JNIOSH

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.
    From the backgrounds described above, safety management and some new trials as preventive countermeasures have been carried out to avoid the electrostatic hazards. However, fundamental data as the minimum ignition energy of new materials being a key to prevent the electrostatic hazards have been hardly investigated quantitatively yet, and also an electrostatic safety assessment in production processes of industries has not been systematically made on the basis of the electrostatic properties of new materials.
    The purpose of the specific research is to develop basic preventive technologies against the electrostatic hazards occurring in the production processes of new materials in industry. Study has been made about test methods for investigating the minimum ignition energy of new materials such as flammable fine powders and mist, and electrostatic safety assessment of industrial activities using real scale experimental facilities and computer simulation. In this report, the following results obtained from experiments and computer simulation are described :
  (1) New test method for measuring the minimum ignition energy of fine powders and mist.
  (2) Data of the minimum ignition energy of typical new materials obtained using test methods developed newly.
  (3) Electrostatic safety assessment of powder handling processes obtained from experiments and computer simulation.
  (4) Measurement of electrostatic charging characteristics on liquids caused by spraying.

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.
    The experiments were carried out using two different test apparatuses. The ultrasonic atomizer type apparatus was used for obtaining the fundamental data on ignition energy and explosion limit in spray clouds for ethanol/water mixtures, liquid alkanes and some halogenated hydrocarbons. The spray gun type apparatus was used for obtaining the practical data on ignition properties for ethanol and some commercially sold glycolic washing agents/water mixtures.
    The following results were obtained through the experiments :
  (1) The minimum ignition energies (MIEs) for the mists with a flash point of lower than a room temperature were smaller than 1 mJ, closer to that of the fuel vapour.
  (2) The MIEs for the nonvolatile alkane mists with a flash point of higher than a room temperature were constant value of about 4 mJ. This value is generally greater than that of gas explosion and smaller than that of dust explosion by one figure, respectively.
  (3) Halogenated hydrocarbon mists such as trichloroethylene were not ignited by sparks with a 12 kV, AC voltage transformer. Those mists can not be ignited by usual electrostatic discharges.
  (4) The lower explosion limits (LELs) for dispersed alkane mists were about 40 mg/litre. This value is equal to the LEL for the alkane gases in air at a room temperature. The LELs for the mists of ethanol/water mixtures were 45 to 70 mg/litre, roughly equal to the LEL (62 mg/litre) of the ethanol vapour/air mixture.
  (5) Ignition hazards of flammable mists can be drastically reduced by dilution with inert liquids such as water or by atomizing with inert gases such as carbon dioxide.

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 :
  (1) An antistatic FIBC made of rubberized or plasticized textile, which are dominantly used in Japan, if not electrically grounded, generated so much electric charge on its body that it was possible to bring about an energetic discharge incendiary even to a dust cloud. When it was properly grounded, the content was given more electric charge than when it was not grounded.
  (2) Another type of antistatic FIBC, which is made up of insulating plastic textile with thin conductive threads, sufficiently reduced static electricity on both the container body and the content when it was grounded. This was probably because of the ions produced by the feeble corona discharges occurring along the conductive threads during emptying process.

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.
    Experimental facility consists of a stainless steel cylindrical silo of capacity measured 3.5 m3 (diameter of 1.5 m and height of 2 m), stainless steel pipeline of 4 inch in diameter, a pressure type air blower driven by an inverter motor, bag filter equipment and air conditioning unit controlling the temperature and humidity of blowing. Electrostatic field strength inside the silo was also measured with an electrostatic field meter. The sensor was mounted on the outside wall which consisted of an opening for measurement. The sensor was a vibrating electrode being driven by a electric tuning fork and the probe was provided with an air blow mechanism for preventing the sensor from being coated by powder particles. Such a kind of field sensor was also mounted on the pneumatic pipe to measure the field strength inside the pipe and electrostatic charge on powder during transportation was calculated from field strength.
    In the experiment, the air blow type field sensors could be applicable for the measurements of both field strength within a silo and specific charge of powder within a pipe. Field strength inside a silo could be estimated from the specific charge and concentration of incoming powder and powder cloud in the silo. As the result, the air blow type field sensor was found to be applicable for safety evaluations during a pneumatic powder transportation. Furthermore, it was suggested that field strength inside a silo concerning the occurrence of incendiary discharge related to the specific charge and concentration of powder including the powder cloud.

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.
    The particle-in-cell technique has been used in the simulation. The motion of charged particles and the electric field inside the vessel have been solved self-consistently. The particle size distribution of the powder has also been taken into account. Four powders, each having a log-normal distribution and different mean particle sizes, have been used to investigate the dependence of particle size on hazards. The probability of incendiary discharges is considered based on calculated local electric fields and electrostatic energies.
    It was found that the motion of particles and the hazards strongly depend on particle size. Lighter particles in the powders were trapped in the upper region by air drag, a self-generated and heap electric fields. Since there was no large charged cloud of dispersed powder in air space, no lightning-like discharge was likely to occur under the simulated conditions. The possibility of incendiary discharges on the top of the heap was demonstrated. The results indicate that the larger size powders, the more dangerous under the simulated conditions.

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.
    The charged cloud consisting of such charged liquid drops and mist raises electrostatic field strength and may generate electrostatic discharges. Furthermore it sometimes causes an ignition of flammable mixtures in case the charged cloud is large in scale and high in concentration.
    In order to prevent such an explosion during spraying of liquid, proper safety evaluation applicable to actual liquid spraying and countermeasures against the ignition are required in industry. The purpose of this research is to clarify the basic characteristics of electrostatic charging and to discuss the evaluation method for ignition risk during spraying of liquid. Using a small scale of experimental apparatus consisting of a two-fluids nozzle we measured the nozzle current from the isolated metal nozzle to the ground, and the mesh current from a metal mesh with 19 cm diameter to the ground using a electrometer, respectively. The mesh was placed at 2 m distance from the nozzle. Electrostatic potential at the edge of the charged cloud was measured with a hand sized electrostatic surface potential meter. Flow rate of liquid drops was calculated from the weight of liquid trapped on a paper filter attached to the metal mesh.
    The results obtained from the experiments are as follows :
  (1) Charge to mass ratio on water drops at the position of 2 m in front of the nozzle was two figures larger than that at the nozzle and reached more than 1μC/kg because of a large amount of charges generated by division of liquid drops after the spraying.
  (2) Electrostatic charging of methanol, gasoline and their mixtures during the spraying was almost the same level and approximately a half of that of water.
  (3) It was found that electrostatic potential at the edge of the charged cloud could be estimated assuming a uniform space charge distribution in the radial direction.
  (4) Incendiary discharges are likely to occur when the charge cloud is large in scale and high in concentration because a large amount of charges generate on liquid drops during spraying.

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.
    The research themes and obtained results are as follows :
  (1) Measurement of minimum ignition energies for combustible fine powders : In the test measuring minimum ignition energies (MIEs) using the test apparatus based on IEC standard, some additives for plastics and bis phenol A dusts showed the MIEs lower than 1 mJ.
  (2) Measurement of minimum ignition energies for flammable mists' MIEs for nonvolatile alkane mists with a flash point of higher than room temperature showed a constant value of approximately 4 mJ.
    Ignition hazard of mist could be drastically reduced by dilution with water or by atomization using carbon dioxide.
  (3) Effect of discharge conditions on measuring minimum ignition energy for dusts : When a resister of 25 kΩ was connected to the discharge circuit in place of the inductor shown in IEC standard, the MIE for powder sugar was 6.8 mJ, much lower than 27 mJ taken with an inductor of 0.94 mH.
  (4) Development of the vibrating type minimum ignition energy measurement system : A newly developed vibrating-type MIE measurement system was applicable for two dusts, lycopodium and polyacrylonitrile, among three reference ones shown in IEC standard, but anthraquinone which has irregular particle shapes.
  (5) Measurement of electrostatic properties of flexible intermediate bulk containers (FIBCs) : In life emptying experiments antistatic FIBCs which were made up of insulating plastic textile with thin conductive threads showed sufficient properties for reducing static electricity on both the container and the content.
  (6) Electrostatic safety evaluation for pneumatic powder transport : A newly developed air-blow type electrostatic field sensor was applicable for electrostatic safety assessment of pneumatic powder transportation in real-size experiments.
  (7) Evaluation of electrostatic hazards caused by charged powders during tank filling using particle simulation-dependence of particle size : Simulation using the particle-in-cell technique clarified the effect of particle size on electrostatic hazard caused by filling of charged powders into a vessel.
  (8) Electrostatic charging on liquid caused by spraying : Charge to mass ratio on water drops in hanged could reached approximately 1μC/kg due to a large amount of charge generation caused by division of liquid after spraying from a two-fluids nozzle with a flow rate of 48 g/min.

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