Abstract of Special Research Report (SRR-No.17)
National Institute of Occupational Safety and Health, Japan
Prevention of Explosions and Fires of New Materials Caused by Static Electricity
Introduction
SRR-No.17-1 |
Yasuyuki TABATA |
: With the remarkable progress in science and technology, new chemical or electronic materials such as fine organic powders, amorphous and thin membrane have been produced recently using industrial robots, a computer integrated manufacturing and factory automation system in industry. However, these new materials and production systems are sensitive to static electricity ; as a result, various industrial hazards have been caused by static electricity. One of them is explosion or fire of new materials due to electrostatic discharges and has brought serious problems on both occupational safety and industrial activity.
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Measurement of Minimum Ignition Energies for Combustible Fine Powders
SRR-No.17-2 |
Toei MATSUDA, Hajime TOMITA and Tsutomu KODAMA |
: Minimum ignition energy (MIE) is used to assess the ignition risk of combustible dusts. It can be conveniently measured using spark discharges, but the MIEs for dust suspensions depend on a number of variables such as particle characteristics, electric circuits and dust-dispersion system. It is then quite troublesome to seek an absolute MIE value for a given material. Recently, International Electrotechnical Commission (IEC) has published a standard test procedure for MIE of dust and a test apparatus (known as MIKE 3) which complies with the IEC procedure has been developed in Europe. Using this test apparatus, approximately 40 sample dusts were tested to compensate for the very limited published data. In the tests, some of samples, additive powders to plastics and bis-phenol A dust, showed the MIEs lower than 1 mJ. Considering the ignition mechanism of a dust-air mixture, it will be not necessarily remarkable that some organic dusts have the MIEs of the same rank as that for gaseous mixtures. The existence of these combustible dusts, however, will require the more severe measure for safety in these dust handling facilities than to attempt to eliminate a system of low energy ignition sources. MIE is strongly dependent on particle sizes, but it doesn't always seem to follow that the smaller particle sizes take the lower MIEs. Some dusts with smaller average particle sizes showed the higher MIEs in the tests, maybe due to particle agglomeration. Since the number of practical test variables is limited, this simple apparatus and method could be used for practical MIE rankings of combustible dusts, provided the selection of dust samples would be made to represent the particle size distributions in the industrial use. |
Measurement of Minimum Ignition Energies for Flammable Mists
SRR-No.17-3 |
Hidenori MATSUI and Teruhito OHTSUKA |
: In industry, flammable liquid sprays are frequently used for the purposes of washing, coating, sterilization and etc. It is very important to assess the explosion hazards of such spraying processes and to take preventive measures against accidental explosions of flammable mists caused by electrostatic discharges.
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Effect of Discharge Conditions on Measuring Minimum Ignition Energy for Dusts
SRR-No.17-4 |
Mizuki YAMAGUMA, Tsutomu KODAMA and Pei-Lan WANG |
: Minimum Ignition Energies (MIEs) for dusts have long been measured using the Hartmann test tube apparatus. In the apparatus, the static energy stored in a capacitor is supplied to the air-gap to cause an incendiary spark discharge. In accordance with the IEC standard, an inductor of 1-2 mH should be serially connected in the circuit to prolong the duration of a discharge in order to facilitate the ignition of dust. In the light of actual conditions where combustible dusts are handled, however, another passive element like a resistor should be taken into account. In this context, resistors of 25 kΩ - 500 kΩ were connected to the electric circuit of an apparatus in place of the inductor, and then the measurements of ignition energies for sugar powder were carried out. The minimum ignition energy was 6.8 mJ, much smaller than the value of 27 mJ taken with the inductor. Discharges resembled glow discharges, which have longer duration and higher gap voltage than arc discharges. About two to twelve percent of the static energy stored in the capacitor, depending on the resistances, was released as a discharge. Generally, higher resistances caused more energetic discharges, contrary to our prediction. |
Development of the Vibrating Type Minimum Ignition Energy Measurement System
SRR-No.17-5 |
Mizuki YAMAGUMA, Tsutomu KODAMA and Wai-Lam CHEUNG |
: It is commonly recognized that the conventional methods for determining the minimum ignition energy (MIE) of a dust cloud are time consuming and require operational skills. As a variety of new flammable powders have recently been developed and produced in industry, there is an urgent need for a quicker and more economical means to measure the MIEs for those powders. To meet this requirement, we have developed a measurement system which employs a novel method to create an air/ dust mixture in a compact combustion chamber. In this system, the powder to be tested is put in a hopper made of a metallic mesh, and, with vibration, successively fed downward to form a dust cloud. With this new apparatus, three types of powders designated by an international standard for evaluation - lycopodium, anthraquinone, and polyacrylonitrile - were put to the test, and their MIE data were compared with those taken with a conventional testing apparatus (the Hartmann tube). The MIEs for two powders, lycopodium and polyacrylonitrile, agreed satisfactorily. For the other, anthraquinone, however, the MIEs were quite different with each other. We finally reached the possible reason the agglomeration of particles which was observed in powders passing through the mesh. It is concluded that the agglomeration was caused by the static charges generated on the particles by passing though the mesh, and also by the irregular shapes of the particles. |
Measurement of Electrostatic Properties of Flexible Intermediate Bulk Containers
SRR-No.17-6 |
Tsutomu KODAMA, Mizuki YAMAGUMA and Shonosuke KAMACHI |
: Because of their reasonable price and usefulness, flexible intermediate bulk containers (FIBCs) are commonly used in industry for handling bulky stuffs such as powders and pellets. They, however, often cause serious accidents such as fire and explosion due to the electrostatic discharges during emptying process. In order to collect scientific data for future regulations or standards for antistatic FIBCs, an intensive research on the electrostatic properties of FIBCs using a life-size apparatus was carried out. The noticeable results obtained from the experiments are as follows :
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Electrostatic Safety Evaluation for Pneumatic Powder Transpor
SRR-No.17-7 |
Tsutomu KODAMA, Koujirou NISHIMURA and Yasuyuki TABATA |
: Recently many kinds of materials and raw materials are being handled in powder form in industries by means of pneumatic transportation because of handling advantage. However, a large amount of electrostatic charge generated by friction within pipe walls and collisions with other components within the system accumulate on powders with a high electric resistivity. Loading powders that carry large amounts of static charge into a silo from the top raises electric field strength inside the silo and may generate electrostatic discharges and cause ignition of flammable dusts in case small particle powders present with higher concentration than the low explosion limit. In order to prevent such a dust explosion from occurring during powder handling, proper safety evaluations applied to real facility are required in industries. So, for the purpose of considering evaluation methods for electrostatic safety during pneumatic powder transport, experiments measuring both static field strength and static charge on powder inside the pneumatic system using full scale experimental facility were carried out. |
Evaluation of Electrostatic Hazards Caused by Charged Powders during Tank Filling Using Particle Simulation — Dependence of Particle Size —
SRR-No.17-8 |
Atsushi OHSAWA |
: It is well known that charge can accumulate on the powder particles during the handling and processing of insulating powders. When these charged powders enter and accumulate in a vessel, a large amount of charge can be stored and can lead to electrostatic hazards. To prevent such hazards, it is important to understand the mechanism of the formation of the hazards. Moreover although a large number of new functional powders have been developed by the recent evolution of technology, it is difficult to evaluate experimentally the hazards in an individual system. In this paper, we present a self -consistent simulation of charged powder entering a vessel for modeling electrostatic phenomena and evaluating electrostatic hazards. |
Electrostatic Charging on Liquid Caused by Spraying
SRR-No.17-9 |
Yasuyuki TABATA, Tsutomu KODAMA and Atsushi OHSAWA |
: Leakage accidents of pressurized flammable liquids and discharges of liquefied gasses or steam, which are accompanied by spraying of liquid, often occur in chemical plants. Also, there are a lot of spraying processes such as coating, cooling, cleaning, chemical reaction, sprinkling, painting and so on in industry. A large amount of electrostatic charges generated by liquid flow, separation from nozzle and division of liquid drop accumulate on liquid drops and mist in spite of electric conductivity of the liquid.
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Conclusions
SRR-No.17-10 |
Tsutomu KODAMA |
: With the remarkable progress of technology, many new materials such as fine organic powders and functional powders have been handled in recent industries. Since such materials are sensitive to static electricity, industrial hazards such as explosions and fires have been caused by static electricity. This specific research was carried out to develop basic preventive technologies against the electrostatic hazards occurring in the handling processes of new materials in industry.
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