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GENERAL INFORMATION

The Homer City mine is near Homer City, Indiana County, Pennsylvania, and the coal produced is transported by overland belt conveyors to the Homer City electric generating station.

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The mine is opened by a double-compartment slope, 2,500 feet long, and a 15-foot-diameter shaft, 608 feet in depth, into the low-volatile Upper Freeport coalbed, which ranges from 44 to 54 inches in thickness in the area being mined.

A total of 138 men was employed and 108 worked underground on 3 shifts a day, 5 days a week. The average daily production of 1,200 tons of coal was mined and loaded with continuous-mining machines.

The immediate roof consists of sandstone or hard shale, and where shale is present, it varies from 10 to 15 feet in thickness. The main roof is sandstone.

The anlaysis of a coal sample taken from the coalbed in this mine, as obtained from the coal company is as follows:

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Numerous tests by the Bureau of Mines have shown that coal dust having a volatile ratio of 0.12 and higher is explosive. The volatile ratio of the coal in this mine as determined from the foregoing analysis is 0.25, indicating that the coal dust is explosive.

The last Federal inspection of this mine was completed December 2, 1969. A partial but representative inspection of the mine was completed April 8, 1970. ' Immediately after the investigation of this explosion was completed, a spot inspection of the entire mine was made because the mine was idle and numerous violations of the Act were cited. Violations that do not relate directly to the explosion are not discussed in this report.

MINING METHODS, CONDITIONS, AND EQUIPMENT

Mining Methods

The mine was in the process of development by a block system, and only entries were being driven. The entries, in sets of seven, were 18 to 20 feet wide, and crosscuts were 80 to 100 feet apart and 18 to 20 feet wide.

Roof-support plans had been adopted and were followed. Roof bolts were being installed in accordance with recommendations made by the United States Bureau of Mines.

Explosives

Mining in the crossover mains section was done with continuous-mining machines. Explosives were not used or stored in the section.

Ventilation and Gases

Ventilation in the mine was induced by an axial-flow fan installed properly on the surface and equipped with all required safety devices. The fan was operated continuously and was exhausting about 168,000 cubic feet of air a minute from the mine. A split system of ventilation was used, employing concrete-block and concrete permanent stoppings, overcasts, and undercasts. The crossover mains section was ventilated by a current of intake air directed through No. 6 entry of south mains and then split right and left at No. 1 entry of crossover mains. The right split ventilated two mechanical units operating in the seven entries of the crossover mains and returned through No. 7 entry. The left split ventilated the seven entries of south mains in which two mechanical units were also operated and returned through No. 1 entry of south mains. See Appendix G.

The air was conducted to the working faces with line brattices and runthrough check curtains constructed of nylon plastic material. At the time of the explosion, the end of the line brattice in No. 7 entry of the crossover mains was 27 feet from the face of a sump cut being driven and about 4 feet from the back end of the continuous-mining machine, a bulky machine which virtually filled the sump cut and was not provided with auxiliary means for ventilating the face. The end of a line brattice in a working place of a nearby section was 54 feet from the face. The development of two sets of seven entries to the south and to the west in close proximity simultaneously necessitated the use of many temporary stoppings, check curtains, and line brattices, all constructed of nylon plastic material. This made control of the ventilation in these sections difficult.

The mine is classed gassy by the Pennsylvania Department of Mines and Mineral Industries. Air samples collected in split returns from the two working sections and the main returns during the inspection completed in December 1969 contained 0.35 to 0.49 percent methane, and those collected in the main returns indicated that methane was being liberated from the mine at a calculated rate of 1,180,000 cubic feet in 24 hours at that time. Preshift examinations for gas and other hazards were made by certified persons before the first operating shift each week, and preshift examinations for succeeding shifts were made by the on-shift officials during their regular tours of duty. On-shift examinations for gas and other hazards were made by certified foremen. Weekly examinations for gas and other hazards were made and recorded. During a hearing conducted after the underground investigation, testimony indicated that methane was detected in the mine with flame safety lamps on a number of occasions, but these findings were not recorded. Methane monitors were not used on the continuous-mining machines. Gas or oil wells were not on the property.

Air measurements taken in the crossover mains and recorded by the section foreman on the shift prior to the explosion indicated that 21,000 cubic feet of air a minute was passing through the last open crosscut between Nos. 6 and 7 entries. This foreman stated during the hearing that he had detected what he estimated to be 2 percent of methane in No. 4 entry during his shift, and this finding also had not been recorded. Analyses of air samples collected in the area during the investigation indicated methane ranging from 0.12 to 0.77 percent, see table 1. Methane could be detected with permissible flame safety lamps and electrical methane detectors at the faces in the section where the explosion occurred and in other sections at various times during the investigation in concentrations ranging from 1 to in excess of 3 percent. From tests made in the face areas after production of coal had been resumed, methane was emitted rather freely from the roof strata when breaks in the roof occurred.

Reportedly, a gas ignition that caused no injuries or property damage occurred in the crossover mains section approximately 3 weeks prior to this explosion. The flame from this ignition was confined to the immediate face area. It was not recorded in the record books at the mine, nor was it reported to the Bureau of Mines.

Dust

During the Federal inspection completed on December 2, 1969, the mine surfaces varied from wet to dry and were free from accumulations of loose coal and coal dust. Dust surveys with sampling points at 200-foot intervals were made in the two areas being developed during the inspection, and analyses of the 50 samples collected during the surveys indicated that

incombustible content of the mine dust ranged from 81 to 100 percent. The continuous-mining machines were equipped with water sprays which appeared to be effective while coal was being mined. The continuousmining machine involved in the explosion was equipped with 19 water sprays to allay dust. Reportedly, the sprays were being used at the time of the explosion; however, the valve for the sprays was found closed after the explosion, the presence of water was not observed in the face area, and testimony indicated that none of the workmen had turned the valve off after the explosion.

During the investigation, the rock-dust applications were found to be 49 to 94 feet from the faces in six working places in the south and crossover mains sections. Dust survey samples were collected at 50-foot intervals in the crossover mains and at 100- to 200-foot intervals in the remainder of the mine. Six dust samples were also collected at spot locations in the mine. of the 135 samples collected, 36 (26.6 percent) which were collected in areas in which the dust was not disturbed to any appreciable extent contained less incombustible material than required. Six dust samples collected in the explosion area contained traces to large amounts of coke indicating that coal dust was a factor in the explosion. See Appendix H.

Transportation

Permissible-type cable-reel shuttle cars were used in the crossover mains section to transport the coal from the face areas. The coal was discharged into coal feeders at the belt-conveyor tailpieces, then transported by belt conveyors to the surface. The men were lowered into and hoisted from the 2,500-foot slope in special man cars. Mine cars pulled by trolley locomotives were used to provide transportation for the workmen between the slope bottom and the disembarking point at the end of the track haulage road. Supply cars pulled by permissible-type batterypowered tractors provided transportation for men to the working faces.

Electricity

Electric power from the public utility system was transformed to 13,200 volts on the surface and conducted underground over nonmetallic sheathed cable. The high-voltage cable entered the mine through the slope portal. Underground, the high-voltage system supplied combination &.c.-d.c. power centers for five continuous-mining machine units, two 150-kv.-8. transformers for conveyor drive motors in 2 right and crossover mains,. an 800-kv.-8. transformer for conveyor drive motors at the slope bottom, and a 300-kilowatt silicon diode rectifier along south mains that furnished 250 volts d.c. power for the trolley system. Short circuit, overcurrent, ground fault, and lightning protection was provided for the resistance grounded high-voltage power circuit which was

equipped with a fail-safe ground check circuit. A disconnecting switch was installed in the circuit near the entrance to the mine, and steelenclosed switch houses with vacuum switches were installed in the system underground near the slope bottom and in all branch circuits.

Electric face equipment in use in the right side of crossover rains at the time of the explosion consisted of a Joy llcM-LAKK continuous miner, Bureau Approval 26-2230-A, and National Mine Service Company 3051234 shuttle car, Bureau Approval 2F-1323-13 in No. 7 entry, and a Galis 300 roof drill, Bureau Approvals 2F-2070A-1 and 25B1471 in No. 6 entry. The continuous miner and roof drill were operated with 3-phase 550 volts a.c., and 250 volts d.c. was supplied the shuttle car by a 150 kw. rectifier in the 600-kv.-a., a.c.-d.c. power center located approximately 400 feet outby the face of No. 7 entry. Short-circuit protection for trailing cables on the face equipment was by circuit breakers in the 13.200/550-volt power center. Ground fault protection was provided for trailing cables on the &.c. equipment.

A thorough examination of this equipment during the investigation disclosed no defects in permissibility or defective trailing cables. The continuous miner and roof drill were effectively frame-grounded through type G trailing cables and the shuttle-car frame was grounded by properly installed diodes. Tests for stray current between the frames of the continuous miner and shuttle car in No. 7 entry and between the equipment frames and earth disclosed a maximum of 7 volts and 4.5 milliamperes with the equipment in operation. Lesser values were obtained when the machines were not being operated. Results of studies made of intrinsic safety of circuits and equipment show that the measured value of current is considerably less than the minimum value of igniting current for methane.

Further examinations revealed that the continuous-miner control switches, with the exception of the conveyor switch, were in the on or running position. The water valve for the spray system was closed. Soot and dust deposits on the controls indicated that they had not been disturbed in the interim following the explosion and the time that they were examined.

Approximately 20 percent of the tungsten-carbide tipped bits in the drums and ripperveyor chain the continuous-mining machine were worn о а point where only about 50 percent of their original cutting length remained.

During the inspection completed in December 1969, tests for methane were made frequently while electrical equipment was being operated in the face areas and just prior to the time such equipment was taken inby the last open crosscuts. Tests were made with permissible flame safety lamps and, according to testimony given during the investigation, these were generally made with walking flames. After the explosion, classes in the safe use and care of permissible flame safety lamps were begun at the mine by the Bureau of Mines. Instructions were also given in the maintenance and use of electrical methane detectors.

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