November 19, 1915. THE COLLIERY GUARDIAN. 1041 AMERICAN COAL DUST EXPERIMENTS.* NEW TYPE OF ROCK DUST BARRIER. By G. S. Rice and L. M. Jones. In addition to the rock dust barriers described in the Colliery Guardian of November 27, 1914, pp. 1121-23, the United States Bureau of Mines has had experiments carried on, at the experimental mine, Bruceton (Pa.), with a rock dust protected ventilating door, a rock dust protected overcast, and a new trough rock dust barrier. Rock Dust Protected Ventilating Door. Inasmuch as ventilating doors are usually placed at the entrance to butt and side entries, points at which it is particularly important to have some means to insure limiting the explosion area, it was thought that advantage could be taken of the well-known vulnerability to wreckage of such doors, to design a device for pro- ducing a thick cloud of rock dust to check any explosion that might start in spite of other protection. The rock dust protected ventilating door consists of a mine door of the usual type. Around the frame of the door, at the sides and overhead, are constructed dust compartments (without the use of spikes or bolts) in such a manner that the walls and overhead comparment floor are held in position by the door frame. The frame is strongly constructed, and its displacement as a whole by an explosion causes the collapse of the compartments and the consequent projection of the rock dust contents into the entry. The floor boards of the upper compart- ments are hung by chains secured to the roof, or roof timbers. The chains allow the boards only a limited fall to different heights, so that the boards delay the fall of some of the dust, and also retain some of the load, which is then available to quench a delayed flame or a d 3 ! 22 8 74 k Fig. 1.—Rock-Dust-Protected Ventilating Door. secondary explosion. This arrangement is primarily intended for a light explosion, which is the most difficult kind to check; as the pressures are low it is necessary' to have a device that will be sensitive to a blast of air. An explosion having a strong pressure offers little difficulty, as then the whole framework and the com- partments are demolished, and the mass of rock dust is launched into the air, quenching the explosion. Fig. 1 shows a plan view of the device, indicating the relation of the side compartments to the door frame, a front elevation of the device, and a vertical section along the line a—b. If the roof is low, it may be neces- sary to take down some of it. The excavation should be tapered off gradually in either direction. Gross bars, 5, are set near the roof upon posts in grooves in the rib. Beneath them, posts, 6, are set up and firmly wedged or nailed to bars 5. The side planks 8 of the side dust compartments are then temporarily nailed during construction to the outside of the posts. The door frame is set up between the planks of the two side compartments, the cross bar 3 being of sufficient length to force or spring in the planks a little, so that they will be held firmly in position when the rock dust presses against them. After the frame is in place the temporary nails in the planks 8 are with- drawn. Braces 14 are placed against the side of the frame on the opposite side to that on which the door will be hung, so that the frame will be enabled to withstand the shocks from the door closing violently, in ordinary mine work, without dislodging the frame. The door is hung by the usual type of hinges, 2, in such a way as * From United States Bureau of Mines Technical Paper No. 84. to be self-closing. The short planks, 7, are set in place on the inside of the posts at the ends of the side com- partments, and are held in position by the dust loading. Crossbars, 10, are then placed on cleats, 9, attached to the posts, 6. The crossbar on the door side of the frame is placed higher than the other, 10, as the free end of a self-closing door needs additional height when open. Short planks, 12, are placed from the crossbar, 3, to the crossbars, 10. End planks, 13, are placed from rib to rib to enclose the ends of the upper compartments. In testing this arrangement in explosions, it was found desirable to fasten the floor planks of the upper dust compartment by chains to the roof, allowing them only a limited drop, adjacent boards dropping different dis- tances. It was also found that the ends of the planks 8 tended to bulge outward when the side compartments were filled with dust. To prevent the spilling of dust through the crack, a 2 in. by 4 in. scantling was nailed near the ends of the planks 8 to fit just inside of planks 7. When the ends of the planks 8 bulge outward, the scantling prevents the spilling of the dust by making a sliding joint. If the explosion advances from the side on which the door is hung, the door is forced tightly against the frame, and even a light pressure over such a large surface will quickly dislodge the frame, unless it is well braced to prevent its being too sensitive. If the explosion advances from the opposite side, the door may blow open, and so not receive the full force of the explosion wave; but as the frame is not braced in that direction, it is dislodged about as easily as from the other direction. It has been found in the tests already made that the frame is dislodged without delay, irrespective from which side the explosion comes. However, it is desirable to make the door frame proper relatively deep and wide to present considerable surface to an explosion, to make sure that it will be blown out, and thus release the rock dust com- partments as described. The effect of an explosion on such a rock dust pro- tected ventilating door in a mine entry, was to blow out the door frame entirely, and scatter the plank of the side compartments into the entry. The explosion was sufficiently strong to break some of the chains and pull down some of the roof in which chain hooks were fastened. The device successfully stopped the explosion. Rock Dust Protected Overcast. In mine explosions, overcasts are almost always dis- rupted or demolished owing to the large extent of surface on which the explosive force can act. For example, if the pressure of the explosion is only 20 lb. per sq. in.—a relatively light pressure—the total lifting pressure on the floor of an ordinary 9 ft. by 12 ft. over- A. ELEVATION OF TROUGH BARRIER WITH SIDE FRAME REMOVED, AND END VIEW 3 LONGITUDINAL ELEVATION OF TROUGH BARRIER. Fig. 3. cast would be 130 tons and about 100 tons pressure on either side. As a result of their destruction, the explo- sion frequently passes to other parts of the mine. It was thought, therefore, that advantage could be taken of the vulnerability of the overcasts to produce a dense cloud of rock dust to extinguish an explosion at such points. The rock dust protected overcast is designed to give such an effect. The device consists of an overcast of any type of con- struction, the floor and sides of which are protected by rock dust compartments. Fig. 3 represents a longi- tudinal section of an overcast, showing the arrangement of dust compartments, A transverse section, and a plan view indicating the arrangement of the device in the mine, as well as certain details of construction are also shown. In A, fig. 2, 1 indicates a main entry, and 2 an air- way, which are separated by the overcast. The side walls are made double, with a space between them 2 to 3ft. wide, which is filled with rock dust; the outer sides may be planned like the rock dust stopping described above. The walls may be made in any suit- able manner, the framing in the diagram showing one method of construction. The framing is preferably made light, so that an explosion will readily disrupt the walls and permit the rock dust to be projected into the entries. In like manner, the floor of the overcast has a compartment 4, built above it with angle dust com- partments 9, connecting it with the side compartments. All of the hollow space is filled with rock dust. If the overcast is built in a place where water is dripping from the roof, it is desirable to place some waterproof cover- ing over the top of the dust compartments in order that the dust will not become wet. No piles of “ slate ” or gob should be permitted to be placed against the side walls of the side compartments, as such piles tend to Fig. 2.—Rock-Dust-Protected Overcast. prevent the most efficient operation of the barrier in the event of an explosion. If the overcasts used in a given mine have hitherto been of the type in which steel beams are used, or if a solid concrete floor has been employed, it is preferable to modify the construction by making the floor system so that it is less rigidly tied together. An excellent form of construction is to use iron beams with small slabs, bricks, or boards over or between the beams, in order that the overcast shall offer less resistance to the disruptive force of the explosion, and thus assist in throwing the rock dust into general suspension to quench the flame. There has not yet been opportunity of testing this type of overcast in an experimental explo- sion, but in view of the success attending the tests of the other kinds of rock dust barriers it is thought by the engineers of the Bureau that the device will operate successfully. Trough Rock Dust Barrier. Another type of enclosed rock dust barrier has recently been designed by engineers of the Bureau. This “ trough rock dust barrier,” was tested at the experi- mental mine, and successfully prevented the passage both of strong explosions of coal dust and of feeble ones, and also of an explosion of dust in air which contained a small percentage of inflammable gas. The essential features of the barrier (fig. 3) are six fixed troughs placed about 6 ft. apart, across the entry and close to the roof, each of which contains longitudinal bars, 1, 2, and 3, or grids which separate the mass of rock dust filling the trough, and, in case of the barrier being dumped, break up the mass of dust and cause it to cascade in falling; a drop shelf 4, which is held up by chains after falling about 6 in.; hinged bottom boards 5, each of which is held up along one edge by a wood strip 12, which acts as a hinge, and by supporting level's 6, and trip levers 7, operated by strong wires 8, from