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Hazards due to electrostatic charging of powders
Journal of Electrostatics, 16 (1985) 175--191
175
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
HAZARDS DUE TO ELECTROSTATIC CHARGING OF POWDERS M. GLOR Ciba-Geigy Ltd., CH-4002 Basle~ Switzerland
ABSTRACT The role of static e l e c t r i c i t y in the field of dust explosions is discussed. A general approach to the assessment of hazards and to the choice of safety measures with respect to electrostatics as an ignition source is outlined. Methods and results describing the charging behaviour of powders are reviewed. The problems to keep the charging level of powders at a safe l i m i t are discussed and compaired to the situation for liquids. The problems and trends in the determination of the minimum ignition energy of powders as a quantity to characterize the ignition sensitivity of powders with respect to electrostatic discharges are discussed. The different types of discharges associated with the handling and processing of powders are reviewed by referring to their phenomenological and theoretical description, their occurrence in practice and their incendivity. Special regard is given to the electrostatic phenomena occurring during the filling of large containers and silos. INTRODUCTION I t took a number of grave dust explosions, occurring in the European foodstuff and in the US grain industry (Bremen 1979, Metz 1981, USA 1976 - 1978) with a large number of fatalities until everybody became aware of the existence and the severity of such explosions. Though the first reports on dust explosions date back to the end of the 18th century (ref. 1) most activities in this field have for a long period of time exclusively been devoted to gas and vapour explosions. Only since the last one or two decades increasing research activities in the field of dust explosions can be observed. According to present statistics from Germany nearly every day one dust explosion occurs within this country (ref. 2). The same statistics claim that 9% of all 381 accidents analysed were caused by static e l e c t r i c i t y i f all different kinds of products are taken into accounts whereas this percentage increases to 34% if only polymeric powders (47 explosions) are included. Several reasons can be indicated which explain why static e l e c t r i c i t y has become an always more serious potential ignition source in powder handling and storage systems. The increase in the transport velocities and the construction of always bigger bunkers and silos led to higher charging rates and to higher levels of charge accumulation. The l a t t e r is additionally favoured by the high resistivity of polymeric powders which are processed in always higher amounts. In addition, i t is presently realized that the sensitivity of many powders with regard to ignition by electrostatic discharges is much higher than was thought only 10 years ago. The values of the minimum ignition energies of certain organic
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© 1985 Elsevier Science Publishers B.V.
176
powders measured in the last years by d i f f e r e n t authors (refs. 3, 4) have become very low, overlapping with the range of values observed for pure gases. However, just because many factors indicate that e l e c t r o s t a t i c hazards in powder handling systems must not be underestimated, the danger exists that static e l e c t r i c i t y is blamed for the actual cause of ignition in all cases where no other obvious ignition sources can be identified. Such a practice is very dangerous since it leads to wrong safety measures and presumably to further explosions. This is p a r t i c u l a r i l y true for powder handling and processing systems in which many other p o t e n t i a l ignition sources may be present. Compared to the situation for liquids, powder handling involves more moving construction parts which may generate mechanical sparks or f r i c t i o n a l heat. Very often a drying operation constitutes one part of the process during which exothermal reactions may lead to a thermal self ignition of the powder. On the other hand i t must be admitted that the present knowledge concerning the control as well as the occurrence and incendivity of electrostatic discharges in large powder handling and processing systems is far from being complete. In contrast to the situation for liquids much more work remains to be done in the field of powders. The present review does not at all pretend to answer all questions concerning the evaluation of electrostatic
hazards associated with
the handling and processing of
powders. It w i l l rather point to the specific problems which are intrinsically related to powders. I t w i l l reveal those questions which nowadays clearly can be answered but i t w i l l also point to the grey regions and comment on the present tendencies within these regions. Despite the c o m p l e x i t y in the assessment of the e l e c t r o s t a t i c risks involved in powder handling many fundamental investigations have been performed and extensive experience from industrial practice has been gained over the last decade by d i f f e r e n t authors
from
universities,
from
chemical
and foodstuff
industries
and from
other
organisations, see reviews by Haase, Hilgner, Gibson, Bright, Maurer and Cross (refs. 5 n). ASSESSMENT OF ELECTROSTATIC HAZARDS - A SYSTEMATIC APPROACH Ignition of an explosible atmosphere by a gas discharge represents the most severe e l e c t r o s t a t i c hazard in industry. To judge the probability of such an event for any given operation in practice the following characteristics must be considered: Rate of charge build-up, rate of charge relaxation, resulting level of charge accumulation, appearance of gas discharges, incendivity of gas discharges and ignition sensitivity of the present explosible atmosphere. Such a systematic approach leads to the well-known scheme represented in Fig. 1. Though this scheme looks rather t r i v i a l at first sight, i t is not always easy to localize each step with respect to space and t i m e for any given process. Of course, i t often happens that several steps occur simultaneously. The level of charge accumulation is determined by the equilibrium between the charge generation and the charge relaxation rate. Since all charge generation processes are in principle processes in which charges of
177
I CHARGESEPARATIONI
I
io...o.0c0.uL..o.L .....
1- ......
IOISCHARGEli
oF CHAROE
...........
I
i
I 'GN,T'°N I
:
EXPLOSIVE MIXTURE L . . . . . . . . . . . .
Fig. 1.
EARTH
I J
Most i m p o r t a n t steps in the evaluation of hazards due to e l e c t r o s t a t i c
charging. opposite polarities are separated, two d i f f e r e n t pathways, one for each p o l a r i t y must always be considered. The most commonly encountered situation when powders are handled, processed or transported in the industrial practice is roughly the following: Due to the continuous separation processes between the product and the construction parts charges of one p o l a r i t y are built up on the product and charges of the other p o l a r i t y are built up on the construction parts or released to earth. The f a c t t h a t for a given powder positive as well as negative charge build-up may be observed, is not relevant for the present discussion. This is of course only true as long as only the net charge is considered which again is reasonable as long as no large domains of positively and negatively charged particles within the bulked or dispersed powder are generated. Since the scheme represented in Fig. 1 allows to identify the steps which may lead to the ignition of an explosible atmosphere by e l e c t r o s t a t i c charge build-up, it is of great help during every discussion on all d i f f e r e n t kinds of safety measures. Clearly, the goal of every safety measure in electrostatics
is to prevent both the charge on the product and the one on the
construction parts to lead to a gas discharge which finally w i l l ignite an eventually present explosible atmosphere. The explosible atmosphere may be generated by the product itself or e m i t t e d from another near-by process. There are basically three d i f f e r e n t approaches to reach this goal: (i)
The level of charge accumulation is kept low by reducing the rate of charge separation and by increasing the rate of charge relaxation. No gas discharges have to be expected.
(ii)
It is well known which types of gas discharges have to be expected. These discharges are not incendive for the explosible atmosphere present.
(iii)
Ineendive discharges cannot be excluded. Other measures of explosion protection must be considered.
178
Measures of the first type are mainly applied in the case of flammable liquids and with respect to contruction parts as soon as f l a m m a b l e liquids or combustible powders are handled. Since i t is very d i f f i c u l t to control the charging level of a powder during an industrial process the measures applied in the case of powders are mainly of the second or third type. This means that every assessment of the e l e c t r o s t a t i c hazards associated with the charging of powders must concentrate on the following two questions: Which types of discharges have to be expected? Will these discharges be able to ignite the presumably present explosible atmosphere?
ELECTROSTATIC CHARGING OF POWDERS Independent of the materials of the installations most powders become charged during transport, handling and processing as soon as the powder resistivity lies above about l 0 8 Ohm • m, a value which is surpassed by most organic products. Since it is d i f f i c u l t to describe amorphous organic substances with the laws of solid state physics, there is still a lack of knowledge concerning the actual e l e c t r i f i c a t i o n mechanisms. But even if there were a general theory predicting the charging behaviour of pure substances with clean surfaces this would be of no practical relevance. Most powders processed in industry are not pure substances and their surfaces
are not clean. Even the slightest
surface
contamination significantly changes the charging behaviour. This fact which is responsible for the famous bad reproducibility of all e l e c t r o s t a t i c phenomena and which often drives the experimentalists nearly to despair can easily be understood from the following considerations. Under the assumption of a distance of several tenths of a nanometer between the molecules on the surface only about one surplus e l e m e n t a r y charge per every 1OSth surface molecule is needed to generate the highest possible surface charge density of about 2.7 • i0 -5C/m2 at a planar interface to air. For powders, the reproducibility of the charging behaviour is often worse than for extended solid surfaces. This can be understood from the fact that the net charge which is measured a f t e r the powder is processed may be the result of a small difference between the charges of highly charged particles of opposite p o l a r i t y . The change in p o l a r i t y which is sometimes observed in practice for powders showing a moderate net charge can easily be understood on the basis of the above considerations. Various authors have published experimental results on the charging of powders (refs. 12 -18) and the l i t e r a t u r e in this field up to 1970 has been c r i t i c a l l y reviewed by Lapple (ref. 19). It is apparent (refs. 20 - 32) that the interest in this subject has recently increased. Most of the methods described in these papers are designed to carry out determinations on one selected powder. Jansen (ref. 20) obtained results from about 20 d i f f e r e n t plastic powders and Boschung and Qior (refs. 31, 32) obtained results from about 200 widely d i f f e r e n t types of industrial powders. In these l a t t e r e x p e r i m e n t a l investigations the charging of the powders was measured in a l a b o r a t o r y apparatus simulating pneumatic transport. The specific charge was determined as a function of parameters such as powder quantity~ specific surface area, air f l o w rate, length, diameter and m a t e r i a l of the tube. When the parameters of the apparatus were
179 kept constant, the variation of the specific charge of all products investigated was in the range of 10-7 to 10-3C/kg with a definite dependence on the specific surface area of the products, as becomes obvious from Fig. 2. The reproducibility and the extent of scatter of the data was such that a classification of the powders according to their charging tendency was possible. Further experiments in which the charging tendency was measured by dropping the powders onto a quickly rotating conical disc showed the same relative classification (ref. 33).
As/kg
100
101
102
103
I
I
F
I
10-3
10-3
5
5
10 - 4
10- 4
5
5 UJ
(3 t¢ <( :E to 10_s ~ tlg
lO-S 5
5
/ u.
Q. 09
1O_S
10--6
5
5
10 - 7
10-7 5
5 /
10 -e
10-1
I
I
I
100
101
102
~
10 -e
m2/kg
SPECIFIC GEOMETRICAL SURFACE
Fig. 2.
Influence of the specific surface area of powders on the charging tendency as
determined by a laboratory scale pneumatic transport device (ref. 3i). Though it is obviously possible to classify powders according to their charging tendency in the laboratory an extrapolation of these results to practice can be problematic. Clearly, the absolute level to which a powder w i l l become charged in practice depends
180
not only on the nature of the product but also very strongly on the operation, as can be seen f r o m Table 1. Nevertheless one would hope that at least the r e l a t i v e charging tendency of the powders determined in the laboratory would be useful for a hazard assessment in practice. TABLE 1 Powder charging by different
industrial
operations according to Gibson (~'efs. 798),
C a r t w r i g h t (ref. 34) and Maurer (ref. 35).
Operation
Specific charge in C/kg
Sieving
10-11
_
Pouring
10-9
_
Scroll Feed Transfer
10-8
Grinding
10-7
_
10-6
Micronizing
10-7
_
10-4
Pneumatic Transfer
10-6
.
i0-4
_
10-9 i0-7 10-6
But even this hypothesis is rather doubtful. There is some recent experimental and t h e o r e t i c a l evidence that p a r t i c u l a r l y powders w i t h a rather moderate charging tendency may represent a hazard~ when they are settling onto the powder heap in a silo, as w i l l be discussed l a t e r in this paper. CONTROL OF THE CHARGING LEVEL OF POWDERS According to the present state of knowledge the charging level of liquids can much b e t t e r be predicted and controlled than the one of powders. This has mainly two reasons. During the last decades much more e f f o r t s have been made to control the hazards associated with charged liquids than with charged powders. The petrol industries have investigated many projects along these lines. However~ it must also be emphasized that in the case of powders the problems are of much higher c o m p l e x i t y . This has to do with the intrinsically d i f f e r e n t nature of liquids and powders. [n order to characterize e.g. a powder transport and storage system many more parameters have to be quantified (particle size distribution, specific surface area, skeletal and bulk density of the powder, resistivity of the bulk powder~ mass f l o w rate9 air f l o w rate~ dimensions of the transport pipe and of the storage silo, design of the powder inlet into the silo etc.) than for a similar system designed for liquids. In addition one must take into account the fact t h a t the powder particles normally become charged in a state where they are dispersed in air. When these particles fall down into a storage vessel the powder material as well as the charge trapped by the particles
181
become closely packed into the powder heap thus giving rise to a high space charge density. Due to this effect also powders exhibiting a very moderate charging tendency may give rise to high electric field strengths when packed into a heap. In contrast to the situation for liquids where no charged mists have to be expected during normal filling operations, charged dust clouds cannot be avoided when large amounts of fine powders are handled. The formation of dust clouds is even favoured by the effect of e l e c t r i f i c a t i o n since the highly charged fine particles are repulsed from the settled powder. Finally, the following important fact should be pointed out. The most common measures in the control of hazards from static e l e c t r i c i t y associated with liquids, namely the decrease of the transport velocity and the increase of the conductivity by the addition of an antistatic additive in very small amounts cannot be applied in the case of powders. A decrease of the velocity in powder transport systems is often not possible for technical reasons. Because of the high specific surface area of powders substantial amounts of antistatic additives would be necessary to generate a reasonably high conductivity of the bulked powder. Moreover, a surface conductivity of the particles which would be sufficient to ensure a quick charge relaxation from the bulked powder (resistivity of the bulked powder below about 108 Ohm • m) would not guarantee that the particles do not become charged at all when they are dispersed in air and make short contacts with each other and with parts of the construction. Since the common measures, such as reduction of transport velocity and increase of product conductivity which allow to control the charge build-up to a level where it ceases to represent a hazard in the case of liquids, cannot be realized in powder processing and transport systems there is a great demand to control the charging level by some other means in all cases where incendive discharges have to be expected. Principally, the methods of charge neutraiisation which are used to reduce the charging level of webs of different kinds of products, offer such an alternative. However, the charge neutraiisation process in a powder transport system must be active over a large volume in contrast to the situation prevailing in the neutralisation of the charges on the surface of a web. To overcome the drawback of short range action Larigaldie et al. (ref. 36) have deve|opped a so called "electro-gas-dynamic generator" which acts as a charge neutralizer by taking advantage of the low mobility of charged aerosols, carried away by a flow of gas. Despite the obvious improvements such neutralizers are not widely used yet to minimize the electrostatic hazards associated with powders. Further research w i l l be necessary to demonstrate whether and for which operations, systems and products incendive discharges actually have to be expected, in these cases such charge neutralization methods could be of great help but, clearly, every decision about their application should be based on a comparison with all other possibilities of explosion protection. HAZARDS ASSOCIATED WITH DIFFERENT MATERIALS AND OPERATIONS Since i t is very d i f f i c u l t to predict or control the charging level of powders, as
182
outlined earlier in this paper, hazard assessment in powder handling and processing systems i s i n most cases based on an assessment
of the occurrence and incendivity of
discharges associated with the given materials and operations. Therefore~ in the present review, the electrostatic hazards prevailing in industry are not discussed under the aspects of the different operations and materials involved but under the aspect of the different discharge types. The relation to practical situations is given by a description of the occurrence of these discharges in the industrial practice. POWDER IGNITION BY ELECTROSTATIC DISCHARGES The ignition sensitivity of a powder with regard to electrostatic discharges is characterized by the minimum ignition energy of the corresponding dust cloud. It is evident that the incendivity of a discharge should therefore be characterized by its energy release. The probability of ignition can then be judged by a comparison of these energies as is demonstrated in Fig. 3.
Incendlvlty
Minimum
E
Ignition energy
uJ
--*]
10s10=.
E
--
103 102 101
r--ll
10o. 1
L--J
F--I
I
i
10~ 10 2
8
(.3
I
\ ExptosiDle atm~ere
Fig. 3.
I
II DiSCIl"arges
Comparison of
minimum
ignition
energies with
energy of
electrostatic
discharges. The minimum ignition energies of organic powders measured in the last few years by different authors (refs. 3, 4, 8, 37 - )9) have become ever lower, overlapping with the range observed for pure gases. As a result of these new findings the basic question must be asked, whether the safety measure "exclusion of ignition sources" can still be accepted as the only protective measure in systems where powders of low minimum ignition energy are processed. According to the present state of knowledge this question cannot be answered in general for several reasons. (i) The ignition potential of newly observed electrostatic discharges is not yet well known. (ii) There is stUl substantial dispute going
183
on amongst the specialists concerning the actual definition and experimental determination of the minimum ignition energy of powders. In all experimental arrangements for the determination of minimum Lgnition energies spark discharges from a charged capacitor are used. The minimum ignition energy of a powder corresponds to that energy W - calculated by the formula W = 1/2 CU 2 with C and U corresponding to the capacitance and voltage of the capacitor respectively - which is just able to ignite the powder at optimal concentration. Though this definition is very simple the experimentally determined values are significantly influenced by a number of parameters, such as= voltage, capacitance and inductance of the whole circuit, electrode gap, shape and material of electrodes, method of spark initiation (voltage increase, electrode gap decrease, preionisation), number of trials (ignition frequency). A working group with experts from different countries is elaborating a standard test procedure. Only minor modifications have to be expected in comparison with the recent first publication of this test procedure by Berthold (ref. 39). In addition to the influence from the spark generator and the electrode arrangement the minimum ignition energy of a powder significantly depends on the particle size distribution, the humidity of the powder, the degree of turbulence within the dust cloud and on the temperature. Dust clouds at low turbulence, formed by dry, fine and well dispersed particles at high temperatures show the lowest values of the minimum ignition energy for a given product (refs. 37, 40). The dependence on temperature is especially important with regard to risk assessment since it may be rather substantial and since many processes in industry are run at elevated temperatures. When the temperature is raised from 20°C to 200°C the decrease amounts to several orders of magnitude for powders w i t h a high minimum ignition energy ( 100 3) at room temperature and to about one order of magnitude for powders with a low minimum ignition energy (10 - 100 m3) at room temperature. Finally i t is important to mention that the minimum ignition energy of powders is also drastically reduced if small amounts of flammable gases or vapours are present. The synergistic effect of powders and gases during explosions has been observed for the first t i m e in coal dust explosions in the presence of methane. The characteristics of such hybrid mixtures have recently been extensively investigated by Pellmont (ref. 41). DISCHARGES - OCCURRENCE AND INCENDIVITY Depending on the m a t e r i a l of the charged objects, their geometrical arrangement and their surroundings the following four main discharge types can be distinguished in pratice (refs. 42 - 44)= Spark, corona, brush and propagating brush. In addition, two l i t t l e known discharge types must be considered. Discharges along the surface of bulked powder during the filling of silos have been observed and lightening-like discharges in dust clouds cannot principally be excluded in very large silos since they have been observed in large dust clouds during the eruption of volcanos. Such a classification of discharges is mainly based on an empirical and more or less phenomenological approach. The differentiation between the various types is not always
184
completely definite, as w i l l become obvious from the following descriptions. Neverr.heless, this classification has been proven to be very useful to evaluate the electrostatic hazards in the industrial practice. The t o t a l energy released in a discharge can in principle be calculated from the difference of the e l e c t r i c field before and a f t e r the discharge. This can easily be done in the case of the so called two electrode discharges such as the spark and, to a certain extent, the propagating brush discharges. In the case of the so called one electrode discharges such as the corona and brush discharges which occur in a more or tess inhomogeneous field such an energy calculation would be very complicated. Because of these d i f f i c u l t i e s the t e r m equivalent energy has been introduced by Gibson (ref. 45) who defined it in the following manner: A certain e l e c t r o s t a t i c discharge has the equivalent energy W if it is just able to ignite an explosive m i x t u r e with the minimum ignition energy W, as determined by capacitor spark discharges. Such an empirical approach to the d e t e r m i n a t i o n of the energy of an electrostatic discharge may be v e r y useful and valuable as long as this discharge has the same characteristics w i t h respect to energy distribution in t i m e and space as the reference capacitor spark discharges. This restriction is based on the reasonable assumption that ignition of an explosible atmosphere does not only depend on the t o t a l energy released in a discharge but also on the energy distribution in t i m e and space. If the energy distribution of the actual and the reference discharge is vastly d i f f e r e n t and if the value of the equivalent energy is applied to explosible atmospheres of d i f f e r e n t ignition characteristics, wrong conclusions concerning the ignition power of the corresponding discharge type may be drawn.
Spark Discharges Spark discharges occur between conductors (construction parts, products, personnel) at d i f f e r e n t potentials. T h e o r e t i c a l l y , spark discharges can most easily be excluded compared to all other discharges, namely by simply earthing all conductors. However, spark discharges are still the most frequent e l e c t r o s t a t i c ignition sources in practice because, very often, earthing of conductors can only be achieved by organisational measures. A d d i t i o n a l l y , due to the always more frequent use of non-conducting construction parts the chance of overlooking an isolated conductor is high. The energy of a spark discharge from an isolated conductor is also calculated by the f o r m u l a W = 1/2 CU 2, as mentioned in the previous section. Depending on the minimum ignition energy of a given powder and on the capacitance and voltage of the isolated conductor one has to expect the ignition of an eventually present dust cloud. More detailed i n f o r m a t i o n on the avoidance of spark discharges in the industrial practice is given by Gibson (ref. 8) and by the Codes of Practice of the British Standards Institution (ref. 43), of the Berufsgenossenschaft der chemischen [ndustrie (ref. 42) and of the Expertenkommission fi.ir Sicherheit in der chemischen Industrie der Schweiz (ref. 44). F r o m a scientific point of view a few remarks concerning the use of t o t a l energies instead of power densities to judge ignition must be added. The incendivity of sparks is
185
influenced by a number of parameters such as gap, shape and material of the electrode and voltage, inductance and resistance of the spark generating circuit. Glarner (ref. 6,0) has shown that for certain powders the incendivity of sparks is considerably increased (by a factor of i 0 or even more) by the addition ofl a small inductance to the discharge circuit. Glarner (ref. 6,0) and Felstead et al. (ref. 46) have measured a pronounced influence of the electrode gap with distances in the range of 6 to I3 mm for the most incendive sparks. For a comparison of the possible energy of sparks occurring in the plant with minimum ignition energies determined i~ the laboratory i t must be kept in mind that the sparks occurring in practice are most probably of pure capacitive nature, whereas the electrode gap cannot be predicted. Results from several authors (refs. 8, 6,7 - b,9) indicate that sparks from the human body are by a factor of 2 to 4 less incendive than pure capacitive sparks for gases, vapours and powders. Brush and Corona Discharges Brush discharges may occur in practice if earthed electrodes with a radius of curvature in the range of about 5 to 50 nlm, such as metallic convex bends within vessels, tanks or silos9 the finger-tips of the personnel or working tools are brought into a strong electric field of several hundred kV/m. Such high field strengths are generated in practice from highly charged non-conducting surfaces (plastic bags, walls of pipes and vessels, liquids), mists, dust clouds and bulked powders. The characteristics of brush discharges have been investigated by Heidelberg (refs. 50, 5I), Gibson (ref. 45) and Glor (ref. 52). The reported values for the equivalent energy were determined with gas/air-mixtures and lie in the range of 1 to 3.6 mJ. The parameters influencing the incendivity most are the radius of curvature of the electrode, the polarity of the electric field and, in the case of a charged non-conducting surface, the surface charge density and the area. Interestingly, a brush discharge from a negatively charged surface is much more incendive than one from a positively charged surface. Although the minimum ignition energies of certain powders lie in the range of 1 to 10 m.], no ignition of a dust cloud clearly caused by a brush discharge has yet been reported. Experiments going on in the laboratory of the author have so far also shown negative results. Therefore, according to the present state of knowledge, one has not to reckon with the ignition of pure dust clouds by brush discharges (in the absence of flammable gases and vapours). The obvious disagreement between the minimum ignition energy of dusts, the equivalent energy of brush discharges and the statement that brush discharges do not ignite dusts is most probably explained by the following arguments: (i)
The energy distribution in time and space is not the same for spark and brush discharges. Since the equivalent energy has been determined w i t h gases and the ignition characteristics of powders are very different this equivalent energy cannot be applied to powders.
(ii)
Different authors (refs. 53, 54) determined a relationship between the incendivi-
186
ty and the amount of charge transferred in a brush discharge. From these investigations a charge transfer in the order of 0.1 ~C in one single brush discharge is considered to be c r i t i c a l with respect to the ignition of hydrocarbons.
Charge transfer measurements from brush discharges in dust clouds clearly show a significant decrease of
the amount of charge transferred
discharge and an increase in the discharge frequency (ref.
in one single
55). These findings
suggest that in a dust cloud less energetic brush discharges do occur at much higher frequency than in a purely gaseous atmosphere. As long as the brush discharges are generated by non-conducting surfaces making part of the installations they can be excluded by the use of conductive materials. If, however, the brush discharges are generated by the powder itself (dust cloud or powder heap) their exclusion is very d i f f i c u l t , as discussed earlier. For this reason the question whether brush discharges are able to ignite powders is of such importance. Corona discharges may principally occur under the same conditions but already at lower field strengths and at electrodes of much smaller radii (sharp tips and edges). The energy released in corona discharges is much too low to ignite any dust cloud (refs. 42 44).
Propagatin 9 Brush Discharges The level to which a non-conducting surface exposed to air can become charged in the form of a charge layer of one single p o l a r i t y is l i m i t e d to about 2.7 • 10-SC/m 2. A t this value of the surface charge density the field strength above the surface reaches the dielectric strength of the air and any additional charges would i n i t i a t e a discharge into the air. However~ when a double layer of charges of opposite p o l a r i t y is generated across a non-conducting layer of l i m i t e d thickness much more charges can be placed on each surface. This is explained by the fact that the e l e c t r i c field in the air due to one layer is, to a certain extent~ compensated by the field of the other layer. Clearly~ the electric field across the non-conducting layer w i l l be very high. Depending on the surface charge density and the extension of the double layer of charges as well as on the thickness and dielectric strength of the non-conducting layer high energies up to several 3oules can be stored in such systems. By an electrical short c i r c u i t of the oppositely charged surfaces a very energetic discharge can be induced which is called propagating brush discharge or, in the older l i t e r a t u r e , Lichtenberg discharge. The electrical short circuit may be achieved either by a p e r f o r a t i o n of the non-conducting layer or by an external electrical connection of the two surfaces. The conditions for the occurence of such discharges are, according to Heidelberg (ref. 50): Surface charge density~-2.7. 10-4C/m 2 and thickness of the non-conducting layer <8 ram. Such high charge densities are only observed in processes generating large amounts of charges at high rates. The counter-charge layer is often created by induced charges on an earthed conducting surface in tight contact with the non-conducting layer.
187
Thus, propagating brush discharges are mainly observed in practice during the following operations: Pneumatic transport of powders through pipes or filling of large containers or silos, all either made from non-conducting materials or having m e t a l l i c walls which are internally coated with a non-conducting layer. In general, paints or layers of deposited, non-conducting product are too porous to p e r m i t a high dielectric strength and therefore w i l l not give rise to propagating brush discharges. According to the author's knowledge there has not yet been carried out a systematic investigation into the incendivity of propagating brush discharges. However, considering the facts that the stored energy may be in the range of several JouLes, that powders with a minimum ignition energy of 10 m3 can easily be ignited in Laboratory tests and that the total energy is released in many different discharge channels spread over the charged surface i t must be expected that propagating brush discharges are able to ignite powders with a minimum ignition energy of about I .] and clearly all flammable gases and vapours. The easiest way to avoid propagating brush discharges is to use metaLLic pipes without internal coating and conducting containers and silos for powders showing a high charging tendency, in the case when non-conducting containers have to be used, a contact between the bulked powder and earth should be provided by inserting e.g. an earthed metal rod into the container (ref. 56). Discharges Associated with the Transfer of Powders into Large Containers and Silos If non-conducting powders are pneumatically conveyed through a pipe or poured through a chute into a large container the electric field from the deposited powder may quickly reach much higher values than the electric field generated by the dispersed powder in the pipe or the chute. This enormous increase of the electric field above the powder heap is caused by two different effects. Firstly, since the particles carry trapped charges along with them, the space charge density increases proportionally to the bulk density when the powder particles become cLoseLy packed in the heap. Due to the high resistivity of the powder the charge cannot be released through the bulked material, even i f the walls of the container are conductive and connected to earth. SecondLy, the electric field at the boundaries of a volume which contains a homogeneous space charge density increases proportionally to the linear extension of this volume. The f i r s t effect of charge compaction typically occurs during the transport and storage of powders whereas the second effect is common to both the transport and storage of powders and Liquids. If the relations between size, density and surface charge density of the powder particles are such that the charged particles still settle against an electric field strength close to the dielectric strength of air discharges at the surface of the powder heap wilt occur. Glor (ref. 57) has calculated the conditions for the occurrence of such discharges under the assumption of a spherically symmetric powder heap. A summary of the results of these model calculations is given in Fig. 4.
188
101
i
8 6 4
i
i
i
I
i
r ~ R=4.0xlo s .~.
2
i
i
i
i
i
i
i
I
i
I I
i
l
i
I
I
I
2
4
6
i
~r~ C2 R=6.SxlO s ,La 12 m 5
lO°si 6 4 2 < I1:
10-1
t0 8
I
I
I
2
4
68
I I
10-5
I
I
I
2
4
68
I J
I
I
I
2
4
68
10-4
10 3
10-2
RATIO o / r ( C / r n 3 )
Fig. 4.
Range of the heap radius R (partially dashed area) where discharges from a
settled powder are to be expected as a function of the charging level of the powder according to the model assumed by Glor (ref. 57). The charging level of the powder is characterized by the r a t i o o/r (surface charge density o and particle radius r).
The most surprising result of these model calculations is the following: Under the realistic assumtion of a very high surface charge density (see Fig. 2) such discharges have only to be expected for rather coarse particles with diameters in the range of 1 - 10 ram. So far such discharges have actually only been reported in the l i t e r a t u r e (refs. 10, 35, 56 59) and observed by the author for granular polymeric materials. Clearly, because of the d i f f i c u l t y to i d e n t i f y such discharges during the handling of fine powders, this cannot d i r e c t l y be considered as an experimental proof of the model calculations. According to the observations of Maurer (refs. 10, 35, 58) the discharges spread over the surface of the settled powder forming radially directed discharge channels. The discharges start at the wall of the container leading in a few distinct steps to the centre. The discharges must in some way be related to the f|ow properties of the product, since an asymmetric powder cone generates an asymmetric discharge pattern. According to the present state of knowledge derived from experimental evidence and theoretical considerations the appearance of discharges along the surface of bulked powder may be drastically influenced by slight variations of the powder properties. The probability of their occurrence increases with the dimensions of the container, with the mass flow rate, and with the r e s i s t i v i t y of the powder. There is strong evidence from theory and pratice that the
189
phenomenon has only to be expected for particles with diameters larger than 100 1~m. The p r o b a b i l i t y of its occurrence is highest for coarse p o l y m e r i c powders with particle diameters in the range of 1 - 10 ram. In a 100 m 3 silo Ivlaurer (ref. 10) has observed an onset of such discharges when the mass f l o w rate was above ca. 4 • 103 I
CONCLUSIONS Due to the grave dust explosions in the European and US industry during the last decade the research a c t i v i t i e s in this field have strongly increased leading, among other results, to the finding that dust clouds are much more sensitive to ignition than had been thought in the past. Therefore, the question has more and more been raised, under which conditions discharges f r o m static e l e c t r i c i t y caused by industrial operations are incendive for dust clouds. Since i t is v e r y d i f f i c u l t to predict and to control the charging level in powder handling operations - in contrast to the situation for liquids - hazard assessment
190
must mainly be based on a prediction of the occurrence of discharges and on an estimation of their incendivity. This approach to the control of static electricity associated with powders has been successful in a wide range of industrial operations, leading to safety measures described in different codes of practice. There exist, however, still some open questions concerning e.g. the conditions for the occurrence and the assessment of the incendivity of discharges from bulked powder during the filling of large containers and silos.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
W. Bartknecht, Proceedings of the International Conference "Safe Handling of Flammable Dusts" organized by VDI, NLirnberg 1983, VDI-Berichte 494, p. 11. Q. Kfihnen and H. Beck, Proceedings of the International Conference "Safe Handling of Flammable Dusts" organized by VDI, NLirnberg 1983, VDI-Berichte 494, p. 25. R. Eckhoff, Combustion and Flame, 24 (19753 53. W. Bartknecht, Chemie Technik, 10 (19793 493. H. Haase, Electrostatic Hazards - Their Evaluation and Control, Verlag Chemie QmbH, Weinheim, 1977. W. Hilgner, 3ahrbuch 1975 der VDI GeseUschaft flit Kunststofftechnik, VDI Verlag, D~.isseldorf, p. 145. N. Gibson, DECHEMA Monogr., 72 (19743 343. N. Gibson, Inst. Phys. Conf. Ser. No. 669 The Institute of Physics, Bristol and London 1983, p. i . A.W. Bright, J. Electrostat., 4 (1977/1978) 131. B. Maurer, Proceedings of the International Conference "Safe Handling of Flammable Dusts", organized by VDI, NCirnberg 1983, VDI-Berichte 494, p. 119. J. Cross and D. Farter, Plenum Press, New York, 1982. W.B. Kunkel, J. Appl. Phys., 21 (1959) 820. E. Bodenstedt, Z. Angew. Phys., IV (1954) 297. K.H. Mirgel, VDI Bet., 19 (1957) 49. K. Min, B.T. Chao and M.E. Wyman, Rev. Sol. Instr., 34 (1963) 529. T.G.O. Berg, G.C. Fernish and W.J. Flood, Aerojet, General Corp. Repts. 039504(8)5P 1963 and 0195-04(14)SP 1963 and Rev. Sci. Instr., 35 (1964) 719. I.N. A]einikova, A.V. Kitaev and B.V. Filipenko, Colloid J. USSR, 28 (19663 505. L. Ramackers, Advances in Static Electricity. I, Proc. 1st Int. Conf. Static Electricity, Vienna, 1970, p. 370. C.E. Lapple, Adv. Chem. Eng., 8 (19703 i. G. Jansen, Der Einfluss elektrischer Felder auf die HBhe elektrostatischer Aufladungen unter besonderer Ber~icksichtigung der Kunststoffe, Dissertation, Universit~t (TH) Fridericiana, Karlsruhe, 1972. B. Maurer, ×I e Congress FATIPEC, 1972, Edizione Ariminum, Milan, 1972. G.A. Turner and M. Balasubramanian, DECHEMA Monogr., 72 (1974) 435; and J. E]ectrostat., 2 (1976) 85. L.B. Schein and 3. Cranch, O. Appl. Phys., 46 (19753 5140. S. Kittaka and Y. Murata, J. Electrostat., 2 (1976) 111. W. Muhr, Chem. Ing. Tech, 48 (1976) 581. J. Kakas and Vigyazo, Staub-Reinhalt. Luft, 36 (1976) 73. A.T. Szaynok, J. Malcher and A. Sycinska-Trojniak, Ill ~me Congr~8 Internationale de l'Electrostatique, Grenoble, April 1977, p. 16a. H. Masuda, T. Komatsu~ N. Mitsui and K. Iinoya, J. Electrostat., 2 (19773 341. S. Kittaka, N. Masui and Y. Murata, 3. Electrostat., 6 (1979) 181. A.G. Bailey, Preprints of the International Symposium: The Role of Particle Interactions in Powder Mechanics, Eindhoven 1983, p. 67. P. Boschung and M. Olor, J. Electrostat., 8 (19803 205. M. Olor and P. Boschung, Swiss Chem. 3 (19813 71. M. Olor, G. LLittgens, B. Maurer and L. Post, unpublished results. P. Cartwright, Sampuran Sing, A.G. Bailey, I.E.E.E. Conference, San Francisco,
191
35 36 37 38 39 b.0 41 42 45 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
LISA, 1982. B. Maurer, Germ. Chem. Eng. 4 (1979) 189. S. Larigaldie and N. Qiboni, 3. Electrostat., 10 (1981) 57. W. Bartknecht, Explosions, 2nd Edition, Springer-Verlag, Berlin, Heidelberg, New York, 1981. P. Field, Dust Explosions, Handbook of Powder Technology, Volume 4, edited by 3.C. Williams and T. Allen, Elsevier Scientific Publishing Company, Amsterdam, Oxford, New York, 1982. W. Berthold, Proceedings of the International Conference "Safe Handling of Flammable Dusts", organized by VD1, N~irnberg, 19839 VDl-Berichte 494, p. 105. T. Glarner, thesis, ETH Z~irich, 1983. Q. Pellmont~ thesis, ETH Z~Jrich, 1979. Richtlinien f~ir die Vermeidung von Z~Jndgefahren infolge elektrostatischer AufLadungen (Richtlinie "Statische Elektrizit~t"), Berufsgenossenschaft der chemischen IndustrLe, Heidelberg, Richtlinie Nr. 4/4.80. 13. S. 5958, Part 1 and 2, 1981/82, British Standards Institution, London. Statische Elektrizit~t, Regeln fiJr die betriebliche Sicherheit. Schriftenreihe der ESCIS, Heft 2, 1978, Verlag Vogt-Schiid AQ, CH-4501 Solothurn. N. Gibson and F.C. Lloyd, Brit. 3. Appl. Phys., 16 (1965) 1619. O.K. Felstead, R.L. Rogers and D.G. Young, Inst. Phys. Conf. Set. No. 66, The Institute of Physics, Bristol and London, 1983, p. 105. N. Wilson, "Electrostatics 1979", Institute of Physics London, 1979, p. 75. P. Tolson, 3. Electrostat., 8 (1980) 289. N. Wilson, Inst. Phys. Conf. Set. No. 66, The Institute of Physics, Bristol and London, 19831 p. 21. E. Heidelberg, Static Electrification Conference, The Institute of Physics and the Physical Society London, 1967. E. Heidelberg, Advances in Static Electricity I, Proceedings of the ist International Conference on Static Electricity, Vienna 1970, p. 351. /vl.Glor, J. E|ectrostat., I0 (1981) 327. H. Kr~mer and K. Asano, 3. Electrostat., 6 (1979), 361. 3.K. 3ohnson, 3. ELectrostat., 4 (1977/78), 55. M. Glor, unpublished results. A.R. Blythe and W. Reddish, Electrostatics 1979, Institute of Physics London, 1979, p. 107. M. Clot, 3. Electrostat., 15 (1984) 223. B. Maurer, Chem. Ing. Techn., 51 (1979) 98. F. Schmalz, private communication. P. Boschung, W. HiLgner, Q. LQttgens, B. Maurer and A. Widmer, 3. Eleotrostat., 3 (1977) 505.
175
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
HAZARDS DUE TO ELECTROSTATIC CHARGING OF POWDERS M. GLOR Ciba-Geigy Ltd., CH-4002 Basle~ Switzerland
ABSTRACT The role of static e l e c t r i c i t y in the field of dust explosions is discussed. A general approach to the assessment of hazards and to the choice of safety measures with respect to electrostatics as an ignition source is outlined. Methods and results describing the charging behaviour of powders are reviewed. The problems to keep the charging level of powders at a safe l i m i t are discussed and compaired to the situation for liquids. The problems and trends in the determination of the minimum ignition energy of powders as a quantity to characterize the ignition sensitivity of powders with respect to electrostatic discharges are discussed. The different types of discharges associated with the handling and processing of powders are reviewed by referring to their phenomenological and theoretical description, their occurrence in practice and their incendivity. Special regard is given to the electrostatic phenomena occurring during the filling of large containers and silos. INTRODUCTION I t took a number of grave dust explosions, occurring in the European foodstuff and in the US grain industry (Bremen 1979, Metz 1981, USA 1976 - 1978) with a large number of fatalities until everybody became aware of the existence and the severity of such explosions. Though the first reports on dust explosions date back to the end of the 18th century (ref. 1) most activities in this field have for a long period of time exclusively been devoted to gas and vapour explosions. Only since the last one or two decades increasing research activities in the field of dust explosions can be observed. According to present statistics from Germany nearly every day one dust explosion occurs within this country (ref. 2). The same statistics claim that 9% of all 381 accidents analysed were caused by static e l e c t r i c i t y i f all different kinds of products are taken into accounts whereas this percentage increases to 34% if only polymeric powders (47 explosions) are included. Several reasons can be indicated which explain why static e l e c t r i c i t y has become an always more serious potential ignition source in powder handling and storage systems. The increase in the transport velocities and the construction of always bigger bunkers and silos led to higher charging rates and to higher levels of charge accumulation. The l a t t e r is additionally favoured by the high resistivity of polymeric powders which are processed in always higher amounts. In addition, i t is presently realized that the sensitivity of many powders with regard to ignition by electrostatic discharges is much higher than was thought only 10 years ago. The values of the minimum ignition energies of certain organic
0304-3886/85/$03.30
© 1985 Elsevier Science Publishers B.V.
176
powders measured in the last years by d i f f e r e n t authors (refs. 3, 4) have become very low, overlapping with the range of values observed for pure gases. However, just because many factors indicate that e l e c t r o s t a t i c hazards in powder handling systems must not be underestimated, the danger exists that static e l e c t r i c i t y is blamed for the actual cause of ignition in all cases where no other obvious ignition sources can be identified. Such a practice is very dangerous since it leads to wrong safety measures and presumably to further explosions. This is p a r t i c u l a r i l y true for powder handling and processing systems in which many other p o t e n t i a l ignition sources may be present. Compared to the situation for liquids, powder handling involves more moving construction parts which may generate mechanical sparks or f r i c t i o n a l heat. Very often a drying operation constitutes one part of the process during which exothermal reactions may lead to a thermal self ignition of the powder. On the other hand i t must be admitted that the present knowledge concerning the control as well as the occurrence and incendivity of electrostatic discharges in large powder handling and processing systems is far from being complete. In contrast to the situation for liquids much more work remains to be done in the field of powders. The present review does not at all pretend to answer all questions concerning the evaluation of electrostatic
hazards associated with
the handling and processing of
powders. It w i l l rather point to the specific problems which are intrinsically related to powders. I t w i l l reveal those questions which nowadays clearly can be answered but i t w i l l also point to the grey regions and comment on the present tendencies within these regions. Despite the c o m p l e x i t y in the assessment of the e l e c t r o s t a t i c risks involved in powder handling many fundamental investigations have been performed and extensive experience from industrial practice has been gained over the last decade by d i f f e r e n t authors
from
universities,
from
chemical
and foodstuff
industries
and from
other
organisations, see reviews by Haase, Hilgner, Gibson, Bright, Maurer and Cross (refs. 5 n). ASSESSMENT OF ELECTROSTATIC HAZARDS - A SYSTEMATIC APPROACH Ignition of an explosible atmosphere by a gas discharge represents the most severe e l e c t r o s t a t i c hazard in industry. To judge the probability of such an event for any given operation in practice the following characteristics must be considered: Rate of charge build-up, rate of charge relaxation, resulting level of charge accumulation, appearance of gas discharges, incendivity of gas discharges and ignition sensitivity of the present explosible atmosphere. Such a systematic approach leads to the well-known scheme represented in Fig. 1. Though this scheme looks rather t r i v i a l at first sight, i t is not always easy to localize each step with respect to space and t i m e for any given process. Of course, i t often happens that several steps occur simultaneously. The level of charge accumulation is determined by the equilibrium between the charge generation and the charge relaxation rate. Since all charge generation processes are in principle processes in which charges of
177
I CHARGESEPARATIONI
I
io...o.0c0.uL..o.L .....
1- ......
IOISCHARGEli
oF CHAROE
...........
I
i
I 'GN,T'°N I
:
EXPLOSIVE MIXTURE L . . . . . . . . . . . .
Fig. 1.
EARTH
I J
Most i m p o r t a n t steps in the evaluation of hazards due to e l e c t r o s t a t i c
charging. opposite polarities are separated, two d i f f e r e n t pathways, one for each p o l a r i t y must always be considered. The most commonly encountered situation when powders are handled, processed or transported in the industrial practice is roughly the following: Due to the continuous separation processes between the product and the construction parts charges of one p o l a r i t y are built up on the product and charges of the other p o l a r i t y are built up on the construction parts or released to earth. The f a c t t h a t for a given powder positive as well as negative charge build-up may be observed, is not relevant for the present discussion. This is of course only true as long as only the net charge is considered which again is reasonable as long as no large domains of positively and negatively charged particles within the bulked or dispersed powder are generated. Since the scheme represented in Fig. 1 allows to identify the steps which may lead to the ignition of an explosible atmosphere by e l e c t r o s t a t i c charge build-up, it is of great help during every discussion on all d i f f e r e n t kinds of safety measures. Clearly, the goal of every safety measure in electrostatics
is to prevent both the charge on the product and the one on the
construction parts to lead to a gas discharge which finally w i l l ignite an eventually present explosible atmosphere. The explosible atmosphere may be generated by the product itself or e m i t t e d from another near-by process. There are basically three d i f f e r e n t approaches to reach this goal: (i)
The level of charge accumulation is kept low by reducing the rate of charge separation and by increasing the rate of charge relaxation. No gas discharges have to be expected.
(ii)
It is well known which types of gas discharges have to be expected. These discharges are not incendive for the explosible atmosphere present.
(iii)
Ineendive discharges cannot be excluded. Other measures of explosion protection must be considered.
178
Measures of the first type are mainly applied in the case of flammable liquids and with respect to contruction parts as soon as f l a m m a b l e liquids or combustible powders are handled. Since i t is very d i f f i c u l t to control the charging level of a powder during an industrial process the measures applied in the case of powders are mainly of the second or third type. This means that every assessment of the e l e c t r o s t a t i c hazards associated with the charging of powders must concentrate on the following two questions: Which types of discharges have to be expected? Will these discharges be able to ignite the presumably present explosible atmosphere?
ELECTROSTATIC CHARGING OF POWDERS Independent of the materials of the installations most powders become charged during transport, handling and processing as soon as the powder resistivity lies above about l 0 8 Ohm • m, a value which is surpassed by most organic products. Since it is d i f f i c u l t to describe amorphous organic substances with the laws of solid state physics, there is still a lack of knowledge concerning the actual e l e c t r i f i c a t i o n mechanisms. But even if there were a general theory predicting the charging behaviour of pure substances with clean surfaces this would be of no practical relevance. Most powders processed in industry are not pure substances and their surfaces
are not clean. Even the slightest
surface
contamination significantly changes the charging behaviour. This fact which is responsible for the famous bad reproducibility of all e l e c t r o s t a t i c phenomena and which often drives the experimentalists nearly to despair can easily be understood from the following considerations. Under the assumption of a distance of several tenths of a nanometer between the molecules on the surface only about one surplus e l e m e n t a r y charge per every 1OSth surface molecule is needed to generate the highest possible surface charge density of about 2.7 • i0 -5C/m2 at a planar interface to air. For powders, the reproducibility of the charging behaviour is often worse than for extended solid surfaces. This can be understood from the fact that the net charge which is measured a f t e r the powder is processed may be the result of a small difference between the charges of highly charged particles of opposite p o l a r i t y . The change in p o l a r i t y which is sometimes observed in practice for powders showing a moderate net charge can easily be understood on the basis of the above considerations. Various authors have published experimental results on the charging of powders (refs. 12 -18) and the l i t e r a t u r e in this field up to 1970 has been c r i t i c a l l y reviewed by Lapple (ref. 19). It is apparent (refs. 20 - 32) that the interest in this subject has recently increased. Most of the methods described in these papers are designed to carry out determinations on one selected powder. Jansen (ref. 20) obtained results from about 20 d i f f e r e n t plastic powders and Boschung and Qior (refs. 31, 32) obtained results from about 200 widely d i f f e r e n t types of industrial powders. In these l a t t e r e x p e r i m e n t a l investigations the charging of the powders was measured in a l a b o r a t o r y apparatus simulating pneumatic transport. The specific charge was determined as a function of parameters such as powder quantity~ specific surface area, air f l o w rate, length, diameter and m a t e r i a l of the tube. When the parameters of the apparatus were
179 kept constant, the variation of the specific charge of all products investigated was in the range of 10-7 to 10-3C/kg with a definite dependence on the specific surface area of the products, as becomes obvious from Fig. 2. The reproducibility and the extent of scatter of the data was such that a classification of the powders according to their charging tendency was possible. Further experiments in which the charging tendency was measured by dropping the powders onto a quickly rotating conical disc showed the same relative classification (ref. 33).
As/kg
100
101
102
103
I
I
F
I
10-3
10-3
5
5
10 - 4
10- 4
5
5 UJ
(3 t¢ <( :E to 10_s ~ tlg
lO-S 5
5
/ u.
Q. 09
1O_S
10--6
5
5
10 - 7
10-7 5
5 /
10 -e
10-1
I
I
I
100
101
102
~
10 -e
m2/kg
SPECIFIC GEOMETRICAL SURFACE
Fig. 2.
Influence of the specific surface area of powders on the charging tendency as
determined by a laboratory scale pneumatic transport device (ref. 3i). Though it is obviously possible to classify powders according to their charging tendency in the laboratory an extrapolation of these results to practice can be problematic. Clearly, the absolute level to which a powder w i l l become charged in practice depends
180
not only on the nature of the product but also very strongly on the operation, as can be seen f r o m Table 1. Nevertheless one would hope that at least the r e l a t i v e charging tendency of the powders determined in the laboratory would be useful for a hazard assessment in practice. TABLE 1 Powder charging by different
industrial
operations according to Gibson (~'efs. 798),
C a r t w r i g h t (ref. 34) and Maurer (ref. 35).
Operation
Specific charge in C/kg
Sieving
10-11
_
Pouring
10-9
_
Scroll Feed Transfer
10-8
Grinding
10-7
_
10-6
Micronizing
10-7
_
10-4
Pneumatic Transfer
10-6
.
i0-4
_
10-9 i0-7 10-6
But even this hypothesis is rather doubtful. There is some recent experimental and t h e o r e t i c a l evidence that p a r t i c u l a r l y powders w i t h a rather moderate charging tendency may represent a hazard~ when they are settling onto the powder heap in a silo, as w i l l be discussed l a t e r in this paper. CONTROL OF THE CHARGING LEVEL OF POWDERS According to the present state of knowledge the charging level of liquids can much b e t t e r be predicted and controlled than the one of powders. This has mainly two reasons. During the last decades much more e f f o r t s have been made to control the hazards associated with charged liquids than with charged powders. The petrol industries have investigated many projects along these lines. However~ it must also be emphasized that in the case of powders the problems are of much higher c o m p l e x i t y . This has to do with the intrinsically d i f f e r e n t nature of liquids and powders. [n order to characterize e.g. a powder transport and storage system many more parameters have to be quantified (particle size distribution, specific surface area, skeletal and bulk density of the powder, resistivity of the bulk powder~ mass f l o w rate9 air f l o w rate~ dimensions of the transport pipe and of the storage silo, design of the powder inlet into the silo etc.) than for a similar system designed for liquids. In addition one must take into account the fact t h a t the powder particles normally become charged in a state where they are dispersed in air. When these particles fall down into a storage vessel the powder material as well as the charge trapped by the particles
181
become closely packed into the powder heap thus giving rise to a high space charge density. Due to this effect also powders exhibiting a very moderate charging tendency may give rise to high electric field strengths when packed into a heap. In contrast to the situation for liquids where no charged mists have to be expected during normal filling operations, charged dust clouds cannot be avoided when large amounts of fine powders are handled. The formation of dust clouds is even favoured by the effect of e l e c t r i f i c a t i o n since the highly charged fine particles are repulsed from the settled powder. Finally, the following important fact should be pointed out. The most common measures in the control of hazards from static e l e c t r i c i t y associated with liquids, namely the decrease of the transport velocity and the increase of the conductivity by the addition of an antistatic additive in very small amounts cannot be applied in the case of powders. A decrease of the velocity in powder transport systems is often not possible for technical reasons. Because of the high specific surface area of powders substantial amounts of antistatic additives would be necessary to generate a reasonably high conductivity of the bulked powder. Moreover, a surface conductivity of the particles which would be sufficient to ensure a quick charge relaxation from the bulked powder (resistivity of the bulked powder below about 108 Ohm • m) would not guarantee that the particles do not become charged at all when they are dispersed in air and make short contacts with each other and with parts of the construction. Since the common measures, such as reduction of transport velocity and increase of product conductivity which allow to control the charge build-up to a level where it ceases to represent a hazard in the case of liquids, cannot be realized in powder processing and transport systems there is a great demand to control the charging level by some other means in all cases where incendive discharges have to be expected. Principally, the methods of charge neutraiisation which are used to reduce the charging level of webs of different kinds of products, offer such an alternative. However, the charge neutraiisation process in a powder transport system must be active over a large volume in contrast to the situation prevailing in the neutralisation of the charges on the surface of a web. To overcome the drawback of short range action Larigaldie et al. (ref. 36) have deve|opped a so called "electro-gas-dynamic generator" which acts as a charge neutralizer by taking advantage of the low mobility of charged aerosols, carried away by a flow of gas. Despite the obvious improvements such neutralizers are not widely used yet to minimize the electrostatic hazards associated with powders. Further research w i l l be necessary to demonstrate whether and for which operations, systems and products incendive discharges actually have to be expected, in these cases such charge neutralization methods could be of great help but, clearly, every decision about their application should be based on a comparison with all other possibilities of explosion protection. HAZARDS ASSOCIATED WITH DIFFERENT MATERIALS AND OPERATIONS Since i t is very d i f f i c u l t to predict or control the charging level of powders, as
182
outlined earlier in this paper, hazard assessment in powder handling and processing systems i s i n most cases based on an assessment
of the occurrence and incendivity of
discharges associated with the given materials and operations. Therefore~ in the present review, the electrostatic hazards prevailing in industry are not discussed under the aspects of the different operations and materials involved but under the aspect of the different discharge types. The relation to practical situations is given by a description of the occurrence of these discharges in the industrial practice. POWDER IGNITION BY ELECTROSTATIC DISCHARGES The ignition sensitivity of a powder with regard to electrostatic discharges is characterized by the minimum ignition energy of the corresponding dust cloud. It is evident that the incendivity of a discharge should therefore be characterized by its energy release. The probability of ignition can then be judged by a comparison of these energies as is demonstrated in Fig. 3.
Incendlvlty
Minimum
E
Ignition energy
uJ
--*]
10s10=.
E
--
103 102 101
r--ll
10o. 1
L--J
F--I
I
i
10~ 10 2
8
(.3
I
\ ExptosiDle atm~ere
Fig. 3.
I
II DiSCIl"arges
Comparison of
minimum
ignition
energies with
energy of
electrostatic
discharges. The minimum ignition energies of organic powders measured in the last few years by different authors (refs. 3, 4, 8, 37 - )9) have become ever lower, overlapping with the range observed for pure gases. As a result of these new findings the basic question must be asked, whether the safety measure "exclusion of ignition sources" can still be accepted as the only protective measure in systems where powders of low minimum ignition energy are processed. According to the present state of knowledge this question cannot be answered in general for several reasons. (i) The ignition potential of newly observed electrostatic discharges is not yet well known. (ii) There is stUl substantial dispute going
183
on amongst the specialists concerning the actual definition and experimental determination of the minimum ignition energy of powders. In all experimental arrangements for the determination of minimum Lgnition energies spark discharges from a charged capacitor are used. The minimum ignition energy of a powder corresponds to that energy W - calculated by the formula W = 1/2 CU 2 with C and U corresponding to the capacitance and voltage of the capacitor respectively - which is just able to ignite the powder at optimal concentration. Though this definition is very simple the experimentally determined values are significantly influenced by a number of parameters, such as= voltage, capacitance and inductance of the whole circuit, electrode gap, shape and material of electrodes, method of spark initiation (voltage increase, electrode gap decrease, preionisation), number of trials (ignition frequency). A working group with experts from different countries is elaborating a standard test procedure. Only minor modifications have to be expected in comparison with the recent first publication of this test procedure by Berthold (ref. 39). In addition to the influence from the spark generator and the electrode arrangement the minimum ignition energy of a powder significantly depends on the particle size distribution, the humidity of the powder, the degree of turbulence within the dust cloud and on the temperature. Dust clouds at low turbulence, formed by dry, fine and well dispersed particles at high temperatures show the lowest values of the minimum ignition energy for a given product (refs. 37, 40). The dependence on temperature is especially important with regard to risk assessment since it may be rather substantial and since many processes in industry are run at elevated temperatures. When the temperature is raised from 20°C to 200°C the decrease amounts to several orders of magnitude for powders w i t h a high minimum ignition energy ( 100 3) at room temperature and to about one order of magnitude for powders with a low minimum ignition energy (10 - 100 m3) at room temperature. Finally i t is important to mention that the minimum ignition energy of powders is also drastically reduced if small amounts of flammable gases or vapours are present. The synergistic effect of powders and gases during explosions has been observed for the first t i m e in coal dust explosions in the presence of methane. The characteristics of such hybrid mixtures have recently been extensively investigated by Pellmont (ref. 41). DISCHARGES - OCCURRENCE AND INCENDIVITY Depending on the m a t e r i a l of the charged objects, their geometrical arrangement and their surroundings the following four main discharge types can be distinguished in pratice (refs. 42 - 44)= Spark, corona, brush and propagating brush. In addition, two l i t t l e known discharge types must be considered. Discharges along the surface of bulked powder during the filling of silos have been observed and lightening-like discharges in dust clouds cannot principally be excluded in very large silos since they have been observed in large dust clouds during the eruption of volcanos. Such a classification of discharges is mainly based on an empirical and more or less phenomenological approach. The differentiation between the various types is not always
184
completely definite, as w i l l become obvious from the following descriptions. Neverr.heless, this classification has been proven to be very useful to evaluate the electrostatic hazards in the industrial practice. The t o t a l energy released in a discharge can in principle be calculated from the difference of the e l e c t r i c field before and a f t e r the discharge. This can easily be done in the case of the so called two electrode discharges such as the spark and, to a certain extent, the propagating brush discharges. In the case of the so called one electrode discharges such as the corona and brush discharges which occur in a more or tess inhomogeneous field such an energy calculation would be very complicated. Because of these d i f f i c u l t i e s the t e r m equivalent energy has been introduced by Gibson (ref. 45) who defined it in the following manner: A certain e l e c t r o s t a t i c discharge has the equivalent energy W if it is just able to ignite an explosive m i x t u r e with the minimum ignition energy W, as determined by capacitor spark discharges. Such an empirical approach to the d e t e r m i n a t i o n of the energy of an electrostatic discharge may be v e r y useful and valuable as long as this discharge has the same characteristics w i t h respect to energy distribution in t i m e and space as the reference capacitor spark discharges. This restriction is based on the reasonable assumption that ignition of an explosible atmosphere does not only depend on the t o t a l energy released in a discharge but also on the energy distribution in t i m e and space. If the energy distribution of the actual and the reference discharge is vastly d i f f e r e n t and if the value of the equivalent energy is applied to explosible atmospheres of d i f f e r e n t ignition characteristics, wrong conclusions concerning the ignition power of the corresponding discharge type may be drawn.
Spark Discharges Spark discharges occur between conductors (construction parts, products, personnel) at d i f f e r e n t potentials. T h e o r e t i c a l l y , spark discharges can most easily be excluded compared to all other discharges, namely by simply earthing all conductors. However, spark discharges are still the most frequent e l e c t r o s t a t i c ignition sources in practice because, very often, earthing of conductors can only be achieved by organisational measures. A d d i t i o n a l l y , due to the always more frequent use of non-conducting construction parts the chance of overlooking an isolated conductor is high. The energy of a spark discharge from an isolated conductor is also calculated by the f o r m u l a W = 1/2 CU 2, as mentioned in the previous section. Depending on the minimum ignition energy of a given powder and on the capacitance and voltage of the isolated conductor one has to expect the ignition of an eventually present dust cloud. More detailed i n f o r m a t i o n on the avoidance of spark discharges in the industrial practice is given by Gibson (ref. 8) and by the Codes of Practice of the British Standards Institution (ref. 43), of the Berufsgenossenschaft der chemischen [ndustrie (ref. 42) and of the Expertenkommission fi.ir Sicherheit in der chemischen Industrie der Schweiz (ref. 44). F r o m a scientific point of view a few remarks concerning the use of t o t a l energies instead of power densities to judge ignition must be added. The incendivity of sparks is
185
influenced by a number of parameters such as gap, shape and material of the electrode and voltage, inductance and resistance of the spark generating circuit. Glarner (ref. 6,0) has shown that for certain powders the incendivity of sparks is considerably increased (by a factor of i 0 or even more) by the addition ofl a small inductance to the discharge circuit. Glarner (ref. 6,0) and Felstead et al. (ref. 46) have measured a pronounced influence of the electrode gap with distances in the range of 6 to I3 mm for the most incendive sparks. For a comparison of the possible energy of sparks occurring in the plant with minimum ignition energies determined i~ the laboratory i t must be kept in mind that the sparks occurring in practice are most probably of pure capacitive nature, whereas the electrode gap cannot be predicted. Results from several authors (refs. 8, 6,7 - b,9) indicate that sparks from the human body are by a factor of 2 to 4 less incendive than pure capacitive sparks for gases, vapours and powders. Brush and Corona Discharges Brush discharges may occur in practice if earthed electrodes with a radius of curvature in the range of about 5 to 50 nlm, such as metallic convex bends within vessels, tanks or silos9 the finger-tips of the personnel or working tools are brought into a strong electric field of several hundred kV/m. Such high field strengths are generated in practice from highly charged non-conducting surfaces (plastic bags, walls of pipes and vessels, liquids), mists, dust clouds and bulked powders. The characteristics of brush discharges have been investigated by Heidelberg (refs. 50, 5I), Gibson (ref. 45) and Glor (ref. 52). The reported values for the equivalent energy were determined with gas/air-mixtures and lie in the range of 1 to 3.6 mJ. The parameters influencing the incendivity most are the radius of curvature of the electrode, the polarity of the electric field and, in the case of a charged non-conducting surface, the surface charge density and the area. Interestingly, a brush discharge from a negatively charged surface is much more incendive than one from a positively charged surface. Although the minimum ignition energies of certain powders lie in the range of 1 to 10 m.], no ignition of a dust cloud clearly caused by a brush discharge has yet been reported. Experiments going on in the laboratory of the author have so far also shown negative results. Therefore, according to the present state of knowledge, one has not to reckon with the ignition of pure dust clouds by brush discharges (in the absence of flammable gases and vapours). The obvious disagreement between the minimum ignition energy of dusts, the equivalent energy of brush discharges and the statement that brush discharges do not ignite dusts is most probably explained by the following arguments: (i)
The energy distribution in time and space is not the same for spark and brush discharges. Since the equivalent energy has been determined w i t h gases and the ignition characteristics of powders are very different this equivalent energy cannot be applied to powders.
(ii)
Different authors (refs. 53, 54) determined a relationship between the incendivi-
186
ty and the amount of charge transferred in a brush discharge. From these investigations a charge transfer in the order of 0.1 ~C in one single brush discharge is considered to be c r i t i c a l with respect to the ignition of hydrocarbons.
Charge transfer measurements from brush discharges in dust clouds clearly show a significant decrease of
the amount of charge transferred
discharge and an increase in the discharge frequency (ref.
in one single
55). These findings
suggest that in a dust cloud less energetic brush discharges do occur at much higher frequency than in a purely gaseous atmosphere. As long as the brush discharges are generated by non-conducting surfaces making part of the installations they can be excluded by the use of conductive materials. If, however, the brush discharges are generated by the powder itself (dust cloud or powder heap) their exclusion is very d i f f i c u l t , as discussed earlier. For this reason the question whether brush discharges are able to ignite powders is of such importance. Corona discharges may principally occur under the same conditions but already at lower field strengths and at electrodes of much smaller radii (sharp tips and edges). The energy released in corona discharges is much too low to ignite any dust cloud (refs. 42 44).
Propagatin 9 Brush Discharges The level to which a non-conducting surface exposed to air can become charged in the form of a charge layer of one single p o l a r i t y is l i m i t e d to about 2.7 • 10-SC/m 2. A t this value of the surface charge density the field strength above the surface reaches the dielectric strength of the air and any additional charges would i n i t i a t e a discharge into the air. However~ when a double layer of charges of opposite p o l a r i t y is generated across a non-conducting layer of l i m i t e d thickness much more charges can be placed on each surface. This is explained by the fact that the e l e c t r i c field in the air due to one layer is, to a certain extent~ compensated by the field of the other layer. Clearly~ the electric field across the non-conducting layer w i l l be very high. Depending on the surface charge density and the extension of the double layer of charges as well as on the thickness and dielectric strength of the non-conducting layer high energies up to several 3oules can be stored in such systems. By an electrical short c i r c u i t of the oppositely charged surfaces a very energetic discharge can be induced which is called propagating brush discharge or, in the older l i t e r a t u r e , Lichtenberg discharge. The electrical short circuit may be achieved either by a p e r f o r a t i o n of the non-conducting layer or by an external electrical connection of the two surfaces. The conditions for the occurence of such discharges are, according to Heidelberg (ref. 50): Surface charge density~-2.7. 10-4C/m 2 and thickness of the non-conducting layer <8 ram. Such high charge densities are only observed in processes generating large amounts of charges at high rates. The counter-charge layer is often created by induced charges on an earthed conducting surface in tight contact with the non-conducting layer.
187
Thus, propagating brush discharges are mainly observed in practice during the following operations: Pneumatic transport of powders through pipes or filling of large containers or silos, all either made from non-conducting materials or having m e t a l l i c walls which are internally coated with a non-conducting layer. In general, paints or layers of deposited, non-conducting product are too porous to p e r m i t a high dielectric strength and therefore w i l l not give rise to propagating brush discharges. According to the author's knowledge there has not yet been carried out a systematic investigation into the incendivity of propagating brush discharges. However, considering the facts that the stored energy may be in the range of several JouLes, that powders with a minimum ignition energy of 10 m3 can easily be ignited in Laboratory tests and that the total energy is released in many different discharge channels spread over the charged surface i t must be expected that propagating brush discharges are able to ignite powders with a minimum ignition energy of about I .] and clearly all flammable gases and vapours. The easiest way to avoid propagating brush discharges is to use metaLLic pipes without internal coating and conducting containers and silos for powders showing a high charging tendency, in the case when non-conducting containers have to be used, a contact between the bulked powder and earth should be provided by inserting e.g. an earthed metal rod into the container (ref. 56). Discharges Associated with the Transfer of Powders into Large Containers and Silos If non-conducting powders are pneumatically conveyed through a pipe or poured through a chute into a large container the electric field from the deposited powder may quickly reach much higher values than the electric field generated by the dispersed powder in the pipe or the chute. This enormous increase of the electric field above the powder heap is caused by two different effects. Firstly, since the particles carry trapped charges along with them, the space charge density increases proportionally to the bulk density when the powder particles become cLoseLy packed in the heap. Due to the high resistivity of the powder the charge cannot be released through the bulked material, even i f the walls of the container are conductive and connected to earth. SecondLy, the electric field at the boundaries of a volume which contains a homogeneous space charge density increases proportionally to the linear extension of this volume. The f i r s t effect of charge compaction typically occurs during the transport and storage of powders whereas the second effect is common to both the transport and storage of powders and Liquids. If the relations between size, density and surface charge density of the powder particles are such that the charged particles still settle against an electric field strength close to the dielectric strength of air discharges at the surface of the powder heap wilt occur. Glor (ref. 57) has calculated the conditions for the occurrence of such discharges under the assumption of a spherically symmetric powder heap. A summary of the results of these model calculations is given in Fig. 4.
188
101
i
8 6 4
i
i
i
I
i
r ~ R=4.0xlo s .~.
2
i
i
i
i
i
i
i
I
i
I I
i
l
i
I
I
I
2
4
6
i
~r~ C2 R=6.SxlO s ,La 12 m 5
lO°si 6 4 2 < I1:
10-1
t0 8
I
I
I
2
4
68
I I
10-5
I
I
I
2
4
68
I J
I
I
I
2
4
68
10-4
10 3
10-2
RATIO o / r ( C / r n 3 )
Fig. 4.
Range of the heap radius R (partially dashed area) where discharges from a
settled powder are to be expected as a function of the charging level of the powder according to the model assumed by Glor (ref. 57). The charging level of the powder is characterized by the r a t i o o/r (surface charge density o and particle radius r).
The most surprising result of these model calculations is the following: Under the realistic assumtion of a very high surface charge density (see Fig. 2) such discharges have only to be expected for rather coarse particles with diameters in the range of 1 - 10 ram. So far such discharges have actually only been reported in the l i t e r a t u r e (refs. 10, 35, 56 59) and observed by the author for granular polymeric materials. Clearly, because of the d i f f i c u l t y to i d e n t i f y such discharges during the handling of fine powders, this cannot d i r e c t l y be considered as an experimental proof of the model calculations. According to the observations of Maurer (refs. 10, 35, 58) the discharges spread over the surface of the settled powder forming radially directed discharge channels. The discharges start at the wall of the container leading in a few distinct steps to the centre. The discharges must in some way be related to the f|ow properties of the product, since an asymmetric powder cone generates an asymmetric discharge pattern. According to the present state of knowledge derived from experimental evidence and theoretical considerations the appearance of discharges along the surface of bulked powder may be drastically influenced by slight variations of the powder properties. The probability of their occurrence increases with the dimensions of the container, with the mass flow rate, and with the r e s i s t i v i t y of the powder. There is strong evidence from theory and pratice that the
189
phenomenon has only to be expected for particles with diameters larger than 100 1~m. The p r o b a b i l i t y of its occurrence is highest for coarse p o l y m e r i c powders with particle diameters in the range of 1 - 10 ram. In a 100 m 3 silo Ivlaurer (ref. 10) has observed an onset of such discharges when the mass f l o w rate was above ca. 4 • 103 I
CONCLUSIONS Due to the grave dust explosions in the European and US industry during the last decade the research a c t i v i t i e s in this field have strongly increased leading, among other results, to the finding that dust clouds are much more sensitive to ignition than had been thought in the past. Therefore, the question has more and more been raised, under which conditions discharges f r o m static e l e c t r i c i t y caused by industrial operations are incendive for dust clouds. Since i t is v e r y d i f f i c u l t to predict and to control the charging level in powder handling operations - in contrast to the situation for liquids - hazard assessment
190
must mainly be based on a prediction of the occurrence of discharges and on an estimation of their incendivity. This approach to the control of static electricity associated with powders has been successful in a wide range of industrial operations, leading to safety measures described in different codes of practice. There exist, however, still some open questions concerning e.g. the conditions for the occurrence and the assessment of the incendivity of discharges from bulked powder during the filling of large containers and silos.
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W. Bartknecht, Proceedings of the International Conference "Safe Handling of Flammable Dusts" organized by VDI, NLirnberg 1983, VDI-Berichte 494, p. 11. Q. Kfihnen and H. Beck, Proceedings of the International Conference "Safe Handling of Flammable Dusts" organized by VDI, NLirnberg 1983, VDI-Berichte 494, p. 25. R. Eckhoff, Combustion and Flame, 24 (19753 53. W. Bartknecht, Chemie Technik, 10 (19793 493. H. Haase, Electrostatic Hazards - Their Evaluation and Control, Verlag Chemie QmbH, Weinheim, 1977. W. Hilgner, 3ahrbuch 1975 der VDI GeseUschaft flit Kunststofftechnik, VDI Verlag, D~.isseldorf, p. 145. N. Gibson, DECHEMA Monogr., 72 (19743 343. N. Gibson, Inst. Phys. Conf. Ser. No. 669 The Institute of Physics, Bristol and London 1983, p. i . A.W. Bright, J. Electrostat., 4 (1977/1978) 131. B. Maurer, Proceedings of the International Conference "Safe Handling of Flammable Dusts", organized by VDI, NCirnberg 1983, VDI-Berichte 494, p. 119. J. Cross and D. Farter, Plenum Press, New York, 1982. W.B. Kunkel, J. Appl. Phys., 21 (1959) 820. E. Bodenstedt, Z. Angew. Phys., IV (1954) 297. K.H. Mirgel, VDI Bet., 19 (1957) 49. K. Min, B.T. Chao and M.E. Wyman, Rev. Sol. Instr., 34 (1963) 529. T.G.O. Berg, G.C. Fernish and W.J. Flood, Aerojet, General Corp. Repts. 039504(8)5P 1963 and 0195-04(14)SP 1963 and Rev. Sci. Instr., 35 (1964) 719. I.N. A]einikova, A.V. Kitaev and B.V. Filipenko, Colloid J. USSR, 28 (19663 505. L. Ramackers, Advances in Static Electricity. I, Proc. 1st Int. Conf. Static Electricity, Vienna, 1970, p. 370. C.E. Lapple, Adv. Chem. Eng., 8 (19703 i. G. Jansen, Der Einfluss elektrischer Felder auf die HBhe elektrostatischer Aufladungen unter besonderer Ber~icksichtigung der Kunststoffe, Dissertation, Universit~t (TH) Fridericiana, Karlsruhe, 1972. B. Maurer, ×I e Congress FATIPEC, 1972, Edizione Ariminum, Milan, 1972. G.A. Turner and M. Balasubramanian, DECHEMA Monogr., 72 (1974) 435; and J. E]ectrostat., 2 (1976) 85. L.B. Schein and 3. Cranch, O. Appl. Phys., 46 (19753 5140. S. Kittaka and Y. Murata, J. Electrostat., 2 (1976) 111. W. Muhr, Chem. Ing. Tech, 48 (1976) 581. J. Kakas and Vigyazo, Staub-Reinhalt. Luft, 36 (1976) 73. A.T. Szaynok, J. Malcher and A. Sycinska-Trojniak, Ill ~me Congr~8 Internationale de l'Electrostatique, Grenoble, April 1977, p. 16a. H. Masuda, T. Komatsu~ N. Mitsui and K. Iinoya, J. Electrostat., 2 (19773 341. S. Kittaka, N. Masui and Y. Murata, 3. Electrostat., 6 (1979) 181. A.G. Bailey, Preprints of the International Symposium: The Role of Particle Interactions in Powder Mechanics, Eindhoven 1983, p. 67. P. Boschung and M. Olor, J. Electrostat., 8 (19803 205. M. Olor and P. Boschung, Swiss Chem. 3 (19813 71. M. Olor, G. LLittgens, B. Maurer and L. Post, unpublished results. P. Cartwright, Sampuran Sing, A.G. Bailey, I.E.E.E. Conference, San Francisco,
191
35 36 37 38 39 b.0 41 42 45 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
LISA, 1982. B. Maurer, Germ. Chem. Eng. 4 (1979) 189. S. Larigaldie and N. Qiboni, 3. Electrostat., 10 (1981) 57. W. Bartknecht, Explosions, 2nd Edition, Springer-Verlag, Berlin, Heidelberg, New York, 1981. P. Field, Dust Explosions, Handbook of Powder Technology, Volume 4, edited by 3.C. Williams and T. Allen, Elsevier Scientific Publishing Company, Amsterdam, Oxford, New York, 1982. W. Berthold, Proceedings of the International Conference "Safe Handling of Flammable Dusts", organized by VD1, N~irnberg, 19839 VDl-Berichte 494, p. 105. T. Glarner, thesis, ETH Z~irich, 1983. Q. Pellmont~ thesis, ETH Z~Jrich, 1979. Richtlinien f~ir die Vermeidung von Z~Jndgefahren infolge elektrostatischer AufLadungen (Richtlinie "Statische Elektrizit~t"), Berufsgenossenschaft der chemischen IndustrLe, Heidelberg, Richtlinie Nr. 4/4.80. 13. S. 5958, Part 1 and 2, 1981/82, British Standards Institution, London. Statische Elektrizit~t, Regeln fiJr die betriebliche Sicherheit. Schriftenreihe der ESCIS, Heft 2, 1978, Verlag Vogt-Schiid AQ, CH-4501 Solothurn. N. Gibson and F.C. Lloyd, Brit. 3. Appl. Phys., 16 (1965) 1619. O.K. Felstead, R.L. Rogers and D.G. Young, Inst. Phys. Conf. Set. No. 66, The Institute of Physics, Bristol and London, 1983, p. 105. N. Wilson, "Electrostatics 1979", Institute of Physics London, 1979, p. 75. P. Tolson, 3. Electrostat., 8 (1980) 289. N. Wilson, Inst. Phys. Conf. Set. No. 66, The Institute of Physics, Bristol and London, 19831 p. 21. E. Heidelberg, Static Electrification Conference, The Institute of Physics and the Physical Society London, 1967. E. Heidelberg, Advances in Static Electricity I, Proceedings of the ist International Conference on Static Electricity, Vienna 1970, p. 351. /vl.Glor, J. E|ectrostat., I0 (1981) 327. H. Kr~mer and K. Asano, 3. Electrostat., 6 (1979), 361. 3.K. 3ohnson, 3. ELectrostat., 4 (1977/78), 55. M. Glor, unpublished results. A.R. Blythe and W. Reddish, Electrostatics 1979, Institute of Physics London, 1979, p. 107. M. Clot, 3. Electrostat., 15 (1984) 223. B. Maurer, Chem. Ing. Techn., 51 (1979) 98. F. Schmalz, private communication. P. Boschung, W. HiLgner, Q. LQttgens, B. Maurer and A. Widmer, 3. Eleotrostat., 3 (1977) 505.