J L Fuentes-Cantillana

J C Catalina

A Rodriguez

J J Orteu

D Dumahu

Use of Computer Vision for Automation ofa Roadheader in Selective Cutting Operation

A research project on automation of a roadheader in selective cutting Operation is being carried out in collaboration by AITEMIN (Spain), CERCHAR and LAAS (France). It implies the recognition of minerals and its distribution at the cutting face, äs well äs automatic generation and execution of adequate cutting trajectories. The case study is a potash mine in northeastern Spain. Computer vision was selected äs the most convenient sensing technique to provide an ore distribution map. A complete System has been developed to perform on-line image processing and boom control at the face.

INTRODUCTION

State-of-the art of automation in roadheaders

Most of the experimental work for roadheaders automation has been centered in the operations which imply cutting a complete section which has a constant profile, or shows only slight changes, and with an arrangement of the cutting sequence subject basically only to the restrictions arising from the geometrical or geotechnical conditions. Nowadays, the market offers Systems able to control automatically the cutting of a fixed profile, which are applied to build roadways or tunnels.

The problem becomes more complex, however, when the distribution of the different ores or rocks at the face has to be taken into account, since the cutting sequence will change according to the geometrical distribution of those Materials, specially in irregulär deposits. The foregoing happens, for instance, in mining operations where ore and waste are found in the face and have to be cut separately to avoid dilution, or if some seams have to be cut before others to improve the cutting Operation efficiency. In these cases, it is necessary to provide the automatic control System with sensing devices able to detect environment conditions, äs well äs with a certain level of intelligence to take appropriate decisions, so that the System can automatically adapt itself to the natural changes in the geology.

Goals of the Project

The goal of the project is, basically, to study the feasibility of an automated System able to control the selective cutting Operation of a roadheader working in an Underground potash mine.

For this purpose, the following specific goals were defined: a) Evaluation of different techniques suitable for ore identification at the face in normal Operation. b) Development of data processing tools capable to produce a "face map" representing ore distribution in the face. c) Development of a cutting plan generator taking into account selectivity, room cross-section, and cutting head dimensions. d) Design and implementation of an adeguate control System for the roadheader, with all required sensors and actuators. e) Development of control Software to conduct a cutting test. f) Test of the whole System Operation in real conditions under human supervision.

DESCRIPTION OF THE TEST SITE

The system has been tested on an Alpine AM-100 roadheader which is working at present in the Potasas de Llobregat Mine, that exploits an ore body about 50 km2 large located some 70 km from Barcelona, in the Northeast of Spain.

The orebody consists of two sylvinite seams, rather dose to each other, located at 500 m depth. Although thickness values are very variable, average values are shown in figure l. Each sylvinite seam actually consists of many alternating sylvinite and halite seams, a few centimeters thick, with very thin clay seams intercalated. On the other hand, the overall orebody is quite undulated and shows a large number of mullocky areas. Due to the foregoing, the mine profitability is rather tight and so productivity and low ore dilution are of major importance.

The Operation is implemented in two successive passes, äs shown in figure l, leaving the carnallite äs roof, and adapting the Operation to the changes in thickness and slope of the seams.

Selective cut is performed in every face and, while the ore is conveyed to the extraction shaft, the salt is dumped in the nearby mined rooms.

The mine is currently eguipped with 13 Alpine AM-100 roadheaders which provide a total output of 270,000 tpy K20. The weight of these machines is 95 tons and cutting motor power is 300 kW.

15-2

••SVLVINnE B 0,7m-lS-/.K20 "INTERMEOiAIE SALT 2.2B -r». «,0 "SylVINlie A" Vm-f/. K;0 Figure 1. Stratigraphic colunn of the deposit

MINERAL EECOGNITION

Several techniques to discriminate the different Minerals present at the face were tested:

Natural gamma radiation showed poor spatial resolution and difficulties to be installed in the moving cutting head.

-Microwave absorption was too much dependent on surface conditions such äs roughness, water film presence, etc. -Vibration and noise analysis did not provide possibilities for discrimination in this case.

Finally, vision was selected äs the most convenient technique for this application and so most of the research work was devoted to it.

COMPUTER VISION Spectral Analysis

The feasibility study of vision äs sensing technique started with an spectral analysis of the ores present in the mine. The choice of CCD solid state video cameras äs sensing devices (for robustness, availability, Standard Output and cost reasons) restrained the study to the visible and near infrared ränge of the spectrum, in which those devices are sensitive.

A computer-controlled spectrometric analysis system was set up to measure, störe and process reflectance spectra (between 350 and 1000 nm) from each ore sample in a set of 70 blocks coniposing the complete Stratigraphic column of the mine.

The study concluded that colour differences among the ores depended on spectrum shape, not on its level. Hallte tends to have a higher relative level in the green ränge than sylvinite. Thus, if the spectra are normalized at a proper wavelength, ore classification becomes evident. Figure 2 shows average spectra for five minerals after normalization at 525 nm.

In view of the foregoing, it was decided to define a discriminating value. K, äs the ratio of:

reflectance at 625 + 25 nm K = reflectance at 525 + 25 nm

Figure 3 shows the values of K computed for all the spectra. The accuracy of such classification is quite significative although there is overlapping between some sylvinite and intennediate salt samples, and carnallite cannot be separated. In order to discriminate the ore distribution in a face it is required to compute the K value at every point. To that end, two monochrome Images should be acquired with narrov passband filters centered at 525 and 625 nm, to be then divided point by point. Another possibility that simplifies camera mechanics is the use of a colour camera, äs the 525 nm and 625 nm filters can be substituted by the in-built green and red filters, with a small degradation of classification accuracy.

The performance of red/green ratio for direct ore classification of individual pixels was too poor to be effective, because of:

-overlapping between K values for the ores, seen in figure 3.

-colour variations in cutting head bit marks on the face. ~ noise introduced by camera, recorder and decoder.

Thus, a new processing algorithm was devised to perform classification after region segmentation, to compensate for the variations of pixel colour through averaging. It took advantage of a key characteristic of the orebody: seams of sylvinite, carnallite and intennediate hallte, all of reddish colours, are always flanked by hallte or clay seaas, of unsaturated colours. Region classification was essayed with several techniques, such äs split and merge, freguency domain analysis, Saturation, R/G ratio, and discriminant analysis, which gave best results.

Image processing algorithm was finally composed of three steps:

-Segmentation: the red/green ratio is combined with a quadratic discriminant function to produce an image highiighting reddish seams, which is thresholded and labelled. -Classification: four quadratic discriminant functions are applied to the original Image over the segmented regions, classifying each of them äs carnallite, A or B sylvinite, or hallte. -Honogenization: regions are merged, their outlines simplified, and very small regions are reaoved, leaving the main searos.

All the above process is perfomied by Software downioaded on the iiaage processor and executed by its own local microprocessor, leaving the main CPU free for other intensive tasks.

Figure 4 illustrates the typical steps in the process of a face colour Image acquired at the mine.

Inage rusion

In the roadheader being used, there is no safe location for the caicera which would provide an obstacles-free view of the whole face. Therefore, it was requiered to sep up two cameras at different locations in such way that they cover areas which caiaplement each other. As a result, the data from two cameras, located at different positions, has to be combined into a single Image at some stage of Image processing.

The problem of Images fusion is solved in two stages:

A calibration of the System composed by cameras, roadheader and face is initially carried out to determine the conversion equations between their respective reference Systems, and to estjiriate the distorsion effects of the wide-angle lenses.

15-6

Through previous equations, any pair of Images is transferred to the same reference System and the data from both Images is combined in the areas where overlapping exists.

Figure 5 shows an example of Image fusion from two different positions and attitudes, with a plane target surface. Face mapping consists of generating a map of the mineral distribution on a face, in the form of a set of polylines which represent the boundaries between the different seams present in the processed Image.

Once the face most adeguate different for desired final

OPERATION PIANNING

map is produced, a Software module generates the cutting trajectory, which is in principle every new cut. This module takes into account the profile, roadheader operational limitations, and 15-7 the constraints imposed by loading procedure, shuttle car capacity, and the mining method itself.

By now, no optimization criteria have been introduced in the operational planning, although the produced trajectory is intended to be an "efficient" one.

As an example, a face map äs shown in figure 6a would be split into regions äs in figure 6b, and cut following the trajectory shown in figure 6c. The control unit located at the machine is a diskless VMEbus industrial Computer dedicated to acquisition and process of all data frorn cameras and sensors, and execution of machine control commands and cutting schedules received from the main unit, through actuators on electric and hydraulic equipment. This unit incorporates dedicated hardware such äs an Image processor and analog/digital I/O and communications boards. The unit is well protected against impacts and vibrations and is equipped with an uninterrupted power supply and its own cooling System since the environmental temperature stays around 37°C.

The main unit at the surface is a VMEbus industrial Computer running UNIX dedicated to Software development work, remote system monitoring and Operation planning and control. To start the system, the main unit downioads the different Software modules to the Underground unit, until the system is up and running. In this first test, however, the system was controlled frorn the man-machine interface at the Underground unit.

MINE TEST

Conditions in the mine where system test has been conducted were hard, due to the dusty and saline environment, high temperature and distance from the surface. Time available for tests could not be äs long äs desired, but has been enough to find out the actual possibilities and limitations of the system.

The basic problem found in the mine test was the low quality of the Images provided by the video cameras, mainly due to insufficient resolution and poor colour quality (most seams of the face are very thin compared to pixel size), which could not be properly processed by the vision system. But in the developing industrial camera market, probably new products will soon appear providing adequate quality at an affordable price.

Positive results were obtained with: -Image acquisition: worked well in terms of camera positioning and face coverage, although stronger mechanical protection for cameras and lighting is required. -Image processing: füll mineral identification process worked successfully on the onboard Computer with videotaped images. -Other programs: camera calibration, image fusion, face mapping and trajectory planning programs were tested on the surface Computer at the laboratory. -Trajectory execution and control: worked well although horizontal movement needs adjustment due to nonlinear boom hidraulics behaviour.

-Software structure developed for Underground unit based on VERSADOS real-time multitasking kernel operated successfully. -System hardware: worked properly without major problems, although the protection of some sensors should be improved. -Communication link between Underground and surface Computers could not be fully debugged to achieve expected data rate.

FUTÜRE WORK

A grant has recently been approved by the Commission of the European Community (DG XII) to develop a second phase of this research project. The aims in this new phase are basically to continue with the development, solving the problems found in the previous one and refining the Systems and programs produced to make them more robust and less dependent on particular conditions.

15-10

Ffgure

  Figure 6a. Face map Figure 6b. Boundaries redefinition

The project is financed by the Commission of the European Community and by the Spanish Institute for Mining and Geology (ITGE).

Image processing

The research on Image processing algorithms was based on a set of Images recorded on videotape in the mine with a Professional colour camera, later decoded to RGB and digitized by the Image processor.


C. MICHOT INERIS R Branka

THE APPROVAL OF FIREWORKStN. FRANCE

on 01/01/91, the LSEV (Laboratoire des Substances Explosives de Verneuil**) is the official laboratory responsible for approval tests on explosive products for civil use.

Since 01/10/90 an order has been in force, fixing the conditions for the approval and use of fireworks in France : no firework may be manufactured, used in or imported into France unless i t confonns with an approved type. This approval is obtained after an Intenninisterial

INTRODUCTION

Since 1972, CERCHAR has, in the civil sector, been responsible for the approval tests to which all explosive products in France must be submitted. This role of official laboratory has been assumed by INERIS since 01/01/91. This Institute has now taken over the main activities of CERCHAR. with its research teams and the installations in Verneuil. Its expertise extends to all major industrial hazards with the exception of those relating to nuclear power and transport. However, the transport of dangerous materials nevertheless constitutes an activity in which INERIS is increasingly involved.

In 1991. its role was extended to cover a further activity : the study of fireworks with a view to their approval.

In effect. an Interministerial Order (dated 01/10/90) establishes the conditions governing the approval and use of fireworks in France. Thus, since 01/04/91, no firework may be manufactured, used in, or imported into France unless it confonns to an approved type.

In parallel with this process, the European Technical Conunittee on Standardisation, CEN TC 212. was created in 1989 under the title of "fireworks". Its aim is the Standardisation of ready-to-use fireworks, particularly from the aspect of their safety in use.

l./ APPROVAL OF FIREWORKS IN FRANCE.

The approval procedure is äs follows :

a) The application for approval is presented to the Ministry for Industry, accompanied by a file, containing, for each product :

-the description of the product and of its variants, -its trade name, -proof of the applicant's ability to guarantee the subsequent confonnity of the product with the model, -the precaution to be observed in use, which will be printed on the product, on its wrapping or on the directions for use.

b) The Minister for Industry arranges for a laboratory approved by him to carry out the examinations and tests that he considers necessary. If appropriate, the results of tests or examinatlons giving equivalent guarantees and carried out in a member s täte of the European Economic Conununity may be taken into account.

c) The decision äs to approval is taken by the Minister for Industry after consultation with the Conunission on Explosive Substances, an Interministerial Commission in which INERIS takes part in its capacity äs a reporter on the examinations and tests carried out on explosive products.

CLASSIFICATION OF FIREWORKS.

With a view to their use, fireworks are classified in the 4 groups defined äs follows : Kl : fireworks presenting only a minimal risk and which may be sold to minors K2 : fireworks, the use of which demands some simple precautions described in the directions for use Note : this classification is independent of the hazard divisions 1.1 to 1.4 imposed by the regulations goveming the transport of dangerous goods.

3./ TESTS CARRIED OUT ON FIREWORKS WITH A VIEW TO THEIR APPROVAL AND CLASSIFICATION IN GROUPS Kl TO K3.

Approval for fireworks may be obtained after examination of the results of safety tests.

All types of fireworks undergo :

-a detailed visual examination. possibly dismantling and comparing the results with the dato given in the technical file ;

-a check on how they function making i t possible to assess whether the description or the directions for use ensure safety in use. The fireworks are likewise submitted to environmental tests (impacts, vibrations, tenperature resistance) betöre a functional fest, in order to assess the resistance of the firework to the usual Stresses encountered when they are handled, transported or stored ;

-measurement of the duration of the ignition phase, in order to guarantee safety in the case of the firework going off by accident.

As a function of the type of firework, three types of test are then carried out :

-for aerial fireworks, examination of effects at a height and of possible dangerous fall-outs ;

-for fireworks designed to go off on the ground or over water, measurement of the ränge of possible projections and of the weight of these ;

-for small fireworks such äs bangers and toy pistol caps, measurement of the noise level.

Finally. if there are any doubts äs to the safety of the product, supplementary tests may be demanded :

-verification that. if the firework goes off accidentally in confinement, this does not cause detonation or any violent deflagration, -examination of the behaviour in fire, of a box or heap of fireworks, -examination of the effects obtained when one ignites an item in a box, these latterly tests being particularly usefui for assessing the safety at the places of use.

The total test results are incorporated in a report, then discussed by the Commission on Explosive Substances with a view to a proposition for approval by the Minister for Industry, if the overall results are satisfactory.

All Information conceming the products will be recorded in a database and the design of the fireworks on an optical database.

See type list of fireworks^ in table l, list of approval tests in tables 2 and 3.

4./ EUROPEAN STANDARDISATION OF FIREWORKS.

The technical committee CEN TC 212 on "fireworks" is made up of three working groups :

-the working group "classification and terminology" which has already compiled a trilingual glossary (English. French and German) of tenns and definitions concerning fireworks, äs well äs a list of all types at present existing. äs far äs is known by the members of this working group ;

-the working group "marking" whose function is to define the terms in which warning notices and instructions for use will be expressed ;

-the working group "test methods" which has to define the conditions in which fireworks must be examined, both for type approval tests and for quality control tests. -SS (1) test to be carried out on a) fireworks that have not undergone any previous test b) fireworks that have previously been subjected to test 6.02 c) fireworks that have previously been subjected to test 7.01 and 6.01 (2) test to be carried out only if there have been projections in test 8.03 0 compulsory test F optional test

0 Q • H g x m 0 • r i v W f-f •ü Q 1-< 0} X Ul 0 • H • V m » "fc< ^s öS •ä ä äl x x x Stability U} 0 • H 4-1 W t -SS ^s 11 x m .3 4-1 01 •• 3 5 • 0 Q 1 1 X Sensitivity M 0 • r i +1 (0 •a • ä' s a E 2 ll x in ü 1-1 4-1 0) •a z; ü« 1-1 +> ll X X Aptitude behaviour • S l X X x x x x x x x ĝ T; r & x x x x

  by the Prefect (Administrative Head) of the Departement after an examination before a Jury' chaired by the Departmental Director of the Urban Police Forces. The training necessary for obtaining the qualification is given in a course run by a body approved by a joint decision of the Ministers responsible for Industry and Public Safety and after consultation with a technical conunission.

K3 : fireworks that can be handled without risk by persons not having the necessary certificate qualifying them to use fireworks of Group K4 and on condition that the ruies given in the directions for use are obeyed K4 : fireworks that can be handled only by persons holding the certificate of qualification or by persons under the direct control of persons holding this certificate. The certificate is issued

Table 3 ?

 3 TABLE OF APPROVAL TESTS

		PRODUCTS
	TESTS
	Ref.	Designation
	5.01	Description
	5.02	Duration of ignition
		phase
	5.03	Functional check
	6.01	Resistance to vibrations
	6.02	Heating test at 50*C in
		atmosphere with 60 %
		rel. humidity
	7.01	Drop test
	8.01	Behaviour in fire
	8.02	Risks of confining
		firework
	8.03	Formation of projections
	8.04	Range of projections
	8.05	Observation of effects
		of fireworks designed to
		go off in the air
	8.06	Noise level
	8.07	Propagation level


Explosives Dans L'

Industrie Du

Gaz A Accorsi

GAS SENSORS FOR CONTROLLING TOXIC AND EXPLOSIVE ATMOSPHERES IN THE GAS INDUSTRY DETECTEURS DE GAZ POUR LE CONTROLE DES ATMOSPHERES TOXIQUES

Keywords:

Nowadays continuous survey of explosive and/or toxic atmosphères is easier thanks to sensors which are more reliable than before and less expensive than industrial analyzers. New technologies developments, like microprocessors, thin layers deposits and integrated optics will lead in the future to miniaturized instruments with high performances. This text will introduce the state of the art in commercially available detectors and trends in new technology developments.

RESUME

La surveillance continue des atmosphères explosives et/ou toxiques est facilitée de nos jours par l'existence de capteurs fiables et d'un coût moins élevé que les analyseurs industriels. Les développements technologiques comme la micro-informatique, les dépôts en couches minces et l'optique intégrée vont permettre dans le futur d'obtenir des appareils miniaturisés très performants.

INTRODUCTION

A gas sensor is a transducer which gives directly an information on gaseous composition of the surrounding atmosphère. It uses a sélective chemical layer and a physical transducer. Originally well différent from an analyzer which gives an information on gaseous composition trough a spécifie treatment, i.e. indirectly, the sensor becomes more and more similar to the analyzer due to its intégration into a computerized multisensor System. The différence between those two éléments remains then in the smaller size and the présent lower précision of the sensor. The qualifies requested from the sensors are sensitivity, précision and reproducibility, specificity, immunity to interfering factors, short response time. This review paper is a short description of sensors presently used for atmosphère control and of developments including new technologies and new materials. Following subjects are developed : reactive tubes, reaction heat sensors, thermal conductivity sensors, semiconductors, electrochemical sensors, then new orientations, first ones yet commercially available (miniaturization, electronics addition), second ones still under development (new materials, new principles, new technologies).

COMMERCIAL SENSORS

Reactive Tubes

Thèse detectors are now more than thirty years old, but they are still in use. They give a semiquantitative measurement without any delay. A holder, silica for example, is impregnated with a product. This product reacts irreversibly with the gas to be detected and gives a coloration. This holder is setted in a graduated glass tube : the sampling of a quantity of air to be analyzed is made with a pump, through the reactive tube. The coloured length dépends on the gas concentration. This method is quite sélective, but some reactions may be interfered by components, like air humidity : so the tubes hâve a dessicating powder at the air inlet. Because of température influence, précision is estimated to be 30 % relative. Response time is short because the reactions are fast : it dépends on pumping rate. Storage life is limited ; high températures must be avoided. Easy maintenance and use, small cost make this method very convenient. It is used especially for gas leaks, pollutants émissions control and individual or collective survey of toxic gases like CO, CO2, NO X , SO2, H2S, 02 deficiency. Many accessories hâve improved their use : prolongator tubes for sampling, electrical pumps, automated reading ... Diffusion tubes allow survey of inhaled gas quantities, that is very useful for work hygiène.

Catalytic combustion sensors

Thèse sensors are dedicated to combustible gases measurements ; ail types of explosimeters are using them, wether portable instruments for the survey of gas leaks, or fixed instruments for plants watching. Two types hâve been available on the market for thirty years : either a simple catalytic wire is used as a heater, reactor and measuring device, or a catalytic pellet is used as the reactor, a metallic wire beats it and is used as measuring device. When the combustible gas oxidation takes place on the sensor, it causes a température rise of the sensor, which is measured by the rising of wire résistance. Most used catalytic wire is a 80 microns diameter platinum wire. It was not very often used in gas industry until now because it had a short life time ; now it becomes a new interest thanks to the use of a new measuring method which gives it more than one year lifetime without important decrease in sensitivity. Its température (900 to 1 000 °C) gives it a good résistance to catalysts poisons like sulfur compounds. Catalytic pellets hâve invaded the explosimetry market. They consist of a ceramic holder which is impregnated with a catalyst (platinum, palladium, rhodium ...) and heated by a platinum wire, up to 450-500 °C.

The sensitivity may be decreased by catalyst poisons like silicium, sulfur, lead, halogens ... That's why many studies hâve been made, especially in Great-Britain, about the pellet structure and résistance to poisoning has been well increased. Metrological quality of explosimeters may be verified trough conformity to the new existing european standards, EN 50-054 to EN 50-055. The instruments hâve to be constructed in conformity with electrical security standards. Nowadays electrical consumption and working température are too high to permit an entirely intrinsically safe instrument. Two drawbacks hâve to be mentioned : one is the response error in low oxygen concentrations (< 12 % v/v) ; another is the response ambiguity in high combustible gas concentrations : oxygen in not sufficient enough to ensure the complète combustion and so the reaction heat is équivalent to the one obtained in low gas concentrations. This ambiguity can be avoided by différent means, one is the use of a thermal conductivity sensor, as described in the next section.

Thermal conductivity sensors

To avoid the ambiguity of measurements previously mentioned, portable instruments are equipped with small non reactive heated beads ; their equilibrium température dépends on thermal conductivity of the ambiant atmosphère. As before, this température is measured by a thermal heater résistance variation. Précision of thèse sensors is a few per cent of gas volume. The most précise measurements are for gases which hâve a very différent thermal conductivity from air, for example hydrogen. As before thèse sensors are calibrated for the gas which has to be measured. When more than two gases are présent measurement is false. Thèse sensors are used for combustible gas leaks détection above LEL, for measurements of binary components, as chromatography detectors for example, and for some particular gases, like hydrogen, not measured by infrared technique, or carbon dioxide, if an infrared technique is too expensive for the application.

Semiconductor sensors

Thèse sensors hâve been on the market for about 10 years, principally in Japan where they are used as gas leaks detectors in domestic appliances. Made of a semiconductor metallic oxide, mostly SnÛ2, they work on this simple way : a gas with oxygen, or a reducing gas is absorbed on the surface of the semiconductor and modifies the O2-O2 vacancies equilibrium. That changes the electrical résistance of the sensor ; this change dépends on the gas concentration. The semiconductor is heated to obtain a réversible reaction. The choice of température is governed by the gas to be measured, this allows some selectivity : an H2S sensor will work at 150 °C, a méthane sensor at 500 °C. Thanks to this sort of reaction, the sensors are adapted to many applications, from a few ppm of toxic gases to combustion surveillance, through fire détection or CO monitoring ... Drawbacks are the interférences of gases or vapors not to be detected, or humidity, from which false alarms occur. Studies are still necessary to get a better selectivity, reproducibility, especially for ail the applications in security, for which reliability is the first criterion.

Electrochemical sensors

Thèse sensors can be splitted into two types : those with solid electrolyte, those with a liquid one. Solid electrolyte sensors now available on the market work at a very high température (> 600 °C). They use the change of the electrochemical potential at the solid-gas interface on a logarithmic way with gas concentrations : two différent concentrations of gas face the two ends of the electrolyte, that gives a potential différence between them. To get a better précision in high gas concentrations a current is forced to pass through the electrolyte, a linear response is then obtained. The best known sensor is the zirconia sensor to rneasure O2 concentrations. There exist SO2 sensors, but thèse use a O2 sensor too, for taking into siccount the SO2/SO3 equilibrium. Précision is good, lifetime too. To hâve a better response time a catalyst métal is placed at the gaselectrolyte interface, but it can create some errors in présence of reducing gases.

The sensors are used in combustion control, and in surveying the pollutants émissions of furnaces. Liquid electrolyte sensors grew very rapidly durîng the last years and are now currently used in individual or collective security, especially for monitoring CO, H2S or O2 deficiency. O»2 sensors exist for about 15 years : a lead électrode is consumed by O2 in air, through a liquid electrolyte before which a semipermeable membrane is placed. Oxidizing current is a direct measurement of O2 concentration. Thèse sensors hâve a limited lifetime, depending on the consumption rate of the électrode. The température variations are easily compensated. Précision is about 1 % relative. Some gases can interfère, either modifying diffusion phenomena, or changing the electrolyte acidity ; CO2 is such an interfering gas. Then other sensors were developped, which use O2 as an oxidising agent, and the gas to be measured as a reducing agent. They allow measurement of 1 to 1 000 ppm gases, like CO, H2S, NOx, SO2 ... At the working électrode the gas diffuses trough the membrane and reduces the electrolyte ; the electrolyte is renewed through oxidation at the auxiliary électrode. Oxidoreduction current is a linear measurement of gas concentration. To get better performances, specially to avoid the decrease of current through électrodes polarization, many sensors hâve 3 électrodes : a third électrode, the référence électrode, keeps the working électrode potential constant ; the current needed is then related to gas concentrations. Lifetime is theoretically not limited, because of the use of oxygen présent in air. In practice, they can live 1 to 3 years, depending on the ambiant conditions. Electrical current is about 0.1 microamps per ppm gas : the sensors are intrinsically safe. Because of high impédances of electrolytes and membranes, manufacturers hâve to pay attention to electromagnetic immunity. Elesponse time dépends on gas : 20 to 30 seconds for CO, it can be one and a half minute for H2s, because thèse molécule diffuse slowly through the semipermeable membrane.

Conclusion

A great diversity of sensors exist on the market. Drawbacks dépend on use. Advantage is always the lower price than analyzers. Through the years they improve on reliability and reproducibility thanks to quality monitoring in manufactures.

NEW ORIENTATIONS

2.1 New developments to be commercialized Miniaturization of electronics and optics, microprocessors invention make the sensors extra rising and permit new sensors types to appear : used for a long time only in analyzing field, optical sensors now enter in the sensor field. So are optical explosimeters, which are miniaturized open path infrared analyzers. Their response time, with mechanical protective parts, is still long ; their précision is not as high as that of catalytic combustion sensors, but they don't suffer from poisoning. Microprocessors allow real time corrélation of différent sensors signais, and interférences corrections. So are portable combustion controllers using electrochemical sensors for O2, CO, NO X and SO2 measurements. Précision is now not sufficient enough because interfering coefficients are changing for one sensor to another, creating about 20 % déviation between the real and calculated values.

New developments in laboratories

New technologies, new materials, new principles are the ferments of a great diversity of research in gas sensors field. New materials are for example semiconductors like metallic phtalocyanines, solid electrolytes working at the ambiant température. New principles are fîeld effect transistors, piezoelectricity, surface acoustic devices. For ail thèse principles a membrane adsorbs, with some selectivity, a gas which modifies the properties of the microelectronic device ori which it is deposited.

To get those sensors being commercially acceptable, the membranes hâve to make progresses in selectivity and humidity immunity. New technologies are integrated optics and optical fibers, microelectronics and thin layers, which allow great decrease in electrical consumption and mass production at a reduced cost. Finally, the advent of microprocessor would make the developments moving towards the gas identification through real-time signais treatments. Further, the miniaturization of electronics allow measurement and treatment technologies to merge through conditionning on a same sensor, signal processing and the sensor itself. Advantages are a better signal-to-noise ratio, thanks to on-site amplifîing, the increase of caracteristics performances (correction of linearity, pressure, température, increase of frequency response), signal conditionning well adapted to chosen digital transmission (AD conversion, in situ impédance change, output formating, mean values calculations).

Conclusion

This short review gives an idea of the multidisciplinarity of gas sensor science : solid physicists, mechanical, electrochemical, electronical, optical, spectroscopical, catalyst or membrane chemical engineers, ail are working for sensors development.

In ail the sciences for sensors, like electrochemistry with its rapid growth, optics with its youth as a sensors science, science of heat, nowadays underestimated, we are going in the direction of miniaturization and intégration.

  Fig. Main sensors used in gas industry


This is a longitudinal study of 3 167 active or retired pneumoconiotic coalminers from Nord -Pas de Calais collieries (HBNPC) recognised through medico-legal decision over three defined periods: 1942-61, 1952-81 and 1982-87. Periodic medical examinations of pneumoconiotics are provided by the medical services of HBNPC for miners and ex-miners. Physicians involved in this follow-up programme are concerned with the high, incidence of progressive massive fibrosis (PMF).

The objectives of this study were to describe pneumoconiosis at time of compensation, IU Uvvulvllvv \Jk TP-sVvf nopli cnVfJ/p»f<• t\ijf\ TUf^Ar^' Before 1962, almost all pneumoconiotics were compensated with small rounded opacities, proportion of type q opacities was 91%. Figure 2 illustrates the progressive change in proportions

S. X «/ A A X few* J. w* \ £-s XX
of small opacity types between first to third period of compensation.

The 645 definite and probable cases of PMF were mainly identified at follow-up.

However, a small number (n=39) were identified as they were first compensated for CWSP; this occured mostly among retired miners (76.9%). Age-specific prevalence rates of PMF increased from 1.3% in die 41-45 age group to 38.6% in the 65 and older.

Progression to PMF was associated both with time since compensation and with category of CWSP at compensation (Fig. 3). Type q small opacities were the most frequently observed at onset. Pneumoconiotics compensated with irregular type opacities were not given equal opportunity to progress.

<Figure 3>

Coalminers compensated for CWSP before 1962compensated for CWSP before 9 between 1962compensated for CWSP before and 1982compensated for CWSP before and in 1982compensated for CWSP before or later started work respectively in 1942compensated for CWSP before , 1946compensated for CWSP before and 1948, on , on (COCKCROFT and ANDERSON 1987), independently of smoking.

The present data suggest that the reduction in severity of CWSP at compensation, the decrease in proportion of rounded opacities (type q) together with an increase in proportion of small irregular opacities, and the increased age at onset may be partly associated with a change in age distribution of coalminers at risk. Indeed, differences in mean year of birth as well as mean year at first employment between the three sub-populations defined in this study, were small compared to mean calendar year at first compensation. Pneumoconiotics compensated between 1982 and 1987 represent a group of coalniiners who remained free of nodular CWSP during most, if not all, of their active life. The disease entity diagnosed at a date close to their retirement appeared different from that diagnosed among young coalminers. Differences in individual susceptibility, personal work practices, cumulated dust exposure or coal rank (SHENNAN et al. 1981;HURLEY et al. 1987) could explain the two clinical pictures of the disease compensated as CWSP. Verification of these hypotlieses would need further investigation.

Twenty years or more follow-up of 506 pneumoconiotics confirmed the importance of category of CWSP at compensation as a factor for the development of PMF [START_REF] Maclaren | Factors associated with the development of progressive massive fibrosis in British coalminer: a case-referent study[END_REF]), Those compensated with irregular type opacities were not given an equal opportunity to progress and therefore, their risk of developing PMF could only be assessed tl ough further follow-up.

Cockcroft

  Films were read by three independent readers blind as to medical and occupational histoiy of cases. They were interpreted mainly according to ILO classification(ILO, 1980), with knowledge of film order but not of dates. The first film was placed on the viewing box and read; both films were then placed side by side for Median readings were used for analysis. Occurence of PMF was considered as possible, probable or definite if one, two or three readers reported presence of large opacities.

	the disease to occur mostly after retirement. The changes in small opacity types reported suggest
	a reduction in incidence of pure silicosis* Occurence of PMF was highly correlated with age and
	Among the group of 326 miners compensated before 1962, 68% had reached category 2
	pneumoconiotics was comparable to that reported in high coal rank collieries (SHENNAN et of CWSP (Fig.l). They had on average 33.6 years of age and were mostly active (97.1%) with
	al.1981; HURLEY et al 1987). 14.6 mean years of coalmining experience. The I 368 miners compensated in 1982 or later had
	a mild form of CWSP. Only 25.1% were active. Mean age was 53.3 and mean coalmining
	experience 29.4 years.
	accompanied by an increase in proportion of small irregular opacities at compensation
	(COCKR-WT and ANDFRSON 19S7) Ap-e-<?nenfic nrevaience rate^ of PMF reached 38 6% in
	pneumoconiotics aged 65 or more« When time interval between films was taken into account,
	profusion of small opacities at compensation greater than 1/1 was associated with a probability
	of 40% or more to develop PMF'.

• • fTtr*fs1 "films \i?« a T*p» £»Y1VQr»?7»r! fr-ntn wsAritr 8 ??, 1 • filps o 'Phuax/ wt&vt* f Ui ksdxril 3UUjCuL? LWii id,u-ll.-• ZplksaA liiLlla, W&IC CAUd^lCu HUIII IllCUlwd.1 lILCb* lil^y W&1C Clllt/Ild 1OI wOUipCIIballOll OdQ. CfldllgtAl OVSI Illiii,-, d IIlalKCO. a predominance of category 1/1 among those compensated, after 1982. Comparison of distribution of small opacity types in the 1942-61 period to that in 1982-87 confinned the progressive fall reported by other investigators of the proportion of predominant micronodular opacities region, born after 1917 and alive on December 31st 1981. From the 31 328 prevalent cases in TTRNPP H9R6WSTAT ANN ]9&%) 75 2^.0 cn<%eK met selection criteria TTsrpp HpfinH ralenHar periods were defined for compensation date (before 1962, 1962-1981 and 1982 or later). Random sampling was done in the two first periods and all 1 594 subjects of the last period were raHioif "• ir's ] filmQ haH to fpv^iQt^/i In 3 TTRNTP 1 to dlafp nf rnrsTn^Ts^fliirin (T+/-^ vpar^ thf* QprnnrS tn flip 1QR7 the closest date prior to 1987 (later than 1982). <rFi OTJTP 1 *>

  A and Anderson N (1987): Radiological irregular opacities and coal work exposure; A case-referent study. Br J Ind Med 44:484-487.Figure 2. Percentage distribution of small opacity types at compensation Figure 3. Proportion of subjects with PMF at follow-up by time to follow-up and by category of CWSP at onset

	1-4	S	-24	2S-29	30 +
			TSma Interval (yean
		18.3%
		13.4%
				57.5%


Claude Davrou

Intrinsic safety principle and phenomenon which influence the intrinsic safety of the circuitry involued

When an electric circuit is getting opening by a switch, we notice of course that the current stopS.to flow but we can also observe a very brief flash more or less bright depending on circumstances between the metallic separating parts of the switch.

This flash, called spark, is the result of a complex phenomenon in which take place simultaneously the burning of metallic particules pulled away from the switch and the ionization of gases situated at this spot and being induced by the electric field existing between the poles of the switch.

Lamp

+

T Figure 1

This spark, which is very hot, can be sometimes able to ignite an atmosphere become flammable by pollution of combustible gas, vapeur or dust.

The idea to reduce the energy dissipated in this spark, to avoid to ignite the atmosphere, is very old, because at the beginning of the century, it was applied in coal mines in United Kingdom to a sounding signal deviee, made of a bell and an electric operating hammer.

Studies carried out on the igniting capability of these sparks have shown the influence of the main following parameters :

-Value of the open circuit voltage -Value of the current flowing through the closed circuit -Stored energy in the inductances and the capacitances -Nature of materials located in the circuit between which sparks are created -Pressure of contact -Nature of the atmosphere Basides, it is also well know since a long time, that abjects or parts of abject being brought up to high temperature, are able to ignite a flammable atmosphere without any spark being necessary.

From Joule effect, an electric circuit is capable to ignite such an atmosphere.

The parameters which ignition hazard by hot surfaces depend on are -the temperature of hot parts -the surface of these parts -the nature of the atmosphere An intrinsic ally safe circuit will be then by principle a circuit in which all the parameters, mentioned above, will have such values that the ignition hazard will be excluded.

-STANDARDIZED SPARK TEST APPARATUS

To study ignition hazard of an electric circuit, it is necessary to use a deviee producing opening and closing of this circuit, in a reproductible manner during the time, simulating realistic cases, not over or insufficiently severe. This deviee, called "spark test apparatus for testing intrinsically safe circuits" is described in IEC publication 79-3 and in European Standard EN 50 020. This apparatus had allowed to study the ignition limits obtained by different types of circuits and for different gases from which are here a few examples taken from EN 50 020 standard.

R>>r

...

___ c

Figure 7 3 -CLASSIFICATION OF ELECTRICAL APPARATUS As we have exposed above, the ignition hazard of an atmosphere depends on the nature of this atmosphere.

To simplify the use apparatus are part groups.

of safety apparatus which the intrinsic ally safe from, the apparatus have been classif ied in two Group I contains apparatus used in gassy mines.

Group II contains apparatus used in industries other than mines.

The variety of atmosphere found in these industries require the classification in 3 subdivisions.

The meaning of this subdivision is exposed in EN 50 014 european standard -Electrical apparatus for potentially explosive atmospheres -General rules -For intrinsically safe apparatus, gases and vapeurs are classified according to the ratio of their minimum igniting currents (MIC) to that of laboratory methane.

The MIC are determined with the spark test apparatus described above.

The atmospheres are so classified in 3 subdivisions :

-Subdivision A -Subdivision B -Subdivision c MIC ratio above 0.8 MIC ratio between 0.45 and 0.8 MIC ratio below 0.45

A second classification of apparatus of group II is done according to maximum surface temperature that they can have got.

The table below taken from EN 50 014 presents this classification.

EN 50014 The maximum ambiant temperature is conventionnally fixed to 40°C except opposite requirements.

For components which the total surface area does not exeed 100 cm2, their surface temperature can exceed those of the class of apparatus if special tests show that the igniting hazard is not possible.

-INFLUENCE OF CONSTRUCTIONAL QUALITY OF APPARATUS

-Infallible components and infallible assemblies of components

Let us devise the circuit below which can be, for example, those of a remote control for a ring or a relay.

R

Bell '

Figure 9 The presence of the resistor R limita the current of the circuit at such a value that the spark produced in the switch, is unable to ignite the atmosphere around the switch.

Where the apparatus is not designed in such a way that the short circuit of the resistor R is impossible during the time, the circuit can become hazardous. The resistor R shall be infallible.

Let us devise now a modification at this circuit.

R'

Bell R'

Figure 10 The short circuit of the resistor R' does not compromize the safety of the circuit, but the short circuit of the two resistors R' is hazardous.

It is not necessary that each resistor be infallible. It becomes only necessary that the assembly of two resistors should be infallible.

The infallibility of a component or an assembly of components complies to definite requirements based under circumstances on :

the nature of components. For example, a single semiconductor cannet be infallible by its own. The EN 50 020 standard precises what kind of components can be accepted as infallible (Resistor, transformer),

-the characteristics of components (minimum voltage, rating, • -The category of the wished safety There. are two categories of intrinslc safety. The first requires from apparatus a reliability higher than the second. The two categories are described below.

-categories of intrinsic safety apparatus

In the above example, the fault of one of the two resistor R' did not induce the production of an explosive hazard.

However, in arder that the apparatus shoud be effectively safe, it must evidently be that the short circuit of R' be the result of very low likely circumstances and over all, it must not be the result of a normal aging or a normally foreseeable damage.

As a matter of fact in the opposite situation, the second resistor could thoroughly, for the same reas ons than the preceeding cnes, be the subject of a second short circuit and then the apparatus should be hazardous.

The number of faults, acceptable before the production of a hazard, depends on the category of the apparatus.

The standard prescribes two categories which, in arder to simplify, are the followings : the category "ia" in which the apparatus remains safe even when two simultaneous faults are considered and the category "ib" in which the apparatus shall remain safe with one fault.

To consider that an event be really a fault and not a normal circumstance, the following requirements shall be complied :

the rating of component is limited to 2/3 of the nominal rating defined by the manufacturer (U,I,P.) -sorne components are excluded (tantalum capacitor for example) -the segregation of bare live parts of circuits in which they belong shall also comply accurate prescriptions. The values of clearences, creepage distances and distances through casting compound cannat be below the third of those prescribed for infallible insulation.

-ASSOCIATED APPARATUS AND INTRINSICALLY SAFE APPARATUS

In the two preceeding examples, the power supply of the ring was not safe ta ignition hazard, only the switch and the conductors up to the switch make up the intrinsic safety circuit.

On the scheme below, the apparatus framed by a dotted line, compose on elementary deviee of what the EN 50 020 standard calls an associated apparatus. In the case of connection 2, it appears cleary that a fault between the intrinsically safe circuit and the ground can be particularly hazardous because no impedance ether than those of the circuit will only limit the fault current.

! ,i t•• j ! • i I l 1 ! 1 R' l ~--~~ $ 1 ! 1 ! ! 1 l~--••-••-••-.. -•-••-~-••-••-••-••-••-••-••-••-••-••-••-••-••-••-••-••-••-••-••-••-••-•:
In one case, as in the ether, the circuit, choosed to be an example, could not be recognized as intrinsically safe.

In the fact, an infallible transformer in the way previously prescribed shall be a part of associated apparatus. Typically barriers are used to connect intrinsically safe transducers to electronic apparatus which are not itselve intrinsically safe.

l 1 i i i l 1 L--•-•-••-••-••-•-••---••-••-••-•---•-••-••-••---•-•---••-••-••-••---•-••-••-••---••-•-.J
The hazard from a power supply connected to ground is the same than those previously studied and the use of a barrier showed above can only be accepted if its terminal C is permanently connected to ground.

-Loop closed by a common return

When a same elementary circuit is recurring, a trouble can appear at the level of an eventual common return.

-------

~------- ,_ ______ _ ------ R T .._4! _________ _.
Figure 16 The circuit drawn shows a control circuit of 4 relaya situated in series in a circuit composed of a push-buttom BP, a resistor R and a voltage supply u.

Each elementary circuit is intrinsically safe, but the current in the common return conductor is not safe.

To avoid that, it is necessary -to install the same number of return conductors than the way out conductors -to keep an eye at the segregation of return circuits in spite of being connected to the same potential mode.

-Induction hazard by an external supply

Let us take again the over-simple preceeding circuit.

R

Bell

I'

Figure 17 For economie hidden motives, the way out conductor A is separated from the return conductor R.

When an alternative magnetic field flows through this loop, an eventually hazardous induced current I can be generated.

The eventual supplies of magnetic field are, in addition to radioemitter, the situation bas been met the electric circuits of high power or those which present high unsymmetric of their conductors such as arcwelding.

To avoid that, it is suitable :

to see that all the conductors of a same intrinsically safe circuit are the least remote as far as possible (cable) -to remote the conductors of electric cables. Beyond a few centimeters, the result from magnetic fields generated by symetrical conductors is of no value.

  ".!1!L-l-~lU-!====1itt.:ft:l=t=::::lnsulated boit ........ --. _,_..,...-Current connection Gear-wheel drive 50:12 ratio r--•--• j Drive mo tor with gears 80 rev/min . . -. . • • • -•---• _. , _ --•--• --1 l -Examplc of a practical design of lest apparatus.

  FIGURE 2 FROM PUBLICATION 79-3 . IEC Recornmendation

Figure 11 In

 11 Figure 11

Figure 14 This+

 14 Figure 14This example sums up a great number of conditions rrequern;..Ly l.n:::.Luy m::n .. espacially in barriere use.The diode safety barrier lies on the following principle

Table 1 .

 1 Classification of maximum surface temperatures for Group Il electrical apparatus

	Temperature class	Maximum surface temperature (°CI
	Tl	450
	T2	300
	T3	200
	T4	135
	T5	100
	T6	85


P Michaelis

G Mavrothalassitis

J P Pineau

CONTRIBUTION TO BOILOVER AND FROTHOVER QUANTIFICATION

INTRODUCTION

Frothing phenomena are related to the presence of water and liquid hydrocarbons in a tank heated by a fire or heaters.

One may quote accidents occurred in Signal Hill (USA), Milford Haven (UK). Pernis (NL), Thessaloniki (GR), Port Edouard Herriot (F), Tacoa (VA), Yokkaichi (JN) ... each of them with important damage.

One first has to distinguish between boilover and frothover, the main difference being in the piston-effect created.

To have a boilover three conditions are necessary : l) generation of a heat wave that enters into contact with water below the hydrocarbon, 2) presence of water to be converted to steam, 3) a viscous hydrocarbon which steam cannot readily pass from below.

To have a frothover similarly : l) a tank heated at a temperature exceeding the boiling point of the accidental fluid, 2) either water or light hydrocarbon being pumped into the tank by mistake, or a layer 7th International Symposium on Loss Prevention and Safety Promotion in the Process Industries, Taormme, 4-8 mai 1992

of water in the tank bottom being disturbed, or an emulsion water to hydrocarbon being dissociated. 3) a viscous hydrocarbon contained in the tank.

Piston Effect

Creation of steam generates a volume resulting from an adiabatic expansion and acting like a piston. The positive displacement of that piston is equal to the ratio of the steam volume by the cross section area of the tank. The displacement velocity of the piston can be quantified by identifying the adiabatic expansion energy with the cinetic energy transmitted to the liquid hydrocarbon contained into the tank.

Volume of steam created can be simply caiculated using the Avogadro and Ampere law, the ideal gas law or the Callendar and Mollier equation.

In case of boilover we can write, VVAP = (.25.Tt.DEQU 2 . EP. ^20-TWAV.R)/PVAP (m3) (eq. l)

with DEQU, the diameter of the tank (m) EP , the thickness of water bottom (m) f^EZO' tne density of water (kg/m3) TWAV, the heat wave temperature (K) is defined hereunder R , the specific ideal gas constant (J/kg.K)

PVAP, pressure at the interface hydrocarbon/water bottom (Pa)

TWAV is caiculated : with, PATM, the atmospheric pressure (Pa)

TWAV = F (t
HLIQo, the liquid hydrocarbon height when boilover occurs, (m)

The piston velocity can be expressed äs follows :

Tank Roof EJection

The cinetic energy of the expelled hydrocarbon is converted into energy for tearing section of the roof from the shell of the tank (case of frothover) and/or into energy of froth expulsion filling and overflowing of the diked area.

Tank roof ejection involves the problem of quantifying trajectory and impact conditions of plate-like fragment or missile.

Three main Steps are developed ;

1) The definition of the roof Fragment : shape, mass, area.

We suggest to consider the missile corresponding to a circular break in the roof, the diameter of which is 75 % of that of the tank, and which is tangent to the tank shell.

2) The caiculation of the initial velocity and elevation angle

The piston effect hereabove described, increases the tank pressure causing the roof to rupture.

The initial velocity of the missile is a function of the available energy at the time of bursting and can be written äs,

VMIS = [(2.F.öp.(VEQU-VLIQ))/(MEQU (l+6pi,)3^-3 (^/-i))]0.5 (m/s) (eq.7)
with : VEQU, the tank capacity (m3)

VLIQ, volume of liquid hydrocarbon at time of frothing (m3) MEQU, mass of tank + liquid content, at time of frothover (m3) ÖP, pressure difference in tank at bursting (Pa) F, yield factor for fragment energy F = 0,2 with brittle fracture F = 0,6 with ductile fracture €pr. fraction of strain of the tank material on fracture \/, ratio of the specific heats in vapor phase.

The System is a cylinder under pressure generating vertical force field within a conical roof : direction of ejection will be quasivertical, and we consider accordingly an elevation angle of 80 .

3) The caiculation of the trajectory Once the roof fragment has acquired its initial velocity, ballistic caiculations take account of gravitational forces and fluid dynamic forces, namely drag and litt components.

Drag coefficient is quantified using CLANCEY's velocity relation with :

CDG, the drag term divided by gravitational acceleration (s/m 2 ) tf, the total time of flight, (s).

Overflowing and spillage

The model proposed to assess overflowing phenomenon inside and outside diked area, assumes a concentric stream tubes configuration, and caiculates for every tube the maximum horizontal impact distance.

being the source of an elementary spillage.

Four steps are developed :

1) The definition of the stream tubes configuration upstream the passage through the tank roof : l central cylindrical stream tube and n-1 annular stream tubes with the condition that every stream tube contains the same mass of hydrocarbon (see fig. l).

2) The alteration of the stream tubes configuration when passing trough the roof hole, due to constriction and asymmetry phenomena : 3) The velocity and angle of elevation distribution.

Initial velocities are assumed to be distributed according a parabolic law. For tube i, the velocity is given by : Vi = (VMAX'/^2)^! -4 (ri^DEQU 2 )) l ä i -s, n (eq. 9)

with, CÜ , the constriction factor (ratio between area of the hole in the roof, and tank cross section) r^, the radius of the circle being the external perimeter of the stream tube i in the roof cross section.

VMAX' = VMAX in case of surface burning tank, the roof being yet Accuracy of the spillage model is increasing with the number of elementary stream tubes. A minimum of four stream tubes is required.

The procedure of quantification is to be iterated in every direction normal to each side of the dike.

Fire ball

Fire ball is a complex phenomenon and not easy to quantify.

At the start of a strong boilover a quickly ascending column of rieh vapor burns at a high elevation when air mixes to support combustion.

A mushroom shaped flame, accompanied by suddenly increased radiant heat, may be the spectacular part of the phenomenon.

Vapor mass available for burning into a fireball is caiculated äs follows :

1) Determination of the liquid mass remaining in the tank when boilover Starts.

In the case of a tank fire, one may write : An order of magnitude for hydrocarbon mass participating to fireball phenomenon is about l percent of the mass contained into the tank betöre fire happens.

Fireball size and thermal radiant heat flux are quantified assuming a spherical model.

Fireball combustion is laminar and assumed to burn at the upper flammability limit. Intensity of the radiation at the source may be about 150 kW/m 2 in this case. Temperature of fireball is about l440 K.

This intensity of radiation is less than the value ordinarily considered for a bleve fireball (200 to 300 kW/m 2 ). This is likely due to smoke environment of the fireball in the case of liquid hydrocarbons.

For estimating effect distances to receivers, one has to quantify the fireball duration, 9 and the threshold values related to radiation effects on human being.

An other approach we propose, is to correlate value of heat flux received on target with prequantified consequence äs a function of the time of exposure :

Methods to prevent boilover or frothover

Boilover or frothover generation is nowadays uncompletely understood.

Some ideas arise when studying the mechanisms.

A frothover can be detected by very high level measurement, increase of presure sensor and froth detector. Modern heated tanks are generally fitted with. The Signals delivered by these sensors are usually actionning to dose valves on the heating and feeding circuits of the tank. These devices reduce the energy available in the System and lessen the potential effects of the phenomenon. Reducing the increase of pressure in the tank may be obtained by weakening the link from roof to shell.

About boilover, few litterature is available, among which Hasegawa and

Risinger. The methods mentioned are :

-draining of water in the bottom of the tank or eliminating it betöre the heat wave reaches it, -cooling of tanks and/or extinguishment of the fire by base foam injection, -use of agitation methods (mechanical or pneumatical).

These methods are to be tested. The devices and procedures they need may rise difficulties and make necessary a modelization of what may happen in the typical case studied.

CONCLUSION

A quantification of boilover and frothover has been proposed. These phenomena need further research, mainly related to fireball aspects.

One of the main features of these phenomena is that they need a delay to occur.

Methods to prevent boilover and frothover are quoted. They have to be tested.

VMAX

  fluide betöre expansion (m3) MLIQO, mass of hydrocarbon expelled by the steam piston (kg).

(

  mainly valid for bulky fragments). Lift coefficient is determined äs a function of the angle of attack. The problem implies the movement of a solid body expelled with an 80° elevation angle into an uniform gravity field and subjected to resisting environment. Equations of motion are written for acceleration in the vertical and horizontal directions. Solving of the differential equations is performed using RUNGE-KUTTA or EULER-CAUCHY methods. Conservative solution may be easily expressed neglecting the litt coefficient ; the maximum impact distance of the roof fragment becomes, RMIS = (.102/CDG)ln (l + 1.703 VMIS. tf.CDG) (m) (eq. 8).

l

  annular external stream tube tangent to the tank shell, l central cylindrical stream tube, and n-2 lens shaped stream tubes between, all of them tangent to the cylindrical stream tube (see fig. l).

  fig. 2) :-annular sector type spillage : normal use for outside diked area spill description ; -ring type spillage : to describe the spillage induced by the annular stream tube tangent to the tank shell ; -circular segment type spillage ; applied to the first stream tube exceeding the diked area in a certain direction ; the chord of it is identified with the dike external slope ; -circular area type spillage : it concerns the stream tube with maximum momentum and elevation angle, when radius inherent in the circular spillage, is neither secant nor tangent to the external perimeter of the preceding elementary spillage.

  MLBO = MLIQ -(.25.n. DENSL. vl. t.DEQü 2 ) (kg) (eq. 15) With MLIQ, the mass of liquid hydrocarbon into the tank at the time the fire Starts, (kg) vl, the hydrocarbon burning rate (m/s) 2) Caiculation of the fictive increase in temperature of the liquid mass MLBO when piston effect generates depressurization to atmospheric pressure : OTI. 3) Caiculation of temperature increase due to heat exchange by radiation and convection through the cylinder of flames of the burning reservoir : üT2. 4) Quantification of the fraction of liquid mass MLBO flashing off :

  , A^ = -.776 B^ = 4.2378 Distances with damage to receivers can then be expressed äs : Ri = l>max 2 (^O.T/^-h 2 :]^ (eq. 21) with R^, distance measured from the tank axis (m) h, center height of fche ball above ground (m) ^max' maximum radius of fireball (m) ^max = (.75.(MVAP/(UEL.DENSG)/7r)-333 (eq. 22) UEL, upper explosion limit of hydrocarbon {% vol) DENSG, gravity of vapour hydrocarbon at fireball temperature (kg/m3) T, attenuation factor.


Min Depc

BENCH STABILITY IN OPEN PIT MINES : A METHODOLOGY FOR JOINTED ROCK MASSES

The methods used to design ehe global angle of open pic mines are noc adapced (o design ehe benches. This paper presents a mechodoiogy in order co scudy ehe scabilicy of ehe benches. This meihodology is based on geomearical modelling of ehe rraccured rock mass wich provide an assemblage of blocks. The block siability is analysed by mechod based on limic equilibrium. In order co take inco accounc ehe s;aciscicai aspect of ehe discondnuides, an opcimal nu-mber of simulacions is recommended. Thus, ehe resul[s can be incerpreced scatisrically. This mechodology was applied for 22 benches in 5 dufereru mines, ehe resuks obcained are consiscenc wich ehe observed instabilides.

When the slopes concain benches and berms is: is noc possible a pdod to elirmnace all risks of failure of ehe same scale äs the benches. Indeed, chis risk is greacer for vedcal or almost venicai facss and ii: would require coo decailed a knowledge of rhe SG""dC;ure of the rock mass, w'nich is noc possible s-c this Stage reasabilicy, for a forecasi co be perfecdy reliable. On ehe ocher hand some degres of risk of failure on a scale comparable wich ehe benches may be accepced if ehe rock falls are haked by horizoncal berms or can be prevenced by local reinforcemenc, installed äs soon äs any excsssive risk is idencified. For these condidons [o be realised, one must be able co predict chese failures and their effects and co have mechods, prevencing chem, available. This paper describes a mean of improving ehe exisring mechods for ehe predicuon and prevendon of bench failures.

L INTRODUCTION

The methods used to determine ehe overall gradiencs of ehe slopes in open pic workings aim to sufficiendy eliminace all risk of failure on a scale cocnparable wich i:he pit wails. For chis ehe slopes musc be made shallow gradual and reinforcing measures must generally be applied. Mos; often these consisi of ensuring the drainage of the slopes and in limidng the effects of blasring (reinforcemenc is generally too cosdy in ehe case ofvery high slopes a-nd deep planes of failure).

TRADITIONAL METHODS OF STÜDYING THE STABILITY OF BENCHES

The traditional method

In order co esdmace ehe scabilicy of benches one generally uses ehe same methods äs chose used for [he pic walls or ehe slopes srudied in civil enginesnng (Hock and[START_REF] Hoek | Rock Slope Engineering -The Insdcudon ofMining an Metallurgy[END_REF][START_REF] Hoek | -Travaux de surface au rocher -Industrie minerale[END_REF]. First chese methods consist of all of idendfying the mosi unfavourable mechanis-ms of potendal failure, based on the siruccure of the rock mass and which is more or less known betöre ehe excavadon (one is normally aware of the general fonn of the sü-aca and the large tectonic irregularities, the smaller discondnuities being ofcen modelled in the form of sets deflned by their average orientadon). Safety factors are then caiculaced for the mechanisms idendfied, then the gradiencs of the slopes are determined in such a way that the lower factors are greater than a cenain value selected by the geotechnician (äs a funcdon, especially, of the Situation of the working, the mechanism of the failure, the caiculadon model used and of the uncercainty affecdng the daca). The values selecred in the case of mining engineering are lower than those selecced in civil engineering. In effect, the life of mines is shoner than that of civil engineering Structures and the latter are noc open to the public, are frequented daily by know[edgeable staff and are regulariy inspecied.

Although the methods are well suiced co the case of the pit walls and intermediace high slopes in the case when one wishes to avoid any risk of failure during the period of exploitadon, we considered [hat they are less well adapced for evaluadng ehe risks relacing co failures ac ehe benches scale.

Limitations of the traditional method

Experience shows on the one hand, thac it is pracdcally impossible co avoid any risk of failure wich respecc to the benches and, on ehe ocher hand, chac a great number of failures are not dangerous, because the fallen blocks are halted by the horizontal berms separadng the benches. Indeed an inquiry (NGOT CONGOLO, 1990) reladng to five mining sices and over a cumuladve bench length of 3775 mecres, made it possible to disdnguish beiween four sraies of bench stabilicy. * The stable stace is deflned by the absence of fallen blocks on the berm situated ac the foot of the bench. It represencs 26 % of the cumuladve length of the benches. (state: very good) * The second siaee corresponds to the case where some isolaced blocks have fallen on the berm. It represencs 33 % of the cumuladve length of the benches. (state: good) * The third snate, which represencs 19 % of ehe cumuladve lengeh of the benches examined.

corresponds to the case where the berm is covered by a large number of blocks. (siace: medium) * The totally unstable state, which represents 22 % of the length observed, is defined by the crumbling of the benches. (stace: bad)

The state of stabilicy thus deflned is direcdy relaied to the danger of rock collapse which could threaten the personnel moving about at the foot of the pit walls. Note that other paramecers recorded, such äs the difference between the angle provided for and the observed angle of the benches, or the percencage of pre-cut relief holes lhat are still visible, seem to be less represencadve of the danger. As a matter of fact the failures relaced to these could have occurred during blasdng or during the loading of the mined macerials and in chis case could never have consdtuced a danger. They correspond to very low safety faccors.

The tradidonal mechod does noc allow one co disdnguish in a sadsfaciory way, between dangerous and safe situadons by taking the example of wedge failures -it gives the largest unsiable volume thac may exisi in the case of a single failure (fig l a) buc it does noc disdnguish becween the case in which a large number of failures of such volume will aceually occur (fig Ib) and ehe case in which several failures of smaller volume will cccur. To evaluace this danger it is necessary co predict the spacial frequency of ehe failures and the probable volumes concemed. For thac ie is necessary eo cake inco consideradon ehe disrribudon in space of the discondnuities and their persisience, 3, PROBABILISTIC APPROACH TO BENCH STABILITY

Necessicy of a probabilistic approach

In order eo dimension ehe pic sides one tries to avoid any possibility of failure by choosing a mean safety coefficient sufficiently high to have a probabilicy of failure negligible or acceptable. One can then explain (or jusrify) the fact thac the probabilisdc aspect of the safety coefficient is rareiy allowed for in ehese studies. Anocher explanadon is provided by [START_REF] Hoek | -Travaux de surface au rocher -Industrie minerale[END_REF]: " The eva-luadon of the stability in eerms of failure probability in itself presenis a problem, most custpmers refuse to accept thac their consuldng engineer or cheir design Office recognises that there exisis a probabilicy, even a smail probabilicy of failure". A cenain value of the safety factor (for example 1.3) may be considered äs sufficient, for one is accustomed to that, and experience indicates that stabilky will be assured, whereas a failure probability of l in 1000 or l in 100 which undoubtedly means exacdy the same thing, will be regarded wich misc-usi.

If one no longer tries to avoid any possibilicy of failure, and this is the case when one is demensioning the benches, it becomes necessary to predict the spadal frequency of the failures and the volumes concemed. A determinisdc method (using the mean values of the input paramaters) would provide unsiable average volumes. Rather than applying a safety factor to [hese volumes, it seems to us more realisdc to malte the probabilisric approach and thus to reason in terms of volume having a low probabilicy (chosen in advance) of being exceeded-Funhermore this type of approach should be more easily accepced by the miners in the contexi of the mine benches for it does noc exclude the possibility that; cercain failures will occur.

Methodoiogy proposed

Scru-c-tural Study and swislical treatment of discontinuines.

The structural study should lead to the idendficadon of the major dicondnuides at the bench scale, for example, the faults and the so-adgraphy äs well äs the different joints set. For the joints set, the orientadon, the extension and the spacing must be deflned stadsdcally. A large number of studies [START_REF] Hudson | -Traicement siadsdque de la fracturadon -Etüde bibliograohique[END_REF]) (KABBAJJ989) have shown thac in the majority ofjointed rock masses the oriencadon is in accordance with a LANGEVIN-FISHER stadsrical law, the extension and the spacing obey exponendal and log-normal laws. The quality and the number of data guarantee a good siadsdcal definidon.

Geometrical modelling of the bench.

Once the discondnuides are determined and betöre scudying the stability of the bench, a seometric model of the bench is preoared with RESOBLOK (HELLIOT. 1988) a block generator Software.

The RESOBLOK Software requires, first of all, the definidon of a "zone of inceresi" in which the discondnuides of the rock mass will be simulaced. This zone is defmed by the heighc, the length and the width of the bench srudied. The discondnuides can then be placed in a determinisdc way, in ehe zone of incerest or creaced by simuladon on the basis of siadsdcal laws defining the distribudon of oriencadon and spacing of each set.,The extension of the discondnuides is incroduced by defining the blocks cuc ouc by the latter. For example, the diaclases of a cenain set can affect only the blocks siruated berween cwo given scradgraphic joints; in ocher words "they will stop" on diese joints: ic is a quesdon of a hierarchic approach in Order to simulate the discondnuous rock masses.

Generally it is difficult to esdmace the discondnuides extension on the field. Simuladon in the geomeo-ic model, by a so-ict method and taking inco account the hierarchy becween sets of fractures is noc easy. Thus, äs a first approximadon, and in the absence of any clear hierarchy between fTacmres, the extension may be considered on the scale of the bench. decennine c and (p by back-analysis. This approach was used by [START_REF] Baroudi | -Udlizadon of the block generator RESOBLOK to complex geologic condidons in an open pit mine[END_REF] and (RODE and ai, 1990); the values measured in the laboratory and those caiculaced by back-analysis were of ehe same order of magnitude. In order co simplify ehe back-analysis [he cohesion of the discononuiües may be assumed to be nil, for the latter are dose to the working faces and have thus been subject to the effects of blasring.

Distribution of unsiable blocks.

For stadstical äata on discontinuities ic is necessary to apply several Simulation processes to the same bench. Each Simulation provides a set of blocks for which the stabilicy has to be investigated in accordance wich the mechod previousiy explained. For each Simulation (i), the tocal volume (v p of the unstable biocks is one of the characteristic papameters of the stäte of srability of the bench. For a number (n) of simulacions, the (v.) consricuces a random variable (v) of mean (m) and Standard deviation (o).

v.) consricuces a

Geomechanical dam andstudies ofsiabuiry.

The geomecric modelling of the bench provides a group of blocks for which the scability has to be studied. The BSA Software has been wricten following RESOBLOK, k enables us to analyse the stabilüy ofsingle bloks . Tne fundamencal aisorichrn is thac of WARBÜRTON [START_REF] Warburton | Vector stabilicy analysis ofan arbio-ary polyhedral rock block with any number of fre& faces[END_REF] and it comprises two stages :

-geometric analysis in Order co identify whecher the block is displaceable.

-mechanical analysis based on the limiting equilibrium of the block using ehe Mohr-Coulomb cricerion.

In the case of insrabilicy, three types of failure can be detennined; free fall, plane failure, or wedge failure. Tne rocadon is then analysed in accordance with ehe algorithm of (LIN and al,198S).

The propenies of the discondnuiries necessary for the analysis of stability are ehe cohesion (c) and the angle of friciion ((p). These are generally detennined in the laboratory by cmpirical (BARTON and 3.1, 1990) It i-s demonstraced (ASOF, 1991) -see APPENDIX A -thac the en-or relarive to the estimadon of (m) on the basis of (n) simuladons is 2 / (n + l), thus for 40 simuladons the eiror is less than 5 %. of the bench.

ANALYSIS OF THE STABILITY OF 22 BENCHES

Inßuence ofscale.

In reality the length of a bench can exceed 50 m or even 100 m. This raises the quesdon of what representadve length of bench should be modelled. This length must be determined by considering two criteria.

* The fracturadon must be homogeneous over the lengch considered. A reladvely long bench must be cuc into homogeneous secdons. * In the case of a homogeneous section the length (L) to be modelled musi be greater than the minimum length (Lmin). The lauer is defined such thac the mean of the total volumes of ehe unsiable blocks (m) is independenc ofL.

For example in the case of the bench previously referred to, (Lmin) is esdmated to be 8m. On five different sites 22 secdons of benches were seudied in accordance with the preceding method [START_REF] Kongolo | Scabilice des gradins des explokadons a. ciel ouvert. Approche siadsdque mecanique et probabiliste[END_REF]. In Order to campare the observadons in situ wich the results obcained by modelling, two global variables were selected.

* The qualitative variable "S täte of bench" introduced in chapter 2.2, which characterises the observed stace of srabilicy of ehe bench-* The variable (m): mean total volume previously discussed. To ensure that (m) was homogeneous througho.ut the different benches k was reduced co a length of 5 mecres. Fig 5 represencs (m) äs a funcdon of the "State of the bench". For 22 benches, äs a whole there is a good degree ofcorreladon. The deviadons from this tendency thac were observed could be explained by: -The subjecdve characcer of ehe variable "Stace of the bench". Only ehe extreme siaces are clearly discriminaced ('"very good" and *'bad"); imermediate Staces are generally noc clearly assessed.

-The geomechanical properdes of the fractures could noc be decermined for all the benches so cenain analyses are approximate.

In'the case of benches in very good condidon HOEK's tradidonal meihod offen gave "pessimisdc" results; greacer instabilides than were actually presenc. By the method proposed the results are more realisdc; this could be explained by the fact that one is ta-king inco account the dispersion around ehe average oriencadon and the average spacing. The angle and the oriencadon of the bench may be optimised by minimising the volume of the unstable blocks. Fig 6 shows, for the previous bench, thac the most unstable siace corresponds co an angle of 80°; this resuk is in effecc foreseable wuhouc special modelling. The most favourable oriencadon is 60°, the volume of unsiable blocks being smallest for the differenc angles.

As for the case of the orientation and the angles, differenc heighcs may be compared in accordance wich the crecerion of minimisadon of ehe volume of unstable blocks.

As to the width of the berm, (HANTZ, 1990) proposes a method -too" long to be explained in this paper -based on the distribudon of the lengch of heap of fallen blocks and inspired by the method of [START_REF] Martin D.C" Piteau D.R | -Select benn widch to contain local failures -E/MJ Operadng Handbook of Surface Mining[END_REF]. This width is determined from the results obcained wich RESOBLOK-BSA.

In the case of unstable blocks BSA provides the plan for reinforcement (number and oriencadon of boics) in accordance wich a method adapced from [START_REF] Shi | Keyblock boldng[END_REF]. Developments are in progress to maJ<e it possible to evaluate a given sysrem of reinforcement. In this case it is a question of decerTnining the disiribution of the total volume of unsiable blocks in the case of the reinforced bench and chus comparmg different sysiems.

To make ehe mechod operacional for dimensioning in a given sice a scage of validadon is necessary. In the absence of quantitative data a comparaison of the condidon of the bench and the resuics of simuladons helps us to determine the validity of the approach.

CONCLUSION

In this paper a methodology has been propoded for studying the stability of the benches in open-pit mines in fractured rock masses. This methodology is based on geometric modelling of the bench, based on characterisarion of the secs of discontinuides. The stability is then considered by analysis ac the limiring equüibrium of single blocks. Several simuladons are carried out providing a stadstical distribudon of the volumes of unstable blocks.

This methodology has proved to be better adapced than the tradidonal mechods used at the slope scale. After validadon, even a summary validadon, the method can help in the dimensioning of the bench and in evaluadon of the reinforcement pattems.

BSA may also be adapted to improve the analysis of single blocks, for example to make it possible to amalgamate a limiced number of blocks and study the stability of ehe resuldng block. Never-theless it is essendal that the principle of the method should remain simple (few parameters to be incroduced) and easy to use.

  Fig.l : Classical meihod resulis andpossible aspeci ofa real bench.

Fig 2

 2 Fig 2 shows an example of a geometric represencadon of a bench in an uranium mine. The

Fig. 2 :

 2 Fig. 2 : Visualisadon of ihe discondnuities nerwork of the bench -Simulation n°.l

  or experimencal mechods. If one has local failures on ehe scale of a given bench one can In a large number of cases (HANTZ, 1990) and for a considerable number of simulations (discussed below), (v) follows an exponencial law. For example, in the case of the bench shown in fig 2, rhe distriburion of (v) for 50 simulations is represenced by fig 3. The validky of ehe exponendal adjustmenc may be verified by comparing the mean (m) and the Standard deviation (o') which are equal in the case of the exponendal law.

Fig. 3 :

 3 Fig. 3 : Distribution of the tocal volume of unstable blocks for 50 simulations.

  Fig 4 shows that above thac limit one can out m = 0.4 L. Average of tocal volumes of imsca.ble blocks (rn3) Length of ehe bench (m) 10 12 Le-ngch of the bench (m)

Fig. 4 :

 4 Fig. 4 : In.fluer.ce of the bench lengch .5. CONTRIBUTION OF THE METHODOLOGY PROPOSED FOR THE DIMENSIONING OF THE BENCHESNote that if the actual length of the bench is less than (Lmin) the siadsdcal simuiadon of the fractures is noc valid. That would indicace thac the disiribudons of the secs o f fractures are noc represencadve of such a bench dimension. In this case one musc introduce all the fractures in a determinisdc manner in order to model the geomeoy The methodology proposed may be of help in dimensioning the benches of open-pic mines-The parameters thac may be studied are: angle, oriencacion, heighc, width of the beim and reinforcement.

Fig. 6 :

 6 Fig. 6 : Optimisanon of orientauon and benchangles.

v. is the total volume of instable blocks for simuladon number i. v. consdtutes a random variable v.

For the (n+l)" simuladon, one can write:

For an exponencial law of v,'\with 95% probability, we have :

Where X is the parameter of ehe law estimated bym":

From ( 2), ( 3) and (4) :

-l/(n+l)<(m^-m^)/mn<2/(n+iy ( 5)

From ( 5) :

Im^-m^ | /m"<2/(n+l) (6).