Article pubs.acs.org/IECR
Analysis of an Explosion in a Wool-Processing Plant Piero Salatino,* Almerinda Di Benedetto, Riccardo Chirone, Ernesto Salzano, and Roberto Sanchirico Dipartimento di Ingegneria Chimica, Università degli Studi di Napoli Federico II and Istituto di Ricerche sulla Combustione − CNR, P.le Tecchio 80, 80125 Napoli, Italy ABSTRACT: A major accident occurred in an Italian wool factory in 2001, culminating with a severe explosion, despite that wool is recognized as the most flame-resistant among the natural textile fibers. The analysis of this exceptional event suggests that, in addition to classical explosion parameters, three key phenomena related to the process jointly contributed to trigger the otherwise unexpected combustion of wool flock suspensions. The first and more important phenomenon is represented by the segregation of dust mixtures occurring during processing of textile fibers and storage of byproduct. Segregation may isolate and concentrate the lighter component of wool processing byproduct as a flammable dust. The main conclusion of our analysis is that, when performing risk assessment, sampling of all materials is a necessary step, since flammability and explosivity of raw materials may not be representative of the safety of the whole process. The second phenomenon is the enhancement of the combustion of the flammable dust layered on nets as they are subjected to cross-flow of air. The enhancement may be such as to promote transition from smoldering to flaming combustion of the dust layer. The third phenomenon is related to the interaction among the flame, the induced turbulence, dust dispersion into clouds and the layout of the plant. The combination of these phenomena promoted a deflagration of unexpected severity. In this paper, the dynamics of the explosion is analyzed in the light of the occurrence of the above cited phenomena. Purposely designed experimental tests have been performed to support the key role of segregation, formation, and ignition of the flammable cloud. Results clarify that real-world dust explosion accidents may be more severe than it could be anticipated on the basis of standard laboratory tests. A procedure for risk analysis is given to predict explosions of flocking materials.
1. INTRODUCTION Dust explosions are a serious source of hazard in many process industries. They may start in mills, dryers, mixers, classifiers, conveyors, and storage silos. Their violence strongly depends on chemical composition, dust concentration, dust size, and turbulence level.1−5 The first documented dust explosion occurred in a flour warehouse in Turin in 1785.2 Since then, the scientific and technical literature has produced a significant advancement in the understanding of dust explosion phenomena that has ultimately resulted in the establishment of internationally recognized standard guidelines for preventing accidents and for mitigating the effects of dust explosions as ATEX Directives 94/9/EC and 99/92/EC, NFPA 68, EN14994, EN14991, and BGIA-GESTIS.6 The present study addresses a severe explosion that occurred in 2001 in a wool factory located in the North of Italy. The explosion resulted in three casualties, the injury of five, as well as considerable damage to the whole factory. Details on the accident, including the layout of the plant and the sequence of events, are reported by Piccinini7 and Salatino et al.8 Only two examples of major accidents in textile plants had been reported in the literature, both not involving wool but rather linen and cotton.4,9,10 Moreover, data on the explosivity of artificial or natural fibers are quite limited, despite that the textile industry is among the most diffused worldwide.11 Wool is considered as a flame-resistant material and, as such, is adopted for fire fighters, because its ignition temperature is relatively high (∼ 570 °C). Furthermore, once ignited, wool fibers display charring phenomena and behave as flame retardant.12 Accordingly, wool dust explosions and even fires © 2012 American Chemical Society
are usually considered unlikely to occur and frequently neglected in industrial risk assessments. The occurrence of a severe explosion in the wool factory in Italy gave rise to a large public debate on its cause and on its unexpectedly severe consequences. As members of the public enquiry, we have focused our attention on the identification of additional key phenomena which have increased the likelihood and severity of the explosion of materials involved in wool processing.
2. BRIEF RECONSTRUCTION OF THE EVENT AND RATIONALE OF THE EXPERIMENTAL TESTING PROTOCOL The explosion occurred in an old multistory industrial building where different operations, starting from the storage of raw wool packages to the last pure wool combing operations, were conducted. The accident started when smoke was observed coming from the ground floor, where large wood boxes (cells) were installed for collecting wool processing byproduct, namely, noils and burr. Wool noils are short fibers removed in the processing of textile fibers, and burr consists of lumps or flakes of wool together with the vegetable fragments present on the sheep’s fleece. Special Issue: Russo Issue Received: Revised: Accepted: Published: 7713
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explosive, even if in the lowest class (St1). The Department of Labour of New Zealand classifies wool as explosive, ranking its severity from “weak to strong”.13 However, these data are not complemented by information about the chemical composition of the analyzed wool. We then performed ad-hoc tests to evaluate the chemical and physical properties and the flammability and explosivity of the dusts involved in the process. In the following, the experimental apparatus and procedures are described. 3.1. Physico/Chemical Characterization. Samples A and B have been characterized to assess their chemical composition. Carbon, Hydrogen, and Nitrogen were determined using a LECO CHN 2000 analyzer. Sulfur was determined by LECO CS144. Moreover, thermo-gravimetric analysis of the samples was performed by LECO TG701. Determination of high/low calorific values was accomplished using a Mahler bomb calorimeter PARR 6200, following ASTM D5865. The morphology of the samples was investigated using a Scanning Electronic Microscope (SEM) (Philips XL30). Samples were loaded on an adhesive carbon stub, treated by gold, and scanned with a constant filament tension of 10 kV. 3.2. Flammability and Explosivity of Dust Clouds. Explosivity of suspended dust was characterized by tests performed in the standard 20 lt spherical apparatus manufactured by Adolf Kühner AG (CH) according to the standard procedure ASTM E681. The equipment is made of a stainless steel spherical bomb surrounded by a water jacket for the control of the internal wall temperature. The explosivity tests were performed according to the standard procedure (ASTM E1226). All tests were carried out with an ignition delay time, tv, equal to 60 ms. The dust-air concentration was varied within the range 125−1000 g/m3, and for each value of dust concentration, the explosion test was repeated three times. The standard deviation and the accuracy of our data are Pmax ≤ ±5% and KSt ≤ ±15%. 3.3. Combustibility of Layered Dust. To characterize the ease of ignition and flammability of layered samples, two different types of combustibility tests were performed. Standard Combustibility Tests on Layered Dust. Combustibility tests on layered dust samples were carried out following the standard procedure proposed by Bartneckht.1 The dried sample is deposited on a ceramic plate and heated by an electrically heated platinum wire which is dipped at the end of the sample for about 5 s. The propagation of a smoldering combustion front and the possible transition to flaming combustion are recorded. Combustibility Tests on Aerated Dust Layers. Considering the peculiar conditions to which sample A was exposed in the cell, discussed in section 2, additional tests were carried out according to a purposely designed, nonstandard, procedure. These tests aimed at characterizing the propagation of smoldering combustion fronts and the possible transition to flaming combustion of dust layers, while the layer was exposed to a cross-flow of air. This experimental procedure reproduced better than the standard procedure the actual conditions that are established in the cells during the loading stage. The combustibility tests under aerated conditions were carried out by means of the apparatus shown in Figure 2. It consisted of a near-cylindrical Pyrex glass container (height = 8 cm, diameter =10 cm), equipped with a sintered pyrex gas distributor and a wind-box at the bottom. The apparatus was equipped with a gas feeding line and a flowmeter/controller, so
While people were checking for the origin of the smoke, an explosion started in one of the cells under the carding machine. It propagated along the ground floor, involving several burr collectors, and the dust accumulated in the neighboring area. Finally, the explosion propagated from the ground to the first floor, through the openings in the ceiling. Some carding machines were heavily damaged. The light roof of building and several structural components of the building were severely damaged.7 According to the most credited reconstructions of the event, the explosion was triggered by combustion of an accumulated dust layer and eventually developed as a deflagration of airborne fine combustible dust. Figure 1A reports a picture of a typical cell for noils and burr storage, where it is likely that the explosion originated.
Figure 1. (left) Burr cell where the explosion was triggered (highlighting the positions where samples have been collected). (right) Schematic representation of the segregation phenomena occurring in the burr cell upon loading via the pneumatic transport line.
Charging of the cells took place periodically by pneumatic conveying of the byproduct as these were removed from the heavy carding machines located on the first floor. Figure 1 (right) outlines the segregation pattern that establishes itself in the cell as the broadly polydisperse mixture of wool processing byproduct is charged into the cells. The burr and noils enter the cell by means of a pneumatic conveying line and eventually undergo a physical process of gravity separation: the lighter/finer materials (burr) disperse into the atmosphere and mainly deposit on the venting nets installed on the side of the cell (Sample A); the heavier/coarser material (noils) falls onto the bottom of the cell (Sample B) where it accumulates. Collection of samples for physico/chemical characterization and selection of the relevant tests were made taking into account this segregation pattern. As shown in Figure 1 (left), sample A was collected from the venting nets, sample B was collected from the floor. When planning the experimental campaign for the characterization of the combustibility of the samples, the following circumstance was taken into consideration: dust accumulated on the nets (sample A) was exposed to a flow of air across the layer while the cell was in the loading stage. Accordingly, in addition to standard dust flammability tests, a series of purposely designed (nonstandard) tests were carried out to assess the combustibility of the dust layer under these peculiar cross-flow conditions.
3. EXPERIMENTAL APPARATUS AND PROCEDURES Published data on explosivity of wool fibers are rather limited. The BGIA-GESTIS database6 reports that wool powders are 7714
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Sample B, which is instead present to a much lesser extent in Sample A.
Figure 2. Combustibility tests of aerated dust layers: the experimental device.
Figure 3. SEM images of burr (Sample A).
that a controlled cross-flow of preset gas superficial velocity could be established across the layer, which was evenly distributed above the sintered plate. Ignition of the layer was accomplished by a flame generated by a lighter for 10 s at the center of the layer. Propagation of smoldering combustion fronts and transition to flaming conditions were recorded by means of a digital video recording system, and eventually analyzed.
4. EXPERIMENTAL RESULTS 4.1. Physico/Chemical Characterization of Samples. Table 1 reports the results of the proximate and elemental Table 1. Chemical Composition and Physical Characterization of the Two Samples sample A Proximate Analysis humidity, % 7.1 volatiles, % 45.3 ash, % 40.4 carbon, % 7.2 upper calorific value, kJ/kg 13,424 lower calorific value, kJ/kg 12,393 Elemental Analysis (Dry Sample) carbon, % 32.0 hydrogen, % 4.3 nitrogen, % 4.2 sulfur, % 0.7 ash, % 43.5 oxygen, % 9.8
sample B 12.6 14.1 4.6 68.7 18,597 17,078
Figure 4. SEM images of noils (Sample B).
4.2. Morphological Characterization of Samples. Figures 3 and 4 report selected SEM micrographs of Sample A and Sample B, respectively. Sample A exhibits a dusty appearance with occasional presence of fibers. Particles are very fine, with sizes ranging from hundreds of μm to a few μm. Sample B is instead characterized by the dominance of the fiber component. Fibers are largely agglomerated into coarse flocks. Altogether, results of the morphological analysis confirm the conclusions drawn after the chemical analysis: Sample A presents a modest content of wool fibers, and a prevailing contribution of inert (ash) and combustible vegetable dust; Sample B essentially consists of wool fibers, contaminated by modest contribution of the other components. The large difference between the properties of the two samples confirms both the significance of the segregation phenomena occurring in the cell, discussed in section 2, and its relevance to the dynamics of the accident. 4.3. Flammability and Explosivity of Dust Clouds. Figure 5 shows the pressure-time histories for Sample A as obtained for different dust concentrations in air. From the
48.6 6.5 15.1 2.8 5.3 21.5
analysis of samples A and B. The calorific values are also reported. Sample A has higher volatile content than sample B. It has a lower calorific value than Sample B, which is largely related to the large ash content. The sulfur and nitrogen contents of the two samples are remarkable: higher values of these parameters are found in Sample B (even on an ash-free basis). Both these parameters may be considered, in the present context, fingerprints of the presence of wool fibers. Accordingly, wool would be the dominant component of 7715
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Figure 5. Pressure vs time for the sample A (burr) by varying dust concentration in air, as obtained in the 20 L bomb.
pressure histories, the maximum pressure (Pmax) and the deflagration index (KSt) are calculated, following the standard procedure (ASTM E1226). The maximum pressure and deflagration index were attained at about C = 500 g/m3 and are equal to 7 ± 0.35 bar and 60 ± 9 bar m s−1, respectively. Sample A may be then classified as St1 dust (KSt ≤ 200 bar m s−1). The minimum explosive dust concentration was found at 60 g/m3. Despite the higher carbon content (proximate analysis, Table 1) and the higher content of C and H (elemental analysis, Table 1), sample B is not explosive. This result is strictly related to physico/chemical properties of Sample A and Sample B. Indeed, it has been widely shown that both the dust particle size1,2,14 and the volatile matter content1,2,15 play a major role in affecting the explosion behavior of dusts. In our analysis, Sample A resulted to be finer (sizes ranging from hundreds of μm to a few μm) than Sample B, which mainly consists of fibers largely agglomerated into coarse flocks. Furthermore, we found that Sample A has a higher volatile matter content (45.3%) with respect to Sample B (14.1%). 4.4. Combustibility of Layered Dust. Standard Combustibility Tests on Layered Dust. The standard combustibility test performed on layered Sample A shows modest propensity for combustibility. We observed propagation of combustion fronts without flame (smoldering regime). The observed course of the reaction suggests that Sample A belongs to the combustion class 4, according to the classification of Bartknecht.1 In this regime, we observed that the stability of the combustion fronts increases as the apparent density decreases. This is in agreement with the results by Kuwana et al.16 Standard combustibility tests on Sample B showed that noils have no propensity to undergo smoldering combustion. Combustibility Tests on Aerated Dust Layers. Figure 6 shows selected snapshots from videorecordings of the time sequence of layered Sample A during the combustibility tests under cross-flow conditions. The gas cross-flow velocity (7 cm/ s) equals the average gas velocity establishing across the nets of the cells under the actual operating conditions of the pneumatic transport line. After ignition (Figure 6A), the spreading of a glowing fire with the production of smoke is observed, suggesting the propagation of a smoldering combustion front (Figures 6B−C).
Figure 6. Fire test for wool collected on the net of burr collector performed by modified version of Bartknecht test for combustibility,1 to allow air flux through sample. Air velocity = 7 cm/s.
Afterward, the production of flames with an increase of the speed of combustion front propagation and a reduction of smoke are observed, suggesting the transition to the flaming regime (Figures 6 D-F).
5. DISCUSSION From the analysis of the reconstruction of the event and the experimental tests performed, it turns out that the unexpected combustion of wool flock suspensions occurred through the sequence of three steps: (i) formation of the flammable dust cloud, (ii) ignition of the dust cloud; (iii) severe flame propagation through the plant. These steps have been triggered by the interaction among three key phenomena: the segregation of the dust materials; the enhancement of the combustion of the flammable dust layered on nets as they are subjected to cross-flow of air; the interaction among the flame, the induced turbulence, and the layout of the plant. In the following, we discuss these three phenomena in the light of the experimental results obtained. 5.1. Segregation: the Formation of an Explosive Dust Cloud. The wool is transformed into the finished textile good producing a waste of the carding process which is a blend of residues of wool, vegetable and organic component, and inert component, hence highly heterogeneous as regards both physico/chemical and granulometric properties. Such material resulted as not being able to produce a flammable cloud. However, as described above, during the pneumatic conveying, this material undergoes a physical process of gravity separation: the lighter/finer material (burr) disperses into the atmosphere and mainly deposits on the venting nets installed on the side of the cell (Sample A); the heavier/coarser material (noils) falls onto the bottom of the cell (Sample B) where it accumulates. According to our experimental results, these samples exhibit completely different physico/chemical and explosivity features. Sample A is very fine, while Sample B is dominated by the 7716
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presence of fibers. Also, Sample A has higher volatile content than Sample B. These differences have a significant impact on the explosivity of the two samples: Sample B is not explosive, while Sample A is explosive, belonging to class St1. The qualitative segregation pattern establishing during the discharge of the waste material has then the crucial role of isolating and concentrating the potentially explosive component of the raw material, the burr, which formed a flammable cloud. As a consequence, in the framework of risk analysis, sampling and characterization of all byproducts is essential: raw materials alone are not fully representative both for design of prevention and mitigation measures. 5.2. Transition from Smoldering to Flaming Regime: The Ignition Source. During the accident, the intense smoke coming from the burr collector was observed few minutes before the severe explosion had occurred. This smoke was attributed to the occurrence of dust smoldering possibly activated by the old electric system installed in the plant. Dust smoldering, however, is not a sufficient ignition source for a dust-air mixture cloud. Gummer and Lunn17 studied the ability of burning nests of dusts as ignition source for dust clouds involved in industrial explosions. They concluded that smoldering nests of dusts are poor ignition sources for most dust clouds. On the other hand, flaming nests are definitely able to ignite clouds of dusts. Our experimental results showed that the Sample A, which is layered on the cell net, is prone to undergo smoldering combustion and also transition to the flaming regime when subjected to an air cross-flow. The dust layer deposited on the cell net is subjected to an air flow crossing the net during the charging phase. The consequent transition from smoldering regime to flaming regime provides the route for ignition of the flammable cloud formed in the cell. 5.3. Interaction between Flame and Vents: Enhancement of Explosion Severity. Following the first severe explosion, the flame traveled along the burr collectors and was sustained by the combustion of other dusts and burrs which were in bags and in the large bag filters installed around the plant. Furthermore, traveling to the upper floor through openings underlying the carding machines, the flame found favorable conditions for secondary explosions resulting in huge damages to personnel and structures. From the explosion tests, it turns out that the burr dust (Sample A) presents a moderate severity (class St1, KSt ∼ 60 bar/m s), a feature being apparently in contrast with the very severe consequences of the secondary explosion. However, the amount of initially unburnt dust which is pushed out of the vent opening, ahead of the flame front, is much larger for dusts belonging to the dust explosion class St1 than for dusts belonging to class St2.1,2 Therefore, it has been reported that the secondary explosion initiated in the free atmosphere by the departing flame may be much more severe for St1 dusts than for St2 dusts. Eventually, it is conceivable that the pressure produced by the secondary explosion obstructed the venting process and raised the level of the reduced venting pressure. Figure 7 reports a block diagram which summarizes the most relevant step of our analysis. In particular, the role of separation and segregation phenomena is highlighted, as they may transform materials which are not inherently flammable (the raw material, selected byproducts of wool processing) into flammable ones. Results show that, in the framework of risk
Figure 7. Flow-sheet for explosion of not flammable raw materials Experimental tests for the analysis of hazards are included.
analysis, sampling and characterization of all the materials involved in the process is a crucial step. Properties of raw materials only may not be sufficient for the setup of appropriate risk prevention and mitigation measures. The flammable material produced has to be analyzed in close relation to actual process conditions to assess the probability that smoldering combustion is initiated and sustained and the possible transition to flaming fire and explosion. Finally, the layout of the plant has to be carefully analyzed to assess the possible interaction among the venting sections and the flow fields associated with the primary explosion. In this respect, compartmentation may turn out to be an effective measure18 to prevent uncontrolled spreading of fire and explosion and to limit the associated damages.
6. CONCLUSIONS The accident of unexpected severity occurred in an Italian wool factory in 2001 was the result of the interplay between the inherent properties of the raw materials and the transformations of such materials and byproducts associated with wool processing. In particular, separation and segregation of powdered materials may change inherently nonflammable materials into flammable ones. Moreover, specific process conditions, such as the establishment of air cross-flow across a layered bed of poorly combustible dust, may significantly enhance its combustibility to the point that the overall scenario of the accident is dramatically affected. The lessons learned are that, in the framework of risk analysis, sampling of all the materials present in the process is a crucial step. Characterization of the combustibility of raw materials only may not be sufficient for the setup of appropriate and effective risk prevention and mitigation measures. Combustibility of airborne and layered dusts must be assessed under actual processing conditions, which may significantly differ from those holding during standard combustibility tests. Finally, the layout of the plant has to be carefully analyzed, as the interaction between primary explosion and the venting sections may lead to flame acceleration and severe secondary explosion.
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. 7717
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Notes
The authors declare no competing financial interest.
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REFERENCES
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