J.-P Bigot
email: bigot@emse.fr

L Véchot
email: vechot@hotmail.fr

D Testa

C Noguera

W Minko
email: minko@emse.fr

M Kazmierczak
email: marc.kazmierczak@ineris.fr

P Vicot

RUNAWAY REACTION: DEVELOPMENT OF A NEW EXPERIMENTAL VENT SIZING TOOL FOR NON TEMPERED SYSTEMS AT THE LABORATORY SCALE

Keywords: vent sizing, untempered reaction, runaway reaction, similarity, cumene hydroperoxide (CHP), adiabatic calorimeter, blowdown experiment, mass loss, non tempered system

In order to protect chemical reactors from runaway reactions, vent sizing methods, based on data obtained from adiabatic calorimetry and two-phase flow models, were built under the guidance of the DIERS (Design Institute for Emergency System Relief). Nevertheless, because of the simplifying assumptions used (mainly conservation of initial reactive mass and homogeneous vessel venting), DIERS methods often lead to unrealistically large vent areas. This is especially the case of non tempered systems (gas-generating and hybrid systems, peroxide decomposition for example).

The United Nation Manual of Test and Criteria (UN) also proposes an experimental method to determine the minimum required vent area using a 10 dm vessel test. The vent area is scaled up on vessel volume. Prior mass loss and two phase flow being experimentally taken into account, this method provides a more realistic vent size.

This type of area to volume scale up can be conservative in the case of non tempered systems because the mass loss at small scale is more important.

Nevertheless, due to the fact that the sample size is large, the UN method has the disadvantage of being time consuming, laborious and requires heavy safety precautions.

This article is about the development of a new experimental vent sizing tool for non tempered system combining the advantages of DIERS method (laboratory scale) and UN method (less overconservative). This new tool consists of an extension of the adiabatic calorimeter Vent Sizing Package II (VSP2).

The reactor in which chemical runaway reaction occurs is the "VSP2 blowdown test cell". In order to simulate the relief system the test cell is connected to a main vent line comprising of regulating valves which allow the simulation of "equivalent ideal orifice" area by volume ratios lying between 102 4 and 5.102 3 m2 1 . A feed bleed system is also installed. The assessment of the vented mass from the test cell is done by adding at the end of the main vent line a glass column half filled with water. During the relief the chemical products are vented in the column (quench of the reaction is realised at the same time) leading to an increase in the differential pressure at the bottom of the column. The initial VSP2 device has also been modified in terms of the rate of pressure rise and fall to allow for these kinds of blowdown experiments.

This new device was tested by running the decomposition of cumene hydroperoxide in 2,2,4-trimethyl-1,3-pentanediol diisobutyrate which is a non tempered system. Pressure evolution and the vented mass profile obtained are presented. The influence of several parameter (A/V ratio, the initial fill level, feed bleed opening) is also observed.

However, heat losses at small scale are much more important, so working in fire simulation mode is generally preferred over the adiabatic mode. This also means that the study of tempered system appears difficult. This method can not be used with initial fill level superior to 80%. It is not adapted to violent runaway reaction (for example very concentrated peroxide solutions).

INTRODUCTION

can lead to too oversized vent areas, especially for Vent sizing methods, based on data obtained from untempered systems (gas-generating and hybrid systems, adiabatic calorimetry and two-phase flow models, were peroxide decomposition for example). DIERS vent designed by the DIERS (Design Institute for Emergency area can be overestimated by one order of magnitude System Relief) in order to protect chemical reactors for xperoxide decomposition runaway reaction [Fauske from runaway reactions. Nevertheless, these methods 2000].

More realistic vent areas are obtained using a "similarity method" such as that proposed by the United Nation Manual of Test and Criteria (UN) for the transportation of peroxide chemicals [Appendix 5]. This latter method has however the disadvantage of being time consuming, laborious and requiring heavy safety precautions because it involves a 10 litre decomposition.

This paper proposes a new similarity vent sizing tool combining the advantages of both DIERS method (laboratory scale) and UN method (less overconservative). This tool allows blowdown experiments to be carried out at laboratory scale. The objectives are both direct determination of the required vent area and real time measurement of the vented mass during relief. This latter will enable a better understanding of blowdown.

NEW "SIMILARITY VENT SIZING TOOL" FEATURES

This tool is an extension of the adiabatic calorimeter VSP2. The reactor in which chemical runaway reaction occurs is the "VSP2 blowdown test cell". The extension of the VSP2 consists in the addition of both a feed bleed and a safety relief system plus the addition of a vented mass measurement device.

ADDITION OF RELIEF SYSTEM

Two venting systems generally exist on chemical reactors or storage vessels:

-a feed bleed system which is designed to avoid pressure changes when filling or emptying the capacity. This feed bleed system is supposed to vent only gas.

-a rupture disc or a safety valve, which is designed to open when the pressure in the reactor reaches the vent set pressure (P s ).

During a runaway reaction, gases produced by the reaction are first vented via the feed-bleed system. This flow maintains pressure in the reactor close to the atmospheric pressure. Then pressure in the reactor starts to increase when volumetric gas generation rate becomes greater. The rupture disc opens when pressure in the reactor reaches P s .

In the new vent sizing tool, the rupture disc is simulated by a relief vent line and the feed bleed system by a feed bleed line. Both lines are composed of a ball valve and a safety valve.

Relief vent line

The relief vent line is connected to the outlet of the blowdown test cell outside of the VSP2 containment vessel (Figure 2). This line is made of 1/80 0 stainless steel tube (inner diameter -1,76 mm). The line includes an actuated 1/80 0 ball valve (C V = 0.2) followed by a 1/80 0 metering valve (needle valve) (C V = 0-0.03).

When pressure in the test cell reaches set pressure P s , an opening signal is sent by the VSP2 software to the ball valve actuator. By this way, the test cell is open to the atmosphere. Contents of the test cell are vented through the metering valve. Venting flow is a function of the metering valve opening.

Feed bleed line

The feed bleed line is installed in parallel to the relief vent line (Figure 2). It's also made of 1/80 0 stainless steel tube and includes a manual 1/80 0 ball valve followed by a 1/8 0 0 metering valve (needle valve). The ball valve is always "open" during a runaway experiment. The vented mass measurement device is composed of a glass column (1 metre high, inner diameter -5 cm) partially filled with water. The relief vent line is ended with a PTFE perforated tube which is immerged in the column water in order to obtain small bubbles. A relative pressure transducer (0-100 mbar) is connected to the bottom of the column.

During the relief, chemicals are vented through the column (quench of the reaction is realized at the same time). Gas is ejected while liquid and condensable matters are collected at the top of the water. This leads to an increase of the relative pressure at the bottom of the column. The PTFE perforated tube reduces noises on pressure measurement.

Assuming that vented mixture is not miscible with water, DP measure d is translated to vented mass measurement (1)

(2)

Dh: height of vented chemical mixture in the glass column A co lumn : glass column cross section DP 0 : initial differential pressure (due to water)

TEST OF THE "SIMILARITY VENT SIZING TOOL": CHP RUNAWAY EXPERIMENTS

The new vent sizing device was tested by running non tempered system runaway experiments. The decomposition of cumene hydroperoxide (CHP), which produces non condensable gases, was chosen.

CHEMICAL SYSTEM AND RUNAWAY SCENARIO 80% (w/w) CHP solution in cumene was diluted in a high boiling point solvent: 2,2,4-trimethyl-1,3-pentanediol diisobutyrate. The composition of the chemical system is thus (w/w): 30% CHP, 7.5% cumene, 62.5% 2,2,4-trimethyl-1,3-pentanediol diisobutyrate.

High boiling point of both peroxide and solvent is the criterion which allowed us to consider this system as untempered (CHP: 1168C at0.02bar, solvent: 2808C at 1.013 bar). A temperature increase rate of 0.58C/min simulates a fire scenario.

EXPERIMENTAL PROCEDURE Blowdown experiments procedure is as follows:

-Assembly of the test cell according to VSP2 documentation. -Vent line characterization.

-Filling of reactant in test cell.

-Feed-bleed line opening.

-Guard heater activation (will always be activated to limit heat losses). -Heating of the sample 20°C à SADT þ 58C ( = 858C) by main heater. -Fire simulation: from 858C heating of the sample (constant heating power until the end of the runaway reaction corresponding to 0,5 8C/min). -Relief vent line opening when test cell pressure reaches set pressure P s .

RESULTS

Figure 4 presents a 30% CHP runaway experiment with similarity vent sizing tool. Test cell pressure (P1) and liquid temperature (T1) histories are plotted. The experimental conditions are:

-Initial reactant mass: 79 g.

-Test cell volume =125 cm3 .

-Initial fill level = 65% (v/v).

-Relief vent line: A/V = 1.36 x 102 3 m2 1 (3 turns opened). -Feed bleed line metering valve -3/8 turn opened.

The whole experiment lasts approximately 3 hours. The runaway period is relatively short (a few minutes). Figure 5 shows a zoom for pressure, temperature and vented mass profiles during the relief period for the same runaway experiment. We can observe two pressure peaks. This is typical for untempered systems. The first one corresponds to relief vent line opening. It has no obvious influence on temperature profile. This behavior confirms that the chemical system is untempered. Dynamic vented mass was measured. 20.4% of initial mass was vented at turnaround.

LIMITS OF THE NEW VENT SIZING TOOL AND ACHIEVEMENT OF SIMILARITY

Our device presents some technical limits. Some of these limits have consequences on the achievement of the similarity concerning hydrodynamic and reaction kinetics.

Technical limits

The following technical limits were observed:

-Working with concentrated peroxide can lead to cell volume swell (uncertainties on A/V) or even to cell failure. -Limited range of equivalent ideal nozzle diameter can be simulated (between 102 3 m2 1 and 3.5 102 3 m2 1 ). -The use of metering valve is convenient (easy to change A/V) but not ideal: vent line has to be characterized before each experiment because of aging.

-When initial fill level is higher than 80 %, liquid enters the feed bleed line (which is supposed to vent only gas)

Technical solutions can probably be found in the future to solve these problems.

Achievement of similarity

The use of a similarity method is not conservative for tempered systems [START_REF] Fauske | Emergency Relief System (ERS) Design[END_REF]. This is the reason why our tool is only devoted to untempered systems. The achievement of similarity is however delicate.

Hydrodynamic similarity

We can distinguish three aspects of hydrodynamic similarity:

-Similarity of the level swell inside the reactor. The vented mass is less important at small scale than at large scale [START_REF] Fauske | Emergency Relief System (ERS) Design[END_REF]. So the similarity of level can not be reached! This however means that at small scale, the relative remaining mass at turnaround is greater than at large scale. This leads to a more violent pressure rise rate. This difference in the level swell makes the new tool to be conservative. -Similarity of the venting flow through the vent. The metering valve in the relief vent line has a complex geometry, very different of an ideal nozzle. To obtain an equivalent ideal nozzle diameter we assumed that the viscous dissipation phenomena introduced by this complex geometry is the same for a gas flow than for a two-phase flow. This needs more investigations. -Moreover, similarity of the flow through the vent needs mass flux (G) to be independent of vent area.

Several authors [START_REF] Fletcher | Flashing flow through orifices and pipes[END_REF]Van den Akker et al., 1984;[START_REF] Kevorkov | A theoritical approach to two-phase critical Influence of the scale factor on the critical discharges of flow. 4th report: Experiments on saturated water discharging saturated water from straight tubes with a sharp inlet edge, through long tubes[END_REF][START_REF] Kevorkov | A theoritical approach to two-phase critical Influence of the scale factor on the critical discharges of flow. 4th report: Experiments on saturated water discharging saturated water from straight tubes with a sharp inlet edge, through long tubes[END_REF]Marviken, 1979] showed that this is true for vent diameter from 3.2 mm to 500 mm. There are however no studies for smaller diameters. In the case of the similarity tool, the vent line pipe has an inner diameter equal to 1.76 mm. Capillarity effect could appear and so lead to decrease of the mass flux. This would, once again, make the similarity tool to be conservative. -Similarity of the gas venting flow through the feed bleed system. Gas venting capacity of the feed bleed system has direct influence on temperature at vent opening (T s ). For an untempered system, if T s at small scale is smaller than T s at large scale, vented mass at turnaround can be more important at small scale, leading to a lower P max . With the similarity tool, a good simulation of the feed bleed system is quite difficult. A feed bleed line settings which allows a vent opening temperature equal or greater to that for a large scale vessel is necessary in order to be conservative.

Globally, when hydrodynamic similarity is not achieved, the similarity vent sizing tool is conservative.

Kinetics (or thermal) similarity

Heat exchanges have to be similar at both scales in order to obtain similarity of reaction kinetics. For our tool, the following points can influence this similarity:

-Heat losses at small scale can be much more important compared to industrial large scale [START_REF] Friedel | Predictive accuracy of simplified vent area sizing methods for the case of thermal runaway reactions[END_REF] because of surface to volume ratio. -Reactant mass in the test cell decreases during blowdown. This leads to an increase of the adiabaticity factor (f). -We ran 15% CHP (w/w) adiabatic runaway reaction with both a closed test cell and a plugged blowdown test cell (plug outside the containment vessel). We observed that dT/dt obtained with the plugged blowdown test cell is lower than the one obtained with the closed test cell (factor of 3 !, Table 1). This means that addition of a vent line to the VSP2 calorimeter generates additional heat losses which can be important in adiabatic mode. We did the same adiabatic tests using a plugged blowdown test cell with a PTFE outlet tube.

The decrease of dT/dt was almost unchanged. This let us think that heat losses do not come from conduction phenomena but from presence of vaporisation/condensation phenomena. The same experiments for a fire scenario (dT/dt -0,58C/min) with or without the blowdown line led to almost identical dT/dt measurements. The thermal bridge effect is masked when external heat flux is added.

It has to be verified that thermal bridge has negligible effect on chemical kinetics before any blowdown experiment with the similarity tool. This tool is not relevant for systems which generate much vapour and for scenarios with a low heat input.

These limits are undoubtedly the most severe ones for this tool. They could lead to less violent runaway reactions at small scale and so an unsafe A/V prediction.

CONCLUSION AND PERSPECTIVES

A new similarity vent sizing tool combining the advantages of both DIERS method (laboratory scale) and UN method (less overconservative) was built by extending VSP2 adiabatic calorimeter. This tool allows blowdown experiments to be carried out at laboratory scale. It was tested with Dynamic mass vented measurement was obtained. That is the second main point of the similarity tool. However the following work has still to be done in order to improve the capacity of the similarity tool:

-improvement of vented mass measurement resolution, -extension of equivalent diameter range by increasing the relief line size, -use of calibrated orifices for a better simulation of relief vent line and feed bleed line, -reduction of heat losses in order to allow for adiabatic tests, -study of vapour and heat losses effect on small scale blowdown measurements for domain of use determination.

Validation experiments will be done by comparison with blowdown experiments at the 10 l scale. An future paper will discuss the behaviour of non tempered system based on vented mass measurement and specific behaviour of CHP system.

Figure 1 .Figure 2 .

 12 Figure 1. New similarity vent sizing tool

Figure 3 .

 3 Figure 3. Picture of vented mass measurement system

Figure 4 .Figure 5 .

 45 Figure 4. Whole 30% CHP runaway experiment with test cell blowdown

Table 1 .

 1 30% CHP runaway experiments in closed test cell and plugged blowdown test cell: comparison between adiabatic and fire simulation modes

				(dT/dt)	(dP/dt) max
	Mode	Test cell	type	8C/s	bar/s
	Adiabatic	closed		1.67	0.5
		Plugged	blowdown	0.56	0.18
		Closed		5.3	1,9
	Fire simulation	Plugged	blowdown	5	1.65


S Duplantier
email: stephane.duplantier@ineris.fr

N Gobeau

MEDIUM-SCALE EXPERIMENTS OF FIRE WAREHOUSES

Keywords: fire, warehouse, experiment, thermal radiation

This study, funded by AFILOG, focuses on the threat caused by the large heat releases from warehouse fires. The objective is to improve the understanding of the propagation of the fire inside the warehouse and to quantify the heat fluxes experienced in the vicinity of the warehouse. This will ultimately help refine the modelling tool presently employed by INERIS [1] to evaluate the thermal effects from warehouse fires on nearby populations and/or other industrial installations. Such an evaluation is important, for example, to help land-use planning.

) were conducted in a purposedly built 3 metres high building with a section of 4.3 x 4 m 2 . One side of the building was open and the top was covered with a plywood. This roof was meant to help the propagation of the fire and then disappear so as to be representative of a fully-developed fire in a large warehouse. Four steel racks were set up inside the building on which were stacked 40 cm x 40 cm x 50 cm high wood cribs made of pine sticks. To monitor the fire propagation, 52 measurements of temperature were undertaken with thermocouples throughout the storage area. Outside the building, heat fluxes were measured at a total of 20 locations in front of the building and on the two sides. The heat flux metres were located at different distances from the building and different heights. Videos were recorded from 4 cameras around the building and an infrared camera. The images allowed to time events such as the appearance and propagation of smoke, the collapse of the roof and stacks and to determine the shape and size of the flames. The fire was set by a burner located beneath the third rack towards the rear of the building. The heat release rate from the fire was deduced from the measurement of the total mass loss.

The influence of a number of parameters on the longitudinal fire propagation along the racks, the lateral propagation from one rack to another and the shape and height of the flame once the roof collapsed was investigated. These parameters were the stockage compacity, the thickness of the roof and the number of rack shelves. The most important results will be presented and compared where appropriate to previous work, in particular to studies on fire propagation in warehouses [eg. 2] Thermal effects of fire warehouses on their surroundings have been less studied and will be analysed here in some details. In particular, the measured heat fluxes will be compared with the values predicted by the in-house tool FNAP. The limitations of FNAP which assumes a solid parallelepipedic flame will be discussed in the light of the observations of the shapes and heights of the flames. Future steps aimed to improve FNAP and to carry out full-scale experiments will be briefly presented.

INTRODUCTION

This study, funded by AFILOG, focuses on the threat caused by the large heat releases from warehouse fires. The objectives are two-fold: to improve the understanding of the propagation of the fire inside the warehouse and to quantify the heat fluxes experienced in the vicinity of the warehouse. This will ultimately help refine the modelling tool presently employed by INERIS [START_REF] Bernuchon | Feux de Nappe[END_REF] to evaluate the thermal effects from warehouse fires on nearby populations and/or other industrial installations. Such an evaluation is important, for example, to help land-use planning.

The influence of a number of parameters on fire propagation, the shape and height of the flame once the roof collapsed and the radiative heat fluxes emitted in the vicinity of the building was investigated. These parameters were the storage compacity, the number of rack tiers and the level of containment (roof thickness and draft curtain depth).

Previous work [for example [START_REF] Mcgrattan | Sprinkler, vent, draft curtain: experiments[END_REF][START_REF] Lonnemark | Fire spread in large industrial premises and warehouses[END_REF] has investigated the effect of various parameters on the fire propagation in warehouses. However, very little work has been carried out on the effect of these parameters on the radiative thermal effects further away from the fire.

EXPERIMENTAL SET-UP

Medium-scale experiments of warehouse fires (see Figure 1) were conducted in a purposedly built 3 metres high building with a section of 4.3 x 4 m2 . One side of the building was open with a 0.6 m deep draft curtain made of a 5 mm thick plywood. The top of the building was covered with a 5 mm thick plywood. This thin roof was meant to help the propagation of the fire and then disappear so as to be representative of a fully-developed fire in a large warehouse. Four steel racks were set up inside the building on which were stacked 40 cm x 40 cm x 50 cm high wood cribs made of pine sticks. The racks were fixed to the building so that they could not collapse. This was safer but it should be born in mind that it is not representative of a real fire in a warehouse. The fire was set by a burner located beneath the third rack towards the rear of the building.

The heat release rate (Q_) from the fire was deduced from the measurement of the total mass loss (m_). Unfortunately, the humidity rate (h) of the wood employed was not negligible and varied with the scenario. This was taken into account by employing the following equation:

Q = (1 -h)m_DH c (1)
This is based on the assumption that the heat release rate will be reduced proportionally to the mass loss throughout the duration of the fire. In reality, the initial mass loss will be due mainly to the vaporisation of water and relatively little to the burning of wood. Once the water is vaporised, the mass loss will correspond to the burning of wood. So the assumption made will tend to make the heat release start a bit early and may under-predict the maximum heat release rate. In some cases, when all the scenarios on which conclusions are drawn have a similar humidity rate, the presence of humidity has little influence on the outcome of this study. If otherwise, this is highlighted.

Videos were recorded from 4 cameras around the building and an infrared camera. The images allowed to time events such as the appearance and propagation of smoke, the collapse of the roof and stacks and to determine the shape and size of the flames. To monitor the fire propagation, 48 measurements of temperature were undertaken with thermocouples, which were uniformly distributed throughout the storage area as can be seen in Figure 2. In addition, 4 thermocouples were placed underneath the roof.

Outside the building, heat fluxes were measured at 9 locations in front of the building, 7 locations on one side and 4 on the other side. Below are considered the measurements obtained the fluxmetres located in front of the building at a height of 1.5 m, corresponding roughly to the heat received by the head of a standing person. The fluxes were measured at five distances from the building: 3, 4, 6, 8.5 and 11m. The time-dependent fluxes are characterised by three values: the maximum flux, the time Tmax at which the maximum flux is reached. It is these three characteristic values that are compared. The maximum flux is plotted against the distance from the building for the different scenarios whilst the values of the characteristic times are given for each scenario since they depend very little on the distance from the building.

The influence of a number of parameters on the fire propagation and thermal radiative effects around the building were studied. These parameters were the storage compacity, the number of tiers of the racks and the level of containment of the building.

EFFECT OF THE STORAGE COMPACITY

The effect of the storage compacity was investigated by comparing the case where the wood cribs are stacked next to each other with the one where a space of 10 cm is left between them. This was done for a 3-tier and a 4-tier storage. The experimental conditions are summarised in Table 1.

The lower the compacity, the sooner the maximum heat release rate is reached (see Figure 3). Also, the peak is much higher in the case of a 4-tier storage whilst it is independent of compacity for a 3-tier storage. The faster propagation of the fire in the case of a storage of low compacity is Time (minute)

Figure 3. Comparison of the heat release rates for the storage compacity confirmed by the events observed (see Figure 4): a hole appears more rapidly in the roof and the fire propagates quicker to the other racks, thus increasing the power of the fire. In the case of low compacity, air can more easily circulate between the cribs, making more oxygen available thus facilitating the burning of the stored goods. This trend was also observed at a smaller scale [START_REF] Carreau | Characterisation of industrial fires[END_REF]. Other studies [START_REF] Block | A theoretical and experimental study of non-propagating free-burning fires[END_REF][START_REF] Delichatsios | Fire growth rates in wood cribs[END_REF] have also shown that the propagation, especially in the vertical direction, depended on the storage compacity.The fire of the low compact 4-tier storage may be particularly fast and powerful due to the larger supply in both oxygen and combustible compared to the other cases. The humidity rates varied greatly between the storages of low and high compacity for a given number of tiers and were higher for the storages of high compacity. It is believed that qualitatively, the trend would remain the same if the humidity rates were similar. However, quantitatively, the differences between the peak times may be higher and the differences between the peak values may be smaller.

As for the radiative effect from the fire, the maximum heat flux received at the front of the building is higher for the 4-tier storage of low compacity than the one of high compacity but the trend is opposite for the 3-tier storage (see Figure 5). In fact, for the 3-tier storage the peaks of the heat release rate for the low and highly compact storage are of the same order but the flame was smaller and narrower for the storage of high compacity. The heat radiated from this flame was thus higher. For the 4-tier storage, the size of the flames were similar for the two different compacities and the peak heat release rate was higher for the low compact storage, leading to a higher radiative flux. On the other hand, for both the 3-tier and 4-tier storages, the times Tmax at which the maximum heat flux are reached (see Table 2) are higher in the case of the high storage compactity. The latter are related to the times at which the peak heat release rates are reached.

Three tiers

Four tiers 3? 6°.

OF THE RACKS

The number of tiers varied from 2 to 4 tiers in the case of a high compact storage and from 3 to 4 in the case of a low compact storage (see Table 3).

For the highly compact storage, the maximum heat release rate is about the same no matter the number of tiers but the time at which it is reached increases with the number of tiers (see Figure 6). It seems that the increased amount of wood available due to the additional tiers does not burn faster, maybe because the supply of oxygen is limited by the storage compacity. So the fire is of similar power but lasts longer when there are more tiers (see Figure 7). On the other hand, for the low compact storage, the additional tier leads to a faster propagation of the fire and the maximum heat release rate is reached relatively early, about 25 minutes after the ignition of the fire. In this case, the fire is not limited by the supply of oxygen The maximum radiative heat fluxes received at the front of the builing are of the same order for the highly packed storage, with slightly higher values for the 3-tier storage than the other two (see Figure 8). For the lower storage compacity, the radiative fluxes emitted by the fire of the 4-tier storage are higher than the ones from the 3tier storage.This follows the trend observed for the maxima of the heat release rates. The times at which the highest radiative flux is received correspond for all cases to the times at which the peak release rates are reached, with a few minutes' delay.

EFFECT OF THE CONTAINMENT

The level of containment was investigated: the thickness of the roof was increased from 5 to 15 mm and the height of the draft curtain from 0.6 to 1.2 m. Two experiments were conducted with a 4-tier highly compact storage (see Table 5).

The behaviour of the fire was drastically different between the two levels of containment: a flashover happened about 20 minutes after the fire started when the building had the highest level of containment. That moment corresponded to the peak of the heat release rate which neared 40 MW and was the highest value of all scenarios. The rapid propagation due to the flashover can be seen in the series of major events shown in Figure 9 which happened within the first 20 minutes compared to 60 minutes Distance from the building (m)

Figure 8. Effect of the number of tiers on the radiative heat in the case of a lower containment. The fire was also observed to be more homogeneous than for any of the other scenarios with a flame that had a shape closer to a parallelepiped whilst in the other cases, the flame had a triangular shape. The humidity rates of the two scenarios under investigation here were quite different. The propagation of Distance from the building (m)

Figure 11. Effect of the level of containment on the radiative heat the fire for the low containment case is likely to have been slowed down by the presence of humidity. However, none of the other cases, that all had a low level of containment, led to a flashover. It is thus believe that the flashover is mainly caused by the relatively high level of containment: the heat from the fire and inflammable gases accumulate in the building and at some point take fire. The radiative heat fluxes received in front of the building are much higher in the case of a high level containment than in the case of a lower containment level (see Figure 11). They are in fact the highest values measured among all the scenarios. Ten metres away from the building, the difference between the low and high level of containment is still significant, of the order of 3 kW/m 2 . This is consistent with the larger amount of heat released by the fire. It is also likely that the rectangular instead of triangular shape of the flame contributed too to the high radiative fluxes. The times at which the maximum radiative fluxes are reached correspond to the times at which the peaks of the heat release rate are reached.

CONCLUSION AND FUTURE WORK

This study showed that the storage compacity and the number of tiers both affect interactively the fire propagation and the heat radiated from the fire. The lower the compacity, the faster the propagation of the fire. However, this trend is more or less emphasised depending on the number of tiers. Reciprocically, when the storage has a low compacity, it is well ventilated and the heat release rate increases with the number of tiers as well as the fire propagation. The fire is more violent and shorter. On the contrary, when the storage has a high compacity, and is thus less ventilated, increasing the number of tiers results in a fire of similar Low High 85 26 strength that lasts longer. Reinforcing the enclosure of the building has also the effect to increase the strength of the fire, to the extent in this study that a flashover happened.

The effects of these parameters on the radiated heat away from the fire followed the trend of the heat released by the fire for the time at which the maximum radiative heat is received. However, the amount of radiative heat received does not necessary follow the trend of the heat released: it also depends on the shape and size of the flame. The latter was found to have a triangular shape in all scenarios but the case of a high level of containment. The height and width varied.

The tools usually employed to predict the thermal radiative effects of a fire on the surroundings are based on the assumption that the flame is a solid parallelepiped. It is thus expected to provide conservative, but not accurate, predictions for most of the cases investigated here. Improvement will need to be made to refine the predictions. INERIS plan to undertake full-scale experiments to check if the trends observed here are the same and to employ a three-dimensional numerical tool to predict the shape of the flame and calculate the radiative effects. Both the full-scale measurements and three-dimensional predictions will help further develop the simple and practical tools for predicting thermal radiative effects.

Figure 1 .

 1 Figure 1. Picture of the experimental set-up (of the 3-tier, highly compact storage with a low containment level)

Figure 2 .

 2 Figure 2. Sketch of the experimental set-up

Figure 4 .Figure 5 .

 45 Figure 4. Effect of the number of tiers on the fire propagation

Figure 7 .

 7 Figure 7. Effect of the number of tiers on the fire propagation

Figure 9 .Figure 10 .

 910 Figure 9. Effect of the number of tiers on the fire propagation

Table 1 .

 1 Experimental conditions for studying the influence of the storage compacity

	Compacity:
	space between		Number of	Roof	Draft curtain	Humidity	Storage	Initial mass
	the cribs			tiers		thickness	depth	rate	height	of dry wood
	No space (high) 10 cm (low) No space (high) 10 cm (low)		3 4		5 mm	0.6 m	23.2% 16.2% 20.6% 17.1%	2m 2.7 m	1850 kg 1480 kg 2623 kg 1915 kg
	40	i • i • i ' i • i • i • i •	-Low compacity, 3 shelves
				High compacity, 3 shelves
				Low compacity. 4 shelves
				High compacity, 4 shelves
	2
	a 2	! 1 V ÏJ VA 4 x -
	0	20	40	.60	80	100 120 140

Table 2 .

 2 Effect of the storage compacity on Tmax

		Compacity	Tmax (minutes)
	3tiers	Low High	40 60
	4 tiers	Low High	25 85

Table 3 .

 3 Experimental conditions for studying the influence of the number of rack tiers

	Number	Compacity: space	Roof	Draft	Humidity	Storage	Initial mass
	of tiers	between the cribs	thickness	curtain depth	rate	height	of dry wood
	2				13.3%	1.35 m	1565 kg
	3	No space (high)			23.2%	2m	2410 kg
	4		5 mm	0.6 m	20.6%	2.7 m	3303 kg
	3 4	10 cm (low)			16.2% 17.1%	2m 2.7 m	1767 kg 2310 kg

Table 4 .

 4 Effect of the number of tiers on Tmax

	Number of tiers	Tmax (minutes)
	High compacity
	Low compacity

Table 5 .

 5

		Experimental	conditions for studying	the level of containment
	Draft	Number of	Compacity: space	Humidity	Storage	Initial mass
	curtain depth	tiers	between the cribs	rate	height	of dry wood
	0.6 m 1.2 m	4	No space (high)	20.6 10.6	2.7 m 2.7 m	2623 kg 2515 kg

Table 6 .

 6 Effect of the containment level on Tmax

	Containment level	Tmax


Keywords: domino effects, jet fire impingement, experiment, radiation, soot concentration

This test campaign aims to validate the physical models concerning the thermal response of a structure to a thermal attack and to quantify the influence of the hot soots conduction in the heat transfers by testing jet fires of gas producing soots more or less.

DOMINO EFFECTS: THERMAL IMPACT OF JET FIRES ON INDUSTRIAL PIPES

S. Patej and T. Durussel Institut National de l'Environnement et des Risques, INERIS, Parc Technologique ALATA, 60550 Verneuil-en-Halatte, France; e-mail: Stephanie.patej@ineris.fr Due to accidental thermal attacks, industrial plants can be damaged or more seriously become themselves the centre of major accidents (domino effects). At the origin of these attacks, it is advisable to mention the "classical" fires such as pool fires, jet fires, but also more exceptional phenomena which generate fire balls with an intense radiation as BLEVE or Boil-Over. Therefore, as part of hazard surveys and more particularly survey of domino effects, some questions are asked about, for example:

-the relevance to consider the explosion of such or such tank as a result of the contribution of heat since a close fire considered besides, -or again the quantitative earnings expected by the implementation of materials of heat insulation or the implementation of means of cooling.

Now it concerns typical structures as pipes or tanks, there are not or few tools which are enough fine to answer the previous asked questions and at the same time there is not enough quick implementation to be compatible with the deadline constraints of usual studies.

Thus a research programme focuses on the thermal impact of different fires on industrial pipes and tanks. Its objective is to develop, to validate and to produce one or several tools of calculation satisfying needs mentioned above in order to approach in a most realistic possible way the thermal impact of fires on equipment such as the industrial pipes and tanks.

This paper presents an experimental campaign aiming to the analysis of the heat transfers being exerted on a pipe submitted at a fire jet. Thus, an experimental apparatus was set up making it possible to determine on the one hand, precisely the characteristics of jet fire and on the other hand, the thermal response of the pipe crossed by water.

To characterise jet fire, measurements of gas temperatures, gas velocities and heat fluxes are realised for three gases that are the methane, propane and ethylene and for various gas release rates. Additionally, test monitoring has also been done, making use of both infrared camera and conventional video camera. These measurements make it possible to define dimensions of jet fires, its surface emissive power as well as the hot gas velocities for then deducing from them the heat transfers received by the pipe. In the second time, the pipe crossed by water is subjected to these various jet fires and the thermal response of pipe is quantified by monitoring the pipe with thermocouples. The experimental apparatus makes it possible to vary various parameters such as:

-the presence or not of an heat insulator like rockwool, -the thermal attack (various gases and heat release rates), -the flow velocity in the pipe going from 0,1 to 1 m/s.

INTRODUCTION

Due to accidental thermal attacks, industrial plants can be damaged and become themselves the centre of major accidents (domino effects). At the origin of these attacks, it is advisable to mention the "classical" fires such as pool fires, jet fires, but also more exceptional phenomena which generate fire balls with an intense radiation as BLEVE or Boil-Over.

To better characterise thermal domino effects, the French Ministry for Ecology and Sustainable Development supports INERIS for the performance of a research program named "FREDRIC". This program focuses on the thermal impact of fires on industrial pipes and tanks. Its main objective is to develop, to validate and to produce one or several tools in order to calculate the thermal response of industrial structures submitted to major fires accidents.

Within this framework, this paper presents an experimental campaign aiming to the analysis of heat transfers being exerted on a pipe impinged by a jet fire. Ultimately,

EXPERIMENTAL APPARATUS

A first experimental campaign consisted in determining precisely the characteristics of jet fire and a second campaign made it possible to define the thermal response of the pipe impinged by a jet fire. Figure 1 presents a sketch of the global experimental apparatus. The experimental campaigns were realised within the closed INERIS fire gallery that provides a confined medium. Thus, there is no influence of meteorological conditions such as wind.

A first experimental apparatus was set up making it possible to characterise initially the phenomenon of jet fire for three various gases chosen for their different propensity to produce soot which are methane, propane and ethylene. Figure 1 presents this apparatus which is made up of a gas cylinder and a warming device of gas to allow the flowmeter to work in its range of operation. This flowmeter is used to control the exit gas velocity (nozzle diameter dj being unchanged during all the tests) in order to obtain a jet fire in stationary regime.

A mass flow rate and an exit temperature define the gas jet. Then, in order to characterise jet fire in term of geometry (lift off, flame length ...) but also radiative power, various instruments of measurements are used such as:

-thermocouples placed in the axis of the jet fire to measure the hot gas temperatures Tg along the flame, -an anemometric bi-directional probe being able to be moved along the axis of the flame to measure the hot gases velocity Ug, -radiative fluxmeters located on the sides and above the flame measuring the radiative flux, -a video camera system to measure dimensions of the flame (by visualisation).

The plan of tests is presented in Table 1 which indicates for all gases, the ranges of the gas mass flow rate m_f, of the heat release rate Q_, of the exit gas velocity Uj, of the Reynolds number Re s , of the Froude number Fr and the effective diameter of the jet D s . The Froude number is often considered to define the characteristics of jet fire.

For the second experimental campaign, the steel pipe is subjected to these various jet fires and the thermal response of pipe is quantified by monitoring the pipe with thermocouples (Figure 2).

It should be noted that measurements are done on the part of the pipe directly impinged by jet fire and symmetrically on the part located in the drag of the flame. The pipe has the following characteristics: internal diameter 22 mm and external diameter 34 m, that is to say a 6 mm thickness. The installation is made of three sections of measurements, the section B located in the axis of jet fire, section A 10 cm upstream and section C 10 cm downstream. The Gas 2). In addition, a collector was also necessary to place the whole of the connectors, and also to ensure the clearing of the extension cables towards outside (Figure 2). An internal fluid that is water at ambient temperature flows in the pipe. This water flow can be modulated until obtaining a maximum mass flow rate of 12 kg/min, that is to say a velocity of 0,5 m/s. The temperature in water is measured using thermocouples upstream and downstream from the sections of measurements. Measurements are done with a rate of 2 seconds acquisition and make it possible to obtain an evolution of the steel pipe temperatures as well as the water temperatures. The variable parameters are as follows:

Methane (CH 4 )//= Propane (C 3 H 8 )/,= Ethylene (C 2 H 4 )f s = 18,9% 17,6% 17% m_f (g/s) 1,02-3,81 1,23-5,31 1,32-6,27
-the jet fire (various gases and the variation of heat release rate), -the water flow within the pipe (water velocity going from 0 to 0,5 m/s), -the presence or not of an heat insulator like rock wool. their important soot concentration. Indeed, the ethylene and propane have a greater propensity to produce soot (see thef s fraction in Table 1).

The fraction of combustion energy radiated h r is important in the calculation of the radiative flux received by a target. To estimate it in experiments, a calculation is carried out while being based on the values of radiative flux given by the fluxmeters.

Figure 4 shows the fraction of combustion energy radiated h r and the radiative power Q_ r (Q_ r = hrQ_) according to the Froude number Fr.

In a general way, the radiative fraction tends to decrease when the gas jet velocity increases. This observation proves to be a recognized characteristic of the turbulent jet fires rather controlled by the forced convection induced by the gas jet. Even if the fraction of combustion energy radiated h r is reduced, the radiative power Q_ r increases with the exit gas velocity. Indeed, more the mass flow rate grows and more the flame is going to radiate.

In addition, the radiative fraction of the methane flame is systematically lower than the radiative fractions EFFECT OF SOOT ON RADIANT ENERGY Whatever the gas tested, the visualisation by video camera shows that jet fire takes a general form of cone (Figure 3) which characterises fully turbulent jet flames whose the Reynolds number is greater than 2000 (Table 1). But the structure of flame differs according to gas. Indeed, the methane flame compared to the propane and ethylene flames has a blue aspect and is very lifted off from the nozzle. The blue color of this flame is characteristic of the radiation in the field of visible of the carbon dioxide and the water vapor. The methane flame is generally considered as a nonluminous flame and soot radiation can be neglected [START_REF] Marracino | Radiation modelling in non-luminous nonpremixed turbulent flames[END_REF]. The soot production in a methane flame is very weak even null under our conditions of tests. At the opposite, the propane and ethylene flames known as luminous are characterised by their yellow color because of of propane and ethylene. Indeed, the radiation of a methane jet fire comes exclusively from the water vapor and carbon dioxide which are bodies known as "semi-transparent". Their emission is very weak compared to the emission of a black body. Conversely, ethylene and propane jet fires have important concentrations of soot, particles radiating as black bodies. Moreover, ethylene jet fire has a radiative fraction more raised than propane one. The radiation of the propane and ethylene flames is due mainly to their consequent soot concentration. The alkenes (ethylene) have a greater disposition to produce soot than the alkanes (propane) [START_REF] Rasbah | Fundamentals of smoke production[END_REF]. The more the flames are concentrated out of soot and the more they tend to radiate until they are saturated with particles. It is completely logical to obtain a radiative fraction of propane flame weaker than ethylene one.

EFFECT OF SOOT ON HEAT TRANSFERS TO THE PIPE

The first tests realised with jet fires put forward the differences existing between tested gases in term of soot concentrations. The second experimental campaign based on the jet fire impingement of the steel pipe apprehended how jet fire interacts with the engulfed equipment.

With this intention, Figure 5 presents the thermal response of the pipe impinged by the flames of three gases (methane, propane and ethylene). The pipe is located at the centre of jet fire and has a water flow of 12 kg/min. The methane mass flow rate is of 3,81 g/s, that of the propane of 4,48 g/s and that of ethylene of 2,48 g/s (Froude number is the same for propane and ethylene).

First of all, an observation can be made as the temperature of the pipe rises very fast. This is caused by its thermal inertia which is very weak. This weak thermal inertia is due mainly to the thermal diffusivity of the steel which is high and about 1,55.10 m /s. Consequently, in less than one minute, the pipe subjected to the methane fire reaches a thermal equilibrium characterised by an asymptotic temperature of 1208C. This thermal response is logical since with thermal equilibrium, absorbed (contributions by radiation and forced convection) and evacuated (losses by forced convection of water) heat fluxes by the pipe are compensated. It should be noted that this is valid only if absorbed and evacuated heat fluxes remain constant. The pipe impinged by propane and ethylene flame does not reach a thermal equilibrium. Indeed, after having reached a temperature peak, the thermocouple returns a temperature which decreases during time instead of stagnating. The forced convection induced by the water flow remaining unchanged during the test, only a modification of the heat absorbed by the pipe can be at the origin of such a phenomenon. The photographs taken after tests showed that the methane fire had caused an oxidation of steel due to the condensation of the combustion steam coupled with the high temperature in the flame. The formation of this ferric oxide residue did not have any impact on the thermal response of steel. On the other hand, after the propane and ethylene tests, it appeared on the pipe a consequent soot deposit. Thus, we deduced from it that this deposit was at the origin of the temperature decrease. This phenomenon is called "thermophoresis" and corresponds to a laminar transport by which the particles (soot) go upstream a temperature gradient. Indeed, the opaque particles (strongly Time (min) Figure 5. Evolution of the thermocouple directly impinged by jet fire and photos showing the consequences of jet fire absorbing) follow usually the heat gradient while escaping from the hot zones to go towards the cold zones. Soot would come to deposit preferentially on the cold pipe and to insulate the pipe thermally as the deposit thickness evolves in the course of time. A test carried out over one forty minutes duration showed that the pipe could go down until a temperature of 408C after having reached a temperature peak of more than 1408C. The soot particles always remain in place condensing and agglomerating on the pipe in thin layer then in the form of aggregates.

How soot can create such a protection? The pipe emissivity changes while passing from 0,2 to 0,95 because of the formation of the soot deposit which radiates roughly as a black body [START_REF] Mcenally | Soot volume fraction and temperature measurements in laminar nonpremixed flames using thermocouples[END_REF]. This modification generates an increased heat loss. In same time, soot absorbs heat due to the radiation and the forced convection but restore only a part with the pipe by conduction. An interstitial medium must remain between soot and the pipe inducing an imperfect contact and thus a thermal resistance of contact, the heat transfer by conduction is carried out less better. Moreover, this soot deposit plays the same role in the long term as a heat insulator because of a very low thermal conductivity.

In addition, this phenomenon which can be characterized by the relationship between the temperature decrease AT and the time over which it is carried out noted DT must be related to the soot quantity produced by jet fire. DT Figure 5 shows that for propane, D TDso ot s -2,78C/min and A7 ^°a = 10,88C/min. However, for these for ethylene, D two tests, the gas velocity and gas temperature are identical (Ug = 8,5 m/s and Tg = 10008C) as well as the radiative power which is 40 kW. If the radiative power is equivalent, the quantities of soot provided by the two flames are about equal. Therefore, the thermal decrease should be similar between two gases and yet, there is a factor 4 between the AT two -ff 20 -. This can be explained by the fact of a different granulometry between ethylene and propane soots that implies a more or less insulator soot deposit.

EFFECT OF HEAT INSULATOR ON THE THERMAL RESPONSE OF THE PIPE

The system of heat insulator is tested in order to consider its effect of fire protection on the pipe. The heat insulator used for the tests consists of a rock wool layer 3 cm thickness. Figure 6 presents two tests carried out, one concerning the impact of the methane fire (m_f -3,81 g/s) on the insulated pipe and the other the impact of the ethylene fire (m_f -4,91 g/s). The insulated pipe is placed in the centre of jet fire and is crossed by a water flow of 2 kg/min.

First of all, it should be noted that thermal equilibrium is reached at the end of 3-4 minutes whereas without heat insulator, less than one minute is enough. This additional time is due to the high thermal inertia of the heat insulator caused by a low thermal diffusivity of about 8.10 7 m /s. Indeed, the heat insulator tends to diffuse heat more slowly. Then, the very low thermal conductivity of the heat insulator of 0,08 W/mK makes it possible to maintain a maximum temperature of the pipe of 408C when the methane jet fire impinges this one. The same test carried out without heat insulator shows that the pipe reaches a maximum temperature of 1408C. The heat insulator decreases the temperature of 1008C that is to say a reduction in the heat transmitted to the pipe of 70%. Consequently, a system of heat insulator is completely eligible as fire passive protection of the equipment. Thus, it can be considered like a means of reducing the risk of rupture in the survey of domino effects.

ETHYLENE METHANE

Heat insulator during and after methane test Heat insulator after ethylene test Figure 6. Tests carried out with the insulated pipe impinged by methane and ethylene jet fire On the other hand, this heat insulator can undergo deterioration during the thermal aggression. Indeed, the photographs show it, during the test with methane, the binder constituting the heat insulator started to burn. With the end of the test, this binder being partly consumed, the structure of the heat insulator is found somewhat faded on the surface. At the opposite, while looking at the photo of the heat insulator after ethylene jet fire, one realises that the rock wool was not degraded and that a soot deposit came to be formed with the flame impingement. And yet, the heat release rate of the ethylene fire (Q = 232 kW) as well as the temperature in the flame (Tg = 11008C) are higher than those of the methane fire (Q = 191 kW; Tg = 9008C).

Thus, soot condensation on the heat insulator by thermophoresis could be at the origin of this different rate of deterioration. The graph of the temperatures shows that with thermal equilibrium, the temperature located in the jet fire drag is more consequent than at the impingement. In the case of the methane fire, these temperatures are identical with a few degrees because, under normal conditions, the heat insulator must restore heat towards the pipe in a more homogeneous way. Soot would act like an additional insulator protecting a part of the heat insulator from the thermal aggression. The phenomenon that appears with the soot deposit is the same one as observed with the pipe without heat insulator. Except any decrease of temperature is not observed, because of the heat insulator thermal inertia which is stronger than steel one.

CONCLUSION AND FUTURE WORK

Two tests campaigns were carried out. One related to the characterization of jet fires and the other the thermal response of a pipe subjected to these jet fires. In this paper, only the effect of soot was investigated.

The tests about jet fires showed that a high molar mass of fuel did not grant a better propensity to produce soot. Propensity is related to the chemical nature of fuel such as alkenes have a better propensity than alkanes. The alkenes flames tend to radiate more than alkanes ones. Thus, the radiative fraction model will have to take into account the soot concentration of the flame. To supplement these investigations, the experimental data will be compared with the existing models of jet fires calculation which are the SHELL model [START_REF] Chamberlain | Developments in design methods for predicting thermal radiation from flares[END_REF] and the API RP 521 model (API, 1997).

Concerning the thermal response of the pipe subjected jet fire, it appears that soot comes to insulate the pipe by a phenomenon of "thermophoresis". The soot deposit plays the same role than a heat insulator. When the pipe is insulated by rockwool, the soot deposit protects rockwool from the jet fire heat. Soot acts as a second heat insulator. Ultimately, this second tests campaign aims to modify the physical models concerning the thermal response of a structure subjected to fires. It will be necessary to determine the thermophoretic soot mass flux settling on the equipment as well as the increase thickness of insulating soot in the course of time. The modification of the physical models will be the subject of the other papers.

Figure 1 .

 1 Figure 1. Sketch of the experimental apparatus

Figure 3 .Figure 4 .

 34 Figure 3. Photos of different gas flames

Table 1 .

 1 Characteristics of jet

			fires tests
	_(kW)	Uj (m/s)	D1s (mm)	Re,=^(-)	Fr = wS-ï
	51-191	25-93	6,6	10000-37330	9600-134030
	62-296	11-47	10,9	27150-117200	1130-21060
	57-246	18-87	8,7	18790-89260	3970-89530

D s : Effective diameter of the gas jet such as: D S = dj /5*.

f s : Fraction which represents the fuel mass fraction at which carbon particles begin to form[START_REF] Beyler | Fire hazard calculations for large open hydrocarbon fires[END_REF].


Carole Dautun
email: carole.dautun@ema.fr

Jérôme Tixier

Jean Chapelain

François Fontaine

Gilles Dusserre

CRISIS MANAGEMENT: IMPROVEMENT OF KNOWLEDGE AND DEVELOPMENT OF A DECISION AID PROCESS

Keywords: crisis management, industrial risk, lessons learnt, decision-making aid

teaching and research institutions in France or abroad, or from public or private research centers.

INTRODUCTION

The major risks which are caused by man made events or natural events, are always inherent in our modern's societies but their apprehensions have changed at the beginning of the eighties with the first researches on the subject. The catastrophic events of the last years such as the industrial disasters in 2001 in France or the various terrorist's attacks have brought about some changes in the concept of risk by integrating dimensions of crisis and threat in a more important way. Nowadays, the European and French policies point out the necessity of the improvement of crisis management in the aim to deal correctly with these news and delicates situations and to protect the populations. The objectives of this study is to propose the development of an expertise usable in a continuous way, to anticipate on a trigger event while detecting the warning signals but also to avoid a crisis of great scale by taking into account the aggravating factors. This methodology of decision making aid will help the decision-makers to anticipate a crisis but also to manage a crisis of great scale by the assessment of a crisis potential index.

The paper is organized as follow. The first part, through a state of the art, puts forward the characteristics of a crisis in the aim to increase the knowledge about this particular situation. Then, the paper presents the different means to learn about a crisis and a case study to illustrate this purpose and the last part will strive to provide the principal phases of the methodology of decision-making aid.

CRISIS CHARACTERISTICS

As for the risk, the notion of crisis is a concept due to the fact that each actor taking part in the crisis management has a different vision according to their perception and their domain of competencies. Indeed, a same situation can be perceived as a crisis for some of them but also, as a simple emergency situation for others. Several definitions of the crisis exist and this notion is used and abused [START_REF] Shrivastava | Crisis theory/practice: towards a sustainable future[END_REF] in several domains. Some characteristics reported in the literature allowing to defined a crisis as a situation due to a sudden, surprising and unexpected event (Boon, 2006), called also triggering event that the probability of occurrence is low [START_REF] Mitroff | The structure of man-made organizational crises -Conceptual and empirical issues in the development of a General Theory of Crisis Management[END_REF] and that causes severe consequences on human, environment, properties, large economics and socials cost [START_REF] Shrivastava | Crisis theory/practice: towards a sustainable future[END_REF]. But the main characteristic of a crisis lies in the impact on an organization which presents some difficulties to manage them. Indeed, the decisionmakers must face to negatives effects as the stress, the lack of information, several uncertainties and a great complexity in the roots causes, in the consequences and in the dynamic of the extreme situation.

A crisis can be understood as a process (Roux Dufort, 2005) that takes place in space and time. In this study, two types of crisis can be defined as an abrupt crisis (1) versus a cumulative crisis (2) in comparison with an accidental situation (3) (Figure 1).

1. The abrupt crisis results from a trigger event, of which the kinetic is instantaneous and of which the hazard potential is high, causing a crisis that develops under the actions of aggravating factors. 2. A contrario, in the case of a cumulative crisis, the situation appeared in a gradual way (during several days or months), after a trigger event, under also the action of aggravating factors. 3. In the case of accidental situation, an immediate response takes place. The management of the situation is without main difficulties. This type corresponds to a classical process of emergency. Various numbers of stages can be defined allowing understanding the crisis process

Vigilance

Emergency

Quantity of difficulties

Cumulative

• The warning signal or incubation period [START_REF] Turner | Man-Made Disasters[END_REF]: This period corresponds to an accumulation of little events or incidents that can be advertised if the organization is able to detect the warning signals.

They correspond to indications that can inform about the occurrence of a trigger event or a crisis. Two type of signals are defined as the weak signal (information imprecise and early that the quantity is poor but that the time of anticipation is greater) and the high signal (quantity of information is important but the time of anticipation is reduced). Generally, various means of detection or alert are set up to inform the population but also the employees of industrial plants of the occurrence of a trigger event. But several crises can be preceded of warning signals but it is noted; after an analysis of lessons learnt from past, that some signals have not been detected or no take into account in time.

• The trigger event: The nature, the intensity but also the numbers of event can initiate an abrupt crisis or a cumulative crisis. As soon as possible, different emergency plans are prompted. A command chain and a crisis organization are set up in order to deal with the situation and to limit the consequences. An emergency phase takes place (Cf. Figure 1). • But, under the action of aggravating factors, the accidental situation can amplify in different steps of time either in abrupt manner (1) or in a slower way (2). They correspond to parameters that amplify [START_REF] Gatot | Crise et cognition: Conception d'une gestion préventive[END_REF] an initial situation to lead to a crisis. They can be many origins as, for example, technological (collapse of communication), organizational origin (lack of coordination). These parameters contribute to create a complex situation and to affect the decision aid process. The decision-makers have to implement news actions and news plans according to these news parameters in order to quickly get out of the crisis. • Other important phase corresponds to the resolution and the end of management. During this transition phase before the return to a normal situation, decision makers should maintain their vigilance and even should strive harder than ever in order to avoid the occurrence of news events inducing a new crisis situation [START_REF] Lagadec | La Gestion des Crises: Outils de réflexion à l'usage des décideurs[END_REF].

For this reason, this study aims to develop a decision-aid process in order to improve the crisis management but also to reduce the impact of the situation on the decision process. The increase of performance must be based on the lessons learnt because that allows to determine and integrate the warning signals, the aggravating factors but also all the negatives and positives parameters that contribute to create a crisis.

HOW LEARNING? LESSONS LEARNT, EXERCISES AND SIMULATIONS

The risk management and particularly the industrial risk assessment can be based on the lessons learnt after an event but this practice is not systematic for natural hazards [START_REF] Ayral | Feedback analysis from flash floods: utilities for fire-fighters management and floods forecasting. The case of the 2002 South East flooding in France, Loss prevention and safety Promotion in Process Industries[END_REF].

The lessons learnt after an accident or a crisis take place in different step of time.

• Immediately after the end of the event, generally one week, a first study is conduced in order to collect the first impressions, positives or negatives, of the actors but also to list damages or indirect consequences [START_REF] Ayral | Feedback analysis from flash floods: utilities for fire-fighters management and floods forecasting. The case of the 2002 South East flooding in France, Loss prevention and safety Promotion in Process Industries[END_REF]. This step can be named the immediate lessons learnt or lessons learnt on the spot.

• The second step of learning takes place several months later. These studies are more complete and have for objectives to analyse the roots causes of the accidents, to describe all the consequences, to try to find the aggravating factors and the warning signals, to highlight the failures, in the aim to improve, in the future, the anticipation and the detection of a crisis, but also to put the organizational failures, in an obvious, without trying to seek responsible.

Beyond the operational aspects that are generated by a crisis, the lessons leant is essential in order to put forward the factors which led to this situation. Indeed, lessons learnt allows to highlight the positive and negative aspects in order to capitalize the knowledge acquired during the crisis, thus to prevent the occurrence of a forthcoming crisis, to improve the vigilance by detecting warning signals [START_REF] Wybo | Mastering risks of damage and risks of crisis: the role of organizational learning[END_REF] and trigger event of a crisis but also the aggravating factors.

The learning can be made also through exercises or simulations of accidental scenarios allowing to improve the coordination between the varied actors via an increase of knowledge of "With which I must work? How must I work?", to test the operational techniques, the emergency plans and to prevent in fine individual error [START_REF] Crichton | Training for emergency management: tactical decision games[END_REF].

So that the learning about crisis was efficiently, the exercises and the lessons learnt must integrated the various characteristics of the crisis that is a description of the accident and the consequences but also the weak signals and aggravating factors in the aim of not starting again the last errors and to capitalize the knowledge as in the following example.

ILLUSTRATIVE CASES STUDIES

The 21 st September, at 10h17, a terrible explosion occurred in Toulouse generating the greater catastrophe in France of the last years.

ACCIDENT DESCRIPTION

The explosion took place in a warehouse used for a temporary storage of "off-specifications" ammonium nitrate. The mass of the substance was estimated by INERIS (Dechy, 2004, a) in a range of 20-40t of TNT that is the equivalence of 20-120t of Ammonium nitrate involved as reactant. The roots causes of the explosion haven't found an agreement among investigators and a controversy about the ignition source of the ammonium nitrate stored.

CONSEQUENCES OF THE ACCIDENTS

The extent of the damages following this terrible explosion is impressive principally on the population of Toulouse and on the buildings.

Human effects

According to the different report [START_REF] Invs | Conséquences sanitaires de l'explosion survenue à l'usine[END_REF]Dechy, 2004,a;[START_REF] Inesc | Explosion AZF de Toulouse-21[END_REF], the first conclusions have identified as 30 fatalities which 21 on the site and 9 off-site, about 9 000 peoples injured with hospitalization (about 860) or medical care The sanitary consequences, in long term, on the population were principally due to three effects: toxicological -transient eyes effects and respiratory effects for the people leaving nearby-, traumatic -blast effects due to the overpressure causing three types of lesion as body wounds (418 persons), eyes injuries and ear traumatism (522 persons) -and physiologic -post-traumatic stress (5 600 persons), psychotropic treatment, depression - [START_REF] Invs | Conséquences sanitaires de l'explosion survenue à l'usine[END_REF].

Material effects

The extent of the materials damages in the city of Toulouse was very important and was estimated of 1 500 million Euros by (Dechy, 2004, b) with 75 000 notifications of disaster including about 30 000 housings and 12 000 families have to be rehoused, several schools and universities were damaged and required to be built again and about 1 300 companies were damaged causing about 7000 persons unemployment.

THE CRISIS MANAGEMENT: EXAMPLES OF AGGRAVATING FACTORS FOR THE ACTORS

During the first days of the crisis, about 1500 persons were mobilized to manage this extreme situation.

Several problems have emerged and have contributed to lead to a complex situation. In the aim to understand recurring problems in crisis management, some examples of difficulties encountered by firemen, Mobile Emergency Medical Service, policemen, are exposed [START_REF] Inesc | Explosion AZF de Toulouse-21[END_REF].

• Problem for the location of the accident area due to a large scale of shock wave impacts and the several calls for terrorist attacks in the centre of the city. Indeed, the explosion has occurred one week after the World Trate Center generating a climate of worries. • Collapse of the entire communication network during 10 minutes for the emergency call and during 8 hours for the rest of the city cutting of Toulouse from the France. • No adequate protection for the first firemen and no gaz detector for toxic cloud • Overloading of the radio networks leading to difficulties for emergencies services to communicate, to transfer information's. This is created a slowdown and a nocoordination of the organization on site. The overloading of the communications ways has also aggravated the management of the accident. • Imbalance between the means and the needs requiring emergencies reinforcements that are arrived 12 hours after. • Definition in emergency of a new strategy of injuries management to deal with the extent of the accident.

What lessons can be learnt about this dramatic accident? This extent of this event has demonstrated a limited response capacity of the first aids (or rescuers) and has highlighted several gaps as a lack of preparation for crisis management when the risk is unknown, a lack of information about the nature of the explosion, difficulties to identify an unknown risk, difficulties to manage the flows of circulation, problem of transmission of information, the lack of emergency-plan for the policeman and some difficulties in hospital for managing a massive surge of victims.

Several lessons and news regulatory plans were created after this greater catastrophe as:

• Reinforcement of emergency response and planning • Reinforcement of risk assessment • News regulation in European Union and in France This case allows defining several aggravating factors can be proposed as for the communications: the default in alert, the collapse or rupture of networks, the lack of communication being able to create a lack of coordination between actors. This dramatic event allows to define and to target the essentials information's useful for the anticipation of various problems that an organization could face during a crisis.

The lessons learnt after an accident are essential in order to highlight the negatives and positives causes and consequences of the crises but also to seek the warning signals and the aggravating factors in the aim to improve the management thought a decision-making aid process.

A SOLUTION TO ANTICIPATE AND MITIGATE A CRISIS: THE DEFINITION OF A DECISION-MAKING AID PROCESS OBJECTIVES

The aim of this study is to propose a framework allowing to the crisis manager, within the case of industrial, natural or intentional crisis, to collect and analyse the information's that coming from their environment, in a continuous way, 24 h/24 h. This methodology will allow an anticipation of changes caused by an event that potentially can produce a crisis while reducing their vulnerability and the uncertainty of the situation.

The general objective of this study is double:

• To anticipate, in vigilance, a trigger event that can induce a crisis of great scale. • To mitigate an important crisis during the emergency phase by detecting the aggravating factors that can amplify the initial situation.

To be done, several indicators will be defined in the aim to establish a level of crisis allowing informing in a clear way, the manager.

This methodology allowing establishing a potential of crisis, is composed of two stages (Figure 2):

Stage 1: The characterization of the crisis system. Stage 2: The definition of indicators and the definition of the potential of crisis. This stage corresponds to the organization of the information, to the definition of the indicators used to assess the potential of crisis.

THE CHARACTERIZATION OF THE CRISIS SYSTEM

The first step of the methodology corresponds to the targeting of the information's characterizing a crisis. For this, a systemic approach is used to structure the problem. A crisis can be defined as a trigger event that impacts the stakes (the humans and their activities, the natural and built environment), the actors who set up various actions, the whole in a political and international context favourable for a crisis. Several resources (cases study, lessons learnt) are used to create different typologies (trigger events, stakes and the potential consequences, actors, weak signals and aggravating factors) that constituting the input data. These different For these, the use of a multi-criteria decision method proves to be adequate. Indeed, the decision making process in crisis management is qualified as complex because several elements of description and analyses of the process of crisis intervene in an interactive and interdependent way. So, to take the right decision in the right time, it's necessary to organize the information' s. For this, a multi-criteria hierarchical method [START_REF] Saaty | Décider face à la complexité: une approche analytique multicritère d'aide à la décision, collection université-entreprise[END_REF] is used because it brings an organization of information and appreciations which intervene in the process of decisionmaking [START_REF] Tixier | Environmental vulnerability assessment in the vicinity of an industrial site in the frame of ARAMIS European project[END_REF]. Five main steps are defined:

Step 1: Identification of criteria and elements qualifying each indicator based on the characterisation of system defined previously.

As underline previously, a crisis can be understood through a systemic approach allowing defining several sub system as the context, the trigger event, the crisis actors and their actions, the consequences on the various stakes, the weak signals and the aggravating factors. Each sub system corresponds to a general indicator. Thus for each indicator is defined according to several criterions and elements. The objective being of to anticipate a crisis through vigilance, the indicator weak signal could be characterised, for example, by criterion that put in exergue an increase of activities in different emergency services by the use the criterion "call" and "activities" for a specific event compared to a normal. The normal will be defined regardless to the activity of the precedents months or years.

After the collect of information allowing the qualification of each indicator, the following step corresponds to the organization of this information.

Step 2: Construction of hierarchical structures

In order to answer to the potential of crisis problematic, the indicators, the criterions and the elements must be organized. Therefore the step two consists to establish several hierarchical structures.

The construction of a hierarchical structure requires the creation or the identification of links between the various levels of this structure [START_REF] Tixier | Environmental vulnerability assessment in the vicinity of an industrial site in the frame of ARAMIS European project[END_REF]. Four levels are defined. The upper level corresponds to the global objective i.e the assessment of the crisis potential (Figure 3). Therefore, the quantification of each indicator (Level 2) is assessed by a set of criterions (level 3) and elements (level 4). But other hierarchical structures take place at all the levels. These structures allow then to compare the importance of each criterion with another one is evaluated by the way of binary comparison.

Step 3: Construction of function describing each hierarchical structures

From each hierarchical structure, the functions are deduced. For example, the function for the crisis potential could be defined in the following way (Eq. 1):

Potential of Crisis = a • IContexte þ b • ITE þ g • + d • IC q þ eIWS þ ZIAF (1)
With TE, trigger event; CQ, consequences; WS, weak signals; AF, aggravating factors. The weight of each indicator will be assessing by expert judgement.

The indicator "weak signal" being composed of two criterions "Call" and "Activities", the function could be defined (Eq. 2 and Eq. 3) as: Step 4: The collect of expert's judgments This step corresponds to the assessment of the priority between each criterion based on pairwise comparisons of elements. The experts assess the relative importance of an element compared to other of the same hierarchical structure through a numerical scale [START_REF] Saaty | Risk-Its priority and probability: The Analytic Hierarchy Process[END_REF]. The expert judgement is collected through the use of questionnaires. At the end of this step, each indicator, criterion and element will be modified by a weigh (corresponding, in this example, to a, b, g, d, e, and z).

I Call = a I call (F) þ b I call (MEMS) + g I call (Hl) þ d I call (...) (2)
I Activities = a I action (Firemen) þ b I act (MEMS) + g I act (H) þ d I act (...) (3)
Step 5: Validation of coherence

To validate the coherence of expert judgement, a ratio of coherence for each hierarchical structure will be calculated.

But several limits emerge and will take into account. By definition, a crisis is an only situation. The lack of information but also the abundance of information can be also a revealing indicator of crisis. Some elements present during a crisis can have any effect whereas in other situation, they can have an aggravating effect and amplify the initial situation.

CONCLUSION

The crisis management is one of the political issues of the European and French governments. Indeed, each year, a crisis of great scale impacts societies causing more and more dramatics damages on the population but also on the economy of a country. The crises correspond to dynamics phenomenon's which can be defined by an event approach (trigger event is the starting point of the crisis in the case of abrupt crises) or by a process approach (the crisis has started before and under the impact of a trigger event, increases in intensity). The improvement of the crisis management can be process by several ways as:

• An increase of the knowledge about these extremes situations through the formalisation of lessons learnt from the past and by the use of simulation in the aim to improve the coordination between actors • The definition of an expertise usable in a continuous way, to anticipate a trigger event while detecting the warning signals but also to avoid a crisis of great scale by taking account the aggravating factors via the creation of a crisis potential index. A crisis represents an opportunity to learn.

Figure 1 .

 1 Figure 1. Conceptualization of dynamics process of major accidents and major crises

Figure 2 .

 2 Figure 2. The definition of the decision making process