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



: 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:
(1) Static electrification associated with sprayers and characteristics of explosive atmosphere caused by solvents
     Experimental investigations were carried out in relation to electrostatic properties (charge, effects of nozzles, temperature, etc.) during the use of sprayers such as airless sprayers and spray cans; mechanisms of explosions and fires; and methods to mitigate static charge. In addition, an innovative testing apparatus for determining the explosion ranges and minimum ignition energies of volatile organic solvents at various temperature was fabricated, and fifteen popular solvents were subjected to testing. The results are analyzed systematically.
(2) Development of safety evaluation methods for spraying process
     In order to investigate hazards of ignition of an explosive atmosphere caused by incendiary discharge from a charged mist cloud created in a large-scale spraying process, charges generated during spraying of water at pressures up to 84 MPa were measured, and numerical simulations were conducted for occurrence of incendiary discharge from a charged mist cloud formed in a container. Experimental results show that tap water mist cannot produce incendiary discharge, and static charge can be reduced by employing a nozzle having a wider bore. According to the results of the numerical simulations, the author proposes a practical risk assessment method for a large-scale charged mist cloud formed in a cylindrical structure.
(3) Risk assessment for ignition of flammable mist caused by accidental leakage of pressurized liquid
     Experimental investigations were conducted to assess the possibility of ignition upon accidental leakage of a pressurized flammable liquid. Ignition energies for mists ejected from an air sprayer were measured and the results show that most liquids having flash points higher than room temperature can be ignited by an electrostatic spark of 10 mJ or less. Styrene mist, especially, can be ignited with a spark as low as 4 mJ. Addition of nitrogen to the air for a sprayer is effective.

Assessment of Static Ignition Risk by a ChargedCloud in Grounded Cylindrical Vessels

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.
Since such calculations give conditions required to prevent such an incendive discharge, those with wide-ranging dimensions in the tank and protrusion may be useful in assessing the static ignition risk.
Consequently, we found that protrusions less than 2 mm in radius of curvatures never lead to incendive discharges; in other words, such protrusions may safely reduce the charges in tanks, but when their radii exceed 2 mm, an incendive discharge is possible. The conditions of ρ≤ 2.3 × 10-6R-1 (ρ: charge density, R: tank radius), and an electric field of ≤ 1.1 × 105 V/m at the side wall of the tank may reduce the static ignition risk caused by a discharge in flammable atmospheres of minimum ignition energy ≥ 0.2 mJ, which corresponds to most hydrocarbons, in tanks up to ≈ 1.5 × 105 m3.

Measurements of Space Charge Density in Clouds Produced by High-pressure Water Sprays

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)

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.
This paper reports the results of experiments dealing with the MIE due to an electrostatic spark of a sprayed liquid under various conditions. As the sample materials, four kinds of the liquid - kerosene, n-decane, m-xylene, and styrene - were used in this study. The spatial distribution of the MIE in spraying liquid under various conditions was also investigated in this study.
    The results obtained from the experiment are as follows:
  (1) The spraying phase of all liquid samples under room temperatures (below 30°C) can be ignited by a spark with a discharge energy of below 10 mJ irrespective of their flash point. The most sensitive sample used in this study was styrene, which was ignited with 4 mJ of spark energy. This suggests that a sprayed liquid must be managed to ensure safety since it is always possible that the energy from an electrostatic spark might exceed the values described above and that it could be generated in the spraying process.
  (2) An optimum region affected by factors such as the velocity, concentration, and particle size of liquid exists for the ignition of a sprayed liquid.

Characteristics of Ignitability of Sprayed Liquids due to Electrostatic Spark (II)

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.
Two types of hand-held spray guns and a supply of air pressure in the range of 0.1 to 1 MPa were used in this study. A liquid was supplied to the spray tube (liquid input line) with air from the pressure supply line. The liquid was automatically sprayed for 10 s and 15 s,respectively to measure the ignitability and the charge amount. With regard to the materials,kerosene (conductivity,ó: 3.50 × 10-14 S/m) was selected for ignition tests while spraying, and water (ó: 0.1 S/m), including sodium chloride (0.5wt% NaCl),was used to investigate the charge amount of the sprayed liquid. The following noteworthy results were obtained: The ignitability of sprayed liquid was dramatically reduced by atomizing with an air changing the percentage of N2 instead of pressurized pure air,and its efficiency increased with increasing the supply of air pressure.
This fact suggests that both conditions, a 90% rate (or over) of N2 in air and 0.3 MPa (or over) of P, are necessary to prevent effectively explosions and fires that are the result of electrostatic sparks in a practical industry. The charge amount values of some obtained in this study were unsafe in the painting industry, and several electrostatic sparks were observed while spraying, even though no incendiary sparks were detected in our tests. These sparks were particularly conspicuous at the nozzle. This fact suggests that the spraying of liquids (especially solvents such as toluene,acetylene,and xylene) under high pressures of several tens of MPa as water jets or airless paint spray must be carefully managed since it is possible that incendiary electrostatic sparks could occur.

Explosible Atmosphere due to Spraying Processes-Ignition Properties of Flammable Vapor by Electrostatic Sparks


: 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.
A spray can also create an explosible mixture, because it contains liquefied gas such as LPG or DME. Therefore, it is necessary to determine the sensitivity of the volatile substance to an electrostatic discharge in order to carry out a safety assessment. However, conventional methods are so complex and cumbersome that data can only be obtained by trained personnel and a lot of time.
In order to solve this problem, we designed a novel, easy-to-operate apparatus to measure the sensitivity of a volatile flammable liquid under various temperature conditions. The ignition energies and explosion limits for 15 solvent vapor/air mixtures were measured using the apparatus.
For all the mixtures, the minimum ignition energies (MIEs) decreased exponentially, and the explosion limits expanded with increasing test temperature. The acetone vapor/air mixture, among others, depended strongly on the temperature — the MIE at 25°C was less than one-third that at 100°C. Empirical equations for estimating the upper and lower explosion limits by an electrostatic spark from those by a more energetic ac spark were derived. A mixture of acetone, water, and air was effectively inactivated when the water concentration was 30 vol% at 100°C.

Electrostatic Properties of Spraying Devices - Spray Cansand an Airless Paint Sprayer


: 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:
(1) Spray cans
    The charge generation was greatly affected by the polarity of the liquid materials, the solid fine particles suspended in the solution, the bore of the nozzle, and the temperature. One of the PT spray cans containing fine silica powder produced current and charge above 50 nA and 30 μC/kg, respectively, at a high temperature (ca 30°C). Such levels are quite dangerous when an operator is not grounded.
When two popular propellents, liquefied petroleum gas (LPG) and dimethyl ether (DME), were compared, DME was found to generate a charge that was considerably higher than that generated by LPG. This was probably because DME is a polar compound that enables dissolution of numerous ions. By slightly enlarging the bore of the nozzle, the charge was drastically reduced, which suggests that an antistatic version of a spray can could be devised with a minor modification of the current nozzle.
(2) An airless paint sprayer
    Paint containing glass flakes was sprayed at various values of liquid pressure while the size of the nozzle orifice was changed. The spraying current and the specific charge of paint mist reached as high as 200 nA and 1.6 μC/kg, respectively.
In some cases, not only the level but also the polarity of the static charge changed when the size of the orifice was changed. When the dilution ratio of the paint was changed to 10 wt% from the original 3 wt%, the spraying current increased ca five fold. The electric potential of the sprayer in operation in conjunction with the human body, when they were electrically isolated, reached over 12 kV within 30 s. These findings revealed that the spraying apparatus must be maintained to assure safety, because it is always possible that a high level of static charge could be generated.

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