JNIOSH

Abstract of Special Research Report (RR-92)

National Institute of Occupational Safety and Health, Japan

Residual Stress Measurements in Butt-Welded Plates by X-Ray and Hole-Drilling Strain-Gage Methods

RR-92-1
Etsuji YOSHIHISA

: Welding residual stresses are frequently of interest in fracture evaluations of structural components. Although some data are available on them in small-sized specimens, data for large sized plates, in particular with a comparison of different measurement methods, are scarce.
    The objects of this study were to investigate the effect of the specimen size on the welding induced residual stress and to compare the data obtained by two different methods. One was X-ray diffraction strain measurement which was the nondestructive technique. The other was hole-drilling strain-gage method which was the semi-destructive one and appropriate in field application. Three pairs of different-sized plates made of mild steel, JIS SB410, were butt-welded in the arc welding process with 5 runs to make three types of specimens. The sizes of the specimens were 1530 mm × 600 mm, 765 mm × 1235 mm and 770 mm × 600 mm. Longitudinal and transverse residual stresses were measured along the weld line and the transverse line which crossed at the midpoint of the weld line. The results of this paper are summarized as follows:
  (1) The shapes of the residual stress distributions in plates butt-welded with 5 runs, can be inferred from those for Bead-on-plate with single run, obtained by Ueda et al. with thermal elasto-plastic analysis of the welding process, and the longitudinal residual stresses along the weld lines are near the yield point level of the basic material.
  (2) In the plates butt-welded under the same condition, the above mentioned residual stresses along the weld lines increase, as the weld lines become longer. On the other hand, the widths of the plates give little influence.
  (3) The shapes of the residual stress distributions in the plates cut in the direction perpendicular to the weld lines remain similar to the shape before cutting, but the longitudial stresses along the weld lines decrease.
  (4) The longitudinal and transverse stresses along the weld lines, obtained by hole-drilling method are much higher than those by x-ray method and most of the longitudinal stresses exceed the yield strength. Local yielding in the periphery of the hole cause errors in experimentally determined residual stresses. In the range near the yield level, hole-drilling method is considered to lose accuracy in the measurement of stresses.

Development of a General Purpose Structural Reliability Analysis Code Jointly Based on the AFOSM Method and the Monte Carlo Simulation with Importance Sampling

RR-92-2
Tetsuya SASAKI

: With the increase of complex engineering systems, structural reliability has become a major consideration in the design of durable machines and structures. In general, structural reliability analysis requires the failure probability of the structural component. However, the exact solution of the failure probability requires the calculation of a multi-dimensional integral which often needs large memory and computing time. For this reason, the availability of a computer code for calculating the failure probability is important for practical use of reliability methods.
    In this study, an efficient and accurate computer code for calculating the failure probability of the structural component is developed. The developed code is jointly based on the advanced first order second moment (AFOSM) method and the Monte Carlo simulation technique with importance sampling. The augmented Lagrangian method and the DFP method, which are known as the nonlinear optimization procedures, are utilized for calculating the coordinates of the design point. Then the failure probability is evaluated using the importance sampling technique weighted around the design point.
    A simple reliability problem is solved to demonstrate the accuracy and the efficiency of the developed computer code. The high performance of the developed code is confirmed.

Fundamental Research of Emergency Stop Mechanism for Power Press

RR-92-3
Shigeo UMEZAKI, Shoken SHIMIZU and Soichi KUMEKAWA

: The half number of industrial accidents caused by power press operation occurs from the structural inappropriateness of positive clutch type presses in Japanese industries. The main fault of this type press is due to the fact that a slide cannot stop until it reaches to upper dead point.
    If an emergency stop mechanism which can stop a slide in any position is developed, the industrial accidents caused by positive clutch type presses may decrease remarkably in number.
    In this study, it is aimed for developing a new type emergency stop mechanism which can be adaptable to positive clutch type presses. As a powerful braking ability is required for the emergency stop, a new type of hydraulic brake which uses compression of fluid (oil) may be necessary for the development.
    The new type of emergency stop mechanism consists of an oil actuator and an electro-magnetic valve as main components. These components are connected in series by oil flowing pipes. The movable part of machinery is coupled with the oil actuator. When safety of an operator is not confirmed, the oil flow is stopped by the valve closing, and the movable part of machinery is locked. The braking ability depends on the physical characteristics of the fluid (oil) itself such as its viscosity and volume elasticity. The characteristics of this emergency stop mechanism developed in this study are summarized as follows;
  (1) The braking time was about 60 ms under the conditions that the flywheel turning radius was 295 mm, the flywheel effect (GD²) was 8.44 kgf·m2 and the number of its revolutions was 60-120 rpm.
  (2) The braking time was independent of the number of revolutions.
  (3) The braking angle increased proportionally with the number of revolutions. On the other hand, in the case of a conventional friction type of brake, the braking angle increased proportionally with the multipled number of revolutions.
    For these reasons, it is considered that this emergency stop mechanism may be able to apply to the power press which is required for the quick emergency stop, or the rotary machine which has a large rotating energy, and so on.

Structural Requisites of Actuator for Grouping Motion and Its Realization

RR-92-4
Hiroyasu IKEDA and Noboru SUGIMOTO

: Normally, the kinetic control of robot actuators can be achieved by providing positional feedback control. In this positional feedback control of actuators, a positional deviation manages a large amount of energy output. Therefore, if a robot comes in contact with humans during groping motion, the positional deviation will be caused and, as a result, force corresponding to this positional deviation is imparted to targets including humans. As the robot touches targets during groping motion, the level of force imparted targets can not be assured of safety by only a touch sensor and a collision avoidance control.
    In relation to the above, this paper deals with structural requisites of a actuator for groping motion. The principal requisite is found that the positional control system of the actuator should have the function as a means to judge whether the energy output of the actuator is appropriate for targets or not. As the contact of targets with the robot can be detected by an increase of the reactive force, or positional deviation, the positional control system can be performed only under the condition that the actuator does not give force larger than the allowable level to targets.
    Furthermore, the actuator should have a soft spring characteristics not to give an excessive force to targets when the robot touches targets, because the targets the robot gropes are unknown to the robot. And after recognition of contact with targets, the variable spring characteristics that can function as a stiff spring is needed to be realized by the control system. As this spring characteristics is virtual, a false actuator output may be generated if the control system fails.
   Then, the actuator is necessary to have a known viscous load as a means to confirm the normality of this spring characteristics before it contacts with targets. For this reason, if the positional control is provided at a constant speed, i.e., by means of lump input during groping motion within a free space with no constraint, a certain positional deviation can be caused, and as a result, the appropriate actuator output can be confirmed. And if the positional deviation can not be confirmed, the viscous load can vanish the energy inevitably by itself without permitting the energy accumulated in the spring due to the spring characteristics, to be emitted to targets.
   In this paper, the ER (Electro-Rheological) actuator for groping motion was developed, in which the actuator has variable spring characteristics exerted by adjusting air pressure and the ER fluid damper as a physical viscous element. By employing this actuator composition and an inhibit in control for pneumatic output by the ER fluid damper, structural requisites of the actuator for groping motion were satisfied, and the functional kinetic control became possible.

Safety Control Theory Based on the Logic of Safety --Energy Transmition of Safety Information--

RR-92-5
Noboru SUGIMOTO and Hiroyasu IKEDA

: Safety control system, which is defined as the machine operation controlled according to safety confirmation information, is explained as an interlocking model that is so functioned that energy output from machine is permitted only while the information (safety information) reports safety. The information transmittion properties presented in this model are applied on all devices in the interlocking system, consistent from the sensor for producing safety information to devices for transmitting/outputting energy. Small energy level of signal signifying safety is normally produced in a pickup element in the sensor. Enormous power made by amplifying the energy of the safety signal is supplied to the machine to conduct its powerful operation.
    This paper discusses on the condition of energy transmittion process in which energy is produced in safety sensor as a signal indicating safety and is amplified in the interlocking system up to the sufficient level to do machine duty. Firstly, in this paper, clarified is the characteristics of information to be provided with the information processing means in the interlocking system.
    Secondly, a logical fail-safe model is proposed for clarifying the production condition of the safety information. Lastly, this fail-safe transmittion of information energy can be applied on not only electrically but mechanically processed safety information.

Evaluation of Noise Control for Impact-Machines for Construction Work

RR-92-6
Yoshiyuki EGAWA

: It is well known that the noise emitted from various impact-machines for construction has a bad influence on machine operation by workers. It has been, therefore, generally carried out to decrease the noise for these impact-machines as the countermeasure of noise control. The noise of these machines consists of two components. One component is the noise which is caused by concrete crushing (hereinafter referred to as "crushing noise") and the other is the noise which is generated from machine's body by the vibration of hitting concrete (vibration noise). These noises emanate nearly at the same time, and the fluctuation of crushing noise for a variety of concrete hardness has a large effect on the vibration noise.
    The most common method of noise control for these machines is to use a high damping material for the machine's body to reduce the vibration noise. However, because of large scatter in vibration noise by the fluctuation of crushing noise due to concrete hardness, it is very difficult to evaluate the effect of a high damping material. This paper deals with the accurate measurement to investigate the effect of these materials by observation of vibration noise under the same condition of crushing noise. The method is as follows;
    Hydraulic concrete breakers were chosen as the examples of impact-machines used in construction work. The noises emitted from many types of breakers were recorded in magnetic tapes. The duration time of crushing noise wave and that of vibration noise wave were examined from these tapes. By using the oscilloscope with memories, it was clarified that the duration times of crushing and vibration noise were about 5 ms and 50 ms respectively, and it was found that the amplitude of noise wave in one breaker's hitting cycle was decreased exponentially. These noise waves were transferred from the oscilloscope to the computer. RMS value of crushing and vibration wave was calculated using the computer. These results were shown with the correlation figures. In the case of the noise controlled type breaker, a good correlation between RMS value of crushing wave and that of vibration wave was obtained. As the result of the correlation tests, "r" value was more than 0.6 and this value was significant in the noise controlled cases.
    Further, a noise comparison simulating system has been developed. This simulating system was capable of making noise comparison between noise controlled breakers and standard type breakers under the same condition of concrete hardness.

Deformation Behavior at Failure of Saturated Cohesive Soil Model Grounds in Geotechnical Centrifuge Model Tests

RR-92-7
Noriyuki HORII, Satoshi TAMATE and Yasuo TOYOSAWA

: In recent years, construction work has been increasing in number for the development of infrastructures such as water and sewage works. Excavation work is very common to construction work since most of the work have been performed on or under ground. For this reason, hazardous situations associated with slope failures during excavation work have been gradually increasing. In fact, many slope failure accidents have occurred in Japan every year.
    This study is focused on the deformation behavior at failure of model grounds. To clarify the failure mechanism of slope is very important to establish the effective countermeasures for the prevention of slope failure accidents.
    Centrifuge model test is a very effective test procedure in which the same failure phenomena can be reproduced in the test facility and it gives comparable results with those in the real scale soil structures such as slopes and trenches.
    Two kinds of experimental studies have been carried out.
(1) Centrifuge model tests of trench excavation using an in-flight excavation simulation system.
    The excavation process was modelled by draining the fluid, having the same specific gravity, from the pre-formed trench in the in-flight conditions.
(2) Centrifuge model tests of vertical slope. In this test, centrifugal force was increased until the slope failure occurred
    The main results obtained in this study can be summarized as follows:
    1) In the failure experiments of vertical slopes, the failure planes designated circular shapes and almost the similar results were obtained at all tests in spite of differences of strength of grounds. On the other hand, in the simulation tests, 3 types of failure patterns could be recognized. First type was a continuous failure from the surface. The others were wedge type failure and circular type failure.
    2) The prototype failure heights were reasonably converted into the real scale heights. When the consolidation stresses were almost uniform at the pre-consolidation stage.
    3) Behavior of γmax up to the failure was observed with photographic measurements. In the progressive failure recognized, the strains are concentrated at the toe of slope in initial stage and the shear band appeared at this area, and finally this shear band developed upward to the ground surface.
    4) The clear sliding line was formed when γmax was reached to around 14%. This results would support the definition of the failure strain which is adopted in the element test in laboratory.

Safety Dimensions of Tread and Stairs in Considering Visual Perceptive Errors

RR-92-8
Hisao NAGATA

: A man seldom looks intently at his feet during walking, except when he wants to avoid an obstacle or a muddy pool. Usually he walks rhythmically while looking at the road surface a few meters ahead. But, sometimes, man becomes absent-minded or distracted while walking, with the result that he stumbles over even small projections or falls down when he mistakenly presumes that there is another step. There are many cases of such kinds of accidents. When descending a stairway, he slows down the pace of his walk just before he takes his first step, in order to adjust his pace to the width of the steps of the staircase, and walk down at an even pace. If the staircase is sloped gently he can see the bottom of it while watching his downward target. However, if the staircase is steep, he can not see anything downward until he arrives at the top of it, so he finds it difficult to adjust his pace. In fact, there are many cases of accidents which occur around this spot. In this paper, relations of dimensions of tread and rise, and human peripheral vision in descending stairs, were theoretically analyzed. A theoretical method for obtaining the required dimensions of a tread was suggested.
    Pedestrians, in descending stairs, are usually looking down the stairs to insure a safer footing on a tread. If the pedestrians vision to their foot is obstructed in descending, they are considered to be liable to stumble. His field of view is limited by part of his body, though people are not usually aware of this fact. For example, while going downstairs, the step on which someone wants to set his foot is temporarily out of his view due to the motion of his thigh. Two male subjects participated in the experiments. The human peripheral vision in descending stairs was analyzed in 24 different combinations of trad and rise. The ratio of vision screened by thigh movements was suggested for evaluating dimensions of tread and rise in regard to human peripheral vision.

A study on the Time Intervals between Occupational Accidents (6)

RR-92-9
Shigeo HANAYASU

: The accident frequency rate has been widely used as a measurement of safety performance in many undertakings over a long period of time.
    In order to explore the significant changes in accident situation in succeeding intervals of time, the time intervals between occupational accidents was utilized as a useful indicator to give expression to safety performance in undertakings having a certain accident risk.
    This paper deals with the stochastic treatment of the number of accidents and the time intervals between occupational accidents to evaluate the safety performance of working places. Emphasis was placed upon the probabilistic analysis of the number of accidents within a fixed interval of time and the time intervals between accidents considering the damage consequences of occupational accidents.
    The damage consequence due to accidents was defined, in this paper, as the number of injured workers involved in an accident, which implies the magnitude of an accident.
    The occupational accident under investigation in this study were those serious occupational accidents involving three or more injured workers within an accident, which took place from the year of 1977 to 1990 in Japan.
    The main results obtained in this study can be summarized as follows:
  1) The frequency distributions of accidents agree with the poisson as well as the negative binomial distribution simultaneously.
  2) The frequency distributions of the magnitude of accidents (number of injured workers involved) agree with the geometric distribution rather than the poisson distribution.
  3) Hence, the frequency of the number of injured workers or fatalities within a fixed of time, can be obtained by mixing the accident frequency distribution with the magnitude frequency distribution, which yields the generalized poisson-poisson or poisson-geometric distribution.
  4) The newly developed accident frequency distributions considering the specific lowest value of accident size, had a good agreement with actual accident distributions.
  5) The frequency of the number of accidents and time intervals between accidents considering the magnitude of accident becomes the poisson and the exponential distribution respectively, whose parameter is the product of the frequency parameter and the magnitude parameter.

An Information Analysis by Statistical Method for the Descriptive Statements of the Occupational Accident Report Concerning Construction Work

RR-92-10
Yoshimi SUZUKI

: In this study, driving a method for making practical and direct use of descriptive statements of the "Occupational Accident Report" concerning Construction Work, statistical analysis methods are applied to examine free-terms used in descriptive statements of theses reports. Especially, statistical quantification analytical method are applied to study free-terms selected from descriptive statements of the "Accident Situation and Circumstances".
    Brief outline of this study and main results are as follows; Firstly, author has developed information retrieval database system about original occupational accident reports, and 9059 kinds of free-terms in total are extracted from the Japanese sentence item of the "Accident Situation and Circumstances" by means of the utility in the Database. The frequency of these extracted free-terms were distributed according to the Bradford's law.
    Secondly, 293 free-terms of high frequencies are selected as keywords for arranging information concerning many factors of occupational accidents occurred in construction work sites. In this procedure, the relationship between these keywords and accident cases are analyzed by the quantification method of third type. As results of this analysis, scatter diagram of keywords (category weight, Fig.3 - Fig.7) and scatter diagram of accident cases (sample score, Fig.8 - Fig.9) are obtained and the information about "Accident Situation and Circumstances" is arranged by kind of accident types and kind of construction works.
    Thirdly, using these 293 keywords, discriminant analysis was made to examine the probability for exploration of accident cases according to various factor of occupational accidents occurred in construction work sites. Classification results of discriminant analysis are obtained as classification result tables (Table 5 - Table 7), and each accident cases are sufficiently classified by accident types or by kind of construction works.
    Then, after these statistical analysis of free-terms extracted from the Japanese sentence item of the "Accident Situation and Circumstances", information structure concerning occupational accident in construction work can be arranged according to a few key concept; accident types, kind of construction works and so on. For distinguishing various information from occupational accident data, statistical methods using free-terms in descriptive statements of "Accident situation and Circumstances" of occupational accident report can be used practically.

Relationship between Chemical Structure and Thermal Stability of Reactive Chemicals (3rd Report) --Thermal Decomposition of Nitrophenylhydrazine Isomers--

RR-92-11
Takayuki ANDO and Yasuhiro FUJIMOTO

: In manufacturing, transporting, and storage of reactive chemicals in recent years, the hazard evaluations have been extremely important. In the chemical industry, the most concerns are focused on thermal hazards such as runaway reactions and thermal decompositions, which are mostly governed by thermodynamics and reaction kinetics of these reactive chemicals in the system.
    The purpose of this work is to establish the method of estimating the thermal hazards of reactive chemicals by investigating the relation between the chemical structure and the decomposition characteristics of these chemicals.
    In our previous reports, the decomposition characteristics of the three respective isomers of fourteen nitrobenzene derivatives were measured with Differential Scanning Calorimeter (DSC) to clarify the isomeric effect on the decomposition of the isomers, and decomposition products of the three isomers of nitophenylacetic acid (NPA) were analyzed with autoclave-Mass Spectrometer (MS), DSC-GC (Gaschromatograph)-MS, and CPP (Curie Point Pyrolyser)-GC-MS to investigate the isomeric effect on the decomposition mechanism for the isomers.
    In this paper, the decomposition characteristics of the two (ortho and para) isomers of nitro phenylhydrazine (NPH) were measured with DSC and ARC (Accelerating Rate Calorimeter), and the decomposition products of these two isomers were analyzed with CPP-GC-MS and DSC-LC (Liquid Chromatography).
    The results are summarized as follows:
  (1) The first exothermic peak in the DSC curve for o -NPH is mainly due to inter molecular dehydration forming 1-Hydroxybenzotriazole (HOBT).
  (2) The second exothermic peak in the DSC curve for o -NPH is mainly due to the decomposition of HOBT formed in the first step of decomposition.
  (3) The exothermic peak in the DSC curve for p -NPH is mainly due to dissociation of hydrazino and nitro groups.

Prediction of Exothermic Onset-Temperature for Benzene Derivatives with Fuzzy Reasoning

RR-92-12
Yasuhiro FUJIMOTO

: Evaluation of chemical hazard is important in the chemical industries because explosions or fires occasionally occur in chemical processes where fine-chemicals including Pharmaceuticals or functional resins are treated. If the evaluation of these chemical hazards is possible by easy calculation, this may save chemical industries money and time. Thermal hazards of chemical substances must be evaluated from the two aspects; sensitivity, which shows how easily chemicals may decompose or ignite, and severity, which shows how much or how fast energy is liberated when chemicals decompose. One of typical methods for the evaluation of thermal hazards is DSC, which gives information on the sensitivity (exothermic onset-temperature) and the severity (decomposition heat) at the same time with easy operation. This report consists of the results of prediction, which were obtained in our laboratory, of the exothermic onset-temperature of benzene derivatives with fuzzy reasoning, using the 141 DSC data. If this result could predict chemical hazards accurately enough, it might be used for the direct evaluation of thermal hazards from chemical structures which are easily known without any experiments.
    In the analysis of fuzzy reasoning in this study, linear model was used as the membership function. We used dissociation energy index (DEI), functional group index (FGI), and inter molecule reaction index (IRI) as variables for the fuzzy reasoning. The membership function of the exothermic onset-temperature was divided into 3 steps namely safe, the intermediate, and danger. The other indexes were divided into two steps (safe and danger). The fuzzy reasoning rule used is shown below.
     1. if DEI, FGI, and IRI are danger, then exothermic onset-temperature (Temp.) is low.
     2. if DEI and FGI are danger, and IRI is safe, then Temp, is the intermediate.
     3. if DEI is safe, FGI is danger, and any IRI, then Temp, is the intermediate.
     4. if DEI is danger, FGI is safe, and any IRI, then Temp, is the intermediate.
     5. if DEI and FGI are safe, and IRI is danger, then Temp, is the intermediate.
     6. if DEI, FGI, and IRI are safe, then Temp, is high.
   The reasoning results didn't sufficiently show a good relationship between the experimental values and the predicted values, so the improvement of the membership function and the index value may be necessary in future.

Laboratory Studies on Spontaneous Combustion of Coal Bed in Pulverizer

RR-92-13
Haruhiko ITAGAKI

: Since the introduction of pulverized coal combustion systems for power plants in the mid-1930's, there has been a great concern about fire/explosion accidents due to coal dust in the pulverizer. Even for some years, the fire/explosion accident in the system has a tendency to increase.
    This paper describes the characteristics of spontaneous combustion of pulverized coal to prevent explosion/fire accident at the pulverized coal combustion system. In this work, small scale tests were performed using 8 kinds of coals. The amount of volatile matter (V.M.) in the coals significantly changed from 5.1% to 43.6%.
    Three results are summarized as follows:
  (1) The exothermic onset temperatures of the coals by Accelerating Rate Calorimeter(ARC) decrease with the increase of volatile matter, and they are nearly the same value in spite of the different oxygen pressure for each coal. For most coals, the onset temperatures are under 80 °C. And, if oxygen is enough for self-oxidation, the spontaneous combustion will occur at the temperature from 151 to 172 °C with rapid pressure increase of over 7 MPa, except for coal H (V.M. 5.1%).
  (2) Temperature changes at center and surface of the coal bed were measured with time in air and in inert gas-air mixture using an infrared image furnace, and it might simulate the oxidation and combustion of coal bed in the pulverizer. In the case of Coal E (V.M. 31.5%), the critical bottom temperatures of spontaneous combustion in air at thickness 5, 13, and 30 mm are about 175, 142, and 122 °C, respectively. In the case of the thickness of 30 mm and the bottom temperature of 130 °C, except for Coal G and H (V.M. 23.8% and 5.1%), the maximum temperatures of the coals are from 500 to 600 °C after 4-5 hour heating.
  (3) Addition of inert gas into air brings the decrease of the critical bottom temperature and the peak temperature at the center and at the surface. Therefore, we need to consider whether the temperature rise of pulverized coal is high enough to cause fire or explosion accidents. On the other hand, in the case of steam-air mixture, the decrease of the surface temperature isn't so much, therefore the peak surface temperature is higher than the peak center temperature. This may be due to the fact that the supply of oxygen is insufficient for self-heating at the center of the coal samples, and the specific heat of steam is about twice as large as that of air.

Ignition Hazards of Hydrogen-Air Mixture Caused by Spark Discharge with Switching in Radio Frequency Resistative Circuit

RR-92-14
Tatsuo MOTOYAMA

: With a progress of electrical technologies, radio frequency circuits including high frequency components are applied to electrical installations. Some of them are used in hazardous locations where explosive or inflammable gases exist: as a result, ignitions of the gases may be induced by a spark discharge when the radio frequency circuits are switched on or off. However, experiments on their ignition hazards are scarcely carried out and problems of the ignition are often discussed to design the high frequency electrical installations used in hazardous area.
    The purpose of this study is to clarify experimentally the ignition hazards of inflammable gases caused by spark discharges occurring from the radio frequency circuits. Based on the experimental method of IEC 79-3 already recommended, experiments are performed to investigate the ignition hazards of a hydrogen-air mixture due to the spark discharge. A high frequency circuit used in this experiment is a 50Ω resistative one consisting of a co-axial cable, resister of 50Ω and power source of 1 to 1000 kHz in frequency. The circuit voltages supplied for inducing the ignition of a hydrogen-air mixture were examined to determine a correlation the minimum ignition voltage versus the frequency.
    Results obtained from experiments indicate that the ignitions of the hydrogen-air mixture caused by spark discharges depend primarily on the frequency of the power source in the resistative circuit. It has been clarified from experiments that the minimum ignition voltage increases gradually with the increase of the frequency, since the duration of the individual spark discharge decreases contrary to the increase of the frequency and that its value at 1000 kHz is 80V in peak voltage. Furthermore, the experimental results suggest that the ignition is induced by a few sparks repeated under the condition of the frequency over 50 kHz.
    As described above, higher voltage and several sparks are required to induce the ignition of the hydrogen-air mixture in the high frequency resistative circuit over 50 kHz. In this paper, experimental data and fundamental factors are presented to evaluate the ignition hazards in the high frequency resistative circuit.

Detoxification of Hazardous Gaseous Substances by Discharge Plasma --Decomposition of Aromatic Organic Solvents by Surface Discharge Plasma--

RR-92-15
Shigeo YAMAGUMA, Atsushi OHSAWA, Tsutomu KODAMA and Yasuyuki TABATA

: There are a lot of industrial processes in which generation of hazardous gaseous substances such as solvent vapor, NOx and SOx, etc. are unavoidable. These substances are also known to cause environmental pollution and have been thought to be difficult to be sufficiently managed by conventional technologies based on heat burning, adsorption by active carbon, etc.
    Recently, as a high performance and low energy consumption environment protection technology, an application of discharge plasma is gaining worldwide attention. This is because that copious high energy electrons generated in discharge plasma directly collide with target molecules and dissociate them, which are then easily changed into other substances by chemical combinations with species in the plasma region, while ion and neutral molecule temperatures are kept comparatively low.
    In this study, in attempt to develop a new application technique of discharge plasma, we employed a kind of discharging method called SPCP (short for Surface discharge induced Plasma Chemical Process). Applications of SPCP have been widely used for years. Compact ozonizers to deodorize household equipments like refrigerators are a part of such applications. We took advantages of the compactness and durability of the SPCP electrode to set up an experimental apparatus for decompositing vapor of aromatic hydrocarbons such as toluene, benzene and xylene, which are major substances given off in painting or washing processes and aggravate working conditions.
    Results obtained with the apparatus are summarized as follows.
  (1) Aromatic hydrocarbon vapor of up to 2,000 ppm is almost thoroughly decomposed at the flow rate of 4 L/min or lower under the discharge with electric power of several hundred Watts.
  (2) In dry air, as the decomposition progresses, tar-like substance deposits on the discharging areas, which deteriorates the decomposition rate in the end. This substance, however, is almost thoroughly removed by keeping discharge in dry air containing no solvent vapor. It has also been proved that using oxygen as carrier gas drastically reduces the generation of liquid and solid substances.
  (3) Adding a discharge enhancing gas like argon is effective to reduce power consumption. For example, about 20% of electric power can be saved by adding 10% of argon to air.

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