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IC 8847 
 
 Bureau of Mines Information Circular/1981 
 
 ^^1/MlI 
 
 Control of Methane by Ventilation 
 of Shafts During Raise Drilling 
 
 By Slavoljub D. Maksimovic 
 
 ^Hi^ 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
imhi^Mh^' fiuM&Aj^ i%au&, 
 
 Information Circular, 8847 
 
 Control of Methane by Ventilation 
 of Shafts During Raise Drilling 
 
 By Slavoljub D. Maksimovic 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 James G. Watt, Secretary 
 
 BUREAU OF MINES 
 
!£> 
 
 2i& 
 
 This publication has been cataloged as follows: 
 
 Maksimovic, S D 
 
 Control of methane by ventilation of shafts during raise drilling. 
 
 (Information circular » Bureau of Mines ; 8847) 
 Includes bibliographical references. 
 Supt. of Docs. I 28.27:8847. 
 
 1. Coal mines and mining— Safety measures. 2. Mines ventilation. 
 3. Methane. 4. Raise drilling— Safety measures. I. United States. Bureau 
 of Mines. II. Title. III. Series: United States. Bureau of Mines. Informa* 
 tion circular ; 8847. 
 
 TN295.U4 [1] 622s [622'. 8] 80-606851 
 
 AACR1 
 
CONTENTS 
 
 Page 
 
 Abstract 1 
 
 Introduction 1 
 
 Pilot hole drilling 2 
 
 Raise drill shafts 2 
 
 Ventilation of raise drill shafts 2 
 
 Methane control during raise drilling 8 
 
 Discussion 9 
 
 Conclusion and recommendations 10 
 
 ILLUSTRATIONS 
 
 1. General location plan during drilling of two raise shafts 4 
 
 2. Ventilation system at bottom of raise shaft.. 4 
 
 3. Alternate use of air compressors and vacuum pump for shaft 
 
 ventilation 5 
 
 4. Simultaneous use of air compressor and vacuum pump 5 
 
 5. Simultaneous use of air compressor and auxiliary blower fan 6 
 
 6. Use of vacuum pump for shaft ventilation 6 
 
 7 . Use of free intake air for shaft ventilation 7 
 
 8. Temporary use of auxiliary blower fan at shaft bottom 7 
 
CONTROL OF METHANE BY VENTILATION OF SHAFTS 
 DURING RAISE DRILLING 
 
 by 
 
 Slavoljub D. Maksimovic 
 
 ABSTRACT 
 
 At some coal mines, methane gas is released during raise drilling. At 
 the request of the Mine Safety and Health Administration (MSHA) , U.S. Depart- 
 ment of Labor, the Bureau of Mines conducted ventilation surveys at different 
 raise drill sites to assess various methods of reducing methane concentration. 
 The resulting data indicate that raise drill shaft cavities are ventilated by 
 air compressors, vacuum pumps, auxiliary blower fans, and free intake air from 
 the surface. The compressors, pumps, and fans operate simultaneously or sep- 
 arately. At the bottom of the shaft cavities, ventilation is controlled by 
 regulators and airlocks. Methane is monitored, measured, and periodically 
 sampled on the surface, at the bottom, and at different levels in the shaft 
 cavities. Maximum concentration of 3.5 percent was measured in one shaft in 
 West Virginia. Air velocity down the raise drill shafts is always very low. 
 Maximum recorded air down the drill stem was 3,000 cfm, which can be increased 
 by adding air compressors. The best way to ventilate shaft cavities is 
 through the simultaneous use of air compressors and vacuum pumps. If the 
 methane concentration cannot be reduced with additional compressors, then the 
 inside diameter of the drill rod should be increased, or larger pilot hole 
 drill bits should be used. 
 
 INTRODUCTION 
 
 In raise drilling, a small-diameter pilot hole is drilled from a higher 
 (surface or underground) elevation to a lower accessible underground eleva- 
 tion. A large cutter assembly is attached to the bottom of the drill rod and 
 enlarges the hole while reaming upward. 
 
 Raise drilling in mining has grown rapidly since its introduction in 
 1957. The mining industry has taken full advantage of the method only 
 recently when the raise drill shaft diameters exceeded 20 feet. Raise drill 
 shafts are used to improve ventilation systems and provide better access to 
 
 Mining engineer, Pittsburgh Research Center, Bureau of Mines, Pittsburgh, Pa. 
 Christensen, R. Raise Boring Experience at Sunnyside Coal Mines. Pres. at 
 
 1971 Coal Conv. , Am. Min. Cong., Pittsburgh, Pa., May 16-18, 1971, 9 pp. 
 
 American Mining Congress, Washington, D.C. 
 
working areas by locating air shafts and miner portals at strategic points. 3 
 This technique offers the possibility of a cheaper, faster, and safer way to 
 build ventilation shafts, and was first applied for this purpose in the coal 
 mining industry about 10 years ago. However, it was found that at some coal 
 mines methane gas is released during raise drilling. This has generated some 
 concern over proper ventilation at raise drill cavities. At the request of 
 the Mine Safety and Health Administration (MSHA) , U.S. Department of Labor, in 
 1977, the Bureau of Mines collected ventilation data at different drill sites, 
 from equipment manufacturers, and from drilling contractors. The results of 
 this survey are presented in this report. 
 
 PILOT HOLE DRILLING 
 
 Pilot holes are drilled from the surface or underground down to lower 
 elevations in advance of raise-drilling operations, employing the same drill- 
 ing machine as for reaming up the shafts. Cuttings are removed from the bot- 
 tom of the holes by compressed air or water. One or more air compressors 
 hooked to a single manifold can be used to supply compressed air. However, if 
 water is encountered during drilling, air cannot lift the cuttings from the 
 bottom of the hole, and water must be used for this purpose. 
 
 The holes range in depth from 50 feet in Indiana to 2,298 feet in New 
 York, and the pilot hole diameters range from 7-7/8 to 13-7/8 inches. 
 
 RAISE DRILL SHAFTS 
 
 For reaming up the raise drill shafts, drilling manufacturers have intro- 
 duced specially designed drilling machines that are ruggedly built to suit the 
 tough restrictive conditions, with large torque and variable speed to suit 
 specific rock conditions. Cutter assemblies used for upreaming can be single 
 or multistage, and can be assembled in stages of increased diameter from the 
 top to the bottom. Drill stems are regularly in 4- and 5-foot sections; 
 larger diameter sections are 11-1/2 feet long. Drill stem outside diameters 
 range from 5-3/4 to 15 inches and minimum inside diameters from 2-11/16 to 
 5-7/16 inches. The shaft diameters range from 3 to 20.25 feet and the depths 
 from 50 to 2,298 feet. 
 
 VENTILATION OF RAISE DRILL SHAFTS 
 
 Raise drill shafts are ventilated by air compressors, vacuum pumps, and 
 auxiliary blower fans operating simultaneously or separately. In some cases, 
 for coal seams wtih little gas, the free intake air from the surface is used. 
 One or more compressors are used depending on the amount of air required. 
 Rated compressor capacities were 600 cfm. Rated capacities at vacuum pumps 
 ranged from 620 to 1,000 cfm. 
 
 ^Wright, W. L. Kaiser's Sunnyside Mines Go All Out for Raise Boring. Coal 
 
 Age, v. 76, No. 1, January 1971, pp. 61-65. 
 ^Davis, H. Raising High Volume Airshafts. Ch. in Coal Age Operating Handbook 
 
 of Underground Mining. McGraw-Hill, Inc., New York, v. 1, 1977, pp. 122- 
 
 125. 
 
Ventilation at the bottom of the raise shafts is controlled by regulators 
 used in normal mine ventilation systems. A general location plan with venti- 
 lation systems during the drilling of two raise shafts is shown in figure 1. 
 Figure 2 shows a typical plan view at the bottom of the raise shaft with ven- 
 tilation systems. Airlocks at the bottom of the raise shafts insure that nor- 
 mal mine ventilation is not interrupted during the breakthrough. 
 
 The survey showed that raise drill cavities can be ventilated in several 
 ways, some of which are not standard practice. 
 
 Alternate Use of Air Compressors and a Vacuum Pump (fig. 3). — This method 
 is generally used in gassy coal seams and gassy overburden. The air compres- 
 sor and vacuum pump are located on the surface near the drilling machine. 
 During hole upreaming, compressed air is forced down the drill stem to the 
 cutterhead area. Air sweeps the cutting surface and moves down the completed 
 portion of the shaft to the mine return entries. The amount of air through 
 the drill stem ranges from 600 to 3,000 cfm, depending on the number of com- 
 pressors used. Air compressors operate during the upreaming and during all 
 idle periods of more than 15 minutes. 
 
 Every 2 to 4 hours, air compressors are shut down, and a vacuum pump 
 operates to prevent downward movement of methane-air mixtures and to bring 
 these mixtures to the surface to make methane concentration measurements. The 
 pump operates for 5 to 30 minutes drawing methane-air mixtures from the cut- 
 terhead area up through the drill stem. A methane-monitoring system installed 
 on the vacuum pump discharge monitors and records methane concentrations while 
 reaming continues. Rated vacuum pump capacity is 1,000 cfm. 
 
 Simultaneous Use of Air Compressors and a Vacuum Pump (fig. 4). — This 
 method is used in mines with an unusual amount of methane. Compressed air is 
 forced down the drill stem to the cutterhead area during the cutting. Some of 
 the air is moved down through the completed portion of the shaft to the mine 
 return entries. Simultaneously, a vacuum pump draws air from the cutterhead 
 area up through the annulus between the drill stem and pilot hole wall. Com- 
 pressed air is also forced down during idle periods of more than 15 minutes to 
 prevent methane accumulations in the shaft cavities. 
 
 A me thane -monitoring system located on the pump discharge monitors and 
 records methane concentrations during the raise drilling. The amount of com- 
 pressed air ranges from 600 to 3,000 cfm. Rated vacuum pump capacity is 
 620 cfm. The amount of air at the bottom of the shaft ranges from 9,000 to 
 13,000 cfm. 
 
 The simultaneous use of air compressors and a vacuum pump allows a con- 
 tinuous downward movement of the methane-air mixture in the shaft, and the 
 methane concentration is monitored continuously at the vacuum pump discharge 
 while cutting advances continuously upward. The disadvantage of this method 
 is the leakage of surface air into the vacuum pump, which requires the instal- 
 lation of a blooie seal (stuffing box) around the drill stem on the surface. 
 
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 Compressed air 
 600- 3,000 cfm 
 100- 120 psi 
 
 To vacuum pump 
 1,000 cfm 
 
 Compressed air 
 600-3,000 cfm 
 
 To vacuum pump 
 620 cfm 
 
 9,000 to 13,000 cfm 
 
 FIGURE 3. - Alternate use of air compres- 
 sors and vacuum pump for 
 shaft ventilation. 
 
 9,000 to 13,000 cfm 
 
 FIGURE 4. - Simultaneous use of air com- 
 pressor and vacuum pump. 
 
 Simultaneous Use of Air Compressors and Auxiliary Blower Fans (fig» 5). — 
 This method is used in some gassy mines in Utah. Compressed air is forced 
 down the drill stem during upward reaming. The air flushes the cutterhead 
 area and moves down the completed portion of the shaft cavity to the mine 
 return entries. One or more compressors are used. If more air is required 
 and the compressors cannot supply it, an auxiliary blower fan on the surface 
 is used to force air down the pilot hole between the outside of the 10-inch- 
 diameter drill stem and the 11-inch-diamter pilot hole. Other auxiliary fans 
 provide additional air which, together with the compressed air, successfully 
 dilutes methane to a safe level. 
 
 ^ork cited in footnote 2. 
 
Compressed air 
 
 From auxiliary fan 
 
 KEY 
 
 - Intake air 
 *• Return air 
 
 s ////////, /\ 
 
 To vacuum pump 
 620 cfm 
 
 -* < 
 
 FIGURE 5. - Simultaneous use of air compres- 
 sor and auxiliary blower fan. 
 
 15,000 cfm 
 
 FIGURE 6. - Use of vacuum pump for shaft 
 ventilation. 
 
 Disadvantages of the simultaneous use of air compressors and auxiliary 
 blower fans are additional expense and the fact that methane concentrations 
 cannot be monitored on the surface as in the previous case. 
 
 Use of a Vacuum Pump (fig. 6). — This method is used in coal seams with 
 low gas. A vacuum pump located on the surface operates continuously during 
 upward reaming, drawing up the methane-air mixture from the cutterhead area 
 through the drill stem. A methane-monitoring system, installed at the pump 
 outlet, is set to flash a light when methane in the pump effluent exceeds some 
 preset value. Rated pump capacity is 620 cfm. Free intake air from the sur- 
 face moves down the annulus and amounts to a few hundred cubic feet per min- 
 ute. The amount of air in the underground mine return entries at the bottom 
 of the shaft cavity is about 15,000 cfm. 
 
The advantage of using a vacuum pump for shaft ventilation is that air 
 compressors are not required. The disadvantages are (1) the methane concen- 
 tration cannot be measured at the bottom of the shaft because of the low rate 
 of free intake air from the surface, (2) considerable dust buildup inside the 
 drill pipe gets on the tool joints when the drill pipe sections are removed, 
 and (3) air samples are not pulled from the face area since inlets may be 8 to 
 10 feet below the upper cutters. 
 
 Use of Free Intake Air (fig. 7). — This method is used in western Ken- 
 tucky, Indiana, and New Mexico where the coal mines are not deep (50 to 750 
 feet) and/or the coal is not very gassy. Free intake air from the surface is 
 moved down the annulus and the completed portion of the shaft. At the shaft 
 bottom, this air is mixed with 15,000 to 30,000 cfm of air from the mine 
 
 Free intake air 
 
 Free intake air 
 
 W/^'« 
 
 vximV', 
 
 p^^^^ 
 
 KEY 
 
 ■*- Intake air 
 -*- Return air 
 
 15,000 to 30,000 cfm 
 
 FIGURE 7. - Use of free intake air for shaft 
 ventilation. 
 
 FIGURE 8. - Temporary use of auxiliary 
 blower fan at shaft bottom. 
 
ventilation system (which is under negative pressure). The amount of intake 
 air from the surface does not exceed a few hundred cubic feet per minute. 
 
 Normally, methane concentrations in the coal seams where free intake air 
 is used for ventilation are low and are not measured during raise drilling. 
 On the surface, a compressor is on standby in case the concentrations exceed 
 the safe level. In some coal seams where this method is used, methane is 
 found only in small pockets in a coal seam or overburden. 
 
 Temporary Use of the Auxiliary Blower Fan at the Bottom of the Shaft . 
 (fig. 8). — This method is applied in Utah during reaming of the first 35 feet 
 of the raise shaft when methane concentrations exceed permissible levels. To 
 dilute methane to a safe level, Neoprene tubing 36 inches in diameter and 
 auxiliary blower fans are used. Tubing is attached to a swivel on the bottom 
 of the cutterhead and air is blown up the tube. This is more than adequate 
 to dilute methane at the cutterhead. Later, when the cutter is changed, holes 
 are drilled into the stinger intersecting the hollow center portion. This 
 allows free air movement from the air compressor and auxiliary blower fan down 
 the completed portion of the shaft to the mine return entries, as described 
 earlier in the discussion of air compressors and auxiliary blower fans used 
 simultaneously (fig. 5). 
 
 METHANE CONTROL DURING RAISE DRILLING 
 
 Methane concentration during drilling of raise shafts varied from site to 
 site. A maximum concentration of 3.5 percent was measured in one raise drill 
 shaft in West Virginia. Concentration during the idle periods at some raise 
 drill sites decreased from 0.50 to 0.25 percent; at others, it increased from 
 0.50 to 1.10 percent. In coal seams with little or no gas, methane concentra- 
 tions were measured only periodically during raise drilling because they did 
 not constitute any hazard. Concentrations were not measured during the drill- 
 ing of pilot holes. 
 
 Methane from the raise drill shaft at gassy mines was monitored, measured 
 and sampled periodically on the surface, at the bottom of the shaft, and at 
 different levels in the shaft. On the surface at gassy mines, a methane- 
 monitoring system was installed at the vacuum pump discharge. It monitored 
 and recorded methane-air mixtures drawn up from the cutterhead area. The 
 system is normally set at 1.0 percent to sound an alarm, and drilling is shut 
 off if the concentration reaches 1.50 percent. Normally, if the concentration 
 is too high, the cutter continues to rotate after reaming is stopped and the 
 cutter has been lowered at least 1 foot. This continues until the methane 
 concentration is reduced to less than 1.0 percent. 
 
 If still higher concentrations are measured, rotation is stopped, and 
 methane is diluted with compressed air and removed through the mine return 
 entries. Simultaneously or separately, a vacuum pump operated on the surface 
 
 Reference to specific trade names does not imply endorsement by the Bureau of 
 
 Mines. 
 Work cited in footnote 2. 
 
draws up methane-air mixtures for sampling; methanometer readings are made 
 every 2 to 4 hours. The pump operates 5 to 30 minutes before methane readings 
 are taken, and while it operates, air compressors are shut off. The results 
 are checked periodically with bottle samples. 
 
 Underground, at the bottom of the shaft cavity and in the mine return 
 entries, methane is monitored and measured regularly at some drill sites. At 
 one location in Alabama, a monitoring system with readout tape was installed 
 at the bottom of the shaft to record the concentration during upward ream- 
 ing. ° Readings were made every 15 to 20 minutes, or every 2 to 4 hours, 
 depending on the methane concentration at the shaft bottom. Bottle samples 
 were taken periodically to check the methane concentration. 
 
 In West Virginia, MSHA has attempted to measure methane concentration in 
 the shaft below the cutterhead as drilling advanced upward. Telescoping 
 probes and inflatable balloons are used for this purpose. A telescoping 
 probe, 8 to 25 feet long, is used at the bottom of the shaft cavities. A 
 methanometer is clamped to the probe by a support bracket, and readings are 
 made periodically. Inflatable balloons 5.5 feet in diameter, filled with 
 helium, are used to take gas samples from different levels in the shaft. 
 Flexible surgical tubing, attached to the balloon, is used to draw the 
 methane-air mixture down to the bottom of the shaft. The balloon can be used 
 only to about 160 feet above the floor level because the weight of the tubing 
 prevents it from rising higher. 
 
 Prior to the beginning of raise drilling, after an idle period of 15 min- 
 utes or more, a methane reading is made at the bottom of the shaft and on the 
 surface. If the methane concentration is 1.0 percent or more, methane is 
 diluted to a safe operating level. A preshift examination is normally made 
 and the concentration recorded on the surface and underground at the shaft 
 bottom. 
 
 DISCUSSION 
 
 Air velocity down raise drill shafts is always very low. If the raise 
 shaft has a 15-1/4-foot diameter, the cross-sectional area is 183 ft 2 . If 
 3,000 cfm of air is blown down the drill stem, the air velocity in the shaft 
 cavity is only 16.4 fpm. 
 
 The raise drill stem can carry more than 3,000 cfm of air, the highest 
 amount recorded during this study. From charts in the Chemical Engineer's 
 Handbook, it was calculated that airflow through a 5-inch-ID clean pipe, 
 
 °Carroll, W. Going Down Deep for Coal in Dixie. MESA, The Magazine of Min- 
 ing, Health and Safety, v. 2, No. 5, October-November 1977, pp. 2-5. 
 Available for consultation at the Bureau of Mines library, Pittsburgh, Pa. 
 
 9 Drew, T. B., H. H. Dunkle, and R. P. Genereaux. Flow of Fluid. Sec. in 
 Chemical Engineer's Handbook, ed. by J. H. Perry. McGraw-Hill, Inc., New 
 York, 3d ed. , 1950, pp. 360-460. 
 
10 
 
 1,000 feet long with 150 psi at one end, is 14,800 cfm, which indicates that 
 3,000 cfm could easily be boosted to 6,000 cfm or more by adding more air 
 compressors. 
 
 The annular space between the raise drill stem and the borehole wall can- 
 not be used to move much air. For a typical case of a 10-inch-OD drill stem 
 and 12-1/4-inch-diameter pilot hole, the cross-sectional area of the annulus 
 is 39.30 in 2 . However, because of the shape, the friction to airflow is much 
 greater than in a round pipe of the same area. 
 
 Mine ventilation air passing under the shaft cavities creates an air 
 swirl at the bottom of the shaft, making precise methane concentration mea- 
 surements difficult. Since the raise shaft air from the compressor is moving 
 downward at a velocity of 3 to 16 fpm, and since the mine ventilation air is 
 moving at a much higher velocity (200 fpm or higher), the swirl of mine venti- 
 lation air could easily extend upward 30 feet. If a telescoping probe is used 
 at the floor level of the shaft cavity, the measured concentration will be 
 that of a mixture of shaft air and mine ventilation air in varying proportion. 
 This was verified by methanometer readings and bottle samples. The results 
 showed higher methane concentrations at the vacuum pump discharge than at the 
 bottom of the shaft cavity. 
 
 Using inflatable balloons for gas sampling in the shaft at different lev- 
 els is not always practical because of the limited lifting capacity and other 
 operational difficulties. 
 
 The air velocity down the completed portion of the shaft is very low. 
 Any increase in methane at the cutterhead level will be registered at the bot- 
 tom of the shaft only after a long delay. 
 
 CONCLUSION AND RECOMMENDATIONS 
 
 The recommended way to ventilate raise drill shaft cavities is through 
 the use of air compressors and vacuum pumps operating simultaneously, blowing 
 air down the drill stem and drawing methane-air mixtures up the annulus for 
 surface measurements. If the methane concentration is too high, more air com- 
 pressors can be added; however, adding more compressors to increase the air- 
 flow can reach the point of diminishing returns because of the shock losses 
 inside the drill pipe and fittings and especially at the rotary swivel on top 
 of the machine. These losses and others occurring at the discharge holes in 
 the cutterhead may result in back pressure sufficient to explode the air tube 
 seals in the machine. 
 
 When a compressor and a vacuum pump are used simultaneously, the compres- 
 sor air volume should be much greater to maintain a downward movement of air 
 in the raise shaft cavities. 
 
 Simultaneous use of air compressors and a vacuum pump requires the use of 
 a blooie seal installation around the drill stem on the surface to prevent air 
 leakage into the pump. 
 
11 
 
 If the methane concentration is not diluted with additional compressors, 
 some other solutions should be found. Possible solutions are a larger minimum 
 inside diameter for the drill rod, or the use of larger pilot hole drill bits. 
 Increasing ventilation through the drill string by increasing the inside diam- 
 eter of the drill pipe will result in increasing the outside diameter, which 
 will require drilling a larger pilot hole. These changes require more expen- 
 sive machines and drill pipes. Increasing the pilot hole diameter may not 
 increase the airflow appreciably in the annulus because there must be several 
 full gage stabilizers immediately above the cutterhead to provide stability 
 for the head. 
 
 More research is needed on raise-drilling ventilation to establish the 
 most practical methods for implementation when it is required. It has yet to 
 be shown that the methods discussed here are practical and effective under a 
 wide variety of circumstances. 
 
 ftll.S. GOVERNMENT PRINTING OFFICE: 1981-703-002/47 
 
 INT.-BU.OF MINES, PGH., PA. 25395 
 
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