Abstract:
A pulverizer  60  includes a spring assembly  10  that urges a grinding roller  72  of a journal assembly  68  onto a grinding surface  66  of a grinding table  64.  The force applied is monitored by a load cell  32  located within spring assembly  10  that creates an electronic signal. A controller  83  receives the electronic signal and stores and/or displays it and alternatively acts to adjust the applied force to a desired value. Alternatively, adjustable forces or mechanical dampening may be applied to journal assembly  68  by controller  83.  Alternatively, additional sensors may measure displacement of the journal assembly and rotation of the grinding table for other calculations.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to solid fuel pulverizers and is more particularly directed to the measurement of forces experienced by solid fuel pulverizers. 
       BACKGROUND 
       [0002]    Solid fossil fuels such as coal often are ground in order to render the solid fossil fuel suitable for certain applications. Grinding the solid fossil fuel can be accomplished using a device referred to by those skilled in the art as a pulverizer. One type of pulverizer suited for grinding is referred to as a “bowl mill pulverizer”. This type of pulverizer obtains its name by virtue of the fact that the pulverization that takes place therein is effected on a grinding surface that in configuration bears a resemblance to a bowl. In general, a bowl mill pulverizer comprises a body portion on which a grinding table is mounted for rotation. Grinding rollers mounted on suitably supported journals interact with the grinding table to effect the grinding of material interposed therebetween. After being pulverized, the particles of material are thrown outwardly by centrifugal force, whereby the particles are fed into a stream of warm and blown into other devices for separation by particle size. 
         [0003]    Grinding rollers are urged toward the grinding table against the fossil fuel being ground by a spring assembly. The force that this exerts may be manually adjusted. The greater the force, the finer the particle size of the fossil fuels being ground. 
         [0004]    There is no feedback relating to the amount of force being applied, or how different this force is from a desired force. 
         [0005]    Currently, there is a need for feedback to more accurately adjust the force used to grind fossil fuels. 
       SUMMARY 
       [0006]    According to aspects disclosed herein, there is provided a spring assembly for urging a grinding roller toward a grinding table with a measured force. The spring assembly has a spring housing that defines an interior area. A preload stud extends at least partially into the interior area and is coupled to the spring housing for movement relative thereto. A stop plate is positioned in the interior area with the preload stud extending through the stop plate. A spring seat is attached to, and is movable with, the preload stud. The spring seat is positioned at least partially within the interior area adjacent to an end of the spring housing. The spring seat extends at least partially through an opening defined by the spring housing. At least one spring is interposed between the spring seat and the stop plate. A load cell is positioned in the interior area of the spring housing for measuring forces exerted by the spring due to spring preload as well as movement of the spring seat relative to the spring housing. 
         [0007]    According to another aspect disclosed herein, a pulverizer for pulverizing solid fuel includes a pulverizer housing having a shaft coupled for rotation thereto. A grinding table is mounted on the shaft and a journal assembly is pivotally mounted on the pulverizer housing. A grinding roll is coupled to the journal assembly. A spring assembly is also mounted on the pulverizer housing and includes a preload stud extending at least partially into the interior area and coupled to the spring housing for movement relative thereto. A stop plate is positioned in the interior area with the preload stud extending through the stop plate. A spring seat is attached to, and is movable with, the preload stud. The spring seat is positioned at least partially within the interior area adjacent to an end of the spring housing. The spring seat extends at least partially through an opening defined by the spring housing. At least one spring is interposed between the spring seat and the stop plate. A load cell is positioned in the interior area of the spring housing for measuring forces exerted by the spring due to movement of the spring seat relative to the spring housing. The load cell creates an electronic signal indicating the force being exerted by the spring assembly at a given time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Referring now to the figures, which are exemplary embodiments, and wherein like elements are numbered alike: 
           [0009]      FIG. 1  is a schematic cross-sectional view of a spring assembly of the bowl mill pulverizer. 
           [0010]      FIG. 2  is a schematic, partial, cross-sectional view of a pulverizer including the pressure spring of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    As shown in  FIG. 1 , a spring assembly generally designated by the reference number  10 , includes a spring housing  12  having a first end  12   a  and a generally opposing second end  12   b.  The spring housing  12  also defines an interior area  13 . The spring assembly  10  is mounted to a support structure  14 . In the illustrated embodiment, the spring housing  12  comprises a spring cup  12   c  coupled to a cylinder  12   d.  However, the configuration of the spring assembly  10  is not limited in this regard as the housing may also have a monolithic construction without departing from the broader aspects of the present invention. A spring seat  16  is movably positioned in the interior area  13  of the spring housing  12  adjacent to the first end  12   a.  A stop plate  18  is also positioned in the interior area  13  of the spring housing  12  adjacent to the second end  12   b  thereof. A first spring  22  and a second spring  24  are positioned within the interior area  13  between the spring seat  16  and the stop plate  18 . In the illustrated embodiment, the first and second springs,  22  and  24  respectively, are coil springs with one of the springs positioned within the other. However, the present invention is not limited in this regard as other coil spring configurations, or other types of springs such as, but not limited to, Belleville washers and elastomeric materials may be substituted. In addition, while a first and second spring  22 ,  24  have been shown and described, the present disclosure is not limited in this regard as a single spring, or more than two springs can also be employed. A preload stud  26  is threadably engaged with the spring seat  16  and extends through an aperture defined by the stop plate  18 . The initial spring force can be varied by varying the position of the pressure spring seat  16  relative to the stop plate  18 , by rotating the stud adjustment nut  46  relative to the preload stud  26 , thus driving the preload stud  26  and spring seat  16  toward or away from the stop plate  18 . Driving the preload stud  26  outward compresses the springs  22  and  24  against the stop plate  18 , whereas driving the preload stud inward decompresses the springs. 
         [0012]    Still referring to  FIG. 1 , the interior area of the spring housing  12  is defined by a cylindrical housing wall  27 . The spring seat  16  is likewise cylindrical and is sized to be slidably positionable within the interior area  13  of the spring housing  12  when it receives a force along the direction of the arrow marked “F R ”. The spring seat  16  may also have a circumferential groove for receiving a piston ring  28 . The piston ring  28  is sealingly engageable with the spring housing  12  to minimize the likelihood of pulverized material passing therethrough. 
         [0013]    An ‘o’-ring  30  or other type of seal such as, but not limited to, a lip seal can be positioned in the aperture defined by the stop plate  18  and be at least partially and slidingly engageable with the preload stud  26  to minimize the likelihood of pulverized material passing between the stop plate  18  and the preload stud  26 . 
         [0014]    The spring assembly  10  includes a load cell  32  positioned to detect the spring forces attributable to the compression of the springs,  22  and  24  respectively, and to generate a signal indicative of the magnitude of the first and second forces. The load cell  32  may comprise, for example, a piezo electric cell that generates an electrical signal in response to an applied compressive force. However, the present invention is not limited in this regard as other types of load cells known to those skilled in the pertinent art to which the present invention pertains may be substituted. In the illustrated embodiment, the load cell  32  is positioned between the springs  22  and  24  and the stop plate  18 , however, the invention is not limited in this regard, and in other embodiments a load cell may be positioned elsewhere in the spring assembly  10  where the initial, the total, and the dynamic spring forces are transmitted from the springs into the load cell. 
         [0015]    In one embodiment, the load cell  32  is a “doughnut” type sensor, i.e., one having a circular body with flat front and rear faces  32   a,    32   b  (disposed toward and away from the spring seat  16 , respectively) and a load cell central aperture configured to allow the preload stud  26  to pass therethrough. The load sensor central aperture may also be sized to accommodate the installation of either an ‘o’-ring for sealing or a wear sleeve (not shown) between the load cell  32  and the preload stud  26 . 
         [0016]    In the illustrated embodiment, the outer circumference of the load cell  32  defines a groove (unnumbered) for receiving a piston ring or ‘o’-ring  34  sealingly engageable with the spring housing  12 . The front and rear faces of the load cell  32  can be made from wear-resistant material, e.g., hardened steel, carbon steel, carbon steel alloy, or the like. 
         [0017]    The load cell  32  includes an output lead  36  on the rear face  32   b.  Output lead  36  includes a power cable to supply the load cell. The output lead  36  passes through an aperture in the stop plate  18  so that the output lead  36  can be connected to controller  83 . This may be, for example, signal processing equipment such as a suitably programmed general purpose computer, programmable logic controller, or the like. Controller  83  monitors the force on the first and second springs,  22  and  24  respectively. The output lead  36  may be equipped with quick-connect fittings to facilitate connection to, and removal from, the signal processing equipment and/or the load cell  32 . In one embodiment, the output lead  36  is a flexible, temperature-resistant, shielded lead that resists failure due to grease and erosion caused by the high velocity pulverized air/coal stream. 
         [0018]    The spring housing  12  is attached to the support structure  14  via bolts  42 . The support structure  14  defines an aperture  14   a.  The spring housing  12  also defines an aperture  12   e  approximately coaxial with aperture  14   a.  A support bushing  44  is attached to the support structure  14  and defines a threaded bore  45  extending therethrough. A support bolt  38  defines a threaded outer surface  49  that threadably engages the threads defined by the support bushing  44 . 
         [0019]    The preload stud  26  extends from the spring seat  16  and through the stop plate  18  and the central bore  51  defined by support bolt  38 , and includes a threaded portion  26   a  that extends out of the spring housing  12 . A stud adjustment nut  46  threadably engages the threaded portion  26   a.  The support bolt  38  also defines a central bore  51  extending therethrough. A bushing  40  can also be positioned in the central bore  51 . As seen in  FIG. 1 , the stud adjustment nut  46  is set on the preload stud  26  so that when the spring seat  16  is in the forward-most position (i.e., farthest from the stop plate  18 , where the first and second springs,  22  and  24  respectively, at their initial degree of compression), the spring seat  16  rests at the offset A from an interior shoulder  12   f  in the spring housing  12 . 
         [0020]    The degree of initial compression of the first and second springs,  22  and  24  respectively, is determinative of the compression force exerted by the first and second springs  22  and  24  on the spring seat  16  and the stop plate  18  when the spring assembly  10  is ready for use. The initial spring force can be varied by varying the position of the pressure spring seat  16  relative to the stop plate  18 , by rotating the stud adjustment nut  46  relative to the preload stud  26 , thus driving the preload stud  26  and spring seat  16  toward or away from the stop plate  18 . Driving the preload stud  26  outward compresses the springs  22  and  24  against the stop plate  18 , whereas driving the preload stud inward decompresses the springs. An optional jam nut  47  helps keep the stud adjustment nut  46  in place on the preload stud  26  after the initial position of the preload stud in the support bolt  38  is set. The initial spring force is transmitted to the load cell  32  that in turn sends information to a controller with which the load cell is in communication. The information is indicative of the magnitude of the initial spring force. 
         [0021]    In the illustrated embodiment, the spring assembly  10  may include a thrust bearing  50  and an optional support bolt seat  52  located between the support bolt  38  and the stop plate  18  and/or there may be a thrust bearing  54  located between the stud adjustment nut  46  and the support bolt  38 . The thrust bearing  50  and the thrust bearing  54  aid in the support bolt  38  being easily turnable using a wrench. Once the support bolt  38  is set in a desired position, the position of the load cell  32  and stop plate  18  is held stationary during operation of the spring assembly  10 . 
         [0022]    The spring housing  12  has an aperture  12   e  located at the first end  12   a,  and the spring seat  16  is configured to partially protrude through the aperture. However, the spring seat  16  cannot exit the spring housing  12  through the aperture  12   e.  In one embodiment, for example, the spring seat  16  includes a flange  16   a  which is configured to slidably engage the interior shoulder  12   f  inside the spring housing  12  to prevent the spring seat  16  from passing through the aperture  12   e.    
         [0023]    Rotating the support bolt  38  relative to the spring housing  12 , i.e., relative to the bolt bushing  44 , will advance or retract the spring seat  16  in the spring housing  12 . Advancing the spring seat  16  into the spring housing  12  causes the preload stud  26  and the spring seat  16  to move forward toward the first end  12   a  of the spring housing  12 , and causes the spring seat  16  to protrude farther out from the aperture  12   e.  The initial compression remains constant as the support bolt  38  advances, unless the offset A between the flange  16   a  and the interior shoulder  12   f  is eliminated and the spring seat and preload stud  26  can no longer advance in the spring housing  12 . Conversely, retracting the support bolt  38  from the spring housing  12  causes the stop plate  18  to move backward in the spring housing, causing the spring seat  16  to withdraw into the spring housing and to protrude less, increasing the offset A. The initial compression remains constant as the support bolt  38  retracts until the stop plate  18  engages the bolt bushing  44 . 
         [0024]    In an illustrative embodiment, a pulverizer  60  in  FIG. 2  is a bowl mill-type pulverizer that includes a pulverizer housing  62  within which a grinding table  64  is situated to provide a grinding surface  66  for a material to be pulverized. In one embodiment, the grinding table  64  is mounted on a shaft (not shown) that in turn is operatively connected to a suitable gearbox drive mechanism (not shown) so as to be capable of being suitably driven for rotation within the pulverizer housing  62 . A journal assembly  68  is pivotably mounted on a pivot shaft  70  that is secured to the pulverizer housing  62 . For ease of illustration, only one journal assembly  68  and associated spring assembly  10  are shown and described, but the invention is not limited in this regard, and in other embodiments the pulverizer  60  may comprise two, three, or more journal assemblies and associated pressure spring assemblies, which may be evenly distributed about the grinding surface  66 . 
         [0025]    The journal assembly  68  carries a grinding roll  72  rotatably mounted thereon and positions the grinding roll to define a gap G 1  between the grinding roll and the grinding surface  66 . The gap G 1  varies when the journal assembly  68  pivots on the pivot shaft  70 . The journal assembly  68  includes a journal stop flange  74  and there is a stop bolt  76  in the pulverizer housing  62  to limit the pivoting motion of the journal assembly toward the grinding surface  66 , thus setting a minimum size for the gap G 1 . As known in the art, selecting the minimum size for the gap G 1  contributes to determining the particle size distribution of the pulverized material produced in the pulverizer  60 . 
         [0026]    The journal assembly  68  also includes a journal head  78 , and the journal assembly and the spring assembly  10  are mounted on the pulverizer housing  62  so that the journal head can engage the spring seat  16  when the journal assembly pivots away from the grinding surface  66 , e.g., in response to the introduction of granule material between the grinding surface and the grinding roll  72 . Optionally, the journal assembly  68  and the spring assembly  10  may be configured so that there is a gap G 2  between the journal head  78  and the spring seat  16 . The gap G 2  is at a maximum when the journal assembly pivots fully forward, i.e., when the gap G 1  is at a minimum. The maximum gap G 2  can be adjusted by advancing or retracting the support bolt  38  as described above. When the journal assembly  68  pivots sufficiently to close the gap G 2 , the journal head  78  engages the spring seat  16  and the spring assembly  10  imposes a spring force upon the journal head. The journal assembly  68  then conveys the spring force onto the granule material to be pulverized via the grinding roll  72 . The more that the granule material causes the journal assembly  68  to pivot away from the grinding surface  66 , the more the springs  22  and  24  are compressed and the greater the spring force that is imposed on the journal head  78 . 
         [0027]    In one embodiment of the use of the pulverizer  60 , the material to be pulverized is coal, to provide coal powder for use as a fuel in a combustion process. Coal granules are delivered onto the grinding table  64 , which is rotated so that the coal granules are crushed between the grinding surface  66  and the grinding roll  72 . Larger granules of coal cause the grinding roll  72  to pivot away from the grinding surface  66  and thus engage the spring seat  16 . If the coal granule is not then immediately crushed, the journal assembly  68  may then pivot further, causing the spring seat  16  to compress the springs  22  and  24 . The load cell  32  generates a signal that indicates the load on the springs  22  and  24 . The signal is emitted via the output lead  36 . Some of the mechanical and operational factors that contribute to the journal assembly  68  movement and spring force change are the depth and location of wear on the grinding roll  72  and grinding surface  66 ; the roundness (circularity) of the grinding roll; the accuracy of the initial clearance set between the grinding roll and the grinding surface (the roll/ring setting procedure); the weakening of the journal spring  22 ,  24  caused by damage or fatigue; depth and granule size of material on the grinding table  64 ; and/or the size and nature of debris contained within the raw material being pulverized. 
         [0028]    When the pulverizer  60  is in operation, the total force created in springs  22  and  24  by the spring assembly  10  as it contacts the journal assembly  68  is the sum of the initial spring force and the dynamic spring force. The dynamic spring force is the force created when the journal assembly  68  pivots upward from the grinding table  64  and compresses the springs  22  and  24  an additional amount beyond the initial degree of compression. The dynamic spring force is transmitted back onto the journal assembly  68  and onto the material to be pulverized. The value of the dynamic spring force can be about 50% to about 70% of the initial spring force, and the dynamic spring force changes with the loading of the pulverizer  60 . As an example, for journal springs  22 ,  24  having a 25,000 pound/inch spring rate (K factor) for an initial spring compression of 1 inch, a further one-half inch compression of the springs resulting from pivoting movement of the journal assembly  68  will produce dynamic spring compression having an additional force of 12,500 pounds, for a total spring force of 37,500 pounds. In one embodiment, the initial spring force of all the spring assemblies  10  in the pulverizer  60  are kept within about 1000 pounds of each other in order to minimize bending and failure of the gearbox components. Accurate spring compression also is helpful for obtaining the desired particle size of pulverized material. For example, a desired size of coal can be selected to contribute to efficient boiler operation, boiler combustion and emissions control. 
         [0029]    The signal from the load cell  32  is conveyed via the output lead  36  to a controller  83  (e.g., suitable data monitor and recording equipment, a programmable logic controller and/or a suitably programmed general purpose computer) that may optionally be positioned in a control room for observation and analysis by a user. The signal processing apparatus can be configured to display and record the initial spring compression force (or, “initial spring force”) that is applied to each spring assembly  10  when the spring compression is set. In addition, the signal from the output lead  36  enables the user to measure, record and display the total dynamic spring force created by the spring assembly  10  as it contacts the journal assembly  68  during operation of the pulverizer  60 . 
         [0030]    In pulverizers that lack a load cell  32  it is difficult to confirm that the respective initial spring force, the dynamic spring force and total spring force that are generated during operation in the spring assembly  10  stay within a desired range of each other. The only information known about the condition of the springs  22 ,  24  is the initial spring force (initial spring compression) that is set on each spring assembly  10  prior to the pulverizer being placed into service. The accuracy with which the initial spring force is set is dependent on the skill of the workers and the condition of the spring compression setting equipment used. The dynamic spring force created by the spring assemblies as they contact the journal assemblies is unknown, except as the spring condition may be estimated visually by observing the vibration of the pulverizer and the movement of the preload stud  26  relative to the support bolt  38 . Based on such observation, a rough assessment of the total force on the spring system and the conditions within the pulverizer is made. This is a crude, subjective and often inaccurate method and the ability to obtain useful results from using such a method is highly dependent on the experience of the personnel that make the assessment. The result is that operational problems or failure of the pulverizer, its grinding components, or its gearbox components can occur before the condition responsible for creating the problem is noticed and repaired or corrected. 
         [0031]    The installation of a load cell  32  into each spring assembly  10  will enable the total spring force created by each spring assembly  10  during operation of the pulverizer  60  to be monitored and recorded. This data will permit the real time detection, analysis and correction of problems with the pulverizer  60  mechanical components and performance during operation. For example, the load cell  32  can be used to detect various conditions in the spring assembly  10  and/or in the pulverizer  60 , such as a weak or broken spring  22  and/or  24 , an incorrectly set initial compression force, an incorrectly set gap G 1 , an out-of-round or broken grinding roll  72 , a badly worn or broken grinding table  64 , and/or the presence of large granules that have become trapped between the grinding surface  66  and a grinding roll  72 . 
         [0032]    The data obtained from the load cell  32  can simplify the work required to equalize the adjustment and setting of the initial spring compression force among each journal assembly  68  and spring assembly  10  in order to reduce the imbalance forces that act on the gearbox components. This, in turn, will extend the service life of the gearbox components. In addition, the data can be used to simplify and improve the accuracy of the adjustment of the pulverizer  60  to achieve a desired fineness (particle size distribution) in the material being pulverized. Attaining a desired particle size of coal facilitates proper combustion and emissions control. Plant safety can also be improved by providing real time detection and analysis of the signal from the load cell  32 , which can indicate several types of mechanical and operation problems in a pulverizer  60 . 
         [0033]    A spring assembly  10  can be installed during the original manufacture of a pulverizer  60 , or in a retrofit process for a prior art pulverizer, by removing a prior art spring assembly and providing a spring assembly  10  as describe herein. 
         [0034]    In an alternative embodiment, spring assembly  10  may be an adjustable actuator controlled by controller  83 . It may include a motor that may screw stud adjustment nut  46  inward or outward increasing or decreasing spring force under the control of controller  83 . Controller  83  may sense the signal from the load cell  32 , calculate a desired amount of force to be supplied by spring assembly  10 , then cause spring assembly  10  to adjustably apply the desired amount of force. 
         [0035]    In still another alternative embodiment, the spring assembly  10  may be replaced with hydraulic or pneumatic actuators operating under the control of controller  83 . 
         [0036]    In another embodiment, a mechanical dampening device  81 , such as a conventional shock absorber, may be attached between pulverizer housing  62  and journal assembly  68  to dampen the motion of journal assembly  68  relative to pulverizer housing  62 . This dampening device  81  may also exhibit variable dampening force that is controlled by controller  83 . 
         [0037]    The terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. 
         [0038]    While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.