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FMG Safety Issues in Particle Handling: Dust Explosions FMG 2.

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Presentation on theme: "FMG Safety Issues in Particle Handling: Dust Explosions FMG 2."— Presentation transcript:



3 Safety Issues in Particle Handling: Dust Explosions FMG 2

4 Credit for Stealing Shamelessly  Our thanks to Bob Gravel and Karen Tancredi of DuPont for their permission to use a number of slides from their presentation to the CCPS Technical Steering Committee. 3 The Miracles of Science

5 Goal of this Presentation  Provide a general overview of dust explosion fundamental concepts, prevention/mitigation methods, and current regulatory trends in the United States  In light of several recent incidents there has been a flurry of activity around combustible dust safety; however, thousands of dust explosion incidents have been documented since 1785 4

6 5 The Washburn Mill, Minneapolis, MN (1878)

7 US Chemical Safety Board Study  Combustible dust incidents in the US from 1980-2005: - 281 events - Wide range of products/industries - Many different unit operations - 119 deaths/718 injuries 6

8 Distribution of Dust Events by Industry Distribution of Dust Events by Material Type (Ref: U.S. Chemical Safety Board Report No. 2006-H-1) Combustible Dust Events in US: 1980-2005 Note: Coal mines & grain handling facilities excluded from study 7

9 8

10 Consequences of Dust Explosions l Potential financial losses – Equipment – Liability – Fines – Lost product/production l Potential for personal injury or loss of life 9

11 Importance of Awareness l 25% of Causes are Unknown in Dust Explosions l 36% of Incidents are Due to "Human Error“ l Knowledge is essential to safe operation! 10

12 Fundamental Concepts 11

13 What is a “Combustible Dust”? Per NFPA-654 (2006 edition): “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.” 12

14 Fire Triangle Fuel Oxidant Ignition Source 13

15 The Dust Explosion Pentagon  In addition to the traditional three components necessary for combustion, dust requires two additional conditions: Fuel (combustible dust), Heat/Ignition (flame), Oxygen in air, Dispersion of dust particles Confinement of dust cloud 14

16 Fundamental Concept of Dust Explosion Log: Difficult To Light, Burns Slowly Kindling: Easier to Light, Burns Quickly Dust: Lights Easily, Burns VERY Fast (Ref: Dust Explosions in the Process Industries, R.K. Echhoff) 15

17 Dust Combustion Consequences  Combustion in closed systems (e.g., vessel or room with few openings) results in pressure rise from confinement of the expanding hot gas and may result in sudden energy release (explosion) from mechanical/structural failure  Combustion in a relatively open area allows fireball expansion with little increase in pressure and poses a flash- fire hazard 16

18 What Materials can give Rise to Dust Explosions? l Materials which are solids, not stable oxides, and can be finely dispersed - Most natural and synthetic organic materials - Some metals (e.g., aluminum, magnesium) 17

19 1.3-Liter Acrylic Dust Explosion Tube Dust Explosivity Screening Tests Expose range of dust concentrations to energetic ignition source Additional tests should be conducted in larger vessel (e.g., 20-liter sphere) with pyrotechnic igniters Initial tests can be conducted in small vertical tube with AC arc ignition source 18

20 Key Dust Explosion Parameters  Minimum Explosible Concentration (MEC): “How much dust is needed to create a hazard?”  Dust Deflagration Index (K st ): “How fast will it burn?”  Limiting Oxygen Concentration (LOC): “How much oxygen is needed to support combustion?”  Minimum Ignition Energy (MIE): “How much energy does it take to make it ignite?” 19

21 Minimum Explosible Concentration (MEC) and Dust Deflagration Index (K st ) 20-Liter Test Vessel Pressure Transducer Port Pyrotechnic Igniters Air/Dust Inlet 20-liter Test Sphere MEC--Measure of the lowest dust cloud concentration capable of sustained combustion K st --Measure of the maximum burning rate of a dust cloud of ideal concentration under turbulent conditions Determine via tests conducted in 20-liter spherical vessel 20

22 A Common Question: “If I can see dust floating around in my work area do I have a dust explosion hazard?” The Answer: Dust suspensions which you can see through are likely to be orders of magnitude below MEC; however, they may still pose a dust explosion hazard if they are allowed to accumulate in layers. 21

23 Explosible Dust Concentration Mass of Powder/Dust per unit Volume [g/m3] (Ref: Dust Explosions in the Process Industries, R.K. Echhoff) 22

24 Optical Density of Explosible Dust Clouds Ref: Dust Explosions in the Process Industries, R.K. Echhoff) 23

25 Dust Classification by K st Explosion violence of dusts classified by value of K st : St-1: 0 < K st < 200 bar-m/s St-2: 200 < K st < 300 bar-m/s St-3: K st > 300 bar-m/s Ref: Dust Explosions, W. Bartknecht 24

26 Limiting Oxygen Concentration (LOC) The minimum oxygen level required for combustion of a dust cloud at any concentration, evidenced by pressure rise or sustained flame propagation Determined experimentally; typically 8-15% LOC depends on the inert gas used Nitrogen, CO 2, water vapor, and combustion gases are commonly used inertants; the higher the molar specific heat the more effective the gas (i.e., less needed) 25

27 Minimum Ignition Energy (MIE) The lowest capacitive spark energy capable of igniting a dust cloud of optimum concentration in a given number of laboratory trials Typically 10-100 mJ; flammable vapor MIEs usually < 1 mJ Threshold of perception ~1 mJ; ‘carpet spark’ in 2-5 mJ range Dust Sample in Bottom of Tube Air to Disperse Sample Glass Tube Spark Electrodes MIE Test Apparatus 26

28 MIE vs. Dust Concentration (ref. ISSA Prevention Series No. 2017) 27

29 MIE vs. Particle Size (Ref: Dust Explosions, W. Bartknecht) Mean Particle Size, microns Minimum Ignition Energy, mJ 28

30 Rate of Pressure Rise (K st ) vs. Particle Specific Surface Area (Ref: Dust Explosions, W. Bartknecht) Surface Area of Dust Rate of Pressure Rise Finer Particles 29

31 The Bottom Line: Size Matters! The finer the average particle size the greater the hazard since finer particles are easier to ignite and burn faster 30

32 Product Moisture l Moisture can effect explosion properties; products usually dried to <2% moisture prior to testing l Testing at actual moisture levels may be warranted if it can be ensured that the level tested is an absolute minimum 31

33 Prevention & Protection 32

34 When to Protect...  Provide protection “…where an explosion hazard exists” (NFPA) - Combustible dust clouds >25% of MEC present - ‘Significant’ combustible dust layers present - Must consider both normal & ‘abnormal’ operation  Consider both ignition source control and preventive/mitigative protection measures  Design of a system which eliminates all possibility of a dust explosion should be considered first (inherent safety) 33

35 Types of Ignition Sources Involved in Incidents 34 27% 18% 17% 15% 7% 4% 3% 2% Unknown Friction/ mechanical Overheating Flames Tramp material Welding & Cutting Static Electrical Other (ref. Partner, B. Dust Explosions - Assessment, Prevention and Protection. IBC Symposium, November 1989)

36 Control of Ignition Sources In general where a fuel and oxidizer are present control of ignition sources should not be relied on as a sole means of explosion prevention, although identification and elimination of such sources should be a high priority 35

37 Mechanical Ignition Sources  Can include impact, friction, or sparks from moving contact or thermite reactions  Can be difficult to quantify--subject of much current research; in general rotating tip speeds <3 ft./sec will not pose a hazard  Preventive maintenance to ensure mechanical integrity is primary means of eliminating this source 36

38 Open Flames Examples are smoking, burning, and welding operations Administrative control via policies & permitting process 37

39 Hot Surfaces l Hot surfaces can be controlled (e.g., electrical equipment design, steam pressure limits) or uncontrolled (e.g., overheated bearing)  Can directly ignite dust clouds or can cause ignition of a dust layer which can then ignite a dust cloud 38

40 Airborne Autoignition Temperature Uniform surface temperature which will cause dust cloud ignition Typically >350 o C for organic dusts (Ref: Dust Explosions in the Process Industries, R.K. Echhoff) Determined in uniform temperature chamber BAM AIT Test Apparatus 39

41 Dust Layer Autoignition Temperature (Ref: Dust Explosions in the Process Industries, R.K. Echhoff) Surface temperature which will cause reaction in a dust layer (ambient temperature surrounding) Standardized tests use either 5 or 10 mm thick layer 100 mm in diameter Hotplate Temperature Datalogger Thermocouple in Center of Layer Dust Layer 40

42 Sources and Hazards of Dust Deposits  Sources Settling of fugitive dust on horizontal surfaces ‘Caking’ of material on internal equipment surfaces Trapping of dust at stagnant points in system  Hazards Dust explosion from re-entrainment of dust Fire caused by self-heating of deposits 41

43 Re-Entrainment Dust Hazard (Ref: Dust Explosions,W. Bartknecht) 42

44 Re-Entrainment Hazard  May result from high air flow, mechanical shock, or from an impinging combustion event  Re-dispersion of relatively thin layers (1 mm) can result in dust clouds >MEC  In many cases the secondary explosion may be worse than the initiating event! 43

45 West Pharmaceuticals Incident: January 29, 2003 44

46 West Pharmaceutical Process 1) Rubber Batch Made in Mixer 5) Water Dried From Sheet 6) Sheet Folded In Box 4) Sheet Coated with Anti-Tack Powder In Water Solution 3) Rubber Rolled Into Sheet 2) Batch Lowered & Dropped into Mill (Source: US Chemical Safety Board) Dust Layer 45

47 Dust Generation  Some dry powder became airborne during drying  Plant personnel did not recognize potential dust explosion hazard posed by this material  No dust explosion potential was found during inspections by OSHA, local officials, and insurance underwriters  Powder dust explosion properties very typical of ‘standard’ combustible dusts 46

48 1) Minor Event Occurs Around Batch Mixer Dust Layer 2) Event Redisperses Dust Layer into Cloud 3) Dust Cloud Ignites And Propagates Through Building The Event… 47

49 Photo Courtesy U.S. Chemical Safety Investigation Board 48

50 The Aftermath: Six Fatalities/38 Injuries Ref: US Chemical Safety Board 49

51 The Moral:  Cleanliness is truly next to Godliness in prevention of dust explosions! The three best ways to prevent dust explosions: HOUSEKEEPING, HOUSEKEEPING, HOUSEKEEPING!  As a ‘rule of thumb’ accumulations of combustible dusts should not obscure the color of painted equipment (<1/32” thick)  Pay particular attention to overhead surfaces: finer particles are present here and they’re more likely to become airborne 50

52 Fugitive Dust Control  Primary emphasis should be on dust containment by design and maintaining mechanical integrity of equipment  Provide adequate ventilation to capture fugitive emissions  Frequently clean deposits by non-dust producing methods (e.g., gentle sweeping or vacuuming rather than ‘blowing off’ deposits with compressed air) 51

53 Other Considerations…  Damage-limiting construction (DLC) may be required for buildings if deposits cannot be adequately reduced: - 1/32” deposit over 5% of floor area based on 75 lb./ft 3 - Maximum area not to exceed 1000 ft 2 - Include overhead horizontal (and possibly vertical) surfaces  DLC involves use of pressure-relieving and resistant walls 52

54 Explosion in Dust Collector Pressure Pulse from Collector Redisperses Layer into Dust Cloud And results in Flame Propagation in Duct Deposits in Equipment & Ductwork can also be a Source for Secondary Events Moral: Minimize stagnation points & provide adequate conveying velocity to keep solids in suspension 53

55 Explosion in Dust Collector Solids in Drum Redisperse into Dust Cloud Back Propagation of Pressure Pulse from Dust Collector Secondary Explosion Pressure Pulses can also cause Secondary Events Moral: Provide isolation device to prevent back propagation of pressure pulse 54

56 Prevention of Spark Discharges  There can be no spark if conductive objects can’t become charged  Grounding of conductors will prevent charging  Personnel grounding recommended if MIE < 30 mJ and there is potential for exposure to combustible dust cloud 55 Bonding & Grounding of all Conductive Objects should be the First Line of Defense!

57 Other Preventive & Mitigative Measures 56

58 Preventive/Mitigative Strategies  Strategies may be preventive or mitigative in nature: 57 - Preventive: remove either the fuel or oxidant leg of the ‘fire triangle’ to prevent combustion (operate below MEC or inert) - Mitigative: accept that an explosion may occur and institute measures that eliminate the potential for injury and/or damage (contain/vent/suppress)

59 Limit ‘Fuel’ Concentration  Operate at maximum ‘acceptable’ concentration at least 25% of MEC (NFPA-69)  Sometimes difficult to achieve in practice due to non- homogeneity of dust clouds  In many cases the average (bulk) dust concentration must be orders of magnitude below MEC for this strategy to be effective 58

60 Inerting by Oxidant Reduction  Addition of a non-oxidizing (inert) gas or operation under partial vacuum may be used to prevent dust explosions by reducing the oxidant below a level where combustion is possible (LOC)  LOC used to establish safe operating limits; must be based on specific inert gas to be used  Do not overlook asphyxiation hazards posed by inerted vessels and processes! 59

61 Containment  Design equipment to withstand internal explosion without catastrophic failure  Generally limited to smaller volume equipment due to cost; applicable only to code-designed and constructed vessels 60

62 Explosion Venting  Use intentionally ‘weak’ elements to relieve the pressure & vent combustion event to a safe location to prevent catastrophic equipment damage or personnel injury  Use value of K st along with appropriate nomographs and/or equations to size vent of proper area (e.g., NFPA-68) 61

63 Length of Flame jet/Fireball Reaction forces Outbreak of fire Pressure waves Explosion Venting 62

64 Effect of Vents on Explosion Pressures 63 Time Pressure P stat P red P max Unvented Explosion Vented Explosion

65 Pressure Relief Panel Failure along score lines at desired pressure (Ref: Dust Explosions in the Process Industries, R.K. Echhoff) Panel ‘As-Installed’ 64

66 Closures set to release at P stat Vent Hatches (Ref: Dust Explosions in the Process Industries, R.K. Echhoff) 65

67 Explosion Suppression  Use fast-responding system to detect incipient explosion and release extinguishing agent to terminate combustion (typically detect in <10 msec, suppress in <100 msec)  Either presence of flame and/or pressure rise may be detected  Agent may be extinguishing powder (e.g., sodium bicarbonate), water, inert gas, or halogenated compound 66

68 S u ppressor Barrier Control Unit Detector Suppressor Explosion Suppression System 67

69 Suppression (cont’d)  Basis of design is generally vendor-specific but K st is always needed  Systems must be periodically inspected to ensure operational integrity  Suppression systems only operate once; process must be interlocked to shut down upon activation 68

70 Isolation  Must be used to prevent propagation of an event in one vessel to other attached pieces of equipment  Always necessary where containment is used and may also be needed where explosion is vented  Either passive or active methods may be used to prevent an explosion from propagating from its point of origin to other pieces of equipment  Process elements or dedicated special devices can be used to isolate the event 69

71 Isolation by Process Equipment (Ref: Dust Explosions in the Process Industries, R.K. Echhoff) Rotary Valve Screw Feeder 70

72 Devices Designed to Isolate Explosions Passive Design Active Design (Ref: Dust Explosions in the Process Industries, R.K. Echhoff) 71

73 Summary Comments  Four of the most critical elements in preventing dust explosions are proper equipment design, good housekeeping, attention to preventive maintenance, and proper grounding of conductive components  In general do not rely on the control of ignition sources as a sole means of protection where combustible dusts are or may be present  Proper training of personnel in the risks posed by combustible dusts and management of change are critical to safety but often overlooked 72

74 Summary Comments… Beware the “We’ve never had an Incident” Syndrome! 73

75 Current Regulatory & Legislative Trends 74

76 US Chemical Safety Board (CSB)  Conclusions from 2006 study on combustible dust hazards: - Combustible dust explosions a serious hazard - Existing OSHA standards not adequate for prevention - Similar problem in grain handling greatly reduced by regulation - Compliance with NFPA standards on combustible dust would have prevented most incidents - Many MSDSs do not adequately address dust hazards 75

77 Common Elements in Incidents  Workers, management and regulators unaware of hazard, leading to: Failure to conform to NFPA standards Inspections by numerous entities failed to identify dust hazard Unsafe accumulations of dust present (HOUSEKEEPING!) 76

78 OSHA Combustible Dust National Emphasis Program (NEP)  Requires 3 annual audits of industries having more frequent/higher consequence dust explosion potential and 1 annual audit of lower risk industry  Reliance on NFPA standards (primarily NFPA-654)  Emphasis on housekeeping, ignition source control, preventive/ mitigative methods  Provision for citations under General Duty Clause as well as other regulations for PPE and hazards communication deficiencies  Outreach activities re. education/training 77

79 Case History: Imperial Sugar Company Port Wentworth, Georgia February 7, 2008 78

80 The Incident  On February 7, 2008, at about 7:15 p.m., a series of sugar dust explosions at the Imperial Sugar manufacturing facility in Port Wentworth, Georgia, resulted in 14 worker fatalities.  Eight workers died at the scene and six others eventually succumbed to their injuries at the Joseph M. Still Burn Center in Augusta, Georgia.  Thirty six workers were treated for serious burns and injuries—some caused permanent, life altering conditions.  The explosions and subsequent fires destroyed the sugar packing buildings, palletizer room, and silos, and severely damaged the bulk train car loading area and parts of the sugar refining process areas. 79

81 The Causes  The CSB investigation identified the following incident causes: 1. Sugar and cornstarch conveying equipment was not designed or maintained to minimize the release of sugar and sugar dust into the work area. 2. Inadequate housekeeping practices resulted in significant accumulations of combustible granulated and powdered sugar and combustible sugar dust on the floors and elevated surfaces throughout the packing buildings. 3. Airborne combustible sugar dust accumulated above the minimum explosible concentration inside the newly enclosed steel belt assembly under silos 1 and 2. 80

82 The Causes (con’t.) 4. An overheated bearing in the steel belt conveyor most likely ignited a primary dust explosion. 5. The primary dust explosion inside the enclosed steel conveyor belt under silos 1 and 2 triggered massive secondary dust explosions and fires throughout the packing buildings. 6. The 14 fatalities were most likely the result of the secondary explosions and fires. 7. Imperial Sugar emergency evacuation plans were inadequate. Emergency notifications inside the refinery and packaging buildings were announced only to personnel using 2-way radios and cell phones. Many workers had to rely on face-to-face verbal alerts in the event of an emergency. Also, the company did not conduct emergency evacuation drills. 81

83 Imperial Sugar Explosion: Wentworth, GA 17 February 2008: 13 Fatalities 82

84 Imperial Sugar Explosion: Wentworth, GA 17 February 2008: 14 Fatalities 83

85 Increased Attention Post-Imperial Sugar Incident  At the Federal level H.R. 5522 was recently passed by the U.S. House of Representatives and is now in Senate committee: - Require interim regulation addressing specific areas of concern within six months after passage - Final standard with federal rulemaking process with guidance from ‘relevant’ NFPA standards within 18 months of passage - Amend Hazard Communication Standard to include ‘combustible dust’ as a physical hazard 84 - -

86 No Current Action  OSHA has just recently moved the new Dust Standard to its long-term agenda, meaning there will be no activity on the standard in 2012.  CSB, under its new chairman, Dr. Rafael Moure Eraso has issued a new call for the standard in the wake of the three incidents with fatalities at the Hoeganaes powdered metals plant in Gallatin, Tennessee.  However, OSHA has a housekeeping standard that, if enforced, would have prevented nearly 90% of all fatalities listed in the CSB report, including those at Imperial Sugar and Hoeganaes. 85

87 No Current Action  CSB Chairperson Rafael Moure Eraso said, “The three accidents at the Hoeganaes facility were entirely preventable. Despite evidence released by the CSB and information that Hoeganaes had in its possession even before the first accident in January 2011, the company did not institute adequate dust control or housekeeping measures. Dust fires and explosions continue to claim lives and destroy property in many industries. More must be done to control this hazard. No more lives should be lost from these preventable accidents.” 86

88 Questions? 87

89 Thank You for your Attendance! 88

90 Resources & References 89

91 Dust Explosion References  Dust Explosions in the Process Industries, 3 rd edition, R.K. Eckhoff, Elsevier, 2003  Dust Explosions: Course, Prevention, and Protection, W. Bartknecht, Springer-Verlag, 1989  Dust Explosion Prevention and Protection, J. Barton (IChemE), Butterworth-Heinemann, 2002 90

92 NFPA Standards/Guidelines  NFPA-61, “Standard for the Prevention of Fires and Dust Explosions in Agricultural and Food Processing Facilities”  NFPA-68, “Guide for Venting of Deflagrations”  NFPA-69, “Standard on Explosion Prevention Systems”  NFPA-77, “Recommended Practice on Static Electricity”  NFPA-484, “Standard for Combustible Metals”  NFPA-499, “Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas” 91

93 NFPA Standards/Guidelines (cont’d)  NFPA-654, “Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing, and Handling of Combustible Particulate Solids”  NFPA-655, “Standard for Prevention of Sulfur Fires and Explosions”  NFPA-664, “Standard for the Prevention of Fires and Explosions in Wood Processing and Woodworking Facilities” 92

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