Christophe Proust
email: christophe.proust@ineris.fr

Proust

D Raveau

Toward Modelling of Frictional Ignition

come

Toward modelling of frictional ignition

Introduction

It is recognised that frictional ignition is still a major cause of explosions in industry with a frequency ranging from 30 to 50 % (Hawsworth et al., 2004a and b). The objective of MECHEX project is to provide new insight and data about the ignition hazard induced by mechanical contacts classically termed as "grinding", "friction" and "impact" so that some sort of classification of mechanical equipments against this specific hazard could be issued that could help to apply the E.U. directives 94/9/CE and 1999/92/CE (Hawkworth et al., 2004a). This project gathers the effort of several major european institutes. Here will only be presented the input of the authors (for a general view of the subject see Hawsworth et al, 2004a and b) mainly devoted to the physical analysis of both the dissipation of the mechanical energy into heat and of the subsequent flame initiation process.

Friction and grinding

Observations and preliminary analysis

Most available knowledge has been gained between 1930 and 1960 with main applications in coal mines. If most influencing parameters have been highlighted (for instance [START_REF] Powell | Ignition of gases and vapors[END_REF], the global phenomenology seems outstanding, again justify ing the present project.

Friction between two solid bodies is a process through which mechanical energy is transformed into heat. "Friction " and "grinding" might be understood as continuous mechanical sollicitation where both heat and effort have reached a steady state and may more or less be analysed by means of standard engineering approach. Experience reveals that heat is produced in the rubbing zone where the material is severely stretched and diffuses outwards. It is well know that the amount of heat evolved is in proportion of the applied force and sliding speed [START_REF] Kragelskii | Friction and Wear[END_REF]. The maximum temperatures are observed after the thermal equilibrium has been reached when the two rubbing bodies do not represent any more a thermal sink as efficient as at the beginning when they were cold. Local liquefaction may be observed. Fragments are often produced either because of classical abrasion (asperities being torn away) or because of « microcutting » action in which process the sliding body acts more or less as a cutting tool. These fragments are the cause of « spark showers » (figure 1). A priori, ignition may either happen at the rubbing zone or around the sparks.

Figure 1 : example of a « spark shower » fromRitter 1984

The analysis reveals that ignition may result from three different processes :

1. In direct contact with the hot rubbing zone whenever the local temperature is large enough according to the mechanism of the hot plate ignition as studied in preceding E.U. sponsored programme [START_REF] Me | Optical Techniques in industrial measurements : safety in hazardous environments[END_REF]Carleton et al., 1999). The experimental and theoretical evidence shows that the critical parameter is the temperature of the hot zone which can be linked to the standard ignition temperature of the ATEX ; 2. The power dissipated in friction may easily be of the order hundreds of Watts. If the friction is occurring in a sufficiently confined area, the mean temperature of the ATEX may rise and reach the autoignition point. This situation is well represented by the standard ignition temperature ; 3. The sparks may be a cause of ignition. However, since the initial temperature at the beginning of the flight should not be different from that of the rubbing zone, the spark are likely to constitute a preferential cause of ignition only if later in their flight, their temperature increases well above that of the rubbing zone. This may occur if the flying particles are able to burn. In this case, the ignition process may be linked to the spark ignition mechanism referring to minimum ignition energies.

A few «reasonable» data (Rae et al

Proposed physical analysis

We separate the mechanical aspects describing the degradation of mechanical power into heat from the ignition process.

The informations available in the literature constitute a sound basis to establish a model. The tangential force, F, is classically linked to the normal force, N, through a friction coefficient, f, such that F= f.N. The prediction of f is somewhat difficult but it depends mainly on the material and, in many cases, 0.2 < f < 1 for non lubricated rubbing. The power dissipated is then q=f.N.V. Jaegger (in Kragelskii, 1965)proposed an excellent mathematical treatment of the local temperature evolution of the heated zone as function of the rubbing parameters (figure 2) separating the fixed friction case where the rubbing point does not move (an axis rotating against a plate for instance) from the classical sliding friction where the rubbing zone moves (fixed slider on a rotating wheel). The problem can be solved if the mean temperature of the bodies can be known which can be done using the classical heat exchange laws with the atmosphere. The typical set of equation is then for a « fixed » friction :

with:

with :

AT f = T f -T bodv l =rc/4. v . q . R / (A . X x )
R : radius of hot spot, A friction area Xi : heat conductivity of body 1 v fraction ofheat transfered to body 1 About the production of fragments, criteria for differentiating abrasion and microcutting may be extracted from the following table. In case of microcutting only very few fragments will be produced (a chip is cut) so that the size of the particle would be given by :

-T am b = v . q / (hi. Si) ibidem for body2 V -h] . S[ . -T am b)] / [h 2 . S 2 . ( TbodY2
|dp#(A.L.p/H) 1/3 | 1/3
If classical wear is expected the size of the fragments should be between r (or e) and dp # (A.L.p/ E) 1/ 3 .

The ignition aspect at the rubbing area (disregarding burning fragments for the moment) should be looked at through the hot spot ignition mechanism which has been extensively studied during the PROPEX project. One of the major findings is certainly that, in most situations, the critical parameter is the hot spot temperature which can be linked to the standard ignition temperature not only for gaseous ATEX but also for dust-air clouds (figure 3).

Comparison against experiments

The recent data from HSL (mid-term report) and those from Ritter (1984) may be used to try and validate this approach. HSL uses a 30 cm wheel rotating against a slider (25 mm

Impacts

"Impact" would be referred to as an instantaneous action such that both the mechanical efforts and thermal phenomena are not in equilibrium. A good representation is the rebound of a flying object striking freely on a fixed target. A transient hot spot is produced in the contact zone. Sometimes, fragments are torn away from the surface.

Apparently, ignition is mostly observed at the contacting point rather than around the flying fragments ("sparks"). Possible exceptions to this general rule arise when these fragments are able to burn either in air or in solide phase ("thermite" reaction). The ignition process seems to be closer to the classical spark-type than to hot surfaces although significant discrepancies with electrical spark-type if ignition seem to arise [START_REF] Thomas | The incendivity of frictional sparks[END_REF][START_REF] Nield | The ignition of gases by the impact of light alloys on oxide-coated surfaces[END_REF][START_REF] Rasuo | Development of device and method for testing of mechanical sparks which might ignite explosive mixtures[END_REF]. The link between this ignition process and the nature of the impact is unknown and reflects the very large scattering of experimental results (table 2).

Table 2 : some mechanical impact ignition results (Dprojectile diameter; Eci incident kinetic energy)

Physical analysis

A model has been developed by the authors based on the production and propagation of pressure waves in the impacting bodies during the rebound. Upon impact (figure 6), the interface between the projectile and the target is slowed down at speed U whereas the rest of the projectile keeps on going at V, the impact velocity. The projectile is progressively slowed down by a pressure wave propagating from the contact area to the opposite extremity of the body. U is a common deformation speed of the target and of the body. This wave is reflected back at the free extremity of the body as an expansion wave leaving the material at speed V-2.U behind.

V-Before impact during impact Very large pressures (10 MPa) may appear transiently (over a few microseconds) which may result in powerful friction if the impacting bodies are allowed to rub during this time. To this purpose the bodies must have the possibility to rub against each other, implying that sliding impact should be much more efficient than normal impacts as indeed observed experimentally. Representing this short friction as a classical friction in terms of speed, forces, deformations and strains, the model is developed and provides a mean to estimate the quantity of thermal energy produced in the contact zone (Ea) and the fraction of this energy which is being really transferred into the atmosphere (Egas).

V-

Beginning of the rebound

Local temperatures, contact areas and some other parameters are also available. Ea = 8V2. Cf/ (e . (l+s) 3/2 ). sin 2a . (cos a) 1 /2. V (V/a,,). Eci

Egas = Ea . 2 . V( Vp g .c g ) /[V(A.I.PI.CI) + V( X2.p2.c2) ] |
Where:

Eci, Ea, Egas stand respectively for the kinetic energy of the impact, thermal energy produced by the rubbing impact and thermal energy transferred into the atmosphere a is the angle of impact V, ap the velocity of impact and of the sound in the projectile 8 the ratio of the acoustic resistivities of the bodies Cfthe friction coefficient Xi, Ci, pi, the thermal conductibility, specific capacity and specific mass of medium i (indice i = 1 and 2 for the bodies and g for the atmosphere)

It appears that only a very small fraction of the incident kinetic energy is really transferred as heat into the atmosphere (order of 1/1000) and that Egas is not a simple and constant fraction of Eci, which may explain at least partly the scattering in figure 6.

Given the nature of the efforts appearing in the contact zone, fragments may be produced under a "microcutting" action. This process need only a very small fraction of the kinetic energy (1/1000) and only a few chips are detached of typical size comparable to that of the contacting zone, leading typically to fragment diameters of a few hundreds of micrometers. These fragments may be launched at the sliding velocity of the impacting bodies and with an initial temperature equal to that in the contact area. If this temperature is large enough, burning of the fragments may be expected.

Classically two ignition modes are distinguished because their underlying physics are intrinsically different (Me Geehin et al, 1994; Carleton et al., 1999): explosive atmosphere may be ignited either by to continuous heating by a hot surface at constant temperature or because of a local and instantaneous deposit of thermal energy ("spark"). For very short friction or impacts such that the thermal energy has been evacuated in laps of time comparable to ignition lags (presumably milliseconds or less), it is believed that the second mode of ignition would be more representative. The literature survey has however suggested that this may not be fully true. One reason may be that for most explosive atmospheres the optimum energy delivery time to obtain a minimum spark energy leading to ignition (MIE as measured experimentally for safety purposes) is of the order of a few microseconds. If characteristic times during impacts deviated strongly from this optimum, there are some indication suggested the required amount of energy would noticeably differ from MIE in an extend depending partly at least of the ratio real/optimum "spark" time. To our knowledge these aspects have not treated extensively up to now and no general rules are available. This work pertains to a further part of the present project. Another important question relevant to burning fragments is to be able to foresee the temperature at which runaway oxidation will occur. The analysis suggests that fusion of the combustible material would be a necessary condition. Further work is also needed on the subject.

Many aspects of the above model are speculative and constitutes a critical issue that cannot be verified by existing data. A specially designed setup had been designed basically to measure the thermomechanical aspects of impacts.

Experimental investigation

An experimental setup has been built. It basically consist of a free fall column thanks to which impacts between rods of different nature and a steel plate can be performed (figure 7).

Figure 7 : views of the experimental setup

Much of the effort has been devoted to insuring a precise control of the conditions of impact and to the development of very fast thermal measurements. Temperatures are measured by means of fast IR pyrometer (15 (is rise time) and the part of thermal energy transferred to the atmosphere is derived from pressure measurements (photograph on the right in figure 7). The impact is also filmed at high speed to measure velocities and observe fragments if any. The target is a steel plate anchored on a rigid thick wood plate impeding flexion during impact. The impacting bodies are rods of different lengths (5 to 20 cm, diameter 15 mm) and different nature (steel, copper, aluminium). Typical impact velocities are in the range 5 to 10 m/s. A very good reproducibility has been obtained. Typical signals are shown on figure 8, in which the temperature is the red curve and pressure the blue one. On the pressure signal many high frequency vibrations can be seen and are attributed to mechanical waves propagating in the solids. Superimposed on them, adis tinct low vibration trace appears which is typical of thermal oscillations of the pocket of gas resulting from heat transfer from the hot spot in the friction zone. This latter part of the signal has been treated to estimate the quantity of thermal energy transferred to the atmosphere. For impact velocities in the range of a few m/s, a transient hot spot with a maximum temperature of the order of 700-800°C is produced with a typical duration measured in milliseconds. The area of the contacting zone is of the order a few mm 2 and the mass loss during the impact about 1 mm 3 . High speed films has shown that very few fragments are produced. Typical value of E gas are in the 1-10 ml range for E c i of the order of 1-10 J such that the ratio is of the order of 1/1000, fully in line with the predictions. The incidence of the nature of the bodies has been shown to be significant : softer material like copper tend to lead to larger deformations but smaller ratio Egas/Eci (three times smaller) as compared to a steel/steel configuration. This trend is coherent with the proposed analysis.

The following graphs (figure 9 and 10) show some comparisons between the calculations based on the proposed theory and the present measurements. The agreement may be judged qualitatively satisfactory, confirming the underlying physics at least about the mechanical aspects. On the ignition side however, the typical time scale in which the thermal energy is being transferred to the atmosphere is in the range of milliseconds, which is much to large to permit the use without care of MIE-based criteria for ignition assessment, but which is much too small to justify the use of AITbased criteria. This fullyjustifies a deeper insight into this ignition domain. An attempt to correlate the present modelling with available experimental data of figure 6 shows that the simple use of MIE to decide ignition occurs does show some links with the experiment if the energy transferred to the gas during the impact is at least 50 times the MIE (figure 11) and, even with this ratio, the comparison is still not very good. An analysis of this would shortly be proposed along with dedicated experiments. Friction (and grinding) are seen as established conditions of mechanical and thermal effects leading to the production of a local hot spot and of fragments with may burn inside the atmosphere. The question arising from the condition of ignition of these and related "thermite" reactions is left to a future part of the work. At for the remaining ignition mechanisms, we identified hot spot ignition and volumetric explosion when the explosive atmosphere happens to be sufficiently encapsulated around the friction zone. With rather simple engineering consideration, we reached a reasonable estimate of the underlying physics. In theory, ignition is possible whenever the local hot spot temperature in open atmosphere reaches roughly 700 °C. Enormous amounts of energy may not required for this (of the order of 1000 W for table 1) and the model does not suggest any clear limit in terms of sliding velocity provided the load is large enough and bearable by the machine. This is in contradiction with other opinions (Bartknecht, 1988) but seems to have been recently confirmed experimentally (Hawksworth et al., 2004b).

Impacti s being defined by opposition to friction to tackle situations where efforts and thermal aspects are all transient. Here again a hot spot is produced during the rebound and fragments also. The physical picture is not as obvious as for friction and mechanical pressure waves have to be invoked. A satisfactory representation of the mechanical and thermal aspect is achievable on the basis of preliminary experiments. These need to be pursued with higher velocities to reach the ignition region. But it seems that the key is inside the ignition mechanism which does not appear to have been greatly investigated. To summarise the difficulty, ignition process is not through a classical hot spot nor a standard spark but in between. Much has to be done on that particular aspect.

Figure 2 :

 2 Figure 2 : Temperature difference (AT$ of the hot spot with the mean body temperature as function of L = V.R/2 . a,w = n. q/A. p. c . V, a is the mean thermal diffusivityofthe bodies, c the specific heat, and R the typical size (radius) of the heated zone.

2 .

 2 cr elast .(r/s) 3 0,3.p.A.L.p/HTable 2 : type of wear (fragment production) with L = friction length, E = Young modulus, H = Hardness, <J e iast=yield strength, r and s radius and height of the asperities

Figure 3 :

 3 Figure 3 : correlation between the standard ignition temperature and the critical hot spot temperature for ignition for various ATEX

Figure 5 :

 5 Figure 5 : comparison ofprediction and experimental data on frictional ignition

Figure 6 :

 6 Figure 6 : deformations during normal impact

Figure 8 :

 8 Figure 8 : typical signals resultingfrom the impact between a steel rod and the steel plate

Figure 9 :Figure 10 :

 910 Figure 9 : hot spot temperatures for steel rods

mm x 8 mm 7 mmx7 mm 6mmx 6mm 6 mmx 6 mm 25 mm x 25 mm 25 mm x 25 mm V (m/s) 1,6 5,3 10,6 16 4,6 9,2 N(N) 444 to 727 115 to 222 53 to 115

				., 1959,	1960) have been extracted from the literature for
	validation purposes (table 1).
	Sourc	ATEX	Wheel	Slider		Rubbing
	e					zone size
						(mm)
	RAE RAE RAE RAE	Methane-air Methane-air Methane-air Methane-	Sandstone : dia. 0.3 m, thickness 30 mm Sandstone : dia. 0.3 m, thickness 30 mm Sandstone : dia. 0.3 m, thickness 30 mm Sandstone : dia.	Limestone cylinder : dia. 20 mm, height 20 mm Limestone cylinder : dia. 20 mm, height 20 mm Limestone cylinder : dia. 20 mm, height 20 mm Limestone cylinder : dia.	8 53 to 115
		air	0.3 m, thickness	20 mm, height 20 mm
			30 mm
	RAE	Methane-	Steel : dia. 0.3 m,	Steel cube : side width =	1300 to
		air	thickness 30 mm	25 mm		2070
	RAE	Methane-	Steel : dia. 0.3 m,	Steel cube : side width =	500 to
		air	thickness 30 mm	25 mm		1300

Table 1 :

 1 a few representative test results (Vis the sliding speed and N the normal application force)

  ). A comparison of the predicted and measured rubbing temperatures (Hawksworth et al., 2004a) is proposed on figure 4.The comparison is very encouraging bearing in mind that the rubbing area has been deduced from the experiments. The difficult point to solve is to be able to foresee this parameter from the theories of contact and to verify the effect of the size of the rubbing system which apparently affects the ignition risk. Work is currently under progress.

	700
	600 •
	500
	400
	300						-T/C Mild steel measured
							-T/C Mild steel predicted
	200
	100
	O.OOE+OO1.00	1.00E+06	2.00E+06	3.00E+06	4.00E+06	5.00E+06	6.00E+06	7.00E+06
	Product p.7000 T"
								• Experiment s
								• Predictions
	Sandstone	Sandstone	Sandstone	Sandstone Steel wheel Steel wheel
	wheel on	wheel on	wheel on	wheel on	on steel	on steel
	limestone	limestone	limestone	limestone	slider V=4,6 slider V=9,2
	slider V=1,6 sliderV=5,3 slider V=10,6 sliderV=16	m/s	m/s
	m/s		m/s		m/s	m/s

V

Figure 4 : measured and predicted rubbing temperatures from HSL data (present project)

About the size of the fragments, the results of Ritter, obtained at moderate loads (10 bars), suggest a classical abrasion and the size of the fragments, a few tens of microns, seem fully in line with the approach suggested above.

The remaining question is to see if the global model is able to reproduce the experimental data pertaining to frictional ignition (data of table

1

). This comparison is proposed on figure

5

and seems again favourable.

Acknowledgement

The authors would like to thank the E.U and French Ministry of Research for their support and are grateful to the other participants of the project inside and outside INERIS for enjoyable and fruitful cooperation.


Ch Proust
email: christophe.proust@ineris.fr

EFFEX: A Tool to Model the Consequences of Dust Explosions

EFFEX : a tool to model the consequences of dust explosions

Introduction

Dust explosions still represent a major threat. Some accidents, such as the explosion of the silo in Blaye in France (August 1997, 12 victims : [START_REF] Masson | Explosion d'un silo de céréales[END_REF]), remind us that explosions of combustible dust can really have serious consequences, not only for the industrial installations concerned but also for the environment.

The necessity of being able to estimate the effects of an explosion of this type became apparent at the beginning of the 1990s. Owing to the complexity of this subject which involves the mechanical resistance strength of structures, flame propagation and fluid mechanics, etc., a simulation tool clearly had to be developed. EFFEX originates from this need and was built brick by brick within a few years, beginning by a flame propagation module (the core of the software), completed later by peripheral tools aiming at predicting turbulence characteristics, combustion rates, mechanical resistance, projection of debris and external pressure effects [START_REF] Proust Ch | Prévoir les effets des explosions de poussières sur l'environnement : EFFEX un outil de simulation[END_REF]. EFFEX is not a CFD code but a rather precise phenomenological software. As it is difficult to envision carrying out full-scale tests, the choice was made to select models which could be qualified separately and to verify overall consistency with respect to accidents. Most of the physics is derived from fundamental research. The operational version of EFFEX (version 3) contains eight modules which solve at least 40 non-linear time-differential equations. Hereafter is briefly described the physics which is being handled and data given for comparison with experimental findings. The simulation case of an accident is proposed and compared to the observed consequences. Due to its operability, this code is being widely used in France to analyse the safety of silos. Sometimes modifications of the structure of the silo are proposed, based on the simulations to reduce the consequences of a potential explosion. Hundreds of simulations have been made, the results of which have been summarised.

Modelling of the basic phenomena

Inside a vessel, a cloud of combustible particles and air is formed around an ignition source. The ignition source triggers the combustion of the nearest particles which are heated to a high temperature by the combustion and become, in turn, an "ignition source" for the surrounding particles. This creates a "combustion zone" or "flame" which propagates itself through the cloud. As it passes, the flame transforms the "cold" reactive medium (20°C) into "hot" combustion products (at temperatures of between 1,000 and 2,000°C), so that a portion of the cloud crossed by the flame undergoes thermal expansion. This generation of volume causes the pressure increase in the equipment in which the explosion occurs. If this equipment is not strong enough, it breaks and fragments are formed and propelled into the environment. The overpressure and the flame which were, up until that point, contained within the vessel, are released and, in this way, can have damaging effects. The purpose of the various modules of EFFEX is to represent these various stages.

Turbulence

In most situations, turbulent movement is caused by jets inside the items of equipment concerned, sometimes due to explosion and pressure development in the neighbouring vessels. A confined jet theory has been implemented (Hinze, 1972) to build a predictive tool ("TURBULENCE" module) with has the ability to predict even transient turbulence. Measurements have been picked up from the literature (Hauert et al., 1994) and extracted from our own measurements in large vessels (Roux and Proust, 2002) and compare very favourably with predictions not only in intensity (figure 1) but also in scale. On that respect a "k-epsilon" model (Tamanini, 1998) does not appear to be as precise.

Figure 1: comparison between turbulence measurements in realistic situations and prediction by

means of the present "jet theory" and "k-epsilon" approach [START_REF] Tamanini | The role ofturbulence in dust explosions[END_REF] The validity of predictions depends to a great extent on whether this parameter is sufficiently accurately estimated. This depends on the speed at which the flame burns the reactants (combustion rate) and, also, the rate of expansion of the combustion products. It is fairly generally accepted that the expansion rate depends almost entirely on the nature of the reactive mixture (Proust and Veyssière, 1988;Rzaletal., 1991).

As shown previously (Amyotte, 1985), the combustion rate ("burning" velocity) depends to a significant degree on the turbulence of the cloud. Fundamental work (Proust and Veyssière, 1988;Rzal et al., 1991) suggests that there are similarities with the behaviour of premixed gas flames. The turbulent combustion rate should depend on the size of the turbulent structures, their intensity and the specific reactivity of the mixture. A correlation ("COMBUSTION" module), similar to that used for premixed gaseous flames [START_REF] Gouldin | An application of fractals to modeling premixed turbulent flames[END_REF][START_REF] Peters | Laminar flamelet concepts in turbulent flames[END_REF]

Flame propagation in a vessel

The simulation module used for this purpose is called EXPBAT and includes conventional equations which can be used to describe the propagation of flames in a vessel [START_REF] Bradley | Mathematical solutions for explosions in spherical vessels[END_REF]; [START_REF] Kansa | Constant volume flame propagation : finite sound speed theory[END_REF]Perlée, 1978, Lewis andvon Elbe, 1987). Recent work has fully confirmed that these equations are perfectly applicable to dust explosions (Roux and Proust, 2002). To illustrate this point, are represented on figure 3, the measured and predicted variations in overpressure with respect to time in the completely closed "1 cubic metre" chamber (cylinder 1.2 m long with a diameter of 0.9 m and as per procedure ISO 6184/4) in the case of an explosion of corn starch dust. The calculated pressure signal is derived from the theoretical equations, using as input data the measured trajectory of the flame (ionisation gages) : a very good level of consistency is found.

Bursting of the vessel

The aim is to estimate the overpressure at which the containment is completely or partially destroyed and, where applicable, evaluating the size of fragments. In general, it is not enough to know the conventional destruction overpressure for an installation (i.e. under static load), as it is very often observed that the structure is set into vibration as a result of the transient nature of the application of pressure (Norris et al., 1959). The STRUCTURE module is used to estimate the destruction overpressure of equipment under static load. Its equations are based on INERIS experiments in this field. Classic formula representing material strength are used. In order to allow for the vibratory aspect, we can use the BIMPACT modules (which simulate the behaviour of the structure represented in the form of a Mass-Spring model) when the application of the explosion impulse is liable to trigger only the fundamental vibration mode, or CIMPACT for more complex situations (for the solution of deformation equations by a spectral method). Basically, these tools are able to reproduce the spectrum of variation of the real bursting pressure upon explosion as compared to the static case. Note that the dynamic bursting pressure might be either higher or smaller than the static value which is fully confirmed by experiments (figure 6).

Projection of fragments

The MISSILE ballistic module is used to calculate the trajectory of fragments. The representative equations are completely conventional, and lift is not taken into account. We compared the software's performances with some rare previous experimental results and had recently the opportunity to compare with our own data (figure 7). We submitted a 10 m 2 steel panel wall to the pressure effects of starch dust explosion (100 m ). We calculated the trajectory of the fragments with the measured pressure pulses and compared to experiments. The agreement is excellent.

External pressure effects

After the failure of containment walls, the stored pressure and explosion products are released. The blast generated by bursting propels a pressure wave in the environment. The corresponding explosion energy is the containment failure energy, described as "Brode" energy, associated with conventional graphs representing the attenuation of air waves (Baker et al., 1991). Obviously, there is a necessity to take into account possible external explosion effects [START_REF] Van | Prediction of pressure and flame effects in the direct surroundings og installations protected by dust explosion venting[END_REF]. However, the level of understanding of these is still not sufficiently good and work is going on. It can however be noticed that if total rupture of the confinement is assumed, then the prediction of the pressure effects ("BRODE" module), at least in the far field, are coherent (figure 8) with the measurements, even in those situations where external explosions have been acknowledged [START_REF] Crowhurst | The external explosion characteristics of vented dust explosions[END_REF].

Procedure

For a given industrial installation, the first step is to describe the various volumes of the items of equipment (geometry, and discharge and communicating orifices). The second step consists in estimating the mechanical strength of the various sections for each item of equipment and the size of the fragments (STRUCTURE and BIMPACT modules). The third step is to imagine the explosion scenario or scenarios likely to lead to significant external effects. The TURBULENCE and COMBUSTION modules are then used in the fourth step. EXPBAT is used for the fifth step, to calculate the maximum overpressure generated and the impulse transmitted to any fragments produced. This initial value of the overpressure is used again as an input parameter in order to repeat steps 4 and 5 for subsequent volumes in which the flame is propagated. The calculation is continued in this way until the scenario is completed. Following this procedure (figure 9), the MISSILE and BRODE modules can be used to assess the external effects.

Organisation of the modules

Steps of the analysis

Performances

The purpose of the comparisons with actual accidents is firstly to demonstrate the suitability of the combination of modules to simulate the development of an explosion. This was done in the case of the several major explosions. The rather well documented (Ronchail et al., 1982) and impressive explosion of the sugar plant at Boiry-Sainte-Rictrude in 1982 has been used. The plant consisted in three large concrete cells(30 to 50 m inner diameter, 50 m high), with dome-like communicating upper spaces (concrete also). The plant was also equipped with 2 towers and an underground gallery (figure 10). The analysis revealed that the explosion began in one tower, propagated through the gallery, joined the other tower and rushed through the domes. The predictions show that the explosion overpressure raises from volumes to volumes especially in the domes where up to 3000 hPa is calculated inducing rupture. Globally the predicted damages are similar to those observed and the debris are predicted to be thrown 450 m away in full agreement with the observations. The first simulations pertaining to silos have been performed in 1992, and up to several hundreds have been performed up to now. The analysis of a single plant requires, for an experimented modeller, a few hours. We tried to extract a classification of the various silo plants on the basis of the potential consequences of dust explosions (major scenarios). Table 1 : Examples illustrating the incidence of the structure of the silo on the possible consequences of a major explosion Conclusions EFFEX is a phenomenological model able to predict the propagation of a confined dust explosion in a complex network of interconnected volumes. Internal and external pressure effects are modelled together with rupturing and projection of debris. The models are based on precise physical concepts and have been checked against experimental results step by step. The global software performance have been assessed with existing data extracted from well documented silo dust explosions. Hundreds of simulations have been performed on existing silos in France and required modifications proposed on the basis of the calculations. EFFEX is often seen as a first step of evaluation. Precise dimensioning of safety equipments like vents is done with dedicated methods and whenever a refined estimation of, for instance, pressure wave interaction with external structures is needed more sophisticated CFD software are used. The data from EFFEX are then considered as a starting point. It is planned to adapt EFFEX to gaseous explosions, following the same methodology. Several fundamental research activities have been decided to this aim including flame folding, turbulence interaction and flame instabilities.

Figure 3 :Figure 4 :Figure 5 :

 345 Figure 3: comparison between calculated and measuredpressure evolution inside the closed ISO 1 m 3 vessel for a starch dust air explosion Other comparisons were made in extremely varied configurations including, in particular, with the vessel fitted with an orifice (figure 4 with the data from Pineau et al., 1978; Eckhoffet al., 1984,1986; Roux and Proust, 2002) or with the simulated explosion originating from another vessel and travelling through a connecting pipe (figure 5 data from Lunn et al., 1996; Holbrow et al., 1996). The level of consistency appears to be satisfactory.

Figure 6 :

 6 Figure 6: Dynamic load factor (DFL = dynamic bursting overpressure/static) measured for thin aluminium membranes (0,5 m 2 ) submitted to dust explosionsof various violences

Figure 7 :

 7 Figure 7: bursting of a 10 m 2 steel panel wall submitted to a 100 m starch dust explosion and projection of debris (curve = prediction ; greyzone = max. observed throw distance)

Figure 8 :

 8 Figure 8: predicted (curve) and measured (squares) external overpressure

Figure 9 :

 9 Figure 9: schematic view of the procedure and chaining of the modules

Figure 10 :

 10 Figure 10: Simulation of the sugar dust explosion in the Boiry Sainte Rictrude plant (1982)

  has been developed and compared favourably (figure 2) to available data (Proust and Veyssière, 1988; Rzal et al., 1991; Tezok et 1., 1985; Pineau et al., 1978; Eckhoffet al., 1984, 1986; Roux and Proust, 2002).

								Predicted
								Measured
	CO Ë	il	l §	O -S CO (D Ë >>	CO o o m	.2 *5> co E 236	ÇO (t u z co o	CO tu z co o o	o È a
	Figure 2: comparison between predicted and measured turbulent burning velocities
				(starch-air clouds)

Acknowledgements

The authors would like to thank the numerous persons inside INERIS who participates in some way to the development and publicity of this software


Jean-Christophe Le Coze
email: christophe.lecoze@ineris.fr

Samantha Lim

In a recent study on learning from experience, INERIS acknowledged the organisational dimensions of major hazard accidents. The aim of this paper is to introduce the approach advocated in order to take into account the organisational aspects of accidents. This approach is based on a review of the literature on the organisational side of major accidents as well as on accident investigation methods.

Among the methods, the Accimap (J. Rasmussen, I. Svedung, 2000) was chosen to illustrate the global dimension of accident, and a method (MORT for management oversight and risk tree, Johnson, 1980) has been chosen as a tool to be developed at INERIS, with the collaboration of the Noordwijk Risk Initiative Foundation. This method integrates a normative organisational model and an approach to the accident investigation process, based on the principle of barriers. These two methods, though not completely compatible, have shown to be relevant for serving different objectives of INERIS.

Integration of Organisational Aspects into Learning from Experience: Illustration with a Case Study Introduction

The organisational dimensions of major accidents has been widely emphasised in the last decades and shown in many accident investigations, as recently stated again in the Columbia accident investigation report (2003):

"In our view, the NASA organisational culture had as much to do with this accidentas the foam. "

Therefore, along with technical issues, the organisational side should be addressed in accident investigation. However the expertise required for technical and organisational investigations are different. Organisational investigation relates more to management, human science, and system science whereas technical investigation rely more one physics, chemistry, etc, the disciplines found at the base of the engineering culture and scientific background. One can say as well that a technical investigation is very analytical and associated to reductionism though organisational aspects are rather holistic and systemic by nature.

Indeed, organisational enquiries are looking for some dynamical understanding of accident, some kind of organisational behaviour towards accident, as J. Rasmussen (1997) quoted:

"It should be considered that commercial success in a competitive environment implies exploitation of the benefits from operating at the fringes of the usual, accepted practice. Closing in on and exploring the boundaries of normal and functionally acceptable boundaries of established practice during critical situations necessarily imply the risk of crossing the limits of safe practices. Correspondingly, court reports from several accidents such as Bhopal, Flixborough, Zeebrugge, and Chernobyl demonstrate that they have not been caused by a coincidence of independent failures and human errors. They were the effects of a systematic migration of organisational behaviour toward accident under the influence of pressure towards cost effectiveness in a aggressive, competitive environment."

Enquiring these aspects means going back into the history of the organisation, into its evolution, its transformation, its context, its wholeness and several dimensions. Such an approach reveals the complexity of the organisational life. In "Managing risk, critical issues for survival and success into the 21 st century", the authors A.Waring and A.I. Glendon, illustrate very well these complexity: industrial failures can't be understood without opening the systems and without taking into account the various dimensions and interactions. The items that would need to be taken into account are suggested in this The structure of this table has already been used in a real accident investigation at INERIS. It proved to be useful for organising the information collected through the interviews and to sort out interrelated dimensions. Indeed, each of these dimensions is relevant and allows relevant insights on the accident.

Formal coping arrangements

Risk management

Risk assessment

Management systems

Approaches to change

Ideally, an organisational investigation should therefore address all these dimensions, in order to sort out what is illustrated in the figure 1.

In 1,the situation is normal, there is no accident, between 1 and 2 an evolution has led the organisation toward a major accident. In 3 there is a technical investigation in order to understand the direct causes of the accident, and the technical causes are found. Between 3 and 4 an organisational investigation is carried out to understand the dynamic of the accident, to go back from the normal situation to the accidental situation. A good example of such an enquiry is the D. Vaughan work (1996) on the challenger accident. Her work shows how the culture of the NASA organisation, through a process of normalisation of deviance, finally produced the context for the decision to launch the shuttle in spite of the probability of having a disaster. Her study took her years to collect and interpret this phenomena, according to several levels and through the help of cross disciplinary perspectives.

0

Explanations of the accident through dynamic concept like for example the " normal deviance concept" of Diane Vaughan cross disciplinary expertise integration of disicplines in order to provide insightful interpretation

Retroactive interpretation Retroactive

Filter between events and observer

Normal situation

Organisational behaviour towards accident 0 0 Description of technological sequence from enquiries bounded representation of the reality -according to the project of the observer Unfortunately, most of organisational investigations can't be carried out that way and there are levels of depth into the organisational investigation. One can represent through the use of filters these degrees of investigation, in terms of disciplinary competencies required and time as well as quality and quantity of information needed.

Levels of investigations

The figure 2 shows the levels of investigation: Technical and physical description of an accident brings information about the installation and its associated safety barriers. This constitutes the first filter. It requires technical competencies and technical type of information about the processes and products. Generally these technical description of accident -mainly often using fault trees -associate the human actions that are linked with it. These human actions are close to the accident in terms of time and space. They can be for example an operator opening a valve, an operator taking some kind of action in a control room following the alert given by warning signals. These are often part of what is called the technical analysis. These human actions are introduced in fault trees and are left at the bottom of them, for most of the time under the "human error" wording. This constitute the second filter of access to the accident. The treatment of this "error" depends on the ability of the investigator to put a context around this error in order to get into a level deeper, as a start to get into the organisational life. From filter 2 we go to filter 3.

The third filter is divided into three parts. The first part reflect the safety management activities that should be implemented, according to state of the art concerning safety management systems, in order to prevent the accident that occurred. These type of systems are normative one and include the safety related activities, like risk analysis, training, maintenance, emergency preparedness, auditing etc for all life cycles (design, commissioning, operating, maintenance, decommissioning) of the installation. Skills needed here are auditing techniques associated with understanding of safety management principles. This type of investigation requires an access to companies documents and traceability. It involves as well the interview of people to understand better the functioning of the organisation and how these safety management activities "live". The second part of this level is what has been called organisational factors as a way to go beyond the safety related activities and going a bit deeper into the way they work., more precisely with deeper assumptions on human and collective behaviour. Often defined as communication, co-ordination, risk perception, conflict, involvement, etc issues, these can be called as well as risk factors. Such a level of details requires skills related to human science to treat accordingly the necessary interviews carried out at that stage. The third part introduces the weak signal hypothesis. This assumption is distinguished from the two others because it raises the question of the ability of the organisation as a whole to generate appropriate representation of risks threatening its activity. The quantity and quality of information required is the same as the previous part.

Finally, the fourth filter would correspond to an organisational investigation that consider the organisation internally and externally with a systemic perspective in order to generate insights into the dynamic, that could be comparable to the work done by D. Vaughan for the Challenger accident. Such an approach is very time consuming and requires unlimited access to huge amount of information.

Selection of methods

This type of decomposition through filters allowed the selection of a certain numbers of methods, among which two were selected for their convenience in various respects. The first method is MORT, for Management Oversight Risk Tree (Johnson, 1980) and the second one is ACCIMAP (J. Rasmussen & I. Svedung, 2000). The advantage of MORT is its technical interface with the organisation based on barriers and control and its powerful normative safety management model. This normative model describe an activity level and a risk factor level that accordingly covers the third filter of our figure. However, such a method does not introduce explicitly environmental constraints of the companies that appears quite useful for certain purpose, for different actors (at the governance level for example). The ACCIMAP method allows graphically this, though it does not specify how to get into these dimensions, at the investigation step. It is nevertheless a very useful illustrative tool to emphasise interaction between various actors and decisions, and is as such, very close to soft system thinking techniques.

It has been therefore chosen to use MORT as a kind of very detailed guidance to get into an organisational analysis after accident and ACCIMAP for its broad perspective and graphic advantage.

An example is given in the next part of this paper. This simple example is illustrated with the ACCIMAP representation, and put in light the main safety management activities that would be investigated through the use of a MORT type of approach. That case has been enquired through extensive research on documents relating the accident like articles from newspapers, the legal judgment, administrative reports, etc.

Illustrated case

The accident investigation starts with the description of the company.

The company

The plant is located in Auzouer-en-Touraine, France about 200 km to the South of Paris. The site is skirted by the Brenne, a secondary effluent of the Loire river. In 1988, the company was in a successful economic situation, employed 150 people and manufactured diverse chemicals (about 800) used in the textile industry and the transformation of plastic materials. This plant was covered by the French law n°76-663 of July 19 th 1976 relating to the classified facilities for the environment protection. This law deals with plants and industrial facilities operated or owned by any physical or moral person, public or private which can represent major hazards either for the environment including health, safety of the people and the protection of the natural resources (agriculture, monuments, etc.).

It can be noticed that the plant was not submitted to the 1 st Seveso Directive.

The plant is divided into two areas: the storage of chemicals and the reactors and other facilities in the North of the plant (Langlais, [START_REF] Langlais | Rapport sur le sinistre de PROTEX du 8 juin 1988[END_REF]Buisson & al, 1988).

Description of the facilities and of the process involved in the accident

In continuous process, at night, the plant had a manpower reduced to a manufacturing worker, a foreman and a supervisor. The accident occurred in the zone Z, precisely in the workshop Z9. Solvents, raw materials and other reactors surrounded it.

In this workshop, the process consisted in manufacturing a waterproofing substance that had already been implemented 221 times. The process was modified in August 1987. On June 8 th , 1988, the day of the accident, the new process was used for the second time. The procedure was to mix different basic alcoholates, called Surfarox, then to add hydrochloric acid before adding silicone oil. This oil is flammable and must stay in an acid pH to avoid release of hydrogen [START_REF] Langlais | Rapport sur le sinistre de PROTEX du 8 juin 1988[END_REF]Buisson, 1988;TGI de Tours, 1992).

Safety systems and safety management

According to our documents, it seems that no specific safety device was implemented in the Z9 area.

The plant had a safety team only trained to use fire extinguishers as safety means. [START_REF] Langlais | Rapport sur le sinistre de PROTEX du 8 juin 1988[END_REF]Buisson, 1988;TGI de Tours, 1992) The accident

During the night, a supervisor and 9 workers were on site. Hired 6 months ago, assigned for a short time at his position and alone on the zone Z, an operator was manufacturing a waterproofing product with silicone oil and additives, like SURFAROX, a very basic alcoholate. The plant had realized this for a long time but the procedure had just been modified. At around 3 a.m., a hydrogen release, due to the rough addition of the alcoholate Surfarox with the silicone oil which is incompatible with bases, led to an explosion and a fire. The operator was found burnt about 10 meters from the reactor. The fire ignited about 500 tons of products, of which a significant amount of alcohol. It seems that some employees tried to fight the fire themselves at the very beginning.

It is a citizen who warned the rescue services. The nearest rescue center was informed but not the professional firemen of the city of Tours. The first means displayed at 3.20 a.m. had no effect. At 4 a.m., about ten people living in houses close to the plant were evacuated. Then, the fire gained in surface and intensity. The high temperatures eased the decomposition of the stored chemicals. The HCN, CO, NOx and halogens concentrations were checked. Only traces of CO and NOx were detected. At around 7 a.m., the fire was controlled by the firemen, requiring lots of fire-fighting facilities. The use of a large amount of foam and water prevented the fire to be propagated to close storage of dangerous products.

But the fire flow, not recovered due to the presence of natural ditches between the site and the river and no containment basin, poured into the Brenne river. During the response period, the lift pumps were still running and filling gradually the basin of 600 m 3 of the wastewater treatment plant situated on the other side of the Brenne. Thus, the effluents loaded the day before were normally evacuated from the plant.

Yet, at the end of the response, an electrical feed loss switched off the working of the lift pumps while the basin was 3/4 full. Whereas the fire had been put out for 3 hours, the restart of the wastewater plant by the company owner led to a new spill during 7 hours of 630 m 3 of toxic effluents.

The following day, dangerous chemical substances were detected in the Loire river which led the Prefect to demand the immediate shutdown of the pumping in the river. The supply in drinking water of the city of Tours and its surroundings (200,000 inhabitants) was stopped. The supplies in water were ensured in about sixty delivery points by trucks ad tank cars. Some producers closed their plants; restaurants and hotels got empty. After ten days, the inhabitants of Tours found back tap water.

ACCIMAP representation of the accident

The ACCIMAP shows the interaction between the several levels and actors and allows to illustrate the accident, with the refering letters.

Accident background

The relations between the company and the competent authorities contributed indirectly but significantly to the accident. Between 1963 and 1987, about fifty minutes were addressed by fishing guards due to pollution statements. Besides, since its foundation, the company had 9 complementary authorization orders. On several occasions, like in 1985, the DRIR 2 reminded the company of bringing efforts on the limitations of consequences after a potential accident. In August 1987, the DRIR gave formal notice to the company but the manager obtained from the Prefect a delay, as it was previously already done for the request concerning an emergency planning in December 1986. The arrival of a new Prefect in January 1988 did not make any change (TGI de Tours, 1992[START_REF] Lascoumes | Risk management in a dynamic society: a modelling problem[END_REF].

a. Thus it was clear that the administration was not inactive, on the contrary: 27 visits by the DRIR between 1978 and 1988. Nevertheless, the argumentation of the DRIR was inadequate towards the manager and towards the Prefects to obtain a regularization of the situation.

b. In the region, the company had a significant power. Besides, it received an important political support and a tolerant listening from the préfectoral authorities. Here, one deals with a politicoadministrative inertia paralyzing any action [START_REF] Lascoumes | Risk management in a dynamic society: a modelling problem[END_REF].

c. One can assume that the manufacturer, without any constraints, focused his policy on productivity to the detriment of safety: no specific safety management system was set up.

On the basis of the structure of the Safety Management System (SMS) which was not compulsory by the legislation at that time, we divided the management system of the company in 1988 into different items discussed below.

Training and organizational management

In 1980, after several accidents within the company, the Departmental Fire & Rescue Services Inspector recommended the creation of a safety engineer position and some safety systems. But nothing was done [START_REF] Langlais | Rapport sur le sinistre de PROTEX du 8 juin 1988[END_REF].

d. On the night of the accident, the operator was left alone without any supervision (TGI de Tours, 1992) whereas the manufacturing process was tricky to carry out. Many reasons for that: additional to the non-existing compensatory measures from the management, the supervisor was sick and not on duty.

e. Personnel's training was insufficient. The operator who manipulated the products has received a short training without any awareness to the risks related to the chemicals used in the processes within the company. Besides, no safety policy was carried out on site [START_REF] Langlais | Rapport sur le sinistre de PROTEX du 8 juin 1988[END_REF], TGI de Tours, 1992).

No risk analysis f. No systematic risk analysis was performed on site.

g. The site layout was inadequate: the anarchic storage of risky products led to the fire spreading, the direct contacts between the plant and the river through natural ditches led to the discharges of fire flow into the river.

h. The lack of risk analysis led to the lack of determination of technical and organizational barriers for that kind of accidental sequence: no prevention and protection systems in the very workshop, insufficient water capacity for the rescue services, no wastewater and fire flow collection network, no containment basin, insufficient separation systems between the plant and the river with dikes, etc.

DRIR stands for Regional Industry and Research Authority. In 1992, it became DRIRE with E for Environment. In the field of the environment protection, the DRIRE works for the Environment Ministry under the authority of the prefect. Its main mission is to control the industrial activities that could have an impact on environment in the framework of the regulation on classified facilities for environment protection.

Management of change

i. The new manufacturing process had several flaws: the products incorporation order had a higher potential danger than the previous one (H 2 release possibility). The material safety data sheet was not with the other manufacturing documents. This lack of dissemination of safety information was approached during the Working Conditions, Health and Security Committee sessions but no efficient measures were taken. Besides, no risk analysis (to check its influence on safety) was performed after the process was modified (TGI de Tours, 1992).

These failures associated with the lack of supervision and training contributing to worsen the poor practice of the new manufacturing procedure by the operator.

Management of emergency situations

j. The setting of an emergency planning in December 1986 was postponed to June 1987 but was still not into practice at the time of the accident [START_REF] Langlais | Rapport sur le sinistre de PROTEX du 8 juin 1988[END_REF].

Learning from experience management k. Two accidents in 1979 and 1980 occurred at night on totally different chemical reactions. No lessons were learnt and the whole manufacturing processes were not assessed at all. It means also that safety systems were insufficient [START_REF] Langlais | Rapport sur le sinistre de PROTEX du 8 juin 1988[END_REF].

Conclusion

This paper intended to present the depth of organisational investigation and the related time, quality and quantity of information as well as type of competence involved. Levels of inquiry have been suggested through the use of 4 filters, each of them going deeper in the understanding of the organisational side of accidents. This representation helped to consider the usefulness of various methods and to select two of them for what appeared to be two complementary angles. MORT is a normative model facilitating the investigation into safety management systems thanks to barriers and control interface and ACCIMAP is a graphic representation that intends to consider the system as a whole and its various actors influencing organisational behaviour.

The example chosen was quite simple to allow a quick description and illustration through the use of the ACCIMAP and the systematic kind of inquiry suggested by MORT.

Investigations has been carried out by INERIS, following such a type of mix approach and proved its interest.

Much work is currently carried out to create an approach integrating these two kind of perspectives.

Filter

  Figure 2: investigation depth, 4filters

Figure 3 :

 3 Figure 3: Description of the accident from an organizational point ofview

table ( table 1 ):

 (1

	Risk contexts
	Organisational environments
	Economies and market
	Public policy, legislation and regulation
	Social and political climate
	Technology
	History, operating territories and conditions
	Human Factors
	Culture
	Power relations, political processes and decision making
	Perception, cognition and meanings of success

Table 1 :

 1

Thematic table linking theoretical aspects of risk with empirical studies

(A. Waring andAl Glendon, 1998)


Emmanuel Ruffin
email: raphael.defert@ineris.fr

DEFERT Raphaël (Civil Engineer -Safety in Transport System)

Industrial activity requires the production and the use of dangerous goods in the chemical plants. It also requires their transport between these installations. These products thus present risks on the plants sites, but also between them, because of their transport. Today, regulations constrain chemical plants to minimise their risks. This is in this context that the GLOBAL project will examine how the logistic strategies of the industrialists can influence the risks due to the transport of dangerous goods. For example, by minimising the quantities stored in the fixed installations, does one increase the risks due to transport and in this case, up to what point? The objective of the present research is to examine new risks assessments methods of quantification that could be applied both for the fixed installations and for transport, and then bring global answers.

Global Assessment of the Technological Risks Relative to the Transport and the Storage of Chemical Products 1 Introduction

The need for development brings forth technological progress but simultaneously induces risks that cannot be completely avoided. The impact on health, environment and economy must thus be approached. One can quote the examples of Seveso, Bhopal, the tunnel fires of the Tauern and the Mont Blanc, etc. This shows the need to analyse and to assess systematically all the types of risks in order to protect the public health and to limit the environmental and economic impacts of potential accidents. However, the impacts (in term of risk) of these technologies should not be treated independently of the benefits that they bring to the society. The respective weight of the risks and of the benefits is fundamental for any decision-making process. The different risk assessment methods provide qualitative and quantitative measurements that can constitute a support for the decisionmaking. The technological risks are treated differently according to the different activities and existing pertaining regulations. Several national and international organisations use methods to be applied in various activities (chemical plants and transport). However, these methods are often specific to the sectors to which they are applied and adopt frequently various definitions, models and approaches to assess the risks.

The problematic

State of the art: a lack of harmonisation

It is obvious that there is a lack of harmonisation in risk assessment. Because of the differences of culture in the different countries and in the different fields (transport, fixed installations, etc.) that requires risk-assessment, it is not simple to find a common methodology. But it would improve the transparency in decision-making.

Technological risks are treated differently in the different fields of activity and in the different countries. For example, the use of deterministic or probabilistic methods depends on where you live and which industry you are looking at. Decision-makers have to deal with a variety of approaches to assess and manage a specific risk, which makes the comparison of risk studies performed by different organisations a difficult task.

In order to illustrate this lack of harmonisation, in the Netherlands, probabilistic risk analysis is a requirement of the safety report. The Netherlands have a clearly defined policy on the maximum levels of risk that are acceptable when considering land-use decisions. In the UK, the probabilistic approach to risk analysis is favoured, but up to now, quantitative risk criteria have been published only as far as the control of land-use in the vicinity of industrial facilities is concerned, whereas criteria for siting of new activities are being developed. In Germany, deterministic approaches are extensively used in the chemical process industry to demonstrate the quality of the measures taken to avoid risk inside and outside the installation. Safety decisions, both with regard to design and operation, are made on the basis of the potential consequences alone, independent of an explicitly defined probability.

How to assess globally the industrial risks?

The industrial activity requires the production and the use of dangerous products and goods in the chemical plants. It also requires their transport between these installations. These products thus present risks in the industrial sites, but also between these sites, because of their transport.

Today, the regulations concerning chemical plants logically constrain the operators to minimise the risks. The risks relative to the transport and to the installations of temporary storage are for their part limited by regulations such as the ADR, the RID, the IMDG, etc. But these risks are not treated by specific studies. New texts of law and European draft Directives are in hand to mitigate partially these lacks. In this context, it is appropriate to examine how the exploitation strategies of the chemical plants can influence the risks due to the Transport of Dangerous Goods (TDG). For example, by minimising the quantities stored inside the sites, does one increase the transport risks and in this case, up to what point? For this purpose, the present project will examine different quantitative risks assessment methods that, applying on chemical plants and also in the fields of transport (road transport, rail transport, etc.), can bring global answers.

3 The GLOBAL project

Context of the project

INERIS has carried out and still carries out many projects linked with the assessments of the risks due to chemical plants and transport of dangerous goods. In this context, the Accidental Risks Division carries out a program entitled "GLOBAL" which means "Global evaluation of the technological risks relative to transportation and storage of chemical products". This project was adopted by the ministry for spatial planning and environment that will bring a part of the financing.

Purpose

The purpose of the project is to examine the ways risk assessments for the systems of transport and the chemical plants are performed and then to analyse the practices of risk management based on these evaluations. The work will consist in setting up a method based on a single risk assessment principle, which will be applicable at the same time on transport activities and on storages. From the criteria of existing risk management in the two fields, the work will consist in proposing new indicators or criteria that could facilitate the arbitrages and allow the comparison of the risks. It will also consist in using the method to give a first opinion on the global benefit that can be withdrawn from strategies consisting in reducing the size of the storages in the sites.

Philosophies and methods of risk management on the chemical plants and on the means of transport are very different. However, the interactions between these fields are strong, and one cannot reasonably claim to manage the whole industrial risks and ignore one of these fields. The project suggested is innovating insofar as only few parallels and comparisons were carried out until today between these fields. Moreover, the probabilistic aspects, which could be approached in this program, are developed very little with regard to the safety reports of chemical plants.

3.3

Content of GLOBAL

The GLOBAL project contains several tasks that are listed below:

• Description of the activities of transport and chemical plants.

The objective is to describe these two types of fields, from a quantitative and a qualitative point of view: flows, means of transport, sizes of the storage, packaging, etc. For a few main products (because of their dangerous character or of their importance in the flews), the methods of production, consumption and transport will be detailed (producing and consuming areas, seasonal character, usual means of transport, etc.)

• Description of the regulation context. Indeed, it is useful to confront these two fields taking into account the regulations applicable. It will make it possible to better understand the differences that exist between the philosophies. The purpose is to point out the historic of the various regulations applied in transport (ADR, RID, etc.) and for fixed installations, then to draw up a synthetic inventory of the current regulations. In this operation, the work will not consist in reproducing in extenso the regulations applying in the two fields, but in revealing the similarities or the differences with regard to these questions of major risks.

• Learning from experience and statistical analyse of the past accidents.

In order tound erline the differences existing between the fixed and the mobile installations, INERIS proposes to undertake a compared study of the accidents resulting from these two fields. That is to say: existing databases (sources, mode of data collecting, etc.), compared uses of these bases, and compared analyse of the accidents (kind of accident, gravity, etc.).

• Current methods of risk assessment in the fields of transport and of chemical plants.

The risk management requires its identification and its assessment with a taking into account of the means able to reduce either gravity, or the frequency of occurrence. Straightaway, the methods used to identify, assess and manage the risks in the field of chemical plants and transport seem notably different. Therefore, this operation will be lead to study the different approaches of risks assessment in the two fields, in particular by presenting the methods used, the principles which justify them and the models most frequently used, by describing their fields and their application limits.

• Analyse of the various possible policies.

The purpose is to study a few cases which induced a transfer of risk from the fixed installations towards freight vehicles of dangerous goods, for example by decreasing the quantity stored in the fixed installations and by increasing the quantity stored temporarily in the bulks. It will consist in understanding the mechanisms of the transfer, judging their interest and their disadvantages, finding means of avoiding them if the disadvantages are significant, and then considering total solutions of management of the dangerous goods. At the end of this operation, several common cases, i.e. installations presenting similar logistic modes (for certain sectors of activity for example), could be highlighted. This operation could also be useful to bring together decisionmakers and industrialists to have a better vision of the existing practices.

• Global method of assessment. In this operation, the purpose consists in thinking about the conditions that would make it possible to compare the risks relative to transport and fixed installations. These activities are different and the risks assessments methods also, but to assess the total risk, a common method, simple but elaborated enough, should be found. It is particularly difficult to achieve, because if the uncertainties are too significant, this kind of method could be unusable to allow decisions. From the criteria existing for the risk management in the two fields, the aim will be to propose new indicators or criteria that could facilitate the decisions and allow the comparison of the risks. If the results of these investigations are sufficiently interesting, a case study could then be then considered.


INERIS Jean Poulleau
email: jean.poulleau@ineris.fr

INERIS Cécile Raventos

Frans Blank

Lukas Emmenegger

Richard Gould

Hakan Kassman

Emile Pilage

Serge Reynaud

Joern Rokkjaer

Michael Waeber

Uncertainty budget and interlaboratory field tests in SO2 and NOx emission measurements

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Many European laboratories are nowadays accredited to the quality assurance standard for the competence of testing and calibration laboratories, ISO/IEC EN 17025 [15]. Among other things, this Standards requires the laboratory to identify all uncertainty components of the implemented method and gives a reasonable estimation of the overall uncertainty attached to the measurement results. For the validation of methods, mainly main techniques are proposed:

-A systematic assessment of factors influencing the result and their associated standard uncertainty on the basis of theoretical principles and practical experience.

-Interlaboratory (in-field) comparisons. Furthermore, a thorough validation of the method will include field tests to verify that its performance data and calculated measurement uncertainty comply with results obtained under field conditions. Appropriate procedures for air quality measurements -whose output is a defined time average -are described in EN ISO 14956. These methods are well suited for the evaluation of emission measurement techniques. Finally, the ISO 5725-2 provides guidelines and tools to plan such intra or inter-laboratory campaigns [START_REF]Application of statistics -Accuracy (trueness and precision) of measurement methods and results -Part 2 : basic method for the determination of repeatability and reproducibility of a standard measurement method[END_REF].

Today, most CEN standards identify the major sources of uncertainty contributing to the measurand and often provide criteria for acceptance related to the main performance characteristics, as well as to the overall uncertainty. Recent standards for ambient air measurements and their concepts with respect to uncertainty have been discussed by E. Sneek [11]. A review of the recommendations given in EN 13005, the ISO Guide to the Uncertainty of Measurement (GUM)(reference?), with special focus on ambient air and stack emission monitoring has been published by R. Beier and R. Kordecki [START_REF] Beier | Uncertainty analysis in the field of ambient air and stack emission monitoring[END_REF]. This paper will focus on emission measurements and give examples of uncertainty budgets determined according to the GUM. The results will then be compared to performance characteristics obtained in inter and intra-laboratory field experiments.

The examples are taken from the work carried out by CEN/TC264/WG16 which is mandated to develop five standards for the measurement of O 2 , CO, NO x , H 2 O and SO 2 , as well as a technical report describing how to evaluate equivalent methods [START_REF]Stationary source emissions -Determination of mass concentration of nitrogen oxides (NO x ) -Reference method -chemiluminescence[END_REF][START_REF]Stationary source emissions -Determination of mass concentration of sulphur dioxide -Reference method[END_REF][START_REF]Stationary source emissions -Determination of mass concentration of oxygen (O 2 ) -Reference method[END_REF][START_REF]Stationary source emissions -Determination of mass concentration of the water vapour (H 2 O) -Reference method[END_REF][START_REF]prEN xxx, Stationary source emissions -Determination of mass concentration of carbon monoxide (CO) -Reference method[END_REF]. This paper discusses the manual reference method for SO 2 and the automatic reference method for NO x . The extensive international laboratory and field testing provide an excellent opportunity to compare the overall uncertainty calculated through an uncertainty budget (GUM) with the field approach yielding uncertainties for (intra-laboratory) repeatability and (inter-laboratory) reproducibility.

Uncertainty budget

Performance characteristics indicate the deviation from a perfect measurement and therefore describe the contribution of a single element to the total uncertainty of the measurement result. The combined impact of the performance characteristics on the measurement result is quantified by the overall measurement uncertainty, the calculation of which is based on the law on propagation of uncertainty as stipulated in the GUM. Calculating the expanded uncertainty then provides a confidence interval within which the accepted reference value is expected to lie, typically with a 95% level of confidence.

To associate the expanded uncertainty with an analytical result, the laboratory has to perform the following steps:

-Determine the analytical function relating the measured value to the input quantities.

-Identify all major sources of uncertainty contributing to any of the input quantities or to the measurand directly.

-Calculate or evaluate uncertainty components expressed as standard uncertainties of input and influence quantities.

-Calculate the combined standard uncertainty and the expanded uncertainty.

The following paragraphs briefly describe the SO 2 and NO x reference methods, each followed by an overview of the specific requirements that are given in the standards and the relevant EU directives with respect to measurement uncertainty.

SO 2 standard (manual method)

The draft European standard prEN 14791 describes a manual reference method. A representative sample of gas is extracted via a temperature controlled probe. It is then filtered and drawn through hydrogen peroxide absorber solutions for a specified time and at a controlled flow rate. The sulphur dioxide in the sampled gas is absorbed and oxidised to sulphate ion. The mass concentration of sulphate in the absorption solutions is subsequently determined using ion chromatography or by titration with a barium perchlorate solution using Thorin as an indicator. The standard describes the specific components and requirements for sampling and analysis. For the sampling system, a number of performance characteristics with associated minimum performance criteria are given as shown in Table 1. When this European standard is used as a reference method, the laboratory has to demonstrate that:

the performance characteristics of the method are lower than the performance criteria given in Table 1, and, -the overall uncertainty calculated by combining values of selected performance characteristics by means of an uncertainty budget is less than ± 20 % at the emission limit value.

The values of the selected performance characteristics shall be evaluated :

for the sampling step : by means of laboratory and field tests in order to determine uncertainty of the calibration and other parameters, and, -for the analytical step: by means of laboratory tests taking the standard deviation of repeatability calculated during an inter-laboratory comparison. A performance better than 7 % of the measured value is required by the standard.

NO x standard (automatic method)

The draft European standard prEN 14792 describes a reference method by means of a continuous analyser using the chemiluminescence principle for sampling and determining the content of nitrogen oxides (NO x ) in ducts and stacks emitting to the atmosphere. A representative sample of gas is taken from the stack with a sampling probe and conveyed to the analyser through the sampling line and gas conditioning system. Five equivalent sampling and conditioning configurations that avoid water condensation in the measuring system are described. The standard provides minimal requirements for both, the sampling system and the chemiluminescence analyser.

When this European standard is used as a reference method, the user must demonstrate that :

the performance characteristics of the method given in Table 2 are lower than the associated performance criteria, and, -the overall uncertainty calculated by combining values of selected performance characteristics by means of an uncertainty budget is less than 10% at the emission limit value, before correction on dry basis and to O 2 reference concentration. This means, that the values of the required performance characteristics, determined by adequate laboratory and field tests, have to be compared to the criteria given in Table 2. The uncertainty budget must be drawn up according to the procedures described in ISO 14956 or GUM.

Field Validation

Field validation is a valuable complementary technique to determine the overall uncertainty based on an uncertainty budget according to the GUM approach. It must be used when some uncertainty components are difficult to evaluate or when the measurement process cannot be modelled (sampling, losses in the line, leakage, etc.). Field validation can facilitate the assessment of influence parameters. Parallel (field) measurements by one or several teams often reveal the existence of systematic deviations which might not otherwise be revealed by the uncertainty budget approach. ISO 5725-2 [START_REF]Application of statistics -Accuracy (trueness and precision) of measurement methods and results -Part 2 : basic method for the determination of repeatability and reproducibility of a standard measurement method[END_REF] describes the method to be followed when carrying out such parallel measurements, and it gives the statistical tools to evaluate the repeatability and reproducibility of the method. Repeatability corresponds to a intra-laboratory validation; however, this particular evaluation does not determine if there is a bias in the result since only one laboratory is involved in the test. Inter-laboratory validation provides information about the reproducibility of a given method. Since the results are provided by several laboratories, the bias between single laboratories is evaluated. For stack measurements, the characteristics of the flue gases are not constant. It is, therefore, not possible to determine repeatability from subsequent measurements of the same sample. To overcome this limitation, the internal (repeatability) confidence interval (CIr) in the field is calculated from results obtained by parallel measurements implemented by the same team and equipment:

r 1 n 0,95; r s . t CI - = [1] r 1 n 0,95; s . t 2 r . - = [2]
where s R is the repeatability standard deviation, t 0.95 is the Student factor at a 95% confidence level and a degree of freedom of n-1, with n being the number of double measurements, r is the repeatability in the field.

The external (reproducibility) confidence interval (CI R ) in the field is determined in a very similar way:

R 1 np 0,95; R s . t CI - = [3] R 1 np 0,95; s . t . 2 R - = [4]
where s r is the reproducibility standard deviation, t 0.95 is the Student factor at a 95% confidence level and a degree of freedom of n-1, with n being the number of double measurements. R is the reproducibility in the field.

Comparison between the uncertainty budget and field test

Repeatability, i.e. the internal confidence interval (CI r ) and reproducibility given by the external confidence interval (CI R ) were determined in six field tests, performed on waste incineration installations, co-incineration installations and large combustion plants. During each field test, four different European teams performed two parallel measurements on twelve 30 min samples. CI r and CI R where calculated from all results after elimination of eventual outliers. CI r and CI R for SO 2 and NO x were then plotted for all field tests as a function of mean stack concentration.

In the following paragraphs, a comparison is made of the expanded uncertainty for SO 2 and NO x for analysers used during tests 5 and 6, with CI r and CI R from all field tests. The final two field tests were chosen because each participating team prepared its own uncertainty budget, based on a standardised protocol, which included the specific site conditions and the individual characteristics of sampling and analytical measurement devices.

SO 2 (manual method)

Tables 3 and4 provide a summary of the relative standard uncertainty for each of the influence parameters on the overall uncertainty. As can be seen, the global quality criteria of overall uncertainty is fulfilled even though lab A (5 th field test), and labs A and B (6 th field test) assume an uncertainty for analysis greater than the 7% required by the standard. For all laboratories, the major contribution to the uncertainty budget comes from the analysis of sulphate, and the second most important factor is the volume of sampled gas.

The tests performed in 6 different stacks lead to the equations: CI r = 0,08 c + 3 mg/m 0 3

[5] CI R = 0,113 c + 5,4 mg/m 0 3 [START_REF]Stationary source emissions -Determination of mass concentration of nitrogen oxides (NO x ) -Reference method -chemiluminescence[END_REF] where c is the measured SO 2 concentration, given in mg/m 0 3 . Figure 1 shows that for the SO 2 manual method the results provided by the uncertainty budget are very close to what has been determined in the field during an intercomparison. The main reason for this is that the most important component of the uncertainty budget is the uncertainty of analysis, which had been determined previously by inter-laboratory tests. All other components have a rather minor contribution to the overall uncertainty and are well known owing to on-going metrology controls, regularly performed in each laboratory.

NO x (automated method)

The field tests performed on 6 different stacks have led to the following equations for repeatability and reproducibility: CIr = 0.029 c + 2.0 mg/ m 0 3 [START_REF]Stationary source emissions -Determination of mass concentration of sulphur dioxide -Reference method[END_REF] CI R = 0.038 c + 4.4 mg/ m 0 3 [START_REF]Stationary source emissions -Determination of mass concentration of oxygen (O 2 ) -Reference method[END_REF] where c is the NO x concentration in mg/m 0 3 . Additionally, all teams involved in field tests 5-6 had developed uncertainty budgets following an exhaustive list of criteria and applying the same calculation routines. The detailed results are given in Tables 6 and7, and a graphical representation of CI r , CI R , and the overall uncertainty from the uncertainty budgets is shown in Figure 2. There are some clear discrepancies between the field results and the uncertainty budgets, the latter ones yielding distinctly higher values for most participating teams. These findings will be discussed in the following paragraphs.

The resulting relative overall uncertainty broadly varies in the range of 5.4 % to 15.5 % for field test 5. The dominating parameters are: lack of fit, sensitivity to temperature, repeatability of measurement and calibration gas. The relatively high value of Lab B is due to both technical problems of the gas analysers, and to the fact that the performance characteristics of its instrument were known and expressed as a fixed value determined at the full range (1000 ppm). All other laboratories estimated similar overall uncertainties between 5.4 % and 7.8 % of the studied concentration.

For field test 6, the resulting relative overall uncertainty broadly varies in the range of 3.2 % to 23.4 %. The overall uncertainty determined by Lab A is quite high, Lab C estimated relative low, and Lab B and D calculated 7,6 resp. 8,6 %. Here too, the laboratory did not know the relevant performance characteristics at the actual measurement range and, therefore applied fixed values determined at full range. Again, this leads to a clear overestimation considering the field results (Figure 2) and the values obtained by the other laboratories. As shown above, the limited information on performance characteristics available from manufacturers and test houses might lead to uncertainty budgets that might be biased and, therefore, irrelevant. To illustrate and quantify the importance of this problem, the example of the performance characteristic lack of fit is considered. Typically, a test house or manufacturer will determine lack of fit, i.e. the deviation of from the true value of linearity, by calibrating the instrument with a reference gas at full range, followed by measurements of the diluted reference gas at 20 %, 40 %, 60 %, and 80 % of the range. For a range of 1000 ppm, a hypothetical set of results is given in Table 5. As observed in Table 5, the maximum deviation is 15 ppm and corresponds to 1,5 % of the range. However, the performance characteristics might be described as follows: (i) lack of fit < 2 %, because this is the performance criterion in the standard, and a typical value that many manufacturers can guarantee, (ii) lack of fit < 15 ppm, or (iii) lack of fit = 0.3 % (200 ppm), 1 % (400 ppm), 1.5 % (600 ppm), 0.5 % (800 ppm), i.e. the full information contained in Table 5.

Considering a hypothetical uncertainty budget at 200 ppm, according to the available information, the contribution of the parameter lack of fit in the calculation of the overall uncertainty may strongly differ according to the different ways of expressing or determining performance characteristics. It would amount to (i) ± 20 ppm, (ii) ± 15 ppm, or (iii) ± 3 ppm. This corresponds to an uncertainty contribution between 1.5 % and 10 % of 200 ppm. In other words, the way of describing the performance characteristic for lack of fit will yield a difference of a factor 7 in its contribution to overall uncertainty. It seems that the very large values of Lab. B (field test 5) and Lab. A (field test 6) of some parameters were mainly due to such unclear documentation of performance characteristics. This phenomenon is of lesser importance when the budget is established at concentrations near to the full range. Another element that often limits the correct establishment of an uncertainty budget is the fact that the laboratory might not know the exact range of variation of influent parameters such as the ambient temperature, voltage, flow-rate or atmospheric pressure encountered during the field experiment. Since these parameters are often not measured, the laboratory might use a standardised range that is wider than the actual field values. Thus, there is a general tendency to overestimate a number of uncertainty contributions.

Conclusions

The establishment of an uncertainty budget is very useful to identify and study the major contribution to the overall uncertainty. As has been shown by the work performed on SO 2 analysis, uncertainty budgets that are based on sound scientific knowledge of the main contributions will give results that are consistent with repeatability and reproducibility confidence intervals determined in field tests. However several conditions must be met to obtain realistic data from uncertainty budgets:

trained personnel to correctly establish and interpret uncertainty budgets, -sufficient information on the performance characteristics of the method at the studied concentration, -knowledge of the variation range of influent parameters in the field, such as temperature, humidity, voltage, etc.

Currently, the performance characteristics determined by test houses are often expressed in % of the range and correspond to the maximum deviation. Therefore, laboratories often miss the relevant figures of performance characteristics at a particular concentration. Because uncertainty budgets must frequently be established at the Emission Limit Value (ELV), it seems essential to provide at least the performance characteristics of an analyser at ELV. In view of future (lower) ELVs and applications over the whole range of an analyser, performance characteristics should be determined and given at least at 20, 40, 60 and 80% of the range. This will yield more trustworthy uncertainty budgets and better consistency with field inter-and intra laboratory tests. The necessity to obtain sufficiently detailed performance data from manufacturers should be considered in future standardisation work, such as the one currently carried out by CEN/TC264/WG22 on a certification scheme for automated measuring systems.

Finally, EN ISO 14956, states that the user of this standard must test the method under field conditions in order to verify that its performance data and measurement uncertainty calculated according to an uncertainty budget, comply with results obtained under field conditions. The results of CEN/TC264/WG16 show that this recommendation is critical for valid uncertainty determinations.

Table 1 Relevant characteristics and minimal performance criteria for the determination of sulphur dioxide [START_REF]Stationary source emissions -Determination of mass concentration of sulphur dioxide -Reference method[END_REF].

Performance characteristic Performance criterion

Volume of the absorption solution ≤ ± 1 % of the volume of solution Volume gas meter:

uncertainty of sample volume (2) uncertainty of temperature (2) uncertainty of absolute pressure (2) ≤ ± 2 % of the volume of gas sampling

≤ ± 2,5 K (1) Absorption efficiency (3) > 95%

Leak in the sampling line ≤ 2% of the nominal flow rate

Value of the field blank ≤ 10 % of VLE Standard deviation of repeatability of analysis ≤ 7 %

Overall uncertainty at the limit value ≤ 20 % (1) Performance criteria corresponding to the uncertainty of calibration.

( 2) The uncertainty of the sampled volume is a combination of uncertainties due to: calibration, drift (random drift, drift between 2 calibrations), resolution, reading. The uncertainty of temperature and absolute pressure at the gas volume meter is a combination of uncertainties due to : calibration, drift (random drift, drift between 2 calibrations), resolution, reading, and repeatability. (3) This characteristic is an assurance quality check to quantify the absorption efficiency in the first absorber; but it doesn't quantify a possible loss of absorption, and therefore it is not included in calculation of overall uncertainty Table 2 Relevant characteristics and minimal performance criteria for the determination of sulphur dioxide [START_REF]Stationary source emissions -Determination of mass concentration of nitrogen oxides (NO x ) -Reference method -chemiluminescence[END_REF].

Performance characteristic Performance criteria

Table 3

 3 Contribution of different parameters to the uncertainty budgets for SO 2 as estimated by the laboratories taking part in field test 5.

	Response time	≤ 200 s
	Detection limit	≤ ± 2 % of the range
	Lack of fit	≤ ± 2 % of the range
	Zero drift	≤ ± 2 % of the range/ 24 hours
	Span drift	≤ ± 2 % of the range/ 24 hours
	Sensitivity to atmospheric pressure	≤ 3% of the range for 2 kPa
	Sensitivity to ambient temperature	≤ 3%of the range/10 K
	Sensitivity to electric voltage	≤ 2 % of the range/10V
	Interferents	Total ≤ ± 4% of the range
	Converter efficiency	≥ 95%
	Losses and leakage in the sampling and conditioning	≤ 2% of the measured value
	system
	Standard deviation of repeatability in laboratory at zero	≤ 1 % of the range
	Standard deviation of repeatability in laboratory at span	≤ 2 % of the range
	level
	Uncertainty of calibration gas	≤ 2 %
	Overall uncertainty at the ELV	≤ 10 %

Table 4

 4 Contribution of different parameters to the uncertainty budgets for SO 2 as estimated by the laboratories taking part in field test 6.

	Lab A	Lab B	Lab C	Lab D

Table 6

 6 Uncertainty budgets for NO x set up by the laboratories taking part in field test 5

	Lab A	Lab B	Lab C	Lab D

Table 7

 7 Uncertainty budgets for NO x set up by the laboratories taking part in field test 6

	Lab A	Lab B	Lab C	Lab D

Acknowledgement

The authors wish to thank the members of working group 16 and its secretariat provided by the French standardization body AFNOR, the secretariat of CEN TC 264 and the CEN Management Centre. The field tests were made possible by financial support of the European.