JNIOSH

Abstract of Special Research Report (RR-2003)

National Institute of Occupational Safety and Health, Japan

Earthquake Resistibility of Floor-Climbing Cranes

RR-2003-1
Masayuki YOSHIMI and Seiji TAKANASHI

: This paper deals with seismic response characteristics of floor-climbing cranes (FCC). The FCC, or jib-climbing crane, is one piece of construction machinery used for the construction of high-rise buildings. It is usually fixed on the upper floor of the building being constructed. In Japan, a crane is required to resist a horizontal static force that is 0.2 times as great as its own weight (0.2G load) while lording its safe working load. This construction code is applied to the FCC as well as to other cranes. Considering that the FCC is fixed on the upper part of a building and is exposed to stronger seismic force amplified by the building, the required property seems to be insufficient. Some FCCs suffered severe damage in the 1995 Kobe earthquake in Japan and the 2002 Taiwan earthquake. These incidents imply that the seismic resistance properties of FCCs are insufficient and that the construction code needs to be revised. It is necessary to evaluate the seismic response characteristics of FCCs.
    In this study, shaking table tests of FCCs on a building and numerical simulations were carried out for evaluation of the seismic forces that act on FCCs. A simple scheme for reducing the acting force is proposed and discussed.
    The shaking table tests and numerical tests showed that the current construction code for the crane is insufficient for the FCC and that the design horizontal static force should be, at least, increased to 0.3-1.0 G in order to assure the quality of the FCC's quake proofing being equivalent to that of other cranes.

A Robot Actuator and a Safety Interlock System Utilizing Magneto-rheological Fluid

RR-2003-2
Tsuyoshi SAITO and Hiroyasu IKEDA

: An innovative robot system, which aims to perform lift assisting for material handling and assembly tasks by its powered motion in response to forces applied by the user directly to its arm or pay load, is expected to be used not only in manufacturing but also in construction, rescue operations, home care for elders and other areas. This robot is termed a "cooperative material handling (CMH) robot." There will no doubt be a bigger demand for the CMH robot than for the conventional robots in the future.
    Since it is inapplicable to the CMH robot that the torque output of its actuators required to lift loads is restricted to less than the human tolerance, a safety interlock system for actuator torque monitoring is essential to achieve safe utilization of the robot. Furthermore, the safety interlock system must be so constructed that its failure never causes an accident even if it causes false trips. An interlock system that meets the above requirements, however, still has not been developed due to the difficulties in dealing with the changes of torque limit values according to robot motions.
    One solution, a torque-monitoring interlock system composed of antagonistic rotary actuators by means of magneto-rheological (MR) fluid and a 3-channel diverse programmable safety controller, is examined in this paper. This interlock system judges the normalcy of the actuator torque output by referring the amount of dead-weight compensation share included among all outputted actuator torques, and then it enables the active motions of the robot to continue. The antagonistic rotary actuators with MR fluid are adopted to materialize the characteristics of the normally closed type of brakes necessary to safely stop operation.
    From results of full-scale testing using a robot arm model, it is confirmed that the proposed interlock system executes the objective interlocking sequence under the condition of the above asymmetrical failure characteristics and achieves safety performance sufficiently to ensure the user's safety.

Development of the Fail-safe Safety Valve and Safety Control System for Boilers

RR-2003-3
Shoken SHIMIZU, Tsuyoshi SAITO and Hiroyasu IKEDA

: Boilers with high-performance control systems recently have been manufactured for power sources and heating and hot water supplies. These kinds of boilers have computerized control systems that enable high efficiency and fine operation without boiler operators. However, accidents such as explosion and destruction caused by over-pressure of boiler drums or high-pressure vessels may happen when sensors and controllers checking temperature, pressure, water level, and flames fail. Many explosion accidents are caused by the failure of sensors for boiler controls. In order to solve this safety problem, failsafe flame detectors for combustion control systems were developed in our former research project.
    In this paper, steam temperature or pressure control, another important boiler control, is used for boiler system safety because the relationship between the temperature and the pressure of steam is unique. The following two safety elements for steam temperature control and steam overpressure have been proposed and applied to an actual boiler experimentally.

1) A new type of thermal switch with a reed-switching mechanism driven by a thermostatic ferrite material and two permanent magnets.
    The thermostatic ferrite and permanent magnets have their own Curie temperatures, which are the changing points of their magnetic characteristics. When the steam temperature is over the Curie point of the thermostatic ferrite, a contact point of the reed switch element opens, and then a combustion valve shuts off. The interlock system using this thermal switch can be separated from the conventional temperature control system. As a result of experimental application of this switch to the actual boiler, the safety interlock function was verified to work even if the switching characteristics of the switch had a small off-delay.

2) A new type of safety valve constructed by a rare-earth permanent magnet with smart fluid.
    Opening pressure of this safety valve can be set only by a magnetic force that makes magnetic or Magneto-Rheological fluid seal the valve seat. As this valve uses a constant magnetic force instead of a conventional spring force for the seal, force adjustment and leakage inspection are not necessary. In a preparatory experiment, it was proved that the seal performance was superior in the case of the mixture of the above two fluids. When this valve, filled with the mixed fluid, was installed in the actual boiler, its performance was confirmed through experiments.

Studies on Dynamic Interactions of Ground, Foundation and Superstructure

RR-2003-4
Satoshi TAMATE, Yasuo TOYOSAWA, Naoaki SUEMASA, Toshiyuki KATADA and Shuji HIRANO

: In the last decade, three large earthquakes, including the Hyogoken Nanbu earthquake, have occurred in Japan and severely damaged many structures. As a result, engineers have rushed to strengthen the seismic resistances of structures in preparation for earthquakes. Countermeasures need to be taken against earthquakes in loading machine structures, such as tower crane systems, as much as in buildings. Although the seismic design generally used for buildings might be applied to the design code of the loading machines, it should not be adopted without careful consideration because seismic characteristics of loading machines are different from those of buildings.
    The purpose of this research is to clarify the dynamic interaction of the ground, the pile foundation and a superstructure such as a loading machine. Three types of centrifuge tests were carried out. In horizontal loading tests of a pile group in clay in a centrifuge, it was observed that the subgrade reaction of the pile was rapidly decreasing as the number of cyclic loadings increased. Dynamic centrifuge tests of a pile foundation in clay were conducted to investigate the behaviors of the subgrade reaction during earthquakes. The results showed that the distribution of the subgrade reaction of the pile was strongly influenced by the vibration mode of the ground. A series of dynamic centrifuge tests for a superstructure with two degrees of freedom supported by a pile foundation in sand was carried out to confirm the influence of the vibration mode of the superstructure with seismic damage of the structure. The results showed that, regarding seismic damage of the foundation, the bending moment at the pile head was increased at the second natural frequency of the superstructure in the case of soft ground.

Stability of Counterfort Sheet Pile Retaining Wall --For Soft Ground--

RR-2003-5
Yasuo TOYOSAWA, Noriyuki HORII, Norio FUJITA, Mitsuo SATO, Yokihiro SHUKURI and Makoto ETO

: About 100 workers die in accidents during excavation at construction sites every year in Japan. There are many construction processes involved in the assembly of retaining walls such as installing wales, struts, braces, etc., as well as dismantling such materials. The complexity of these processes is one of the main causes of such accidents.
    Cantilever sheet pile retaining method is the simplest method of earth support, as there is no need to install and dismantle earth supports inside the excavation site. Although this method is an effective way to avoid accidents, it can only be used for shallow excavation. Especially in soft ground, the penetration depth tends to be deeper and the costs higher when designed by some standard designs.
    In order to overcome these problems, the counterfort sheet pile method (SCB method) was developed. This method uses counterfort retaining walls with some intervals along the conventional cantilever retaining wall. These counterfort retaining walls stabilize the retaining wall and allow deeper excavation. However, there have been few researches on the mechanisms of the deformation and collapse of retaining walls. The safety and risks of this method should be clarified and a reasonable design method developed. The effectiveness of counterfort retaining walls for sandy ground was proved in the previous study. In this study, centrifuge model tests in soft clay ground (Cu = 17 kN/m2) were carried out to clarify the mechanisms of deformation and collapse, and the following conclusions were drawn.
    1) The counterfort retaining wall effectively reduces the inclination of the wall and bending moment generated due to excavation in soft clay ground as well as in sandy ground.
    2) The results of the centrifuge tests showed that the counterfort retaining wall has greater stability and resistance to toppling failure of the retaining wall. Allowable excavation depth, which means the depth before large deformation occurred, for the counterfort retaining wall was two times deeper than for the conventional cantilever sheet pile wall. It is suggested that the penetration depth can be decreased by using the counterfort retaining wall.
    3) The installation interval between counterfort retaining walls greatly affects the stability of the retaining wall.
    4) To evaluate the effect of the counterfort retaining wall when designing a retaining wall, the value of active earth pressure should be reduced. It is not sufficient to increase the stiffness of the wall in order to reflect the effect of the counterfort retaining wall when designing such walls.

A Circuit Model to Estimate Electric Shock Hazard

RR-2003-6
Eiki YAMANO

: The allowable current of electric shock accident depends on the body current properties (waveform, duration and frequency), contact locations and other factors of the human body. Until recent years, only "commercial AC or DC" was presumed as a body current in electric shock accidents. The use of electric power converters recently has increased in electric power systems, such as solar power generators, and may be a possible source of fault current (or body current) besides "commercial AC or DC". Moreover, the allowable limit of current is often unknown except for "commercial AC or DC". The limits for other currents must be made available in order for a circuit model to be used to estimate whether a particular body current is allowable. A trial of such an approach is reported herein. A circuit model was composed for the above use and was checked for the known data of the threshold current of ventricular fibrillation as follows.
 (1) Circuit constants were optimized to the IEC's data of threshold current by the least squares method.
 (2) The DC threshold current was calculated from the model. The calculated values were compared with the data of the threshold DC current.
 (3) The maximum error of the calculated value to IEC's data was 3.19 % in the section of 10 ms - 10 s.
 (4) As for frequency characteristics, the maximum error was 11.2 % in the section of 1 - 1000 Hz. In conclusion, this type of approach has possibilities, though it is not applicable immediately.

Electrostatic Properties of Dust Blown from Bag-Filters and Static Discharge from Layers of Metal Dust on Filter

RR-2003-7
Mizuki YAMAGUMA and Tsutomu KODAMA

: In recent years, there have been many serious industrial accidents resulting in severe injuries and deaths as a result of metal dust explosions in bag-filter dust collecting devices. According to the investigation reports, some cases seemed to be caused by electrostatic sparks as devices were manipulated by the workers, although the mechanisms of the charge generation and the ignition of dust by a electrostatic spark were unknown. For making these mechanisms clear, we proposed an explanation that frequent mechanical interaction between the dust and the bag-filter during the manipulation could generate a great deal of static-charges both on the dust and the filter, which ultimately discharged in an incendiary form. To prove this hypothesis, electrostatic charges on dust detached from the surface of a bag-filter by a high pressure blast of air were measured, and then the discharge phenomena from electrically isolated bag filters with charged dust were observed. The results are summarized as follows :
(1) By removing the coarse and/or round dust particles from the fuzzy filter surface, larger charges were obtained than those of fine and/or irregularly shaped particles. This was probably made possible because the friction was more vigorous after the dust was removed. A roundish aluminum dust, for example, got as high as 70μC/kg of static charge.
(2) No relationship between the resistivity of the filter and the bulk resistivity of the dust was found when a fuzzy surface was used; however, a relationship was identified when a flat, smooth surface was used.
(3) Two types of antistatic filters containing conductive fibers mitigated the charge generation by the process of dust removal probably because of the corona discharges. One of the filters, where the conductive fibers are sparsely dispersed throughout its fabric, greatly decreased the charge.
(4) The charge density on the dust was also dependent on the number of sequential removal of the dust by the blasting of air. In general, a larger charge density was obtained as the cycle progressed.
(5) If aluminum dust were to accumulate densely on a filter that was electrically isolated and highly charged, it could produce an incendiary electrostatic discharge.

Development of a Static Charge Neutralizer using an Atmospheric Pressure Glow Discharge

RR-2003-8
Atsushi OHSAWA

: The management of static charge is very important for preventing electrostatic hazards in many industries. Charge elimination using neutralizers (sometimes referred to as charge eliminators or ionizers) is one way to reduce the risk of static charge, and it is often necessary especially for electrification on electrically insulated materials. However an imbalance between the currents of positive and negative charged carriers from the neutralizer itself leads to a charging potential on the object to be neutralized. In particular the static potential for small devices becomes higher because of the low stray capacitance of the devices, and the required level of the static potential to protect ESD (electrostatic discharge) sensitive devices becomes lower, e.g., 10 V. In practice, ESD sensitive devices have had serious problems caused by static charges even when an ionizer is used. Therefore, a balanced neutralizer is intensively required to increase the production rate of ESD sensitive devices.
    At present, there are some types of bipolar ion source used in the ionizers, e.g., a corona discharge, a soft X-ray, an ultraviolet and an alpha ray from radioisotope. The corona discharges, however, are widely used because they can easily, economically, and safely (without radiation hazards) produce bipolar ions including electrons in air at atmospheric pressure. On the other hand, atmospheric pressure glow discharges (APGDs) have been well established. Because APGDs having attractive properties, such as no use of a vacuum system and a lower discharge sustaining voltage of several hundred volts, some applications have been sought. Therefore, we have predicted that by using the APGDs, a different type of neutralizer could be developed in terms of a lower source voltage and easily electronic control of the discharge owing to low discharge sustaining voltages. The other merit of the utilization of the glow discharges for a neutralizer is that there is no polarity dependence, while the feature of corona discharge is significantly different in terms of polarity known as positive and negative corona discharges.
    In this paper we report the development of our neutralizer using APGD in air. We have succeeded in precisely controlling the ion balance of the glow neutralizer by controlling the discharge itself or the ion flow extracted from the discharge.

Computational Assessment of Electrostatic Hazards in Silo Filling Operations

RR-2003-9
Atsushi OHSAWA

:It is well known that charges accumulate on powder particles by frictional contact electrification during the handling, processing and transport of powders. In filling operations with the charged powder, charges stored in vessels can lead to electrostatic hazards. This is because the charges often build up strong electric fields to produce electrostatic discharges including incendiary discharges that can induce ignition of any flammable powder dusts or vapors present in vessels. To prevent such electrostatic discharge (ESD) hazards, it is important to understand the mechanisms of the formation of the strong electric fields and the occurrence of the incendiary discharges. Computer simulation may be powerful to explore the mechanism and to assess the ESD hazards because it is easy to change the conditions of operations and the properties of powder in simulations rather than experiments.
    This paper presents the computational assessment of electrostatic hazards associated with filling processes with charged powder. Since the particle-in-cell technique has been used in the simulation, the simulation has been able to give self-consistent solutions for both the motions of charged particles and the electric fields inside the vessels. However, in the previous model of the simulation, charge dissipation processes were assumed to be ignored. The assumption caused an overestimation of the electrostatic hazards because the charge dissipation processes, i.e., charge relaxation of powder heap and discharges, reduce the electrostatic hazards as long as discharge energies are low. Therefore, in this paper we propose models of the charge dissipation processes and present a simulation for a vessel of 1 m in diameter and a mono disperse powder of 1 mm in diameter and of 1μC/kg in charge-to-mass ratio. The models gave a much truer simulation than the previous ones and the results of the simulation were comparable with those of experiments. Under the simulation conditions studied in this paper all discharges occurred on a heap surface, and the simulation estimated that incendiary discharges could be produced.

Suppression of Incendiary Electrostatic Discharge in a Silo using a New Electrostatic Charge Eliminator for Powder

RR-2003-10
Tsutomu KODAMA, Teruo SUZUKI and Tomofumi MOGAMI

: In a pneumatic powder transport system, large amounts of electrostatic charge generated by friction within pipe walls accumulate on powders with high electric resistivity. Loading such powders into a silo from the top raises the electric field strength inside the silo and may generate electrostatic discharges, causing the ignition of flammable dusts when fine powders are present at a concentration higher than the lower explosion limit. In order to prevent such a dust explosion during the loading of particulate products, we have developed a new nozzle-type electrostatic charge eliminator to neutralize the static charge on products before entering a silo, and we have confirmed the suppression performance against incendiary electrostatic discharges in a silo by using a full-sized experimental facility for pneumatic transportation and 350-400kg of polypropylene pellets. The eliminator is installed at the end of the loading pipe inside a silo. The eliminator used in the experiment consists of an SUS short pipe with an inner diameter of 10cm and a length of 10cm, as well as 16 nozzle-type ionizer arranged along the double circumference line of the SUS short pipe. In each ionizer, high voltage is impressed between a needle electrode and a grounded metal nozzle to produce air ions by corona discharges. The ions are blown inside the SUS short pipe by compressed air at a pressure of 250-300 kPa to neutralize the static charge on pellets within it. The experimental facility consists of an SUS silo with a diameter of 1.5m and a capacity of 1.5 m3, an SUS pipeline with a diameter of 10cm and a length of approximately 20 m, an air blower with an air volume of 15 m3 / min and an air conditioning unit. Incendiary discharges which occur along the pellet heap in the test silo are observed using a camera / CCD camera with an image intensifier set on the window of the silo roof. The charging tendency on pellets is mainly evaluated using an airblow-type electrostatic field sensor set on the side wall of the silo. The experimental results are as follows:
   i) Applying 50Hz ac high voltage to the eliminator results in an elimination shortage. On the other hand, applying dc high voltage requires control of the impressed voltage because of reverse charging.
   ii) Elimination ability increases as the frequency of the ac voltage increases. However, control of the frequency or voltage is also needed because of reverse charging.
   iii) Incendiary discharges can be satisfactorily suppressed by feedback control like ON-OFF control or proportional control utilizing the output signal of an electrostatic field sensor.

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