Abstract:
An unloader system for a reciprocating gas compressor includes an unloader valve assembly which may be installed as the suction valve assembly or the discharge valve assembly of the compressor. The valve assembly includes a valve seat having one or more seat passages formed therethrough and a valve guard having a number of valve members movably mounted thereon equal to the number of seat passages. One of the valve guard and valve seat is rotatable relative to the other. The rotatable member is driven by a constant torque motor or other power source which stalls when a retarding force caused by pressure differentials across the valve members overcomes the torque supplied. Once pressures across the valve members equalize, the rotatable member can resume rotation. A method of unloading a compressor using the unloader system includes selecting a rotational speed for the rotating member to allow backflow.

Description:
This application claims the benefit of provisional application Ser. No. 60/847,233 filed Sep. 26, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to unloaders for reciprocating gas compressors, and in particular to an unloader having a valve assembly with a valve guard and valve seat mounted for relative rotation. The rotatable member is driven by a constant torque motor or other power source which stalls or slips when the torque supplied is overcome by forces on the valve assembly caused by pressure differentials across the valve. Once pressures equalize, the rotatable member is free to resume rotation. 
     2. Description of the Related Art 
     In my earlier patent, U.S. Pat. No. 5,695,325, entitled Synchronized Unloader System and Method for a Gas Compressor, I disclosed an unloader system for a reciprocating gas compressor which includes an unloader valve assembly having a valve seat with multiple seat passages extending therethrough and arranged in a seat passage circle. A valve guard is rotatably mounted on the valve seat and includes a plurality of valve members arrayed in a valve circle and movable between open and closed positions with respect to the seat passages. An unloader actuation system includes a controller connected to a control system for the compressor and a stepper motor drivingly connected to the valve guard. In use, the valve guard is incrementally rotated in synchronization with the compressor crankshaft by increments corresponding to the spacing between the valve members and the seat passages. The closings of the valve members are delayed by varying amounts to achieve varying amounts of unloading. 
     SUMMARY OF THE INVENTION 
     The present invention is an unloader system which utilizes a valve assembly similar to those described in U.S. Pat. No. 5,695,325. Instead of being synchronized with the compressor crankshaft by means of a stepper motor and electronic control system, however, the valve guard is rotatably driven by a power source having a constant or steady torque and the ability to stall or slip when the resistance on the valve guard exceeds the torque supplied by the power source. As the valve guard rotates, the valve members will periodically come into alignment with the valve seat passages. When the pressure acting on the valve member is sufficient to resist the torque of the power source, the power source will slip, causing a delay in the rotation of the valve guard. When the pressure equalizes across the valve member, the guard will resume rotation. The speed of rotation of the valve guard may be selected to cause the valve members to next align themselves with the valve seat passages at a point in the compressor cycle wherein some amount of gas is allowed to flow backward before the valve member can close, thereby partially unloading the compressor. The amount of backflow can be adjusted by adjusting the speed of rotation of the guard. Unloading is achieved by decreasing capacity by intentionally allowing either late closure of a suction valve or a discharge valve. 
     In addition to unloading, the use of rotational valves, such as the valve of the present invention also improves efficiency of the compressor. Many compressors now have up to 30% of the compressor horsepower that results from just the resistance to flow through the valves at the velocities required. Efficiently operating reciprocating compressors may have as little as 5%-7% of the horsepower used to overcome the resistance to flow through the valve. The majority of the horsepower in both cases goes to getting the gas from the lower pressure to the higher pressure. 
     One factor in the operation of the unloader of the present invention is that efficiency is improved as the sealing element is out of the gas stream during a significant part of the intake or exhaust stroke. In some cases, it would be possible to show an improvement of as much as 15% to 20% in the operating efficiency of a compressor if any or all valves in the compressor were equipped to allow this reduced resistance to flow. This would be referred to as “active valves” as they would have an operating mechanism, and would not be dependent strictly on a pressure differential to open the valves as is the case with conventional valves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary cross sectional view of a reciprocating gas compressor showing a constant torque unloader system according to the present invention operating as a suction valve of the compressor. 
         FIG. 2  is a cross section of an unloader valve which forms a part of the unloader system taken along line  2 - 2  in  FIG. 1  and showing a valve guard thereof. 
         FIG. 3  is a cross section of the unloader system taken along line  3 - 3  in  FIG. 2  and showing valve members mounted on the valve guard in an aligned orientation with valve seat openings in a valve seat. 
         FIG. 3(   a ) is a view similar to  FIG. 3  showing an alternative embodiment of the unloader system having a rotatable guard mounted in a stationary carrier. 
         FIG. 4  is a cross section of the unloader system taken along line  4 - 4  in  FIG. 2  and showing the valve guard in an orientation wherein the valve members are not aligned with the valve seat openings. 
         FIG. 5  is a cross section of the unloader system taken along line  5 - 5  in  FIG. 3  and showing the valve seat thereof. 
         FIG. 6  is a partially schematic fragmentary view of the unloader system showing an alternative embodiment of the constant torque power source for the unloader system. 
         FIG. 7  is a cross sectional view similar to  FIG. 3  and showing the unloader system operating as a discharge valve. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention, which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. 
     Certain terminology will be used in the following description for convenience in reference only and will not be limiting. For example, the words “upwardly,” “downwardly,” “rightwardly,” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the embodiment being described and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof and words of a similar import. 
     Referring to the drawings in more detail, and in particular to  FIG. 1 , the reference number  1  generally designates a constant torque unloader system according to the present invention. The system  1  is for use on a reciprocating compressor  3  including a cylinder  5  slidably receiving a piston  7  connected to a crankshaft (not shown). A suction valve assembly  9  mounted in a suction of the compressor  3  selectively communicates the cylinder  5  with a suction line  11 . Similarly, a discharge valve assembly  13  selectively communicates the cylinder  5  with a discharge line  15 . The compressor  3  generally operates to moves gas from the suction line  11  to the discharge line  15  at increased pressure. 
     The system  1  includes a valve assembly  21  which may be installed in the compressor  3  to act as either a suction valve assembly  9  (as shown in  FIGS. 1-5 ) or as a discharge valve assembly  13  (as shown in  FIG. 7 ). For purposes of simplicity, the valve assembly  21  will primarily be described and depicted herein as a single deck suction valve assembly  9  selectively controlling communication between the cylinder  5  and suction line  11  of the compressor  3 . It is to be understood, however, that the current invention may be equally well applied to radial valve assemblies and multi-deck valve assemblies, which may be either suction valve assemblies  9  or discharge valve assemblies  13 . These other types of valve assemblies  21  are generally described in U.S. Pat. No. 5,695,325, the disclosure of which is hereby incorporated by reference. 
     Referring to  FIGS. 2-5 , the valve assembly  21  includes a valve seat  23  and a valve guard  25  rotatably mounted on the valve seat  23 . The valve seat  23  includes one or more valve seat passages  27  extending therethrough. The valve guard  25  includes one or more valve members  29  movable between open and closed positions with respect to the valve seat passages  27  when the valve guard  25  rotated relative to the valve seat  23  such that the valve members  29  are in alignment with the valve seat passages  27 . The valve guard  25  further includes a plurality of bypass openings  30  around the valve members  29 . As the valve guard  25  rotates on the valve seat  23 , the valve members  29  move cyclically in and out of alignment with the valve seat passages  27 . When the valve members  29  are aligned with the valve seat passages  27  (as shown in  FIG. 3 ) the valve seat  23  the valve members  29  control flow through the valve assembly  21 . When the valve members  29  are out of alignment with the valve seat passages  27  (as shown in  FIG. 4 ), gas flows freely through the valve assembly  21  by way of valve seat passages  27  communicating with the bypass openings  30 . 
     An exemplary valve guard  25  is shown in  FIG. 2  as having eight poppet type valve members  29  arranged in a circle and equally spaced apart (at 45 degree increments). A compatible valve seat  23  is shown in  FIG. 5  as having eight valve seat passages also arranged in a circle and equally spaced apart (at 45 degree increments). As best seen in  FIG. 3 , each poppet valve member  29  has a head  33 , a stem  35  and urged against the valve seat  23  by a valve spring  36 . It is to be understood, however, that the number of valve seat passages  27  and valve members  29  may be more or less than the eight shown and that they may be arranged in several concentric circles. Furthermore, it is to be understood that other known types of valve members  29  may be used in place of the poppet type valve members  29  shown. 
     Referring again to  FIG. 3 , the valve assembly  21  is mounted in a suction valve pocket  10  of the compressor  3  such that, when the valve members  29  are aligned with their respective seat passages  27 , pressure in the suction line  11  acts on the heads  33  of the valve members  29  and urges them toward their open positions. After Bottom Dead Center (BDC), pressure in the cylinder  5  acts on the stems  35  of the valve members  29  through openings  37  in the valve guard  25  and urges the valve members  29  toward their closed positions. When the pressure in the cylinder  5  is less than the pressure in the suction line  11 , the valve members  29  move into their open positions. When the pressure in the cylinder  5  is greater than the pressure in the suction line  11 , the valve members  29  move into their closed positions. 
     A cap  39  covers the suction pocket  10  and retains the valve assembly  21  in position. A first end of an unloader drive shaft  41  is fixedly connected to the valve guard  25  in axial relation to the circle of valve members  29 . The shaft  41  extends through a shaft receiver  43  in the valve seat  23  and is rotatable relative thereto. A second end of the unloader drive shaft  41  extends outwardly from the suction pocket  10  through an opening  45  in the cap  39 . 
     A constant torque power source  47  is operatively connected to the second end  42  of the unloader valve drive shaft  41  and is operable to rotate the valve guard  25  relative to the valve seat  27 . As shown in  FIG. 3 , the constant torque power source  47  may be, for example a motor  47   a , such as a pneumatic, hydraulic or electric motor (such as a direct current electric motor) having the ability to slip or stall when the resistance to rotation exceeds the torque being produced. The rotational speed of the power source  47  is preferably adjustable, such as through variation in the current or fluid flow supplied to the motor  47   a , so that the amount of unloading may be varied as discussed below. 
       FIG. 3(   a ) shows an alternative embodiment  1   a  of the present invention wherein the valve guard  25  is mounted in a stationary carrier  26  having a cylindrical recess sized to rotatably receive the valve guard  25 . As with the previous embodiment, the guard  25  is fixed to the shaft  41  and carries the moveable valve members  29 . The carrier  26  is fixedly clamped between the valve seat  23  and a lower shoulder of the valve pocket  10 . Mounting the guard  25  in a separate carrier  26  allows for easier rotation of the guard  25  relative to the valve seat  23 . The operation of the system  1   a  is identical to the operation of the system  1  as described below. 
     As shown in  FIG. 6 , the constant torque power source  47  may also be a flywheel  47   b  acting on the unloader valve drive shaft  41  through a slip clutch  49 . The flywheel  47   b  may be, for example, driven by a motor  51 . In this embodiment, the slip clutch  49  may be adjusted to vary the torque transmitted to the valve guard  25 . The slip clutch  49  will begin to slip, pausing rotation of the valve guard  25  when the set torque limits of the clutch are overcome by forces on the valve assembly  21  caused by pressure differentials across the valve members  29 . The flywheel  47   b  will continue to rotate. It should be noted that  FIG. 6 , which is partially schematic, shows the motor  51  acting on the flywheel  47   b  through a simple belt and sheave arrangement, however it is to be understood that the flywheel could also be driven using any known drive system, including a gear drive, and that any drive system used would incorporate sufficient reduction to allow the flywheel  47   b  to rotate the valve guard  25  at the proper rotational speed for the compressor. 
     Operating Example 
     The operation of the system  1  may be shown by assuming a 320 RPM Compressor operating with 50% Suction Volumetric Efficiency (“VE”) and looking at a cycle of the compressor  5  starting with the piston  7  at top dead center (“TDC”) and the valve members  29  in their closed position sealing the valve seat passages  27 . As described above, the valve assembly  21  of the system  1  is a suction valve assembly  9  having eight valve members  29  equally space apart (at 45 degree intervals) around a circle. A constant torque is applied on the suction unloader drive shaft  41  by the power source  47 . The valve guard  25  does not move initially since at TDC the cylinder pressure pushes the valve members  29  against the valve seat  23  with sufficient force to resist the torque supplied by the power source  47 . However, when the pressure equalizes across the suction valve assembly  9  (at mid-stroke with a 50% suction VE), the valve guard  25  will start to rotate (after being delayed for one quarter revolution of the compressor crankshaft  8 ). If the valve guard  25  is rotated at one half of the speed of the compressor crankshaft, the valve members  29  will line up with the valve seat passages  27  at the same time that the piston  7  hits bottom dead center (“BDC”). If the pressure rise in the cylinder  5  is fast enough, the valve members  29  will not be able to move out of the valve seat passages  27  and compression will start. Because the valve members  29  close virtually simultaneously with the piston  7  reaching BDC, there is little or no backflow from the cylinder  5  to the suction line  11  and therefore the compressor  3  is operating in a fully loaded condition. 
     In order to partially unload the compressor  3 , the rotational speed at which the unloader drive shaft  41  is driven by the power source  47  would be reduced. This would cause the valve guard  25  to arrive at the point where the valve members  29  realign themselves with the valve seat passages  27  at some point after BDC. This would allow some backflow from the cylinder  5  into the suction line  11  until the valve members  29  realign themselves with the valve seat passages  27  and are seated. To reduce the load even more, the speed of the unloader drive shaft  41  would be reduced even more. 
     If the valve assembly  21  is installed as a discharge valve assembly  13  (as shown in  FIG. 7 ), the operation of the system  1  is essentially the same except that the valve members  29  are installed such that pressure in the cylinder  5  acts on the heads  33  of the valve members  29  urging them toward their open positions and pressure in the discharge line  15  acts on the stems  35  of the valve members  29  urging them toward their closed positions. As the valve guard  25  rotates on the valve seat  23 , the valve members  29  periodically align with valve seat passages  27 . If the pressure in the discharge line  15  pushes the valve members  29  closed with sufficient force to overcome the torque of the power source  47 , the valve guard  25  will cease to rotate until pressure across the valve members  29  equalizes. Once the pressure equalizes, the valve guard  25  is free to resume rotation. As before, the degree of unloading is changed by adjusting the speed of rotation of the valve guard  25 . By slowing the speed of rotation of the valve guard  25 , the valve members  29  can be made to not align with the valve seat passages  27  again until some point after the piston  7  reaches top dead center, thereby delaying closing of the discharge valve members  29 . This will allow some gas to flow back from the discharge line  15  into the cylinder  5 , thereby delayed opening of the suction valve members, resulting in less gas coming into the cylinder during the suction event and partially unloading the compressor. 
     It is to be understood that while certain forms of the present invention have been illustrated and described herein, it is not to be limited to the specific forms or arrangement of parts described and shown. For example, the valve assembly  21  has been described as having a valve guard  25  rotatably mounted on a stationary valve seat  23 , however it is foreseen that the valve guard  25  could be held stationary and the valve seat  23  rotated to produce the same result.