yield approximately 1 gal of water per ft of cross section or 0.01 gal/ft3 of mine volume between rows.

The unmodified barriers should be suspended by their rims on a metal frame made of 3/4- to 1-1/2-inch square or rectangular tubing. The empty barriers can be placed in their frame on the mine floor and the total assembly lifted into place at the roof. The frame can be fastened to hooks or rods extending from the rib and/or roof. After the tubs are filled with water, cover plates furnished with the tubs should be installed. A similar metal frame should be assembled for the modified barrier; however, in this case, the frame can be attached to the rib and roof first, followed by installation of the tubs, filling them with water and covering them with lids. As the section advances, new rows of barriers should be erected, maintaining the change in the barrier types in alternate rows, and the barrier rows furtherest outby can be removed, first draining the water by aspiration techniques.

SUMMARY AND CONCLUSIONS

Three experimental water barriers, designed especially for the suppression of slow-moving coal-dust explosions, were developed and tested by the Bureau of Mines. One barrier is a modification of a West German barrier and responds to dynamic pressures generated ahead of an explosion to tilt the tub and release its water for the suppression of the oncoming explosion. The second and third barriers operate in response to an increase in static pressure, also developed ahead of the explosion. Tests indicate the first barrier begins to release its water at air speeds as low at 50 ft/sec and the second and third barriers will operate at a rise in static pressure as little as 0.5 psi.

The barriers were found to be effective in stopping coal dust explosions propagating at speeds as low as 100 ft/sec. One barrier (180 lb of water) was sufficient to suppress explosions in a single entry with an average cross section of 55 ft. Results of the tests indicated that the minimum distance between the barrier and explosion initiator should be of the order of 75 ft. It appeared that the barrier system's efficiency to suppress explosions remained high when as little as 50 pct of the water of a single barrier (90 lb) is spilled prior to the flame arrival. Its effectiveness is diminished, however, when the barrier is placed large distances (>300 ft) from the explo

sion initiation.

Based on our present experience and knowledge gained from researchers abroad, a plan for installing a water barrier system in a working coal mine for the protection of a beltway is described.

APPENDIX.--RESPONSE OF MODIFIED BARRIER

Response of the barrier to a dynamic pressure force is described, and accidental tipping of the barrier by a static and impulse force is analyzed. To aid in the analyses, barrier dimensions are given in figure A-1, and a side view sketch of the barrier during motion resulting from gravity and dynamic pressure is shown in figure A-2, where A is the faceplate; A1, the faceplate lip; B, the rear tub face; B1, the rear tub lip; C, the hinge; and D1 and Da, the support bars. The two methods of supporting the barrier are considered

FIGURE A-1. Dimensions of modified water barrier. A, Front view; B, side view.

FIGURE A-2.- Side-view sketch of modified water barrier showing motion and force vectors.

=

1.0) between A, and D.; = 0.4 (small CT. The and 1/8 inch. Thickness

in the analyses: (1) A neoprene tape is attached to the underside of A1, resulting in a large coefficient of friction (Cr and (2) A rests directly on D2, resulting in C treatment is given for two thicknesses of A, 1/4 of A, remains fixed at 1/4 inch. Α1

The analyses is begun by first determining the torque on the tub due to gravity and the torque on A due to the dynamic pressure force. Referring to figure A-2, the dynamic pressure forces act to swing A in an upward arc about C. Simultaneously, gravity forces on the tub act to swing C downwards in an arc about its pivot at B. Gravity forces on the water tub to result in a torque (L) about B1 as follows:

where I is the moment of inertia of the tub about B (about 210 lb-ft), t is the time in seconds, С is the horizontal distance from B1 to the tub center of gravity (10 inches) m is the tub mass (190 lb when filled with water), and g is the gravity force. The acceleration of C about B1 is there fore

(A-1)

(A-2)

(A-3)

The wind forces on A result in a torque (L2) around the pivot C as follows:

where I is the moment of inertía of (A+A1) about C (2.6 and 1.9 lb-ft3), C2 is the horizontal distance from C to the center of gravity of (A+A1) (2 and 2.1 inches), and m2 is the mass of (A+A,) (6 and 3.6 lb). Values in parentheses correspond to A equal to 174 and 1/8 inch thick, respectively. Co is the drag coefficient assumed to be 1.0, A1 is the vertically projected area of A (352 in), p is the air density (0.081 lb/ft3), v is the wind velocity in feet per second, and h is the vertically projected height of A (11 inches). The acceleration of A about C is therefore

The preceding treatment of wind forces against A, is based on the supposition that C is spatially fixed whereas, owing to gravity forces on the tub, C does move concurrently with the motion of A. However, the assumption is justified since the swinging motion of C is nearly parallel with A and therefore has little effect on the angle y.

Barrier Response With Neoprene Strip (Large C、)

When d2 E,/dt>d S, /dt, the force of the barrier holding A down on D2 is nullified and A will lift, separating entirely from D2 as it swings in an arc about C. This mode of barrier response would be expected to occur when the coefficient of friction (C) between A1 and D2 is large and fits the case of the modified barrier (with neoprene strip attached to A1) for C2 = 1.0. Calculations indicate that A will lift at a wind speed of 88 and 75 ft/sec

for

faceplate A thickness of 1/4 and 1/8 inch, respectively. As A1 moves, angle B (fig. A-2) increases and d2E,/dt decreases, which would indicate a need for an increase in air speed for A1 to remain free of D2. However, there is a corresponding decrease in ds, /dt and C2, and calculations show that the wind speed should increase about 8 pct over the previous calculated values near the end of A's swing.

Time (t) for A1 to move past D2 to allow the tub to fall free can be estimated by integrating equation A-4:

where x = 0.8 inch is the distance that A must swing to be free of D2• Assuming an average value for C2 and a constant wind speed of 90 and 77 ft/sec for A equal to 1/4 and 1/8 inch thick, respectively, yields times of 33 and 27 msec, respectively.

Barrier Response Without Neoprene Strip (Small C, )

The barrier will also respond to lower air speeds during which

d2 E1/dt2 <d2 S1/dt. This is the case for the modified barrier without neoprene strip, where C is small (0.4) and A1 slides along D2. However, d2 E/dt is still effective in reducing the force of the barrier holding A to D2 and thereby lessening the required air force to set A, in motion. Referring to the vector diagram of figure A-2 for the analyses, f is depicted as the barrier force holding A, to D2 with its vertical and horizontal components shown as f, and fh, respectively; f, forces A1 down on D2, whereas facts to slide A off D2. The vertical acceleration component of the wind on A

(d3E, /dt) reduces the barrier force holding A down on as follows:

down on D2 from f1, to f1

and the equation that then governs the conditions for the barrier to respond to a dynamic pressure force is

When the tub was filled with 180 lb of water, f, was measured to be 90 lb for the 1/4-inch-thick A and about 2 lb less for the 1/8-inch-thick plate; a computation for f yielded 28.6 and 28 lb, respectively. The first step in

solving equation A-9 was to calculate the ratio da Evdas,

dta

=

dt2 dt for which 11⁄2 (£,1è̟£n) 0. It can be shown that this is true when the barrier responds to a d2 Ev minimum air speed; dt

is then determined from this ratio

dta

d2.
dt2

d2 Sv

is calculated

from equations A-1 through A-3) and used to resolve 12. Results showed v equal to 47 and 40 ft/sec for the 1/4- and 1/8-inch-thick A, respectively.

Time for A, to slide off D2 is approximated by integrating equation A-9, resulting in

1

t

= 2

픔 I2

2 12 / [1/2 CA2 ov2 + - Com 8 - 12 (4,1q3 - £)], (^-10)

where C, is the kinetic coefficient of friction equal to about 0.3 and

1

£n) 0.

11⁄2 (£,1 C
47 and 40 ft/sec.

Times of 35 msec were found for the two air speeds of

When equation A-9 is used also for the case of C = 1.0 and A is 1/4 inch thick, wind speeds of about 75 ft/sec are obtained. The previous and present analyses therefore suggest that A can lift entirely from D2 or merely slide along its surface when C, is large. However, since the wind is normally accelerating within the speed range that these barriers operate, A may respond initially by sliding on D2 and then lift as the wind speed increases. High-speed photographs confirm the latter mode of operation.

Because of the accelerating nature of the wind, the calculated times for A1 to leave D2 are overestimates. Total time interval from the initial motion of A to beginning of tub fall (about 1/3 inch) was measured to be in the range of 30 to 50 msec.