Mizuki Yamaguma

Characteristics of the vibrating-mesh minimum ignition energy testing apparatus for dust clouds

Abstract A novel apparatus for testing the minimum ignition energies of flammable dusts is introduced. Unlike the conventional apparatus (the Hartmann tube), this new apparatus employs a vibrating mesh to produce a dust cloud. Using three kinds of powders, namely lycopodium, anthraquinone and polyacrylonitrile, which are designated as the samples for calibration by the International Electrotechnical Commission ( IEC, 1994 ) standards, fundamental characteristics were experimentally investigated. Concerning the m inimum i gnition e nergies (MIEs), the new testing apparatus worked well for two samples, lycopodium and polyacrylonitrile. The MIE for anthraquinone, however, was by far larger than the expected value. We concluded that the aggregation of anthraquinone particles is the main cause of the difference and is attributable to both the tribo-charges acquired by the particles when passing through the mesh and the filamentary shape of the crystal. Other essential factors for characterizing the testing apparatus, such as the concentration of dust, the shape and spacing of the sparking electrode system, and the impedance of the sparking circuit are discussed.

Ignitability of sprayed liquids due to an electrostatic spark

The minimum ignition energy (MIE) is a reasonable and practical index to assess the ignition risk of flammable materials. This paper reports the results of experiments dealing with the MIE due to an electrostatic spark of a sprayed liquid under various conditions. Four kinds of liquid (kerosene, n-decane, m-xylene and styrene) were used as the materials in this study. The liquid was pneumatically conveyed to the spray gun and automatically sprayed for 7 s to measure the MIE. The spatial distribution of the MIE in the spraying liquid under various conditions was also investigated in this study. The following results were obtained. (i) In the normal temperature range 0–30°C), all sprayed liquids can be ignited by the spark with a discharge energy below 10 mJ irrespective of their flash point. In particular, styrene was ignited at 4 mJ. (ii) An optimum region for ignition in a spraying liquid was observed, and it depended on the velocity, concentration and particle size of the liquid. Experimental details and a discussion are presented.

Electric spark ignition eneries of coating polymer powders

In this paper, we report on experimental results dealing with the minimum ignition energy (MIE) of coating polymer powders. An important result was found that some of the samples examined were so sensitive that even a spark with a very low energy, such as 2 mJ, could ignite them. The values of the discharge spark energy of ignition testing set by the BSI (British Standards Institution) standards and the FM (Factory Mutual) regulations related to the safe operation of electrostatic powder coating systems are sufficiently high to result in the ignition of some of the coating powders. Therefore, it is imperative that more appropriate discharge spark energy values in testing be defined for safety assessment in electrostatic powder coating systems.

Experimental Study on Electrostatic Hazards in Sprayed Liquid

In this study, to evaluate ignition hazards in a paint process, electrostatic sparks in the sprayed area and the amount of charge while spraying were observed. With the objective of preventing accidents involving fires and/or explosions, we deal also with the ignitability due to an electrostatic spark of a sprayed liquid relative to the percentage of nitrogen (N2), including compression in an air cylinder. For this study, an air-spray-type handheld gun with a 1-mm-internal-diameter orifice and a supply of air pressure in the range of 0.1 to 1 MPa were used. With regard to the materials, water, including some sodium chloride, was used to investigate the charge amount of the sprayed liquid, and kerosene was selected for ignition tests while spraying. Several electrostatic sparks in the sprayed region were observed while spraying. Some values of the electrostatic charge observed in the course of this study would be unsafe in the painting industry. Thus, if any of the conductive parts of the equipment are not grounded, incendiary electrostatic sparks can result. The ignitability of sprayed liquid was markedly reduced; the percentage of N2 in the air was substituted for pressurized pure air, and its efficiency increased with air pressure.

Experimental Study on Electrostatic Spark Ignitability of Coating Polymer Powders in Electric Field with Corona Discharge

This paper is a report of the ignitability of coating polymer powders due to an electrostatic spark in an electric field with a corona discharge. The ultrasonic vibration-type apparatus for measuring the ignitability of an electrostatic spark of coating polymer powders was used in this study. The dust concentration in all tests was always 0.60 kg/m3. Polyester powder with grains of 32 µm used in practical electrostatic powder-coating plants was employed as the powder material. The corona discharger, which was located in the explosion chamber, was controllable in the range of DC 5 to 8 kV (bipolar). As a result, the ignitability of coating polymer powders was confirmed to decrease in an electric field created by corona discharge. The reduction was dependent on the voltage applied to the wire electrode of the corona discharger. This result was attributed to the changes in the dispersion of the powder and the free ions produced by the corona discharge. Therefore, the influence of the corona discharge must be considered in the minimum ignition energies of powder paints when using electrostatic powder coating systems.

Experimental Study on Safe Performance of Nozzle-Type Electrostatic Neutralizer

The safe performance of a nozzle-type electrostatic neutralizer was examined experimentally in inflammable gas (ethylene)–air mixtures. A 100 MΩ-coupled electrode produced ion pairs safely for reducing static charges from polymer materials, and incendiary discharges were suppressed by the resistor and voltage supplied to the electrode. The performance of the neutralizer (flange-type) driven with an AC (50 Hz) or DC high-voltage power source was also evaluated with a full-size pneumatic powder-transport system. It was confirmed that the nozzle-type neutralizer is markedly effective for reducing electrostatic charges from polymer materials; however, several disadvantages were also noted.

Electrostatic hazards of charging of bedclothes and ignition in medical facilities

We investigated the charge generated on bedclothes (cotton and polyester) during bedding exchange with different humidities and the ignitability of an alcohol-based hand sanitizer (72.3 mass% ethanol) due to static spark with different temperatures to identify the hazards of electrostatic shocks and ignitions occurring previously in medical facilities. The results indicated that charging of the polyester bedclothes may induce a human body potential of over about 10 kV, resulting in shocks even at a relative humidity of 50%, and a human body potential of higher than about 8 kV can cause a risk for the ignition of the hand sanitizer. The grounding of human bodies via footwear and flooring, therefore, is essential to avoid such hazards (or to reduce such risks).