index” or “Dust Constant”. It is calculated as the rate of pressure increase multiplied by the cube root of the test vessel volume. KSt = (dP/dtmax)(V)1/3 • KSt is proportional to the speed of the flame front through the dust cloud. • KSt is measured in units of bar-meter per second (barm/sec). KSt is just a way of classifying dust to estimate the anticipated behavior of the dust during deflagration. KSt • Dusts with a KSt less than or equal to 200 bar-m/sec are classified as “ST-1” dusts. • Dusts with a KSt between 200 and 300 barm sec are classified as “ST-2” dusts. • Dusts with a KSt greater than 300 barm/sec are classified as “ST-3” dusts. • These classifications have NOTHING to do with hazardous area classifications under the NEC. 7 Dust Classification Based on KSt Combustible dusts are classified by the numerical value of KSt. Measured KSt Classification <=200 St-1 * 200 – 300 St-2 >300 St-3 ** • * THIS CLASSIFICATION KILLS THE MOST EMPLOYEES • ** THIS CLASSIFICATION IS USUALLY MADE UP OF METALS – Al, Mg, Be, etc. Recognition •Review the materials in the facility that are utilized and/or manufactured. If any the material is reduced to fines, dust, chips, chunks, flakes or fibers then research the material further because it may be a combustible dust. •Keep in mind, any material containing “carbon” and/or un-oxided metal will most likely explode. •Check your material safety data sheet (MSDSs) concerning any unusual hazards such as explosibility. •Review the online German database http://www.dguv.de/bgia/en/gestis/expl/index.jsp. The database is operated by BGIA - Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung. It contains combustion and explosion characteristics of more 4000 dusts samples from virtually all sectors of industry. •If you are unable to locate any information about your material, then try using Google.com to look for such hazards. Recognition (cont.) • • • • • Take a sample of the dust in your plant from the highest points when possible. Have the material tested for its KSt. This will at least give you an idea whether the material will explode. Many locations need to be considered in an assessment. One obvious place for a dust explosion to initiate is where dust is concentrated. Equipment such as dust collectors may contain a combustible mixture whenever the equipment is operating. Other locations to consider are those where dust can settle, both in occupied areas and in hidden, concealed spaces. A thorough analysis would include all possible scenarios in which dust can be disbursed, both in the normal process and potential failure modes. A hazard may exist without visible dust in the air. Meaning an initial explosion may occur in dust collector and create secondary explosion in areas with poor housekeeping. A sure sign of close encounters may be a plant history of fires. While evaluating the dust in your facility, keep in mind the potential ignition sources such as flames, sparks from welding/cutting operations, friction or impact sparks, electric sparks, electrostatic discharges and hot surfaces. 8 Combustible Dust Explosion Pentagon Fire Triangle - Dust Explosion Pentagon FIRE TRIANGLE: DUST EXPLOSION PENTAGON: Ignition Source Dispersion Confinement Explosion Combustible Dust Oxygen in Air Combustible Dust Explosion Pentagon: Five Elements – ALL Necessary 1. Combustible Dust 2. Oxygen in Air 3. Dispersion 4. Confinement 5. Ignition Source IMPORTANT NO DUST EXPLOSION OCCURS if one or more elements are missing 5. Ignition Source 3. Dispersion 4. Confinement Explosion 1. Combustible Dust 2. Oxygen in Air 9 Element 1: Combustible Dust Explosion Agricultural Products such as: • Corn Starch, Dry Milk, Sugar, Wood Flour, Powered Milk Agricultural Dusts such as: • Cocoa Powder, Hops (malted), Rice Flour, Wheat grain dust Carbonaceous Dusts such as: • Petroleum Coke, Pine Soot, Bituminous Coal, Wood Charcoal. Chemical Dusts such as: • Lactose, Sulfur, Calcium Acetate, Methyl-Cellulose Plastic Dusts such as: • Phenolic Resin, (poly)Propylene, (poly)Vinyl Chloride, Melamine Resin Metal Dusts such as: • Aluminum, Magnesium, Zinc, Bronze Combustible Dust Particles may break into or Combustible Dust Particles • Size – Pellets > 2mm diameter – Granules 0.42mm – 2mm – Dust Particles <0.42mm 10 Combustible Dust Particles • Hazard increases as particle size decreases – Larger surface area for combustion – Fine particles may have a larger role in dust cloud ignition and explosion and propagation Aluminum Dust dP/dtmax [bar/sec] Combustible Dust Particle Size Distribution Eckhoff Eckhoff Decreasing Particle Size Increases dP/dtmax Element 2: Oxygen in Air Explosion • The Oxygen content in air is all that is necessary to support an explosion. 11 Element 3: Dispersion Explosion • Dust needs to be dispersed in the air Combustible Particulate Smallest particles Largest particles •Generally the finest (most hazardous) fraction of the accumulated fugitive dust settles highest in the compartment of building. •Every large compartment is a particulate separator, separating particles by mass. Concentration • Fact: When particles are suspended, a concentration gradient will develop where concentration varies continuously from high concentration to low concentration, Concentration Gradient Non-ignitable Dust Cloud Ignitable 12 Dust Combustibility • 25 watt light bulb probably can not be seen through six feet of a mixture of combustible dust in air > Minimum Explosible Concentration What bulb? 6 feet 40 g/m3 concentration of comb. dust suspended in air 25 watts Glass Glass Dust Combustibility Health Hazard Range Explosible Range 15 mg/m3 2.5 ug/m3 50 g/m3 3000 g/m3 0 Concentration 3000X Element 4: Confinement Explosion • Confinement can be provided by buildings, process equipment, ducting, piping, and dust collection equipment. 13 Element 5: Ignition Source Can be Electrical • Static – • • Human - 20-30 mj Grain – filling – 10-25 mj • Lightning • Generated Explosion Can be Mechanical • Match/lighter • Spark All 5 Elements = EXPLOSION Explosion The “Typical” Explosion Event • Dust explosions occur as a series of deflagrations leading to a series of explosions in stages. – While a single explosion is possible it is the exception rather than the rule. – Most injuries are the result of the “secondary” deflagrations rather than the initial event. 14 A Dust Explosion Event Primary Deflagration inside process equipment 0 25 50 75 100 125 150 175 200 225 250 300 325 Time, msec. (Timing of actual events may vary) A Dust Explosion Event Shock wave caused by primary deflagration 0 25 50 75 100 125 150 175 200 225 250 300 325 Time, msec. A Dust Explosion Event Shock waves reflected by surfaces within the building cause accumulated dust to go into suspension 0 25 50 75 100 125 150 175 200 225 250 300 325 Time, msec. 15 A Dust Explosion Event Dust clouds thrown in the air by the shock waves 0 25 50 75 100 125 150 175 200 225 250 300 325 Time, msec. A Dust Explosion Event Primary deflagration breaks out of the equipment enclosure – creating a source of ignition 0 25 50 75 100 125 150 175 200 225 250 300 325 Time, msec. A Dust Explosion Event Secondary deflagration ignited 0 25 50 75 100 125 150 175 200 225 250 300 325 Time, msec. 16 A Dust Explosion Event Secondary deflagration is propagated through the dust clouds 0 25 50 75 100 125 150 175 200 225 250 300 325 Time, msec. Secondary deflagration bursts from the building 0 25 50 75 100 125 150 175 200 225 250 300 325 Time, msec. A Dust Explosion Event Collapsed building with remaining fires 0 25 50 75 100 125 150 175 200 225 250 300 325 Time, msec. 17 Equipment & Operations Blenders/Mixers • Heat Generation due to – Rubbing of Solids – Rubbing of internal parts • Electrostatic Charging of the Solids • Dust Formation inside of the equipment Source: http://www.fedequip.com/abstract.asp?ItemNumber=1 7478&txtSearchType=0&txtPageNo=1&txtSearchCriter ia=ribbon_mixer Dryers • Direct-Heat Dryers – Convective Drying System – Heat provided by heated air or gas – Moisture is carried by drying medium • Indirect–Heat Dryers – Heat transfer by Conduction – Steam for Jacketed Dryers Source:www.barr-rosin.com/products/rotary-dryer.asp 18 Dust Collectors Fabric Filters (Baghouses) • Presence of easily ignitable fine dust atmosphere and high turbulence • Experienced many fires over the years due to broken bags. • Ignition source is electrostatic spark discharges • Another ignition source is entrance of hot, glowing particles into the baghouse from upstream equipment Dust Collector • If burning material is introduced into the dust collector, a deflagration can result from the operation of the bag or filter element cleaning cycle. • Dust collector system located inside a building Note: 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 cause 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. Dustexplosion explosioninin Dust equipment equipment Dust Collector With dispersal and ignition of 2 kg dust by the flame jet 19 Pneumatic conveying system • Downstream equipment have high rate of risk for fires and explosion – Static electricity is generated from particle to particle contact or from particle to duct wall contact. – Heated particles which are 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 also cause frictional heating. – Charged powder may leak from joints to the atmosphere and electrostatic sparking can occur resulting in an explosion. Figure source:www.flexicon.com/us/products/PneumaticConveyingSystems/index.asp?gclid=COa2kKWK 4o8CFQGzGgodikc9Dg Size Reduction System • Size reduction equipment is regarded as a possible ignition source because of friction and hot surfaces arising from grinding • Entrance of metal into the equipment • Too slow feed rate can increase the possibility of fire/explosion hazard Bucket Elevators • Belts Slipping • Belts and lagging are not fire and oil resistant • Belts and bearings are not outside the casing • Lack of preventive maintenance Head pulley Lagging 20 Silos and Hoppers • No inter-silo Venting • Silos and hoppers shall be located outside the buildings with some exceptions • Air cannons not to be used to break bridges in silos • Detection of smoldering fires in silos and hoppers can be achieved with methane and carbon monoxide detectors • Pressure containment, inerting, and suppression systems to protect against explosions • Venting is the most widely used protection against explosions Explosion Hazard Management Methods • There are three ways of handling combustible dust hazards: – Prevention • Explosion suppression • Dilution with non-combustible dust • Oxidant concentration reduction – Mitigation • Controlling the explosion energy • Venting through a listed dust retention and flame arresting device – Combination of Prevention and Mitigation 21 Processes • Combustible particulate solids should not pass through fans. • All conveyance ducts shall be steel or other conductive material. • All bends smooth. • Changes in diameter with 10o(max) taper transitions. Processes This Not this This Not this Explosion Venting • The vent opening must be sized to allow the expanding gases to be vented at a rapid rate so that the internal pressures developed by the explosion do not compromise the structural integrity of the protected equipment • The volume of the equipment to be protected • The maximum pressure during venting (Pred) • The KSt of the dust (or fundamental burning velocity of a gas) • The burst pressure of the explosion 22 Deflagration Relief Venting (Explosion Venting) Dust Collector Return Air Filter Media Vent Dusty Air Into Collector Deflagration Relief Venting (Explosion Venting) Dust Collector Return Air Filter Media Vent Dusty Air Into Collector Deflagration Relief Venting (Explosion Venting) Dust Collector Return Air Filter Media Vent Dusty Air Into Collector 23 Deflagration Relief Venting (Explosion Venting) Dust Collector Return Air Filter Media Vent Dusty Air Into Collector Deflagration Relief Venting (Explosion Venting) Dust Collector Filter Media Vent Vent Duct Wall Deflagration Suppression • Reaction times are generally in the 30 to 40 millisecond time domain. • Pressure sensors are generally calibrated to 0.5 psi above ambient. • Extreme care should be used when Deflagration Suppression is used on vessels also equipped with Deflagration Relief Vents. 24 Deflagration Suppression • Must be designed, installed and maintained per NFPA 69. • Relies on high speed pressure sensors and high rate discharge extinguishing units to detect initial shock wave from the deflagration and discharge suppressing agent before the pressure has gotten large enough to cause damage. Deflagration Suppression Specialized Agents • Gaseous agents • Dry chemicals • Water spray or mist • Others Pressure Sensor / Automatic Dust Explosion Suppression System 25 Detectors Detection & Supression System Deflagration Suppression Pressure Sensors Return Air HRD Extinguishing Unit Dusty Air Into Collector Control Unit 26 Deflagration Suppression Pressure Sensors Return Air Dusty Air Into Collector Control Unit Deflagration Suppression Pressure Sensors Return Air HRD Extinguishing Unit Dusty Air Into Collector Control Unit Deflagration Suppression Pressure Sensors Return Air HRD Extinguishing Unit Dusty Air Into Collector Control Unit 27 Spark Detection and Extinguishment • Spark detectors are sensitive infrared sensors that detect burning material on a conveyor or in a pneumatic conveyance duct. • Spark detection is usually used in conjunction with automatic water-spray extinguishment. Spark Detection and Extinguishment • When the spark enters the field of view of the detector it sends a signal to the control unit which energizes the valve. • The valve allows a spray of water to commence in the duct. • The spark collides with the water spray and is quenched. Dust Collector Filter Media Return Air Spark Detectors Fan Vent Solenoid Valve Control Unit Pump Spark Detection and Extinguishment 28 Dust Collector Return Air Filter Media Spark Detectors Fan Vent Solenoid Valve Control Unit Pump Spark Detection and Extinguishment Dust Collector Return Air Filter Media Spark Detectors Fan Vent Solenoid Valve Control Unit Pump Spark Detection and Extinguishment Dust Collector Filter Media Return Air Spark Detectors Fan Vent Solenoid Valve Control Unit Pump Spark Detection and Extinguishment 29 Dust Collector Filter Media Return Air Spark Detectors Fan Vent Solenoid Valve Control Unit Pump Spark Detection and Extinguishment Deflagration Relief Venting – Flame Arrestor • Deflagration through a listed dust retention and flame-arresting device Cut-away of deflagration vent • Burned and unburned dust is retained by filter and flame arrestor • Combustion gases are cooled • No flame emerges from the system • Nearby blast effects are reduced • May be vented indoors • Change burst disc and clean filter after each event (Europe) No Isolation Device 30 Isolation Devices for Ductwork Ventex Valve Bursting Disc /or other Vent Cover Passive device for interrupting dust explosion Device for interrupting dust explosion Back Flash Dampers for Ductwork Isolation Devices Chemical & Mechanical Isolation Devices 31 Isolation Devices • Airflow control valve, NFPA 654 (7.8) • Diverter valves seal mechanically and close all other directions from air or material leakage Flame front Diverter Positive shut off flap valve Hinged device Normal flow of material Isolation Devices • Segregation of the hazards (isolate with a diverter) Flame front diverter Air Roof Line Isolation Devices • Segregation of the hazard (isolate with a diverter) 32 Isolation Devices • Segregation of the hazard (isolate with a barrier) Automatic Fast Acting Isolation Valve Is Shown Isolation Devices • Segregation barrier – Chemical Suppressor Isolation Devices • Segregation of the hazard (chemical isolation) Flame front is halted by barrier released from chemical suppressor 33 Without Return Air Abort • As the flame and hot gases fill the space beneath the ceiling/roof deck sprinkler heads begin fusing. • Often more heads fuse than the riser is designed to support. • The excessive demand deprives entire facility of required delivered density and fires are not controlled. Dust Collection Without Return Air Abort Return Air Duct Dust Collector Radiant flux Dust Collection Return Air Abort • The diversion of return air to building exterior is usually implemented with spark detection and a fast-acting abort gate. 34 Dust Collection Return Air Abort Return Air Duct Abort Gate Dust Collector Return Air Abort • Upon actuation of the return air spark detection the automatic bag cleaning is shut-down. • DO NOT shut down the air movement as this can cause a deflagration • If fire expands the sprinklers in the D/C will fuse and dissipate the heat generated by the fire. Return Air Abort • In lieu of using an abort gate, fast acting valves can be used to prevent fire and smoke from returning to the occupied space. 35 Spark Detectors Valve or Flame-front Diverter Dust Collector Return Air Filter Media Vent Dusty Air Into Collector Control Unit NFPA 664-2007 Section A 8.2.2.6 Minimum Compliance Design Chokes for Screw Conveyors Isolation Devices Rotary Valve Isolation Device Rotary Air Lock 36 Explosion Management – Pressure Containment Vessel must contain the calculated maximum explosion pressure equal to 2/3 yield Allows vessel strength re-use 2/3 catastrophic Contains the failure pressure event, but the vessel must be scrapped Wet Scrubber/Collectors • Air is drawn 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. Process Equipment - Bucket Elevators • Deflagration venting • Inside bucket elevators must be provided with deflagration venting ducted to the outside or otherwise properly protected against explosion • Elevators must be dust tight and noncombustible • Inlet and discharge hoppers are to be accessible for cleaning and inspection deflagration venting test at mock-up lab 37 Bucket Elevators • Power cutoff • Is a device used to cut off power when the drive motor drops to <80% of normal • Feed is to be stopped or deflected when the power to the motor is stopped Bucket Elevators • Belts • Non-slip material and (lagging) is to be provided on the head pulley to minimize slippage • Belts and lagging are to be fire and oil resistant Head pulley Lagging Bucket Elevators • Belts • Bearings are to be outside the casing • Provide openings at the head and tail pulley for – Cleanout – Inspection – Alignment Dirty Really dirty and “partially inside” construction 38 Bucket Elevators • Drive components are to be engineered to: – Handle full rated capacity – Start the unchoked elevator under full load Bucket Elevators • Monitors are to be provided • Monitor the head and tail pulleys for: Head – High bearing temperature – Vibration – Head pulley and belt alignment • An alarm requiring corrective action to sound at the operator’s station • Exception: Conveyors <500 ft/minor <3750 ft3/hour Tail Bucket Elevators • Emergency Controls • Bins where material directly discharges from the elevators must be provided with: – Automatic high-level shutdowns – Visual indicators – Audible alarms sounding at the operator control station 39 Explosion Management • Oxidant Concentration Reduction • Outside the combustible range through oxidant concentration reduction, fuel enrichment or both • Detailed engineering requirements • Life safety hazard • Metals problematic Normal Limiting Oxygen Concentration for combustion of many dusts Explosion Prevention Techniques • Elimination of Ignition Sources Involves: – Control of Heat Sources -NFPA 654 – Control of Friction -NFPA 654 • Mechanical friction • Friction sparks – Control of Electrical Sparks -NFPA 497, NEC – Control of Static Electricity -NFPA 77 Ignition Control • Separate heating systems from dusts • Proper use of cartridge activated tools • Adequately maintain equipment. Hot work permit _________ _________ _________ _________ Kaboom! 40 Ignition Control • Control mechanical sparks and friction; • Use separator devices to remove foreign materials capable of igniting combustibles from process materials; • Separate heated surfaces from dusts; Non - Magnetic rotating drum Magnetic Core Ignition Control • Use appropriate electrical equipment and wiring methods • Control static electricity, including bonding of equipment to ground • Control smoking, open flames, and sparks No! 41 Ignition Control • Other ignition sources – OSHA 29 CFR 1910.178 (c) regulates powered industrial trucks in dust areas – Coal handling operations must comply with OSHA 29 CFR 1910.269 Class I & II Group D & G Explosion Prevention Techniques - Control of Heat Sources • If the material is subjected to heat as part of the normal process (e.g. during drying), the temperature should be maintained below the self heating temperature (for solids) • Preventing the overloading of processing plant (grinders, conveyors, etc.). Internal buildup will BOTH reduce heat loss from material AND increase operating temperature above “normal”. Consider the installation of overload protection devices on drive motors • Isolation or shielding of hot surfaces • Prevention/removal of dust accumulations on hot surfaces • Use of approved electrical equipment (correct temperature rating) Explosion Prevention Techniques Control of Friction Mechanical equipment and component could be a source of ignition due to: – Mechanical impacts producing: • Small flying fragments of hot/burning material • A pair of hot spots where impacting bodies touch – Mechanical friction caused by objects rubbing against each otherand producing hot surfaces – Thermitereaction caused by impacts involving aluminum and rust 42 Explosion Prevention Techniques Control of Friction • • • • Prevent overheating due to misalignment, loose objects, belt-slip/rubbing etc. by regular inspection and maintenance of plant Prevent foreign material from entering the system when such foreign material presents an ignition hazard. Consider use of screens, electromagnets, pneumatic separators, etc. Floor sweepings should not be returned to any machine Impact sparks can occur when for example operators use, drop, orotherwise strike metal equipment with metal tools or objects. Minimize the likelihood of impact sparks through: – Proper tool selection – Techniques to prevent dropping tools e.g. wrist straps – Operator training • 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 may be used to ensure flammable vapors/gases are not present Explosion Prevention Techniques – Control of Static Electricity Contact (Frictional) Charging 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 flammable atmosphere can be ignited. 43 Electrical Area Classification of Dust Locations • 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 Housekeeping • Facilities shall be maintained dust free. • No blow-down unless ALL electrical power and processes have been shut-down. • No welding, cutting or grinding unless under a hot-work permit per NFPA 51B. • Comfort heating equipment shall obtain combustion air from a clean outside source Housekeeping – Rule of Thumb • If you can write your name in the dust there is probably sufficient dust present to blow the building away. 44 Housekeeping •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 elimination of accumulated fugitive dust is CRITICAL and the single most important criterion for a safe workplace. Housekeeping • Include: – Overhead beams, joists, ducts, – Tops of equipment, and other surfaces – Vertical surfaces if they are dusty – Structural members – Conduit and pipe racks – Cable trays – Floors – Above ceiling tile – Equipment (leaks around dust collectors and ductwork.) ? Housekeeping • Some recommendations: – Minimize the escape of dust from process equipment or ventilation systems to minimize the time needed for housekeeping – Use dust collection systems – Utilize surfaces that minimize dust accumulation and facilitate cleaning Spots are not raindrops 45 Housekeeping • Develop and implement a hazardous dust plan – – – – Inspection Testing Housekeeping Control program • Program should be written with established frequency and methods Housekeeping • Use cleaning methods that do not generate dust clouds • Only use vacuum cleaners approved for dust collection HAZ LOC Dust Layer Thickness Guidelines 1/8” in grain standard Rule of thumb in NFPA 654 1/32” over 5% of area Bar joist surface area ~ 5% Max 20,000 SF Idealized 46 Training • They need to know – The safe work practices applicable to their job tasks – The overall plant programs for dust control and ignition source control • Training must be – Before they start work – Periodically to refresh their knowledge – When reassigned – When hazards or processes change Training • Training needs were identified for: – Employees – Supervisors – Managers Electrical 47 Hazardous (classified) Locations 1910-307 Subpart S Electrical – Design Safety Standards for Electrical Systems 29 CFR 1910.307(c) Electrical Installations • Equipment, wiring methods, and installations of equipment in hazardous (classified) locations shall be intrinsically safe, approved for the hazardous (classified) location, or safe for the hazardous (classified) location ! Electrical Equipment for Hazardous Occupancies • 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. 48 Class II Hazardous Occupancies • Occupancies containing combustible dusts. • Division 1: – Dusts suspended in the air under normal operating conditions. – Production system upset, equipment failure or maintenance can produce BOTH dust suspension and ignition source. – Where combustible dusts are electrically conductive. • Division 2: – Compartments where ignitable dust suspensions do not normally exist outside process equipment. – Where dust accumulations are not normally sufficient to cause electrical equipment to overheat. Class II Locations 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. Group E Dust • Could cause a short in the electrical equipment • Electricity may find the path of least resistance through a dust layer, heating up the dust particles in it path and thus providing a source of ignition. The resulting electric arc could ignite a dust layer or dust cloud. *NFPA 499 . . . Hazardous (Classified) Locations (2008) Sec 4.4 49 Class II, Division Locations • Equipment must be marked to identify suitability for use with Group E, F and/or G. • Where suitable for ambient temperatures exceeding 40º C (104º F) marked with both – The maximum ambient temperature and – The operating temperature or temperature range at that ambient temperature. Nationally Recognized Testing Laboratories (NRTLs) • NRTLs are third party organizations recognized by OSHA • Provide product safety testing and certification services to manufacturers on a wide range of products • Testing and certifications are based on product safety standards developed by U.S. based standards developing organizations • Often issued by American National Standards Institute (ANSI) 50 NFPA 654 Guide to Area Electrical Classification – A.6.6.2 Negligible dust -color discernable Not much dust Negligible dust to < 1/32 inch (paper clip) Infrequent – Negligible dust to < 1/32 inch Continuous to frequent – N/A being released Unclassified – general purpose Clean up Unclassified – Episodic release, during same general purpose not > 2-3 times shift yearly Continuous with < 1/32 inch per 24 hours or Episodic release, > 3 times yearly Clean as needed – maintain average < 1/64 inch (puffy cloud with each step) Unclassified – dust tight NEMA 12 enclosures and /sealed non- heat producing enclosures (For existing plants – New to be Class II, Div. 2) NFPA 654 Guide to Area Electrical Classification – A.6.6.2 1/32 to 1/8 inch (Two Quarters stacked) Infrequent Clean up during same shift Unclassified – Dust tight NEMA 12 enclosures and /sealed non- heat producing enclosures (For existing plants – New to be Class II, Div. 2) 1/32 to 1/8 inch Continuous to frequent Clean as needed – maintain average < 1/16 inch Class II, Division 2 >1/8 inch Infrequent Immediately shut down and clean Class II, Division 2 Clean frequently – minimize accumulation Class II, Division 1 > 1/4 inch Continuous to frequent Electrical Classification and Cleaning • If Clean-up is constant and – Dust layer is not apparent; – Surface color is discernible; – e.g.: Storage area with bags, drums, or closed hoppers; – No dust around. Classify as unclassified Electrical Equipment Metal Dust is always Class II Div I 51