Abstract:
An accumulator is connected to a source of hydraulic fluid, comprising a movable barrier which separates the hydraulic fluid from a compartment containing two distinct energy absorbing medium—a first energy absorbing medium and a second energy absorbing medium. The movable barrier, when acted upon by an increase in pressure of the hydraulic fluid, will move in a first direction to initially contact and begin compressing the first energy absorbing medium. When the movable barrier moves further in said first direction it will contact the second energy absorbing medium and will begin compressing the second energy absorbing medium while still compressing the first energy absorbing medium.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Vertical roller mills, especially those common for grinding of cement raw materials, typically employ a hydraulic-pneumatic system to apply a grinding force to the material bed. During operation, these systems will contain pressurized hydraulic fluid in an isolated branch of the circuit consisting principally of cylinders and accumulators. This trapped pressure, along with the cylinder and accumulators, creates a hydraulic “spring”. The hydraulic spring serves two purposes. First, it provides the grinding force to the rollers for the purpose of comminution. Second, it acts as a suspension system so the grinding rollers can accommodate changes in material depth and strength.  
         [0002]     Typical vertical roller mill geometry has the rod side of the cylinder pressurized to create the grinding force. Various possibilities exist for the piston side. Some systems have non-pressurized oil which freely flows between the cylinder and tank. Other systems have means to evacuate this area, and operate with a partial vacuum. A third type, relevant to this invention, employs pressurized oil on the piston side. These counter-pressure hydraulic systems for vertical roller mills are well known in the cement industry. Pressurization of the piston side, at a much lower level than on the rod side, has been demonstrated to improve operational stability of vertical mills grinding cement raw materials.  
         [0003]     During normal grinding, it is desirable to have a relatively flat force-displacement curve, i.e., a soft hydraulic spring. This softness, or low spring stiffness, contributes to maintaining a low mill vibration level. However, to prevent potentially damaging mill vibration or tire-to-table contact, the grinding force should be reduced or even removed completely if the material bed becomes unstable. This cushioning effect (that is, a decrease in grinding force at low bed depths) is one of the major benefits of counter pressure systems.  
         [0004]     In traditional counter pressure systems, the cushion effect comes at the expense of increasing system stiffness.  FIG. 1  illustrates force displacement curves A-D in such traditional counter pressure systems utilized in a roller mill. Since the cushion effect is directly proportional to the counter pressure magnitude, as the cushion effect is increased, that is, as one goes from the system depicted in curve A toward the system depicted in curve D, the system stiffness, or steepness of the force displacement curve, is also increased. It is one object of the invention, therefore, to eliminate the need to make trade offs between system stiffness and cushion effect. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a graph showing the force displacement curve in a traditional counter pressure system utilized in a roller mill.  
         [0006]      FIG. 2  is a graph showing a comparison of the force displacement curve in a traditional counter pressure system utilized in a roller mill, a roller mill system which utilizes no counter pressure, and the system of the present invention.  
         [0007]      FIG. 3  is a graph showing the force displacement curve in the system of the present invention which illustrates respective values at various points in the system.  
         [0008]      FIG. 4  illustrates a portion of a roller mill of the present invention in which there is depicted the use of an accumulator assembly of the present invention.  
         [0009]      FIG. 5  is a more detailed illustration of an accumulator assembly of the present invention.  
         [0010]      FIG. 6  depicts another embodiment of an accumulator which can be utilized in the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0011]      FIG. 2  illustrates the force displacement curves of the traditional, prior art, counter pressure system (curve E) a system in which there is no counter pressure (curve F) and the proposed system of the present invention (curve G).  FIG. 3  displays the force displacement curves of the proposed system at various points in the system, as will be explained in more detail below.  
         [0012]     By utilizing the accumulator system of the present invention, it is possible to create a hydraulic spring suspension with a transition point. This point defines a material bed level below which there is substantial risk for either high vibration or tire-to-table contact. For material bed depths greater than the transition point, the hydraulic spring is soft. When the material bed is lower than the transition point, the hydraulic spring becomes progressively stiffer, partially relieving the net grinding force and inhibiting both vibration and tire-to-table contact.  
         [0013]     The present invention describes a system of accumulators to achieve the desired effect. While it is possible to realize such spring characteristics in other ways, these systems require additional valves, transducers, or other components. The proposed system can, using a novel arrangement of accumulators, provide improved cushioning effect without the drawbacks of either complex hydraulics or increased system stiffness.  
         [0014]     With reference to  FIG. 4 , the various parts of which are not necessarily drawn to scale, the vertical roller mill  20  of the present invention comprises rotating table  21 , supported by gearbox  22  which is powered by an electric motor (not shown). Material is fed to the center of table  21 . A plurality of grinding rollers  23 , only one of which is depicted in  FIG. 4 , are equally spaced about table  21 . Each grinding roller  23  includes tire  25 , which is free to turn about axle  26 . Axle  26  is held by lever  27 , which pivots on shaft  28 . The grinding force is created by hydraulic cylinder  29 , attached to the lever  27 . A hydraulic power unit (not shown) provides and maintains pressurized fluid to both the rod side  30  and piston side  31  of the cylinder.  
         [0015]     Due to the centrifugal force of rotating table  21 , the material is distributed to rollers  23 , where it forms a grinding bed  24  which is ground between roller tire  25  and table liners  33 .  
         [0016]     Accumulator assembly  35 , which is the assembly of the present invention, is connected by hydraulic fluid conduit  36  to piston side  31  of cylinder  29 . Optional standard accumulator  32  is connected by hydraulic fluid conduit  37  to rod side  30  of cylinder  29 . Both accumulator assembly  35  and standard accumulator  32  serve to store and supply pressurized fluid to and from the cylinder  29  as it moves in response to the material grinding bed fluctuations. The accumulators are typically precharged with gas, typically an inert gas that is preferably nitrogen, for energy storage, that is, as an energy absorbing medium, but mechanical energy absorbing media such as mechanical springs or other energy storage mechanisms known in the art may be employed.  
         [0017]     The accumulator assembly of the present invention can be connected to either or both the piston side or the rod side of the vertical roller mill&#39;s hydraulic cylinder. The accumulator assembly may be used by itself or in conjunction with a standard accumulator, as is depicted in  FIG. 4 .  
         [0018]     The accumulator assembly of the present invention comprises at least two accumulators that are hydraulically interconnected to the same source of hydraulic fluid. Each accumulator contains an energy absorbing medium. The medium is compressible when a movable barrier which separates the hydraulic fluid from the energy absorbing medium is acted upon by an increase in pressure of the hydraulic fluid.  
         [0019]     At least one of the accumulators in the accumulator assembly of the present invention contains a compressibility limiter which interrupts the compressibility of the energy absorbing medium within the accumulator. That is, through the use of the compressibility limiter the compressibility of the medium is stopped at less than its natural state of compression. At least one of the accumulators in the accumulator assembly of the present invention does not contain a compressibility limiter so that the energy absorbing media therein may be fully compressed to its natural state by the hydraulic fluid. Thus, if there are only two accumulators in the accumulator assembly of the present invention one must contain a compressibility limiter and the other one must not.  
         [0020]     Typically, the movable barrier in the accumulator that contains a compressibility limiter is a movable piston which, when acted upon by an increase in pressure of the hydraulic fluid, moves and compresses the energy absorbing medium. Alternatively the movable barrier can be a diaphragm or a bladder.  
         [0021]      FIG. 5  depicts one embodiment of an accumulator assembly  50  of the present invention. The assembly contains a first accumulator  40  and a second accumulator  41 , which are both depicted as being a piston style, having movable pistons  43   a  and  43   b . Both pistons can move in the direction specified by arrow a (when there is an increase in hydraulic pressure) or arrow b (when there is a decrease in hydraulic pressure). When each piston moves in the direction specified by arrow a they thereby compress gas located in compartments  47   a  and  47   b . First accumulator  40  contains compressibility limiter  45 , which in this instance in a piston stroke limiter which serves to limit the stroke of piston  43   a  in the direction of travel indicated by arrow a and thereby interrupt the compressibility of gas located in compartment  47   a . Compressibility limiter  45  can have many forms. Preferably it is externally adjustable, which is the version depicted in  FIG. 5 , wherein compressibility limiter  45  can move in the direction specified by arrow a or arrow b. In another embodiment, compressibility limiter  45  can be an internal retainer set in a fixed position. As depicted in  FIG. 5 , first accumulator  40  has a larger internal volume than second accumulator  41 . This is an optional embodiment.  
         [0022]     A second accumulator  41 , which can be any style, must also be present in accumulator assembly  50 . The second accumulator  41  must allow the gas located in compartment  47   b  to be freely compressed, i.e., no limiter as described for first accumulator  40  may be present. Accumulator assembly  50  may have more than two accumulators, with each additional accumulator being chosen from a version of an accumulator which contains a compressibility limiter or one that does not.  
         [0023]     Accumulator assembly  50  operates as follows (this is in reference to the depicted embodiment when accumulator assembly  50  is as depicted, i.e. attached to piston side  30  of hydraulic cylinder  29 ): during normal grinding operation, there are only small variations in the material bed  24  depth. Fluid flows between the cylinder and the accumulators on the piston side (assembly  50 ) and rod side (accumulator  32 ) of hydraulic cylinder  29 . The accumulators  40  and  41  in accumulator assembly  50  act jointly, sharing the displaced hydraulic fluid. Piston  43   a  in the stroke limited accumulator  40  will float between the retainers  44  and stroke limiter  45  without contacting either. The piston  43   b  in the second accumulator  41  will also move freely, and is limited only by the compressibility of gas in compartment  47   b.    
         [0024]     During unstable operation, there can be a sudden reduction or loss of material bed  24 . Roller  23 , under force of hydraulic cylinder  29 , will push downward towards the table  21 . This motion will push a large volume of hydraulic oil through the common manifold  46  into accumulators  40  and  41 . Piston  43   a  of accumulator  40  will be forced upward until it contacts stroke limiter  45 . Once the piston  43   a  contacts stroke limiter  45 , accumulator  40  will no longer accept any displaced hydraulic fluid. Thus, the system&#39;s effective accumulator volume is reduced. Any and all additional oil must then flow into the second accumulator  41 . The reduced effective volume results in a stiffer hydraulic spring, characterized by the steep section of the plot in  FIG. 3 .  
         [0025]      FIG. 6  illustrates another embodiment of the present invention, in which a single accumulator  60  replaces accumulator assembly  50 . Single accumulator  60  incorporates a mechanical spring  63  or other energy absorbing device. The action is similar to the previously described system. During normal grinding, piston  62  will freely travel between piston retainers  64  and spring  63 . When the piston moves in the direction of arrow c, moving from retainers  64 , it will initially contact a first energy absorbing medium, in this case inert gas or nitrogen located within compartment  67 . Should, as previously described, bed instability or another reason cause the grinding roller to move sharply downward, the piston  62  will move upwards in direction c and, at a later point in its travel, contact a second energy absorbing medium, in this case mechanical spring  63 . At this contact point, any further upward motion will be resisted by both the second energy absorbing medium, that is, the compressed gas, and mechanical spring  63 . Again, the result is a stiffer system.  
         [0026]     This invention has the advantage of not requiring additional valves, transducers, or electronic components to achieve the desired effect.  
         [0027]     A roller mill incorporating the system of the present invention has the further advantage that it is self-compensating for wear of the grinding components. Internal leakage is inherent to virtually all hydraulic systems. Therefore, oil must be added to the system periodically to maintain the prescribed nominal grinding pressure setpoint. This occurs on a much shorter time scale than wear of the grinding parts, that is, grinding tire  25  and table segments  33 . While mechanical stoppers for limiting travel of the grinding lever are well known, these mechanical stoppers engage the roller at an absolute roller position. Wear of the grinding parts must be compensated for by adjustment of the mechanical stoppers. Through the use of the present invention, the transition point is a function solely of hydraulic pressure changes. As such, the transition point will always occur at a predetermined level below the nominal grinding bed depth. This feature eliminates the need to adjust mechanical stoppers to compensate for wear.  
         [0028]     While there are shown and described present preferred embodiments of the invention, it is distinctly to be understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.