TESTING TO ASSESS EXPLOSION CHARACTERISTICS OF DUST CLOUDS

Transcription

1 TESTING TO ASSESS EXPLOSION CHARACTERISTICS OF DUST CLOUDS Vahid Ebadat, Ph.D. Chilworth Technology, Inc. 250 Plainsboro Road Plainsboro, New Jersey 08536, USA INTRODUCTION Statistics suggest that fire/explosion hazards exist in facilities or equipment that handle or process combustible dusts (ref. CSB Investigation Report: Combustible Dust Hazard Study Nov 9, 2006). The consequences of a dust explosion can range from short-term disruption of production to loss of facilities and injury or fatality of personnel. A dust deflagration will occur if the concentration of the combustible dust that is suspended in air is sufficient to propagate flame when ignited by a sufficiently energetic ignition source. The oxygen in air is the most common oxidant however other oxidants such as fluorine, chlorine, and bromine can also be associated with deflagration events. The ignition sources that have been found to be the cause of the majority of explosions in dust handling/processing plants include (this is not an exhaustive list) hot work, open flames, mechanical friction and sparks, hot surfaces and equipment, thermal decomposition, electrical arcs (sparks), and electrostatic discharges. A systematic approach to identifying dust cloud explosion hazards and taking measures to ensure safety generally involves: Understanding the explosion characteristics of the dust(s) Identifying areas of the facility where combustible dust cloud atmospheres could exist under normal and/or abnormal conditions Identifying potential ignition sources that could exist under normal and/or abnormal conditions Taking measures to eliminate/control ignition sources, control the spread of combustible dust clouds, control oxidant supply (application of inert gas purging and/or padding); Taking measures to protect against the consequences of potential dust cloud explosions. Explosion protection measures include explosion relief venting, explosion suppression, explosion containment, and explosion isolation; and Regular inspection and maintenance of equipment and facilities to minimize ignition sources and dust releases As indicated above testing to characterize the powder s fire and explosion properties is an essential step in identifying potential ignition sources, assessing the risk and consequences

2 of igniting the dust cloud, and establishing the most appropriate basis of safety. A basis of safety is the management of the fuel, oxidant, or ignition sources to prevent ignition and/or the mitigation of the potential explosion to prevent or limit damages. 2.0 LABORATORY TESTING TO ASSESS EXPLOSION CHARACTERISTICS OF DUST CLOUDS To assess the possibility of an explosion in a facility and to select the most appropriate basis of safety, explosion characteristics of the dust(s) that are being handled/processed in the facility should be determined. The explosion characteristics of powders normally fall within one of two groups, likelihood of an explosion (ignition sensitivity) and consequences of an explosion (explosion severity). The combination of likelihood and consequences will define the explosion risk. Two groupings of dust tests are used to define these aspects of dust explosion risk as discussed below: 2.1 Laboratory Tests to Determine the Likelihood of an Explosion (Ignition Sensitivity) Explosion Classification (Screening) Test (US Bureau of Mines Report of Investigations 5624, Laboratory Equipment and Test Procedure for Evaluating Explosibility of Dusts) The Explosion Classification test determines whether a dust cloud will explode when exposed to a sufficiently energetic ignition source. The test results in a powder being classified as either explosible or non-explosible. This test answers the question Can this dust explode? The Explosion Classification Test is usually conducted in a modified Hartmann Tube apparatus. The apparatus consists of a 1.2-Liter vertical tube mounted onto a dust dispersion system. Dust samples of various quantities are dispersed in the tube and attempts are made to ignite the resulting dust cloud by an approximately 10Joule electrical arc ignition source. If the material fails to ignite in the Modified Hartmann Tube apparatus, the testing is continued in a 20-Liter Sphere apparatus where the dust cloud is exposed to a 10,000Joule ignition source. Minimum Explosible Concentration - MEC (ASTM E1515, Standard Test Method for Minimum Explosible Concentration of Combustible Dusts) The Minimum Explosible Concentration (MEC) test determines the lowest concentration of dust cloud in air that can give rise to flame propagation upon ignition. This test answers the question How easily can an explosible dust cloud be formed? The test involves dispersing a sample of the dust in a 20-Liter Sphere apparatus and attempting to ignite the resulting dust cloud with an energetic ignition source. Trials are repeated for decreasing sample sizes until the MEC is determined. The MEC of a given dust cloud is influenced by the energy of the ignition source. An

3 increase in the energy of the ignition source will result in a lower MEC value. With the exception of certain steady state processes, it may prove difficult in practice to maintain dust concentrations below the lower limit of flammability. Limiting Oxidant Concentration - LOC (EN , Determination of the Limiting Oxygen Concentration of Dust Clouds) The Limiting Oxidant Concentration (LOC) test determines the minimum concentration of oxygen (displaced by an inert gas such as nitrogen or carbon dioxide) capable of supporting combustion. An atmosphere having an oxygen concentration below the LOC is not capable of supporting a dust cloud explosion. The LOC test is used to study explosion prevention or severity reduction involving the use of inert gases and to set oxygen concentration alarms or interlocks in inerted vessels. LOC testing can be performed using the 20-Liter Sphere apparatus. Dust samples of various sizes are dispersed in the vessel and attempts are made to ignite the resulting dust cloud with an energetic ignition source. Trials are repeated for decreasing oxygen concentrations until the LOC is determined. The LOC of a given dust cloud is dependent on the type of inert gas that is used to replace the oxidant of the atmosphere as well as some process conditions such as temperature. Therefore, LOC testing should simulate the process conditions and be performed by using an inert gas that is representative of the inert gas used in practice. Minimum Ignition Temperature of a Dust Cloud MIT Cloud (ASTM E1491, Standard Test Method for Minimum Auto-ignition Temperature of Dust Clouds) The Minimum Ignition Temperature (MIT) test of a dust cloud determines the lowest temperature capable of igniting a dust dispersed in the form of a cloud. The MIT cloud is an important factor in evaluating the ignition sensitivity of dusts to such ignition sources as heated environments, hot surfaces, electrical devices, and friction sparks. Varying dust concentrations (ranging from lean to rich) are dispersed into a furnace and the minimum furnace wall temperature capable of igniting the dust cloud at its optimum concentration for ignition is determined. The MIT cloud value, as with most dust explosion parameters, is strongly influenced by the particle size and moisture content of the dust. A decrease in particle size and moisture content of the dust particles results in a lower MIT cloud value. Minimum Ignition Temperature of a Dust Layer MIT Layer (ASTM E2021, Standard Test Method for Hot Surface Ignition Temperature of Dust Layers) This Minimum Ignition Temperature (MIT) test of a dust layer determines the lowest temperature capable of igniting a dust layer of standard thickness (5 to 12.7 mm). The MIT layer is used in evaluating the ignition sensitivity of powders to ignition by hot surfaces. The lower value of MIT cloud or MIT layer is used to specify the maximum surface temperature of Class II electrical devices in hazardous areas, per the National Electric Code.

4 Minimum Ignition Energy - MIE (ASTM E2019, Standard Test Method for Minimum Ignition Energy of a Dust Cloud in Air) The Minimum Ignition Energy (MIE) test determines the lowest electrostatic or mechanical spark energy that is capable of igniting a dust cloud at its optimum ignitable concentration. The test is used primarily to assess the susceptibility of dust clouds to ignition by electrostatic discharges (sparks). Dust samples of various sizes are dispersed in a 1.2-Liter vertical tube and attempts are made to ignite the resultant dust cloud with discrete capacitive or inductive sparks of known energy. Capacitive sparks are used to assess electrostatic discharge sensitivity whilst inductive sparks are used to assess mechanical discharge sensitivity. MIE values measured with inductance tend to be lower than the corresponding capacitive values for the same powder. The MIE value is influenced by particle size and moisture content of the dust and by process conditions such as temperature and oxidant content. A decrease in particle size and moisture content of the dust particles results in a lower MIE value. Increasing the temperature of the atmosphere where the dust cloud is suspended will result in a lower MIE value as well. Electrostatic Volume Resistivity (General Accordance with ASTM D257, Standard Test Methods for DC Resistance or Conductance of Insulating Materials) Volume Resistivity is a measure of the electrical resistance for a unit volume of material and is the primary criterion for classifying powders as low, moderately, or highly insulating. Insulating powders have a propensity to retain electrostatic charge and can produce hazardous electrostatic discharges. The method involves placing a powder sample into a standardized electrode cell. A voltage is applied to the cell and the current through the powder is measured. Volume Resistivity is calculated using the known voltage, the measured current, and the geometrical relationship between the electrodes. Because of the effect of atmospheric and absorbed moisture on volume resistivity this test is usually performed at both ambient and low relative humidity conditions. Electrostatic Chargeability (General Accordance with ASTM D257, Standard Test Methods for DC Resistance or Conductance of Insulating Materials)) Electrostatic Chargeability is a measure of the propensity of powder particles to become charged when flowing through conveyances or when handled in containers. Electrostatic Chargeability is measured by flowing samples through a pipe and measuring the resultant electrostatic charge per unit mass. This test provides data that can be used to develop appropriate material handling guidelines from an electrostatic hazards point of view. Because of the effect of atmospheric and absorbed moisture on powder chargeability, this test is usually performed at both ambient and low relative humidity conditions.

5 Self-Heating (Prevention of Fires and Explosions in Dryers, Institute of Chemical Engineers, 1990) Ignition of bulk powders can occur by a process of self-heating when the temperature of the powder is raised to a level at which the heat liberated by the exothermic oxidation or decomposition reaction is sufficient to exceed the heat losses and to produce a runaway increase in temperature. At some elevated temperature, ignition can occur. The minimum ambient temperature for self-ignition of a powder depends mainly on the nature of the powder and on its dimensions. If these variables are predictable, a reliable assessment of the onset temperature for self-ignition and also the induction time to self-ignition can be made by appropriate small-scale laboratory tests. o o o o Bulk Powder Test: Simulates bulk powder in IBCs, bags, bottom of hoppers Aerated Powder Test: Simulates fluidized bed processing Powder Layer Test: Simulates powder deposits on dryer walls/surfaces and tray drying Basket Test: Simulates large-scale storage or transport conditions 2.2 Laboratory Tests to Determine the Consequences of an Explosion (Explosion Severity) Maximum Explosion Pressure, Maximum Rate of Pressure Rise, Deflagration Index (K st Value) (ASTM E1226, Standard Test Method for Pressure and Rate of Pressure Rise of Combustible Dusts) The Maximum Explosion Pressure and Maximum Rate of Pressure Rise values are determined by using a 1 m 3 or 20-Liter Sphere test apparatus. The dust sample is dispersed within the sphere, ignited by chemical igniters, and the pressure of the resulting explosion is measured. The cloud concentration is varied to determine the optimal dust concentration. The maximum explosion pressure and maximum rate of pressure rise are measured and used to calculate the Deflagration Index (K st ) value of the dust cloud. These data can be used for the purpose of designing dust explosion protection measures such as explosion relief venting, suppression, and containment and to classify a material s explosion severity. This test answers the question How bad is it if it happens? 3.0 MATERIAL SAFETY DATA SHEETS (MSDS) According to the US Chemical Safety and Hazard Investigation Board (CSB) (ref. CSB Investigation Report: Combustible Dust Hazard Study Nov 9, 2006) Material Safety Data Sheets (MSDS) are less than adequate regarding combustible powder properties. Quantitative combustible dust fire and explosion properties are not specifically and clearly required to be included on Material Safety Data Sheets by the existing OSHA Hazard

6 Communication Standard (HCS) or the American National Standards Institute consensus standard for MSDS format and preparation, ANSI Z As a result, when explosivity information is included, it is often in the form of less than adequate qualitative statements such as Powder may form explosive dust/air mixtures. Qualitative statements give no hint as to the conditions required to create an explosion hazard, or the relative violence of the resulting deflagration. Good information on the hazardous properties of materials is critical to an adequate process hazards analysis. Therefore, all pertinent fire and explosion, electrostatic, and thermal instability information should be included in the MSDS in order to adequately understand and control the hazards associated with combustible dusts. 4.0 APPROACH TO PROCESS SAFETY TESTING The following table specifies the type of data that might be required for some common unit operations involving powders (Reference 1) Table 1 Dust Explosion Test Data Requirements for some Specific Unit Operations Unit Operation Manual Handling / Pouring Explosion Screening (A/B) 1 MIE (mj) MIT Cloud ( C) MIT Layer ( C) Explosion Severity Kst (bar.m/s) LOC 2 (%) MEC (g/m 3 ) Volume Resistivity 3 (Ω.m) Chargeability 4 (C/Kg) X X X X Self-Heating ( C) Sieving / Screening X X X X Tumble / Double Cone Blending X X X X X X Ribbon Blending X X X X X X Milling X X X X X X X X X Jet Milling X X X X X X Spray, Fluidized Bed, Tumble, Flash Drying X X X X X X Tray Drying X X X X Pneumatic Conveying X X X X X Screw Conveying X X X X X Transfer to Hopper / Bin / Tote / Container Dust Collector and Exhaust Ventilation X X X X X X X X X X X 1. Explosibility Screening test is only conducted if the combustibility of the powder/dust (as being present in the process/facility) is not yet established. If the powder is found to be noncombustible, other tests in the table may not be required. 2. LOC is determined if the basis of safety is inert gas blanketing. 3. Volume Resistivity should be considered if the Minimum Ignition Energy is less than 25mJ. 4. Chargeability should be considered if the Minimum Ignition Energy is less than 25mJ.

7 5. OSHA Combustible Dusts National Emphasis Program (NEP) - CPL , March 11 th, 2008 The purpose of this NEP is to inspect facilities that generate or handle combustible dusts which pose a deflagration or other fire hazard when suspended in air or some other oxidizing medium over a range of concentrations, regardless of particle size or shape. OSHA Combustible Dusts NEP Inspection and Citation Procedures Include: Assessment of the combustible dust threat to employees o Are the dust and management practices hazardous? o What is the site history of fires involving dust? o Does the MSDS indicate a dust explosion hazard? o Are dust accumulations hazardous? Collection of samples of combustible dusts for laboratory analysis o From high places o From floors and equipment surfaces o From within ductwork Audit of dust management practices and equipment including dust collectors, ductwork, and other dust containers. Audit of room safeguards Audit of ignition source management Table 2 lists the tests, test apparatus, and sample conditions which may be performed by OSHA to determine the explosibility and combustibility parameters of dust samples collected during their site inspections. Table 2 Combustible Dust Tests Conducted by OSHA Test Particle Size Moisture Content Test Apparatus % Through 40 Mesh (420µm) As Received As Received 40 Mesh (420µm) Sieve % Moisture Content Less than 420µm As Received Drying Oven at 75ºC for 24 hrs % Combustible Material Less than 420µm As Received Muffle Furnace at 600ºC for 1 hr % Combustible Dust Less than 420µm - (Calculation) Max. Normalized Rate of Pressure Rise - Kst As Received Less than 5% 20-Liter Sphere (2500J Igniters) Min. Explosible Concentration (MEC) Less than 420µm Less than 5% 20-Liter Sphere (2500J Igniters) Class II* Less than 75µm 1-Liter Hartmann Bomb for Explosion Severity (E.S) Volume Resistivity As Received As Received Minimum Ignition Energy (MIE) Less than 75µm?? Hartmann Lucite Chamber Minimum Ignition Temperature (MIT) Less than 75µm?? Godbert-Greenwald Furnace

8 * Class II Test The U.S. Bureau of Mines (see NFPA 499 section A.3.3.9) classifies dust by its properties compared to a standard material, Pittsburg Coal, and defines ignition sensitivity and explosion severity as follows: IgnitionSensitivity [ MIE MIT [ MIE MIT MEC] MEC] Pittsburg Coal Sample ExplosionSeverity [ P [ P max max dp dt dp dt max max ] ] Sample Pittsburg Coal If the ignition sensitivity is greater than 0.2 and/or the explosion severity is greater than 0.5, the dust is defined as Class II. If the ignition sensitivity is less than 0.2 and the explosion severity is less than 0.5, the dust explosion hazard is not significant. OSHA will only characterize a sample sufficiently to prove (or disprove) that the sample meets the definition for Class II dusts based on the ignition sensitivity or the explosion severity test results. It should be noted that the dust sample preparation and test methods/apparatus suggested by OSHA may not comply with the current ASTM test methods and suggestions for sample preparations as described in section 4 of this paper. For example, ASTM E1226 (test method for the determination of the Maximum Explosion Pressure and Maximum Rate of Pressure Rise (K st )) recommends a 10,000J ignition source and particle size less than 75µm compared to an ignition source of 2,500J energy and particle size of as received according to the OSHA NEP test protocol. Only appropriate Pittsburgh Coal data including explosion severity obtained using a Hartmann Tube apparatus should be used in the explosion severity calculation in the NEP protocol. 6. BASIS OF SAFETY FROM DUST CLOUD EXPLOSIONS - Overview Safety from dust cloud explosions could include taking measures to avoid an explosion (explosion prevention) and/or designing facilities and equipment so that in the event of an explosion people and facilities are protected (explosion protection) from injury. Selection of explosion prevention and/or protection measures is usually based on: The availability of information on the sensitivity of the powder(s) to ignition and the resulting explosion severity The nature of the processes and operations The level of personnel s knowledge and appreciation regarding the consequences of a potential dust explosion and the reliability of administrative controls

9 The environmental effects of a dust explosion The business interruptions resulting from a dust explosion The site/corporate Safety Culture: The desire to protect personnel and property 6.1 Explosion Prevention and Protection Measures To ensure safety from dust explosion hazards one of the following measures is taken: An explosible dust cloud is never allowed to form o The atmosphere is sufficiently depleted of fuel so that it cannot support combustion o The atmosphere is sufficiently depleted of oxidant (normally the oxygen in air) so that it cannot support combustion All ignition sources capable of igniting the dust cloud are eliminated Explosion protection measures mitigate ignition of a dust cloud o Explosion Containment o Explosion Suppression o Explosion Relief Venting o Explosion Isolation Measures 7. CONCLUSION The National Fire Protection Association (NFPA) standard # 654, Standard for the Prevention of Fire and Dust Explosions from the Manufacturing, Processing and Handling of Combustible Particulate Solids recommends that the evaluation of the hazard of a combustible dust should be determined by the means of actual test data by evaluating each situation and selecting applicable tests. According to NFPA 654 the factors that are sometimes used in determining the deflagration hazard of a dust include: Particle size distribution Moisture content as received and as tested Minimum explosible concentration Minimum ignition energy Maximum explosion pressure, Maximum rate of pressure rise, and Kst Layer ignition temperature Dust cloud ignition temperature Limiting oxidant concentration (LOC) to prevent ignition Electrical volume resistivity Chargeability A testing protocol (see table 1) exists for classifying powders as to their suitability for particular handling/processing operations. By identifying the powder characteristics, suitable preventive and protective measures (a basis of safety) can be determined for each unit operation.

10 8. REFERENCES The following references provide further detail on the topics that have been discussed in this paper: 1. Handling Dusts and Powders Safely A Strategic Guide to Characterization and Understanding, Published by Chilworth Technology, American Society for Testing and Materials (ASTM), Testing Standards, West Conshohocken, Pennsylvania, USA. 3. National Fire Protection Association (NFPA), 1 Batterymarch Park, PO Box 9101, Quincy, MA Center for Chemical Process Safety (2005), Guidelines for Safe Handling of Powders and Bulk Solids, American Institute of Chemical Engineers, New York, New York. 5. Eckhoff, Rolf K (1997), Dust Explosion in the Process Industries, 2 nd Edition, Butterworth-Heinemann Linacre House, Jordan Hill, Oxford.