Quantitative assessment in safety reports of the consequences from the detonation of solid explosives

Journal of Loss Prevention in the Process Industries 26 (2013) 974e981

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Quantitative assessment in safety reports of the consequences from the detonation of solid explosives Davide Manca* PSE-Lab, Process Systems Engineering Laboratory, Dipartimento di Chimica, Materiali e Ingegneria Chimica “Giulio Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 July 2013 Received in revised form 13 October 2013 Accepted 14 October 2013

Safety reports are mandatory documents in member states of European Union whenever any threshold limits of amounts of either stored or processed hazardous substances are exceeded. After a short introduction to EU Seveso Directives on major-accident hazards involving dangerous substances and to the transposition and implementation by member states, with a brief comment on last 2012/18/EU Directive (also known as Seveso III directive), the paper focuses on drafting of safety reports for industrial activities involving solid explosives. Specifically, the quantitative assessment of consequences from detonation is tackled respect to the side-on overpressure and the debris production. Both direct and inverse problems are illustrated to determine respectively the overpressure value at a given distance, and the explosive amount that allows respecting the regulations. Their solution is based on either analytic or numerical techniques and being based on recent scientific publications on the matter either evaluates or zeroes nonlinear algebraic equations. The availability of these equations avoids grounding the consequences assessment on diagrams and nomograms that otherwise would lead to interpretation and usage errors besides avoiding the automatic solution of the inverse problem. The paper focuses also on details such as embankment, crater, munitions, rocket propellant, building structure, and wall material that, at different levels, play a role in the assessment of detonation consequences. A discussion on debris formation, the available literature, and the evaluation of the impact probability of fragments on both fixed and moving targets closes the paper. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Safety report Solid explosive Detonation Overpressure Debris 2012/18/EU Directive

1. Introduction Since 1982, European Community has been active in delivering Directives on the safety of industrial activities. The so-called “Seveso Directives” focus on the control of major-accident hazards involving dangerous substances. The accident occurred in Seveso (Italy) in 1976 inspired the first directive, 82/501/EEC, six years later. Since then, the Bhopal, Schweizerhalle, Enschede, Toulouse, and Buncefield accidents motivated the successive replacement and amendment of previous Directives, as reported in last 2012/18/ EU Directive, commonly known as “Seveso III”. EU member states have to transpose and implement this Directive by June 1st, 2015.

Abbreviations: AP, ammonium perchlorate; AGM, above ground magazine; APCP, ammonium perchlorate composite propellant; DoD, US Department of Defense; ECM, earth covered magazines; EU, European Union; IBD, inhabited building distance; PETN, pentaerythritol tetranitrate; PTRD, public traffic route distance; TNT, trinitrotoluene; USA, United States of America. * Corresponding author. Tel.: þ39 02 23993271; fax: þ39 02 70638173. E-mail address: [email protected]. 0950-4230/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jlp.2013.10.010

Seveso III directive does not change significantly the contents of Seveso II directive (i.e. 96/82/EC in turn amended by directive 2003/ 105/EC) that was instrumental in reducing the likelihood of consequences of industrial accidents. As a matter of facts, the most important innovation of Seveso III directive is to increase the involvement of the population, potentially exposed to industrial accidents, through an informed participation to the decision processes about public safety and land-use planning. It is worth observing that most of the accidents that inspired and motivated the delivery of either new or amended Seveso directives were explosions. Two, i.e. Enschede and Toulouse, implied the detonation of solid explosives, whilst a series of deflagrations, originated by the ignition of a hydrocarbon unconfined vapor cloud, characterized the Buncefield accident. After the Toulouse detonation in 2001, the 2003/105/EC directive introduced an explicit reference to risks arising from storage and processing activities in mining, pyrotechnic, production, and manipulation of explosive substances. According to Seveso directives, the industrial activities that store and/or process amounts of explosive substances above a given

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threshold must compile a safety report. Seveso III directive requires that “.to prevent major accidents, and to prepare emergency plans and response measures, the operator should, in the case of establishments where dangerous substances are present in significant quantities, provide the competent authority with information in the form of a safety report. That safety report should contain details of the establishment, the dangerous substances present, the installation or storage facilities, possible major-accident scenarios and risk analysis.”. Hence, a safety report must quantify also the consequences of the so-called top events (together with their identification, description, and probability assessment). This paper focuses on the definition of the methodology to identify and quantify the consequences of possible top events in industrial processes involving solid explosives. The consequences depend also, but not only, on the typology and amount of solid explosive taking part to the detonation. The paper introduces both analytic and numerical methods to determine the consequences and to define the admissible amount of solid explosives that can be stored/processed. This is done according to the safety thresholds of EU member-states laws, and in compliance with the surroundings in terms of population distribution, and land-use planning. 1.1. Decomposition of the detonation event A top event involving solid explosives starts with a source of ignition, either mechanical or thermal, that triggers the detonation. The detonation takes to the following physical and chemical phenomena:

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inverse problem to determine the maximum allowed quantity of solid explosive to appear also in the safety report. That quantity depends on a number of elements that entail both the industrial and the environmental features. Usually, the direct problem has an analytic solution whereas the inverse problem requires the solution of an implicit equation often performed by a numerical algorithm. 2. Quantification of the consequences This Section focuses on the methodologies to quantify the consequences of detonation from solid explosives in the optics of drawing up a safety report. The proposed methods will tackle the solution of both direct and inverse problems as defined in Section 1.1. Most of the formulas used in the proposed methods are drawn from the scientific literature. The paper contribute consists in putting together, analyzing, and commenting the available alternatives. The paper discusses also the limitations and weaknesses, outlining the missing issues. Different investigation fields contribute to the whole vision, which is devoted to understanding and quantifying the effects of possible top events. For the sake of space and with reference to Section 1.1, the paper focuses only on the first two phenomena produced by the detonation of solid explosives, i.e. the side-on overpressure and the debris formation. A forthcoming paper will finalize the analysis of the remaining three phenomena, i.e. combustion of the degradation products from ignition, radiation from the emitted hot gases, and seismic wave produced by the detonation. 2.1. Overpressure

    

overpressure (i.e. shock wave, air blast); primary and secondary fragmentation (i.e. debris formation); combustion of the degradation products; thermal radiation; seismic wave.

According to 2012/18/EU Directive, a safety report must analyze and quantify the outcomes of a detonation so to define an internal emergency plan and allow drafting an external emergency plan by the competent public authority. In addition, the competent public authority analyzes and eventually validates/rejects the safety report to authorize/forbid the establishment to operate. The above reported list of phenomena must be considered in its totality since there is not a priori any hierarchy of effects produced by the detonation of solid explosives. In case of detonation, all those phenomena occur and may have different impacts on surrounding equipment and people (i.e. operators and population). Only a quantitative assessment of the consequences allows identifying the phenomenon that, according to the specific event and the surrounding features, produces the most relevant effects. In general, carrying out a safety report requires the solution of two distinct problems. The former is the “direct problem”, i.e. determine the detonation consequences when the amount of explosive is known. The latter is the “inverse problem”, i.e. determine the maximum amount of explosive that, when detonated, allows respecting the threshold limits prescribed by law. The inverse problem identifies the critical quantity of solid explosive that can be stored/processed to preserve the exposed population, equipment, and environment from severe consequences. The transposition and implementation of EU directives by member states define and in case update the safety thresholds. Consequently, a safety report must respect the safety thresholds of the specific member state where the industrial activity is run. When the solution of the direct problem shows that either the stored or the processed quantity of a hazardous substance produces damages higher than the law thresholds, it is necessary to solve the

The release of energy from the detonation of solid explosives produces a shock wave whose velocity in the very first moments reaches a few thousands of meters per second as a function of the explosive nature (Baker, Cox, Westine, Kulesz, & Strehlow, 1983; Lees, 2012). The sudden volume expansion of the degradation elements, which eventually burn to oxidized products, is due to the solid-gas phase transition and to the increase of both the number of moles and temperature (both the degradation and combustion reactions are highly exothermic). Kuhl et al. (Kuhl, Bell, Beckner, & Reichenbach, 2011; Kuhl et al., 1998) showed that the pressure blast takes a few milliseconds to deploy inside a pressure-proof chamber. They measured a significantly higher pressure when the explosive detonates in presence of air (oxygen plays the role of comburent), whereas the pressure increase is lower (roughly 1/3 for a 0.8 kg detonation of TNT in a 17 m3 chamber) when the surrounding atmosphere is inert, e.g., nitrogen. In case of TNT, the combustion process delivers almost twice the energy released by the degradation process. This energy ratio depends on the typology of explosive and therefore on the nature of the degradation products. In presence of air, which mixes turbulently with the degradation products, the temperature reached by the detonation of either TNT or PETN is 2900e3200 K. These high-energy releases explain the sudden increase of pressure and the shock wave that propagate from the epicenter of the explosion. To quantify the peak overpressure and the impulse of the shock wave, the scientific literature produced over last decades a series of diagrams and nomograms that correlate these values to the so-called scaled distance, which accounts for the TNT equivalent mass involved in the detonation (Baker et al., 1983; Casal, 2008; Lees, 2012). These diagrams had some significant limitations, often being logarithmic on both axes. They required the user to identify by hand the input values on some parametric curves and guess the output result. The reading and interpretation errors could be significant. Even the same user would read repeatedly different output values from a constant input one. Finally, these diagrams

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allowed solving only the direct problem. In case of inverse problem, the user had to find the solution manually with pencil and ruler by applying a trial and error approach. Rather recently, Alonso et al. (Alonso et al., 2006, 2007, 2008) overturned the situation by introducing two equations that evaluate analytically the side-on overpressure and the impulse. These equations are a regression of the experimental data that the literature reports as scaled values in the abovementioned diagrams and nomograms. The availability of explicit equations allows solving the direct problem analytically (see also Eq. (1)). Hence, the reproducibility and consistency of the output values are assured and any manual approximation and parallax (reading) error are avoided. As far as the inverse problem is concerned, the available nonlinear algebraic equations can be solved numerically respect to the TNT equivalent mass (see also Eq. (2)) by setting the overpressure threshold (the same applies to the impulse threshold) and assigning the safety distance (for instance from the nearest exposed settlement). Alonso et al. (2006) propose the following equation for the side-on overpressure:

Ps ¼ aðz0 Þ

b

with

z0 ¼ d=WTNT 1=3

(1)

The inverse problem becomes:

a

d 1=3

WTNT

!b  Ps ¼ 0

would be so small to violate significantly that lower bound. The extrapolated value would therefore be inconsistent and unreliable. 2.1.1. Further details and specific issues This subsection focuses on further issues that play a role on the formation and range of the blast wave. These issues are either quite peculiar or rather common. Nevertheless, a safety report must tackle such points to quantify the derived consequences and determine eventually the safety thresholds in terms of distances and stored/processed quantities. 2.1.1.1. Embankments. Alonso’s equations regress experimental data from hemispheric explosive detonations at ground level and in the open (Alonso et al., 2006). Such equations do not take into account the damping effect produced by buildings and embankments since they refer to experimental data measured for detonations of solid explosives leant on the soil of an open environment. Embankments are quite common in storage magazines and processing units since in case of detonation they allow:  absorbing a fraction of the blast wave and partially deviating it vertically;  blocking a fraction of both the primary and secondary fragments;  avoiding domino effects on surrounding buildings.

(2)

where the over-bars show the values assigned by the user. Formby and Wharton (1996) and Wharton, Formby, and Merrifield (2000) report a detailed treatment of the TNT equivalent mass, WTNT. Usually, the safety report focuses only on the overpressure value since most regulations of member states do not take into account the impulse contribute. In case of explosion (either detonation or deflagration), Italian laws (i.e. D.M. 9-May-2001, D.Lgs. 238/2005) prescribe safety thresholds only for overpressure. Experts agree that both the duration and the profile of dynamic overpressure on the structures play a role on their blast resistance. Therefore, the impulse should not be neglected in safety reports. This is matter for future amendments and improvements of the member states legislations. Back to Alonso’s equations, they are valid within a given range of the scaled distance. Theoretically, it is not suitable to extrapolate the response of such equations beyond those limits. For very short scaled-distances, real experiments cannot measure the real overpressures since the measuring devices (e.g., piezometers) would be destroyed immediately. For very long scaled-distances, the overpressure is so small that either it is not measurable anymore or it is negligible (a good point for the safety report). However, the apparently critical point represented by very short scaled-distance values is a non-problem as far as safety distances are concerned. This is due to very high values of overpressure at the lower bound of the scaled distance. So high overpressures are well above the law thresholds and in real applications correspond to quite short distances from the epicenter. These distances are plentifully inside the battery limits of the plant. On the contrary, law thresholds lead usually to much-longer safety distances and preserve the external structures and people from the detonation consequences. In case of adoption of Alonso’s equations to design alternative solutions, the user is advised not to infringe the corresponding lower limit of scaled distance. For instance, it would be wrong to evaluate the side-on overpressure on the walls of an explosive magazine for structural dimensioning purposes. Given the amount of explosive stored in the magazine and the small distance between the explosive and the surrounding walls, the reduced distance

In addition, a fraction of the detonation energy is devoted to the destruction of the building walls, which host the explosive, and the formation of the ground crater. The amount of energy absorbed by walls may vary significantly according to their structure and material. Murtha (1998) discusses the importance of walls structure (in segregating the explosive loads and avoiding the sympathetic detonation) and material (in avoiding the formation of big debris). Murtha shows how a proper design of the wall thickness and the selection of lightweight structural concrete, to absorb a fraction of the blast wave (by crushing into small fragments), allow reducing significantly the maximum credible event. Murtha discusses also the contribution of soft matter (such as water and sand in support to concrete) to absorb and disperse further the blast wave. It is then possible to determine the damping coefficient of the overpressure when one implements a well-designed storage/processing building and a protective embankment. The damping coefficient is evaluated by comparing the overpressure value predicted by Eq. (1) in the open air with the experimental values measured at several distances from the epicenter of the detonation of the storage magazine surrounded by the embankment. Specifically, Murtha (1998) reports the details about the detonation of 27216 kg of TNT in storage magazine and the overpressure values measured at a few distances. Eq. (3) defines the formula of the damping coefficient as the average value of the ratios between the real mass of TNT detonated in Murtha’s experiment and the mass inferred from Eq. (2):

PNM

xw;e ¼

W exp i ¼ 1 W calc i

NM

(3)

Eq. (3) takes to xw,e ¼ 1.36 when Murtha’s experimental values are used. In other words, Eq. (3) determines the corrective coefficient to evaluate the TNT equivalent mass that in the open would produce the same overpressure measured in the field by the piezometers (the arithmetic mean is called for by the experimental data that are intrinsically affected by error). The damping coefficient is greater than one since in the open a smaller amount of explosive produces the same consequences as the detonation of a

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larger amount of explosive stored inside a building surrounded by an embankment. In case of safety reports, which have to evaluate the consequences from the detonation of explosives stored/processed inside buildings similar to Murtha’s magazine and with surrounding embankments, the effective mass of explosive to be used in Eq. (1) is then:

Wair

blast

¼

Wdeton

xw;e

(4)

A conservative value of the damping factor to be used in safety reports can be rounded to x ¼ 1.3, which translates the rough concept that 30% of the explosion energy is absorbed by the disruption of deposit walls and embankment deformation, whereas 70% produces the air blast. As far as below-ground-level storage/processing buildings are concerned, their buried layout dampens the overpressure discharged to the environment. At the same time, the layout of these buildings suggests adopting soft roofs to control and give vent to the pressure growth. Hence, a conservative approach to safety reports suggests considering below-ground-level buildings as ground magazines surrounded by embankments. 2.1.1.2. Crater. Cooper (1996) focuses the attention on the detonation energy dispersed in the crater formation. This energy does not contribute to the blast wave. Lees (2012) reports also some formula to determine the diameter and the depth of the crater as a function of the explosive mass. Nonetheless, it is worth observing that the experiments run to quantify the explosion features reported the total mass of detonated explosive (TNT equivalent) and measured the overpressure dynamics. Consequently, the TNT equivalent mass used in Eqs. (1) and (2) should not be corrected to take into account the energy absorbed by the crater formation since it is already accounted for by the experimental data. 2.1.1.3. Munitions. Even if military depots are exempted from compiling a safety report (according to Seveso directives), there are civil industrial activities that either produce or demilitarize munitions. Cooper (1996) and Mendonca-Filho, Bastos-Netto, and Guillardello (2008) report that for metal munitions characterized by a metal/explosive mass ratio M/C ¼ 1 the energy spent for the expansion and fragmentation of the case is about 2090 kJ/kg. Consequently, only a fraction (i.e. 2760 kJ/kg) of the original energy of TNT (i.e. 4520 kJ/kg) encased in munitions contributes to the air blast. In analogy with Eq. (4) the effective amount of explosive contributing to the air blast is:

Wair

blast

¼

Wdeton

xmun

(5)

where xmun ¼ 4520/2760 if M/C ¼ 1. For conventional munitions such as bombs and grenades, the M/C ratio is even larger. For heavy artillery and mortar bombs M/C ratios belong to the 4e6 interval. Therefore, the proposed xmun value is fully conservative. 2.1.1.4. Rocket propellant. Among the munitions handled and processed by the demilitarization industry there are weapons propelled by rockets. One of the most used rocket propellants is ammonium perchlorate composite propellant, APCP, which contains a large amount of ammonium perchlorate, AP. This propellant is solid and dense. APCP bounds AP to some inert plastic binders, which account for 25% of the total weight. Hence, for industrial activities involving APCP, the AP effective mass is 75% of APCP. In case of APCP, the safety report must then focus on the AP effective

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mass. According to a rather recent report from the U.S. Department of Justice (DoJ, 2009), AP is no more an explosive substance and must be considered as a combustible element. In addition, APCP should be stored in explosive magazines only for security reasons (i.e. no more for safety reasons). Even if some safety cards define AP as a combustible substance that does not detonate, nonetheless such cards add a R2 risk phrase that, according to 2001/59/EC Directive, means that the substance may detonate when ignited. This is clearly a contradiction but to preserve the conservative approach of safety reports, until some forthcoming EU directive demotes AP from explosive to combustible substance, AP should be regarded as an explosive. Therefore, Eqs. (1) and (2) hold provided that one evaluates the TNT equivalent mass of AP by multiplying the amount of AP stored/processed by 2040/4520 ¼ 0.45, which accounts for the ratio of detonation energies released respectively by AP and TNT (i.e. 2040 kJ/kg and 4520 kJ/kg). 2.2. Primary and secondary fragmentation The role played by fragments and debris in the detonation of solid explosives may be relevant in terms of consequences on both surrounding equipment and exposed people. This section focuses on the possible consequences on exposed people and on their probability of occurrence. It is worth underlining that there are neither EU directives nor laws from member states to discipline the fragments production from explosions. Safety thresholds are also missing. According to a strict application of available legislation, a safety report could then neglect this subject. At the same time, a serious and responsible approach to drafting of safety reports calls for dealing with fragmentation and its connected risks. In addition, it may happen that the governmental body in charge to examine and validate the safety reports requests an addendum covering the risks connected to fragmentation. In case of munitions, the rupture of the metal case produces primary fragments. In general, the side-on overpressure on deposit walls and the possible impingement of primary fragments produce secondary fragments. Secondary fragments include debris such as those from structural elements of the facility and from non-confining equipment that are likely to rupture into enough pieces so to contribute significantly to the total number of expected fragments. The US military is the main actor in the technical literature who dealt with fragmentation (see for instance (DoD, 2004; HNC-EDCS-98-1, 1998; HNC-ED-CS-98-2, 1998). According to the US Department of Defense (DoD, 2004), a fragment is a solid piece of debris whose mass is at least 134 g; a fragment is dangerous if its impact energy is higher than 79 J. Given that the impact energy is equal to the kinetic energy, the smallest fragment (according to DoD (2004) definition) must have an impact velocity of at least 34.34 m/s to be dangerous (the higher the mass of the fragment the lower the minimum impact velocity to make it dangerous). The DoD does not spend any words on the shape and on the density of the fragment. It is straightforward observing that the minimum-weight fragment of 134 g has an equivalent diameter respectively of 4.5 cm and 3.2 cm if it is made up of concrete or steel. Nonetheless, by increasing significantly the impact velocity, fragments much lighter than 134 g can have devastating effects on human beings (e.g., bullets fired by guns; bolts, nuts, and nails projected by rudimentary bombs). DoD (2004) introduces also a safety threshold in terms of fragments density, which is assigned a value of 1 fragment per 600 ft2 of soil (i.e. 1 fragment per 56 m2). Since the number of possible fragments projected from the epicenter of the detonation decreases with the distance, it follows that the safety distance starts from the 1/56 fragment/m2 density threshold. The same document (DoD, 2004) prescribes two safety distances for inhabited buildings (IBD) and for public traffic routes

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D. Manca / Journal of Loss Prevention in the Process Industries 26 (2013) 974e981

(PTRD) respect to possible detonations of munitions stored either in above ground magazines (AGM) or in earth covered magazines (ECM). For instance, in case of ECMs storing from 227 to 20,412 kg of explosive, the IBD is 381 m and the PTRD is 229 m (i.e. 60% of the IBD value). It is rather staggering to observe that both the distances are constant since they do not depend on the amount of detonated explosive. Such constant values are also in contrast with the threshold of fragments density reported in DoD (2004). This inconsistency can be reconciled through the work of Murtha (1998) carried out at the USA Naval Engineering Service Center. Murtha reports the results of a full-scale experiment on the detonation of 27,216 kg of TNT in a military depot storing munitions (i.e. bombs and grenades). Besides the data reported in Section 2.1.1, the experimenters classified, counted, and weighed the fragments burst in the surroundings. Working on concentric areas progressively distant from the epicenter of the explosion, they counted the fragments and determined their superficial density. A nonlinear regression of measured data allowed evaluating the distribution of fragments as a function of the distance from the epicenter:

  f ¼ 74:376 exp  3:763515$103 d

(6)

where f is the number of fragments found in an area of 56 m2 and d is the distance [ft] from the detonation epicenter. Eq. (6) can be zeroed respect to the safety threshold of the fragments density:

  74:376 exp  3:763515$103 d  1 ¼ 0

(7)

The solution of Eq. (7) allows determining the safety distance of 349 m. It is worth observing that the real full-scale experiment (Murtha, 1998) showed that an amount of explosive (i.e. 27,216 kg) larger than the upper limit (i.e. 20,412) produced a shorter safety distance (i.e. 349 m) respect to the suggested IBD (i.e. 381 m). The only limitation of Murtha’s work and specifically of Eq. (6) is that it was determined for a specific case study. As a matter of fact, it applies to the exact amount of TNT detonated and to the exact typology of military magazine surrounded by an embankment (Murtha, 1998). Thus, at first sight, Eqs. (6) and (7) could seem not applicable to other cases. Really, the safety distance of 349 m can be seen as a conservative value, as long as the typology of building where the explosive is stored/processed is similar to the one described in Murtha (1998) and that the equivalent amount of detonating explosive is lower than 27,216 kg of TNT. At the same time, when the amount of solid explosive is significantly lower than Murtha’s experiment the 349 m safety distance becomes over conservative. In this case, even if the fragments density threshold is well-grounded, it appears unusable unless a destructive test is run and a curve similar to Eq. (6) is regressed. The only theoretical equivalence that can be implemented reasonably is the evaluation of the TNT equivalent mass participating to the detonation so to check whether it is respects the 27,216 kg upper bound. Obviously, the company responsible for the safety report cannot run any destructive tests on storage/processing magazines due to economic, safety, and authorization reasons. It is then desirable that either over-national or national authorities run experimental tests to determine the consequences of catastrophic detonation of different amounts of explosives within well-defined typologies of magazines (in terms of both structure and materials). Subsequently, the official results from the experiment activity should be collected in guidelines and eventually in EU directives to drive the construction of buildings that store/process solid explosives. Such experimental activity could also tackle a further challenging issue (beyond the scope of this Journal) which consists in determining the typology, structure, layout, material, and thickness of magazine

walls as a function of the explosive quality, case, and amount together with the presence/absence of an embankment. 2.2.1. Role of containment buildings on fragments production The idea of containing the explosion burst by means of very thick walls made of reinforced concrete and putting heavy roofs on magazines and depots to dampen the overpressure is fatal. Zhang and Tang (1993) report the outcomes of a catastrophic explosion of 40 ton of TNT occurred in 1991 in an explosives production factory in Liaoning province, China. Besides the large number of fatalities and destruction of buildings and equipment, most of projected fragments were found within a 300 m distance from the epicenter of the detonation. However, a steel rod 0.8 m long and 8 cm in diameter (i.e. weighing more than 31 kg) was thrown at a distance of 1685 m. A 50-ton chunk of reinforced concrete was projected at 487 m from its original position. Fragments of concrete wall, weighing 50 kg each, flew 310 m, penetrated the roof of a nearby building, and injured two workers. The plant section, where the detonation occurred, was a three-layer reinforced concrete building, with thick roofs, and surrounded by a 3 m high soil barricade. The lesson learned from Liaoning accident is avoiding any attempts to contain the air blast and block the overpressure discharge. As the phenomenology of that accident event shows, when the building gives away structurally the consequences are catastrophic due to the amplification of the pressure build up exerted by the initially overresistant case. Murtha (1998) suggests a different approach to magazine/depot/building design to minimize the explosion consequences. The main containment walls should not be made of reinforced concrete. Quite the reverse, Murtha recommends a large use of lightweight structural concrete with some soft matter interposed (e.g., water, foam, pumice, sand) to absorb and dampen the blast wave. Equally, the roof should be made of sandwich plates and composite materials (Bahei-El-Din, Dvorak, & Fredricksen, 2006) to be weatherproof but at the same time yielding to side-on overpressure. Conceptually, this approach is quite similar to the pressure relief valves and rupture disks widely adopted in chemical processes to dampen/control the consequences of overpressures. Both lightweight concrete and composite materials have the capacity to crush into small and light fragments whose offending capacity is significantly reduced respect to common civil construction materials. The photos reported in Murtha (1998) show the aftermath suffered by the lightweight concrete walls of the depot after the TNT detonation of 27,216 kg in munitions. The containment walls were literally crushed into fine fragments, resembling even a kind of powder, and were capable of avoiding any sympathetic detonation. In fact, Murtha (1998) maintains also that lightweight structural concrete avoids sympathetic detonation. The prevention of sympathetic detonation, coupled with an appropriate division into compartments of the explosive masses (either stored or processed) plays an important role in reducing the consequences of top events. 2.2.2. Exposure probability to fragments In case of detonation, a person exposed to air blast and debris has two distinct probabilities to undergo their consequences. A conservative hypothesis is assuming a radially isotropic distribution of both air blast and debris from the epicenter of detonation. Under this hypothesis, the exposure probability to air blast is 1 (i.e. 100%). Conversely, the exposure probability to debris must be calculated since, as reported in Eq. (6), fragments are discrete elements that are projected from the epicenter according to a surface density that decreases with the distance. This paragraph is devoted to the quantitative assessment of probability that fragments, produced in a detonation, hit a target. The probability of a fragment hitting a target depends on the target dimension. For the sake of simplicity, the following

D. Manca / Journal of Loss Prevention in the Process Industries 26 (2013) 974e981

treatment focuses on a person as the target. The following formulas can be extended to any physical target provided that its proper shape area, Ashape, is used. A person shape can be bounded above by that of a rectangle having the same height and width. Hence, the frontal shape of a person has an area approximately equal to 0.9 m2 (i.e. 1.8 m height and 0.5 m width). In case of building surrounded by an embankment, a fraction of the projected fragments is blocked by the embankment. This fraction depends on the angle subtended by the blast epicenter to the embankment. Only the fragments projected with a take-off angle greater than the minimum angle amin of Fig. 1 fly over the embankment and may hit the exposed target. Consequently, Eq. (6) must be corrected by the following multiplicative parameter, be, which quantifies the fraction of fragments effectively projected to the surroundings:

p  2amin be ¼ p fe ¼ be f

(9)

Under the ballistic hypothesis of parabolic flight, the impact angle of the debris with the soil is equal to the take-off one. Then, the exposed area of a person can be bounded above by the following inequality:

(10)

Where the take-off angle satisfies the following inequality:

atakeoff  amin

year interval is usually adopted as the reference one since the probability of occurrence of a given event is the frequency of that event per year (i.e. 1/y). More realistically, people exposed to debris are moving targets who may travel along roads in proximity of the plant where the top event occurs. The case study reported in Fig. 2 shows the critical tract of a road where the fragments density is higher than the threshold limit proposed by DoD (2004). To determine the average fragments number along the critical tract one has to solve the following line integral:

ZL fe ðdðxÞÞdx fave ¼

0

L

(14)

(8)

The fragments density in case of embankment becomes:

  Aexposed ¼ Ashape cos atakeoff  Ashape cosðamin Þ

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Where x is the line coordinate that describes the road profile along the critical tract and L is the critical-tract length. One can suppose three typologies of people who travel along the critical tract: a pedestrian, a cyclist, and a motorcyclist with average speeds of respectively 6, 20, and 50 km/h. It is then straightforward to determine the time, tj, taken to cover the critical tract by each people typology. To determine the cumulative exposure probability over a well-accepted time interval, which is one year (to comply with the occurrence probability of explosion events measured in events per year), one can hypothesize that every working day the exposed people cover that distance to go to work. By assuming 200 working days per year, the overall probabilities to be hit by a fragment for the proposed people typologies (j) are:

(11)

Amax exposed

tj 200 $ $ Asampled 86; 400 365

The conservative approach to safety reports calls for adopting the maximum exposed area:

Pj ¼ 2fave

Amax exposed ¼ Ashape cosðamin Þ

Factor 2 in Eq. (15) accounts for the there-and-back trip. Finally, by multiplying Pj by the probability of occurrence of a specific top-event involving the detonation of a given amount of solid explosive, one obtains the probability that a person exposed to the blast is hit by a fragment. The evaluation of the probability of

(12)

For a fixed person at a distance d from the detonation occurring in a building surrounded by an embankment the probability of being hit by a fragment is:

Pfixed ¼ fe ðdÞ

Amax exposed Asampled

(15)

(13)

Where Asampled is the reference area of 56 m2 of soil respect to which the number of fragments are counted (as in Eq. (6)). However, the hypothesis of fixed person is simplistic and unreal since it would correspond to a statue continuously exposed to the risk of detonation. Even if a person stands still at a given distance from the epicenter, his/her permanence in that position covers a fraction of the whole time used as the reference interval for the probability assessment. For the sake of clarity, it is reasonable assuming fixed the person’s position for a maximum of either some minutes or some hours respect to a time interval of one year. The

Fig. 1. Minimum projection angle of debris for a building (light gray) surrounded by an embankment (brown, dark gray). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2. The pink (light gray) portion of road shows the critical tract where the fragments density is higher than the threshold limit reported in DoD (2004). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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D. Manca / Journal of Loss Prevention in the Process Industries 26 (2013) 974e981

occurrence of specific top-events is based on risk-analysis methods that are extensively discussed in the literature (see Lees, 2012, for a list of references). It is worth adding two comments to this treatment on fragments. The former comment is about fave which depends on fe. Obviously, if the building involved in the detonation lacks a surrounding embankment then f replaces fe in Eq. (14). The latter comment is about the availability of a dedicated f(d) equation consistent with the typology of building and the amount of explosive detonated. At present, the fragments-density formula available in the literature is only the one of Murtha (1998) reported in Eq. (6). Hence, Eq. (6) can be extended only to buildings similar to the military magazine described by Murtha (1998) and for explosive amounts as high as 27,216 kg of TNT. When the explosive amount involved in the detonation is significantly lower than that value, the consequence analysis can still be carried out, but it risks to become too much conservative and little informative. 3. Conclusions The paper presented some methodologies to determine the consequences of solid explosive blasts in industrial activities for drafting of safety reports. For the sake of space, the paper focused only on the overpressure evaluation and the fragments formation. A forthcoming article will tackle the degradation/combustion, the thermal radiation, and the seismic wave effects originated by a detonation. Besides the conventional approach to quantitative assessment, the proposed methodologies allow designing for safety by determining the maximum amounts of stored/processed explosive that respect the safety thresholds. In addition, they suggest a probability approach to evaluate the exposure of targets to debris. The paper discussed a number of uncertainties and open issues on the quantification of the consequences from blasts. Both EU and USA are urged to produce official and shared data to fill the gaps in building structure, construction material, wall thickness, mitigation devices, and division into compartments as a function of the typology and amount of solid explosive either stored or processed. The desirable production of guidelines and specific directives can increase significantly the safety of industrial activities involving solid explosives and define a common basis for both the design of new sites and the revamping of existing ones. Acknowledgments The author acknowledges the fruitful discussions on detonation of solid explosives with Matteo Giantomaso of Esplodenti Sabino company. Symbology

A C d E f L M NM Pfixed Pj Ps t W

area [m2] explosive charge in munitions [kg] distance from the detonation epicenter [m, ft] degradation and combustion energy [J/kg] fragments density [m2] length of the critical tract [m] mass of the metal encasing of munitions [kg] number of experimental measurements exposure probability for a fixed target exposure probability for a moving target of given typology side-on overpressure [bar] exposure time [s] explosive mass [kg]

WTNT x 0 z

equivalent mass of TNT [kg] line coordinate [m] reduced distance [m kg1/3]

Greek letters a coefficient of side-on overpressure Equation in Alonso et al. (2006) amin minimum projection angle of fragments over an embankment b exponent of side-on overpressure equation in Alonso et al. (2006) be fragments fraction projected over the embankment x rounded damping coefficient in case of embankment xmun damping coefficient in case of munitions xw,e damping coefficient in case of containment walls and embankment Subscripts and superscripts air blast participating to the air blast ave average calc calculated deton detonated e embankment eq equivalent exp experimental exposed exposed to fragments i component index j typology index of exposed target max maximum sampled referred to the sampled area of 56 m2 shape shape of the exposed target take-off take-off angle of the ballistic trajectory References Alonso, F. D., Ferradás, E. G., Pérez, J. F. S., Aznar, A. M., Gimeno, J. R., & Alonso, J. M. (2006). Characteristic overpressure-impulse-distance curves for the detonation of explosives, pyrotechnics or unstable substances. Journal of Loss Prevention in the Process Industries, 19(6), 724e728. Alonso, F. D., Ferradás, E. G., Pérez, J. F. S., Aznar, A. M., Gimeno, J. R., & Alonso, J. M. (2007). Consequence analysis by means of characteristic curves to determine the damage to humans from the detonation of explosive substances as a function of TNT equivalence. Journal of Loss Prevention in the Process Industries, 20(3), 187e193. Alonso, F. D., Ferradás, E. G., Miñarro, M. D., Aznar, A. M., Gimeno, J. R., & Pérez, J. F. S. (2008). Consequence analysis by means of characteristic curves to determine the damage to buildings from the detonation of explosive substances as a function of TNT equivalence. Journal of Loss Prevention in the Process Industries, 21(1), 74e81. Bahei-El-Din, Y. A., Dvorak, G. J., & Fredricksen, O. J. (2006). A blast-tolerant sandwich plate design with a polyurea interlayer. International Journal of Solids and Structures, 43, 7644e7658. Baker, W. E., Cox, P. A., Westine, P. S., Kulesz, J. J., & Strehlow, R. A. (1983). Explosion hazards and evaluation. Amsterdam: Elsevier. Casal, J. (2008). Evaluation of the effects and consequences of major accidents in industrial plants. In Industrial safety series (Vol. 8). Amsterdam: Elsevier. Cooper, P. W. (1996). Explosives engineering. New York: Wiley-VCH. DoD. (2004). Ammunition and explosives safety standards. USA: U.S. Department of Defense. DoD 6055.9-STD. DoJ. (2009). Open letter to all federal explosives licensees and permitters. USA: U.S. Department of Justice. 17-Jul-2009. Formby, S. A., & Wharton, R. K. (1996). Blast characteristics and TNT equivalence values for some commercial explosives detonated at ground level. Journal of Hazardous Materials, 50, 183e198. HNC-ED-CS-98-1. (1998). Methods for predicting primary fragmentation characteristics of cased explosives. Huntsville, AL: U.S. Army Corps of Engineers Engineering Support Center. HNC-ED-CS-98-2. (1998). Methods for calculating range to no more than one hazardous fragment per 600 square feet. Huntsville, AL: U.S. Army Corps of Engineers Engineering Support Center. Kuhl, A. L., Forbes, J., Chandler, J., Oppenheim, A. K., Spektor, R., & Ferguson, R. E. (1998). Confined combustion of TNT explosion products in air. LLNL Report UCRLJC-131748 (pp. 1e52).

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