Title of Invention

"AN IMPROVED GRINDING ROLL ADAPTABLE TO HIGH PERFORMANCE BOWL MILL"

Abstract

The invention relates to an improved grinding roll acaptable to a high performance bowl mill, the grinding roll rotatable on a bowl generally designed as an inverted cone, the bowl being rotatable by a motor, coal lumps inputted in the rotating bowl being crushed by the grinding roll which derives its rotational force from the rotating bowl via the medium of crushable coal lumps, at least one passive roll mounted on a journal shaft, a biasing member when activated causes the grinding roll to press on the crushable coal lumps disposed on the bowl. Configuration of the device features is selected such that the grinding roll effectively crushes the coal lumps and having a longer operational life.

Full Text

The invention generally relates to a dynamically optimised grinding roll adaptable to bowl mill for grinding large pebble size coal to very fine pulverized coal particles. More particu¬larly? the invention relates to an improved grinding roll adapt¬able to a high performance bowl mill. Low efficiency boilers like in steam engines use larger pebble size coal for burning. With the advent of modern large thermal power plants high efficiency boilers are employed which require very fine pulverized coal particles. There are various pulverizing mills used for this purpose like bowl Mill? Ball and Tube Mill, E-type Mill etc. Majority of the power plants in India employ bowl mills for coal pulverization? which has been found to be more economical than the other types of mills mentioned above. The main problem faced by these mills is grinding Indian coal containing highly abrasive ash (20-6Ǿ4/) consisting of very hard quartz particles which make the rolls wear out very fast. Improvement in metallur¬gy of the rolls by making it harder like the high chromium rolls has not yielded significantly better life while grinding high ash coal. In order to increase the life of rolls HP (High Perform¬ance) series Bowl mills were introduced. The HP series mills employed rolls with higher thickness of hard grinding material. There are more than one hundred such mills operating in India today. Another object of the present invention is to propose a grinding system with an improved roll where the rolls maintains a better parallelity between the roll and the bowl giving uniformity distributed stresses. Yet another object of the present invention is to propose a system of grinding with an improved roll where the power consumption is reduced. Still another object of the present invention is to propose a system of grinding with an improved roll where rejection of coal is reduced. Further object of the present invention is to propose a system of grinding with an improved roll with improvement in grinding efficiency. According to the present invention there is provided an improved grinding roll adaptable to a high performance bowl mill, the grinding roll rotatable on a bowl generally designed as an inverted cone, the bowl being rotatable by a motor, coal lumps inputted in the rotating bowl being crushed by the grinding roll which derives its rotational force from the rotating bowl via the medium of crushable coal lumps, at least one passive roll mounted on a journal shaft, a biasing member when activated causes the grinding roll to press on to the crushable coal lumps disposed on the bowl; the improvement is characterized in that the grinding roll is so configured to satisfy the following relationships of the device features: a) ratio of an outside diameter of an upper end (F) and a lower end (c) of the roll (1) = 1.12 b) ratio of a radius (R ) of an outside corner of the upper end and a radius (R2 ) of an out¬ side corner of the lower end = 0.90. 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 a non—limiting examplary embodiemnts of the invention represented in the accompanying drawings. Figure 1 dynamically optimised bowl mill roll, Figure 2 shows the dynamically optimised roll (1) with its various parameters. The roll is concial and truncated. From the bottom (A) represents maximum inside diameter of the roll (B) diameters of the interference between the roll and the outer layer (Z) and (C) the outside diameter of the roller at the bottom. The corresponding diameters are presented by D, E and F. The height of the roll is shown by (H) and the radius at bottom is ( R ) and at top as (R ) . The HP Bowl Mill used in Power Plants work on the principle three conical rolls, rolling on a bowl of inverted cone shape of (not shown). The coal lumps of average one inch are crushed between the bowl rotated by motor and passive rolls mounted on a separate journal shafts. The rolls derive their rotation from the bowl through the layer of coal to be crushed. The rolls are also pressed on the coal bed on the bowl by springs (not shown). The presently used rolls are of truncated cone shape. The roll has a outer layer hard centrifugally cast wear resistant material which take part in grinding. The average life of these rolls is 2000 to 3000 hours depending upon the ash content in coal and motor current consumption is around 43 ampere for full load operation. The forces acting upon the rolls and the bowl consists of (1) static component of spring force (pre compression) applied to the roll and (2) dynamic component of inertia and spring force. The inertia component consists of the roll assembly mass multiplied by acceleration of motion. The static and dynamic forces are responsible for grinding but they need to be optimised as greater force cause excessive wear out of the grinding elements (undesirable) and lessr force results in inadequate grinding with lowering of output and increase of reject quantity. The optimisation of force to give better mechanical property has been achieved in this case by decreasing the roll mass and the spring pre compression acting on the roll and the spring compression from 11 tons to 7 tons. To bring about this mass reduction, the roll geometry had to be changed without compromising its other functions like parallelity, hard material depth, fouling characteristics etc. Measurement of forces carried out on the various sizes of rolls operating in actual mill has shwon the reduction in dynamic forces for dynamically optimised roll and these rolls have registered improved wear characteristics. The improved dynamically optimised rolls are constructed (1) to avoid fouling with each other (2) to have maximum amount of grinding material available for wear out (3) to have uniform gap with Bull ring segment (mating part) through out the length of contact. The grinding of coal takes place by the process of attrition. The normal load required for grinding coal is produced by the weight of the roll assembly and the applied spring force. To achieve required fineness, the spring force can be varied within a certain range. The spring compression reduced by 40% within a range of 20% to 60%. The stresses existing within the grinding zone are the static stresses as described above and the dynamic stress produced by the spring and acceleration of the roll mass. The alpha quartz particles can be present either outside the coal lumps or can be contained within the coal. Quartz in either of the form is present in the grinding zone and cause abrasive wear out of the grinding material as they get around along with coal. From the detailed studies of the wear of the rolls and measurement of forces on the rolls of existing design, it was concluded that wear was because of high stress abrasion. The presence of high quantity of alpha quartz in coal being ground under high dynamic stresses lead to high rate of wear. It was also found that it was possible to obtain similar output of pulverised coal even under reduced load condition which give reduced stresses in the rolls. In other words the rolls of existing construction were subjected to higher static and dynamic forces than was necessary for grinding. Hence, to achieve better life of the rolls, a new design of roll was evolved which would produce optimum grinding condition with reduced but uniform dynamic stresses on the rolls. At the same time computer aided design analysis was carried out to optimize the roll dimensions such that uniform gap between roll and bowl could be achieved. In the new construction it was also seen that uniform stresses existed through out the contact. The new dynamically optimised rolls had the following features for a typical application. a) The outside diameter of the upper end of the roll was fixed at 1100mm (F) compared to 1168 earlier. b) The outside diameter at the lower end of the roll was fixed at 981mm (C) compared to 1063 earlier. c) The radius at the outside corner of the upper end of the roll is kept at 50mm (R.) no change. d) The radius at the outside corner of the upper end of the roll is kept at 55mm (R2) compared to 90mm. The following main improvements in the performance of grinding rolls of new design has been observed in the trial runs at the 210 MW Thermal Power Plants compared that of the earlier design rolls under similar working conditions. 1 . The life of rolls has been found to be between 4500 to 5000 hours compared to the 2000 to 3000 hours obtained using earlier design rolls. 2. The current drawn by motor driving the Mill has been found to have reduced from 43 amperes to 37 amperes. 3. The quantity of reject from the mill during grinding has been observed to be qualitatively less. 4. The new roll has optimum grinding material volume of dynamic stress ratio to give increased life of about 1.5 to 2 times than that obtained with earlier design of rolls with no reduction in output. The trials of the new design roll at power stations have given a life of 4500-5000 hours consistently compared to 2000-3000 hours with earlier design of rolls. Because of reduction in roll mass, the dynamic forces have been reduced to cause less wear. 5. The new rolls maintains better parallelity between the roll and the bowl giving uniformly distributed stresses compared to that obtained from earlier design. 6. The weight of each roll is reduced by 365 Kg. from 1380 Kg. to 1015 Kg. This constitutes a material saving of around 26%. 7. The power consumption has been reduced. The current drawn by motor driving the mill has come down from 43 amperes to 37 amperes. 8. The rejected quantity of coal from mill has been observed to be less compared to using earlier roll. 9. Due to reduction in overall dimensions in the new roll, it is possible to use the roll in different fixing positions without fouling with other two rolls in the mill during operation. This is beneficial for better utilization of the grinding element in the roll. 10. There is a decrease in collar formation of the roll. This is beneficial for improvement in grinding efficiency. The invention described hereinabove is in relation to a non-limiting embodiment and as defined by the accompanying claims. WE CLAIM: 1. An improved grinding roll adaptable to a high performance bowl mill, the grinding roll rotatable on a bowl generally designed as an inverted cone, the bowl being rotatable by a motor, coal lumps inputted in the rotating bowl being crushed by the grinding roll which derives its rotational force from the rotating bowl via the medium of crushable coal lumps, atleast one passive roll mounted on a journal shaft, a biasing member when activated causes the grinding roll to press on the crushable coal lumps disposed on the bowl, the improvement is characterized in that the structural parameters of the grinding roll is configured to satisfy the following relationships: a) ratio of an outside diameter of an upper end , and a lower end (c) of the roll (1) * 1.12, and b) ratio of a radius (R ) of an outside corner of the upper end, and a radius (R ) of an outside corner of the lower end = 0.90. 2. The grinding roll as claimed in claim 1, wherein the outside diameter of the upper end (F) of the grinding roll (1) is 1100 mm. 3. The grinding roll as claimed in claim 1, wherein the outside diameter of the lower end (c) of the grinding roll (1) is 981 mm. 4. The grinding roll as claimed in claim 1, wherein the radius (R ) of the outside corner of the upper end (F) of the grinding roll (1) is 50 mm. 3. The grinding roll as claimed in claim 1, wherein the radius of the outside diameter of the lower end (c) of th» grinding roll is 35 mm. &• The grinding roll as claimed in claim 1, wherein the roll mass is 1015 Kg. 7. The grinding roll as claimed in claim 1, wherein the compression force exerted by said biasing element is 7.OS Ten* 5. The grinding roll as claimed in claim 1, wherein the roll (1) comprises an outer layer (2) having a thickness 130 to 140 mm. 9. The grinding roll as claimed in any of the preceding claims, wherein the grinding roll is shaped as a truncated cone, aftd wherein said outer layer (2) comprises hard centrifugally cast wear resistant material. 10. An improved grinding roll adaptable to a high performance bowl maill substantially as herein described and illustrated with reference to the accompanying figure.

Documents:

3164-del-1998-abstract.pdf

3164-del-1998-claims.pdf

3164-del-1998-correspondence-others.pdf

3164-del-1998-correspondence-po.pdf

3164-del-1998-description (complete).pdf

3164-del-1998-form-1.pdf

3164-del-1998-form-19.pdf

3164-del-1998-form-2.pdf

3164-del-1998-form-4.pdf

3164-del-1998-form-5.pdf

3164-del-1998-gpa.pdf

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