Title of Invention

"GROOVED ROLLS FOR BOWL TYPE PULVERIZING MILL"

Abstract

This invention relates to grooved rolls for bowl type pulverizing mill (1) comprising a bowl like grinding table (3) mounted on a driven shaft (4) to rotate said grinding table (3), plurality of replaceable bull ring segments (5) being placed on said bowl like grinding table (3) to co-act with plurality of grinding rolls (6) each supported on a shaft (7) and spaced equidistantly for rotation within the interior of a separator body (2) and having a body portion (17) characterized in that said grinding rolls (6) are provided with grooves (18) at both sides of the said grooved grinding roll (6), the said grooves (18) on the two end faces of the roll being kept at a pre determined radio us from the axis of the roll on the respective axial end plane and so dimensioned that the collars (19) on rolls (6) formed during operation of the mill, breaks of during the grinding process itself.

Full Text

This invention relates to a grooved rolls for Bowl Type Pulverising Mills used in pulverizing coal which can work more efficiently in terms of uniform output through out its useful life and controlling of unwanted rejects. Coal fired boilers used in thermal power plants require pulverised coal in a suitable fineness range about 75 microns size for efficient combustion. Various types of pulverisers are employed in Thermal Power Stations. Raymond Bowl mill is one commonly used pulveriser for this purpose. This type of mill is also used in other industries like cement. The main components of bowl mill consists of a cylindrical body, usually known as separator body, within which a conical bowl like grinding table is mounted for rotation, three equally spaced grinding roll in the shape of inverted frustum of a cone are mounted so as to interact with the rotating bowl to grind the coal lumps fed on to the centre of the bowl into fine powder. The three rollers are mounted on to inclined stationary axis journal shafts by bearings so that the roll surface is aligned with the bowl surface and are able to rotate about the axis by the friction force derived from the rotating bowl. Preselected force can be applied between the bowl and the roll surface by means of spring or hydraulic arrangement to assist grinding. The ground particles are thrown out of the bowl by centrifugal force derived from rotation of the bowl. The hot primary air stream entering from the bottom of the mill, commonly knownas air mill, lifts the ground coal particles from the rim of the bowl to the outlet on top through a classifier and heavier stone or quartz particles fall down through annular space between the bowl and the body on to the mill base as rejects. The requirement of pulverised fuel for boiler requires a number of such mills and usually one or two mills are kept as stand by to take up maintenance and roll changing when the roll life for a mill is over. These mills are also required to operate at part load depending upon boiler load and are required to grind different types of coal as per availability. There has been many patents embodying different types of construction arid method of manufacture of pulverising mills. " . A • ,., P.* ,.: .-. i*.. > US Patent No. 3,409,236 for " Segmented Grinding Roll Assembly" there is provided a grinding roll that is made up of a number of pieces or segments, which segments are secured to an inner cylindrical member by means of nuts and bolts. In another granted US Patent No. 4, 181, 264, in 1980 entitled " Static Cast Grinding Rolls", the invention is related to a grinding roll made up of removable rolls sleeve, which is securely held on the journal bearing housing by means of wedge lock rings located at each end of the roller sleeve. Another US Patent No. 4,389,767 entitled "Method of Manufacture Pulverizer Rolls" issued in 1983 provides a method of manufacturing a grinding roll by applying a substantially uniform layer of wear resistant material on to the outer surface of a body with a contour that of an work out roil. In another U.S.Patent No. 4,604,781 entitled " Highly Abrasive Resistant Material and Grinding Roll Surfaced Therewith" issued in 1986, it has been advocated to provide the external surface of the grinding roll fabricated from an alloy which is characterized by highly abrasive resistant qualities. In yet another US Patent WO 97123292, dated 3rd July, 1997, titled " An energy efficient grinding rolls for coal pulverizers", provides a different construction of rolls with rib like or tread like means which is claimed to achieve 15% reduction in energy required to pulverize solid material when compared with the use of standard grinding rolls. An earlier application No.3164/DEL/98 filed for Indian Patent in 1998 by Bharat Heavy Electrical Limited for a patent titled "Dynamically Optimized Bowl Mill Roll for Grinding Coal in Vertical Bowl Mill" states a certain size configuration of a bowl mill roll optimized dynamically to perform efficient grinding at minimum power consumption and long life of rolls. From the observations made on the profile of the grinding surfaces of the rolls during the course of its useful life, it has been found that after certain time of operation,depending on the type of coal used, amount of coal ground, material of roll etc, the wear of material is more in the middle span of the roll along its axial length. As a consequence the material which has not worn out near the edges of the end faces i.e. at the outer diameter of the bigger end and the outer diameter at the smaller end of the roll, form humps or collars. These collars create a non-uniform gap along the axial length between the bull ring segments on the bowl and the roll. This non-uniform gap makes grinding process inefficient. This gap increase with increasing running hour making the process more inefficient. There are disadvantages associated with the present type of pulverizing mills as their life is short and become inefficient in operation, consumes higher energy with low output and costly to operate.The drive motor current consumption goes up to maintain the same level of fineness in pulverized fuel. The fineness fractions changes reducing the output of finer particles. The amount of the rejects from the mill increases. Another disadvantge associated with the present type of pulverizing mills is that due to the nature of high abrasive content of Indian coal the wear and tear on the rolls are very high. Indian coals used in thermal power plants in India generally have a high percentage of hard abrasive^ -quartz particles, shortening the useful life of grinding rolls. Therefore the main object of the present invention is to provide a new and improved form of grinding roll for bowl type pulveriser, and in particular bowl mills, for the purposes of pulverisation of materials, for example, coal. Another objective of the present invention to provide a grinding roll, with longer life giving the same quantity and quality of output at a negligible additional cost. Still another object of the present invention is to provide a roil which will provide overall energy saving by reducing motors current consumption throughout the life of rolls and better utilization of the hard grinding material, than presently available rolls with collar combination. Yet another object of the present invention is to reduce the amount of undesirable rejects, which occur due to formation of collars. Further object of the present invention is to provide an energy-efficient grinding roll, which is suitable for installation in new bowl mills and also capable of being retrofitted into existing bowl mills. Yet still another object of the present invention is to provide an energy efficient grin ding roll, which is relatively inexpensive is easy to employ. According to the present invention there is provided Grooved rolls for bowl type pulverizing mill comprising a bowl like grinding table mounted on a driven shaft to rotate said grinding table, plurality of replaceable bull ring segments being placed on said bowl like grinding table to co-act with plurality of grinding roils each supported on a shaft and spaced equidistantly for rotation within the interior of a separator body and having a body portion characterized in that said grinding rolls are provided with grooves at both side of the said grooved grinding roll, the said grooves on the two end faces of the roll being kept at apre determined radio us from the axis of the roll on the respective axial end plane and so dimensioned that the collars on rolls formed during operation of the mill, breaks of during the grinding process itself. The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to non-limiting exemplary embodiments of the invention represented in the accompanying drawing. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a typicl worn-out surface profiles of rolls of the prior art after different running hours. Figure 2 a schematic diagram of a bowl mill. Figure 3 shows the detail of the grooved roll of the invention Figure 4 shows the roll and bull ring segment of prior art with increased bed height with collar formation. Figure 5 shows the grooved roll of the invention with a different embodiment of the grooves. Figure 6 shows the grooved roll of the invention with another embodiment of the groove. SUMMARY OF THE INVENTION The object of the present invention is to provide a grinding roll suitable for employment in pulverizers which enables some of the drawbacks associated with the known grinding rolls to be eliminated or at least reduced, while still keeping their advantages. The roll or a set of these rolls of the invention when fixed to a bowl mill will co-act with another surface of the bowl to pulverize material such as coal. The nature of construction of the subject grinding roll embodies basically the configuration of a frustum of a right circular cone. Namely, the subject grinding roll, generally speaking is circular in shape with one end there of being of lesser diameter than the other end thereof. Continuing the subject grinding roll is provided with circular grooves or cavities on the two end faces construction of the subject grinding roll embodies basically the configuration of a frustum of a right circular cone. Namely, the subject grinding roll, generally speaking is circular in shape with one end there of being of lesser diameter than the other end thereof. Continuing the subject grinding roll is provided with circular grooves or cavities on the two end faces of the subject roll. The center line grooves or cavities on the end faces are situated near the outer periphery, at a predetermined radial distance from the axis of roll depending on the size of the roll and also on the type of hard abrasion resistant material of the roll. The size, depth and the shape of the groove or cavity on the end faces are also determined based on the size and material of roll as described above. The grooves or the cavities on the hard abrasion resistant material of the roll are to be made by machining with special tools or by casting in a die suitably designed to produce the predetermined grooves or cavities in the casting. The casting can be either static casting or centrifugal casting. The rolls produced with the grooves or cavities as described above will have the advantage of gradual breakage of collar as and when they form during the operation of grinding material such as coal and thereby increasing the grinding efficiency, reducing the driver motor current consumption and the rejects from the mill and also increasing the useful life of roll with full utiliztion of the hard abrasion resistant outer shell. DETAILED DESCRIPTION OF THE INVENTION Figure - 2 shows a schematic diagram of the essential components of a bowl mill required for describing the nature of construction and the mode of operation of a bowl mill, generally designated by reference numeral 1, specially with reference to the energy efficient, grooved grinding rolls developed in the present invention. The Figure -2 shows a substantially closed separator body, which encloses the grinding zone of pulverisation. A bowl like grinding table (3) is mounted on a drive shaft (4), which in turn is connected to drive machines ( not shown) which can rotate the table at the desired speed. A set of replaceable wear part called bull ring segments (5) made of hard abraisve resistant material are placed on the bowl to co-act with the grinding roll (6). A plurality of grinding rolls (6), preferably three in number as per conventional practice, are suitably supported within the interior of the separator body (2) so as to be spaced equidistantly one from another around the circumference of the latter. The nature of construction of the grinding rolls (6) comprises the subject matter that forms the essence of the present invention. A description of the nature of the construction of the grinding rolls (6) will be found set forth hereinafter. First however, note is made here of the fact that in the interest of maintaining clarity of illustration in the drawing only one of grinding roll (6) has been shown in Fig. 2. Each roll is suitably supported on a suitable shaft (7) for rotation relative thereto. Additionally the roll shaft assembly can move in the vertical plane by rotation of the shaft assembly about the trunion shaft (8) which is in turn suitably mounted in the separator body (2). The vertical movement of the roll is restricted by application of force by a spring mounted in a spring housing assembly (9). It is known to those skilled in the art of bowl mill, that the spring is used to exert the requisite amount of force through the grinding roll on to the material, e.g. coal that is disposed on the grinding table. The material to be ground such as coal lumps of specified size range are fed through the vertical feed pipe on to the rotating bowl. Under the action of centrifugal force the coal lumps or similar solid material to be ground, move radially outward with respect to the bowl. Being simultaneously under rotating motion along with the bowl, these solid lumps as a part of a moving bed of these lumps and other recirculated material has to pass through the roll and bull ring segments on the bowl. The gap between roll and the bowl being less than the coal bed height, the coal lumps in the bed get pulverised between the roll and the bull ring segments by attrition and also direct coaction. The spring force adds to the weight of the roll assembly to crush the coal. The coal particles of various sizes of flow out of bowl rim as they reach it. Pressurized hot primary air coming through the air mill housing (11) and flowing upward passes between the bowl rim and the separtor body at a high velocity to carry the coal particles upward toward the classifier inlet (12). The particles or lumps of the ground material, which can not be lifted from the edge of the bowl by the primary air fall down through the gap between the bowl rim and the separator body on to the air mill bottom as reject. The reject gets out of the system through discharge hopper ( not shown ). Sufficiently heavy coal particles in the upward stream of air/coal mixture which are not supported by the air velocity fall back on the bowl before reaching the classifier inlet (12). In the classifier (13), the finer particles required for proper combustion in the boiler go out through the outlet port do) -) and the heavier undesired particles fall back on the bowl through the cone (14) and gets mixed with the incoming coal lumps . The amount of fines produced by grinding depends on the amount of precompression applied in the spring for a given quality of the material to be ground that is characterised by the grindability of coal and other factors like moisture and the abrasives in the coal. The amount of force required to grind the coal to required fineness also depends on the depth to which coal is disposed on the grinding table, which in turn is function of the input/output rate at which the bowl mill is being operated. The wear pattern of roll of the prior art is shown in Fig. 1. It is generally observed that with time the difference in wear depth and between the middle span of the rolls along the axial length and the end portion of the roll goes on increasing leaving more material at two end faces. This gives rise to collar like shape shown in the Figure 1 . The profile of the roll looks like an inverted 'bath tub' curve. Similar profile is generated in the mating part on the bowl, that is the bull ring segment. The net result is that in the zone of higher wear, the normal gap between the roll and the bull ring segment becomes much larger than the gap at the collar portion. General depiction of the gap profile is shown in Fig. 4. The (19) wear profile in the collar portion^restricts the adjoining roll surface and bull ring segment to reduce the maximum gap. Since roll cannot be lowered down further directly because of the (20) collar, a greater depth of coal bed/ forms between the roll and the bull ring segment (The present method of lowering the roll through a six position cam action is cumbersome and does not allow the collar to be broken). The force applied cannot maintain sufficient grinding pressure for this depth of coal bed resulting in inefficient grinding. There is also greater slip between the roll and the bowl resulting in higher consumption of drive motor current compared to that under normal condition. This also results in less of desired fineness to be produced resulting in more recirculation of coal particles, less output and finally increased energy consumption. Another effect is also increased quantity of rejects as higher size coal particles can pass through the increased gap between roll and the bull ring segment. After going over the bowl rim under centrifugal action, these higher size particles either fall down in the air mill section or get recirculated through classifier for further grinding. In totality,the grinding process becomes less efficient. Practically it has been observed in operating mills, that the mill motor current goes up with wear of roll in the manner described above. In view of the above, the present invention aims at breaking the collar in the process of grinding. A cavity or groove as shown in Figure 3 is provided in the end faces of the roll during manufacturing. The size and shape of the groove/cavity is designed based on the size and material of the roll, so that force grinding f-ee-e-s during operation can gradually break the collar in small portions. The wear profile is taken into account and the stresses are calculated for the design of the groove/cavity. The cavity or the groove is positioned and dimensioned such that cracks can propagate in the weakest section as soon as collar starts forming and the collar portion is quickly broken. Once the collar is chipped away, the roll functions normally till again small collar is formed. Two examples of the groove/cavity is given in Fig. 5. Thus, the absence of predominant collar prevents the grinding process from becoming inefficient. The groove roll (6) of our invention comprises a body .portion (17) embodying the configuration of a frustum of a right circular cone. The body portion (17) is formed of a first type of material which comprises a relatively soft easily machining material. An external surface (16) is provided on the body portion (17). The external surface (16) is made of a second type of material which is having good abrasion-resistant characteristic. A groove (18) like cavity of suitable shape as shown in figures 3, 5 and 6 on the two faces of the roll (6) at the two ends of the roll. The centre line ftr)- of the groove ( 18) or the inner boundary of the cavity, on each face is at a predetermined radius from the roll axis on the axial plane and such centre line/boundary line is circular in configuration. The shape of the groove/cavity (18) can be semi circular or a variation thereof suitable to be produced by machining, grinding or casting using suitable die without significant cost of production. The said grooves (18) on the two end faces are at a predetermined radius from the axis of the roll on the respective axial end planes, such that the collar ( 19) on rolls (6) formed during operation, breaks off during the grinding process itself. Neither separate shutdown of the mill for any operation is required nor any separate adjustment in the mill while running is required for breaking the collar (14). The width (w) and depth (h) of the groove/cavity (18) is chosen such that the forces encountered by roll (6) during grinding find the weakest section between the roll surface and the groove ( 18) to break off only the collar(19) like formation on the roll (6) and no other useful part of the roll (6). The width (w)and the depth (h) of the groove/cavity (18) are chosen such that a weak section is available throughout the useful life of the roll (16) to break off the collar (19) between the roll surface and groove (18) such that the collar (19) formed breaks off at regular intervals without affecting the normal operation of the roll. The groove/cavity width (W) can be 5 cm to 15 cm and the groove/cavity depth (h) can be 5 cm to 15 cm depending on the size of the roll. In case of semicircular groove (18), as shown in Fig. 5 the radius of the center line of the groove (18) from the axis of the roll may be located from the boundary between the abrasion resistant material ( 16) and the soft material (17) of the roll (6) to the outer abrasion resistant material (16) region depending on the size and material of the roll (6). In case of cavity (18),as shown in Fig. 6,the inner radius may be located at the boundary between the abrasion resistant material (16) and the soft material (17) of the roll (6). The outer radius may extend up to the outer diameter of the abrasion resistant material (16) depending on the size of the roll (6) and material of the roll (6), conducive to breaking of the collar (19). The grooved rolls (6) give higher efficiency of grinding maintaining more or less uniform depth of coal between the roll (6) and the bull ring segment (5) throughout the axial length roll contact and not allowing the bed height (20) to be excessive which is not conducive for efficient grinding. The grooved rolls (6) will be free of harmful collar and consequently give efficient and uniform grinding output with less and uniform motor current throughout the life of the roll. The grooved rolls (6) give less undesirable rejects compared to rolls in which collars ( 19) have formed. The grooved roll (6) can be retrofitted into the existing bowl mills (1) and can be installed in new bowl mills also. The invention described hereinabove is in relation to a non-limiting embodiment and as defined by the accompanying claims. WE CLAIM: 1. Grooved rolls for bowl type pulverizing mill (1) comprising a bowl like grinding table (3) mounted on a driven shaft (4) to rotate said grinding table (3), plurality of replaceable bull ring segments (5) being placed on said bowl like grinding table (3) to co-act with plurality of grinding rolls (6) each supported on a shaft (7) and spaced equidistantly for rotation within the interior of a separator body (2) and having a body portion (17) characterized in that said grinding rolls (6) are provided with grooves (18) at both sides of the said grooved grinding roll (6), the said grooves (18) on the two end faces of the roil being kept at a pre determined radio us from the axis of the roll on the respective axial end plane and so dimensioned that the collars (19) on rolls (6) formed during operation of the mill, breaks oi£during the grinding process itself. 2. The grooved rolls for bowl type pulverizing mill as claimed in claim 1 wherein said grooved rolls a body portion (17) embodying the configuration of frustum of aright circular cone, 3. The grooved rolls for bowl type pulverizing mill as claimed in claim 1 wherein the body portion (17) is formed of a relatively soft material from an external surface (16) being made of abrasion resistant material provided on the body portion. 4. The grooved rolls for bowl type pulverizing mill as claimed in claim 1 wherein the said groove/cavity (18) is semi-circular 5. The grooved rolls for bowl type pulverizing mill as claimed in claim 5 wherein the (w) and the depth (h). is 5cm to 15cm/ depending on the size of the roll. 6. Grooved rolls for bowls type pulverizkg mill as hereindescribed and illustrated with the accompanying drawkgs.

Documents:

546-del-2001-abstract.pdf

546-DEL-2001-Claims.pdf

546-del-2001-complete specification (granted).pdf

546-del-2001-correspondence-others.pdf

546-del-2001-correspondence-po.pdf

546-del-2001-description (complete).pdf

546-del-2001-drawings.pdf

546-del-2001-form-1.pdf

546-del-2001-form-19.pdf

546-del-2001-form-2.pdf

546-del-2001-form-3.pdf

546-del-2001-gpa.pdf

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