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U.S. Department of Labor | |
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| Occupational Safety & Health Administration | ||||||
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| Safety and Health Topics > Mineral Processing Dust Control > Reducing Respirable Dust Levels > Testing Equipment and Procedures | |||
Testing Equipment and Procedures | |||
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The B&BCD was evaluated at two silica sand
plants. The primary goal of each evaluation was to compare
respirable dust concentrations after installation to previous ambient dust
levels. At the first evaluation site, the pallet loading process was
performed by two bag stackers. At the second evaluation site, this
process was performed automatically by a palletizing machine. The respirable dust monitoring equipment and analysis were similar for both plants. Real-time aerosol dust monitors (RAM-1)6 were the primary evaluation tool. Owing to the ability to perform preinstallation and postinstallation testing, gravimetric dust samplers were also used at the first evaluation site. Both instruments used 10-mm Dorr-Oliver cyclones to classify the respirable dust fraction, particles having an aerodynamic diameter of 10 µm or less (6-7). In both evaluations, a RAM-1 device was located at each predetermined dust-sampling position. This device measures respirable dust concentrations by light scattering within a sensing chamber. The RAM-1 has been shown to closely simulate gravimetric measurements when calibrated for a specific dust type (8). This instrument was ideal for evaluating the effectiveness of the B&BCD. Gravimetric dust samples were used at the first evaluation. Various studies have shown that gravimetric sampling can simulate dust deposition in workers' lungs (9). Gravimetric dust filters were weighed before and after us at the USMB's Pittsburgh Research Center. All gravimetric sampling pumps were calibrated to 1.7 l/min according to procedures specified in "Chapter D-USDOL, Mine Safety and Health Administration (MSHA) Handbook Series, Metal/Nonmetal Health Inspection Procedures, Handbook No. PH90-1V-4, November 1990. Although both gravimetric and RAM-1 dust monitoring devices measure respirable dust concentrations, the data obtained by each method was very different. Gravimetric samplers provided a single dust concentration value while the unit was operated. They are used by MSHA for compliance measurements to determine a worker's respirable dust exposure over the entire work day. In this work effort, gravimetric devices had a more limited role than the RAM-1 device because they provided only a single value of the time-weighted average concentration and gave no indication of peak values or fluctuations in dust levels during the monitoring period. The RAM-1 provides a continuous real-time output of respirable dust concentration. With the RAM-a device, it is easy to determine how changing parameters impact dust levels, making it an excellent tool for performing comparative evaluations. Because of the large volume of dust data obtained, a datalogger unit was used to store the data from each RAM-1 monitor. The datalogger was then dumped to a lap top computer after each day of testing. This information was then processed using various software programs that specialized in data analysis. One last analysis technique performed at both evaluation sites involved cleaning a specific quantity of bags with and without the B&BCD in operation using a vacuum cleaning device. This was done to measure the amount of product material on the outside of the bags. To do this, ten 45.4 kg bags and twenty 22.7 kg bags were randomly removed from the conveyor line and vacuumed in each test. These bags were set on a piece of plastic and manually vacuumed using a high-efficiency portable vacuum cleaner to remove all product on the outside of each bag. By preweighing and post weighing the vacuum bags to the nearest tenth of a gram on an analytical balance, the amount of product removed from these bags was determined. Using the above measurement techniques and comparing identical product and bag sizes at each site, the ability of the B&BCD to remove external product and dust was determined. EVALUATIONS SITE 1 Testing The first evaluation of the B&BCD was at a silica sand operation. Testing was performed at the ground silica mill on material formerly called "silica flour," with the product sizes from 120 to 325 mesh. A bag operator manually loaded product into 22.7 or 45.4 kg paper bags at a four-station, fluidized air fill machine. As the bags were ejected from the fill station, they traveled a short distance on a chain conveyor and were then dumped onto a second conveyor and traveled out the conveyor belt to the loading platform. Two methods were used to palletize the bags, but in both, bag stackers manually performed the palletizing process. The first method was to load the bags onto pallets at the edge of a loading dock. Once a pallet was completed, it was taken by forklift to either a trailer truck or the warehouse, and a new pallet started. The second method was to load the bags directly onto flatbed trailer trucks. This involved lengthening the conveyor belt and having the bag stackers load the bags directly on the trailer. During the dust analysis, all loading was performed using a modified version of the previous method. A platform was constructed to extend the loading dock and to permit the installation of the B&BCD. This platform was used to load all pallets during this 2-week test period. In addition, the entire outside loading dock area was enclosed using wood framing covered with nylon reinforced plastic material. This enclosure was fabricated to more precisely measure dust levels in the conveying and pallet loading area by minimizing the effects of background dust sources. Phase 1 of the evaluation was performed for 5 days to determine respirable dust concentrations without the B&BCD. The B&BCD was then installed and the same dust analysis was performed for an additional 5 days of testing (phase 2). Respirable dust was liberated by the movement and handling of bags during the palletizing process. The amount of dust liberated is dependant on the mesh and bag size, these factors were therefore evaluated separately. Figure 3 shows the test layout including the four dust-monitoring locations and the location of the B&BCD during phase 2 of testing. The performance of the B&BCD at this first test site was impacted by the insufficient exhaust air volume provided to the unit. The device was designed to operate with an exhaust volume of approximately 34m3/min. When the unit was originally started, the exhaust volume was 13.4 m3/min. To increase this value, a van axial fan was installed and increased the exhaust volume to 20.5 m3/min. a Even at this quantity, the B&BCD was operating at only 60 pct of the designed exhaust volume to 20.5 m3/min. Even at this quantity, the B&BCD was operating at only 60 pct of the designed exhaust volume. The unit was under insufficient negative pressure, resulting in dust leaking from the device into the work area. Results The results of testing at this first evaluation site indicate a reduction in dust levels with the B&BCD. Dust levels were compared at the four sample locations during phase 1 (baseline conditions), and phase 2 with the B&BCD. With the gravimetric dust samplers, respirable dust concentrations ranged from 1/10 to 3.72 mg/m3 in phase 1, compared to 0/13 to 2.19 mg/m3 with the B&BCD in phase 2. The respirable dust concentration for both phases can be seen in appendix, table A-1. Figure 4 shows the average respirable dust concentrations at all four sample locations. Average dust reductions of 35.0, 19.1, 30.0, and 21.4 pct were obtained with the B&BCD for sample locations 1 through 4, respectively, Gravimetric dust concentrations grouped all product mesh sizes and bag sizes together for each sample location. It would have been preferable to look at each separately, as was done with the RAM-1 technique, but this would have been difficult considering the manpower and the amount of equipment that would have been required. Dust concentrations measured by gravimetric devices were higher than those obtained for compliance sampling for two primary reasons. The first was that the dust-monitoring devices were only operated during bag stacking compared to compliance sampling where they are operated for the entire shift. The second was the enclosing of the work area to minimize the effects of background dust sources. This caused dust levels to increase inside the enclosure because dust liberated during the palletizing process was contained in the area. The next analysis examined RAM-1 dust monitor results. RAM-1 dust concentrations from each sample location were analyzed, but unlike the gravimetric samples, product mesh sizes were compared separately. Respirable dust concentrations measured with RAM-1 dust monitors for the various mesh sizes and bag sizes in phases 1 and 2 can be seen in appendix, table A-2. Average respirable dust concentrations ranged from 0.99 to 2.02 mg/m3 in phase 1; compared with 0.63 to 1.89 mg mg/m3 with the B&BCD during phase 2 of testing. Figure 5 shows the reduction in respirable dust concentrations with the B&BCD versus normal dust concentrations prior to installation for 45.4 kg bags. Both gravimetric and RAM-1 dust monitor results were likely affected by the insufficient volume of exhaust air provided to the B&BCD during testing. As dust was cleaned from the bags inside the device, some of this dust would escape and flow out into the test chamber. Another indication of the effectiveness of the B&BCD was provided by the results of product vacuumed from the bags. The average weight gain per run for phase 1 ranged from 46 to 171 g. This compares to a weight gain of between 9 and 17 g per run during phase 2 with the B&BCD (appendix, table A-3). The comparison can be seen in figure 6 for each mesh and bag size. The reductions in the amount of product on the outside of the bags with the B&BCD were 77.6, 81.2 and 89.9 pct for 200 mesh per 45.4 kg bags, 325 mesh per 45.4 kg bags, and 200 mesh per 22.7 kg bags, respectively. These results should not have been significantly influenced by the lack of exhaust air volume to the B&BCD and provide an indication of the actual reduction in the amount of product on the outside of the bags. Following this first evaluation, several modifications were made to improve the effectiveness and safety of the B&BCD. These included:
EVALUATION SITE 2 Testing The second field evaluation on the B&BCD was at another silica sand plant that bagged product that ranged from 140 to 325 mesh. A bag operator loaded ground silica into 22.7 or 45.4 kg paper bags at a four-station fill machine. As the bags were ejected from the fill machine, they traveled on a chain conveyor, rotated 90°, then traveled a short distance on a conveyor belt before entering a 4-m-long inclined bag flattener that removed excess air pressure from with the bag. After exiting the flattener, the bags traveled through the B&BCD. A dust monitor was located inside the B&BCD. The bags then traveled down a short section of conveyor belt before going over a bag weighing and printing device. The second dust monitor was placed immediately before this bag weighing and printing location. If a bag of product was not within an acceptable weight tolerance, it was mechanically removed. Bags that were acceptable were printed with the necessary information, then traveled a short distance on another conveyor belt before reaching the automated palletizer. The first function of the palletizer unit was to properly align each layer of bags. The third dust monitor was positioned above this bag alignment process. Once aligned, a mechanical arm would slide the entire layer of bags approximately 4 m where it was ready to be loaded onto the pallet. The layer was loaded by holding the bags in place while a dual opening sliding trap door opened and allowed the bags to stack onto the pallet. The fourth dust monitor was positioned above this bag-loading process. The rap door would then close and the pallet would slightly raise to squeeze the upper layer of bags against the sliding door. This compacted the bags by removing additional air from within the bags. After an entire pallet was stacked, it was lowered and transported to a position where it was picked up by a fork lift. A new pallet would then automatically be positioned to repeat the process. It was not possible to perform preinstallation and post installation testing of the B&BCD at this operation because numerous modifications were being made to the bag loading and palletizing process at the same time as the installation of the B&BCD. Because of this, dust concentrations were recorded with and without the B&BCD operating during similar periods for each specific mesh and bag size. The test layout showing the various dust monitoring locations is seen in figure 7. Results Testing at this field evaluation site also provided results indicating the effectiveness of the B&BCD at reducing respirable dust concentrations. The actual respirable dust concentrations measured with RAM-1 dust monitors for the various mesh and bag sizes with and without the B&BCD can be seen in appendix, table A-4. These results need to be separated in two different areas. Sample location 1, (inside the B&BCD) should be looked at independently of the other locations. Sample locations 2-4 were similar to those locations at the first field evaluation site and lower dust concentrations should occur with the use of the B&BCD. Figure 8 shows the increase in dust concentrations inside the B&BCD (location 1). Since the device removed dust and product from the exterior of the bags, dust levels were higher when the device was operating. Respirable dust increased between 180 and 1000 pct inside the B&BCD during its use. Figure 9 shows the dust reduction with the B&BCD for sample locations 2 to 4. Note the slight increase with 140 mesh per 100 lb bags at sample location 4, as well as the marginal reduction for the other tests at this location. This was due to the dust generated as the layers of bags were compressed by the palletizer. During this squeezing process, a dust cloud was often observed escaping from the bag valve area. This overwhelmed any dust reductions due to cleaner bag surfaces, figure 10. There was also a significant increase in respirable dust levels at sample location 3 for the 325 mesh per 50 lb bags. This increase can be attributed to several broken bags during testing. When a bag breaks, it is rejected at the bag-weighing station. Once rejected, it travels down a conveyor to be disposed. The conveyor that carries these bags from the palletizer unit traveled directly beneath sampling location 3. Dust liberated from broken bags had a measurable effect on this sample location because 325 mesh silica sand is the smallest sized material and has the highest fraction of respirable size material. Again the results obtained by vacuuming the exterior of the bag provided a good indication of the effectiveness of the B&BCD. The average weight gain per run of bags before going through the B&BCD was between 43.7 and 72.8g. This compares to a weight gain of between 5.3 to 10.7 g per run after going through the B&BCD. The actual comparison can be seen in figure 11 for each product mesh size and bag size. Reductions in the amount of product on the exterior surfaces of the bags with the B&BCD ranged from 82.6 to 92.7 pct for the five tests.
The goal of this research project was to design and test a device to clean the exterior of bags of product material and the conveyor belt between the bag-loading station and the palletizing location. Throughout this research project, modifications were continually made to improve the operational effectiveness and safety of the B&BCD. The final design configuration as listed in this report has shown itself to be very effective from an operational standpoint over this time. One area of this research project that was not as successful as hoped, was the ability to effectively evaluate the performance of the B&BCD at cleaning the exterior of bags of product material and the belt during the two field evaluations. Conditions at both field evaluation sites had some aspects that caused a degree of negative bias in the analysis. At the first evaluation site, the insufficient exhaust volume impacted on the evaluation of the unit. Several attempts were made to increase the air volume, but even with this, the exhaust volume was still only 60 pct of the designed exhaust volume. At this level, the B&BCD was not operating under sufficient negative pressure to prevent dust from leaking out of the device and into the test enclosure. This negatively affected testing. Two backup RAM-1 dust monitors located close to the B&BCD for spot checks measured respirable dust concentrations up to 64 pct higher with the device operating (phase 2) compared to normal conditions in phase 1. Test results showed that respirable dust reductions ranged from 19.1 to 35.0 pct with gravimetric samplers and from no reduction to 49.7 pct with the RAM-1 dust monitors. Knowing that dust was being emitted from the B&BCD because of the inadequate exhaust volume, this indicated a significant reduction in the amount of dust liberated during the bag-stacking process. The 77.6 to 89.9 pct reductions in the amount of dust and product on the outside of the bags of the product showed the effectiveness of the B&BCD. Determining the effectiveness of the B&BCD was more difficult at the second evaluation site because of the various parameters that were changing and impacting dust levels in the area. The B&BCD was installed simultaneously with a new four-station bag loading station, a bag flattener, a bag weighing and printing station, and a new automated palletizing system. Because of these other components, it was difficult to determine improvements in dust levels because of the B&BCD. One significant bias at this second evaluation site was a bag breakage. This problem was created by a combination of two factors being the new bag filling machine along with faulty bags. In fact, due to the significant number of bags that were breaking at the bag filling station, it was approximately 1 year after the installation of the B&BCD before testing could be performed. Even during testing, there were several broken bags through the 5 days of testing that had a measurable impact on contaminating the test area. The USBM's past research on bag breakage showed that significant dust contamination can occur with each broken bag and just a few broken bags in a work day can cause a worker to be overexposed. As with the first evaluation site, the bag vacuuming technique provided a good analysis of the effectiveness of the B&BCD at removing dust and product from the bags and belt. Reductions at the second evaluation site ranged from 82.6 to 92.7 pct. Considering the various factors that affect respirable dust levels in these operations, it was not possible to determine the influence from other components at this second field evaluation site.
The USBM-designed B&BCD is effective at reducing product contamination of the bags of product material and the conveyor belt at mineral processing operations. The B&BCD uses a combination of air nozzles and mechanical brushes to clean the exterior of the bags and the belt. The system is designed to be under negative pressure so that all the dust and product removed is contained with the system. Product either falls into a hopper at the bottom of the unit and is recycled back into the process using a screw conveyor or is exhausted and filtered from the air using a baghouse dust collector unit. The system is adjustable allowing it to handle two different bag sizes (normally 22.7 and 45.4 kg paper bags) by simple changing a switch that operates two hydraulic pistons to change the bag width. Because of these various parameters that affected testing of the B&BCD at both field evaluation sites, recorded dust reductions in and around the device were not at levels that would have been anticipated if these negative influences were not present. The bag vacuuming technique was not believed to be negatively impacted by these parameters and provided dust reductions that were significantly higher than the other techniques. We believe that the other evaluation techniques would have recorded similar reductions without the biases as indicated. Dust levels inside the device during operation were measured up to 1,000 pct higher than with the system not operating. Dust cleaned from the bags with the device operating means there is less potential for dust generation and contamination of mill workers. Significant dust reductions were recorded at both field evaluations. Using the bag vacuuming evaluation technique, dust and product on the bags were reduced between 77.6 and 92.7 pct at both evaluation sites. The unit has been proving itself over time to be a very reliable and effective technique to clean bags of product and the belt at mineral processing plants, thereby improving worker health.
1. Cecala, A.B., A. Covelli, and E.D. Thimons. Reducing Dust Exposure of Workers During Bag Stacking in Enclosed Vehicles. USBM IC9148, 1987, 13pp. 2. Cecla A.B., and A. Covelli. Pallet Loading Dust Control System. USBM RI 9197, 1988, 12 pp. 3. Stahura, R. Using Belt Cleaners to Avoid Conveyor Shutdowns. Plant Eng., v.37, July 7, 1983. pp.70-71 4. Rappen, A. Systematic Cleaning of Belt Conveyors. Bulk Solids Handling. V.4, No.1, Mar. 1984, pp. 115-124. 5. Mencacci, M.C. Belt Buildup? Scrape it Off. Rock Prod., v.93, June 1990, pp.34-39. 6. Breslin, J.A. and R.L. Stein. Efficiency of Dust Sampling Inlets in Calm Air. Am. Ind. Hyg. Assoc. J., v.36, Aug. 1975, pp. 576-583. 7. Caplan, K.J., L.J. Doemeny, and S. Sorenson. Evaluation of Coal Mine Dust Personal Sampler Performance. U.S. Dep. of Health, Education, and Welfare, Contract PHCPE-R-70-0036, Nov. 1973, 16 pp. 8. Williams, K.L., and R.J. Timko. Performance Evaluation of a Real-Time Aerosol Monitor. USBM IC 8968, 1984, 20 pp. 9. American Conference of Governmental Industrial Hygienists. Threshold Limit Values Committee. Documentation of the Threshold Limit Values for Substances in Workroom Air. 3rd ed., 1971, 286 pp. 6Reference to specific products does not imply endorsement by the U.S. Bureau of Mines. |
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