Abstract:
An unloader includes a first volume bottle with a cavity formed therein of predetermined volume, and a port in the forward end with a plug operably mounted to selectively open and close the port. A second volume bottle is stacked on the first volume bottle and has a port in the forward end with a plug operable to open and close the second bottle port. The second bottle is arranged such that the second port will fluidly connect the first and second bottles. Actuators are connected to the plugs in the first and second bottles to selectively open only the first bottle or both bottles together, to thereby provide a two stage internal clearance for the reciprocating compressor cylinder.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     (Not applicable). 
     STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     (Not applicable). 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates generally to a reciprocating compressor utilized to pump fluids, and more particularly to an improved unloader for varying the internal volume of a reciprocating compressor cylinder. 
     (2) Background Information 
     Natural gas is distributed through pipelines throughout the country. A series of pumping stations are located at predetermined intervals along the pipelines to move the gas throughout the system. Each pumping station includes one or more compressors with a plurality of reciprocating cylinders for moving the gas through the pipeline. 
     An unloader is a device which will selectively increase or decrease internal volume/clearance of a reciprocating compressor cylinder in order to increase or decrease the unit load and accommodate changing suction and discharge pressures. It is beneficial to maintain a compressor engine loaded near its rated horsepower to obtain the lowest specific fuel consumption rate and the highest compressor throughout. This is accomplished by constantly varying the unit load depending upon various operating conditions. Such conditions include: (1) the type/quality of gas being compressed; (2) operating pressures and temperatures; (3) compressor speed; (4) compressor cylinder size (diameter of the bore and length of the stroke); and (5) fixed and variable clearance volumes. 
     Single stage unloaders are currently used to permit the clearance volumes in the compressor cylinders to be varied, to thereby attempt to keep the engine loaded near its rated horsepower. However, single state unloaders still suffer several drawbacks. 
     First, there are a limited number of locations around a compressor cylinder for installation of an unloader. Thus, the amount of clearance volume available to a cylinder is also limited. 
     In addition, if the single stage unloader provides a large volume change, the large volume change may lead to cylinder single acting operation where gas is compressed and expanded without the gas flowing through the cylinder. This operation leads to high temperatures that may exceed material limits. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore a general object of the present invention to provide an improved multistage unloader for reciprocating compressor cylinders. 
     Another object of the present invention is to provide improved unloader operation, with multiple volumes in place of a single volume, which permits the selection of smaller clearance volumes, allowing a compressor to maintain high volumes while keeping the engine loaded near its rated horsepower. 
     A further object is to provide an improved multi-stage unloader which permits a greater selection of internal clearances in the same space as a single stage unloader. 
     These and other objects of the present invention will be apparent to those skilled in the art. 
     The unloader of the present invention includes a first volume bottle with a cavity formed therein of predetermined volume, and a port in the forward end with a plug operably mounted to selectively open and close the port. A second volume bottle is stacked on the first volume bottle and has a port in its forward end with a movable plug to open and close the second bottle port. The second bottle is arranged such that the second port is connected to the gas stream of the first and second bottles. Actuators are connected to the plugs in the first and second bottles to selectively open only the first bottle or both bottles together, to thereby provide a two stage internal clearance for the reciprocating compressor cylinder. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The preferred embodiment of the invention is illustrated in the accompanying drawings, in which similar or corresponding parts are identified with the same reference numeral throughout the several views, and in which: 
     FIG. 1 is a perspective view of a compressor for pumping natural gas in a pumping station; 
     FIG. 2 is a perspective view of two reciprocating compressor cylinders of the compressor of FIG. 1, with one cylinder and prior art manually operated single stage unloader shown in partial sectional view; 
     FIG. 3A is a perspective view of the two stage unloader of the present invention; 
     FIG. 3B is a perspective view similar to FIG. 3A, with portions shown in sectional view to show the interior of the two stage unloader; 
     FIG. 4 is a vertical sectional view through the two stage unloader, taken at lines  4 — 4  in FIG. 3A, with the unloaders in a “closed” position; 
     FIG. 5 is a sectional view similar to FIG. 4, but with the first stage of the unloader moved to the “open” position; and 
     FIG. 6 is a sectional view similar to FIGS. 4 and 5, but with the second stage of the unloader moved to the “open” position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, and more particularly to FIG. 1, a plurality of two stage unloaders are designated generally at  10 , and are shown mounted on a plurality of cylinders  12  of a compressor  14 . Natural gas is supplied to each cylinder  12  through supply pipe  16  to an intake port  18  on each cylinder  12 . Intake ports  18  are located on the upper, suction side of each cylinder  12 . Discharge ports  20  are provided on the lower discharge side of each cylinder  12 , and lead to a discharge pipe (not shown) for further distribution through the distribution network. 
     Referring now to FIG. 2, two cylinders  12  of compressor  14  are shown in more detail. Each cylinder  12  is provided with a reciprocating piston  22  which reciprocates between proximal and distal positions within a cylinder bore  24 . A pair of prior art single stage unloaders  26  are shown on cylinder  12 ′, located at the proximal and distal ends of bore  24 , such that each stroke of piston  22  in each direction will move gas from the intake port  18  to the discharge port  20  (not shown). A pair of two stage unloaders  10  are mounted in similar locations at the proximal and distal ends of cylinder  12  in FIG.  2 . 
     Referring now to FIGS. 3A,  3 B and  4 , the two stage unloader  10  of the present invention includes a first stage volume bottle  26  having a bore  28  extending through the length thereof. Volume bottle  26  is cylindrical in shape, and has an outer diameter designed to slide within the existing opening  30  in the cylinder head  32 , as shown in FIG. 4. A radially projecting flange  34  at the outward/distal end of first stage volume bottle  26  is bolted to the cylinder head  32  to secure the unloader  10  in position. 
     The second stage volume bottle is designated generally at  36 , and includes a cylindrical transition piece  38  mounted to the distal end of first stage bottle  26 , and an end cap  40  mounted to the distal end of second stage bottle transition piece  38 . A plurality of bolts  42  extend through a plurality of apertures  44  around the perimeter of end cap  40  and thence through the side wall of transition piece  38  to secure second stage volume bottle  36  to first stage volume bottle  26 . Transition piece  38  has a cylindrical forwardly projecting end  46  with an outer diameter sized to slide within bore  28  of first stage bottle  26 . The forward face  46   a  of forward end  46  forms the rearward end of the first stage cavity, designated generally at  48 , within bore  28 . A disk-shaped plate  50  is mounted in the forward/inward end of bore  28  to form a forward wall for cavity  48 . 
     Second stage volume bottle  36  includes a central bore  52  defining a second stage cavity designated generally at  54  within bore  52  extending between end cap  40  and the transition piece forward end forward face  46   a . A port  56  is formed in the forward end forward face  46   a  to permit gas within first stage cavity  48  to enter second stage cavity  54 . Similarly, a port  58  in plate  50  permits gas from the cylinder head  32  to enter first stage cavity  48 . 
     A plug  60  is mounted on the forward end of and elongated actuator shaft  62 , and is sized to be securely seated in the first stage port  58 , to seal the same. Actuator shaft  62  extends through the second stage cavity  54  and thence through an aperture in end cap  40  to project rearwardly outwardly from end cap  40 . 
     A second stage plug  64  has an aperture formed therethrough and is slidably mounted on actuator shaft  62 . Second stage plug  64  is sized to be seated and seal second stage port  56 . Second stage plug  64  is mounted on the forward end of an elongated sleeve  66  which extends through aperture  68  in end cap  40  to project rearwardly therefrom. Actuator shaft  62  is slidably journaled through sleeve  66 , to permit slidable movement of actuator shaft  62  independently of movement of sleeve  66  within aperture  68 . 
     An actuator assembly is designated generally at  70  and is operable to selectively shift actuator shaft  62  and sleeve  66  so as to open or close first stage port  58  with first stage plug  60  and/or second stage port  56  with second stage plug  64 . Actuator assembly  70  includes a second stage cylinder  72  mounted on the outward face of end cap  40  and coaxial with actuator shaft  62 , and a first stage cylinder  74  stacked coaxial atop second stage cylinder  72 . A disk-shaped top  76  is mounted on the outward end of first stage cylinder  74 , to enclose cylinders  72  and  74 . A disk-shaped splitter plate  78  is juxtaposed in between top plate  76  and end cap  40  to separate the interior bore of actuator assembly  70  into a first stage cavity  80  within first stage cylinder  74 , and a second stage cavity  82  within second stage cylinder  72 . 
     Actuator shaft  62  extends through an aperture  84  in splitter plate  78 , and thence through an aperture  86  in top plate  76  to permit the actuator shaft to project outwardly from top plate  76  when moved to an open position as described in more detail hereinbelow. A piston  88  is secured to actuator shaft  62  by a locknut  90 , to slide within first stage cavity  80  and slides with actuator shaft  62 . Piston  88  has a diameter to provide a sliding seal between the piston and the inner wall of first stage cylinder  74 , and is positioned spaced slightly rearward of splitter plate  78 , when first stage plug is in the closed position. A hole  92  through the side wall of first stage cylinder  74  and located between piston  88  and splitter plate  78 , permits the entry of fluid under pressure to force piston  88  outwardly towards top plate  76 , thereby shifting actuator shaft  62  rearwardly and outwardly, along with first stage plug  60 , to thereby open first stage port  58 . A hole  94  in top plate  76  permits entry of fluid under pressure to shift piston  88  forwardly, thereby shifting actuator shaft  62  and plug  60  back to the closed position shown in FIG.  4 . 
     A second piston  96  is secured to the rearward projecting end of sleeve  66 , by locknut  98  such that piston  96  will slide with sleeve  66  along the axis of actuator shaft  62 . Piston  96  also has a diameter to provide a sliding seal between the piston and the inner wall of second stage cylinder  72 , and is positioned spaced rearwardly of end cap  40  when second stage plug  64  and sleeve  66  are in the closed position shown in FIG. 4. A hole  100  through the side wall of second stage cylinder  72  and located between piston  96  and end cap  40 , permits the entry of fluid therebetween to push piston  96  rearwardly until locknut  98  contacts splitter plate  78 . A second hole  102  located between piston  96  and splitter plate  98  permits the entry of fluid under pressure to return a piston  96 , sleeve  66 , and plug  64  to the closed position shown in FIG.  4 . 
     An indicator pin  104  is connected to second stage piston  96 , and extends rearwardly therefrom, and is journaled through aligned apertures in splitter plate  78 , first stage piston  88 , and top plate  76 , to project from top plate  76 . In this way, when second stage piston  96  is shifted rearwardly to move second stage plug  64  and open second stage port  56 , indicator pin  104  will be moved rearwardly and project outwardly from the rearward end of top plate  76 . It is thereby possible to visually ascertain whether the second stage port is open or closed. 
     In use, the two stage unloader  10  of the present invention may be mounted in the same location and restricted space as a single stage unloader, as shown in FIG.  2 . As shown in FIGS. 4,  5 , and  6 , the two stage unloader  10  may be selectively operated to close both first and second stage ports  58  and  56  (shown in FIG.  4 ), open first stage port  58  (shown in FIG.  5 ), or open both first and second stage ports  58  and  56  (shown in FIG.  6 ). 
     A pneumatic operator (not shown) is connected to holes  92  and  94 , and activated to force gas into hole  92  and out of hole  94 , to thereby shift first stage piston  88  rearwardly. This in turn shifts the entire first stage actuator shaft  62  rearwardly such that the end projects out of top plate  76 , as shown in FIG.  5 . This movement of actuator shaft  62  also moves first stage plug  60  out of seated engagement with first stage port  58 , to fluidly connect first stage cavity  48  with the bore  24  of the compressor cylinder  12  (shown in FIG.  2 ). Thus, the internal volume/clearance of the compressor cylinder is increased by the volume of first stage cavity  48 . If operating conditions call for even greater clearance, a second pneumatic operator, connected to holes  100  and  102  in second stage cylinder  72  is activated. As shown in FIG. 6, this second operator will inject gas through hole  100  and out hole  102 , to force second stage piston  96  rearwardly within second stage cylinder  72 . This in turn will shift sleeve  66  rearwardly and the attached second stage plug  64  to thereby open second stage port  56  and increase the total clearance of the compressor cylinder by the volume of second stage cavity  54 . Indicator pin  104  will project outwardly through top plate  76 , to visually indicate that second stage port  56  is open. 
     Whereas the invention has been shown and described in connection with the preferred embodiment thereof, many modifications, substitutions and additions may be made which are within the intended broad scope of the appended claims.