Abstract of Special Research Report (RR-98)

National Institute of Occupational Safety and Health, Japan

A Trial Improvement of Energy of efficiency of Plasma Reactor by Superposing Two Heterogeneous Discharges --Characteristics of Surface and Corona Discharge Combined Plasma Reactor--

RR-98-1

Mizuki YAMAGUMA and In-Sung WOO

: In order to cope with environmental problems caused by harmful gases emitted from various industrial sources, a new technology which employs discharge plasma formed in ordinary atmospheric pressure has been intensively investigated in many industrialized nations. Although a plenty of useful outcomes and suggestions have been made public by scientists in this field, few commercial products which effectively decompose pollutant gases have appeared as yet. This is partly because that the energy efficiency of a most effective plasma reactor has not reached a satisfactory level in comparison with those of devices using conventional technologies.
    In an attempt to solve the problem mentioned above, we noticed to combine heterogeneous electrical discharges. This concepts is based on that each plasma reactor has its specific spatial region in which chemical reactions are active and by electrically mutually affected with another reactor of different type, the activated region would increase - which may lead to cutting down the energy consumption.
    To prove this concept experimentally, two different discharge equipments, a plane ceramic based surface discharge electrode and a corona electrode with tungsten needle array, are selected and combined to fabricate a hybrid plasma reactor.
    The results are summarized as follows;
  (1) Ozone concentration generated in the plasma region drastically increases when the positive corona discharge is added to the surface discharge. The rate of increase of ozone depends on the frequency of the surface discharge. The negative corona, however, does not contribute to the improvement of the ozone generation.
  (2) NO (nitrogen monoxide) decomposition rate also improves by simultaneously applying the surface and the positive corona discharges. The effect of the corona superposition is more evident when the level of the surface discharge is moderate.
  (3) By adjusting the corona level, the net energy efficiency during NO decomposition improves in comparison with the simple surface discharge reactor.

Electromagnetic Noise generated by Spark Discharge --Dependence of Spark Current and Induced Voltage on Sparking Voltage--

RR-98-2

Hajime TOMITA

: Recently, electronic equipment, such as industrial robot, is commonly used for enhancement of productivity and reduction of labor. Semiconductor devices used for those equipment are susceptible to electromagnetic noise because signal level and current are several volts and mA, respectively. Among many electromagnetic noise sources, electric discharge which is generated, for example, due to surge is considered to be one of the main noise sources. Spark discharge, which has a significant influence on electronic equipment, occurs at the beginning of electric discharge as a transition phenomenon. Before the occurrence of spark discharge, an electric dipole is formed at a discharging point and the neutralization of the charge occurs through the spark discharge thereafter. But this spark discharge phenomenon is not fully understood because conventional current sensors are not available without disturbing the spark discharge generated in the air.
In this report, one method for measuring spark current generated by the spark discharge for neutralization of the charge was proposed by devising the grounding of a discharging part. Spark discharge of static electricity charged on a metal object was used to simplify electric discharge. Dependence of spark current and voltage induced by magnetic field due to the spark discharge on sparking voltage was measured to understand the fundamental characteristics of electromagnetic noise generated by the spark discharge. As the result, the peak and maximum time change of spark current due to the spark discharge decreased with sparking voltage. Voltage induced on a load of a loop antenna due to the spark discharge decreased with discharging gap length and almost coincided with that calculated by measured spark current. One reason for the difference of strength of electromagnetic noise generated between when a charged matter and a grounded matter are fixed and when a charged matter approaches a grounded matter after charging with the same potential as the former one was considered by paying attention to discharging gap length.

Investigation of Overturning of Mobile Crane due to Penetration of Outriggers induced by Failure of Ground

RR-98-3

Satoshi TAMATE, Noriyuki HORII, Yasuo TOYOSAWA, Naoki SUEMASA and Tosiyuki KATADA

: Overturning of mobile cranes frequently occur on construction sites. Mobile cranes are comprised of hoisting machinery combined with both a base carrier and a revolving super structure. Outriggers are equipped at the base carrier. The mobile crane usually stands on the outriggers which are extended laterally and set on the ground during hoisting operation to keep the crane level. Investigation of these accidents confirmed that one cause of the accidents was penetration of the outriggers into the ground. This study focuses on the fact of both penetration of the outriggers due to failure of the ground and overturning accidents of mobile cranes. Procedures and results in this paper are summarized as follow;
  1) Investigation using reports of occupational accidents was carried out to clarify conditions at the overturning. Penetration of the outriggers was seen in about 40 % of the reports investigated. Although the net rated load is prescribed for safety of stability, about 20% of the overturning occurred where hoisting weight was less than the net rated load.
  2) Bearing capacity tests on model grounds were carried out to investigate both strength and deformation characteristics due to the penetration. Soil used in the tests was Kanto loam which is a kind of volcanic cohesive soil for the partially saturated ground. Model footing simulating float of outrigger settled with increasing acting stress on the model ground which had uniform strength. It was found, however, the footing settled rapidly on a layered model ground which had a hard surface layer overlying soft ground. The hard surface has characteristics of brittle failure considering condition of the soil dried. A wedge failure under the footing was developed in the ground due to penetration. Vertical displacements around the wedge were predominant comparing to the horizontal displacements.
  3) A theoretical stability analysis was performed by both static and kinetic formulations. The mobile crane tilt due to penetration of the outriggers. At the same time, an arm length of overturning moment increases with decreasing a boom angle. This mechanism is considered as the static instability of the mobile crane due to penetration. Meanwhile, where the ground fails suddenly and the outriggers penetrate rapidly, moment of inertia due to penetration is important in addition to the static instability. The results indicate that penetration of the outriggers into the ground (i.e. depth of penetration and its speed) greatly influences the stability of mobile cranes.

Evaluation of Audibility for Voice Sounds

RR-98-4

Yoshiyuki EGAWA

: Miss hearing for task information often would happen labor accidents. For example, work performances for working instructions between "Tai ki seyo" and "Tai hi seyo" are very different. By mistake in hearing between "ki " and "hi ", worker might be involved in accidents. So this study has dealt with the evaluation for the audibility of voice sounds. By the audiometer test for sound pressure level and resolution test for sound frequencies, subjects were examined for the audibility of vowel and consonant sounds.
    For the audibility of vowel sounds, three type-groups of subjects were observed. The first type group of the subjects, who had high threshold level for high frequency band of audiometer test, had the characteristics of low distinction between /i/ and /e/ of vowels. The second type-group of them, who had high threshold levels for high and low frequency bands of audiometer test, had the low distinction for the vowels of /i/, /u/ and /e/. The third type-group of them had the good measurement for audiometer test. However, this type-group was divided into two classes. One class of the subjects could distinguish the each vowel, and the other class couldn't. So the third type group of subjects was examined for resolution test of sound frequencies. This test has examined the resolution of "360 vs 381 Hz", "660 vs 670 Hz" and "1335 vs 1350 Hz". These frequencies was formant frequencies constructed the vowels. As the result of this frequency test, the class which could distinguish the each vowel had good resolutions for this frequency test, and the other class had bad resolutions for the test.
    For the audibility of consonant sounds, subjects were divided into two groups. One group could obtain 100% levels of hearing for consonant articulation and the other group couldn't obtain. On the subjects of less audibility for consonants, it was difficult to distinct /de/, /ba/, /ha/ and /ne/ of consonant sounds. These four consonants had very short time to discern themselves from other consonants. And, as the result of analysis for these four consonant waves, frequencies and sound pressure levels had changed rapidly at very short time. The subjects of less audibility for /de/, /ba/, /ha/ and /ne/ of consonant sounds couldn't perceive rapid change of frequencies and sound pressure levels.
    As the result of this study, to evaluate of audibility for voice sounds, it was very important to carry out both audiometer test and frequency resolution test.

An Evaluation of Fall Protection Performance of an Air Bag System

RR-98-5

Kiyoshi FUKAYA

: In general there are two ways in safety measures: one is prevention of the accident and another is reduction of damage by the accident. The former is necessary and the latter is important, too. An air bag system for personal protection against falls, which was evaluated in this research, is an example of the latter. The air bag system consists of an air bag that expands in case of fall, a fall sensor system that detects fall, an air filler that supply gas and a vest that contains other components. The expanded air bag covers the neck, backbone and waist and absorbs the shock of the impact to the ground and reduces the injury of the worker who falls.
    In order to evaluate the shock absorbing performance of the air bag, fall tests of dummy with/ without the air bag were conducted and deceleration of impact on to the ground were measured. The fall tests were conducted in various conditions such as fall height and fall angle of the dummy. In case of horizontal fall the maximum deceleration is reduced by the air bag remarkably.
    The criterion of the car crash test and the criteria of protective helmets for vehicular users were applied to the estimation of the shock absorption performance. In case of higher fall height an injury is unavoidable. Mechanical analysis showed that thicker air bag is needed.
    Fall sensor is also important. There is similar phenomenon as fall such as jump, and a distinction is necessary.

Survey on Improving Occupational Environments in the Rapidly Aging Society --Survey for Workers from 40 to 49 Years Old and the Elderly People over 60 Years Old--

RR-98-6

Hisao NAGATA, Akihiko SASAKI, Kazuhiro SAKAI and Sunyoung LEE

: The elderly population increases rapidly while younger population becomes decreasing year by year in Japan. People over 65 years old are estimated to reach over 27 % of population by the year 2025. In order to facilitate the participation of elderly individuals and to integrate them with other generations in the rapidly aging society, we need various strategics to solve social problems. The present occupational environments are not always fit to elderly workers. It is an urgent issue that we should redesign occupational environments. First of all, inquiries to the elderly people and to workers from 40 to 49 years old are helpful to obtain hints to solve these problems and to redesign occupational environments.
    66 elderly people and 430 workers from 40 to 49 years old completed questionnaires. Questions were done about the maximum possible age for working, major contributing factors to extend elderly employment, anxiety about their own physical deterioration, preferred age-group to work with, future elderly image of workers from 40 to 49 years old, etc. The elderly group was interviewed about daily life in their community and desires to improve working conditions. The following conclusions were obtained.
  (1) Most of workers wish to work until 65 years old. But barriers for continuing their work are regarded as quantity of tasks, time period of working hours, loads of commuting to or from work.
  (2) Workers from 40 to 49 years old guess that barriers for their own work at the age over 60 would be mental workload from new job assignments and physical workload from tasks.
  (3) Workers from 40 to 49 years old guess that their own physical deterioration would be eyesight, loss of judgement and deterioration of the leg muscle.
  (4) Workers from 40 to 49 years old are reported to give more contributing influence than the present elderly people and they would be accustomed to use personal computers and word processors.
    On the contrary their physical performance would not be superior to the present elderly people and especially their patience is underestimated by the present elderly people.
  (5) Most of workers want to work not only in a group of aged people, but together with younger generations.

Autoignition Limit of Flammable Liquids when Rapidly Compressed by High Pressure Oxygen

RR-98-7

Haruhiko ITAGAKI

: When the valve for high pressure gas is operated quickly, the temperature of gas rises partially at the other valves or at the end of pipe by rapidly compression of gas. If there is some combustible substance near the valve or the end of pipe, it can ignited. In the worst case, whole pipeline explodes. We can easily calculate the temperature of gas after compression easily, if there is no heat conduction to outside, that is, adiabatic condition. But, the calculated temperature under adiabatic condition is excessive high, if it compares with the ignition temperature of combustible substance to decide the possibility of ignition.
    The actual temperature to ignite is obviously lower than the calculated temperature under adiabatic condition. Because, there are heat conduction to the vessel wall and the mixing of gases, so that the duration time of high temperature spot is very short.
    In the experiment, various volume with a drop of flammable liquid was compressed rapidly by high pressure oxygen was carried out to determine the minimum pressure ratio of ignition. Besides, the heat conduction factor that describes the heat flux per unit volume was introduced to be able to decide whether ignition occur or not. The results of the experiment, and consideration are as follows.
  (1) The minimum pressure ratio of ignition is affected by the diameter and length of vessel. The lowest minimum pressure ratio of ignition is obtained when the ratio of length to diameter is between 10 and 15.
  (2) The influence of the length and diameter on the minimum pressure ratio is caused by the duration time of high temperature spot. It is supposed that the heat flux per unit volume affects the duration time.
  (3) The modified heat conduction factor was introduced to describe the heat conduction between compressed gas and the vessel, and the mixing of compressed gas and expanded gas.
  (4) The relationship between the minimum pressure ratio of ignition and the modified heat conduction factor is was shown by a linear equation for each flammable substance.
  (5) The possibility of estimation whether ignition occur or not for other case was found by using the modified heat conduction factor.

Quenching Behavior of a Propagating Flame in a Duct with Water Sprays

RR-98-8

Masaaki YASHIMA

: An experimental study on extinction limits of premixed flame has been performed in order to develop a new explosion suppression system applied to gas and dust explosions. This kind of study on explosion suppression system started from the later term of World War II in the U.K. Since then many practical studies have been conducted to suppress gas or dust explosions in coal mines and chemical engineering plants. However, they have not been clarified how the propagating flame in a duct is quenched by suppressant agents dynamically. Elucidating the quenching mechanisms by suppressant agents is indispensable to make a suppression model and to develop the system.
    The experiment was conducted in a vertical duct of 0.2 m x 0.2 m cross section and 2 m long, with both-ends-opened, top-end-closed and bottom-end-opened. The behavior of the flame propagating in the middle of the duct was observed through two glass plates of 0.38m long serving as duct walls. Methane - air and LPgas (propane 98.1 vol.%)- air premixed gas were used and water was used as suppressant agent, which was harmless and could be obtained with ease. The results are as follows.
  (1) It is found that the propagating flame is laminar state without water sprays, and when the flame can not be quenched with the sprays, the flame front is disturbed and propagating velocity increases.
  (2) When the case of flame is quenched by the sprays, it does not extinguish instantaneously; it takes some time to be extinguished.
  (3) The optimizing conditions of the water sprays effect on the flame quenching, are examined in various concentrations of the mixture. In case of both - ends - opened,

Φ = 0.7; D_SMD = 175μm (D_AV = 92μm); T_i <= 660 ms † (V_f <= 3.7 m/s),
      q_w >= 108 mg/cm²s, T_D >= 280 ms.
    ; D_SMD = 293μm (D_AV = 172μm); T_i <= 660 ms † (V_f <= 3.7 m/s),
      q_w >= 144 mg/cm²s, T_D >= 850 ms.
      † T_i = 660 ms is the maximum value in the present condition.
Φ = 1.0; D_SMD = 215μm (D_AV = 97μm); T_i <= 260 ms (V_f <= 2.9 m/s),
      q_w >= 135 mg/cm²s, T_D >= 500 ms.
    ; D_SMD = 235μm (D_AV = 130μm); T_i <= 210 ms (V_f <= 2.4 m/s),
      q_w >= 144 mg/cm²s, T_D >= 850 ms.
    Where Φ Equivalence ratio, T_i Spray start time from ignition, V_f Flame Propagating velocity, D_SMD Sauter mean diameter, D_SMD Arithmetic mean diameter, q_w Water flux of sprays (mean), T_D Duration of sprays.

Questionnaire Survey to Construction Workers concerning Failling-accident

RR-98-9

Yoshimi SUZUKI, Shin-nosuke USUI, Yoshiyuki EGAWA and Takuro SHOJI

: In Japan, the number of casualties(deaths) in construction industry is reckoned about 1000 victims each year and about 40% of these accidents are caused by falling from high elevated place.
    In order to establish the ergonomic counter-measures against these falling-accidents in construction works, questionnaire survey to construction workers is carried out. Statistical analysis have been made on 239 subjects which were obtained by this questionnaire survey to investigate latent factors concerning falling accident in construction work sites.
    Main results of these investigations are as follows;
  1) According to worker's occupation or worker's age, there are some remarkable differences in answer patterns for some items of questionnaire, for example, item choice patterns for reasons of taking/non-taking safety belt by scaffold workers are different from workers of other occupations.
  2) Some uneven tendencies in free descriptive answers according to worker's occupation are also extracted by statistical quantification analysis.
  3) Concerning questions about non-taking safety-belt, task-efficiency factor and psychological factor are extracted by factor analysis.

JNIOSH