Abstract of Special Research Report (SRR-78-1)

National Institute of Occupational Safety and Health, Japan

Special Research on Integrated Preventive Countermeasures against Natural Gas Explosion and Firre in Tunnelling Work


Yoshitada MORI

: The pneumatic caisson method has a history of about 50 years in Japan. As the caisson method or shield method is recently employed in many construction works, various sorts of labour accident occur as follows :
  (a) poisoning by carbon monoxide occurring unexpectedly in compressed air supplying system;
  (b) fire caused by gas cutting torch under compressed air atmosphere;
  (c) sudden sinking of caisson by air leakage caused by fracture of bolt joining caisson shafts.
    A object of safety in pneumatic construction method was only the disease by high pressure atmosphere hitherto, but it is necessary to establish urgent counter-measures against accidents as mentioned above.
    The following studies, therefore, have been carried out in our research institute for safety in pneumatic construction method ;
  (1) study on gas evolution from lubricating oil heated in compressed air ;
  (2) study on the combustion hazard of combustible materials under compressed air atmosphere ;
  (3) study on the optimum dimensions of vertical fixed ladder ;
  (4) study on the reliability of compressed air supplying system;
  (5) study on the strength of bolt joining caisson shafts.
    The report of these studies appears from next chapter to chapt.6.

A Study on Gas Evolution from Lubricating Oil Heated in Compressed Air

Kougaku KOMAMIYA and Takashi KOTOYORI

: Unexpected accidents such as explosion, fire or poisoning often occur in air compressor systems. For example, on Feb. 20th in 1976, six workers were dead due to carbon monoxide poisoning in a caisson for a bridge base construction in Tochigi prefecture. The cause was carbon monoxide evolution as the result of heating and combustion of activated carbon held in an air cleaner brought about by the cooling deficiency of the air compressor. As a matter of fact the cooling deficiency of compression heat of air can cause easily the thermal decomposition and combustion of lubricating oil or carbonaceous sludges in compressed air lines, accompanied by the evolution of carbon monoxide. Furthermore the combustion of lubricating oil itself may provide an ignition source for combustion of activated carbon or other organic substances that may be present in the compressed air lines.
    Thus, in this study an experiment was undertaken on ignition of lubricating oil and its poisonous gas evolution phenomena. The instrument used is a combination of a high pressure differential thermal analyzer of gas flow type (ref.9), in which a specimen of lubricating oil is heated in excess of its ignition temperature in compressed air, and an infrared absorption gas analyzer, by which carbon monoxide and dioxide concentrations of gas evolved from the specimen are continuously recorded under atmospheric pressure. The standard experimental conditions are as follows: sample amount, 15 mg; heating rate, 20 °C/min, air flow, 1 liter/min. The purpose of the measurement is to record simultaneously the change of sample temperature, temperature difference between sample and reference substance, carbon monoxide and dioxide concentrations (e.g. Fig.6).
    Main conclusions gained are as follows:
  1) amount and composition of gas evolved from heated lubricating oil is affected by various factors such as sample amount, coexistent substance, or flow rate, composition and pressure of atmosphere :
  2) gas evolution increases naturally with an increase in sample amount (Table 3).
  3) the increase in air flow rate results in the rarefaction of evolved eras, and so results in the lowering of poisonous gas concentration, al though the total amount of evolved gas is kept almost constant in each case; in such cases a rise in ignition temperature was also observed owing to the suppression of heat accumulation (Table 4) :
  4) by addition of copper dust and alumina the ignition temperature of lubricating oil drops by about 90 °C at maximum; moreover in this case a considerable amount of carbon monoxide was observed to arise before ignition (Table 5).
    The following preventative measures are recommended: overheat alarm system and carbon monoxide alarm system should be installed in compressed air lines to prevent the fatal accidents.

Combustion Hazard of Combustible Materials under Compressed Gas Atmospheres

Kougaku KOMAMIYA and Shigeru MORISAKI

: Danger of combustion will be increased in atmospheres at work places such as caisson work, as oxygen partial pressure is higher than air at atmospheric pressure.
    Experiments on ignition temperature, flame velocity and quenching distance were carried out at these conditions to estimate the combustion danger in these gas atmospheres.
    From the experiments, it was revealed that the increase of a few pressure had not so much influences on the combustion danger. However, as a caisson work is proceeded at a confined room, it is necessary to pay more attention for prevention of fire.

Experimental Study on the Optimum Dimensions of Vertical Fixed Ladders

Kinichi KINOSHITA, Yoshimasa KAWAJIRI and Hisao NAGATA

: Vertical fixed ladders are commonly used at walls of buildings, chimneys, towers, etc. as a simple facility for elevation. Dimensions of vertical fixed ladders are empirically applied without any definite standards to date. This research is aimed at setting optimum dimensions of vertical fixed ladders through analyses of various experiments.
    The research was accomplished by going through four processes, namely , a survey of ladder dimensions presently in use which were empirically established, preference questionnaires of ladder dimensions and kinesiological studies. The results of survey of presently employed dimensions of fixed ladders are acquired as in Fig.4.2. The results of the preference questionnaires are shown in Fig.4.4. These results are applied to determine the optimum dimensions of rung spacement and diameter. Ladder width, cage dimensions and clearance in back of ladder are computed from digitized data derived from motion pictures of human movement on a ladder as shown in Fig.4.18 and Fig.4.17. Various characteristic points related to the difference of rung spacement are derived from the joint angle, stride time, external work values, force exerted on the rung, and movement of each gravity point of individual segments of the body. Utilizing these data enable us to compare optimum rung spacement with other less things.
    The following suggestions for ladder dimensions are made in this paper : (1) rung spacement -30 cm ; (2) diameter of rung -22 - 25 mmφ ; (3) width of rung-minimum 40 cm ; (4) clearance in back of ladder-minimum 15 cm ; (5) cage dimensions-see Table 4.5.

Reliability of Compressed Air Supplying System

Yoshinobu SATO, Taiji KONDO, Noboru SUGIMOTO and Kiyoshi FUKAYA

: On pneumatic engineering method, reliability of compressed air supplying system is closely related to the safety of work in pneumatic workrooms.
    Therefore in order to improve the system, its reliability was studied in this paper.
    The system was grasped and set in the following manner through field investigations. System mission is always to provide the place like pneumatic workrooms with (i) proper pressure and (ii) suitable quality air coping with the proper consume.
    Four central functions directly linked to the system mission were extracted and named System Object Functions. (S.O.F.).
    These functions are System Managing Function, Compressed Air (C.A.) Producing Function, C.A. Sending Function and C.A. Regulating Function. The first function is mainly composed of human activity and the others are mainly composed of mechanical equipment. Each function was broken down to the tip functions and their failure modes and effects were analyzed.
    The final system state caused by malfunctions was evaluated in five steps, and these are S1 : none of the S.O.F. is stopped and the malfunctions are corrected rapidly, S2 : none of the S.O.F. is stopped but the malfunctions are not corrected rapidly, S3 : one or more of the S.O.F. are stopped but they are recovered rapidly, and the malfunctions are corrected rapidly, S4 : one or more of the S.O.F. are stopped but they are recovered rapidly, but the malfunctions are not corrected rapidly, S5 : one or more of the S.O.F. are stopped and they are not recovered rapidly.
    In order to calculate the probability that the final system state caused by malfunctions becomes SI(I=1,2,3,4,5) and to clear the time or logic series of the relation between malfunctions and human activity, a kind of probability tree named Evaluation Map of Man-Machine System Reliability was proposed.
    It is to pass the routes of the map choosing the courses according to the probability that t le incident on the path will occur, and to get the final system state having some probability. On this hereditary stochastic process the probability on the path is calculated by FTA based on reliability data. By using this map, not only the reliability of the system can be calculated but also it is easy to evaluate the effect of safety devices on the system.
    When some probable problems happened in this system, the probability of the final system state was calculated, and the effect of safety devices on system reliability was evaluated

On the Strength of Bolt Joining Caisson Shafts

Yoshio KITSUNAI, Etsuji YOSHIHISA and Masazumi TANAKA

: Tensile and fatigue tests of bolts were performed to decide a criterion for strength of the bolts which are used to connect caisson shafts. Three groups of the bolts which had been) actually used in some caisson shafts were supplied from different sites. Strength of these bolts were compared with that of the same kinds of new bolts. The bolts were made of low carbon steels and the diameters were 3/4 and 7/8 inch. Tensile tests of the bolts were carried out using a 50 ton capacity Amsler universal testing machine. Fatigue tests of the 3/4 inch dia. bolts were conducted on a 10 ton Vibrophore machine operating at 140 Hz. The tests of the other bolts were performed on a 20 ton closed-loop servo hydraulic fatigue machine at a frequency of 10 Hz.
    It was found that degrees of scatter in tensile strength of the old bolts are larger than in those of new ones. Endurance limits of the new bolts are the range from 3.3 to 4.6 kg/mm2 ; but the values of old ones are 3 to 4.3 kg/mm2 . Therefore, the fatigue strength of the old bolts reduces as compared with that of the new bolts. The differences in the fatigue strength of the both bolts may be explained by the fact that the minor diameter of external thread is slightly reduced by corrosion or plastic deformation during service. From above results, it is recommended to reject the bolt that had been used once. Some of the new bolts with 7/8 inch dia., which are created fissures by machining, are fractured at head-to-shank radius by fatigue tests. These defects are easily found by visual inspection. Then, it is necessary to inspect the bolts before use.

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