TN295 v.<^ ♦ '. -^^n^ 4 o ■oK ^^^-^, :. -^^^^ •*bv^ ^^-n^. * »>» o <]5 ^ '^ '"■> /.w^-x .o°^•i^l./='. >\c:«i:.\_ •" c/..^-.\'->\..^ .^ ..^ <>.. *' . .T* A • Ay o * <^9^ :>'' » *bv" ^^^^ J> ,•1 c. - ■ *. '-V^--,*"' o.--?'^ ^^'•d' £-^ . o, *.,..' IC 9068 Bureau of Mines Information Circular/1986 Dust Reduction Capabilities of Five Commercially Available Bag Valves By Andrew B. Cecala, Anthony Covelli, and Edward D. Thimons UNITED STATES DEPARTMENT OF THE INTERIOR Information Circular 9068 Dust Reduction Capabilities of Five Commercially Available Bag Valves By Andrew B. Cecala, Anthony Covelli, and Edward D. Thimons UNITED STATES DEPARTMENT OF THE INTERIOR Donald Paul Model, Secretary BUREAU OF MINES Robert C. Horton, Director no, '^oG^ Library of Congress Cataloging in Publication Data: Cecala, Andrew B Dust reduction capabilities of five commercially available bag valves. (Information circular ; 9068) Supt. of Docs, no.: I 28.27: 9068. 1. Ore-dressing plants — Dust control. 2. Valves. I. Covelli, Anthony. II. Tliimons, Hdward D. III. Title. IV. Series: Informa- tion circular (United States. Bureau of Mines) ; 9068. TN295.1J4 [TN5011 ()22s [622'.79l 85-600328 CONTENTS Page Abstract 1 Background 2 Acknowledgments 4 Testing 4 Discussion 8 Conclusion 10 ILLUSTEiATIONS 1. "Rooster tail" of product discharged from bag valve as bag falls from fill nozzle 2 2 . Product accumulation on outside of bags 3 3. Product that has escaped from bag valve during conveying process 4 4. Sampling setup for field testing 5 5. Sampling setup to monitor worker's dust exposure 6 6. Dust reductions with extended polyethylene and foam valves compared with that of standard paper valve 8 TABLES 1. Results from initial 2-week test at various locations using 325-mesh product 7 2. Results from final test at various locations using 200-mesh product 8 3. Increase in valve cost above the cost of standard paper valve 9 UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT cfm cubic foot per minute min minute in inch mm millimeter lb pound pet percent mg/m^ milligram per cubic meter DUST REDUCTION CAPABILITIES OF FIVE COMMERCIALLY AVAILABLE BAG VALVES By Andrew B. Cecala,^ Anthony Covelli,^and Edward D. Thimons^ ABSTRACT The dust-reduction capabilities of five commercially available bag valves were evaluated by the Bureau of Mines for use in mineral process- ing plants. The five valves were made of (1) standard paper, (2) poly- ethylene, (3) extended polyethylene, (4) double trap (p^^per) , and (5) foam. The valves were evaluated at a mineral processing plant during the bag loading, conveying, and palleu loading process. For the overall process, the extended polyethylene valve was the most effective at re- ducing product blowback, and resulted in lower dust concentrations for bag-generated dust. When compared to the standard paper, it gave a 62- pct reduction in respirable dust at the bag operator location, and a 66-pct reduction at the bag stacker location. The cost of the extended polyethylene valve is less than 1 cent per bag higher than that of the standard paper valve. The high dust reductions and slight cost increase make it an attractive choice for the mineral processing industry. 'Mining engineer. ^Physical scientist technician. -^Supervisory physical scientist. Pittsburgh Research Center, Bureaii of Mines, Pittsburgh, PA. BACKGROUND The intent of this Bureau of Mines work was to determine the dust reduction capa- bilities of various bag valves available to the mineral processing industry. Many different types of ground mineral products are packaged into 50- or 100- Ib paper bags. Although bag technology is continually improving, contamination from bags still remains a significant source of dust exposure for workers. Most of the bag contamination comes from the bag valve. The bag valve is simply a means of inserting the fill nozzle into a bag. When the bag is full and falls to the conveyor, the product within the bag forces the valve closed. The valve usu- ally does not close properly, however, because product is usually trapped inside it during filling and ejection. As a re- sult, the valves leak product, and, therefore, dust is generated during the conveying and pallet-loading process. This dust contaminates the work environ- ment and is a health hazard to persons working in the area. P TU IPWIF-' T.ffi^"^ '"" ^ """ m 1 ^^ VALVE Jj 4^ i^ M ' 1 FIGURE 1. - "Rooster tail" of product discharged from bag valve as bag falls from fill nozzle. Bag-generated dust contaminates the work area in a number of ways. One source is product blowback during the fill cycle. During filling, the inside of the bag becomes pressurized by the fluidizing air, which is used to carry the product into the bag. This pressure is relieved by air and product blowing out of the bag valve. This produces a considerable amount of airborne dust to which the bag operator is exposed. When the bag has finished filling and falls from the fill station, a "rooster tail" of product is discharged from the valve (fig. 1). As the bag hits the conveyor belt, additional product is discharged from the valve, further contaminating the work environment and exposing the bag operator. These dust sources to the accumulation of outside of the bag (fig. of product are conveyed loading area, this accumulation is dis- persed into the air, exposing workers, especially the bag stackers. In addi- tion, as the bags drop from one conveyor to another or from the conveyor to the bag slinger, dust is again emitted from the valve (fig. 3) . also contribute product on the 2) . As the bags to the pallet- I' FIGURE 2. - Product accumulation on outside of bags. f I" t-C-f V f < € < ^ f f f •€!* f t € f HT -« ^■r-i 4t t< i-r -t'i ftttf c » r-r-< "I'-i : *• f c f c f f c-€ '•ft* rC-i-C*"'^ < * f * tf * t t * * ? '^ ' <• f-r FIGURE 3. - Product that has escaped from bag valve during conveying process. ACKNOWLEDGMENTS The authors thank the members of the National Industrial Sand Association's Committee on Engineering and Technology for its guidance and cooperation. Spe- cial thanks also to James P. Snider, Vice President and General Manager of Central Silica Co. , for his cooperation and as- sistance in the actual testing at one of the Central Silica facilities. We would also like to thank Jon C. Volkwein, Bu- reau of Mines, for helping to establish this project. TESTING A 3-week study was performed to evalu- ate five different bag valves that were considered either to be the most common- ly used, or to be potentially the most effective. Initially, 2 weeks of testing were performed to compare the following five bag valves: 1. Standard paper (used as the normal). 2. Polyethylene. 3. Extended polyethylene, 4. Double trap (paper). 5. Foam. The standard paper and polyethylene valves are the two most common valves used in the mineral processing indus- try. The last three valves were believed to have the greatest dust reduction potential. In preparation for testing, the con- veyor and pallet-loading area were en- closed in a tunnel made of wood framing and thick plastic. A 1,300-cfm blowing fan directed air through this enclosure. By knowing the dust concentration coming into the enclosure, the amount of dust generated from bag contamination and valve leakage could be determined. The enclosure and fan provided a controlled atmosphere and reduced contamination from outside sources. Five real-time aerosol monitors (RAM) were placed at various locations through- out the bagging and pallet-loading area to measure respirable dust concentra- tions. The RAM-1 monitors, developed by GCA Corp. 3 under a Bureau contract, use a light-scattering device to calculate the dust concentration of a sample drawn in from the environment. The monitors can be sensitive to changes in the dust con- tent (size, shape, refractive index), but if calibrated to a specific dust content, their accuracy is within ±10 pet of gravimetric samplers equipped with the standard 10-mm cyclone to measure respirable dust.^ Before the RAM-1 moni- tors were taken into the field, they were Reference to specific manufacturers does not imply endorsement by the Bureau of Mines. ^VTilliams, K. L. , and R. J. Timko. Performance Evaluation of a Real-Time Aerosol Monitor. BuMines IC 8968, 1984, 20 pp. calibrated side by side in a dust box. The RAM-1 dust monitors were used in this study for a comparative evaluation. Since the dust concentration for each valve type was compared to that of each of the other four valves, it was not nec- essary to use the specific dust type for calibration. The output from these dust monitors were continuously recorded on a data logger and dual-pen strip-chart re- corders. The following sampling posi- tions were monitored (fig. 4) : Intake. — Located upstream of the blower to measure intake dust concentrations. Operator. — Located above the bag opera- tor to measure the dust in the bag pit area. Transfer. — Located slightly downstream from the conveyor transfer point to mea- sure dust generated at that point. Right and left stacker. — A cyclone at- tached to lapel of each bag stacker to give a direct indication of their dust exposure. Stackers KEY • Sample location -^Airflow — Enclosure Transfer Bag room intake '^f^ — L_ { Intake % 7 / / / / -7-} \ Operator fir/j7illI iii/J7///7 -7-r FIGURE 4. - Sampling setup for field testing. In all cases, the 10-mm cyclone was physically separated from the monitor and connected by Tygon vinyl tubing. To mon- itor a worker's dust exposure, a cyclone was attached to the worker's lapel, as shown in figure 5. The cyclone was con- nected to the dust monitor by the vinyl tubing to allow the worker to perform his job function with minimal interference. Since the tubing length at each sample location remained the same, deposition on the inner tubing walls was neglected, especially since the monitors were used for a comparative evaluation in comparing one valve type to another. Each valve type was tested separately on a truck-by-truck basis (480 bags). After one truck was loaded, the bags were changed to a different valve type, until each valve type had been tested; then the cycle was repeated. At the conclusion of the 2-week test period, the data were an- alyzed for each valve type. The results of the 2-week test are shown in table 1 for 325-mesh product. The values in the table represent a time- weighted average concentration for the dust measurements at each location. The measured dust concentration for each truck was multiplied by the run time. The value for each truck was then summed to give the average concentration for each monitoring location. The intake dust concentrations were subtracted for each of the sampling locations. The mea- sured dust concentrations for each posi- tion were ranked from one to five (lowest to highest). These ranking values were totaled to indicate an overall compara- tive rating for each valve. The lower the number, the more effective the valve was at reducing bag-generated dust. The results from the foam valve were limited to one sampling period because the valve was undersized, which caused difficulty in placing the bag on the nozzle, and would not allow the bag to eject automatically after filling. This under- sized valve created production delays and prevented additional testing of the foam valve. The extended polyethylene valve showed the greatest dust reduction during the conveying and pallet-loading process. The foam valve also gave substantial dust reductions, but it must be remembered that this testing was limited. Dust con- centrations with the double trap and the polyethylene valves were higher than with the standard paper valve except at the bag stacker location for the polyethylene valve. Based on these results, it was decided to perform some additional testing on the standard paper, the extended polyethyl- ene, and the foam valves. Additional foam valve bags were acquired with the correct valve size. Due to financial contraints by the cooperating plant for this study, it was only possible to order 500 bags with foam valves. Therefore, the results of this final analysis are based on one truckload (480 bags) of each valve type (standard paper, extended polyethylene, and foam), which takes ap- proximately 50 min to load. To eliminate fluctuations due to different work pat- terns and style, each employee at the ground-silica-bagging area was asked to work the same position for the entire TABLE 1. - Results from initial 2-week test at various locations using 325-mesh product Valve Operator Transfer Bag stackers Total of Cone, mg/m^ Rank Cone, mg/m^ Rank Cone, mg/va? Rank rankings Double trap 0.20 .04 .02 .24 .05 4 2 1 5 3 0.67 .15 .25 .47 .41 5 1 2 4 3 0.76 .32 .56 .46 .55 5 1 4 2 3 14 Extended polyethylene Foam 4 7 Polyethylene 11 Standard paper 9 NOTE. — Intake dust levels have been subtracted from measured dust concentrations. FIGURE 5. - Sampling setup to monitor worker's dust exposure. test. An additional sample location was included and one sample location was im- proved, as follows: Operator. — The cyclone was relocated to the bag operator's lapel to give a direct indication of dust exposure. Bag room intake. — The cyclone was lo- cated at the intake window into the bag room to indirectly measure the amount of dust liberated during the conveying process. Due to truck schedules the week of test- ing, it was more advantageous to evaluate the bag valves using 200-mesh product than the 325 mesh used for the 2-week evaluation. The results of this final analysis are shown in table 2. Because the intake concentrations did not fluctuate sig- nificantly during the 2-week evaluation and because of a faulty pump in a RAM-1 dust monitor, the intake sample location was eliminated, and thus the intake con- centrations were not subtracted as in the first test; thus, the values are the ac- tual recorded concentrations. The ex- tended polyethylene valve had the lowest dust concentrations at all sample* loca- tions, and the foam valve showed an im- provement over the standard paper valve at each location. Figure 6 shows the percent dust reduction with these two valves over that of the standard paper valve. The two most critical sample lo- cations, those of the bag operator and the bag stacker, showed a 62-pct and a 66-pct reduction, respectively, using the extended polyethylene, as compared with a 49-pct and a 35-pct reduction, respec- tively, for the foam valve. Transfer Operator Bag room intake Bag stackers SAMPLE LOCATIONS FIGURE 6. - Dust reductions with extended poly- ethylene and foam valves compared v/ith that of stan- dard paper valve. TABLE 2. - Results from final test at various locations using 200-mesh product, milligrams per cubic meter Valve Oper- ator Trans- fer Bag room intake Bag stack- ers Extended polyethylene. Foam 0.36 .48 .94 0.69 .73 1.28 0.28 .35 .66 1.03 1.94 Standard paper 2.99 NOTE. — Intake dust levels have not been subtracted from measured dust concentrations . DISCUSSION The significant portion of bag- generated dust begins in the bag valve area. This leakage from the bag valve either directly contaminates the work environment, or soils the outside of the bag, which contaminates the work environment later during the conveying and pallet-loading process. The effec- tiveness of the different bag valves ap- pears to depend on two factors: the valve length and the material. The first factor that influences the effectiveness of each valve type is the length of the valve. The lengths of the valves used for this study are as follows, in inches: Double trap 4.5 Extended polyethylene... 6.0 Foam 4.0 Polyethylene 4.5 Standard paper 5.5 For the same valve material, the longer the valve, the more effective it is at reducing leakage. This is most clearly demonstrated by the substantial differ- ence between the extended polyethylene and the normal polyethylene valve. The additional 1.5 in of length significantly reduced leakage from the valve, and thus bag-generated dust. However, the valve should never be longer than the fill noz- zle because this could substantially in- crease product blowback. The second factor that influences the valve effectiveness is the material used in the valve: namely, paper, polyethyl- ene, and foam. Polyethylene is a plastic material, lighter than paper, and thus may provide for a more effective seal. The idea behind the foam valve is that the open cells in the foam allow excess air to exit the bag but retaining the product. If the same bag valve length were tested for each valve material, the speculated ranking of material in this test would be foam, polyethylene, and paper, from the most to the least effective. The foregoing two factors justify the order of effectiveness in the bag valve testing. For instance, the double trap, which was initially expected to be an ef- fective valve, proved to be the least ef- fective, because it was the second short- est valve and made of what we believe was the least effective material (paper). Another example is the foam valve, which was ranked as the second most effective valve. Foam is believed to be the most effective material, but the valve was the shortest one tested. If the foam valve were lengthed by an inch or two, it might be more effective than even the extended polyethylene valve. A major consideration in selecting a specific valve is the increase in cost over that of the paper valve. A number of factors influence the cost of a valve. The first is the inherent cost of the ma- terial and the quantity of material re- quired to make the valve. The second factor would be the amount of work neces- sary to make the valve, such as folding it in a specific way and the difficulty in actually gluing it in place. Table 3 shows the increased cost of the different valves over that of the standard paper valve. These costs represent the average of prices obtained from three different manufacturers, with the exception of the foam valve, which is manufactured by only two companies at the present time. When a company selects a valve type, it chooses the most cost effective valve possible, which incorporates the effec- tiveness of the valve in reducing product blowback and bag-generated dust and the cost of the valve. These tests have shown that for the five bag valves test- ed, tne extended polyethylene valve ap- pears to be the most effective valve for reducing product blowback from the bag valve, thus resulting in lower dust concentrations for bag-generated dust for the conveying and pallet-loading process. It also has the lowest cost increase (the same as the normal polyethylene) over that of the standard paper valve. The extended polyethylene, therefore, was the most cost-effective valve tested. TABLE 3. - Increase in valve cost above the cost of standard paper valve Price per Price Valve 1,000 bags. per bag, dollars cents Polyethylene 6.85 0.7 Extended polyethylene 6.85 .7 Double trap 11.17 1.1 Foam 214.98 21.5 686 277 10 CONCLUSION An evaluation was performed to deter- mine the effectiveness of five commer- cially available bag valves in reducing dust generated during bag filling, con- veying, and pallet loading. The five valves tested included standard paper, polyethylene, extended polyethylene, dou- ble trap, and foam. Two factors appeared to determine the effectiveness of these bag valves. The first was valve length, in which the longer the valve, the more effective it was in reducing product blowback and bag-generated dust. The second factor was the valve material. It is speculated that the foam was the most effective material for reducing dust gen- eration, followed by the polyethylene, and then standard paper. Considering both factors , the extended polyethyl- ene was the most effective valve tested, and resulted in a 62-pct reduction in respirable dust exposure for the bag op- erator, and a 66-pct reduction for the bag stackers. The extended polyethylene was also the most cost-effective bag valve tested. The increase in cost for the extended polyethylene valve is ap- proximately $6.85 per thousand bags (0.7 cent per bag) over that of the standard paper valve. ■i us GOVERNMENT PRINTING OFFICE 1966-605-01 7/40,008 INT.-BU.O F MIN ES,PGH.,P A. 28217 U.S. Department of the Interior Bureau of Mines— Prod, and Distr. Cochrans Mill Road P.O. Box 18070 Pittsburgh, Pa. 15236 AN EQUAL OPPORTUNITY EMPLOYER OFFICIAL BUSINESS PENALTY FOB PRIVATE USE. $300 I I Do not wi sh to receive this material, pleose remove from your mailing list* I I Address change. Please correct as indicated. ^ JJ °- ,/.-i;fe-.\. o°*..i^-.% .**\.i:^X /.'ji;;-X"v.-^..\'°-' ^o*^... *bv" ,^'^^^ °.,'^m¥; .^"^^.^ ^."^i^« .rN^-^^ o^^^m^- c^^. ^^^m^^i ..V^. *° ^'^'^ ■.^' T7^'\ *3 ^.^^'^^ '^"H^^.* ^ **'% .^^\ .^' "4. A" o *,.■,• ^.. v-^*y' %*^-'%o'' V"^'*.^^ >^ .. .* <=lK ,*^' *^ . ,'. '-^u-n^ ov^^^^iia'. ^»•..■^^ o . » - .0 o v-0^ LIBRARY OF CONGRESS '0662955 8851 ■^