Abstract of Special Research Report (RR-2005)

National Institute of Occupational Safety and Health, Japan

Study on Ignitability of Coating Powder Used in Electrostatic Powder Coating System (Part 1)

RR-2005-1

Kwang-Seok CHOI, Mizuki YAMAGUMA and Jae-Hee JOUNG

: Electrostatic powder coating has been successfully implemented in the painting industry as an environmentally friendly process. However, there have been many accidents involving dust fires and/or explosions in electrostatic powder coating systems using corona discharge. One of the methods for avoiding fires and/or explosions in electrostatic powder coating plants is to investigate the Minimum Ignition Energy (MIE) of coating powders due to the capacitive discharge, such as the electrostatic discharge. This paper reports the experimental results dealing with MIEs of the coating polymer powders (polyester, epoxy, epoxy-polyester copolymer, nylon, and polyacrylonitrile). The MIEs of five kinds of polyester powders, which were also investigated in this study, differed with respect to pigment type, non-combustible mass fraction, and particle size. The Hartman vertical-tube (1.2 L) apparatus (MIKE-3) was used for the ignitability (MIE) testing of dust clouds. The important results were found as follows: (1) the ignitability of epoxy powder related to the thermal decomposition and surface conditions was higher than that of other powders used in this study, (2) the particle size of coating powders is more important than other factors, such as the pigment type and a non- combustible mass fraction, with regard to their ignitability, (3) some of the sample was so sensitive that even a spark with very low energy, such as 2 mJ, could ignite them. The values of the discharge spark energy of ignition testing set by the BSI standards and the FM regulations related to safety of the electrostatic powder coating system are high enough 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.

On the Assessment Methods of Electrostatic Hazards during Liquid Spraying

RR-2005-2

Atsushi OHSAWA

: During liquid spraying, for example, cleaning by water jets or solvents, charged clouds consisting of droplets in space are built up. The clouds create high electric field causing electrostatic discharges leading to ignition hazard. The hazard depends not only on the spray liquid and spraying parameters but also on the geometry of the spray space. The aim of this study is to construct a method to estimate the electrostatic hazards during liquid spraying, i.e., whether or not the charged sprays can lead to conditions enabling discharges to occur. Following the evaluation methods with the aid of computer calculation to estimate the maximum electric field have been examined;
  (1) Measuring space charge density in a cylindrical cage with a field-meter;
  (2) Measuring space charge density by a suction Faraday tube;
  (3) Measuring space potentials by a floating probe.
    The measured values by each method above are used to compute the electric field distribution in a given space based on the Poisson's equation and E = - ∇V , and the methods are compared.
    In this paper, a spraying with two phase flow (purified water and compressed air) is used to compare the evaluation methods. The method using the suction Faraday tube gives the charge distribution with high resolution but measurements at many points are required, so that it may be useful for detailed evaluation. The floating probe method, however, is inadequate for the assessment because of inaccuracy. Consequently, the method using the cylindrical cage with a field-meter may be useful and reasonable for the hazard assessment to determine the maximum electric field because of easy measurement and a good agreement with the method with the suction Faraday tube giving the high resolution measurement, so that in future work, it will be employed in comprehensive experimental study with wide ranging parameters of spraying.

Centrifugal Modelling of Rockfall Simulation Associated with Rock Fracturing

RR-2005-3

Kazuya ITOH, Yasuo TOYOSAWA and Osamu KUSAKABE

: Rock-fall is the rapid movement of rock from a steep slope. The types of movement may be classified into four types: free falling, bouncing, rolling or sliding. Rockfalls along highways often occur where natural rock slopes are cut. When a piece of rock reaches the roadway below, it might cause a hazard to roadway users. In Japan nationwide inspection on potentially dangerous slopes has been carried out every five years and hundreds of millions of yen are annually spent on rock slope maintenance, and rockfall hazard mitigation on both new and existing slopes. Although rock-fall protections works are applied, rock-fall prone areas are increasing every year because of road extension work and degradation of slope materials. In order to implement a rockfall protection work, it is important to evaluate and estimate the response of a recognized rockfall site, its scale and time, the rockfall (i.e., velocity, bounce height, falling course, final position), rockfall forces (i.e., impact force, kinetic energy), etc. Several researchers have developed computer simulations to investigate rockfall mechanism. These provide estimates of probable bounce heights and velocities for rockfall. Recently, additional statistical data have been added providing probability distributions for velocity, energy and bounce height of rockfalls. The computer program is applicable to almost all slope configurations. It is more flexible than design criteria that require slopes of given configurations. Simulation, however, requires detailed site conditions and slope geometry input data and assumptions; therefore accuracy varies, depending on the quality of the input data. It is expected that centrifuge modeling could provide a more consistent set of data, properly taking both stiffness variation with depth and damping characteristics of soil into account.
In this study, a series of centrifuge model tests were performed to observe the movement of falling rocks during the falling process. The test program includes the measurements of trajectory of falling rock captured by a high speed camera. A series of centrifuge model tests confirmed that falling rocks exhibit various types of falling modes; free falling, rolling or sliding, sometimes associated with rock fracturing during the falling process. As a result, in the case of model crushable rock, it was found that the observed movement patterns were rolling with small bounces, and rolling with large bounces, occurring fracturing.

Study on Prevention Methods of Falling from the Eaves of the Roof

RR-2005-4

Yasumichi HINO

: In Japan, there are about one hundred fatal falling accidents at construction sites of residential houses. Especially, about 20 to 30 of fatal falling accidents from roofs periodically happened over the last decade and the decreasing trend of the accidents has not been observed.
    In recent years, construction work has changed from the construction of new houses to extensions or repair work. Extension or repair work in general has a short construction period and has small budgets. Therefore, countermeasures against falling accidents have been difficult. However, the effective and useful countermeasures against falling accidents from roof have not been explored.
    In this study, causes of falling accidents and fundamental data for establishing countermeasures against falling accidents from roofs were analyzed from the accident data. Also, the requirements of prevention equipment against falling from eaves were discussed by the experimental results.
    The major findings obtained in this study can be summarized as follows:
  1) Most of the falling victims fell from eaves. But quite a few victims also fell from the verges. Therefore, the prevention equipment against falling from roof need to set up, not only at the eaves, but also at the verges.
  2) The safety performance of the scaffold erected by the present scaffold installation method is not sufficient for prevention of falling from roof. Some falling accidents from roofs also occurred at the construction site, where it was used.
  3) Characteristics of the actual falling mechanism of the victims can be reproduced by experimental studies. The results show that the prevention method by using a small number of guide rails is ineffective.
  4) The effective assembly method of safety nets for prevention of fatal accidents due to falling from the eaves of roofs was investigated by experimental studies. From the results, the models of victims in some experimental tests had strong impact on their heads, because they rebounded back and bashed their heads against the roof surface after they hit the safety net. Therefore, it is necessary to prevent the rebound to the roof surface.
  5) The safety net needs to be set up with a gentle descent for prevention of severe injuries by the rebound to the roof surface.

A Study on an Accident Prevention System Using RFID for the Restart of Machinery in the Spacious Dangerous Areas

RR-2005-5

Kiyoshi FUKAYA

: New monitoring system for human detection in a spacious movable area of machinery is necessary, and workers' position detection system using RFID (Radio Frequency Identification) was developed and evaluated in this paper.
    It is dangerous for a person to enter this area, and the safety control, such that the machinery is stopped when a person enter this area, is needed. To realize this control, it is necessary to know a worker's presence in the dangerous area. There are some defects in the conventional means of detecting human presence. For example, the direct monitoring method, in which dangerous area is monitored, is advisable, but there are technical difficulties for monitoring spacious area and the method has only limited application. The boundary monitoring method, in which the boundary of dangerous area is monitored, is used in many cases. The boundary monitoring is essentially monitoring of the change of human presence, and it needs other monitoring system for detecting initial safety condition. Because of these defects new monitoring system was needed.
    A workers' position detection system was developed. This system uses RFID devices to detect a worker's position. In order to identify the position RF tags are attached to equally spaced posts. A RF reader that a worker carries, read the position and the data are sent to the safety control system through wireless LAN (Local Area Network). This system can help an operator of machinery to know whether men are in a dangerous area or not.
    There are two types of RFID; active tag type and passive tag type. Because receivable range of active tag type is longer than that of passive tag type, active tag type RFID was used. The RF reader can change detection threshold of the tag and for each threshold measurements of the receivable area of the RF reader were conducted. And it was confirmed that:
  (1) Generally, the shorter the distance between a reader and a tag it was probable to detect tag.
  (2) There are cases in which a distant tag is detected without detecting a near tag.
    Measurements of a walking worker's position were conducted. Mean of positions of detected tags was used as an estimated position of a worker and it showed a worker's walking route. But there were errors of estimated positions of workers, because of the fluctuation in the receivable range and time-lag.
    This system is an experimental system and the devices are not suitable for carrying, i.e. it is big. But theses devices will become small and it will be put to practical use in the near future, when all workers will carry some IT (Information Technology) devices such as PDA (Personal Digital Assistance).

Static Eliminating Properties of Incendiary Discharge-Free, Nozzle-Type Electrostatic Neutralizer

RR-2005-6

K.S.CHOI, M.YAMAGUMA, T.KODAMA, T.SUZUKI and T.MOGAMI

: Most industrial operations with powders, such as transport, filling, and emptying, involve collision and separation between the particles themselves and between particles and the walls of the installations. Thus, electrostatic charges are often generated on powdery substances. Such a charging phenomenon may cause electrostatic discharges (ESD) inside the silo and result in the ignition of flammable dusts. As one method to protect a dust explosion due to ESD, we have developed a new nozzle-type electrostatic neutralizer with a corona discharge. In this study, it was investigated experimentally whether the developed neutralizer can be used in an explosive atmosphere. The ignition testing apparatus, which consists of a testing needle electrode and an ignition vessel (2500 cm³) filled with an ethylene-air mixture (6.5 Vol.%), was used for evaluating the ignitability of the high-voltage electrode system. The performance of the neutralizer (flange-type) was also evaluated with a full-size pneumatic powder transport system. The results obtained from the experiments are as follows:
(1) The needle electrode system with a 100 MΩ-coupling for the developed neutralizer can be used in the explosive atmosphere with a MIE (minimum ignition energy) of more than 0.1 mJ up to the applied voltage of AC or DC 7 kV.
(2) The nozzle-type neutralizer can be used for producing air ions to reduce effectively the static charges on polymer materials in a pneumatic powder conveying system by controlling the applied high-voltage.

Fatigue Crack Monitoring of the High-Strength Aluminum Alloy Lap Joint and its Fatigue Fracture Properties

RR-2005-7

Tetsuya SASAKI, Takashi HONDA

: To prevent fatigue fracture of machines, equipments or structures is essentially important to assure safety of workers at industrial sites because most fracture accidents are attributed to fatigue. Basically, prevention of fatigue fracture used to be attained by employing safe life design or damage tolerance design. However, these conventional anti-fatigue design concepts are not perfect because predicted fatigue life is inaccurate and all cracks are not always detected by non-destructive inspection. These fundamental problems of the safe life design and the damage tolerance design can be eliminated if fatigue damage which may result in a fatal accident can be monitored at any time using some kind of damage sensor. Since fatigue fracture generally occurs in the stress concentration area, developing fatigue damage monitoring technique in joints is important for this approach. In this point of view, the authors proposed new fatigue damage monitoring technique for bolted joints which uses strain gauges embedded in bolts as damage sensors. The previous study revealed that this method is applicable to both shear type bolted joints and flange type bolted joints. However, in shear type specimens, the monitored fatigue damage was limited to visible cracks initiating in outer plates and the change of strain gauge signal due to fatigue crack propagation was relatively small.
    In this study, the proposed fatigue damage monitoring method is applied to the high-strength aluminum alloy lap joint specimens in order to confirm the monitoring ability for fatigue damage at any locations, and the more sensitive monitoring method is also examined. Two types of specimens, notched and unnotched, are used to investigate the effect of initial notches at outer plate bolt holes, and fatigue fracture properties of the lap joint are investigated. Following conclusions are obtained from this study:
  (1) There are three different fatigue fracture locations in the high-strength aluminum alloy lap joint specimens that have no initial notches at bolt holes. However, fatigue lives of these specimens are not seriously effected by the fracture location.
  (2) With propagating of a fatigue crack, the fluctuating range of strain gauge signal changes further than the mean value of gauge signal does.
  (3) Using the range of strain gauge signal, fatigue cracks can be detected before final fracture of lap joint specimens regardless of fatigue fracture locations.

Development of NIIS-Mark II Centrifuge

RR-2005-8

Kazuva ITOH. Satoshi TAMATE. Yasuo TOYOSAWA. and Noriyuki HORII

: In 1988, Centrifuge called Mark-I was established into the National Institute of Industrial Safety with an effective radius of 2.3 m. Since then, NIIS has conducted many centrifuge tests to investigate the failure mechanism which are mainly related to the labor accidents. The results of fundamental and practical research projects have been applied and reflected to some safety guidelines given out by several construction safety groups.
In 2003, NIIS started to reconstruct its Mark-I centrifuge and it was completed in 2004. A new centrifuge "Mark-II" was named for this reconstructed centrifuge. This medium sized centrifuge is believed to be one of the most effective and multi-functional type of centrifuge in the world.
This paper presents the detailed outline on the structural and mechanical design of NIIS Mark-II centrifuge along with its performances. As in other centrifuge, it also has a main shaft, a drive unit, two arms, two swinging platforms, a signal and power supply interface and a control box. But unlike in other centrifuge, its arms (forms) are asymmetric, which is its special feature. One side of the arm is provided with a bridge plate where the swinging platform which could be fixed to its inner end plate when required by using a pair of hydraulic suspension jacks when the platform is lifted up. This type of system is called "Touch-down system". This facilitates the simulation of strong earthquake motions and mounting of shaking table on to the platform. Other side of the arm is not provided with the bridge plate and hence no end plate. In order to balance this portion (weight of the end plate) in this side of the arm while the swinging platform is moved up, two counter-weights are overhung on the two sides of this arm. This platform is used for non-shaking or static tests. In this type of middle size centrifuge, where bridge plate is not provided in one of its arm, longer radius and larger platform could be obtained. In addition, it makes the working on the platform easier and more efficient.

JNIOSH