No. 490 June 2001

An Inexpensive Device for Monitoring Explosions

in Sealed Areas of Underground Mines

Objective

To develop a passive, inexpensive technique for monitoring

explosion overpressures within sealed areas in underground coal

mines.

Background

From 1994 to 2000, seven explosions of methane and/or coal

dust occurred within the worked-out, sealed areas (gobs) of underground U.S. coal mines. These explosions, believed to have been

started by lightning, destroyed many seals and caused much damage

external to the sealed area. One gob explosion went undetected until

the mine operator reopened the sealed area and discovered the

damage. Fortunately, these gob explosions did not cause any fatalities or injuries.

Because lightning occurs in just about all mining areas, several

questions arise. Could gob gas explosions be occurring in other

sealed areas and going undetected? If so, what is the magnitude of

the resulting explosion overpressures? The Pittsburgh Research

Laboratory (PRL) of the National Institute for Occupational Safety

and Health (NIOSH) undertook a study to identify and evaluate

passive measuring devices to monitor and quantify the overpressures

associated with such explosions. This work was done in parallel with

full-scale explosion experiments for evaluating the strength characteristics of seals used for isolating areas in underground coal mines.

Approach

Evaluation studies were done in PRL's 20-liter explosion test

chamber and in the full-scale Experimental Mine at Lake Lynn

Laboratory to evaluate the response and accuracy of various

commercially available automobile tire pressure gauges to

overpressures generated from gas and dust explosions. The first

series of tests was done in the 20-liter chamber, which is used to

study the explosibility and inerting of fuel dusts and gases. Data

from the tire pressure gauge tests were compared to the pressure-time

data obtained from the 20-liter chamber 's traditional electronic strain

gauge pressure transducer recorded on a high-speed data acquisition

system. To evaluate the tire pressure gauges, the valved inlet (tip) of

the candidate pressure gauge was removed and the body of the gauge

was cemented with epoxy into a 0.25-in pipe elbow and attached to

the outside of the test chamber.

A second series of tire gauge evaluation tests was done at Lake

Lynn during explosion-resistance testing of seals used in

underground coal mines. Full-scale seals (20 ft wide by 8 ft high)

were constructed as one wall of a 2,000-ft3

 chamber and exposed to

a series of methane explosions. Average overpressures ranged from

20 to 80 psi. The seals were built with various strength materials

ranging from 7 to 48 in thick. The modified tire gauges are attached

on the outside of the chamber to a 0.25-in NPT pipe that extends into

the chamber (figure 1). The axis of the gauge is mounted at a right

angle to the plane of the seal to minimize the coupling of any seal

movement to the stem of the gauge that may cause inaccurate

readings. The internal explosion pressure history is recorded with a

strain gauge pressure transducer mounted inside the chamber.

Methane is injected into the chamber, mixed with air, and ignited

with an electric spark located at the geometric center of the chamber.

Maximum explosion overpressures are recorded and compared with

the tire gauge pressure.

Results

Several constant volume explosions were conducted in the

20-liter chamber in order to vary the maximum pressure and the

time-to-peak pressure. The comparison between the maximum

pressure measured by the strain gauge and that determined by the tire
Chamber Explosion Pressure

0

2

4

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16 18 20

Tire Gauge, psi

Explosion Pressure, psi

Figure 2.–Comparison of chamber explosion pressure with tire gauge response.

Figure 1.—Tire gauge installed on mine solid

block seal.

pressure gauge is shown in figure 2. There is fairly good agreement

over the pressure range from 5.9 to 18.7 psi. Maximum pressure

comparisons from large-scale chamber tests are also very encouraging. Maximum pressures recorded with internal electronic

strain pressure gauge agreed within 1 psi of the tire gauge reading for

all tests without seal failure. Results show that this simple, inexpensive, passive device can be used to monitor overpressures

contained within sealed areas of underground mines or to record the

gob explosion overpressures if one or more seals fail during the

explosion.

For in-mine use, a single pressure gauge could be installed on

one seal in a set of two to six seals. If desired, two gauges covering

different ranges could be attached to the same seal. Existing

sampling tubes could be used to attach the gauge where seals are

already built. During the construction of a new seal, a dedicated or

additional 0.25-in-diam pass-through tube could be installed. Any

pressure rise can be identified during routine weekly examinations of

the seals or shortly after a lightning strike in the area. Such an

inexpensive device is useful as part of a surveillance study on the

number and magnitude of undetected explosion events that occur in

sealed areas of underground coal mines.

For More Information

For more information on this study, contact Michael J. Sapko or

Eric S. Weiss, NIOSH Pittsburgh Research Laboratory, Cochrans

Mill Rd., P.O. Box 18070, Pittsburgh, PA 15236-0070, phone: (412)

386-6619 or (412) 386-5050, fax: (412) 386-6718, e-mail:

MSapko@cdc.gov or EWeiss@cdc.gov.

To receive more information about occupational safety and

health problems, call 1-800-35-NIOSH (1-800-356-4674), or

visit the NIOSH Web site at www.cdc.gov/niosh

Mention of any company name or product does not constitute

endorsement by the National Institute for Occupational Safety

and Health.