|
|
|
|
|
 |
 |
|||
VI. THE INVESTIGATION OF THE EXPLOSION
Detailed inspection of the explosion area was delayed by the difficulties encountered in restoring the ventilation, but I am satisfied that this had no adverse effect on the investigation, and in particular that the condition of apparatus and samples did not deteriorate to a significant extent, if any, before examination. The coincidence of the explosion taking place at the time when, according to his evidence, one witness was operating an exploder was not allowed to limit the field of investigation.
Blast was felt near the air compressor in the Main Loco Drift, approximately 750 yards from the face of the Main Drift. and the air separation canvasses were displaced in the stentons outbye of No. 4 Stenton, but the extent of real violence was limited and is shown in detail on Plan II.
The auxiliary fan in No. 4 Stenton and its air tubes were displaced, but the adjacent electrical switch was not. The lamps belonging to Banton and Armstrong were severely damaged and other articles were moved. Nevertheless, violence was not great, nor was it extensive.
The effects of flame were visible to the naked eye for a distance of 26 yards from the face, and burning had been severe m and around No. 5 Stenton. The passage of flame could be traced for a further 57 yards outbye by microscopic examination of samples of wood splinters and pieces of canvas air tubing, etc. Determination beyond this was difficult due to wet conditions, but the two men at or near the firing station in No. 4 Stenton were certain that they saw no flame.
There was no sign of coking, but coal dust was not present under the wet conditions. The explosion, therefore. was purely one of firedamp.
The Source of the Inflammable Gas
Coal seams were met, as shown in the sections on Plan II, but there was no statutory report of inflammable gas being encountered. As mentioned previously, one deputy stated that he had detected a non-inflammable percentage about three weeks prior to the explosion.
During the investigation a feeder of firedamp was found issuing from the roof about 30 yards from the face of the drift, in the position of the most inbye piece of coal encountered.
Due to the time required to restore the ventilating current and to clear away the firedamp which had collected and persisted near the roof. it was not until Tuesday, 3rd December, however, that it was feasible to estimate with reasonable accuracy the rate of emission from the feeder.
Measurements of the air current and determinations of the percentage of firedamp it contained were made on that day at the inbye and outbye sides of the feeder, and these gave the quantities of firedamp passing the points as 3.4 cubic feet per minute and 33 cubic feet per minute respectively, indicating an emission from the feeder zone of some 30 cubic feet per minute.
A similar set of measurements on Friday, 6th December. gave 0.26 cubic feet per minute and 30 cubic feet per minute respectively, again indicating a make of approximately 30 cubic feet per minute. Another survey on the same lines on Friday, 3rd January, 1958, showed contents of 0.05 cubic feet per minute and 35 cubic feet per minute in the inbye and outbye sides respectively, with an estimated make at the feeder zone of 35 cubic feet per minute.
A sample of the coal at the feeder was taken on 2nd December and its volatile matter content on a dry ash free basis was slightly over 36 per cent.
The analysis in general did not suggest any change in the character of the coal which could be indicative of an abnormal gas emission.
The barometric pressure was steady throughout the day of the explosion, with a slight tendency to rise during the afternoon.
Sudden emissions of firedamp are not common occurrences in crossmeasure drifts, and although the feeder was in a "heavy" zone with a soft wet floor in which the arch girder supports were set at half the normal spacing, I am of the opinion that an emission did not suddenly occur here but that the feeder was present for an appreciable period before the explosion took place. The steadiness of the subsequent rate of emission of the feeder would appear to support this.
Nor is it necessary to postulate a sudden outbreak of firedamp.
The evidence in regard to the shift during which the explosion took place was to the effect that inflammable gas was present in considerable quantity in the Main Drift at the beginning of the shift and during the afternoon when the auxiliary fan was not working.
Immediately following an inspection which he made, the deputy went to find the overman, whom he met not long afterwards at about 6.30 p.m. The overman stated that the deputy did not tell him about having found inflammable gas, but informed him that the fan had been stopped. Thereupon both went together to the Main Drift in which the overman did not detect firedamp when testing as high up as he could reach.
At 8 p.m. the charging of the round of shotholes was commenced and when an attempt to fire it was made at 8.50 p.m. there was then very definitely sufficient firedamp present to cause an explosion of some violence.
The events during the shift and the evidence in regard to the standard of gas testing leads me to the belief that a layer of firedamp persisted near the roof over a considerable length of the Main Drift, particularly inbye from the feeder from which it emanated. Unfortunately, there was no evidence to indicate how long before the explosion the feeder had been present, or how large it was, but there would appear to be little doubt that it was the cause of the fouling of the Main Drift when the auxiliary fan was not working at the beginning of and during the shift.
The ignition occurred during an attempt to fire a round of shots which did not explode during shotfiring operations in a cross-measure drift in and about which there were several possible sources of ignition.
| (a) |
Main Drift Auxiliary Forcing Fan: Examination in the mine and subsequent inspection and test on the surface revealed nothing in the condition of the fan to suggest that it was the means of ignition, although the standard of its maintenance was not high. The fan motor and the associated electrical apparatus were in safe condition. Moreover, all were situated outside the area covered by the flame of the explosion.
| |
| (b) |
Haulage Signalling System: The electric signalling apparatus, which also was beyond the limits of the explosion flame, was found by examination and tests at the Safety in Mines Research Establishment to be in accordance with the relevant approval specifications and to be in safe working order, incapable of igniting a methane / air mixture.
| |
| (c) |
Compressed Air Range: The joints in the range in the explosion area were generally of the screw type. No evidence was found of air leakage which might cause heating or formation of sparks.
| |
| (d) |
Safety Lamps: All the lamps found in the explosion area were carefully examined at the Safety in Mines Research Establishment.
| |
|
The electric cap lamps belonging to the deputy and the drifter who were fatally injured were so extensively damaged by the explosion that no firm conclusions could be formed as to their state previously.
| ||
|
The deputy’s flame safety lamp was also badly damaged and the vital parts which had enclosed the flame were missing, but they would undoubtedly be destroyed by a violent impact which had deformed the fuel vessel. The wick was in a low flame position and, accordingly, suggested that there had been no attempt at relighting at the time of the explosion.
| ||
|
None of the other lamps showed defects which were likely to constitute a hazard, and accordingly there was no evidence that any lamp provided the means of ignition.
| ||
| (e) |
Contraband: No sign of contraband was found and there was no indication that it was involved.
| |
| (f) |
Shotfiring: Explosive and Detonators: Samples of the explosive and detonators, similar to those with which the round of shots was charged, were tested and found to be according to specification. Visual inspection of the round of shots gave no indication that any shot had exploded, even partially. Later the round was fired successfully, after the detonator leads, which had been adversely affected by the heat of the explosion, had been repaired. No other measure affecting the round was taken.
|
Exploder
The exploder was examined and tested at the Safety in Mines Research Establishment and was found to be in first class condition and in accordance with its specification.
Experiments were made at the Safety in Mines Research Establishment to investigate the possibility of the exploder igniting a methane/air mixture by sparking in the shotfiring circuit without at the same time firing a round of shots coupled in it.
Circuits were arranged to simulate a shotfiring circuit with a round of 26 shots:
| (a) |
with a shunt connection across the firing circuit consisting of a variable resistance and including a connection with bare wires resting lightly in contact, the latter being surrounded with an explosive methane / air mixture;
| |
| (b) |
with a variable resistance and a spark gap in an explosive methane / air mixture in series with the firing circuit.
|
The tests proved that the exploder was capable of igniting firedamp without firing a round of 26 shots, provided that the shotfiring cables were in such a condition that certain shunt paths or series gaps were present.
Shotfiring Cables
The two single-core shotfiring cables in use were made to the National Coal Board Specification P.115/1955 and each consisted of 3/0.036 T/C. V.I.R. conductors taped, braided and impregnated with fire resisting compound. Each was suspended about 4 feet or 5 feet above the floor on each side of the roadway, generally by being hung over the steel struts between the arches or by being tied to the arches with old pieces of cable. The positions of the cables are shown on Plan II in the sections, which also give the large number of uninsulated joints and bare parts observed in the mine. Several of these were found to be in contact with, or in very close proximity to, steel arches or struts.
Tests were made in the mine with the cables in situ. Using a Circuit Testing Ohmmeter it was found in several instances that the resistance of a joint was in excess of the scope of the instrument, i.e., over 300 ohms.
Analysis of the water coming from the roof of the drift showed that it contained a high content of sodium chloride and was appreciably corrosive.
The cables were carefully removed to the surface for test and examination at the Safety in Mines Research Establishment.
The left hand cable, approximately 145 yards long, had 20 very crudely made joints, all exposing bare conductors, and in eight other places the conductors were uncovered, the rubber insulation was split or broken wires were piercing the insulation.
The right hand cable, about 123 yards long, possessed eighteen crude joints and in eleven other places the cable was faulty due to exposure of conductors or the insulation being split or pierced by a wire.
Some of the "joints" were loose, providing a good opportunity for sparking when shotfiring was in progress.
Resistance measurement of the cables with a testing instrument gave the following readings:
| Left hand cable | 2,000 ohms | |
| Right hand cable | 1,500 ohms |
When an instrument capable of applying 1,000 v. D.C. was used, however, the resistance of the left hand cable was measured as 18.8 ohms.
The specified resistance of the shotfiring cable was 8.6 ohms per 1,000 yards and the above values confirm convincingly the extremely poor state of the cables in use in the Main Drift at the time of the explosion, and evidently for a considerable period of time before then.
There was no evidence that the Safety Ohmmeter at the colliery was used for the purpose of testing the circuit for continuity before the attempt was made to fire the round of shots, as laid down as a precaution by the Approval, and as required since 1st January, 1957, by Regulation 32 (7) of the Coal Mines (Explosives) Regulations, 1956.
A thorough search in the explosion area, including the vicinity of the shotfiring station, failed to find one.
For several weeks there had been only one ohmmeter in use in both the Loco Drifts and the Lower Three Quarters Drifts, one of the two previously available having been damaged and not replaced.
Evidence was to the effect that the one and only ohmmeter was at the firing station for the Loco Drifts during the whole of the shift in which the explosion took place.
The resistance values given above, of course, show that the proper use of an ohmmeter would have given definite evidence of the unsatisfactory state of the shotfiring cables.
There was no point at which metallic fusion was found definitely as evidence of sparking from the cables.
However, it must be borne in mind that in view of the low standard of insulation of cables there could exist a multiplicity of shunt paths with more than one in which sparking could occur. With several sparking points the energy available at any one could be adequate to ignite firedamp without any evident sign of fusion. Furthermore, the sparking need not have occurred at a shotfiring cable itself, as they may have served merely to conduct current to a gap in the steel structure formed by the arches and struts.
Indeed, tests with an ohmmeter gave resistances as high as 25 ohms across fish plated joints in arch girders in situ and lend weight to the possibility of there being sparking elsewhere than at a shotfiring cable itself.
Contains public sector information licensed under the 
