Abstract of Special Research Report (SRR-No.27)

National Institute of Occupational Safety and Health, Japan

Comprehensive Study on Preventive Techniques for Industrial Explosions in Chemical Processes


Hidenori MATSUI

: Many kinds of chemical substances are processed in industries. Industrial explosions and fires related to chemical processes have shown a tendency to increase since 1996 in Japan. Although, the rate of total number of occupational casualties caused by explosion and fire is not so high in the total occupational casualties, the number of serious accidents (more than three causalities) due to the industrial explosion, fire and poisoning shows the second highest rank next to the traffic accidents. Some results of the industrial explosions such as the accidental explosion of the hydroxyl amine distillation plant in Gunma prefecture and the accidental explosion of the hydrogen peroxide tank truck on the metropolitan expressway gave a great influence on the public. It is very important to assess the explosion hazards in chemical processes and to take preventive measures against accidental explosions caused by chemical substances.
    The objectives and research subjects of this specific comprehensive research are as follows:
  (1) To develop the techniques of chemical reaction hazard evaluation by considering the factors such as chemical and physical parameters in chemical reaction, method of reaction control and operation procedures.
  * A survey on evaluation techniques of reaction runaway hazards in chemical processes
  * Real time optimization of reaction parameter and measurement prediction
  * A new prediction method of heat release rate for heterogeneous liquid-liquid bach reactions with the agitation speed
  (2) To reveal the dust flame propagation mechanism and to develop a dust explosion suppression devices for prevention of dust explosion in chemical processes.
  * Propagation behavior and mechanism of the dust flame in a duct
  * Suppression of dust flames propagating in a duct
    -Quenching with water sprays, wire Gauze, and isolation by inert gas-
  (3) To develop a method for evaluation of the insulation in a mounted wiring board, and also to develop a fail safe design in gas detection system and safety control design for computerized chemical plants.
  * The evaluation of the insulation in a mounted wiring board by the monitor using wiring board of the comb-pattern
  * Basic requirements and a construction method for a gas detection system with asymmetrical failure characteristics
  * Safety control design and safety evaluation for computerized plant system
  (4) To develop a computer soft ware which aids investigation of industrial explosions and planning a explosion preventive measures based on the chemical process accident database. The soft wares can be used for safety education.
  * Development of the chemical process accident database
  * Development of the support system for prevention of explosions at chemical processes

A Survey on Evaluation Techniques of Reaction Runaway Hazards in Chemical Processes

Takayuki ANDO, Yasuhiro FUJIMOTO and Mieko KUMASAKI

: The thermal runaway process of chemical reactions is characterized particularly by a progressive increase in heat generation rate, temperature and pressure. It is initiated when heat generation from particular chemicals undergoing exothermic reaction or decomposition becomes greater than cooling capacity of facilities.
    In this report, we have enumerated the factors to affect the balance of heat generation and dissipation in chemical processes, and compared the importance of each factor.
    Subsequently, some evaluation equipments for the characteristics of chemical reactions, the most important factor of all, such as DSC (Differential Scanning Calorimeter), ARC (Accelerating Rate Calorimeter), RC1, and SuperCRC were introduced and the feature of each equipment was compared.
    Finally, two examples of the application of evaluation equipments for investigation of reaction runaway accidents which occurred recently in Japan were shown.

Real Time Optimization of Reaction Parameter and Measurement Prediction

Teruhito OHTSUKA, Takaaki MIZUTANI, Ou-Sup HAN and Yasuhiro FUJIMOTO

: The most important thing for safety is "precise prediction". But experiment under exactly same condition is impossible for almost all case. Therefore, various extrapolation is used for estimation. And factor of safety is also adopted without any explanation.
    The problems are, data acquisition and analysis are not performed on the same process-plant and in real-time. Now a days, computers become faster and faster. They will allow to solve such problems.
    In this study, real-time optimization was carried out with BFGS method that can converge to optimum rapidly. The modified algorithm of BFGS hessian update is also reported. Cause getting inverse hessian with BFGS has some numerical unstability, the modified algorithm can calculate update hessian numerically stable and inverse matrix quite easily.
    The experiment was performed with 20 L bench scale batch plant. Acetic acid anhydride and ethanol esterification reaction was chosen for thermal run-away model reaction. Only reactor temperature was measured and second order reaction equation and Newton type heat transfer were assumed.
    The results are summarized as follows;
  1) When uncertain parameter is included in the model, the optimization makes it appropriate. Therefore prediction can be going on.
  2) The parameters are dynamically fit by using onsite measurement data. The data extrapolation has better match than ordinary static fitting.
  3) The variables in the system can be easily gotten without direct measurement.
  4) When the constants in the system slightly or suddenly changed, the optimized value is following such change automatically.

A New Prediction Method of Heart Releasa Rate for Heterogeneous Liquid-Liquid Batch Reactions with the Agitation Speed


: Some industrial explosions, which occurred on the heterogeneous liquid-liquid reaction process, have been reported. To progress such reaction smoothly, reactants need to be mixed well by mechanical agitation. And it is the failure of controlling the mixing that causes unusual reaction and sometimes leads a runaway situation.
    An objective of this study is to simulate accident stories on heterogeneous batch reactions which are caused by the inappropriate mixing condition. The example is a liquid-liquid two-phase heterogeneous reaction of which rate is controlled by only agitation speed for mixing. That is, the faster mixing speed makes the bigger reaction rate. In this case, the accident could be triggered by the faster agitation speed than that in the former successful batch. Such a change might be decided due to improving productivity by the faster reaction. As the result, the reaction temperature could finally achieve to the boiling point and the bubbling could start. After that, the slower agitation couldn't make the reaction rate smaller any more because the bubbling could stir up the reactants instead of the mechanical stirrer.
    First of all, to simulate such an accident, the heat release rate in the heterogeneous reaction must be estimated under any mixing condition.
    In this paper, in order to predict the heat release rate at any time, the method, which could lead the equation to predict it, was proposed using an empirical equation of time versus particle size with the experimental results from a Reaction Calorimeter RC1. The hydrolysis of anhydride was selected as a model reaction, whose reaction rate is suitable to do it.
    It was proposed that the experimental data in the liquid-liquid heterogeneous reaction, in which agitation speed would change time and time again, could be summarized with Qr/φvalue. The fitted parameters would be useful to simulate the similar reaction. Furthermore, it was also proposed that, when the agitation speed increases instantly, the Qr on the faster speed is predicted from the evaluation of the ratio Qrs with the ratio of the lower and the faster speed, which could support to run the simulation.

Propagatation Behavior and Mechanisms of the Dust Flame in a Duct

Ou-Sup HAN, Masaaki YASHIMA and Toei MATSUDA

: The aim of this study is to investigate experimentally the flame structure in dust flame propagation and to provide the fundamental knowledge for development of explosion suppression systems applied to dust explosions. Insufficient information on dust flame propagation is mainly due to experimental difficulties in the generation of a uniform dust suspension, as well as the fact that particle size and size distribution. Upward propagating laminar flames in a vertical duct of 1800 mm height and 150 mm × 150 mm square cross-section are observed. From the experimental result, it is found that propagating lycopodium flame front shows the double flame structure in which individual burning particles and ball-shaped flames surrounding several particles are included. The thicknesses of preheated and reaction zone have been also determined by a schlieren photography. The flame in spatial area between independent flames and individual burning particles is not observed. This means that the flame can not propagate continuously in comparison with premixed gaseous flames. The thickness of lycopodium dust flame is observed to be 20 mm, about several orders of magnitude higher than that of premixed gaseous flames. By means of the microscopic visualization, it was found that the flame front propagating through lycopodium cloud is discontinuous and not smooth. The experiments using a PIV (Particle Image Velocimetry) system with a high-resolution video camera have been conducted to clarify the motion of particles during the flame propagation. Due to convective flow induced by the flame, a part of gravitational settling particles is shifted to its surrounding sides and the rest of the particles changed their movements to upward in front of the flame. Such particles movement causes a dynamic variation in dust concentration ahead of the flame, which propagates at lower dust concentration rather than the mean concentration. Considering the movement of single particle ahead of the flame, a certain residence time of the unburned particle in preheat zone is needed to generate combustible gas from the particle. The residence time will depend on preheat zone thickness, particle velocity and flame propagation velocity.

Suppression of Dust Flames Propagating in a Duct (Quenching with Water Sprays, Wire Gauze and Isolation by Inert Gas)


: It is necessary to conduct protective measures for ignition prevention in powder handling processes since ignition may cause dust explosions or fires. Moreover explosion protection and explosion suppression systems are also required so that the damage may not expand even ignition occurs. Explosion pressure release (vent for deflagrations), automatic fire extinguishing systems and rapid valves (mechanical isolation and chemical isolation) have been known as equipment to mitigate the damage of dust explosions. The goal of this study is to develop a new explosion suppression system applied to dust explosions, which relates to the latter two terms of equipment described above. An experimental study has been conducted to obtain fundamental knowledge on suppression of dust flames in a duct by (a) water sprays, (b) wire gauges and porous plates, and (c) inert gas isolation. Lycopodium was used as a sample powder to compare with previous studies.
*Experiment (a):
    The experimental apparatus consisted of a vertical duct having 200 × 200 mm in square cross section and 3 m in height, a fluidized bed and a water spray system. Lycopodium was mainly used at the concentration of 50 g/m3, which is lower than the stoichiometric concentration for the dust cloud. An infrared sensor was mounted on the surface of the duct to detect the propagating flames and made water spray with time delay.
*Experiment (b):
    The experimental apparatus consisted of a vertical duct (pipe) having 60 mm in diameter and 1.1 m in height, a fluidized bed and a mechanical insert system of quenching elements. Different sizes of wire gauzes with more than 0.46 mm opening and porous plates were used as quenching elements. In order to examine quenching behavior of the flame, the quenching elements were rapidly inserted into the duct from horizontal direction before arrival of the flame at the element position but after a spark ignition was given.
*Experiment (c):
    The experimental apparatus is similar that of Experiment (b) except for the injection system of inert gas from the duct wall. Argon (Ar) was used as inert gas.

The Evaluation of Insulation in a Method Wiring Board by the Monitor Using Wiring Board of the Comb-pattern


: Recent progress on wiring boards has made them more weather-resistant, and narrow pitched wiring boards have been used in severe environments, such as chemical processes, for instrumentation and control equipment. Such equipment is vital for production systems, and related maintenance problems can spread to other parts of a system and increase the risk of an industrial accident. The failure in wiring board is generally caused by loss of conductivity of wire and loss of insulation between wirings. The causes of loss of insulation are primarily dust adhesion, electrochemical migration, and aging of polymers. Loss of insulation is greatly dependent on environmental factors. The evaluations of insulation between wires can be accomplished satisfactorily in electric power fields; however, no adequate technique exists for use in printed wiring boards. In a mounted wiring board, it is technically difficult to apply the methods mentioned above to the evaluation of loss of insulation since electronic components and elements are mounted between wirings.
    Under such background, the method for evaluating loss of insulation of mounted wiring board by the method using wiring board of the comb-pattern as a wiring board for the monitor was studied on. With this method, the frequency characteristics of the impedance of the wiring board for the monitor evaluate the environment in which the mounted wiring board is set. Furthermore, the loss of insulation of the mounted wiring board is indirectly estimated by the frequency characteristics. Alternating current was used as a measuring source, because the main cause of the loss of insulation is the fouling between wirings. And the frequency f in proportion to the loss factor D between wirings, which is similar to the loss factor of the capacitance, was applied as the index to the loss of insulation.
    The loss factor D is defined in the following equation, and it is also defined as a function of the measuring frequency f, where Rp or Cp is the parallel equivalent resistance or the parallel equivalent capacitance on parallel equivalent circuit of the impedance between wirings, respectively.
  D = 1 / (ωCp ·Rp) = 1 / (2π·Cp ·Rp ·f)
    The loss factor is also defined as a function of (Cs /Cp ), where Cs is the series equivalent capacitance on a series equivalent circuit.
  D = (Cs /Cp - 1) 1/2
    In this study, the frequency characteristics of Cp and Cs are measured, and the loss of insulation of the wiring board is evaluated by the frequency in which (Cp /Cs ) nearly equals 1 or 2, where (Cs /Cp )=1 or 2 corresponds to D = 0 or D = 1, respectively.
    The frequency obtained by the experiment is almost proportional to the resistance Rp of the parallel equivalent circuit at 1 Hz at the negative gradient. Therefore, the frequency may be adequate for an evaluation index of the loss of insulation because Rp is stable in a lower frequency range in spite of the dependency on the frequency in the high-frequency range. The frequency range in which Rp is an adequate index is under consideration.

Basic Requirements and a Construction Method for a Gas Detection System with Asymmetrical Failure Characteristics

Tsuyoshi SAITO, Hiroyasu IKEDA and Noboru SUGIMOTO

: Gas detection systems in chemical plants are utilized for many purposes, e.g., to prepare material gas, to control process reactions, to monitor product quality and so forth. In a case that a gas detection system is used as a safety interlock system, which compels the plant operation to stop when it detects the leakage or the accumulation of flammable or toxic gas into the work environment, its structure must be essentially different from other systems so that its failure never causes an accident even if it causes false trips. The gas detection system with such asymmetrical failure characteristics, however, still has not been developed because it is basically impossible to generate the energy of sensor signal output from the state that objective dangerous gas does not exist in a monitored area atmosphere.
    In this report, a construction method to realize the gas detection system having the asymmetrical failure characteristics is deduced from logical considerations concerning generalized structural requirements of hazard detection systems. This method is also examined by applying to a trial system composed of a gas-chromatograph with a thermal conductivity detection unit (TCD) and a 3-channel diverse programmable logic controller. In this method, the asymmetrical failure characteristics are functionally achieved by automatically and frequently testing the normalcy of the TCD before every gas detection operation. The normalcy of the TCD is judged by comparing a preset pattern and a response waveform of the TCD corresponding to a dummy input of the dangerous gas. The gas-chromatograph is adopted to obtain the high reproducibility of the response waveform in the normalcy diagnosis operations and to confirm the normalcy of gas filtering function to separate interference gases from the atmosphere. From experimental results, it is confirmed that the trial system is able to execute the objective operation sequence composed of the gas detection and the normalcy diagnosis under the condition of the asymmetrical failure characteristics. Moreover, the safety performance of the trial system is quantitatively evaluated on the basis of the frequency of the operation sequence.

Safety Control Design and Safety Evaluation for Computerized Plant System

Hiroyasu IKEDA, Tsuyoshi Saito and Noboru SUGIMOTO

: With advancement of large-scaled plants, the use of a computer increases in the control, and there is a tendency that programmable equipment including a micro-processor is introduced into a hardware safety related system based on conventional electromagnetic relays. Because reliability of normal hard-wired system is low, and a fault diagnosis capability is uncertain and insufficient, it is a trend to replace the hard-wired with a flexible and functional software. However, as for such a programmable controller, indistinct characteristics remain in the use in safety security still, and, for example, potential systematic failures which can not be identified may appear and cause unsafe conditions.
    A functional safety standard of programmable electronic safety systems is established and this standard divides instrumentation of an automatic control system into safety related part and control part. This standard requires expressing the safety integrity as unreliability. Therefore, general-purpose programmable controllers shall not be used as safety related parts when the characteristics of the controllers can not be analyzed and evaluated. The safety interlock system must be consisted with special consideration on safety in order to apply programmable equipment in a safety related parts.
    This report proposes layered control systems with the functional controller and the safety interlock controller based on the concept of independent protection layers in case of adopting programmable equipment. And the role of each controller and safe performance required for safety integrity levels are examined. In addition, a safety function of process control is confirmed by a simple heat exchanger model. This model has three process controllers and one safety interlock system with a three channel programmable controller. Powerful self-checking functions and a diverse structure of the programmable controller can improve total safety integrity level of the heat exchange process.

Development of the Chemical Process Accident Database

Haruhiko ITAGAKI

: An explosion and a fire often happened recently in the chemical processes. Worker's casualties and great damage arose due to such accidents. To prevent an explosion and a fire, it is required to develop the advanced safety technology corresponding to recent technological innovation and working conditions such as an advanced safety countermeasure, fail safe technology, and to diffuse the safety
information and etc.
    National Institute of Industrial Safety manages explosion/fire database based on the accident information provided by the Ministry of Health, Labour and Welfare. But, there was no database about the detailed information about the chemical processes. So, an object was limited to the chemical process, and it was developed "the chemical process accident database" specified for the explosion/fire in the chemical processes. At first, type of industry related to the chemical process was extracted from the explosion/fire database. Next, information items were reconsidered in the items for the chemical processes, and modified them, and each item was classified by the contents and encoded. The purpose of this database is to provide the foundation data to extract the present problem of the accident by analyzing past accidents in the chemical processes. Furthermore, this database composes the support system for prevention of explosions at chemical process which aimed at the clarification of the explosion/fire hazard in the chemical processes and the support of the investigation of the accident with the case reference, total program and the countermeasure database. This report showed the information items and classification of the chemical process accident database, and as a result of an analysis of the accidents in the chemical processes, the transition of the number of explosions/fires and a result of a total in the type of the accident and each information item.

Development of the Support System for Prevention of Explosions at Chemical Processes

Haruhiko ITAGAKI

: Explosion and fire often happen at the chemical plant. A search for the problem, the selection of the effective countermeasure and accurate cause study are very useful to prevent these accidents. The chemical process accident database based on the accident information provided by the Ministry of Health, Labour and Welfare was created, and accidents were analyzed in the National Institute of Industrial Safety. However, there is a little useful literature when a countermeasure to the specific device is selected at the factory. Therefore, the cooperation of the specialist who has knowledge and an experience is often necessary. So, the software named "Support system for prevention of explosions at chemical process" for the purpose of the support of the clarification of the explosion hazard and the cause investigation at the chemical process plant was developed. This system is composed by the chemical process accident database, the countermeasure database of chemical process accident and the program which refers to two databases. The main targets of this system are the business worker and the safety manager who are not a specialist of the safety. This system searches the accident cases which contains input search words at a moment. Then, this system shows the statistic of the countermeasure with the ignition source, the cause and other factors. This report was mentioned about the frame of the support system for prevention of explosions, the contents of the countermeasure database and the interactive search program.
    This system can't replace a specialist. However, this system will suggest a useful countermeasure based on the statistics on about 500 recent accident which happened in the Japanese chemical process plant. This system is probably a good adviser to the business worker and the safety manager when a countermeasure is selected in the various industry which is not only a chemical process plant.

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