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Explosions and static

Jonassen, Niels M

Published in: Electrical Overstress/ Symposium Proceedings, 1995

Link to article, DOI: 10.1109/EOSESD.1995.478301

Publication date: 1995

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Citation (APA): Jonassen, N. M. (1995). Explosions and static electricity. In Electrical Overstress/Electrostatic Discharge Symposium Proceedings, 1995 (pp. 331-337). IEEE. https://doi.org/10.1109/EOSESD.1995.478301

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EXPLOSIONS AND STATIC ELECTRICITY

NIELS JONASSEN PHYSICS DEPARTMENT, BUILDING 307 TECHNICAL UNIVERSITY CIF DENMARK DK-2800 LYNGBYj DENMARK PHONE: +45 45 88 24 88, FAX: +45 45 93 27 66

ABSTRACT The paper deals with the problem of electrostatic oxy

7.1.1 EOS/ESD SYMPOSIUM 95-331

Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 16,2010 at 05:29:02 EST from IEEE Xplore. Restrictions apply. EXPLOSIVE MIXTURES Vapors and gases It appears that it is possible to ignite a mixture of ether vapor and pure oxygen with a little more than 1 pJ if the concentration of ether vapor is about 16 %, while it takes about 0.2 mJ, or about 200 times more, to ignite a mixture of ether vapor and inition energy 10-2 atmospheric air, and then only if the ether concentration is very close to 6 %. The main reason it takes more energy to ignite a mixture with atmospheric air than with oxygen, is that it also takes energy to heat the nitrogen in the air without it participating in the combustion 10-3 process, and this energy is therefore wasted. It also appears that the necessary ignition energy of ether vapor with atmospheric air increases rapidly if the concentration deviates only slightly from the 6 % minimum-energy value. Consequently only mixtures between maybe 4 and 10-4 8 % are therefore in practice explosive. The curves shown in Fig. 1 are typical, in shape as well as in energies, for vapors of many organic compounds, cyclic as well as aliphatic, like hydrocarbons, ketons etc.

10-5 The 0.2 mJ value may thus be taken as a good lower energy limit for vapor/air mixtures. The vapor concentration of the easiest ignitable mixture, however, depends upon the chemical composition of the vapor and varies from 80 % (methane) to maybe 170 % (heptane) or more of 10-6 I I I the stoichiometric mixture3. 10 20 30 L *Ether vapor Although a spark with an energy near the minimum ignition energy is potent enough to cause an explosion in mixtures within a narrow Fig. 1. concentration range, it should be kept in mind, that Ignition energy of ether vapor mixtures if one has a container with a liquid with vapors with oxygen and atmospheric air like the ones discussed above, the vapor concentration at the surface of the liquid is If an atmosphere contains oxygen and vapor of an probably far too high for the mixture to be inflammable liquid a reaction (combustion) explosive, and at height of maybe half a meter between the two components is possible if enough above the surface, the concentration may be far too energy is delivered in the right way. low. Some mixtures, however, are near to impossible'to Therefore, somewhere in between the concentration make react. has exactly the critical value, requiring only the This is illustrated in Fig. 1 showing the ignition minimum energy for the mixture to blow up. energy for mixtures of ether vapor and pure oxygen or atmospheric air.

7.1.2 EOS/ESD SYMPOSIUM 95-332

Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 16,2010 at 05:29:02 EST from IEEE Xplore. Restrictions apply. Powders and gases Solids It is a well known fact that explosions may happen Generally speaking the tendency of giving rise to not only in vapor/gas mixtures but also under high charge separation will increase with the certain conditions in clouds of dust or powders. surface resistivities of the materials involved. On the anecdotical level it can be mentioned that 0fte:n solid materials are divided into normally in the 1930s explosions in grain siloes were three: (overlapping) groups after their chargebility reported with a rate of approximately one per week surface resistivity in the midwestem US states. While the (minimum) ignition energy for a static materials > 1014 Q vapor/gas mixture is only a question of the nature and concentrations of the vapor and the gas, as astatic materials 1010 - 1014 Q illustrated in Fig. 1, the situation is much more antistatic (dissipative) 1O1O Q complicated in the case of powders. materials First of all, while mixtures of vapors and gases are The division of materials into the above groups normally homogeneous with the vapor concen- should be taken only as a rule of thumb. It is true tration (in closed containers) being the same that most measurements of chargings of solid throughout the whole mixture, the concentration of materials show that the level of charges separated powder particles in a cloud may easily vary from do increase with the resistivity, but definitely not point to point, making the determination of a mini- in an unambiguous way. mum ignition energy very difficult. Whille low resistive materials never charge unless Further, the ignition energy normally depends upon the materials are separated extremely fast, there factors like the grain size of the powder involved. are, on the other hand, many examples of high Generally speaking however, it takes more energy resistive materials being brought into contact to start an explosion in a cloud of powders than in without showing any significant charges after an explosive vapor/gas mixture. While minimum sepairation. ignition energies for vapors in atmospheric air are Undoubtedly the nature of the contact play a role lower than 1 mJ, powders will normally require a for the charging process. minimum of 10 to 100 mJ to combust.

IGNITION SOURCES The energy necessary to start an ignition may be delivered to the vaporlgas mixture in a variety of ways, but we will here limit the interest to electrical discharges as an ignition source, and further especially such ones caused by static electric charge accumulations. Charging parameters A necessary, but not sufficient, condition for a static electric discharge to be incendive is that it originates from a large, or rather concentrated, charge distribution. Although the course of a charging process is diffi- cult, if at all possible, to predict from the know- +I ledge of characteristic parameters of the materials involved, certain simple rules may be formulated concerning the likelihood of encountering major Fig. 2 charge separations under given circumstances. Charging between conductor and non-conductor

7.1.3 EOS/ESD SYMPOSIUM 95-333

Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 16,2010 at 05:29:02 EST from IEEE Xplore. Restrictions apply. Normally the charges separated will be greater the many types of powder, the current increases with more intimate the contact and friction is, and the square of the linear velocity or even stronger, therefore smooth surfaces will often charge more and may have values of several PA for a velocity than do rough ones. of a few meter per second. When it is stated that low resistive materials do not If the powder is collected in an isolated metal charge, this is generally true only when both container, the current will charge the container to materials have low resistivity. If for instance an a voltage where the unavoidable decay current is insulating sheet of plastic, Fig. 2, is guided by a equal to the charging current. metal roller, the sheet as well as the roller may be charged, and if the roller is not grounded, both Liquids polarities of charge may be retained after sepa- It has been shown that an electrical double layer ration. exists on the 'surface of many liquids and that the charges in this layer can be partly separated by Dust - powders ffow and spraying.

Fig. 3 Fig. 4 Charging of a powder Charging of a liquid by flow Dust and powders may be charged by contact and If the liquid is flowing through a tube, the friction between the particles, especially if the in- separated charges constitute a current to the con- dividual particles have different properties, for tainer in which the liquid is being collected, Fig. 4. instance regarding size and/or material. A charging Since the specific charge (charge per unit volume) of this type may result in the particles sticking of the liquid often depends very little on the together. More common, however, are the pro- velocity, the charging current is more or less cesses where a powder is being transported through proportional to the flow rate of the liquid. a system of tubes, Fig. 3, and the powder as a The specific charge, in general, increases with whole is being charged by the friction with the decreasing tube diameter and flow through a filter, walls of the tube system. which can be considered as a large number of This kind of charging may take place if either the (parallel) narrow tubes, will consequently often powder or the tube or both are insulating. cause chargings. The flow of charged powder can be considered as The specific charge also depends strongly on the a current and it has been demonstrated that for resistivity of the liquid.

7.1.4 EOS/ESD SYMPOSIUM 95-334

Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 16,2010 at 05:29:02 EST from IEEE Xplore. Restrictions apply. As a rule of thumb only liquids with resistivities discharge. above lo7 Qm will give rise to essential static The value of the breakdown field strength in air at chargings because of flow. atmospheric pressure is normally given as 3.106 But also small amounts of additivies may change V" the specific charge rather drastically. This, however, is only (almost) true for the field As an example, it can be mentioned that adding of between plane parallel electrodes. In order to get a water to for instance toluene, to form a disperse discharge started at a sharp point or around a thin mixture, makes the specific charge increase wire the field strength at the surface has to be considerably although the resistivity is nearly considerably higher. constant. Nevertheless, sharp points are the most likely As examples of specific charges, diesel oil may starting places for electrical discharges. And the have values of 10 -6 - 10 -5 C.m'3 and aviation reason for this apparent paradox is that at a given gasoline about 10-~~-m-~. voltage, the field strength will be higher at areas While only high-resistive liquids charge by with low radius of curvature, such as points or flowing, almost any liquid may give rise to charge protrusions. separation by spraying. The spray may charge any The course of the resulting discharge, however, insulated conducting system it hits or form space depends strongly, among other factors, upon the charges. The resulting fields may under special geometry of the body where the discharge starts circumstances give rise to dangerous discharges, as and the nature of the material through which it is believed to have been the case with explosions develops. in oiltankers during tank washing. Types of discharges Gases Finally it should be remembered that flow of pure gases, contrary to earlier popular belief, does not give rise to any charge separation.

DISCHARGES As indicated above, a concentrated charge distri- bution is necessary for an electrostatic discharge, but the incendiveness depends strongly on the type of discharge. in air Normally, atmospheric air is considered a good Fig. 5 insulator because the concentration of mobile Corona discharge charge carriers, atmospheric ions, created primarily by natural radioactive radiation, is very low. If a conducting electrode in the shape of a sharp An electric field of, say, 100,OOO Vm-' will thus point (or a thin wire) is held at a sufficiently high produce a current with a density of only a few potential, maybe 2 - 20 kV, the breakdown field r~A.m-~. strength may be exceeded in a region of a few If, however, the field strength exceeds a certain millimeters around the electrode, Fig. 5. critical value (the breakdown field strength, E&,), In this region positive and negative ions will be electrons freed by the natural ionization processes formled, as explained above, and ions of opposite will be accelerated to such high velocities and polarity of the electrode voltage will be attracted energies that they themselves will create new ions towards the electrode, where they will be neutra- by collision in the whole region where the field lized, while ions of the same polarity as that of the strength exceeds E,,. electrode will be repelled. Once outside the ionizat-

The resultingv current is what we call an electrical ion iregion" the ions move awav towards Ygrounded

7.1.5 EOS/ESD SYMPOSIUM 95-335

Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 16,2010 at 05:29:02 EST from IEEE Xplore. Restrictions apply. surroundings with velocities rapidly decreasing called , characterized by moving with the distance from the electrode from about irregular luminiscent discharge paths. 100 meter per second to maybe 1 - 2 cm per second. This kind of discharge is called a corona or silent It should be mentioned that discharges may also discharge. take place to or from charged insulators. If watched in the dark a bluish luminiscence may Such discharges will always be corona or brush be seen in the region of ionization. discharges.

Spark discharge

Fig. 6. Fig. % Spark discharge Brush discharge

The best known type of electric discharge is the INCENDIVENESS OF DISCHARGES spark. The ability of an electrical discharge to start a Although the name is commonly used for almost combustion or explosion depends on the energy any kind of charge transfer through dielectrics density of the discharge. (including air), it ought to be reserved for the dis- The total energy dissipated in a corona discharge charge between two conductors (without sharp may in principle be rather high if the discharge protrusions) at different potentials. takes the form of a continuous current. In a spark, ionization takes place along and the If for instance, a discharge current of 1 FA is charge is transferred through a narrow channel established by a voltage of 5 kV, an energy of 5 between the two conductors. In this channel, most mJ is dissipated in the discharge volume every of the energy stored in the field between the con- second (a power of 5 mW). ductors will be dissipated. If the (partial) capaci- The density of this dissipation, in time as well as tance of the two conductors is C and their potential in space, however, is too small to start a com- difference is V, the energy W dissipated in the bustion process in any known vapor/gas mixture. discharge is given by: Also, a brush discharge may be maintained as a continuous current, but is in most cases, a short 2 time event, and the energy density is much higher than in a corona discharge. It has so far not been Brush discharge possible to accurately measure or calculate the If the discharge takes place between a small elec- energy dissipated in a brush discharge, but it is trode (radius of curvature in the order of milli- estimated that total energies of about 1 mJ may meters) and ground, the discharge may be a so- occur under certain circumstances, which is enough

7.1.6 EOSIESD SYMPOSIUM 95-336

Authorized licensed use limited to: Danmarks Tekniske Informationscenter. Downloaded on February 16,2010 at 05:29:02 EST from IEEE Xplore. Restrictions apply. to ignite many vaporlgas mixtures with (un)suitable If we assume Wmin= 0.2 ml and the capacitance of mixing ratios. the system is C = 300 pF we find It should be pointed out, however, that such high brush-energies require extreme1y high charge accu- mulations, and that consequently discharges from charged insulators normally are not incendive. An exception to this "rule" are the so-called stem- med branch brush discharges from highly charged thin layers of insulators, backed by a grounded It shaluld be stressed that safe-voltage levels only conductor. refer to explosion risks. When dealing with elec- The charge neutralized in such discharges and tronic ESD-problems the acceptable levels are hence the energy dissipated may be very high (in often considerably lower. the order of tenths of ml). Also, the safe-voltage levels only apply to insu- The Cape Kennedy accident may have been the lated conductors, since voltage can not (or hardly result of such a discharge, but normally it takes ever) be meaningfully determined for an insulating carefully planned laboratory experiments to create material. a stemmed branch brush discharge. The most incendive of all types of discharges is the CONCLUSION spark discharge. The discharge volume is much Electrostatic discharges may cause fires and smaller, and consequently the relative energy den- explosions. It has, however, been demonstrated that sity much higher, than in a corona or brush dis- only certain types of discharges may be incendive, charge. A real spark always occurs between fairly and even when such discharges do occur, a series flat regions of two conductors when the breakdown of limiting conditions have to be fullfilled in order field strength is exceeded. When this happens the to maike the discharges become harmful. energy W dissipated is determined by the voltage Generally speaking, discharges from insulators are V and by the partial capacitance C between the less incendive than those from and between two conductors, as given by: conductors. 1 In the case of the potentially most harmful type of = -cv= discharge, the spark, i.e. the discharge between two w 2 conductors, it is possible to define an explosion- safe upper limit of the potential difference. If thus, the minimum ignition energy W,, for a given atmosphere is known, we can define an REF'IERENCES "explosion-safe" voltage Vexfor a system with the 1. US. Department of Agriculture, Bull. 379. capacitance C as 2. Tidsskrift for danske sygehuse, nr. 6, 1954. 3. U.S. Bureau of Mines, Bulletin No. 520, 1953.

7.1.7 EOS/ESD SYMPOSIUM 95-337

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