Disclosed is a device that includes a natural-gas compression system having a natural-gas powered combustion engine, a compression cylinder configured to receive natural gas and output compressed natural gas, a piston disposed in the compression cylinder and configured to translate through the compression cylinder in response to mechanical power received from the natural-gas powered combustion engine, and a variable-volume head mounted to the compression cylinder and configured to vary a compressed volume of the compression cylinder. In some instances, the variable-volume head includes a head body, an adjustment screw rotatably coupled to the head body, a plug moveable in threaded engagement with the adjustment screw, and an anti-rotation device coupled to the plug, the head body, or both. The anti-rotation device may be configured to impede the plug from rotating relative to the head body as the adjustment screw rotates.

FIELD OF THE INVENTION

Embodiments of the present invention relate generally to compressors. More particularly, some embodiments of the present invention relate to compressors having variable-volume heads.

BACKGROUND

Reciprocating compressors are frequently used to compress and transport fluids, such as natural gas. Generally, a reciprocating compressor includes a piston and a cylinder. During compression, inlet valves temporarily open to allow the fluid to flow into the cylinder. Then, the inlet valves close, and the piston is driven through the cylinder, reducing the volume of the cylinder in which the fluid is disposed and elevating the pressure of the fluid. The change in the volume of the cylinder during the compression stroke of the piston is referred to as the “swept volume.” Near the end of the piston's travel, an outlet valve is opened and the compressed fluid flows from the cylinder.

Compressors are often characterized by their volumetric compression efficiency. This parameter is the ratio of the swept volume to the total volume of the cylinder that houses the fluid being compressed. A high volumetric efficiency generally correlates with a larger outlet pressure, as a substantial portion of the volume of the cylinder is swept by the piston, and a low volumetric efficiency generally correlates with a lower outlet pressure, as the percentage reduction in the cylinder's volume during a piston stroke is lower.

The volumetric compression efficiency of a given compressor may not be matched to the system in which the compressor operates. A compressor design may be used in a variety of systems that expose the compressors to different conditions. For example, across systems, the compressor may be subject to varying inlet pressure or outlet pressure, as components upstream or downstream from the compressor may impede flow to or from the compressor to differing degrees in different applications. These variations and others can affect the performance of a compressor. Accordingly, it would be useful to be able to tune a compressor's volumetric compression efficiency according to characteristics of upstream and downstream components.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

When introducing elements of various embodiments, the articles “a,” “an,” “the,” “said,” and the like, are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” “having,” and the like are intended to be inclusive and mean that there may be additional elements other than the listed elements. The use of “top,” “bottom,” “above,” “below,” and variations of these terms is made for convenience, but does not require any particular orientation of the components relative to some fixed reference, such as the direction of gravity. The term “fluid” encompasses liquids, gases, vapors, and combinations thereof.

FIG. 1illustrates an elevation view of an embodiment of a compressor10. In this embodiment, the compressor10includes a variable-volume head12that adjusts the volume of a cylinder14of the compressor10. As explained below, the variable-volume head12consumes relatively little space compared to conventional designs and is adjustable with relatively little force. Additionally, the illustrated variable-volume head12is relatively resistant to ambient corrosive elements. Not all embodiments, however, provide all of these benefits, and some embodiments may provide other benefits. Before describing the variable-volume head12in detail, other features of the compressor10are described.

The illustrated compressor10includes an engine16, a crank case18, a flywheel20, a piston22, a rod24, valves26and28, and a skid30. In this embodiment, the engine16is a generally horizontally-mounted, internal-combustion engine with a reciprocating piston. The engine may be a spark-ignition engine or a compression-ignition engine, e.g., a diesel engine. Other embodiments may include other sources of mechanical power, such as electric motors or pneumatic drives. In operation, the engine16drives a rod coupled to a crankshaft in the crankcase18via a crosshead. The crankshaft in the crankcase18is coupled by an axle to the flywheel20, which provides an inertial mass that functions as a reservoir for angular momentum. The crankshaft in the crankcase18is also connected to the piston22via the rod24and another crosshead. The piston22and the cylinder14have a generally complementary generally right circular cylindrical shape that is generally concentric about a central axis32. The cylinder14includes inlet passages34and outlet passages36that are in fluid communication with the valves26and28. The valves26and28may include a variety of types of valve members, such as a plurality of poppet valves that are biased against openings connected to the passages34and36. In some embodiments, the valves26and28are check valves configured to open in response to a pressure in the cylinder greater than a threshold pressure or less than a threshold pressure.

In operation, the engine16drives the flywheel20and the piston22. As the flywheel20rotates with the crankshaft in the crankcase18, the movement of the crankshaft causes the piston22to oscillate axially, back and forth through the cylinder14, as illustrated by arrow38. While the piston22moves back toward the crankcase18, the valve28opens in response to the drop in pressure in the cylinder14, and fluid is drawn into the cylinder14through the inlet passage34. Then, as the piston22translates away from the crankcase18, the valve28closes in response to the increase in pressure, and the piston22decreases the volume of this cylinder14in which the fluid is disposed, thereby elevating the fluid's pressure. As noted above, the portion of the cylinder14through which the surface of the piston22translates is referred to as the swept volume of the cylinder14. As the piston22nears the end of its travel away from the crankcase18, the valve26opens in response to the increase in pressure, and pressurized fluid exits the cylinder14through the outlet passage36. In some embodiments, the valves26and28may be pressure-actuated valves, such as check valves. For instance, the valve28may be a check valve configured to open in response to a fluid pressure in the cylinder14below some threshold, e.g., a partial vacuum, corresponding to an intake stroke, and the valve26may be a check valve configured to open in response to a pressure in the cylinder14above some threshold corresponding to the end of a compression stroke.

FIG. 2illustrates a cross-sectional elevation view of an embodiment of the variable-volume head12. In this embodiment, the variable-volume head12includes a head body40, a plug42, an adjustment screw44, a collar46, and a cap48. Each of these illustrated components40,42,44,46, and48is generally concentric about the central axis32and is made of steel or other appropriate materials.

In the illustrated embodiment, the head body40includes a flange50, a tubular portion52, and a distal portion54. The illustrated flange50is a generally annular member shaped to couple to a distal surface56of the cylinder14. The flange50may include a threaded aperture58for receiving an eye-bolt or other structures configured to support the variable-volume head12during installation. A mating surface60of the flange50may be generally orthogonal to the central axis32. Near the inner diameter of the mating surface60, the illustrated flange50includes a groove62and a lip64that extends axially beyond the mating surface60. A seal66, such as an O-ring seal or a T-ring seal, is disposed in the groove62. The lip64overlaps a shoulder68near the distal portion of the cylinder14. A plurality of bolts70extend axially through the flange50and couple the variable-volume head12to threaded apertures in the cylinder14. In other embodiments, other coupling mechanisms, such as a weld, a lock ring, or threads, may be used to secure the variable-volume head12to the cylinder14.

The tubular portion52, in this embodiment, extends generally perpendicular from the flange50and defines an interior72with a generally right-circular-cylindrical shape. The interior72may be generally concentric about the central axis32and generally coaxial with an interior74of the cylinder14. In other embodiments, these volumes72and74are not coaxial or are not right circular cylinders, e.g., the variable-volume head12may be mounted to the side of the cylinder14and may extend generally radially or at an angle. The interior72may have a diameter76between about 5 inches and about 28 inches. In other embodiments, the interior72may have other shapes, e.g., the interior72may be a generally right elliptical cylinder or it may have some other curvilinear or non-curvilinear shape. The distal portion54extends radially inward from the tubular portion52and is generally orthogonal to the central axis32. In this embodiment, the distal portion54includes an aperture78and a mating surface80. The aperture78generally defines a right-circular-cylindrical volume that is generally concentric about and coaxial with the central axis32. The illustrated aperture78extends through the distal portion54to the interior72of the head body40. The mating surface80is generally perpendicular to the central axis32and is shaped to mate with a complementary surface on the collar46.

As illustrated byFIG. 2, the plug42is disposed in the interior72of the head body40. In some embodiments, the plug42includes an outer tubular member82, a base84, and an inner tubular member86. The outer tubular member82, in this embodiment, is generally concentric about, and coaxial with, the central axis32. The outer tubular member82includes a sealing surface88, generally annular grooves90and92, and a recessed portion94. The sealing surface88is generally complementary to the surface of the interior72of the head body40. The grooves90and92house seal members96and98that are configured to seal against the surface of the interior72of the head body40. In some embodiments, the seal members96and98are carbon-filled-Teflon ring seals that are biased against the surface of the interior72of the head body40. The recessed portion94has a smaller diameter than the sealing surface88and is generally complementary to the interior74of the cylinder14.

The illustrated base84extends radially between the outer tubular member82and the inner tubular member86and is generally orthogonal to the central axis32. In this embodiment, the base84includes a recess100in which a distal portion of the adjustment screw44may be disposed.

In the present embodiment, the inner tubular member86extends generally axially from the base84and is generally concentric about, and co-axial with, the central axis32. The inner tubular member86is disposed within the outer tubular member82. The illustrated inner tubular member86includes a fillet102near where the inner tubular member86meets the base84. A threaded aperture104extends through the inner tubular member86to the recess100. The threaded aperture104is generally coaxial with, and concentric about, the central axis32. Additional details of the plug42are described below with reference toFIG. 3, which illustrates a perspective view of the plug42.

In this embodiment, the adjustment screw44is a generally right-circular-cylindrical member that extends generally coaxial with the central axis32. The illustrated adjustment screw44extends through the plug42, the collar46, and into the cap48. The presently described adjustment screw44includes a threaded portion106, a sealing surface108, another threaded portion110, another sealing surface112, and a tool interface114. These features of the adjustment screw44are described further below with reference toFIG. 4, which illustrates a perspective view of the adjustment screw44. With reference toFIG. 2, it should be noted that the threaded portion106is configured to mate with the threaded aperture104of the plug42, and the threaded portion110is configured to mate with complementary threads on the collar46. The sealing surface108is shaped to form a sliding and rotating seal with the collar46and the sealing surface112is shaped to form a generally static seal with the cap48. In some embodiments, the sealing surfaces generally define right-circular cylindrical volumes.

In the illustrated embodiment, the collar46includes a tubular portion116, a flange118, and another tubular portion120. The tubular portion116includes a distal sealing surface122that is generally orthogonal to the central axis32. The illustrated tubular portion116also includes a breather aperture124that extends to a threaded aperture126through the collar46and a grease fitting128for lubricating the threaded aperture126. The breather aperture124may include a check valve or some other device configured to relieve pressure in the threaded aperture126. As explained below with reference toFIG. 4, which illustrates complementary structures on the adjustment screw44, the threaded aperture126, in this embodiment, is threaded in an opposite direction relative to the threaded aperture104in the plug42. Further, as is also explained below, the threads in the threaded aperture126may have a finer thread pitch then the threads in the threaded aperture104to reduce the movement of the adjustment screw44relative to movement of the plug42.

The flange118is a generally annular member that extends generally perpendicular to the central axis32. A plurality of bolts130extend axially through the flange118and secure the collar46to threaded apertures in the head body40. The bolts130also transmit loads from the plug42to the head body40via the collar46. These forces are transmitted through the adjustment screw to the collar46by the coupling formed between the threaded portion110and the threaded aperture126. A seal member132, such as an O-ring seal, is biased against the mating surface80of the head body40to form a seal.

The illustrated tubular member120extends generally axially through the aperture78in the head body40. The interior of the tubular member120includes a non-threaded aperture134that is an extension of the threaded aperture126and a seal member136, such as a T-ring seal. In the state illustrated byFIG. 2, the distal portion of the tubular member120extends into the outer tubular member82of the plug42and abuts the inner tubular member86. However, as explained below, the position of the plug42may be shifted relative to the collar46as the variable-volume head12is adjusted.

In this embodiment, the cap48includes an interior138and seals140and142. The cap48may be threaded to the adjustment screw44or it may be secured to the adjustment screw44with a friction fit or other coupling mechanism. The interior138of the cap48includes a generally conical tip144and a generally right-circular-cylindrical portion146extending through the remainder of the cap48. The seals140and142may include a variety of types of seals. For example, the seal142may be a T-ring seal, and the seal140may be an O-ring seal. In some embodiments, the seals140and142are formed with elastomers. The cap48may be removable from the adjustment screw44with or without tools. For example, the cap48of the illustrated embodiment is removable by hand. As explained below, the cap48may be removed to access the tool interface114of the adjustment screw44. When the adjustment screw44is not being adjusted, the cap48is returned to the adjustment screw44to protect the adjustment screw44from the environment.

FIG. 3illustrates additional details of the plug42. In this embodiment, the plug42includes a groove148. The illustrated groove148is recessed generally radially into the plug42, orthogonal to the central axis32, and extends generally parallel to the central axis32. The illustrated groove148does not penetrate entirely through the outer tubular member82, but it does extend axially along and radially into both the sealing surface88and the recessed portion94. The groove148includes a deeper portion150and a shallower portion152that produce a bottom surface154of the groove148that is a generally uniform distance away from the central axis32. As explained below, the groove148is part of an anti-rotation device that impedes the plug42from rotating while allowing the plug42to translate axially. Some embodiments include a plurality of grooves like the groove148distributed around the plug148, e.g., two grooves 180 degrees apart.

FIG. 4illustrates additional details of the adjustment screw44. As illustrated, the threaded portion106and the threaded portion110include threads with different pitches and different orientations. In some embodiments, the threaded portion106is right-handed, and the threaded portion110is left-handed. Consequently, rotation of the adjustment screw44in one direction tends to bring objects coupled to the threaded portions106and110toward one another, and rotation in the other direction tends to drive those objects away from one another. In other embodiments, the orientation of the threads106and110may be reversed, or it may be the same. Having oppositely oriented threaded portions106and110is believed to enhance the mechanical advantage of the threaded couplings formed by the threaded portions106and110, as both threaded portions106and110cooperate to drive the plug42(FIG. 2) axially. As the adjustment screw44is rotated, the threaded portion110pushes against the collar46(FIG. 2), and the threaded portion110pushes in the same direction against the plug42(FIG. 2).

The pitch of the threaded portion106may be substantially greater than the pitch of the threaded portion110, such that a given amount of rotation of the adjustment screw44produces more movement in an object coupled to the threaded portion106than in an object coupled to the threaded portion110. For instance, the threaded portion110may have more than 4 threads per inch, e.g., generally equal to 8 threads or more per inch. In addition, the threaded portion106may have fewer than 4 threads per inch, e.g., generally equal to or less than 2 threads per inch. As explained below, having relatively fine pitched threads on the threaded portion110may result in relatively little axial movement of the adjustment screw44through the collar80(FIG. 2) during adjustment, thereby reducing the volume of space consumed by the variable-volume head12(FIG. 2) as the adjustment screw44is adjusted between its maximum and minimum positions. Further, including relatively coarse threads in the threaded portion106tends to produce a relatively large movement of the plug42(FIG. 2) for a given amount of rotation of the adjustment screw44, which also tends to reduce the amount of space consumed by the variable-volume head12, as the plug42(FIG. 2) can reach its maximum and minimum positions with relatively little movement of the adjustment screw44.

In some embodiments, the adjustment screw44does not translate axially as it rotates. For example, the threaded portion110may be omitted (which is not to suggest that other features may not also be omitted), and an annular flange may extend generally radially from the adjustment screw44near where the threaded portion110is positioned. The flange may mate with an annular groove in the collar46(FIG. 2), and together, these components may allow the adjustment screw44to rotate while generally axially constraining the adjustment screw44. In another example, the adjustment screw44may include an annular groove, and the collar46(FIG. 2) may include an annular flange that extends generally radially inward into the groove, thereby impeding axial movement of the adjustment screw44while allowing rotation.

FIG. 5illustrates the variable-volume head12being adjusted. To shift the position of the plug42, and thereby adjust the volume of the interior74of the cylinder14(FIG. 2), the cap48is removed, and the adjustment screw44is rotated. In some embodiments, the cap48is removed by un-threading the cap48from the adjustment screw44, or in other embodiments, the cap48is removed by applying an axial force to the cap48and overcoming friction that secures the cap48to the adjustment screw44.

In some embodiments, the cap48includes a gauge156that correlates axial movement of the adjustment screw44with changes in the volume of the interior74of the cylinder14(FIG. 2) produced by movement of the plug42. To use the gauge156, the cap48is inverted and placed on the flange118of the collar46to axially align the gauge156with the variable-volume head12. Then, an initial volume of the interior74of the cylinder14(FIG. 2) is determined by identifying which mark on the gauge156corresponds with an indicator on the adjustment screw44, such as the top of the adjustment screw44.

After taking an initial reading, the adjustment screw44is rotated to shift the position of the plug42. The range of motion of the plug42is illustrated byFIGS. 2 and 6, which illustrate the plug42retracted and extended, respectively. The adjustment screw44may be rotated manually by applying a tool, such as a wrench or a wheel, to the tool interface114and rotating the tool about the axis32, as indicated by arrows158inFIG. 5. In some embodiments, the adjustment screw44may be rotated with a powered device, such as an electric motor or a pneumatic motor. As the adjustment screw44is rotated, the threaded portion110(FIG. 4) cooperates with the threaded aperture126(FIG. 2) to axially shift both the adjustment screw44and the plug42(FIG. 6). As the adjustment screw44rotates and translates axially, it both carries the plug42and rotates within the plug42. The plug42is impeded from rotating with the adjustment screw42by a member, such as a guide pin (an example of which is described below with reference toFIG. 7), disposed in the groove148and mounted to the head body40. The rotation of the threaded portion106within the threaded aperture104(FIG. 2) of the plug42causes the plug42to translate axially along the adjustment screw44, which is itself also translating axially due to the threaded portion110(FIG. 4). Thus, the axial movement of the adjustment screw44relative to the collar46and the axial movement of the plug42relative to the adjustment screw44add together to yield a net movement of the plug42that is larger than either individual axial movement.

After adjusting the adjustment screw44, the gauge156(FIG. 5) may be used to determine the change in volume of the interior74of the cylinder14(FIG. 2). The new axial position of the adjustment screw44is correlated with a volume of the interior74by positioning the cap48on the flange18(FIG. 2) and determining which mark on the gauge156generally corresponds with a given point on the adjustment screw44. The mark indicates the volume of the interior74(FIG. 2) corresponding to the position of the adjustment screw44.

As mentioned above, the range of movement of the plug42is illustrated by comparingFIG. 2andFIG. 6.FIG. 2illustrates the plug42in its retracted position. In the retracted position, the volume of the interior74of the cylinder14is generally increased or maximized. As a result, in the state illustrated byFIG. 2, the compressor10(FIG. 1) operates with a relatively low, e.g., minimized, volumetric compression efficiency. In the equation for volumetric compression efficiency, i.e., the swept volume divided by the total volume of the cylinder14(FIG. 1), the denominator is increased by retracting the plug42to the state illustrated byFIG. 2. That is, the total volume is increased in the state illustrated byFIG. 2. Thus, retracting the plug42and increasing the volume of the cylinder14reduces the volumetric compression efficiency, as the swept volume remains generally constant and the total volume of the cylinder is increased.

In contrast,FIG. 6illustrates the plug42in its extended position. In this state, the plug42has penetrated into the interior74of the cylinder14, and as a result, the volume of the interior74is reduced. Decreasing the volume of the interior74increases, e.g., maximizes, the volumetric compression efficiency of the compressor10(FIG. 1), as the denominator in the equation for volumetric compression efficiency is reduced by reducing the total volume of the interior74. That is, while the swept volume may remain generally constant, the total volume decreases.

As illustrated byFIG. 6, the plug42moves a larger axial distance160than the axial distance162moved by the adjustment screw44. This is due to the difference in the thread pitch of the threaded portions106and110(FIG. 4). In some embodiments, the distance160is larger than or generally equal to 2 times the distance162, 3 times the distance162, 4 times the distance162, or five times the distance162.

The seals96and98and the threaded coupling between the threaded portion106and the threaded aperture104may impede or seal fluids from flowing between the interior74of the cylinder14and the interior72of the head body40. Should the pressure in the interior72rise, the seals136and132tend to prevent that pressure from driving fluid to the atmosphere.

Further, the threaded coupling between the relatively fine threads of the threaded portion110and the threaded aperture126in the collar46are believed to prevent debris from penetrating the collar46and entering the interior72of the head body40. This is believed to extend the useful life of the variable-volume head12.

FIG. 7illustrates another cross-section of the variable-volume head12that is generally orthogonal to the view illustrated byFIG. 6. This view illustrates the operation of the groove148and a guide pin164to impede the plug42from rotating with the adjustment screw44. The illustrated guide pin164is a generally right-circular-cylindrical member that radially extends through the flange50, generally orthogonal to the central axis32into the groove148. The guide pin164applies a torque to the sidewalls of the groove148to impede or prevent the plug42from rotating. The guide pin164translates axially through the groove148as the plug42translates axially. In some embodiments, the plug42may be characterized as having a single degree of freedom relative to the head body40. Some embodiments may include multiple grooves and guide pins. For example, another groove and guide pin may be disposed opposite the guide pin164and groove148, e.g., about 180 degrees around the plug42.

The groove148and guide pin164, together, may be referred to as an anti-rotation device. Other embodiments may include other types of anti-rotation devices. For example, the plug42and interior72may have a generally non-circular shape, such as a generally right-elliptical-cylindrical shape, that tends to impede rotation about the central axis32. In another example, the guide pin164may be positioned on the plug42, near the distal portion of the plug42, extending generally radially outward, and the groove148may be disposed in the inner walls of the cylinder14(FIG. 2). In some embodiments, the groove148is not necessarily straight, e.g., the groove148may spiral, causing the plug42to rotate as it translates axially, though the rotation may be less than the rotation of the adjustment screw44.

FIG. 8illustrates an example of a compression system166that includes the variable-volume head12described above. The system166includes a natural-gas well168, the engine16, a compressor10that includes the above-described variable-volume head12(FIG. 2), and a pipeline, storage, or other fluid destination169. The gas well168may be a subsea or a surface natural gas well. The engine16may be a two-stroke combustion engine having between 40 and 800 hp, e.g., between 40 and 200 hp.

In operation, natural gas flows from the gas well168to the compressor10, as illustrated by arrow170. A portion of this flow is diverted to the engine16, as illustrated by arrow172. The diverted flow of172may be conditioned by removing moisture or changing the gas pressure before being introduced to the engine16. The engine16combusts the diverted fuel172and drives a shaft174or other mechanical linkage, such as a crankshaft and rods, that powers the compressor10. The compressor10compresses the flow170from the gas well168and produces an outlet flow176at a higher pressure. The volumetric compression efficiency of the compressor10may be adjusted with the variable-volume head12(FIG. 2) to account for the pressure of the inlet flow170or the outlet flow176. The outlet flow169flows to a fluid destination, such as a pipeline, storage, refining equipment, or other fluid destinations.