M19: A PREMIUM SUGAR MILL ROLLER SHELL MATERIAL FOR BETTER PERFORMANCE OF CONVENTIONAL AND HIGH PERFORMANCE INTERNAL BLEEDING ROLLERS

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1 REFEREED PAPER M19: A PREMIUM SUGAR MILL ROLLER SHELL MATERIAL FOR BETTER PERFORMANCE OF CONVENTIONAL AND HIGH PERFORMANCE INTERNAL BLEEDING ROLLERS SABNIS D AND SHAIKH M S.B. Reshellers Pvt. Ltd., Kolhapur, India. dvsabnis@sbreshellers.com; mmshaikh@sbreshellers.com Abstract S. B. Reshellers Pvt. Ltd. in India has developed a revolutionary alloy specifically catering to sugar millers worldwide. This paper explores the material, the SBR Alloy, a cast iron alloy which has the benefits of open grain cast iron, but has higher strength, excellent weldability, and higher wear and corrosion resistance than cast iron. This paper will compare the abovementioned properties with traditional cast iron, used for its rough surface which is conducive to better traction. However, the cast iron material has many shortcomings, including low elongation, and as a result the roller teeth tend to break under high concentrated loads. The traditional cast iron suffers from low strength and low weldability. Economically, the SBR Alloy is preferred by many sugar factories as the slight increase in cost is immediately offset by longer life and better performance of the rollers. SBR Alloy also enhances the performance of the Kamal internal bleeding rollers which, by design, is itself much better than the Lotus design prevalent in most parts of the world. This paper discusses the operation of internal bleeding Kamal rollers and their advantages. Keywords: sugar mill roller, internal bleeding, cast iron, SBR Alloy, Kamal, wear resistance, weldability INTRODUCTION A sugar milling unit typically consists of three rollers, each of which has been open grained cast iron shell shrunk over a forged steel shaft. The roller shell surface is grooved circumferentially to increase the contact surface area to get better traction on the shredded cane blanket that is passed between the rollers. The grooved surface also provides additional traction force and mechanical grinding of the fibre blanket due to inclined faces. During crushing, as the cane blanket moves through the mill, extracted juice comes out and flows to collection chutes. The better the traction of the shells over the cane blanket, the lesser the slip, the lesser the wear and hence the better the extraction. Open grained cast iron has historically been the preferred shell material because of its relative low cost and because it provides a rough surface finish to aid the traction and hence the feeding of the cane blanket into the nip of the rollers. Additionally, if a large tramp iron piece falls into 435

2 the mill, the brittle nature of the cast iron often causes the grooved construction to break so that further damage, such as bending of the roller, might be avoided. However, open grained unalloyed cast iron has low strength, very poor weldability and low abrasion resistance. As mill sizes grew with the passage of time and more power was introduced for driving the mills, higher strength shell material became a necessity. The wear due to corrosion as well as abrasion also hampers the working of rollers and causes frequent regrooving and shell changes. As the arcing (hardfacing by weld deposition) of the roller shells became standard practice for the roughening of the shell surface, poor weldability of cast iron became a hindrance in surface engineering of the shells. Thus shell material, having higher strength, higher wear resistance and better weldability, was desirable. SBR Alloy Properties SBR Alloy was developed after intensive research and years of trials by the development team at S.B. Reshellers Pvt. Ltd to address the shortcomings of the open grained cast iron. Although the chemical composition of SBR Alloy does not vary much from the cast iron, it is the way it is treated that makes all the difference. It is a semi-nodular cast iron, having almost all the benefits of cast iron but with higher strength, better wear resistance and far superior weldability. Table 1 compares the properties of SBR Alloy to conventional cast iron. Note that since sugar mill roller shells have thicknesses ranging from 150 mm to 300 mm, comparison also includes thick sections that are not commonly assessed. Table 1. Comparison of properties between cast iron and SBR Alloy based on analysis by SBR Sr. No. Property Cast Iron SBR Alloy 1 Std. tensile strength of test bar of dia. 30 mm 2 Tensile strength of dia. 30 mm test bar made from section cut from sugar mill roller shell. 3 Std. yield point strength of dia. 30 mm bar 200 to 280 MPa 450 to 600 MPa 100 to 150 MPa 300 to 400 MPa 200 to 280 MPa 350 to 450 MPa 4 Elongation Nil 1.5 to 4% approximately 5 Modulus of elasticity 125 X 10 3 MPa 175 X 10 3 MPa 6 Brinell hardness 150 to 220 BHN 180 to 240 BHN 7 Weldability Poor Excellent 8 Wear resistance Good Better 9 Corrosion resistance Good Good 436

3 Table 1 highlights the advantages of SBR Alloy over cast iron. The increase in the modulus of elasticity and the increase in the BHN are particularly noteworthy. Wear resistance Tables 2, 3 and 4 provide information on the wear resistance of SBR Alloy compared to conventional cast iron as reported by factories using SBR Alloy shells. Sr. No. Table 2. Wear resistance at Appasaheb Nalwade Gadhinglaj Taluka SSK [3] (1 September 2006) Description Crushing Diameter at the start of 1 SBR Alloy 5th mill top roller No. A- 5468/05 2 Conventional 6th mill top roller tons with 13% 5.5 ph Diameter at the end of Remark 825 mm 822 mm No appreciable wear of hard facing after 825 mm 807 mm - Sr. No. Table 3. Wear resistance at Nandi SSK [3] ( 22 January 2009) Description Crushing Diameter at the start of 1 SBR Alloy top roller no. A-8024/07 2 Conventional bottom roller no. A-7888/ tons Diameter at the end of Remark mm mm Regrooving not done after 1190 mm 1180 mm - Sr. No. Table 4. Wear resistance at the Ugar Sugar Works (2 December 2008) Description Crushing Diameter at the start of 1 SBR Alloy top roller during Above roller during tons tons Diameter at the end of Remark mm mm Regrooving done after to mm mm mm - 437

4 The following conclusions can be drawn from Tables 2, 3 and 4: 1. Wear of the SBR Alloy rollers is far less than conventional cast iron rollers. Sometimes no wear is observed; 2. Hard facing remains intact even after crushing for complete, indicating better adhesion of the arcing due to better weldability; and 3. Regrooving can be completely avoided after first. Weldability Figures 1 and 2 are photographs of arcing (hardfacing) using KST TUB A 1001 M OA open arc hardfacing continuous wire dia. 2.4 mm diameter. The Figure 1 photograph was taken after usage on conventional cast iron and the Figure 2 photograph was taken after usage on SBR Alloy material with identical settings. Considerably more deposit can be seen on the SBR Alloy roller. Figure 1. Arcing of conventional cast iron roller [Source SBR Datafiles] Figure 2. Arcing of SBR Alloy roller [Source SBR Datafiles] 438

5 Benefits of using SBR Alloy: 1. The wear of the shell is as a result of many factors other than wear resistance of the shell material. However, it can be seen from the above tables that wear of the conventional cast iron shells in identical conditions is more than double the wear of the shell in SBR Alloy; Hence, as a result of lower wear, SBR Alloy shell has approximately double the life expectancy of a conventional cast iron roller because of: 2. The reduced wear, annual regrooving can be avoided. Only the roots of the grooves need to be corrected for scraper adjustment if required; 3. The better adhesion of the arcing, subsequent weld deposition can be minimised; 4. The better adhesion, initial weld deposition (hardfacing/arcing) of the roller, requires lesser electrodes than usage on conventional cast iron, reducing waste of material; 5. Increased life of arcing implies less frequency of arcing, and less downtime resulting from arcing; 6. The ability to withstand weakening of shell due to multiple drilling for high performance internal bleeding rollers against cane crushing stresses; and 7. Better wear resistance which is seen as another major advantage. It is widely accepted that worn shells have a negative effect on pol extraction. At Manuelita mill in Colombia, extensive research was carried out on shell wear and extraction performance using historical milling data and an extraction model based on reabsorption coefficient (Gomez et al., 2005). It was concluded that wear of the grooving is the main factor that affects the performance of the milling units. Estimation of loss of brix extraction was between 4% and 12% for first mill. The study also indicated that there may be additional effects of roller wear such as imbibition efficiency impairment. Figure 3 shows wear patterns of conventional as well as SBR Alloy shells, based on the reports received. For similar conditions, at least three times the wear is observed in cast iron roller shells as compared to SBR Alloy roller shells. This reduced the low pressure area to get better performance all over, including pol extraction over time as per research carried out in Colombia. Figure 3. Wear Patterns [5] on cast iron and SBR Alloy 439

6 Figure 4. Wear Pattern [5] on Grooves Figure 4 shows typical wear of the cast iron roller shell. Unevenness of the wear contributes to considerable efficiency of the crushing. Considering the benefits of SBR Alloy, which has been explained in detail, it is better to opt for SBR Alloy in conventional and Kamal rollers. Internal bleeding rollers: Background The internal draining roller is a recent innovation in the long history of sugar cane milling. When higher and higher crushing rates were sought in cane milling and problems such as flooding, reabsorption of juice in bagasse and high final bagasse moisture had to be dealt with, Jean Bouvet, (Bouvet, 1978) brought this concept of internal bleeding/drainage to the attention of the sugar industry. This original design had some inherent defects like clogging of drilled holes by bagasse, simultaneous fouling of scraper tips in drainage holes, etc. and required certain modifications and improvements in the geometry and shell material which were accomplished to make a suitable product for commercial use. This Kamal design substantially improved on Bouvet s basic concept and is widely used in Indian industry. Internal bleeding Kamal rollers One major problem with the Lotus design was that the slippage would affect proper gripping of the cane blanket. When cane is compressed between the rollers the pressure in the zone becomes very high in the case of internal bleeding rollers, resulting in expressed juice being forced through the nozzles and into the horizontal drainage system which ultimately exits in the juice tray. This removal of the additional shifts the semi-liquid zone, increasing the gripping strength of the rollers. Following are the details of shifting of semi-liquid zone. Figure 5 shows the bagasse entering the nip of the two rolls. The cane touching the roll at A.A., is gripped by the rolls and starts being compressed. This progressive compression removes the 440

7 air space between the cane fibres. At the same time, the available free juice is expressed and the ratio of expressed juice/fibre increases gradually. Figure 5. Bagasse entering the nip of the two rolls of conventional rollers [5] Eventually, at some region delineated by the dotted line B1 B1 there is nothing but juice and fibre. This is the start of the semi-liquid zone. From that point on, the roller surfaces cannot grip the cane fibres any more as the material is in a semi-fluid state. All frictional force required to offset the high pressure zone is developed by the roll surfaces in the region A A to B1 B1. Figure 6 shows a mill with a top internal bleeding roller. Here the juice is removed through radial nozzles connected to axial horizontal passages. As more of the juice is removed from this high pressure area, less juice will be in semi-liquid zone hence the object of the internal bleeding roller is to remove as much of this juice as possible and to advance the semi-liquid zone from B1 B1 to BB. This advance increases the roll surface area that grips the bagasse and makes it available to increase the frictional force capable of forcing more cane between the rollers. This not only reduces the power required, due to reduction in juice pressure and reduction of the quantity of reabsorbed juice moving bagasse further, or increases the milling capacity, but as the slippage of cane blanket on the roller surface is reduced, wear of the roller is also reduced considerably. Flooding and reabsorption Figure 7 shows crushing in a conventional mill. As the expressed juice does not have a route to progress to the juice basin there is flooding on the feed side of the top roller. This flooding initiates reabsorption of the juice on the bagasse blanket. 441

8 Figure 6. Bagasse entering the nip of the two rolls of Kamal rollers [5 Figure 7. Crushing in a conventional mill [5] 442

9 In the case of the Kamal internal bleeding roller, the expressed juice gets way through nozzles and juice passages to juice rings/juice guard and, consequently, to the juice basin, as shown in Figure 8. This reduces flooding considerably and hence reabsorption. Figure 8. Crushing in Kamal Mill [5] Consequently, the following are the benefits that may be derived from using Kamal geometry: 1. Increased juice extraction; 2. Lower shell wear; 3. Increased mill capacity; 4. Lower power consumption; 5. Lower bagasse moisture; and 6. Lower pol in bagasse. However, most of the above indicated benefits are interdependent. If mill capacity is to be increased by using the Kamal roller, shell wear may not be reduced. Wear per ton of cane crushing will be less in the case of the Kamal roller since slipping of the cane blanket over the roller surface, which is wear, is reduced. If lower power consumption is desired, the benefit of increased juice extraction may be lost. Features of the Kamal design: 1) The juice passages in the Kamal design are cast in place in the shell itself. Since these are of steel construction, they act as reinforcement and add to the strength of shell which is otherwise compromised due to the holes; 443

10 2) Juice passages are circular in design and do not have corners or angles for bagasse particles to get lodged hence blockage due to accumulation of fibre is avoided. Circular passages are also easy to clean by cylindrical brush or pressurised steam; 3) Juice nozzles are taper force fitted. The holes for the nozzles are drilled with special taper drill, tapered at 1.25 degrees. Tapered nozzles are press fitted in the holes. For additional sealing ANR adhesive is applied to both nozzles and holes; and 4) The geometry of the nozzle is such that it has a reverse taper, i.e. it has larger opening inside the juice channel than on the outer surface making it virtually clog free. The benefits of the Kamal roller depend on the amount of juice that is extracted through the nozzles and the amount of juice extracted is a function of the number of nozzles per groove. The Kamal roller traditionally has one nozzle per groove per juice channel. Figure 9. Typical Kamal roller design with single nozzle. [5] After some experiments, it was found possible to have double nozzle geometry without compromising the strength of the roller. Double nozzles increased the juice extraction, which proved to be beneficial, especially in first or last mill, for reducing moisture in bagasse. In the double nozzle design (Figure 5), the distance between the nozzles is half that of the single nozzle design. Referring to X in Figure 9 Y in Figure 10 Y=X/2 444

11 Figure 10. Double nozzle design [5] In most of the large rollers, the circumferential gap between the two radial nozzles is considerable and it was thought desirable to accommodate one more nozzle midway so that the evenness of circumferential distance is maintained (Figure 6). With success of double nozzled geometry, experiments were conducted for triple nozzle construction. Because of additional nozzles and the machining thereof, the price of the roller increased marginally. Referring to X in Figure 4 and Z in Figure 11 Z=X/3 Figure 11. The triple nozzle design 445

12 Triple nozzle geometry s additional drilling weakened the shell cross section. In the case of cast iron shell, it was very difficult, if not impossible, to devise a geometry of juice channels to avoid excessive weakening of the shell. However, with the use of SBR Alloy, being of stronger shell material, it was found possible to use the triple nozzled geometry. Conclusion 1. SBR Alloy shell material, because of higher wear resistance, has higher life than conventional cast iron, and requires less regrooving with less reshelling frequency. 2. SBR Alloy shell material, because of higher wear resistance, reduces loss of brix to the extent of 4% to 12 %. 3. SBR Alloy, because of better weldability, has denser arcing, which remains firmer during crushing and requires lesser frequent arcing. 4. SBR Alloy, because of higher strength, is easily adopted to double/triple nozzled internal bleeding rollers, entailing higher extraction efficiency. 5. The Kamal internal bleeding roller reduces flooding and reabsorption of juice in bagasse. 6. The Kamal internal bleeding roller reduces juice pressure in the nip, consequently reducing input power and increasing extraction. 7. The Kamal internal bleeding roller reduces shell wear and increases mill capacity by increasing the gripping area. 8. The double and triple nozzle Kamal roller reduces bagasse moisture, increasing boiler efficiency and power in the case of co-generating mills. REFERENCES Patented shell material of S. B. Reshellers Pvt. Ltd., Patent no. 1253/MUM/2004 dated 22nd Nov Jean Bouvet : Inventor of Internal Bleeding Lotus Rollers US Patent No Reports received from M/s Appasaheb Nalwade Gadhinglaj Taluka SSK, The Nandi SSK and The Ugar Sugar Works through letters. Gomez AL et al. (2005) Research Paper ISSCT Vol An approach to a relationship between roll shell wear and Pol extraction 446