Document VJkGbDn83By3QGBwYLMNyZwag

CONTINUOUS RESPIRABLE MINE DUST MONITOR DEVELOPMENT Bruce K. Cantrell,1 Kenneth L. Williams,1 Stephen W. Stein,2 David Hassel,3 and Harvey Patashnick3 'National Institute for Occupational Safety and Health, Pittsburgh Research Center, Pittsburgh, PA; 23M Co., Inc.; 3Rupprecht and Patashnick Co., Inc. ABSTRACT In June 1992, the Mine Safety and Health Administration (MSHA) published the "Report of the Coal Mine Respirable Dust Task Group, Review of the Program to Control Respirable Coal Mine Dust in the United States," As one of its recommendations, the report called for the accelerated development of two mine dust monitors: (1) a fixed-site monitor capable of providing continuous information on dust levels to the miner, mine operator, and to MSHA, if necessary, and (2) a personal sampling device capable of providing both a short-term personal exposure measurement as well as a full-shift measurement. In response to this recommendation, the National Institute for Occupational Safety and Health (NIOSH), Pittsburgh Research Center, initiated the development of a fixed-site machine-mounted continuous respirable dust monitor. The technology chosen for monitor development is the Rupprecht and Patashnick Co., Inc. tapered element oscillating microbalance. Laboratory and in mine tests have indicated that, with modification, this sensor can meet the humidity and vibration requirements for underground coal mine use. NIOSH is continuing that effort by developing pro totypes of a continuous dust monitor based on this technology. These prototypes are being evaluated in underground coal mines as they become available. This effort, conducted as a joint ven ture with MSHA, is nearing completion with every promise of suc cess. The immediate benefit of this effort will be to researchers, regulatory personnel, and mine personnel, by permitting evalua tion of specific mining practices to see which expose mine work ers to excessive dust levels. Using this information, mine person nel can optimize mining procedures to reduce dust exposure. MSHA and the operators will also be able to use the dust concen tration data to judge whether dust plan parameters are adequate to continuously maintaining environmental dust levels below the applicable standard. The second development recommended by the Dust Task Group was a person-wearable version of the continuous dust monitor. It will be a monitoring system designed to provide a measurement of worker exposure to respirable dust, both during and at the end of the shift. INTRODUCTION The current gravimetric approach to measurement of shift av erage respirable dust concentrations in underground coal mines (1), with its inherent delays, cannot provide dust level data quick enough to allow on-site correction of inadequate dust control practices. Furthermore, in April 1991, the Secretary of Labor al leged widespread tampering at several hundred coal mines with respirable dust samples taken by mine operators for compliance assessment. These allegations and a directive from the Secre tary prompted MSHA to appoint a special Respirable Dust Task Group to study options to improve monitoring and control of respi rable coal mine dust. In June 1992, MSHA published the report of the dust task group (2). This report called for the accelerated development of two mine dust monitors: (1) a fixed-site monitor capable of pro viding information on dust levels to the miner, mine operator, and to MSHA, if necessary, and (2) a personal sampling device ca pable of providing both a short-term personal exposure measure ment as well as a full-shift measurement. In response to MSHA's request, the former U.S. Bureau of Mines (USBM) investigated several sensor technologies for con tinuously monitoring respirable coal mine dust. Most instruments currently on the market for measuring aerosol concentration sense some property of the particles other than their mass. Con verting the sensor reading to equivalent mass requires tacit as sumptions about the relationship between aerosol mass and the property sensed. These assumptions can lead to significant error. A direct aerosol mass sensing instrument eliminates the potential for error that is associated with converting sensor measurements to equivalent mass concentrations. One such technology is Rupprecht and Patashnick Co., Inc. (R&P) proprietary tapered element oscillating microbalance (TEOM) sensor. In USBM laboratory tests of sensor response to vibrations and humidity (3), the TEOM dust monitor exhibited excellent accuracy and stability. Consequently, the TEOM dust sensor has been chosen for use in the high humidity and vibration environment of underground coal mines. Prototypes of a fixedsite machine mountable continuous respirable dust monitor (MMCRDM) based on this sensor are currently being developed, and evaluated in underground coal mines as they become avail able. The first such evaluation was completed in 1995; the sec ond is scheduled for mid 1996. A commercial version of the moni tor is due in early 1997. An effort to use the technology devel oped, for this monitor in the design of a person-wearable version is being planned. It will provide short-term personal exposure measurement as well as end-of-shift dust exposure measurement. MMCRDM REQUIREMENTS This fixed-site monitor will be mountable on mobile mining machines, most likely continuous miners or longwall shearers, and will continuously sample dust in the vicinity of the machine operator. While MSHA has not finalized how data from such a monitor would be used, it would provide more complete and im mediately available information regarding how well dust control practices are working. With the concurrence of MSHA, the USBM formulated target specifications (Table I) for the MMCRDM (4). At the request of MSHA, the sample preclassifier was designed to pass size se lected aerosol to the tapered element filter based on the interna tional standards organization (ISO) definition of respirable dust (5). In addition to measuring respirable dust concentration in the mine environment, the monitor is to be tamper-resistant and store dust exposure information for 30 days. In operation, the unit will be programmed to display dust concentrations in real time and indicate whether the shift average permissible exposure limit will be exceeded during the work shift. TAPERED ELEMENT OSCILLATING MICROBALANCE The TEOM sensor uses the inertial behavior of a vibrating element to measure the mass of sampled dust (6). The active element of the system, depicted in figure 1, is a specially tapered hollow tube constructed of metal or an elastic, glass-like material. The wide end of the tube is firmly mounted on an appropriate base plate. The narrow end supports a replaceable collection medium such as a filter and is permitted to oscillate. Particle-laden air is drawn through the collection medium, where particles are depos ited. The filtered air is then drawn through the hollow tube. Air flow is controlled by an automatic mass flow controller. As the collection medium collects dust, its mass increases, causing a decrease in the frequency of oscillation. By measuring the change in frequency, one can determine the gain in the mass of dust on the collection medium. An electronic feedback system initiates and maintains the os cillation of the tapered element. The details of the feedback sys tem have evolved over the years, but typically, a light emitting di ode (LED)/photo-transistor pair aligned perpendicular to the plane of oscillation of the tapered element, detects the frequency of os cillation. The light-blocking effect of the oscillating element posi tioned between the photo/transistor and the LED modulates the output signal of the photo-transistor, which is then amplified. Part ControUar Figure 1. Schematic of the tapered element oscillating microbalance (TEOM) dust sensor. of the amplified signal is used by the feedback system to provide sufficient force to overcome any amplitude damping of the ta pered element oscillation. The other part of the amplified signal from the LED/photo-transistor pair is sent to a counter and data processing stage. Here, the frequency of oscillation of the ta pered element is calculated and stored in memory. Benefits/Challenges Unlike many other aerosol measurement technologies that Table 1. Target Performance and Environmental Specifications for a Fixed-site MMCRDM Performance Specification____________________ Standard______________ ___________________________________________ Measurement units Measurement range Accuracy Overload tolerance Measurement period Maintenance cycle Safety certification Temperature range (anticipates both underground and some surface operation) Mass concentration of respirable dust, mg/m3 0.5 to 2.0 mg/m3 25% 25% of reading with 95% confidence 100 mg/m3 for 10 sec 25 mg/m3for30sec 10 mg/m3 for 120 sec 30 min cumulative shift average (shift length: 8,10, or 12 hrs) 30 days unattended, data storage for a minimum of 90 shifts Must be certifiable by MSHA for use in permissible areas of coal mines -40 to 40 C, typically 0 to 30 C Thermal shock range 40to 0 C Temperature excursion rate Operational altitude/pressure equivalent range Humidity Mechanical shock (shipping) Mechanical shock (operating) Vibration (continuous miner) Vibration (haulage vehicle) 10 C/min Sea level 10,000 ft 0 to 100% (typical operation range 30 to 95%) 1 m drop equivalent, 11 ms period sawtooth impulse shock of 20 g Sine vibration, 5 to 2,000 Hz, 1.5 g Sine vibration, 5 to 92 Hz, 2.5 g and 92 to 500 Hz, 3.5 g Power fluctuation____________________________ 25%____________________________________________________________ Pre-Classifier Inlet Figure 2. Cross-section view ot machine mounted, continuous respirable dust monitor. measure an aerosol parameter correlated with mass, the TEOM technique measures mass directly. With the appropriate preclassifier, the instrument collects and measures respirable mass. Sampling at 2 L/min for 30 min, the typical measurement accuracy is 15 pg/m3. The measurement remains accurate as long as the mass on the filter remains below about 5 to 10 mg. The TEOM aerosol monitors would measure any water drop lets reaching the collection filter as aerosol mass. Changes in mine air humidity and temperature could also affect the response of the instrument. However, the monitor uses a 50C tempera ture-controlled inlet conditioning system to eliminate or reduce humidity and temperature variations of the sensor. Under these sampling conditions, collected water aerosols evaporate, leaving only solid particulate on the filter. Since TEOM instruments operate by measuring the change in frequency of a vibrating element, vibrations from external sources can interfere with the measurement. In the machinemounted unit, however, vibrations from the machine are damped using vibration isolation mounting of the monitor sensor. (b) Begin design of two pre-production prototypes. The pre- pro duction prototypes will include prototypes of all the compo nents that will be included in the production models. Phase 2 Complete the design and construction of the two pre-produc tion prototype MMCRDM's. The design was to incorporate tamper-resistance. The prototypes are machine mountable and certified for experimental permits by MSHA. These prototypes will be evaluated during laboratory and field tests. During the field test, one unit will be mounted on a continuous miner and the other unit will be used in a longwall mining section. Phase 3 Delivery of ten production model MMCRDM's built to commer cial production specification. CONTINUOUS RESPIRABLE DUST MONITOR DEVELOPMENT Under a USBM contract, R&P Co., Inc. embarked on a pro gram to design, build and test ten production MMCRDM's based on the TEOM aerosol mass sensor. Three phases are required to complete this work: Phase 1 Phase 1 has two parts: (a) Design a research proof-of-concept prototype MMCRDM to be tested using tethered electronics. The prototype must be cer tifiable by MSHA for an Experimental Permit for use in permis sible areas of coal mines. Two of these prototype MMCRDM's were evaluated by PRC and MSHA in laboratory and field tests, primarily for response to machine vibration. Field evalu ation was conducted in a continuous miner section of an oper ating coal mine. PHASE 1 FIELD EVALUATION Phase 1 laboratory and in-mine evaluation of the prototype of the machine-mounted monitor was conducted as a joint venture with MSHA. Laboratory tests of the monitor sensor were con ducted by the USBM in mid 1995. The protocol followed has been detailed elsewhere (7). Field evaluations were conducted in a continuous miner section from August 10 to 24, 1995. The monitor configuration tested in the Phase 1 field evalua tion is illustrated in figure 2. As designed, this configuration mea sured 0.3 x 0.55 x 0.2 meters. During the tests, however, the computer control, data logging, sample flow control, and sample pump were removed from the case and placed at a remote loca tion. Only the sample inlet, sample preclassifier, and TEOM aerosol mass transducer/sensor were attached to the canopy of the continuous miner. This arrangement placed the sensor as sembly within 1 m inby of the machine operator while leaving his view of the face unobstructed. As indicated in figure 3, a 215-m umbilical cable containing instrument power, signal, and vacuum lines, connected the sensor through a group of electrical barriers to the electronics located at a data and control center near the criteria used by the continuous monitor. Table II displays the measurements made during the field evaluation of the continuous monitor. Measurements were taken for seven days over a two-week period. Comparison of continuous dust monitor and personal dust monitor measurements of dust concentrations were made for three of the four sample days. The data from the fourth day were not used due to monitor data control damage during the measure ment period. A pre-filter (PF) was placed on the monitor on the other three days to explore the effect machine vibration had on the instrument's baseline. Vibration measurements were taken on all sampling days. RESULTS During the Phase 1 test, the continuous monitor measured re spirable mass concentration in mg/m3 every 1.7 sec. A 30-sec moving average was applied to these data. As shown in figure 4, this average was displayed graphically in real time by the com puter data acquisition system. The figure shows typical results during two coal cutting cycles and one bottom cleaning pass by the continuous miner. Mass concentrations drop to background levels between periods of activity, suggesting that the measure ment process is not severely affected by machine vibration. Also, total collected mass values and trends indicated no dependence on water or humidity. Table II. Primary measurements conducted during the field evaluation of the MMCRDM Figure 3. Location of MMCRDM sensor (Monitor) and auxiliary electronics (DC) during phase one evaluation. section's power center. In this way, the intrinsically safe sensor assembly could be operated at the face, while the nonintrinsically safe equipment was operated in fresh or intake air. During the field tests, measurements were also taken of ma chine vibration and respirable dust levels at the position of the monitor. The accelerometers used to measure machine vibration were also connected through a 300-m umbilical cable and electri cal barriers to signal amplifiers and a power supply at the data and control center. Data logging for both dust monitor concentration and vibration measurements was done using personal computerbased data loggers. Gravimetric respirable dust measurements were collected for comparison with the continuous monitor mea surements using Mine Safety Appliances (MSA) personal respi rable dust samplers operated at a flow rate of 1.7 Lpm. This flowrate was used to better approximate the ISO respirable dust Date (August 1995) Continuous Monitor Measurement Personal Vibration Sampler 15 'PF 16 Yes 17 Yes 18 Yes 21 2No 23 PF 24 PF Yes No Yes Yes Yes Yes Yes Yes Yes Yes Yes No Yes No 'Continuous monitor operated with pre-filter. Continuous monitor data/control cable damaged; data not valid. On the last two days of the study, a pre-filter was inserted into the sample path of the continuous monitor The monitor was oth erwise operated normally. At the end of this period, which in- 00 4700 4000 1100 1300 0M.MC Figure 4. Dust concentrations measured by the MMCRDM on 21 August 1995. *100 1700 5900 RMS Acceleration on Continuous Miner (8/21/95) Figure 5. Vibration measured at the MMCRDM on 21 August 1995. eluded one overnight shutdown of the monitor and a cold start half-way through the period, baseline drift for the unit was less than 1 pg of collected mass. This indicates that the sensor is not sensitive to environmental factors such as vibration and humidity. Typical results for vibration measurements, in units of the gravitational constant (G, rms), made during the last part of the period illustrated by figure 4, is shown in figure 5. These data, collected during the three sump cuts, show in detail the machine operations during the cuts. In general, accelerations were an or der of magnitude less than those cited in the target specifications of Table I. Table III includes collateral respirable dust measurements made during the sensor evaluation. The comparison points to a positive bias in the TEOM results, an average of 20 13%. This bias can be partially explained by the difference in the preclassifiers used in the TEOM sensor and the personal respi rable dust sampler. Using the measured penetration efficiency of the monitor's preclassifier and the published penetration effi ciency of the Dorr-Oliver cyclone used with the personal sampler in combination with an assumed particle size distribution for the collected mine aerosol, a net bias of 12% is predicted. Although not determined, the remaining bias has been attributed to the position of the inlet on the side of the sensor box. With this inlet position, the monitor samples from a volume where air may recir culate, concentrating aerosol mass in the TEOM sample and producing the noted bias. PROJECT STATUS Phase 2 of the project to produce and test a redesigned, selfcontained monitor is now underway. Using the results of the Phase 1 evaluation, the MMCRDM has been redesigned. The insensitivity of the unit to vibration and the lower than expected vibration levels permitted a modification in the vibration isolation member that has reduced the size of the sensor assembly. Be cause of the success using the umbilical configuration, the MMCRDM will be installed in two parts: (1) an intrinsically safe unit containing the sensor and a filter changer, and (2) an explo sion-proof enclosure (XPE) containing the computer, pump, power supplies, and electrical barrier circuits. This arrangement will permit a smaller profile sampling unit in front of the machine operator. Also, the use of an XPE preserves the ability to install the monitor components in the existing miner XPE. The inlet bias problem has been solved by moving the inlet from the side to the top of the sensor assembly, thus putting it in the unrestricted sample airstream. Referee sampling will be done using a newly designed personal respirable dust sampler that in corporates the same inlet configuration and preclassifier as that used on the MMCRDM. This will eliminate the need to correct one of the measurements to obtain a valid comparison of respirable dust mass concentration. The completed monitor, as shown in figure 6, is scheduled for laboratory and in-mine evaluation on both a continuous miner and a longwall section in late 1996. Laboratory evaluation of the moni tor will follow the protocol followed in Phase 1 of the project. Dur ing the in-mine evaluation, the monitor will be mounted on the mining machine or, as an option in the case of the longwall tests, on a shield. The monitor will derive electrical power from the miner. It will provide the machine operator with graphical and numeric information on dust concentrations as illustrated in figure 7. A series of collateral measurements, like those performed in the Phase 1 evaluation, will be made during the evaluation. These will include measurements of machine vibration, referee shift average respirable dust concentration collected with per sonal samplers, face ventilation, water usage, and for a portion of the personal samples, determination of silica fraction of the col lected mass. Table III. Comparison Between Integrated Continuous Dust Monitor Mass Concentrations and Those Obtained Using the MSA Personal Respirable Dust Sampler Date (August 1995) TEOM mass concentration, mg/m3 Personal sampler mass concentration, mg/m3 Difference % 16 1.26 1.19 6 17 3.09 2.10 32 18 0.40 0.29 23 n21 0.46 NA 'TEOM data not used because of cable break during test day. NA Not applicable. The data developed during this phase of the project will be used to finalize the design of the machine-mounted monitor. Fol lowing delivery of ten production units in Phase 3, MSHA will use them in in-mine evaluations to develop use strategies for the monitor and will explore its use in MSHA's respirable coal mine dust control program. SUMMARY In response to MSHA's Respirable Dust Task Group report, the USBM initiated an aggressive research effort to develop mineworthy devices to continuously monitor respirable coal mine dust mass concentration. The dust sensor technology chosen for development is the TEOM manufactured by R&P Co., Inc. This technology is desirable in that it measures collected aerosol mass directly. Results from laboratory and initial in-mine evaluations of a TEOM sensor modified to reduce response to environmental vibrations indicate that the sensor will operate well within the en vironmental and performance specifications of Table I. Two pre-production prototype MMCRDM's will be fabricated in mid 1996. After certification by MSHA, this MMCRDM system will be evaluated in both continuous miner and longwall sections of an underground coal mine. MSHA will be involved in these tests as part of a joint venture with NIOSH-PRC. As a final phase in this development, ten production prototype continuous respirable dust monitors based on the TEOM sensor will be fabricated for in mine evaluation. The production versions of the instruments will be used underground to develop use strategies for the monitor. If successful, this NIOSH-PRC research will foster develop ment of a family of commercial continuous respirable dust moni tors that can be mounted on mining machines, used as portable dust monitors, incorporated in mine atmospheric monitoring sys tems, and perhaps even be used as personal exposure monitors for mine workers. Such instruments could make accurate, con tinuous records of dust concentrations in the workplace, a signifi cant development for occupational health and the mining industry. Using this record, researchers, regulatory personnel, or mine personnel could evaluate specific mining practices to see which ones expose mine workers to the least dust. This information would permit mine personnel to optimize mining procedures to reduce dust exposure. It will also provide mine personnel with an immediate record of daily dust concentration levels, enabling dust exposures to be maintained below federally mandated levels. FUTURE WORK MSHA has endorsed the development of a personal version of the continuous dust monitor. This project will be a contract re- Figure 6. Design of MMCRDM for phase two in-mine evaluation. Figure 7. Real-time MMCRDM data display. search effort with in-house evaluation of the research product by NIOSH-PRC. The personal monitoring system would be de signed to operate in two modes. The first would be in the form of a personal dust dosimeter that can be interrogated underground or at the end of a shift using a portable reading system. The sec ond configuration will provide continuous monitoring of respirable dust concentrations and would be used as a portable monitor. The system will provide immediate dust exposure information for the user and permit the mine operator to evaluate his dust control system and take effective measures to correct problems as they arise. These efforts would be part of the expanded FY96 MSHA/ NIOSH joint effort and would continue through FY97 into FY98. REFERENCES U.S. Code of Federal Regulations, 1995, Title 30-Mineral Re sources; Chapter 1-Mine Safety and Health Administration, Department of Labor; Subchapter O-Coal Mine Safety and Health; Part 70-Mandatory Health Standards-Underground Coal Mines; 1995, pp. 412-418. Mine Safety and Health Administration (U.S. Department of La bor), 1992, "Review of the Program to Control Respirable Coal Mine Dust in the United States-Report of the Coal Mine Respi rable Dust Task Group," June 1992, 60 pp. Williams, K.L., and Vinson, R.P., 1986, "Evaluation of the TEOM Dust Monitor". USBM 1C 9119, 1986, 11 pp. Trelewicz, K., 1982, "Environmental Test Criteria for the Accept ability of Mine Instrumentation," (Contract JOI00040, Dayton T. Brown, Inc.). BuMines OFR 1-82, NTIS PB 82-146325; 1982. Soderholm, S.C., 1993, "Proposed International Conventions for Particle Size-Selective Sampling," Ann. Occup. Hyg., Vol. 33, 1989, pp. 301-320. Williams, K.L., Fairchild, C., and Jaklevic, J., 1993, "Dynamic Mass Measurement Techniques," K. Willeke and P. A. Baron, Eds. Van Nostrand Reinhold, New York; 1993, pp. 296-312. Cantrell, B.K., et al., 1996, "Status of a Tapered Element Oscillat ing Microbalance-Based Continuous Respirable Coal Mine Dust Monitor," Appl. Occup. Environ. Hyg. 11(7), July 1996, awaiting publication.