Downloaded from orbit.dtu.dk on: Sep 26, 2021 Explosions and static electricity Jonassen, Niels M Published in: Electrical Overstress/Electrostatic Discharge Symposium Proceedings, 1995 Link to article, DOI: 10.1109/EOSESD.1995.478301 Publication date: 1995 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit 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 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. 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<gen.A discharge happened, probably from or to discharges as causes of ignition of vaporlgas and the "hair pin" causing an explosion. The patient did dustlgas mixtures. not survive2. A series of examples of staticcaused explosions In 11989 an operator in a Danish chemical plant will be discussed. was pouring resin into a mixing mill containing The concepts of explosion limits, the incendiveness acetone (vapor). The resin was guided by a metal of various discharge types and safe voltages are funnel resting on the opening of the mill. An explained. explosion happened that injured the operator seriously. It turned out that neither the funnel nor INTRODUCTION the operator were properly grounded. The funnel Probably the most dramatic and extensive known could thus be charged by the direct contact with effect of an electrostatic discharge was the serious the resin. The operator was holding an insulating accident at Cape Kennedy in 1964 where bag from which the resin was poured, arid he could discharges between charged plastic sheets and the be "charged" by induction, and both processes body of a space vehicle started a fire that killed could give rise to a discharge and possibly the three people, injured eleven others and caused ignition of the vaporlair mixture. damages for about 55 M$. But static related accidents had been known long CONDITIONS FOR EXPLOSIONS before this. The above examples suggest that static electric In the fall of 1915 the US midwest states shook by pro'blems have been around for a long time and the an astonishing number of explosions in threshing number of known static caused accidents is legion. machines. On the other hand, static electric discharges do An investigation of 94 cases revealed that in about happen very frequently (in atmospheres containing 75 % of the accidents, electrostatic charging had vapors which may form explosive mixtures with air occurred and that in about half of all the cases or other gases) without giving rise to ignitions or other causes but electrostatic discharges could be explosions. ruled out as the ignition source. In about 30 % of The reason obviously is that most discharges do the cases, the machinery was totally destroyed by not have enough energy (or rather energy density) the explosion or the resulting fire'. to start the necessary interactions between enough In January 1954, an explosion happened during an molecules of the vapor and the oxygen of the air. operation in a hospital in Arhus, Denmark. The Or, maybe more precisely, that the incidental ratio anaesthetist assisted the patient's breathing by between the concentrations of the vapor and the rythmically compressing a respiration bag (of oxygen requires more energy dissipated per unit ordinary insulating rubber) hereby rubbing the bag volume than can be delivered by a likely static against a metal "hair pin" inside the bag. The discharge. anaesthetic was a mixture of cyclopropane and 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.