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Paper pulp refiner long-duration wear monitoring with polymer replicas
Wear 267 (2009) 1095–1099
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Paper pulp refiner long-duration wear monitoring with polymer replicas W. Frazier a , D.R. Danks b,∗ , B. Hodge c a
Andritz Inc. (Ret), Portland, OR, United States Danks Tribological Services, Portland OR, United States c Andritz Inc., Muncy, PA, United States b
a r t i c l e
i n f o
Article history: Received 3 September 2008 Received in revised form 25 January 2009 Accepted 29 January 2009 Keywords: Field test Wear Replica Paper mill Long-duration test
a b s t r a c t Between 2001 and 2004 a wear test was conducted in an operating Oregon paper mill. The objectives were to determine if there was a significant wear difference between two refiner alloys and to monitor and record wear while minimizing the impact on production. Four complete circles of plates were installed in a Sprout refiner. One circle was all Andritz A10 alloy, one was 17-4PH and the other two were alternating 17-4PH and A10 plates. Before installation and after removal bar height was measured with a micrometer. Wear surface replicas were taken five times during and after the test which started in August 2001 and ended in October 2004. It was found that 17-4PH wore at a slightly higher rate than the A10 in this application. The replica method of measuring and recording wear was very effective, producing data with good precision and accuracy and also minimizing disruption to the operating mill. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Wear testing in operating industrial environments is difficult. Variations in raw materials, operating parameters, personnel, and corporate economic requirements affect and confuse seemingly simple technical evaluations [1]. This study was designed to produce permanent, accurate records of wearing surfaces in the form of polymer replicas. The replicas needed to be produced rapidly to (1) minimize production interruption and (2) enable evaluation away from the plant in a more controlled environment. The project took place in an operating paper pulping mill testing the wear of refiner plates. Chemical wood pulp fibers are modified mechanically to produce optimal paper properties. Sets of plate segments may last for several years and be subject to a variety of pulp types and force loading conditions so that it is difficult to compare alloys and their wear performances unless they are used in one refiner filling. A slurry of pulp at 4–5% consistency (dry weight/wet weight) is passed through refiners where the rotating plates pass fixed plates, both with sharp, radially oriented raised bars on the surfaces. Opposing bars on the surfaces trap, stretch, abrade and cut fibers to improve their flexibility and bonding characteristics when formed into paper. This process is most efficient when the leading edges of passing radial bars are sharp and unworn [2,3]. As these leading edges wear and round over, primarily from hard contaminants in the pulp slurry,
∗ Corresponding author. Tel.: +1 503 318 3877. E-mail address: [email protected] (D.R. Danks). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.01.017
more electric power is required to meet the pulp quality required thus increasing costs to the paper maker [4]. The general wear rates and more specifically the edge rounding of the refiner plate material is of interest to the refiner manufacturers and paper plants alike [5]. This study compared the wear rates of a standard alloy, 17-4 PH and a similar alloy with a surface hardening treatment called Andritz A10. The leading edges are cast into the surface of the metal plate segments and appear as approximately radially oriented bars. Only the face and back of these segments are ground smooth leaving the sides of the bars and the grooves between them in the rough, ascast state. Wear affects two bar dimensions, the reduction of bar height and rounding of the leading edges. Eventually the capacity of the refiner is reduced by the loss of bar height and the required pulp quality cannot be achieved with any amount of motor load. The complete set of plate segments is then replaced requiring 6–8 h of downtime. 2. Experimental 2.1. Timeline The test was commenced in August 2001. The refiner plate bar heights were measured by dial calliper and by the first set of replicas prior to the plates being installed in the refiner. Approximately 1 month later (September 2001) the refiner was shut down and partially opened and a second set of replicas was taken. The third, fourth and fifth replica sets were taken in January 2002, May 2002 and October 2004. Although the mill was unable to find the exact
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Table 1 Refiner plate configuration. Location in refiner
Plate configuration
Adjustment end
Stator Rotor
All A10 Alternating A10/17-4PH
Motor end
Stator Rotor
All 17-4 PH Alternating A10/17-4PH
date the plates were removed from service, records indicate they were probably taken out of the refiner in September 2003. Not counting down time the plates were in service for about 25 months. The last bar measurements took place on 1 October 2004. 2.2. Refiner plate configuration The refiner has two refining zones and the pulp slurry is fed under hydraulic pressure to the center of each zone and forced radially outward through the refiner plate area. The rotor can move axially to accommodate wear and small differences in flow rates. Loading is accomplished by moving the adjustment end stator toward the rotor until both refining zones are actively refining pulp. Four complete circles of refiner plates were specially made the Durametal Corporation (presently Andritz Inc.) and installed in a Sprout 300 kW (400 horsepower), 700 RPM refiner. The configuration was designed to provide data on the wear of two alloys, the differences between the rotors and stators, and the differences between the motor and adjustment ends. The D7E155/156 plate arrangement for the 26 in. Sprout is detailed in Table 1. Table 2 lists the available operating data of the refiner over the course of the test. Normally the refiner would be run with all one material in both zones but this experiment tested whether A10 could wear more slowly working against 17-4PH or A10. According to Blickensderfer this would be classified as a concurrent group test. 2.3. Measuring bar height with a dial calliper Bar heights were measured with a dial calliper prior to installation (6 August 2001) and after removal (1 October 2004). Calliper measurements were taken in the same areas from which the replicas would be taken (Fig. 1). It was not possible to place the calliper in precisely the same location each time. 2.4. Replicas
Fig. 1. Bar height measurements and replicas were taken on the test plates at locations marked.
A standard kit costs $62.00 (2002) and will produce approximately 25 each 75 mm diameter replicas. To take replicas of the refiner plates the refiner was shut down and opened just far enough to allow access to the plate surfaces, approximately 18 cm (6 in.). Any pulp between bars was removed with a small screwdriver. A golf ball sized lump, half by volume parts A and B each, was mixed immediately prior to placing on the refiner plate in each test location (Fig. 2). The replica was pushed into spaces between the bars insuring that the material was forced all the way to the bottom of the grooves. A specimen number was impressed into the back side of the replica. It was then allowed to set for approximately 10 min. Once the polymer had set it was easily peeled from the plate in a single piece. The replicas were taken on 6 August 2001, 5 September 2001, 16 January 2002, 31 May 2002 and 22 October 2004. For identification the two monitored segments from each circle were marked with grooves ground into the periphery of the segment. Although precise data has not been collected, the replicas do not appear to change dimension appreciably over a 3–4-year interval. 2.5. Bar replica wear measurement After removal from the refiner the replicas were cut with a scalpel perpendicular to the bar orientation. Each cross-section was placed on a metallograph with the top of the bar in view (Fig. 3). A digital image of the bar cross-section was opened in an image
The dental replica material used in this project was Exoflex made by G.C. America (1-800-323-3386, www.gcamerica.com). It is a two-part mix, typically 500 g part A, 500 g part B, with a set time of approximately 5 min. It was selected based on its set time, precision, and ability to work in conditions similar to the temperature, humidity and pH conditions that exists in a low-consistency refiner. Table 2 Sprout refiner operating information. Parameter
Data
Pulp type Production range Consistency Fiber processed Motor load Gross specific energy Net specific energy Operation time Tons processed KM/Rev
Mixed Pacific softwoods 1135–1514 lpm (300–400 gpm) 3.9–4.2% 63,500–90,100 kg/day (70–100 oven dry tons/day) 300 kW (400 horsepower) 4.2–5.5 HPD/T 3.6–4.6 NHPD/T 3 years (±2 months) or 26,000 h 82 × 106 ODkg (91,000 ODT) 12.96
Fig. 2. Replica on refiner plate inside refiner.
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Fig. 3. (a and b) Replicas of a refiner plate bar in new condition (left) and worn (right) conditions.
analysis program. In the image a 50 mm × 50 mm box was drawn on the bar with the bar top and side forming two sides of the box. In the box the amount of bar that had worn away was light (replica material) and the portion of the bar that had not worn away was black (the replica is a negative of the refiner plate bar). The image analysis system quantified the amount of light and dark areas. This gave a percent of total area that had been lost to wear. Since the same magnification (50×) and area were used for all the replicas it was possible to compare the different bars, segments, wheels and alloys. Each replica covered up to seven bars. To improve the objectivity of the wear measurements, each replica was known only by specimen number during the measurement process. The sample location and alloy were not associated with the specimen until after the measurements had been taken and the statistical data calculated.
3. Results 3.1. Recovery of worn plates The segments were recovered in October 2004. Precise service hours and tons of pulp processed were not recorded due to personnel changes, a common event in long-term field testing. Plant records indicate the plates were removed in September 2003. Based on that date and general plant operating conditions, they ran approximately 17,000 h and processed about 61,000 oven dry short tons of pulp. The plates occasionally had bailing wire jammed between the bars, approximately one piece per segment, but the plates appear to have never been clashed or run against each other without pulp. 3.2. Dial calliper bar height measurements
2.6. Analysis of wear data Between 2 and 7 bars were duplicated by each replica. Therefore each segment wear data consisted of measurements of 2–7 wear data points. An average and standard deviation of the missing bar areas (wear) were calculated for each segment based on the image analysis data. Several analyses were conducted on the wear data. The first was to use the replicas from new plates to generate data on the baseline precision and accuracy of the replica technique. Since the plate bars were new and unworn, the replica data would ideally indicate 0% wear. Therefore the averages and standard deviations on the new plates were used as a gauge of precision of the replica and the image analysis procedures. Once the baseline precision had been established with new plates, the wear rates of the two alloys and the wear rates of the complete circles by location (adjustment end rotor, etc.) were compared. Finally the standard deviation of the wear averages was used to quantify the data variability over the life of the testing. The replica data was also compared to the calliper bar height measurements. To check the repeatability of the replica/image analysis process, the first replica measured in the last set was measured twice, once at the start of the measurement session and then at the end.
Table 3 lists the changes in height for each measurement and averages by circle and alloy. Some measurements show an increase in bar height which is impossible but probably attributable to the as-cast surface roughness in between the refiner plate bars. Using the student t test the difference in bar heights from August 01 to October 04 was highly significant even with the negative values caused by the rough as-cast surfaces and inability to measure at the exact same spot. Further testing of the data showed the wear rates of the 17-4PH and A10 were not significantly different although it appears from these data the A10 wore slightly slower than the 17-4. In the mixed alloy rings the nearly equal wear of adjacent plates would indicate that neighbor segments generally did not shield or shelter the other. 3.3. Replicas Fig. 4 is a typical replica cross-section. Figs. 5–7 illustrate the Baseline precision, comparative wear rates of the two alloys and rotor vs. stator wear. The replicas made accurate permanent negatives of the plate bars. Minute details such as sharp, as-machined surfaces including grinding marks were reproduced. The replicas duplicated plastically flowed metal flash that often formed on the trailing edge of the bar. The rounding that occurred on bar leading edges was also
Table 3 Average bar height loss from 8/2001 to 10/2004. Circle, location and alloys
D7E156 Adjustment End Rotor (mixed) D7E156 Adjustment End Stator (A10) D7E155 Motor End Stator (17-4PH) D7E155 Motor End Rotor (mixed)
Changes in bar height (mm) Average of circle
17-4PH average
A10 average
0.07162
0.0272
0.0297 0.0142
0.0391
0.0148 0.0397
0.0375
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Fig. 7. Refiner plate bar edge average wear, rotor and stator, by months in service.
Fig. 4. Replica of worn plate bar, leading edge is upper right corner.
Fig. 5. New (unworn) bar edge baseline measurement average and standard deviation.
accurately reproduced. The polymer functioned well in the refiner environment, even if the refiner was opened immediately after shutdown and the surface temperature exceeded 38 ◦ C. The wear measured by the replicas increased continuously over the life of the plates. The unworn baseline, measured on new, unworn bars produced average variations from a low of 1% to a high of about 4.5% (Fig. 5). Ideally the “wear” of a new plate should be zero. On all plates the baseline variation was significantly different from the wear measured on used plates. In addition, the overlap between new and worn bar dimensions was limited to a few tenths of a percent. The standard deviation of new plate baseline was about 2% or less of the total area measured. In a sample that was measured
Fig. 6. A10 and 17-4 PH refiner plate bar edge wear rate by months in service.
twice (specimen #10-2) at the start and end of the last replica image analysis session, the results were nearly identical—14.54 (1.51) and 14.96 (2.14), average % area measured loss (and standard deviation), first and second measurements, respectively. The wear of bars that had been in service for about 2 years was between 12 and 22% of the measured area with the standard deviation remaining near or less than 5% (Fig. 6). In general it appears that, except for the first set of replicas, A10 wears slightly slower than does 17-4 in this application. A10 occupies the lower end of the measured wear range for all but the pre-installation set. When sorted by rotor vs. stator wear, rotor wear tended to exceed stator data but there was no definitive pattern (Fig. 7). Comparing the wear on the motor vs. adjustable ends of the refiner, the results were mixed. The adjustable end rotor wear appears to be highest and the adjustable end stator lowest, especially at the 25 month interval. The motor end wear, both rotor and stator are generally somewhere in between. 4. Analysis and discussion Generating accurate and precise data in industrial environments is very difficult. The list of uncontrollable and changing conditions that affect the data is endless. The value of such data, however, makes such an endeavour worthwhile. This project attempted two things: (1) test the relative wear rates of Durametal A10 and 17-4PH and (2) further the art of long-term wear monitoring of an operating system, in this case a low-consistency paper mill with a replica sampling technique. Comparative wear data on the two alloys is complicated by the fact that they are very similar, differing only in surface treatment. The replica technique was used to minimize the impact on the mill production while producing data that could be quantified and evaluated away from the mill setting. The wear test results indicate that in this application A10 has a slightly lower wear rate than 17-4PH. The precise reason why there was a measureable wear difference was not investigated. Both bar height measurement and replica bar edge rounding support the wear rate difference, although the bar height measurement technique is not as conclusive. In addition, some data on rotor vs. stator and motor end vs. adjustable end was generated. Although the calliper bar height measuring method indicated slightly less relative wear than the replicas, it was invaluable because it is more direct than taking replicas and also because it provided a standard against which to judge the replicas. The “growth” of some bars indicates improvements in the calliper method are possible. For example, more accurate data would result if the measurement location could be precisely and repeatedly accessed. Unless some form of location referencing equipment (e.g. CMM) is accessible, returning to the original measurement point will be
W. Frazier et al. / Wear 267 (2009) 1095–1099
less than perfect. In addition, it would be beneficial to prepare the surface so casting feature effects are minimized. The as-cast grooves between the bars are not designed to be measurement references and changes in the groove condition over 2 years can be significant. Baked-in oxides, mold and mold wash materials, variability in molds or cores will all tend to confound the measurement datum. The direct bar height measurement is also more difficult or impossible if the segments are not removed from the refiner for measurement. In general the replica technique worked very well. The replicas reproduced very fine detail, e.g. manufacturing surface grinding grooving, that was preserved and available for later analysis. The image analysis method of quantifying the amount of wear from refiner bar leading edges appears to produce significantly accurate and precise data. Using the standard deviations as a gauge, the measurement variability remained significantly lower than the actual wear averages. If the same operator makes the measurements variability is low as demonstrated by the multiple measuring of a single sample. Error sources of the replicas are the draft angle of the refiner plate bar and operator judgment on locating the analysis area. A square-cornered box cannot be perfectly superimposed on a refiner bar cross-section due to the draft angle of the refiner bar. Therefore judgments are made on where to position the image analysis box onto the non-90◦ angle bars. Another source of error with the replica technique is the irregular, as-cast refiner plate surface. Because the cast surfaces are not perfectly flat, the imposed measurement box sometimes crosses imperfections on the casting surface. It is also possible to compensate for this by intentionally locating the analysis area such that the areas of irregular bar and space were again approximately equal. A more flexible image analysis software program that followed the contrast line along the edge of the replica would be a more precise solution. Another issue than can be addressed with the replica data is whether or not bar edge rounding gets progressively more severe or if it reaches some point and further plate wear occurs only as bar height loss. This trial indicates that bar rounding gets progressively worse throughout the life of the plate and does not reach some equilibrium and stabilize. This has obvious ramifications for production efficiency and quality.
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5. Conclusions 1. The calliper bar height measurements and replica bar edge rounding data indicate that 17-4PH wears somewhat more rapidly than A10 in this refiner. 2. The data indicate wear in this refiner on the motor end is somewhat lower than that on the adjustable end and that rotor wear is slightly higher than stator wear. 3. The replica technique generates accurate data with minimal refiner disassembly. Using the technique it is not necessary to completely disassemble the refiner or remove plates to generate good wear data throughout the life of the plates. 4. The image analysis aspect of the procedure is also a viable measurement technique. 5. Both the bar height and replica data indicate similar wear rates. “Growth” of bars as measured by initial and final bar height is an indication of the difficulty of the bar height measurement technique. Improvements in location and measurement reference points would improve the bar height data. 6. Bar replica analyses indicate the continual increase of edge rounding in addition to height loss throughout plate life. Acknowledgements The authors wish to acknowledge Andritz for permission to publish and also the staff of Dr. J. Mattson, DMD, Portland OR, with special appreciation to Brenda, for their kind assistance with the replica materials. References [1] Blickenderfer, R., Design Criteria and Correction Factors for Field Wear Testing, Wear of Materials 1987, Houston, TX, April 5–9, 1984, pp. 877–884. [2] C.B. Thompson, The cavitation erosion resistance of refiner plate alloys, Journal of Pulp and Paper Science 16 (January (1)) (1990) j20–j27. [3] Jia, Y., Failure Analysis and Material Evaluation of Thermal-Mechanical Pulping Refiner Plates, Ph.D. dissertation, Oregon Graduate Institute of Science and Technology, October 1995. [4] C.B. Thompson, Gardner, The resistance of refiner plate alloys to bar rounding, in: Technical Association of the Paper and Pulp Institute (TAPPI) Engineering Conference, 1987, pp. 189–198. [5] P.L. Wasikowski, Properties of refiner plates used in thermo-mechanical pulping, Pulp and Paper 90 (12) (1989) 209–212.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Paper pulp refiner long-duration wear monitoring with polymer replicas W. Frazier a , D.R. Danks b,∗ , B. Hodge c a
Andritz Inc. (Ret), Portland, OR, United States Danks Tribological Services, Portland OR, United States c Andritz Inc., Muncy, PA, United States b
a r t i c l e
i n f o
Article history: Received 3 September 2008 Received in revised form 25 January 2009 Accepted 29 January 2009 Keywords: Field test Wear Replica Paper mill Long-duration test
a b s t r a c t Between 2001 and 2004 a wear test was conducted in an operating Oregon paper mill. The objectives were to determine if there was a significant wear difference between two refiner alloys and to monitor and record wear while minimizing the impact on production. Four complete circles of plates were installed in a Sprout refiner. One circle was all Andritz A10 alloy, one was 17-4PH and the other two were alternating 17-4PH and A10 plates. Before installation and after removal bar height was measured with a micrometer. Wear surface replicas were taken five times during and after the test which started in August 2001 and ended in October 2004. It was found that 17-4PH wore at a slightly higher rate than the A10 in this application. The replica method of measuring and recording wear was very effective, producing data with good precision and accuracy and also minimizing disruption to the operating mill. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Wear testing in operating industrial environments is difficult. Variations in raw materials, operating parameters, personnel, and corporate economic requirements affect and confuse seemingly simple technical evaluations [1]. This study was designed to produce permanent, accurate records of wearing surfaces in the form of polymer replicas. The replicas needed to be produced rapidly to (1) minimize production interruption and (2) enable evaluation away from the plant in a more controlled environment. The project took place in an operating paper pulping mill testing the wear of refiner plates. Chemical wood pulp fibers are modified mechanically to produce optimal paper properties. Sets of plate segments may last for several years and be subject to a variety of pulp types and force loading conditions so that it is difficult to compare alloys and their wear performances unless they are used in one refiner filling. A slurry of pulp at 4–5% consistency (dry weight/wet weight) is passed through refiners where the rotating plates pass fixed plates, both with sharp, radially oriented raised bars on the surfaces. Opposing bars on the surfaces trap, stretch, abrade and cut fibers to improve their flexibility and bonding characteristics when formed into paper. This process is most efficient when the leading edges of passing radial bars are sharp and unworn [2,3]. As these leading edges wear and round over, primarily from hard contaminants in the pulp slurry,
∗ Corresponding author. Tel.: +1 503 318 3877. E-mail address: [email protected] (D.R. Danks). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.01.017
more electric power is required to meet the pulp quality required thus increasing costs to the paper maker [4]. The general wear rates and more specifically the edge rounding of the refiner plate material is of interest to the refiner manufacturers and paper plants alike [5]. This study compared the wear rates of a standard alloy, 17-4 PH and a similar alloy with a surface hardening treatment called Andritz A10. The leading edges are cast into the surface of the metal plate segments and appear as approximately radially oriented bars. Only the face and back of these segments are ground smooth leaving the sides of the bars and the grooves between them in the rough, ascast state. Wear affects two bar dimensions, the reduction of bar height and rounding of the leading edges. Eventually the capacity of the refiner is reduced by the loss of bar height and the required pulp quality cannot be achieved with any amount of motor load. The complete set of plate segments is then replaced requiring 6–8 h of downtime. 2. Experimental 2.1. Timeline The test was commenced in August 2001. The refiner plate bar heights were measured by dial calliper and by the first set of replicas prior to the plates being installed in the refiner. Approximately 1 month later (September 2001) the refiner was shut down and partially opened and a second set of replicas was taken. The third, fourth and fifth replica sets were taken in January 2002, May 2002 and October 2004. Although the mill was unable to find the exact
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Table 1 Refiner plate configuration. Location in refiner
Plate configuration
Adjustment end
Stator Rotor
All A10 Alternating A10/17-4PH
Motor end
Stator Rotor
All 17-4 PH Alternating A10/17-4PH
date the plates were removed from service, records indicate they were probably taken out of the refiner in September 2003. Not counting down time the plates were in service for about 25 months. The last bar measurements took place on 1 October 2004. 2.2. Refiner plate configuration The refiner has two refining zones and the pulp slurry is fed under hydraulic pressure to the center of each zone and forced radially outward through the refiner plate area. The rotor can move axially to accommodate wear and small differences in flow rates. Loading is accomplished by moving the adjustment end stator toward the rotor until both refining zones are actively refining pulp. Four complete circles of refiner plates were specially made the Durametal Corporation (presently Andritz Inc.) and installed in a Sprout 300 kW (400 horsepower), 700 RPM refiner. The configuration was designed to provide data on the wear of two alloys, the differences between the rotors and stators, and the differences between the motor and adjustment ends. The D7E155/156 plate arrangement for the 26 in. Sprout is detailed in Table 1. Table 2 lists the available operating data of the refiner over the course of the test. Normally the refiner would be run with all one material in both zones but this experiment tested whether A10 could wear more slowly working against 17-4PH or A10. According to Blickensderfer this would be classified as a concurrent group test. 2.3. Measuring bar height with a dial calliper Bar heights were measured with a dial calliper prior to installation (6 August 2001) and after removal (1 October 2004). Calliper measurements were taken in the same areas from which the replicas would be taken (Fig. 1). It was not possible to place the calliper in precisely the same location each time. 2.4. Replicas
Fig. 1. Bar height measurements and replicas were taken on the test plates at locations marked.
A standard kit costs $62.00 (2002) and will produce approximately 25 each 75 mm diameter replicas. To take replicas of the refiner plates the refiner was shut down and opened just far enough to allow access to the plate surfaces, approximately 18 cm (6 in.). Any pulp between bars was removed with a small screwdriver. A golf ball sized lump, half by volume parts A and B each, was mixed immediately prior to placing on the refiner plate in each test location (Fig. 2). The replica was pushed into spaces between the bars insuring that the material was forced all the way to the bottom of the grooves. A specimen number was impressed into the back side of the replica. It was then allowed to set for approximately 10 min. Once the polymer had set it was easily peeled from the plate in a single piece. The replicas were taken on 6 August 2001, 5 September 2001, 16 January 2002, 31 May 2002 and 22 October 2004. For identification the two monitored segments from each circle were marked with grooves ground into the periphery of the segment. Although precise data has not been collected, the replicas do not appear to change dimension appreciably over a 3–4-year interval. 2.5. Bar replica wear measurement After removal from the refiner the replicas were cut with a scalpel perpendicular to the bar orientation. Each cross-section was placed on a metallograph with the top of the bar in view (Fig. 3). A digital image of the bar cross-section was opened in an image
The dental replica material used in this project was Exoflex made by G.C. America (1-800-323-3386, www.gcamerica.com). It is a two-part mix, typically 500 g part A, 500 g part B, with a set time of approximately 5 min. It was selected based on its set time, precision, and ability to work in conditions similar to the temperature, humidity and pH conditions that exists in a low-consistency refiner. Table 2 Sprout refiner operating information. Parameter
Data
Pulp type Production range Consistency Fiber processed Motor load Gross specific energy Net specific energy Operation time Tons processed KM/Rev
Mixed Pacific softwoods 1135–1514 lpm (300–400 gpm) 3.9–4.2% 63,500–90,100 kg/day (70–100 oven dry tons/day) 300 kW (400 horsepower) 4.2–5.5 HPD/T 3.6–4.6 NHPD/T 3 years (±2 months) or 26,000 h 82 × 106 ODkg (91,000 ODT) 12.96
Fig. 2. Replica on refiner plate inside refiner.
W. Frazier et al. / Wear 267 (2009) 1095–1099
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Fig. 3. (a and b) Replicas of a refiner plate bar in new condition (left) and worn (right) conditions.
analysis program. In the image a 50 mm × 50 mm box was drawn on the bar with the bar top and side forming two sides of the box. In the box the amount of bar that had worn away was light (replica material) and the portion of the bar that had not worn away was black (the replica is a negative of the refiner plate bar). The image analysis system quantified the amount of light and dark areas. This gave a percent of total area that had been lost to wear. Since the same magnification (50×) and area were used for all the replicas it was possible to compare the different bars, segments, wheels and alloys. Each replica covered up to seven bars. To improve the objectivity of the wear measurements, each replica was known only by specimen number during the measurement process. The sample location and alloy were not associated with the specimen until after the measurements had been taken and the statistical data calculated.
3. Results 3.1. Recovery of worn plates The segments were recovered in October 2004. Precise service hours and tons of pulp processed were not recorded due to personnel changes, a common event in long-term field testing. Plant records indicate the plates were removed in September 2003. Based on that date and general plant operating conditions, they ran approximately 17,000 h and processed about 61,000 oven dry short tons of pulp. The plates occasionally had bailing wire jammed between the bars, approximately one piece per segment, but the plates appear to have never been clashed or run against each other without pulp. 3.2. Dial calliper bar height measurements
2.6. Analysis of wear data Between 2 and 7 bars were duplicated by each replica. Therefore each segment wear data consisted of measurements of 2–7 wear data points. An average and standard deviation of the missing bar areas (wear) were calculated for each segment based on the image analysis data. Several analyses were conducted on the wear data. The first was to use the replicas from new plates to generate data on the baseline precision and accuracy of the replica technique. Since the plate bars were new and unworn, the replica data would ideally indicate 0% wear. Therefore the averages and standard deviations on the new plates were used as a gauge of precision of the replica and the image analysis procedures. Once the baseline precision had been established with new plates, the wear rates of the two alloys and the wear rates of the complete circles by location (adjustment end rotor, etc.) were compared. Finally the standard deviation of the wear averages was used to quantify the data variability over the life of the testing. The replica data was also compared to the calliper bar height measurements. To check the repeatability of the replica/image analysis process, the first replica measured in the last set was measured twice, once at the start of the measurement session and then at the end.
Table 3 lists the changes in height for each measurement and averages by circle and alloy. Some measurements show an increase in bar height which is impossible but probably attributable to the as-cast surface roughness in between the refiner plate bars. Using the student t test the difference in bar heights from August 01 to October 04 was highly significant even with the negative values caused by the rough as-cast surfaces and inability to measure at the exact same spot. Further testing of the data showed the wear rates of the 17-4PH and A10 were not significantly different although it appears from these data the A10 wore slightly slower than the 17-4. In the mixed alloy rings the nearly equal wear of adjacent plates would indicate that neighbor segments generally did not shield or shelter the other. 3.3. Replicas Fig. 4 is a typical replica cross-section. Figs. 5–7 illustrate the Baseline precision, comparative wear rates of the two alloys and rotor vs. stator wear. The replicas made accurate permanent negatives of the plate bars. Minute details such as sharp, as-machined surfaces including grinding marks were reproduced. The replicas duplicated plastically flowed metal flash that often formed on the trailing edge of the bar. The rounding that occurred on bar leading edges was also
Table 3 Average bar height loss from 8/2001 to 10/2004. Circle, location and alloys
D7E156 Adjustment End Rotor (mixed) D7E156 Adjustment End Stator (A10) D7E155 Motor End Stator (17-4PH) D7E155 Motor End Rotor (mixed)
Changes in bar height (mm) Average of circle
17-4PH average
A10 average
0.07162
0.0272
0.0297 0.0142
0.0391
0.0148 0.0397
0.0375
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Fig. 7. Refiner plate bar edge average wear, rotor and stator, by months in service.
Fig. 4. Replica of worn plate bar, leading edge is upper right corner.
Fig. 5. New (unworn) bar edge baseline measurement average and standard deviation.
accurately reproduced. The polymer functioned well in the refiner environment, even if the refiner was opened immediately after shutdown and the surface temperature exceeded 38 ◦ C. The wear measured by the replicas increased continuously over the life of the plates. The unworn baseline, measured on new, unworn bars produced average variations from a low of 1% to a high of about 4.5% (Fig. 5). Ideally the “wear” of a new plate should be zero. On all plates the baseline variation was significantly different from the wear measured on used plates. In addition, the overlap between new and worn bar dimensions was limited to a few tenths of a percent. The standard deviation of new plate baseline was about 2% or less of the total area measured. In a sample that was measured
Fig. 6. A10 and 17-4 PH refiner plate bar edge wear rate by months in service.
twice (specimen #10-2) at the start and end of the last replica image analysis session, the results were nearly identical—14.54 (1.51) and 14.96 (2.14), average % area measured loss (and standard deviation), first and second measurements, respectively. The wear of bars that had been in service for about 2 years was between 12 and 22% of the measured area with the standard deviation remaining near or less than 5% (Fig. 6). In general it appears that, except for the first set of replicas, A10 wears slightly slower than does 17-4 in this application. A10 occupies the lower end of the measured wear range for all but the pre-installation set. When sorted by rotor vs. stator wear, rotor wear tended to exceed stator data but there was no definitive pattern (Fig. 7). Comparing the wear on the motor vs. adjustable ends of the refiner, the results were mixed. The adjustable end rotor wear appears to be highest and the adjustable end stator lowest, especially at the 25 month interval. The motor end wear, both rotor and stator are generally somewhere in between. 4. Analysis and discussion Generating accurate and precise data in industrial environments is very difficult. The list of uncontrollable and changing conditions that affect the data is endless. The value of such data, however, makes such an endeavour worthwhile. This project attempted two things: (1) test the relative wear rates of Durametal A10 and 17-4PH and (2) further the art of long-term wear monitoring of an operating system, in this case a low-consistency paper mill with a replica sampling technique. Comparative wear data on the two alloys is complicated by the fact that they are very similar, differing only in surface treatment. The replica technique was used to minimize the impact on the mill production while producing data that could be quantified and evaluated away from the mill setting. The wear test results indicate that in this application A10 has a slightly lower wear rate than 17-4PH. The precise reason why there was a measureable wear difference was not investigated. Both bar height measurement and replica bar edge rounding support the wear rate difference, although the bar height measurement technique is not as conclusive. In addition, some data on rotor vs. stator and motor end vs. adjustable end was generated. Although the calliper bar height measuring method indicated slightly less relative wear than the replicas, it was invaluable because it is more direct than taking replicas and also because it provided a standard against which to judge the replicas. The “growth” of some bars indicates improvements in the calliper method are possible. For example, more accurate data would result if the measurement location could be precisely and repeatedly accessed. Unless some form of location referencing equipment (e.g. CMM) is accessible, returning to the original measurement point will be
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less than perfect. In addition, it would be beneficial to prepare the surface so casting feature effects are minimized. The as-cast grooves between the bars are not designed to be measurement references and changes in the groove condition over 2 years can be significant. Baked-in oxides, mold and mold wash materials, variability in molds or cores will all tend to confound the measurement datum. The direct bar height measurement is also more difficult or impossible if the segments are not removed from the refiner for measurement. In general the replica technique worked very well. The replicas reproduced very fine detail, e.g. manufacturing surface grinding grooving, that was preserved and available for later analysis. The image analysis method of quantifying the amount of wear from refiner bar leading edges appears to produce significantly accurate and precise data. Using the standard deviations as a gauge, the measurement variability remained significantly lower than the actual wear averages. If the same operator makes the measurements variability is low as demonstrated by the multiple measuring of a single sample. Error sources of the replicas are the draft angle of the refiner plate bar and operator judgment on locating the analysis area. A square-cornered box cannot be perfectly superimposed on a refiner bar cross-section due to the draft angle of the refiner bar. Therefore judgments are made on where to position the image analysis box onto the non-90◦ angle bars. Another source of error with the replica technique is the irregular, as-cast refiner plate surface. Because the cast surfaces are not perfectly flat, the imposed measurement box sometimes crosses imperfections on the casting surface. It is also possible to compensate for this by intentionally locating the analysis area such that the areas of irregular bar and space were again approximately equal. A more flexible image analysis software program that followed the contrast line along the edge of the replica would be a more precise solution. Another issue than can be addressed with the replica data is whether or not bar edge rounding gets progressively more severe or if it reaches some point and further plate wear occurs only as bar height loss. This trial indicates that bar rounding gets progressively worse throughout the life of the plate and does not reach some equilibrium and stabilize. This has obvious ramifications for production efficiency and quality.
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5. Conclusions 1. The calliper bar height measurements and replica bar edge rounding data indicate that 17-4PH wears somewhat more rapidly than A10 in this refiner. 2. The data indicate wear in this refiner on the motor end is somewhat lower than that on the adjustable end and that rotor wear is slightly higher than stator wear. 3. The replica technique generates accurate data with minimal refiner disassembly. Using the technique it is not necessary to completely disassemble the refiner or remove plates to generate good wear data throughout the life of the plates. 4. The image analysis aspect of the procedure is also a viable measurement technique. 5. Both the bar height and replica data indicate similar wear rates. “Growth” of bars as measured by initial and final bar height is an indication of the difficulty of the bar height measurement technique. Improvements in location and measurement reference points would improve the bar height data. 6. Bar replica analyses indicate the continual increase of edge rounding in addition to height loss throughout plate life. Acknowledgements The authors wish to acknowledge Andritz for permission to publish and also the staff of Dr. J. Mattson, DMD, Portland OR, with special appreciation to Brenda, for their kind assistance with the replica materials. References [1] Blickenderfer, R., Design Criteria and Correction Factors for Field Wear Testing, Wear of Materials 1987, Houston, TX, April 5–9, 1984, pp. 877–884. [2] C.B. Thompson, The cavitation erosion resistance of refiner plate alloys, Journal of Pulp and Paper Science 16 (January (1)) (1990) j20–j27. [3] Jia, Y., Failure Analysis and Material Evaluation of Thermal-Mechanical Pulping Refiner Plates, Ph.D. dissertation, Oregon Graduate Institute of Science and Technology, October 1995. [4] C.B. Thompson, Gardner, The resistance of refiner plate alloys to bar rounding, in: Technical Association of the Paper and Pulp Institute (TAPPI) Engineering Conference, 1987, pp. 189–198. [5] P.L. Wasikowski, Properties of refiner plates used in thermo-mechanical pulping, Pulp and Paper 90 (12) (1989) 209–212.