JNIOSH

Abstract of Special Research Report (RR-17)

National Institute of Occupational Safety and Health, Japan

Elimination of Static Electricity by Use of Radioisotopes --Electric Poential Decay of Charged Body--

RR-17-1
Yasuyuki TABATA, Katsuhiro SAKANUSHI and Tsutomu KODAMA

: Static electricity charged on the insulating materials leads to spark ignition in the explosive gas or the vapour atmosphere. It can often cause serious accidents in the various industries and consequently several counterplans are considered in order to eliminate static electricity.
    One of the safe methods for eliminating positively static electricity is to produce a number of ion pairs in the atmosphere. Writers have tried to do it by the use of radioisotopes and neutralize static electricity. It has been definitely shown by the previous experiments that it is able to eliminate static electricity on running vinyl sheet, in powder or in oil by using radioisotopes. This eliminating mechanism, however, has not been well established. It has been vague, in other words, how ion pairs produced by radioisotopes behave in the static electric field.
    This experiment is carried out with a view to define the eliminating mechanism of static electricity in case of using radioisotopes, Polonium-210. When the charged body is applied radioisotopes to, writers, as the first steps, investigate quantitatively how the electric potential of the charged body is decaying with elapsed time. Also we tried to formularize the eliminating mechanism and analyze approximately the electric potential decay of charged body, too.
    It has become clear from the above experimental and analytical results that the electric potential decay of the charged body is converging to the zero potential through the following three regions during several time.
  (1) The decaying region shown in the trigonometric function or the hyperbolic function.
  (2) The decaying region shown in the linear function.
  (3) The decaying region shown in the exponential function.
    On the other hand, on the occasion that the charged body is not applied radioisotopes to and left alone in the air, it has been led that the electric potential decay of the charged body is roughly indicated by the hyperbolic function. Those detail data are shown in the appendix.
    Following those experimental and analytical results, we, as the next steps, will further research on the fundamental data owing to develop the eliminator using radioisotopes in a simple experimental model of the charged system.

Study of the Safe Gaps with Naphtha Cracked Gas/Air Mixtures for Flanged Joints

RR-17-2
Heizaburo TSURUMI and Toei MATSUDA

: Experimental safe gaps with 1 inch flanges for naphtha cracked gas/air mixtures under atmospheric pressure have been determined in the standard spherical vessel.
    The naphtha cracked gas used in the experiments composed of hydrogen, carbon monoxide, acetylene, ethylene and the others as shown in Table 1.
    This experiment was carried out in the spherical vessel with the flanges and the parameters of the experiment were the concentration of the naphtha cracked gas/air mixtures and gap widths.
    The spherical vessel was made of stainless steel and the inner volume of it was 8,000 cc.
    Electrical sparks were discharged between electrodes of an ignition plug installed in the center of the spherical vessel.
    The explosion-pressure vs. time diagram in the outer and inner of the spherical vessel was recorded with strain gauges and by this diagram whether ignition to the gas of the external chamber occurred or not was determined.
    The maximum explosion pressure obtained with the standard spherical vessel with naphtha cracked gas/air mixtures is given in Fig.5, which shows the maximum pressure of 8.7 kg/cm2 at 18.1 vol.% naphtha cracked gas/air mixtures.
    The results of measurements for the experimental safe gap are shown in Table 2.
    The conclusions as to the results of this experiments are as follows.
  1. The experimental safe gaps depend on the concentration of the naphtha cracked gas/air mixtures as shown in Fig.6.
  2. The maximum experimental safe gap was 0.365 mm at 13.6 vol.% to 15.2 vol.% naphtha cracked gas/air mixtures.
  3. The naphtha cracked gas/air mixtures would be estimated to belong to the explosion grade 3.

Joint of Strut in Excavation Work

RR-17-3
Yoshitada MORI and Ikuo MAE

: The constitution of joint in the member of temporary structure is simplified by comparison with that of permanent structure, therefore, such a joint is prone to have some structural defects. If the incomplete joint is used in the principal member such as a strut in excavation work, the buckling strength of strut is reduced, and the reduction of strength may bring about the failure of the structure.
    Previously, the model tests on the joint effect were carried out, and suggested that the desirable joint in the strut of temporary structure was butt-plate and splice plate joint.
    This paper presents the results of the further studies on the problem of joint effect for the buckling strength of compression members, involving the theoretical analysis and the flexural rigidity tests of joints. The investigations are made on the following types of joints in H steel strut ( section 300 x 300 x 10 x 15 mm) :
  (a) butt-plate and splice plate type
  (b) butt-plate type
  (c) reinforced concrete type
    In the case of (c) type joint, butt-plates of members are separated each other, and are connected with four long bolts and concrete filler, as if they form a reinforced concrete member.
    The theoretical analysis is studied on the buckling strength of an axially compressed member having a joint at the midpoint, its results are shown as follows.
  1. (a) and (b) type joints ; these cases are treated as a problem of column having an elastic hinge at the midpoint, then the buckling strength of column can be expressed
    P = μ2 EI / l 2                 (A)
where EI is the flexural rigidity of column, l is the length of column, and μ is the coefficient which can be obtained from the below equations.
  i) Both ends of column hinged ;
    μ/2 tan(μ/2) = kl / EI           (B)
 ii) Both ends of column fixed ;
    μ/2 cot(μ/2) = - kl / EI         (C)
where kl / EI is the rate of the spring constant of elastic hinge to the flexural rigidity of column, then it may be called a rigidity ratio.
  2. (c) type joint ; this case is treated as a problem of column with varying sections, then the buckling strength of column can be expressed
    P = μ12EI / l 2        (D)
where μ1 is the coefficient which can be given from the below equations.
  i) Both ends of column hinged ;
         μ1 cot ξ μ1 - μ2 tan(1/2 - ξ ) μ2 = 0     (E)
 ii) Both ends of column fixed;
         μ1 tan μ1 + μ2 tan(1/2 - ξ ) μ2 = 0        (F)
In the above equations (E) and (F), the relation between μ1 and μ2 can be expressed as the following equation
         μ1 / μ2 = √( EI ' / EI )                        (G)
where EI ' is the flexural rigidity of joint and ξ is the value determined from the length of the column except the joint.
    Meanwhile, in order to find the flexural rigidity of the abovementioned three types of joints, the bending tests on the specimens having those joints are carried out, and the following results are obtained.
  3. The spring constant for (a) type joint ;
         k = 0.02525 EI (t·cm)                     (H)
  4. The spring constant for (b) type joint ;
         k = 0.01079 EI (t·cm)                     (I)
  5. The flexural rigidity of (C) type joint varies with the age (compressive strength) of concrete, the value of them are shown in the below table.

Age of concrete (day)Flexural rigidity EI ' (t cm2)
10.0235 EI
3 0.0432 EI
7 0.0587 EI
7 0.0732 EI
14 0.0904 EI
14 0.0982 EI

    On the basis of the results from the theoretical analysis and the experimental data, the buckling strength of struts having the various types of joints are calculated, and the rate of decrease in buckling strength are estimated as follows.
Type of jointThe rate of decrease of buckling strength
(a)10 %
(b)22 %
(c)Concrete age 1 day71 %
3 days60 %
7 days55 %
7 days49 %
14 days43 %
14 days43 %

    In conclusion, (a) type joint needs no anticipation of the decrease of buckling strength in practical uses, (b) type joint should be dealt with consideration of the decrease of buckling strength, and (c) type joint should be set near the restraint point such as the intersecting point of structural members.

The Quenching Ability of Flame Arresters for n-Hexane

RR-17-4
Kougaku KOMAMIYA

: For the prevention of unwanted fires and gas explosion disasters in chemical industries, flame arresters are used mounting on oil tanks or other similar installations. However, their effectiveness has not been well established in experiments.
    The writer made an investigation into the quenching abilities of flame arresters used in practice on his bench.
    The experimental equipments are shown in Fig.1. Inflammable gases used were n-Hexane/air mixtures. The concentration of which were determined with an interferometer having the length of gas chamber of 48 mm, taking a value of 1.002032 for the refractive index of hexane. The constituting wires of arresters were made of stainless steel having a nominal size of S.W.G 26 and mesh of 16.
    The directions of flame propagation were selected upwards and downwards.
    The quenching ability of the arresters obtained in the experiments are shown in Fig.4 to 6 from which some knowledge on the effectiveness and design of flame arresters are available.

Fundamental Study of Testing Appratus for Determining Minimum Igniting Current for Explosive Gas Atmospheres

RR-17-5
Ryuji TANAKA

: In designing intrinsically safe electrical apparatus and circuits, the incendivity of the spark discharges under the atmospheric pressure is a vital problem. Many reports have so far been published on the incendivity of electrical sparks for methane-air mixtures, while a few for many other inflammable gases or vapours.
    Works at the Electrical Research Association have determined minimum ignition currents for typical various gases and vapours with a simple series inductive circuit having a 95 mH inductor. And it is also a fact that Blanc et al. showed the minimum ignition energy for some gases and vapours by means of H.V. condenser discharges.
    One of the most desirable characteristics for a testing apparatus in determining the limit of ignition or non-ignition is that it can well simulate spark discharges encountered in practice. From this standpoint, the minimum ignition energy with H.V. sparks by Blanc et al. is not appropriate to be used for designing intrinsically safe apparatus and circuits. On the other hand, the m.i.c. obtained at the E.R.A. are seemed to be rather large compared with those obtained with wire-break apparatus. In 1967, I.E.C. adopted as a testing apparatus the one proposed by the German delegations which had 4 tungsten wires (anodes) and a cadmium disc (cathode). However, the I.E.C. testing apparatus is blamed for too high ignition sensitivity because of the use of cadmium disc.
    The author investigates into the m.i.c. in low-voltage inductive and non-inductive circuits by means of a copper-wire break apparatus which has a high separation speed and a little quenching effect of the electrodes with a view to meeting with the wire-break accidents.
    The results obtained are compared with those of the I.E.C. apparatus, Break-flash No.3 apparatus which ignition sensitivity is similar to that of E.R.A. No.2 Apparatus, and Intermittent Break apparatus which is used for testing the incendivity of slow break or capacitive sparks at the Safety in Mines Research Establishment (Sheffield).
    The copper-wire break apparatus, hereinafter referred to as copper apparatus, has a stretch of copper wire the length of which is 40 cm. Each end of the wire is led round a metal roller and when the roller is rotated manually, the wire is stretched to be broken and discharge sparks occur between the wire ends. After ascertaining non-effect of the wire diameter 0.1 to 0.5 mm on the ignition currents with inductive and non-inductive circuits, only 0.4 mm - wires are used for determining m.i.c. for hydrogen and methane-air mixtures.
    With inductive circuits, a 24 V d.c. supply, an air-cored inductor, a current limiting resistor and the copper apparatus are connected in a simple series, and with non-inductive circuits, the inductor is short-circuited.
    Fig.4 shows the comparison of the m.i.c. for hydrogen- or methane-air mixtures between the spark producers. As the m.i.c. for hydrogen-air mixtures with copper apparatus are less than the minimum arcing currents of the electrode materials, the possibility of occurring arc discharges may be ignored. Hence, the discharge energy required for the ignition is approximately equal to 1/2 LI2 . The values of 1/2 LI2 are compared with reference to the B/F No.3 apparatus. The relative discharge energy with the copper apparatus is 0.6 to 0.8 and that with the I.E.C. type 0.15 to 0.55. depending on inductances.
    Results with non-inductive circuits, as seen from Fig.6, the use of I.E.C. type for determining the m.i.c. is limited to more than some 20 V, and within this limit the ignition sensitivity of the I.E.C. type is shown to be the highest. Up to about 20 V, the copper apparatus is more sensitive to ignition than the Intermittent break apparatus.
    The difference of the sensitivity may be attributed to electrode separation speed, quenching effect, discharge characteristics etc.
    More are discussed on the I.E.C. type, namely, it concerns with the electrode materials. The cadmium disc is replaced with iron, nickel or brass electrode and each m.i.c. is compared. For inductive circuits with larger inductances the m.i.c. are nearly the same, while differences between them gradually appear with decreasing the inductance.
    Effect of electrode polarity on the m.i.c. of the I.E.C. type is also investigated. For inductive circuits with no arc discharge occurring the m.i.c. does not depend upon the polarity. On the other hand, for non-inductive circuits, the effect is clear and the m.i.c. of a circuit voltage of 48 V increases by approximately 20 % with a cadmium anode and tungsten wire cathodes. However, when the cadmium is replaced with iron electrode, there is no effect of reversing the polarity. Ignition sensitivity of the I.E.C. type seems to increase if the new surface of the cadmium disc is more or less scratched to "aging" by the tungsten wire ends.

The Minimum Igniting Currents for Explosive Gas- or Vapour-Air Mixtures by Break-Spark in Low-Voltage d.c. Inductive Circuits

RR-17-6
Ryuji TANAKA

: The author previously made clear the ignition sensitivity of the IEC type apparatus for testing intrinsically safe electrical apparatus and circuits for use in explosive gas atmospheres and that the ICE type could appropriately simulate break-sparks of low voltage inductive circuits and resistive circuits with a supply voltage of not less than some 20 volts as regards the spark ignition.
    The minimum igniting currents for various explosive atmospheres by break-sparks of inductive circuit having a 95 mH inductance and working at 24 volts d.c. have been reported from the Electrical Research Association, using the "Break-flash No.2" apparatus with a pair of platinum alloy electrodes. The data obtained at the ERA, however, are required to be reevaluated with a view to finding new criteria for designing intrinsically safe apparatus and circuits, since the IEC type has been found to be much more sensitive to spark ignition than the Break-flash No.2.
    From this standpoint, the author determined the minimum igniting currents for typical explosive gas- or vapour-air mixtures by break sparks of low voltage d.c. inductive circuits using the IEC type which electrodes are composed of four tungsten wires and a grooved cadmium disc under the various experimental conditions.
    The test circuit used in this series of experiments consists of a battery, an air-cored inductance, a non-inductive variable resistance for current regulation and the IEC type apparatus. The air-cored inductances range from 0.1 mH to 10 H, stepping at six values.
    The calibration of the apparatus is made before and after each determination of minimum igniting currents, and if the performance of the apparatus is found to have changed it is brought back to a standard calibration.
    Table 3 shows the comparison of most easily ignitable concentration of the atmospheres by breaksparks of 95 mH inductance working at 24 volts d.c. and the minimum igniting currents obtained with the IEC type and Break-flash No.2 apparatus. The ratio of the minimum igniting current for each combustible between the two are 2.95 for hydrogen, 2.17 for ethylene and 1.6 to 2.0 for the others.
    The effect of value of the inductance on the most easily ignitable concentration is examined for hydrogen, methane and ethylene.
    With a circuit voltage ranging from 6 to 96 volts d.c., the minimum igniting currents I for the various air-cored inductances L are determined and although the results are basically given in the form of LIn = const, the deviation of the data from the equation occurs with smaller inductances. Where the circuit voltages are more than 48 volts, the inductance less than about 3 mH has little effect on the minimum igniting current for hydrogen and methane.
    The effect on the minimum igniting current of spark suppressors such as shunt ohmic resistors, a non-linear resistor, air-condensers and rectifiers are also briefly examined. The ohmic resistors used in this test are ranged from 500 Ω to 20 kΩ and the condensers from 0.01μF to 1μF. The non-linear resistor used is composed of silicon carbide and in the form of annular disc, and the rectifiers used are of selenium, germanium and silicon types with the peak inverse voltage of 50 volts.
    In general, the increases of minimum igniting current with the spark suppressor appear with, inductance of more than some tens mH, and especially the rectifiers used show almost equally the large increase of the minimum igniting current in the range of several mH to 1 H.

Basic Study of Intrinsically Safe Circuits for Methane-Air Mixtures under Hyperbaric Pressure

RR-17-7
Ryuji TANAKA

: The minimum ignition limits in low-voltage inductive, resistive and capacitive circuits are determined under hyperbaric methane-air mixtures using the IEC-type spark-producing apparatus.
    In inductive circuits the minimum igniting current decreases with increase of the pressure and reaches the lowest value under a certain pressure each depending upon the circuit inductance, and after that the m.i.c. begins to increase with the pressure. Particularly with the circuit inductance of 1 H, the m.i.c. in the range of pressure examined is shown to be larger than that under the atmospheric where the pressure exceeds 10 kg/cm2 in gauge pressure.
    In resistive circuits, the m.i.c. also decreases at first with increase of the pressure and the lowest value is attained under the atmospheres of some 4 kg/cm2. Following that the m.i.c. increases with the pressure and gets into a region of "plateau" corresponding to supply voltage under the pressure of 10 kg/cm2 or more.
    The minimum igniting voltage of capacitive circuits, however, is shown to decrease monotonously with increasing the pressure, eventually arriving at a certain constant value corresponding to the capacitance, which is smaller than that under the atmospheric. The constant values seem to be attained again under the mixtures of 10 kg/cm2 or more.
    Finally, some problems are discussed on designing and testing intrinsically safe circuits under hyperbaric mixtures.
    Showering discharge duration with the pressure under particular inductive circuit conditions are appended.

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