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Safe handling of combustible powders during transportation, charging, discharging and storage
Journal of Loss Prevention in the Process Industries 13 (2000) 253–263 www.elsevier.com/locate/jlp
Safe handling of combustible powders during transportation, charging, discharging and storage夽 Tom Hoppe, Norbert Jaeger *, John Terry Safety Testing Laboratory, Ciba Specialty Chemicals, Additives Division, McIntosh, Alabama, USA
Abstract The knowledge of the ignition behavior of dust–air mixtures due to electrical sparks (MIE, Minimum Ignition Energy) and hot surfaces (MIT, Minimum Ignition Temperature) is important for risk assessments in chemical production plants. The ignition behavior determines the extent and hence the cost of preventive protection measures. This paper describes the use of the minimum ignition energy and minimum ignition temperature as very important safety indexes in practice. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Minimum ignition energy; Hazard evaluation; Electrostatic discharges
1. Explosion hazards in production plants An explosion hazard can exist when flammable gases, liquids or dusts are produced, stored or processed in a plant and these materials are present as a mixture in air. An explosive mixture is present when combustible gases vapors or dusts are present in such quantities in air that an explosion occurs after an ignition (Bartknecht, 1993). An explosion requires three conditions to exist simultaneously (see Fig. 1): 앫 fuel or flammable material in sufficient quantity and effectively mixed with 앫 air and 앫 an effective ignition source Using preventive measures against explosions requires at least the reliable exclusion of one of the conditions necessary to generate an explosion as shown in Fig. 2. An explosion can thus be excluded with certainty by either
夽 Prepared for presentation at the International Symposium of Hazards, Prevention and Mitigation of Industrial Explosions, Sohaumberg, IL, September 21–25, 1998 * Corresponding author. Tel.: +1-334-436-2709; fax: +1-334-4365044. E-mail address: [email protected] (N. Jaeger).
Fig. 1.
Requirements for the occurrence of explosions (ISSA, 1996).
앫 avoiding the development of explosible mixtures (combustible dusts, flammable gases), or 앫 replacing the atmospheric oxygen with an inert gas, working in a vacuum or using inert dust, or 앫 preventing the occurrence of effective ignition sources. All three measures are summarized under the heading “preventive explosion protection”. It should be ensured that at least one of the three conditions is eliminated or so strongly reduced that an explosion is no longer poss-
0950-4230/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 4 2 3 0 ( 9 9 ) 0 0 0 3 6 - 4
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T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
2. Nature and origin of static electricity in production plants
An electrostatic charge by itself does not necessarily represent an ignition hazard. Such a hazard exists only when the charge is so high that discharges occur owing to the high electric field (ISSA, 1996). The individual steps, which lead to the occurrence of charge build up and discharge are always the same: Fig. 2.
Hazard triangle.
ible or at least very rare. Their appropriate elimination prevents an explosion from starting. The use of “Avoidance of Ignition Sources” as a protective measure requires a comprehensive hazard evaluation to determine all possible ignition sources that may occur during production. There are a large number of different ignition sources that one must consider in industrial operations (see Fig. 3). Not every ignition source has sufficient energy to ignite all types of explosible atmospheres. Therefore, it is necessary to investigate the ignition sources in detail in order to determine the ignition hazard in conjunction with the expected explosible mixtures. Trivial ignition sources (welding, smoking etc.) must be excluded by using organizational measures. Ignition sources, which could result from the process itself introducing energy into the product being handled (e.g. mechanical, friction energies), are not the topic of this paper but must be considered during a risk analysis. This holds particularly true for products that have a tendency to form glowing particles and, if in the course of the process, glowing particles could be formed or entrained. Electrostatic discharges are ignition sources that are often underestimated in industrial operations. These discharges occur frequently in most product handling processes and will be considered in detail in this paper.
Fig. 3.
Examples of possible ignition sources (ISSA, 1996).
Charge separation:
Separation process (usually between product and plant units) lead to charging of the surfaces in contact. Charge Charges can accumulate on products, accumulation: plant units, packaging containers and operators, etc. Charge As soon as a connection of sufficient dissipation: conductivity is established between the ground and the sites of the accumulated charge, the charge can dissipate to the ground. Discharge: If the charge continues to accumulate because the charges formed in the separation processes cannot flow to ground quickly enough, a discharge will occur when the breakdown field strength is reached.
In a powder handling production plant, static charges can occur at the surface of solids and powdery substances. At this point it is important to make a distinction between conductors of electricity (e.g. metals) and nonconductors or insulators (e.g. plastics). Electrically charged particles (electrons) can move freely on a conductor, whereas on an insulator they are fixed in one place. If a substance has an excess or a deficiency of charged particles, it is said to be charged. If two uncharged (neutral) objects, of which at least one is an insulator, are brought into close contact with each other and then rapidly separated, both of the objects become charged. This occurs because charged particles will first of all pass from one object to the other but then, during separation, cannot return fast enough. An electrostatic discharge can be incendive when the energy released is equal to or greater than the minimum ignition energy (see Section 3.1 for definition) of a mixture. The energy released depends, among other things, on the type of discharge. This in turn depends on the geometry and material of the participating surfaces as well as on certain other conditions. The following overview summarizes the ignition behavior of several types of electrostatic discharges.
T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
Ignition sources: Brush discharge
255
Incendivity for MIE⬍3 mJ1
Conical pile discharge
MIE⬍1 J2
Spark discharge
MIE⬍1 J
Propagating brush discharge
MIE⬍10 J Fig. 4.
Charge accumulation (ISSA, 1996).
3. Basic information Thorough knowledge of the ignition sensitivity of dust-air-mixtures towards electrostatic discharges (Minimum Ignition Energy, MIE) and hot surfaces (Minimum Autoignition Temperature, MAIT) is extremely important for assessing the hazards in dustcarrying plants. The ignition behavior essentially determines the extent, and hence the cost of the protective measures, to be used. This is especially true for the use of the protective measure “prevention of ignition sources” and also for the understanding of the ignition phenomena regarding static electricity e.g., brush discharges, bulk surface discharges, spark discharges, propagating brush discharges. Knowledge of a powder’s minimum ignition energy, particle size distribution and specific resistivity allows one to define the necessary protective measures for different operations based on the vessel size and the conductivity of the material of construction (Figs. 4 and 5).
Fig. 5.
Basic scheme of electrostatics (ISSA, 1996).
3.1. Minimum ignition energy The minimum ignition energy, MIE, of a combustible substance is the lowest value of the electrical energy stored in a capacitor, which on discharge just suffices to ignite the most readily ignitable fuel-air-mixture at atmospheric pressure and room temperature. To help
1
It is assumed that dusts are not ignitable from a brush discharge. The initial assumption that a dust cannot be ignited from a brush discharge is justified by considerable industrial experience with handling powders with low MIEs and extensive laboratory testing. 2 Estimated value from Glor and Maurer, 1993.
assure a standardized test procedure, a test apparatus known as MIKE 3 (Fig. 6) of the third generation has been specially developed by Ku¨hner AG, Switzerland and has been made commercially available (Cesana & Siwek, 1992; Siwek, 1995a,b; Siwek & Cesana, 1995). Other test apparatus are also available to determine the MIE and are included in an American Society for Testing Materials (ASTM) standard on MIE of dusts which will be issued in the near future. The MIE is usually quoted as a range: The lower value represents the highest energy at which no ignition is found in at least 10 experiments. The higher value, on
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T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
Fig. 6. MIKE 3 apparatus for determination of the minimum ignition energy MIE of dusts.
the other hand, is the lowest energy at which the dustair mixture is just ignited:
3.3. Powder volume resistivity
no ignition⬍MIE⬍ignition
To characterize the static dissipative properties of a material, its powder volume resistivity, rR, has to be determined (see Fig. 8). It must be kept in mind that resistivity is not an absolute property of a powder and depends very strongly on moisture content and on the method used for measurement.
The method for the determination of MIE is described in the international standard of the International Electrotechnical Commission (IEC) (IEC, 1992) and in the near future will be available as an ASTM standard. The MIE is generally obtained with an inductance in the discharge circuit. However, in order to assess the incendivity of electrostatic discharges in industrial operations towards dust-air-mixtures, the MIE must also be determined without an inductance in the discharge circuit. With flammable gases and easily ignitable dusts, the influence of the inductance is generally not detectable. 3.2. Minimum autoignition temperature, MAIT The Minimum Autoignition temperature, MAIT, is defined as the lowest temperature of a heated surface at which the most readily ignitable mixture of a dust with air just ignites (Bartknecht, 1993; IEC, 1964; Jaeger, 1996; Jaeger & Siwek, 1996; Siwek & Cesana, 1995a,b; Siwek, 1996). It provides information on the ignition behavior of a dust suspension when quickly passing over a hot surface (see Fig. 7).
Fig. 7. BAM furnace.
4. Protective measures When combustible dusts are handled, avoiding an explosive atmosphere by keeping the dust concentration outside the explosive range is rarely possible due to sedimentation or whirling up of the material being handled. Thus, as a matter of principle, an explosive atmosphere can only be avoided with certainty by reducing the oxygen concentration, e.g., inerting. In practice, however, the possibilities to apply inerting are also limited. For
Fig. 8.
Powder resistivity test chamber.
T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
such situations, avoidance of effective ignition sources and/or explosion-proof design are the only measures available (Jaeger & Siwek, 1998; VDI, 1990). In the following, protective measures are outlined for typical powder handling operations and processes. As previously stated, the following product and plant properties are important for an accurate hazard assessment: 앫 Minimum ignition energy, MIE, of the bulk material (measured without inductance in the discharge circuit), 앫 Minimum Autoignition Temperature, MAIT, of the bulk material, 앫 Volume resistivity SR of the powder, 앫 Particle size distribution of the bulk material and mean value, M, 앫 Volume and shape of the silo or container (volume and shape of the product heap and of the dust cloud). Unless otherwise stated, the following sections are based on the assumption that the bulk materials are handled without flammable gases or vapors being present. 4.1. Filling and emptying operations In filling and emptying operations, electrostatic ignition hazards are of prime importance due to the electrostatic charging occurring during separation process (Glor, Maurer & Rogers, 1995; Lu¨ttgens & Glor, 1988). The hazard comprises possible charge accumulation not only on plant units, including drum and container, but also — in the case of insulating bulk material — on the bulk material itself which is shown in Fig. 9. Assuming the insulating bulk material carries a charge, filling represents the more hazardous of the two operation for the following reasons. In a filling operation, the bulk material undergoes dispersion (e.g. gravity feed, pneumatic transport, etc.) and can therefore be charged in the separation processes occurring in transport. The bulk material, and hence its associated charge, is then “packed” in a small space in which the charge
Fig. 9.
Filling (charging) and emptying (discharging) operations.
257
is not able to flow to ground sufficiently quick, even with a conductive and grounded receiver. This generates a high space charge density and hence a high electric field. In addition, consideration must also be given to the problems associated with heat accumulation and the possibility of entrainment of smouldering lumps. Based on the product and plant properties mentioned above, the decision tree and the matrices on the next pages provides guidance for the required safety measures. The matrices mentioned in the decision tree are shown in the following Tables for different container volumes. 1. Containers capable of discharge: The leakage resistance to the ground point from any point on the surface of the container must be less than 100 Mega Ohm. (⬍106 Ohm) 2. Charge dissipative (ⱖ108ⱕ1011 Ohm) containers having at least on one side a surface resistance ROA measured according to ASTM method D257-93 (Surface Resistivity) of less than 100 Giga Ohm at 30% rel. humidity and more than 100 Mega Ohm at 65% rel. humidity. 3. Use only conductive containers, 1 MOhm (⬍106 Ohm), or those capable of discharge (see above), and ground. 4. Fines in concentrations greater than the LEL are not to be expected for dust free forms during normal discharge or charging operations involving packaging.
4.2. Loading bulk materials into flammable solvents Whenever it is possible, bulk materials should be fed into flammable solvent under closed conditions and in an inert atmosphere (see Fig. 10). This can be realized by means of a rotary air lock, a two-valve system, a feed screw or by conveying by inert gas. If such a closed introduction system is not possible, as the next best thing, the solvent should be cooled at least 5°C below its flash point before loading the bulk material, or the bulk material introduced first followed by the flammable solvent. If the open addition of bulk material into previously loaded flammable solvent at a temperature greater or less than 5°C the flash point is unavoidable, then the following points must be considered: 앫 operator must be grounded. 앫 all aids to introduction, such as funnels, charging chutes, etc., must be made in conducting materials and grounded during the transfer. 앫 containers for solvent and powders must be of conductive material and grounded during the charging process. 앫 conductive vessels for solvents and bulk material with
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T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
an insulating internal coating of up to 2 mm maximum may be used, if they are grounded during filling and emptying. 앫 normal loose plastic sacks or plastic liners must not be used. Plastic liners or plastic sacks with at least one-sided non-chargeable (surface resistance according to ASTM ⬍1011 ohms at 30% relative humidity) are allowed (e.g. paper sacks or paper sacks lined one side with a normal plastic coating, if the coating thickness is less than 2 mm).
Even when handling highly sensitive dusts, the occurrence of effective ignition sources in mixing is unlikely, provided the following conditions are met: 앫 In the filling and emptying of the mixer, the measures applied are the same as those in the filling and emptying of containers. 앫 In the filling and emptying of the mixer, the mixing
The formation of a solvent/air mixture must be expected, and corresponding safety measures (e.g. inerting) must be taken
START
Flammable Solvent Content > 0.5 % w/w
4.3. Mixing
Is the product stored for long periods in large volume in a silo or without special ventilation possibly after a size reduction
yes
yes
no
Flammable Solvent Content between 0 and > 0.5 % w/w
no yes The formation of solvent/air or hybrid mixtures must be expected, and corresponding safety measures inerting) must be taken
no
Particle Size greater than 0.5 mm and fines below explosive (< 10 g/m3)
yes
The presence of an explosive atmosphere is not to be expected
no
MIE of the bulk solid is greater than 1 J
Ignition due to static electricity is not to be yes
no Earth all conductive plant components, vessels and containers including those capable of
MIE of the bulk solid is less than 30 mJ
yes
Personnel must be grounded by means of conductive and flooring where explosible concentrations are
no
Additional safety measures relevant to the situation to be taken are shown in the following matrix
T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
259
ated in a mixer with a product fill height of less than 70 vol %, provided the combination of material values as listed in the following table are present. MIEa [mJ] ⬍1
1–3
3–10 10–30 30–100 100–300 300–1000 ⬎1000
MAIT
do not
530
500
[°C]
process
a
465
430
395
360
325
The MIE values must be determined with an additional inductance in the dis-
charge circuit.
앫 Nauta mixers with bottom support of the helical screw can heat up during operation and care must be exercised with substances capable of spontaneous decomposition.
4.4. Dust separation
elements must be off or run at a circumferential speed (tip speed) which does not exceed 1 m·s⫺1. This restriction must be assured by technical safeguards. 앫 If the mixer is closed and is filled to fill level of 70 vol. % or more, the circumferential speed of the mixing elements is no longer restricted (see Fig. 11). 앫 Any insulating coating must have a breakdown voltage of less than 4 kV. Product build-up must be checked if a homogeneous layer can be formed. 앫 Circumferential speeds up to 10 m·s⫺1 can be toler-
In the case of dust separators, especially in filters, the dust explosion hazard must not be underestimated. The probability of occurrence of a fine dust atmosphere sensitive to ignition is large. In addition, the entrainment of ignition sources (e.g. formation of smouldering lumps) and the danger of ignition through an electrostatic charging is of prime importance (see Fig. 12). Electrostatic charging must be prevented by the following measures: 앫 Grounding of all conductive apparatus parts. Particular attention must be paid to the grounding of all conductive parts which could possibly be insulated from ground if a filter cloth made of insulating
260
Fig. 10.
T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
Powder charging into a vessel with a precharged solvent.
must be specially checked after repair and maintenance work. 앫 With a MIE⬍3 mJ or in the presence of flammable gases or vapors in the air being cleaned, electrically conducting filter materials must be used. An exception to this rule can be applied when the protective measure “inerting” is employed. Continuity of the conductivity and safe grounding must be checked. Multiple washings can have an adverse effect on the continuity of the filter material conductivity and thus require repeat checking. 앫 All inner walls on which dust can impact at high speed must not have any insulating inner coatings with a high electrical breakdown strength (breakdown voltage must be less than 4 kV; periodic checks are required). In general, with dusts with a MIE⬍10 mJ it is advisable to implement explosion protection measures which go beyond the avoidance of effective ignition sources or to consult the responsible specialist departments. It should further be noted that the fan must be installed on the clean air side of the filter and that dust deposits must be avoided in the pipe and fan housing (periodic check or install a dust control unit). 4.5. Use of flexible intermediate bulk container FIBC
Fig. 11. Mixing operation.
material is used (e.g. filter supports, clamps). This
Flexible intermediate bulk containers are used on an ever increasing scale in the powder handling industry. Depending on the hazard situation at the location where they are used, they must meet different requirements (see
T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
Fig. 12.
261
Electrostatic ignition sources (dust collector).
following table) in order to avoid ignition hazards caused by electrostatic charging.
grounding strap. The conductivity and the necessity for grounding must
Environment bulk No explosible
Explosible dust Flammable gases
material
atmosphere
atmosphere
or vapors
MIE ⬎1 J
C
Most FIBC’s on the market today are made of polypropylene ribbon fabric. To pass the type B classification the following requirements are recommended:
*a
A
B
3 mJ⬍MIE*a⬍1 J B
B
C
MIE*a⬍3 mJ
C
C
C
a
MIE*, measured without inductance in the electrical circuit. A: No special
requirements; B: Breakdown voltage of the FIBC wall material must not exceed 4 kV in order to prevent propagating brush discharges; C: The bag
be clearly marked.
앫 any inner PE coating/liner present is not thicker than 20–30 µm 앫 and the inliner is not made of plastic. FIBC’s on the market that meet the requirement of a type C are constructed as follows:
material including the slings must be electrostatic conductive. The resistance to the ground measured at any bag location (inside and outside) must be less than 100 Mega ohm (108 ⍀). The flexible bulk bag must have a
Fig. 13.
앫 The basic fabric consists of conductive material (e.g., plastic with sufficient admixture of carbon)
FIBC requirements.
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T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
앫 The basic fabric consists of non-conductive material, but web contains interwoven threads of conductive plastic material which are interconnected. 앫 The basic fabric consists of non-conductive material, but the web contains interwoven metal threads, which are interconnected. 앫 The basic fabric consists of non-conductive material, but the FIBC has an internal conductive coating. To meet the specification as a type C bag, the following requirements are recommended: 앫 The FIBC must be clearly labeled indicating that it is conductive and that grounding is required during charging and discharging. 앫 The FIBC must have a clearly marked area for the attachment of the grounding clamps. 앫 The lifting straps must also be made of conductive material and have a leakage resistance of less than 108 ⍀ to the FIBC body (see also Fig. 13).
Fig. 14. Electrostatic discharges from an ungrounded type C bag.
Fig. 15.
Electrostatic discharge inside a grounded type C bag.
It is extremely important to keep in mind that the discharge from an ungrounded bag can occur at a single point. Such a discharge is strong enough to ignite dust clouds (see Fig. 14. Using a type C bag requires a permanent grounding of the bag during the entire time that the bag is filled or discharged (see Fig. 15). The generated charge in the product pile cannot fully dissipate to ground. Small discharges can occur along the surface of pile. These electrostatic discharges are too weak to ignite dust clouds if the volume of the bag is less than 2 m3 but strong enough to ignite solvent vapors.
References Bartknecht, W. (1993). Explosionen, Grundlagen und Anwendungen. Berlin: Springer-Verlag. Cesana, Ch., & Siwek, R. (1992). MIKE 3 minimum ignition apparatus. CH-4127 Birsfelden: Kuhner AG. Glor, M., & Maurer, B. (1993). Ignition tests with discharges from bulked polymeric granules in silos. J. Electrostatic, 30, 123–134. Glor, M., Maurer, B., & Rogers, R. (1995). Recent developments in the assessment of electrostatic hazards associated with powder handling, Proceedings of the 6th Int. Symposium “Loss Prevention and Safety Promotion in the Process Industries” Antwerp, Amsterdam: Elsevier Science Publishers, B.V. IEC 31 H (CO) 4: (1964). Methods for determining the minimum ignition temperature of dusts. Part 2: Dust cloud in a furnace at a constant temperature. IEC (International Electrotechnical Commission). (1992). Electrical apparatus for use in the presence of combustible dust, Part 2: Test methods, Section 3: Method for determining minimum ignition energy of dust/air mixtures (draft). ISSA (International Section on the Prevention of Occupational Risks) (1996). Static electricity — ignition hazards and protection measures. Germany: Heidelberg. Jaeger, N. (1996). Explosionen und emissionen, handbuch des umweltschutzes und der umweltschutztechnik, Band 3 (649–670). Berlin: Springer Verlag. Jaeger, N., & Siwek, R. (1996). Safe handling of easily ignitable dusts, 5th World Congress Of Chemical Engineering, San Diego. Jaeger, N., & Siwek, R. (1998). Determination, prevention and mitigation of potential hazards due to the handling of powders during transportation, charging, discharging and storage. Process Safety Progress, 17 (1), 74–81 Spring. Lu¨ttgens, G., & Glor, M. (1988). Understanding and controlling static electricity. Ehringen, Germany: Expert Verlag. Siwek, R., (1995a). Moderner explosionsschutz. Brasil. Siwek, R. (1995b). Determination of technical safety indices and factors influencing hazard evaluation of dusts. Journal of Loss Prevention in the Process Industries, 9 (1), 21–31, Elsevier Science Ltd. Siwek, R., & Cesana, Ch. (1995). Ignition behavior of dusts: meaning and interpretation. Process Safety Progress, 13 (5) April. Siwek, R., Cesana, Ch. (1995a). Methods for determination of the explosion characteristics according to international standards. Proceedings of the “First International Seminar on Fire and Explosion Hazard of Substances, Venting of Deflagrations”, July 17–21. Russia: Moskau. Siwek, R., Cesana, Ch. (1995b). Practical application of the minimum ignition energy and the minimum ignition temperature. Proceedings of the 2nd International Conference and Exhibition on Health, Safety and Loss Prevention in the Chemical and Process Industries, Singapore, 4–8 December.
T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
Siwek, R. (1996). Dusts: explosion protection. In 7th edition of Perry’s chemical Handbook for Chemical Engineering. USA: McGraw-Hill.
263
VDI guideline 2263: (1990). Dust Fires and Dust Explosions, part 1; Test Methods for the Determination of the Safety Characteristic of Dust. Dusseldforf: VDI-Verlag.
Safe handling of combustible powders during transportation, charging, discharging and storage夽 Tom Hoppe, Norbert Jaeger *, John Terry Safety Testing Laboratory, Ciba Specialty Chemicals, Additives Division, McIntosh, Alabama, USA
Abstract The knowledge of the ignition behavior of dust–air mixtures due to electrical sparks (MIE, Minimum Ignition Energy) and hot surfaces (MIT, Minimum Ignition Temperature) is important for risk assessments in chemical production plants. The ignition behavior determines the extent and hence the cost of preventive protection measures. This paper describes the use of the minimum ignition energy and minimum ignition temperature as very important safety indexes in practice. 2000 Elsevier Science Ltd. All rights reserved. Keywords: Minimum ignition energy; Hazard evaluation; Electrostatic discharges
1. Explosion hazards in production plants An explosion hazard can exist when flammable gases, liquids or dusts are produced, stored or processed in a plant and these materials are present as a mixture in air. An explosive mixture is present when combustible gases vapors or dusts are present in such quantities in air that an explosion occurs after an ignition (Bartknecht, 1993). An explosion requires three conditions to exist simultaneously (see Fig. 1): 앫 fuel or flammable material in sufficient quantity and effectively mixed with 앫 air and 앫 an effective ignition source Using preventive measures against explosions requires at least the reliable exclusion of one of the conditions necessary to generate an explosion as shown in Fig. 2. An explosion can thus be excluded with certainty by either
夽 Prepared for presentation at the International Symposium of Hazards, Prevention and Mitigation of Industrial Explosions, Sohaumberg, IL, September 21–25, 1998 * Corresponding author. Tel.: +1-334-436-2709; fax: +1-334-4365044. E-mail address: [email protected] (N. Jaeger).
Fig. 1.
Requirements for the occurrence of explosions (ISSA, 1996).
앫 avoiding the development of explosible mixtures (combustible dusts, flammable gases), or 앫 replacing the atmospheric oxygen with an inert gas, working in a vacuum or using inert dust, or 앫 preventing the occurrence of effective ignition sources. All three measures are summarized under the heading “preventive explosion protection”. It should be ensured that at least one of the three conditions is eliminated or so strongly reduced that an explosion is no longer poss-
0950-4230/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 5 0 - 4 2 3 0 ( 9 9 ) 0 0 0 3 6 - 4
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T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
2. Nature and origin of static electricity in production plants
An electrostatic charge by itself does not necessarily represent an ignition hazard. Such a hazard exists only when the charge is so high that discharges occur owing to the high electric field (ISSA, 1996). The individual steps, which lead to the occurrence of charge build up and discharge are always the same: Fig. 2.
Hazard triangle.
ible or at least very rare. Their appropriate elimination prevents an explosion from starting. The use of “Avoidance of Ignition Sources” as a protective measure requires a comprehensive hazard evaluation to determine all possible ignition sources that may occur during production. There are a large number of different ignition sources that one must consider in industrial operations (see Fig. 3). Not every ignition source has sufficient energy to ignite all types of explosible atmospheres. Therefore, it is necessary to investigate the ignition sources in detail in order to determine the ignition hazard in conjunction with the expected explosible mixtures. Trivial ignition sources (welding, smoking etc.) must be excluded by using organizational measures. Ignition sources, which could result from the process itself introducing energy into the product being handled (e.g. mechanical, friction energies), are not the topic of this paper but must be considered during a risk analysis. This holds particularly true for products that have a tendency to form glowing particles and, if in the course of the process, glowing particles could be formed or entrained. Electrostatic discharges are ignition sources that are often underestimated in industrial operations. These discharges occur frequently in most product handling processes and will be considered in detail in this paper.
Fig. 3.
Examples of possible ignition sources (ISSA, 1996).
Charge separation:
Separation process (usually between product and plant units) lead to charging of the surfaces in contact. Charge Charges can accumulate on products, accumulation: plant units, packaging containers and operators, etc. Charge As soon as a connection of sufficient dissipation: conductivity is established between the ground and the sites of the accumulated charge, the charge can dissipate to the ground. Discharge: If the charge continues to accumulate because the charges formed in the separation processes cannot flow to ground quickly enough, a discharge will occur when the breakdown field strength is reached.
In a powder handling production plant, static charges can occur at the surface of solids and powdery substances. At this point it is important to make a distinction between conductors of electricity (e.g. metals) and nonconductors or insulators (e.g. plastics). Electrically charged particles (electrons) can move freely on a conductor, whereas on an insulator they are fixed in one place. If a substance has an excess or a deficiency of charged particles, it is said to be charged. If two uncharged (neutral) objects, of which at least one is an insulator, are brought into close contact with each other and then rapidly separated, both of the objects become charged. This occurs because charged particles will first of all pass from one object to the other but then, during separation, cannot return fast enough. An electrostatic discharge can be incendive when the energy released is equal to or greater than the minimum ignition energy (see Section 3.1 for definition) of a mixture. The energy released depends, among other things, on the type of discharge. This in turn depends on the geometry and material of the participating surfaces as well as on certain other conditions. The following overview summarizes the ignition behavior of several types of electrostatic discharges.
T. Hoppe et al. / Journal of Loss Prevention in the Process Industries 13 (2000) 253–263
Ignition sources: Brush discharge
255
Incendivity for MIE⬍3 mJ1
Conical pile discharge
MIE⬍1 J2
Spark discharge
MIE⬍1 J
Propagating brush discharge
MIE⬍10 J Fig. 4.
Charge accumulation (ISSA, 1996).
3. Basic information Thorough knowledge of the ignition sensitivity of dust-air-mixtures towards electrostatic discharges (Minimum Ignition Energy, MIE) and hot surfaces (Minimum Autoignition Temperature, MAIT) is extremely important for assessing the hazards in dustcarrying plants. The ignition behavior essentially determines the extent, and hence the cost of the protective measures, to be used. This is especially true for the use of the protective measure “prevention of ignition sources” and also for the understanding of the ignition phenomena regarding static electricity e.g., brush discharges, bulk surface discharges, spark discharges, propagating brush discharges. Knowledge of a powder’s minimum ignition energy, particle size distribution and specific resistivity allows one to define the necessary protective measures for different operations based on the vessel size and the conductivity of the material of construction (Figs. 4 and 5).
Fig. 5.
Basic scheme of electrostatics (ISSA, 1996).
3.1. Minimum ignition energy The minimum ignition energy, MIE, of a combustible substance is the lowest value of the electrical energy stored in a capacitor, which on discharge just suffices to ignite the most readily ignitable fuel-air-mixture at atmospheric pressure and room temperature. To help
1
It is assumed that dusts are not ignitable from a brush discharge. The initial assumption that a dust cannot be ignited from a brush discharge is justified by considerable industrial experience with handling powders with low MIEs and extensive laboratory testing. 2 Estimated value from Glor and Maurer, 1993.
assure a standardized test procedure, a test apparatus known as MIKE 3 (Fig. 6) of the third generation has been specially developed by Ku¨hner AG, Switzerland and has been made commercially available (Cesana & Siwek, 1992; Siwek, 1995a,b; Siwek & Cesana, 1995). Other test apparatus are also available to determine the MIE and are included in an American Society for Testing Materials (ASTM) standard on MIE of dusts which will be issued in the near future. The MIE is usually quoted as a range: The lower value represents the highest energy at which no ignition is found in at least 10 experiments. The higher value, on
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Fig. 6. MIKE 3 apparatus for determination of the minimum ignition energy MIE of dusts.
the other hand, is the lowest energy at which the dustair mixture is just ignited:
3.3. Powder volume resistivity
no ignition⬍MIE⬍ignition
To characterize the static dissipative properties of a material, its powder volume resistivity, rR, has to be determined (see Fig. 8). It must be kept in mind that resistivity is not an absolute property of a powder and depends very strongly on moisture content and on the method used for measurement.
The method for the determination of MIE is described in the international standard of the International Electrotechnical Commission (IEC) (IEC, 1992) and in the near future will be available as an ASTM standard. The MIE is generally obtained with an inductance in the discharge circuit. However, in order to assess the incendivity of electrostatic discharges in industrial operations towards dust-air-mixtures, the MIE must also be determined without an inductance in the discharge circuit. With flammable gases and easily ignitable dusts, the influence of the inductance is generally not detectable. 3.2. Minimum autoignition temperature, MAIT The Minimum Autoignition temperature, MAIT, is defined as the lowest temperature of a heated surface at which the most readily ignitable mixture of a dust with air just ignites (Bartknecht, 1993; IEC, 1964; Jaeger, 1996; Jaeger & Siwek, 1996; Siwek & Cesana, 1995a,b; Siwek, 1996). It provides information on the ignition behavior of a dust suspension when quickly passing over a hot surface (see Fig. 7).
Fig. 7. BAM furnace.
4. Protective measures When combustible dusts are handled, avoiding an explosive atmosphere by keeping the dust concentration outside the explosive range is rarely possible due to sedimentation or whirling up of the material being handled. Thus, as a matter of principle, an explosive atmosphere can only be avoided with certainty by reducing the oxygen concentration, e.g., inerting. In practice, however, the possibilities to apply inerting are also limited. For
Fig. 8.
Powder resistivity test chamber.
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such situations, avoidance of effective ignition sources and/or explosion-proof design are the only measures available (Jaeger & Siwek, 1998; VDI, 1990). In the following, protective measures are outlined for typical powder handling operations and processes. As previously stated, the following product and plant properties are important for an accurate hazard assessment: 앫 Minimum ignition energy, MIE, of the bulk material (measured without inductance in the discharge circuit), 앫 Minimum Autoignition Temperature, MAIT, of the bulk material, 앫 Volume resistivity SR of the powder, 앫 Particle size distribution of the bulk material and mean value, M, 앫 Volume and shape of the silo or container (volume and shape of the product heap and of the dust cloud). Unless otherwise stated, the following sections are based on the assumption that the bulk materials are handled without flammable gases or vapors being present. 4.1. Filling and emptying operations In filling and emptying operations, electrostatic ignition hazards are of prime importance due to the electrostatic charging occurring during separation process (Glor, Maurer & Rogers, 1995; Lu¨ttgens & Glor, 1988). The hazard comprises possible charge accumulation not only on plant units, including drum and container, but also — in the case of insulating bulk material — on the bulk material itself which is shown in Fig. 9. Assuming the insulating bulk material carries a charge, filling represents the more hazardous of the two operation for the following reasons. In a filling operation, the bulk material undergoes dispersion (e.g. gravity feed, pneumatic transport, etc.) and can therefore be charged in the separation processes occurring in transport. The bulk material, and hence its associated charge, is then “packed” in a small space in which the charge
Fig. 9.
Filling (charging) and emptying (discharging) operations.
257
is not able to flow to ground sufficiently quick, even with a conductive and grounded receiver. This generates a high space charge density and hence a high electric field. In addition, consideration must also be given to the problems associated with heat accumulation and the possibility of entrainment of smouldering lumps. Based on the product and plant properties mentioned above, the decision tree and the matrices on the next pages provides guidance for the required safety measures. The matrices mentioned in the decision tree are shown in the following Tables for different container volumes. 1. Containers capable of discharge: The leakage resistance to the ground point from any point on the surface of the container must be less than 100 Mega Ohm. (⬍106 Ohm) 2. Charge dissipative (ⱖ108ⱕ1011 Ohm) containers having at least on one side a surface resistance ROA measured according to ASTM method D257-93 (Surface Resistivity) of less than 100 Giga Ohm at 30% rel. humidity and more than 100 Mega Ohm at 65% rel. humidity. 3. Use only conductive containers, 1 MOhm (⬍106 Ohm), or those capable of discharge (see above), and ground. 4. Fines in concentrations greater than the LEL are not to be expected for dust free forms during normal discharge or charging operations involving packaging.
4.2. Loading bulk materials into flammable solvents Whenever it is possible, bulk materials should be fed into flammable solvent under closed conditions and in an inert atmosphere (see Fig. 10). This can be realized by means of a rotary air lock, a two-valve system, a feed screw or by conveying by inert gas. If such a closed introduction system is not possible, as the next best thing, the solvent should be cooled at least 5°C below its flash point before loading the bulk material, or the bulk material introduced first followed by the flammable solvent. If the open addition of bulk material into previously loaded flammable solvent at a temperature greater or less than 5°C the flash point is unavoidable, then the following points must be considered: 앫 operator must be grounded. 앫 all aids to introduction, such as funnels, charging chutes, etc., must be made in conducting materials and grounded during the transfer. 앫 containers for solvent and powders must be of conductive material and grounded during the charging process. 앫 conductive vessels for solvents and bulk material with
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an insulating internal coating of up to 2 mm maximum may be used, if they are grounded during filling and emptying. 앫 normal loose plastic sacks or plastic liners must not be used. Plastic liners or plastic sacks with at least one-sided non-chargeable (surface resistance according to ASTM ⬍1011 ohms at 30% relative humidity) are allowed (e.g. paper sacks or paper sacks lined one side with a normal plastic coating, if the coating thickness is less than 2 mm).
Even when handling highly sensitive dusts, the occurrence of effective ignition sources in mixing is unlikely, provided the following conditions are met: 앫 In the filling and emptying of the mixer, the measures applied are the same as those in the filling and emptying of containers. 앫 In the filling and emptying of the mixer, the mixing
The formation of a solvent/air mixture must be expected, and corresponding safety measures (e.g. inerting) must be taken
START
Flammable Solvent Content > 0.5 % w/w
4.3. Mixing
Is the product stored for long periods in large volume in a silo or without special ventilation possibly after a size reduction
yes
yes
no
Flammable Solvent Content between 0 and > 0.5 % w/w
no yes The formation of solvent/air or hybrid mixtures must be expected, and corresponding safety measures inerting) must be taken
no
Particle Size greater than 0.5 mm and fines below explosive (< 10 g/m3)
yes
The presence of an explosive atmosphere is not to be expected
no
MIE of the bulk solid is greater than 1 J
Ignition due to static electricity is not to be yes
no Earth all conductive plant components, vessels and containers including those capable of
MIE of the bulk solid is less than 30 mJ
yes
Personnel must be grounded by means of conductive and flooring where explosible concentrations are
no
Additional safety measures relevant to the situation to be taken are shown in the following matrix
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259
ated in a mixer with a product fill height of less than 70 vol %, provided the combination of material values as listed in the following table are present. MIEa [mJ] ⬍1
1–3
3–10 10–30 30–100 100–300 300–1000 ⬎1000
MAIT
do not
530
500
[°C]
process
a
465
430
395
360
325
The MIE values must be determined with an additional inductance in the dis-
charge circuit.
앫 Nauta mixers with bottom support of the helical screw can heat up during operation and care must be exercised with substances capable of spontaneous decomposition.
4.4. Dust separation
elements must be off or run at a circumferential speed (tip speed) which does not exceed 1 m·s⫺1. This restriction must be assured by technical safeguards. 앫 If the mixer is closed and is filled to fill level of 70 vol. % or more, the circumferential speed of the mixing elements is no longer restricted (see Fig. 11). 앫 Any insulating coating must have a breakdown voltage of less than 4 kV. Product build-up must be checked if a homogeneous layer can be formed. 앫 Circumferential speeds up to 10 m·s⫺1 can be toler-
In the case of dust separators, especially in filters, the dust explosion hazard must not be underestimated. The probability of occurrence of a fine dust atmosphere sensitive to ignition is large. In addition, the entrainment of ignition sources (e.g. formation of smouldering lumps) and the danger of ignition through an electrostatic charging is of prime importance (see Fig. 12). Electrostatic charging must be prevented by the following measures: 앫 Grounding of all conductive apparatus parts. Particular attention must be paid to the grounding of all conductive parts which could possibly be insulated from ground if a filter cloth made of insulating
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Fig. 10.
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Powder charging into a vessel with a precharged solvent.
must be specially checked after repair and maintenance work. 앫 With a MIE⬍3 mJ or in the presence of flammable gases or vapors in the air being cleaned, electrically conducting filter materials must be used. An exception to this rule can be applied when the protective measure “inerting” is employed. Continuity of the conductivity and safe grounding must be checked. Multiple washings can have an adverse effect on the continuity of the filter material conductivity and thus require repeat checking. 앫 All inner walls on which dust can impact at high speed must not have any insulating inner coatings with a high electrical breakdown strength (breakdown voltage must be less than 4 kV; periodic checks are required). In general, with dusts with a MIE⬍10 mJ it is advisable to implement explosion protection measures which go beyond the avoidance of effective ignition sources or to consult the responsible specialist departments. It should further be noted that the fan must be installed on the clean air side of the filter and that dust deposits must be avoided in the pipe and fan housing (periodic check or install a dust control unit). 4.5. Use of flexible intermediate bulk container FIBC
Fig. 11. Mixing operation.
material is used (e.g. filter supports, clamps). This
Flexible intermediate bulk containers are used on an ever increasing scale in the powder handling industry. Depending on the hazard situation at the location where they are used, they must meet different requirements (see
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Fig. 12.
261
Electrostatic ignition sources (dust collector).
following table) in order to avoid ignition hazards caused by electrostatic charging.
grounding strap. The conductivity and the necessity for grounding must
Environment bulk No explosible
Explosible dust Flammable gases
material
atmosphere
atmosphere
or vapors
MIE ⬎1 J
C
Most FIBC’s on the market today are made of polypropylene ribbon fabric. To pass the type B classification the following requirements are recommended:
*a
A
B
3 mJ⬍MIE*a⬍1 J B
B
C
MIE*a⬍3 mJ
C
C
C
a
MIE*, measured without inductance in the electrical circuit. A: No special
requirements; B: Breakdown voltage of the FIBC wall material must not exceed 4 kV in order to prevent propagating brush discharges; C: The bag
be clearly marked.
앫 any inner PE coating/liner present is not thicker than 20–30 µm 앫 and the inliner is not made of plastic. FIBC’s on the market that meet the requirement of a type C are constructed as follows:
material including the slings must be electrostatic conductive. The resistance to the ground measured at any bag location (inside and outside) must be less than 100 Mega ohm (108 ⍀). The flexible bulk bag must have a
Fig. 13.
앫 The basic fabric consists of conductive material (e.g., plastic with sufficient admixture of carbon)
FIBC requirements.
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앫 The basic fabric consists of non-conductive material, but web contains interwoven threads of conductive plastic material which are interconnected. 앫 The basic fabric consists of non-conductive material, but the web contains interwoven metal threads, which are interconnected. 앫 The basic fabric consists of non-conductive material, but the FIBC has an internal conductive coating. To meet the specification as a type C bag, the following requirements are recommended: 앫 The FIBC must be clearly labeled indicating that it is conductive and that grounding is required during charging and discharging. 앫 The FIBC must have a clearly marked area for the attachment of the grounding clamps. 앫 The lifting straps must also be made of conductive material and have a leakage resistance of less than 108 ⍀ to the FIBC body (see also Fig. 13).
Fig. 14. Electrostatic discharges from an ungrounded type C bag.
Fig. 15.
Electrostatic discharge inside a grounded type C bag.
It is extremely important to keep in mind that the discharge from an ungrounded bag can occur at a single point. Such a discharge is strong enough to ignite dust clouds (see Fig. 14. Using a type C bag requires a permanent grounding of the bag during the entire time that the bag is filled or discharged (see Fig. 15). The generated charge in the product pile cannot fully dissipate to ground. Small discharges can occur along the surface of pile. These electrostatic discharges are too weak to ignite dust clouds if the volume of the bag is less than 2 m3 but strong enough to ignite solvent vapors.
References Bartknecht, W. (1993). Explosionen, Grundlagen und Anwendungen. Berlin: Springer-Verlag. Cesana, Ch., & Siwek, R. (1992). MIKE 3 minimum ignition apparatus. CH-4127 Birsfelden: Kuhner AG. Glor, M., & Maurer, B. (1993). Ignition tests with discharges from bulked polymeric granules in silos. J. Electrostatic, 30, 123–134. Glor, M., Maurer, B., & Rogers, R. (1995). Recent developments in the assessment of electrostatic hazards associated with powder handling, Proceedings of the 6th Int. Symposium “Loss Prevention and Safety Promotion in the Process Industries” Antwerp, Amsterdam: Elsevier Science Publishers, B.V. IEC 31 H (CO) 4: (1964). Methods for determining the minimum ignition temperature of dusts. Part 2: Dust cloud in a furnace at a constant temperature. IEC (International Electrotechnical Commission). (1992). Electrical apparatus for use in the presence of combustible dust, Part 2: Test methods, Section 3: Method for determining minimum ignition energy of dust/air mixtures (draft). ISSA (International Section on the Prevention of Occupational Risks) (1996). Static electricity — ignition hazards and protection measures. Germany: Heidelberg. Jaeger, N. (1996). Explosionen und emissionen, handbuch des umweltschutzes und der umweltschutztechnik, Band 3 (649–670). Berlin: Springer Verlag. Jaeger, N., & Siwek, R. (1996). Safe handling of easily ignitable dusts, 5th World Congress Of Chemical Engineering, San Diego. Jaeger, N., & Siwek, R. (1998). Determination, prevention and mitigation of potential hazards due to the handling of powders during transportation, charging, discharging and storage. Process Safety Progress, 17 (1), 74–81 Spring. Lu¨ttgens, G., & Glor, M. (1988). Understanding and controlling static electricity. Ehringen, Germany: Expert Verlag. Siwek, R., (1995a). Moderner explosionsschutz. Brasil. Siwek, R. (1995b). Determination of technical safety indices and factors influencing hazard evaluation of dusts. Journal of Loss Prevention in the Process Industries, 9 (1), 21–31, Elsevier Science Ltd. Siwek, R., & Cesana, Ch. (1995). Ignition behavior of dusts: meaning and interpretation. Process Safety Progress, 13 (5) April. Siwek, R., Cesana, Ch. (1995a). Methods for determination of the explosion characteristics according to international standards. Proceedings of the “First International Seminar on Fire and Explosion Hazard of Substances, Venting of Deflagrations”, July 17–21. Russia: Moskau. Siwek, R., Cesana, Ch. (1995b). Practical application of the minimum ignition energy and the minimum ignition temperature. Proceedings of the 2nd International Conference and Exhibition on Health, Safety and Loss Prevention in the Chemical and Process Industries, Singapore, 4–8 December.
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Siwek, R. (1996). Dusts: explosion protection. In 7th edition of Perry’s chemical Handbook for Chemical Engineering. USA: McGraw-Hill.
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VDI guideline 2263: (1990). Dust Fires and Dust Explosions, part 1; Test Methods for the Determination of the Safety Characteristic of Dust. Dusseldforf: VDI-Verlag.