Abstract of Special Research Report (SRR-No.38)
National Institute of Occupational Safety and Health, Japan
A Comprehensive Study on the Prevention of Explosions and Fires Associated with Static Electricity during Liquid Spraying
Introduction
SRR-No.38-1 |
Mizuki YAMAGUMA |
: Static electricity generated during spraying of liquid may often become an ignition source for a flammable gas or vapor mixture, even for mists. In this comprehensive study, a variety of experiments and numerical simulations were carried out in relation to electrostatic phenomena and ignitions associated with spraying. Themes of this study were selected according to surveys and analysis of labor accidents, and the topics that are considered most important and urgent were examined intensively. This study is divided into three sub-themes; results for the respective sub-themes are summarized as follows:
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Assessment of Static Ignition Risk by a ChargedCloud in Grounded Cylindrical Vessels
SRR-No.38-2 |
Atsushi OHSAWA |
: This paper presents the criteria of space charge density and wall electric field required to prevent incendive discharges produced by a charge cloud in cylindrical tanks. To obtain them, the threshold charge density of a homogenous cloud for initiating a discharge between the cloud and grounded protrusion is numerically calculated, and then the transferred charge and energy of the discharge are obtained to investigate the incendivity. In addition, the maximum electric field at the side wall of the tank at the threshold charge density is obtained for evaluating and monitoring the risk. |
Measurements of Space Charge Density in Clouds Produced by High-pressure Water Sprays
SRR-No.38-3 |
Atsushi OHSAWA |
: Charged clouds produced by water spraying used in industrial operations are potential hazards of ignition by electrostatic discharges. Since this ignition risk can be assessed by electric fields created by the charges in a cloud, estimation of the charge density of the cloud is necessary to determine whether the electric fields can initiate an incendive discharge. This paper presents the results of measurements of charge densities in clouds during water spraying at water pressures up to 84 MPa. The results show that use of tap water, which is highly conductive, effectively reduces the risk in washing operations; in addition, relatively thicker nozzles reduce the charge in clouds. |
Characteristics of Ignitability of Sprayed Liquids due to Electrostatic Spark(I)
SRR-No.38-4 |
Kwang Seok CHIO, Mizuki YAMAGUMA and Atsushi OHSAWA |
: Electrostatic charge often presents in typical liquid processes and operations, such as filling tanks and containers, road and rail tanker deliveries, and insulation pipes through which liquids pass. In particular, a liquid charges rapidly when it spews out through a small hole or a crack that occurs unexpectedly due to some problems in a pipe. This charging phenomenon may give rise to an explosion or ignition of the liquid. The minimum ignition energy (MIE) is a reasonable and practical index to assess the ignition risk of flammable liquid.
Keywords;Electrostatic spark, Spraying liquid, Ignitability, Minimum Ignition Energy |
Characteristics of Ignitability of Sprayed Liquids due to Electrostatic Spark (II)
SRR-No.38-5 |
Kwang Seok CHOI, Mizuki YAMAGUMA and Atsushi OHSAWA |
: Explosions and/or fires involving liquid while spraying frequently occur due to electrostatic sparks in various chemical processes. Accordingly, the quantitative research into the ignitability and the electrostatic charge of sprayed liquids will be necessary to assess hazards in industrial operations under various conditions and to propose effective countermeasures. In the current paper, we deal with Minimum Ignition Energy (MIE) due to an electrostatic spark of a sprayed liquid relative to the percentage of nitrogen (N2),including compression
in an air cylinder. Four different mixtures, A (O2: 20 %,N2: 80 %),B (O2: 15 %,N2: 85 %), C (O2: 10 %,N2: 90 %),D (O2: 5 %,N2: 95 %) were used. The charge amount while spraying and electrostatic sparks in a sprayed region were also observed. |
Explosible Atmosphere due to Spraying Processes-Ignition Properties of Flammable Vapor by Electrostatic Sparks
SRR-No.38-6 |
Mizuki YAMAGUMA |
: In a spraying process, flammable liquid such as a volatile organic solvent is usually contained in the spayed substance, and often forms a flammable atmosphere if it evaporates in a poorly ventilated space. Actually, numerous explosions and fires occur each year — one of the main causes of which is an electrostatic discharge. Occurrence of a fatal explosion in a shipyard, for example, has been reported, where an airless sprayer was used to paint the wall of a tanker under construction. |
Electrostatic Properties of Spraying Devices - Spray Cansand an Airless Paint Sprayer
SRR-No.38-7 |
Mizuki YAMAGUMA |
: According to our survey on accidental explosions and fires due to electrostatic sparks in industrial processes, about half of these are related to spraying devices, such as hand-held spray cans and paint sprayers. Spray cans used for the liquid-penetrant testing (PT) and airless paint sprayers, especially,are found to be the dominant ignition sources. A variety of measurements were carried out in order to clarify the electrostatic properties of these devices. The results for spray cans and an airless paint sprayer are summarized as follows:
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