Abstract:
An improved bypass valve assembly for a plunger lift apparatus comprises a bypass valve cage having improved flow characteristics and a simplified clutch assembly having enhanced durability and low cost.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/720,451 filed Oct. 31, 2012 by the same inventors and entitled PLUNGER APPARATUS. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention generally relates to oil and gas production operations, and more particularly to gas-lift plunger devices for lifting production fluids to the surface to restore production to shut in wells. 
         [0004]    2. Description of the Prior Art 
         [0005]    Gas lift plunger apparatus has been in use for many decades and has a long history of development. In one recent example, U.S. Pat. No. 7,438,125, Victor, a bypass assembly of a plunger lift device employs a bypass valve assembly having both an internal cage and an outer cage. The internal cage, when rotated using an adjustment performed with tools at the surface, operates to vary the size of the bypass orifices of the bypass valve and thus vary the bypass fluid volume. In addition a clutch within the lower part of the outer cage is used to maintain the valve push rod in a fixed position within the valve assembly until the push rod is forced to change the valve from an open (bypass) configuration to a closed (no bypass) configuration. The clutch tension is provided by a plurality of small metal coil springs wrapped around the clutch bobbin that surrounds the push rod. Some disadvantages of this design are its complexity that increases its cost and the effects of corrosion which predisposes the clutch assembly to premature failure. Another drawback is that the bypass orifices are cut at right angles through the inner and outer cages, which impedes the flow of fluid through the plunger as it descends through the tubing. 
         [0006]    In another example, U.S. Patent Application Publication No. 2010/0294507, Tanton (See also U.S. Pat. No. 6,467,541, Wells) discloses two different free piston embodiments in which one or both of their components are made of materials that are at least partly buoyant. One embodiment is a simple combination of a sleeve having a seat to receive a ball at its lower end, as in a ball-check valve. In operation the ball is allowed to fall through the fluid in the well bore, followed by the sleeve at some time interval. The ball reaches the bottom of the well first. When the sleeve arrives it contacts the ball, which seals the well bore. Gas pressure can then lift the ball and sleeve together to the surface, pushing the production fluid ahead of them upward through the well bore. The other embodiment eliminates the separate ball or plug and closes the lower end of the sleeve, thus presenting a closed face to whatever material is in the well casing during descent. While simple in configuration, the first lacks predictability because the sleeve and ball operate independently until they reach the bottom of the well bore, and the second lacks broad utility because of its buoyancy and is not able to bypass fluids as it descends to the bottom of the well. 
         [0007]    Variations in the ball-check valve concept have been in the art for decades, as for example is illustrated in U.S. Pat. No. 2,001,012 patented May 14, 1935. 
         [0008]    What is needed is an improved plunger bypass valve mechanism for a gas lift plunger device that is simple and durable, as well as reliable in operation and low in cost to manufacture. 
       SUMMARY OF THE INVENTION 
       [0009]    It is an object of the present invention to provide a clutch assembly for a plunger lift bypass valve having an axial valve stem slidingly disposed within a valve cage attached to a plunger lift, the clutch assembly comprising a split bobbin sized to surround less than 360 degrees of the perimeter of the valve stem; a resilient tension band formed of synthetic rubber and surrounding the split bobbin; and a predetermined surface roughness applied to the valve stem. In another aspect the tension band may be configured as two or more tension bands used together. 
         [0010]    In another embodiment there is provided an improved bypass valve assembly for a plunger lift apparatus, comprising a bypass valve cage with at least one elongated opening or port formed through a side wall of the bypass valve cage, the opening outwardly relieved at a lower end thereof; a valve stem disposed within a longitudinal bore of the valve cage and having a predetermined surface roughness; and a split bobbin clutch assembly including a resilient tension band formed of a synthetic rubber having an A Scale durometer characteristic between 60 and 90, the clutch assembly disposed around the valve stem. 
         [0011]    In yet another embodiment of the invention there is provided a plunger lift apparatus having an improved bypass valve assembly, comprising a plunger body having a plurality of annular sealing rings and a full diameter upper body portion with shortened taper at an upper end thereof, the plunger body configured at a lower end thereof for threadable engagement with the bypass valve assembly; and a bypass valve assembly comprising a valve cage and a valve stem having a clutch assembly disposed there around, the valve stem disposed within a longitudinal bore of the valve cage, the clutch assembly configured as a split bobbin having a synthetic rubber or elastomer tension band disposed around the split bobbin, and the valve stem surface configured with a predetermined surface roughness. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  illustrates a perspective view of one embodiment of a plunger lift apparatus—a rotary bypass plunger—according to the present invention; 
           [0013]      FIG. 2  illustrates a cross section view along a longitudinal axis of the embodiment of  FIG. 1 ; 
           [0014]      FIG. 3A  illustrates a side view of an embodiment of a bypass valve cage portion of the embodiment of  FIGS. 1 and 2 ; 
           [0015]      FIG. 3B  illustrates an end view of the embodiment depicted in  FIG. 3A ; 
           [0016]      FIG. 3C  illustrates a cross section view of the embodiment of  FIGS. 3A and 3B  along the line  3 C- 3 C as shown; 
           [0017]      FIG. 4  provides a perspective view of a bypass valve cage  30  as it may appear in one embodiment of the present invention; 
           [0018]      FIG. 5  illustrates a perspective view of one embodiment of a clutch assembly used in the rotary bypass plunger according to the present invention; 
           [0019]      FIG. 6  illustrates a perspective view of a resilient tension band for use in the clutch assembly embodiment depicted in  FIG. 5 ; 
           [0020]      FIG. 7  illustrates a perspective view of a bypass valve stem and clutch assembly for use in the embodiment of  FIGS. 1 through 5  of the present invention; 
           [0021]      FIG. 8  illustrates a perspective view of an alternate embodiment of a clutch assembly used in the rotary bypass plunger according to the present invention; and 
           [0022]      FIG. 9  illustrates a perspective view of a resilient tension band for use in the clutch assembly embodiment depicted in  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The drawings that accompany the following description depict several views of a bypass valve assembly for a gas lift plunger apparatus according to one embodiment of the rotary bypass plunger apparatus provided by the present invention. It has been discovered that significant improvements can be made to the bypass valve assembly that utilizes a clutch-controlled “dart” or valve stem that reciprocates within a bypass valve “cage” and provides a mechanism for sealing the fluid passages through the bypass valve. One of the functions of the bypass valve is to allow fluid to flow through the valve in a controlled manner to control descent of the plunger assembly to the bottom of the well. Another function of the bypass valve assembly is to switch the valve configuration to seal the passages that allow the flow-through of fluid so that the plunger acts as a piston to seal the well bore and permit the gas pressure in the well to force the piston and accumulated fluids above it to the surface so that production from the well can resume. 
         [0024]    The present invention incorporates design features that substantially improve the performance and durability of the bypass valve assembly in a gas lift plunger. Descent of the plunger assembly is faster and better controlled, which cuts the shut-in time approximately in half, thus more quickly restoring the well to production. Moreover, the superiority of the valve stem and clutch assembly configuration that is disclosed herein, which enables the switch from plunger bypass/descent to gas lift/ascent at the bottom of the well, is confirmed by performance in the field. The success of the improved design of the present invention is demonstrated by sales volume exceeding 1200 units during the first six months of its availability, without a single reported instance of failure. In addition, the reliability and durability of the plunger and the bypass valve assembly is extended by the features to be described herein, thereby reducing downtime and maintenance costs. 
         [0025]    To achieve the aforementioned advantages, the following features are preferably and most advantageously used in combination in the bypass valve assembly described herein: (a) elongated bypass openings or ports that are relieved at the upper and lower ends at an angle to reduce turbulence and improve flow as the plunger descends, providing a smoother and a more rapid descent; (b) helical disposition of the bypass openings around the body of the bypass valve assembly to impart a torque to the plunger, causing it to spin within the well casing as it descends, ensuring more uniform wear and longer life while providing a smoother descent; (c) a valve stem clutch with an elastomeric tension band (or bands) that is more resistant to high temperatures and corrosive chemicals than metal and thus much less prone to failure; (d) calibrated surface roughness of the valve stem surface to improve the friction characteristics of the valve stem clutch as it arrives at the bottom of the well and configures the plunger for its ascent to the surface; (e) machined grooves on the inner surface of the clutch bobbin to allow sand particles to be flushed away from within the clutch, thereby preventing undesired lock-up; and (f) shortened taper of the upper end of the plunger body that utilizes the improved bypass valve assembly, to ensure a more complete seal with minimum leakage of production fluids during ascent of the plunger to the surface. 
         [0026]    Variations in the above features are contemplated to adapt the bypass valve assembly to different well circumstances. For example, the number of bypass openings or slots may be varied to provide different flow rates. The tension in the tension band (or bands) of the clutch assembly may be varied or adjusted to adapt the clutch clamping force to different descent velocities as the plunger contacts the bumper at the well bottom. The helical pitch may be varied within narrow limits to control the amount of spin imparted to the plunger. The profile of the machined grooves in the clutch bobbin may be varied to accommodate different sand particle sizes. The surface roughness of the valve stem may be varied to optimize the friction applied by the clutch. The tapered profile of the plunger body at the upper end may be varied to optimize ascending performance with different fluid viscosities, etc. Persons skilled in the art will understand that the bypass valve assembly described herein—the assembly of the cage, valve stem and clutch—may be constructed in a variety of combinations of the above features and interchanged with other combinations to suit particular conditions of individual oil or gas wells. For example, the plunger and bypass valve assembly may be produced in several diameters for use in different size well tubing. Also, different length plungers may be provided. For example, a shorter bypass plunger is better able to negotiate well tubing that have curves or elbows, and because of its lower weight, it places less stress on the bumper spring at the bottom in wells that are relatively dry. A longer casing falls more easily through more fluid and provides a better sealing action. This adaptability is yet another advantage of the present invention. As is well known, performance of a gas lift plunger may be reduced if the configuration of the plunger is not well-matched to the conditions of a particular well. 
         [0027]    One important component of the clutch assembly to be described herein is the elastomer tension band. In this description the use of the singular form of the term “tension band” is intended to mean that the tension band may be composed of one such band or a plurality of individual bands used together. The tension band (or bands) may be fabricated of an elastomeric material, a broad category of synthesized polymer materials that are commonly known as synthetic rubber. Among the properties required in the tension band is resistance to high temperatures and corrosion, elasticity, reversibility—ability to return to and maintain its unstressed or relaxed configuration after being stressed, and excellent stability. Some examples of such materials include neoprene, buna-N, respectively polychloroprene and acrylonitrile butadiene. An alternative is hydrogenated nitrile rubber. Another example, preferred for the present invention, is a fluoroelastomer such as a fluoronated hydrocarbon better known as Viton®, a registered trademark of the E. I. DuPont de Nemours and Company or its affiliates of Wilmington, Del., USA. In particular, the preferred material will have a Shore A durometer of 60 to 90, and for most applications a Shore durometer of 75 on the A scale is preferred. In some applications where the tension band needs to be thicker or wider (greater cross sectional area), the durometer figure may be reduced. Similarly, if the tension band needs to be thinner or narrower (lesser cross sectional area), the durometer figure should be increased. 
         [0028]    When installed on the valve stem, the split bobbin segments are disposed around the valve stem shaft, held in a clamping action against the valve stem shaft by the action of the elastomer tension band. The elastomer tension band has been found during tests to provide superior durability in down-hole conditions to other ways such as metal springs to provide the needed clamping force. The clamping force provided by the tension band resists by friction of the bobbin segment against the valve stem the movement of the valve stem through the clutch assembly. This friction arises because of the clamping force from the tension band and the predetermined surface roughness formed into the surface of the valve stem shaft along the greater portion of its length. The function of the clutch assembly is to ensure that the valve stem remains in either (a) the lower-most position within the valve cage during descent of the plunger so that the plunger will fall freely through the fluid in the well casing and cause it to rotate smoothly during the descent; and (b) the upper-most position within the valve cage during ascent of the plunger to seal the bypass valve assembly so that the gas pressure in the well will cause the plunger to rise through the well casing, pushing the production ahead of it. The clutch assembly enables the valve stem to be held in the appropriate position during descent and ascent, and also to change the position of the valve stem from the lower-most position to the upper-most position when the plunger reaches the bottom of the well to configure the plunger for its ascent. 
         [0029]    In the drawings to be described each structural feature is identified with a reference number. A feature bearing the same reference number in more than one figure may be assumed to be the same feature. Turning now to  FIG. 1  there is illustrated a perspective view of one embodiment of a plunger lift apparatus—a rotary bypass plunger—according to the present invention. The plunger  10  includes two main sections—the plunger section  12  and the rotary bypass valve assembly  14 . The plunger section  12  includes the plunger body  16  having an upper end  18 , a series of concentric outer rings  20  and a tapered portion  26 . The outer rings  20  around the plunger section  12  provide a seal against the well casing (not shown) and reduce friction (because of reduced surface area of the plunger section  12 ) as the plunger  10  descends or ascends through the well casing. The sloped surface  22  on the upper side of each ring facilitates ascent by reducing friction due to turbulence of the fluid. The underside  24  of the outer rings  20  may optionally be configured to serve a purpose such as minimizing drag, improving sealing, providing a flushing action upon descent, etc. In some applications the outer rings  20  may be formed as a continuous helix instead of concentric rings, for example. 
         [0030]    The rotary bypass valve assembly  14  (also: bypass valve  14 ) includes a valve cage  30 , and end cap  34 , and a valve stem  102 . The body  32  of the valve cage  30  may be threaded (See  FIG. 2 ) onto the lower end of the body  16  at threads  41  and may be secured with a set screw in a threaded hole  40 . The end cap  34  may be similarly threaded (See  FIG. 2 ) into the lower end of the valve cage  30  at threads  43  and may also be secured with a set screw in a threaded hole  42 . An optional socket  44  for a spanner wrench for removing the bypass valve assembly  14  and the end cap  34  is shown in the outer surface of the end cap  34 . The valve cage  30  includes bypass ports  46 , to be further described below, which are disposed at equal radial intervals around the valve cage  30 . 
         [0031]      FIG. 2  illustrates a cross section view along a longitudinal axis  60  of the embodiment of  FIG. 1 .  FIG. 2  is a side cross section view of the assembled bypass plunger  10  showing the sealing rings  20  formed along the axis  60  of the bypass plunger  10 . The bypass valve assembly  14  is shown to the left in the figure, and the upper end  18  of the plunger body  16  having the shortened taper  26  is shown at the right in the figure. The shortened taper  26  permits the upper portion of the plunger body  16  of the bypass plunger  10  to retain its full diameter over a maximum portion—at least 70% thereof—of its length. This feature provides improved sealing performance as the bypass plunger  10  rises within the well bore while lifting the production fluids to the surface. The plunger body  16  of the plunger section  12  is hollow—formed with a cylindrical bore  28  in this example to permit the flow of fluid through it during descent of the bypass plunger  10 . During descent, fluid flow enters the lower end of the bypass plunger  10  through the bypass ports  46  and the cylindrical bore  50  in the bypass valve cage  30 , and through the cylindrical bore  28  of the plunger body  16 .  FIG. 2  also depicts a cross section view of the valve stem  102  with the clutch assembly  70  installed including the split bobbin  72  and the elastomer tension band  76  disposed around the split bobbin  72 , as these components appear when assembled in the bypass valve cage  30 . The clutch assembly  70  is further described in  FIGS. 5 ,  6 , and  7 . 
         [0032]    Also clearly visible in  FIGS. 1 and 2  is the bypass valve assembly  14 . As shown in the cross section view of  FIG. 2 , the bypass valve assembly  14  includes the valve stem  102  disposed within a bore  36  through the end cap  34 , a clutch assembly  70  encircling the valve stem  102 , and an elongated bypass port  46 . Three such ports  46  are depicted in the preferred embodiment shown in the drawings, although for example without limitation other embodiments may include two or four such ports  46 . The details of the port  46  will be described in  FIGS. 3A through 3C . The profile of the ports  46  features relieved areas to facilitate the flow of fluids during descent of the bypass plunger  10 . This relieved port configuration provides less resistance and turbulence to the flow of fluids as the bypass plunger  10  falls through the well bore. The valve stem  102  includes an enlarged head  68  at its upper end that includes a chamfered perimeter  66  formed to mate with a similarly beveled seat  64  formed in the lower end of the bore  28  through the plunger body  16 . This configuration provides a poppet-type valve to regulate the flow of fluid through it. The poppet valve configuration thus provides for sealing the bypass valve assembly  14  against the passage of fluids as the plunger  10  ascends through the well casing. 
         [0033]    Continuing with  FIG. 2 , the clutch assembly  70  to be described maintains the valve stem  102  in an extended, open-valve position during the descent of the bypass plunger  10 . The clutch assembly  70  is held in place in the lower end of the bypass valve cage  30  between a circumferential internal ridge  38  and the end cap  34 . When the plunger  10  reaches the bottom as the lower end of the valve stem  102  contacts a bumper at the well bottom, the inertia of the plunger  10  overcomes the frictional clamping force of the clutch assembly  70 , enabling the valve stem  102  to move upward (to the right in the figure) through the bore  50  in the bypass valve cage  30  and against the seat  64  in the plunger body  16  to seal the bypass valve assembly  14 . Thus sealed, the bypass plunger  10  functions like a piston, allowing the gas pressure in the well to lift the bypass plunger  10  upward, carrying accumulated fluids above it to the well surface. 
         [0034]    Preferred materials for fabricating the rotary bypass plunger  10  described herein include the use of type 416 heat treated stainless steel for the bypass valve stem  102  and the clutch bobbin segments  72 A/ 72 B. The remaining parts—plunger body  16 , valve cage  30 , and end cap  34  may be fabricated of type 4140 heat treated alloy steel. In alternative embodiments, the 416 heat treated stainless steel may be used to fabricate all of these parts. Both materials are readily available as solid “rounds” in a variety of diameters, as is well known in the art. 
         [0035]      FIGS. 3A ,  3 B, and  3 C illustrate a bypass valve cage  30  of the present invention in several views to depict the profile of a bypass port  46 . The actual shape of the bypass port  46  is somewhat complex because of the tapered cylinder or conical configuration of the body  32  of the valve cage  30  and the helical alignment of a port  46  around the valve cage  30 . The views in  FIGS. 3A and 3C  illustrate the basic parameters of the profile of the port  46 . The port  46  is an elongated slot with rounded ends  54 ,  56  cut through the wall of the body  32  of the valve cage  30 . As will be described, the port  46  may be substantially aligned with a continuous helix disposed around the tapered cylinder valve cage  30 . In addition, both ends  54 ,  56  of the port  46  are cut at the same angle of approximately (but not limited to) 45° in the illustrated embodiment with the centerline  60  of the valve cage  30  as shown in  FIG. 3C . 
         [0036]    This nominal 45° angle results in an inward slope of the ends  54 ,  56  of the port  46  with both ends  54 ,  56  oriented toward the upper end  18  of the bypass plunger  10  as it is positioned within a well casing. This relief of the ends  54 ,  56  of the port  46  facilitates the flow of fluid through the port(s)  46  as the bypass plunger  10  falls through the well casing by gravity. In alternate embodiments, this nominal angle of 45° may be varied to suit a particular implementation of the bypass valve assembly  14 . For example, the angle may be different at opposite ends of the port(s)  46 , they may be larger or smaller acute angles relative to the longitudinal axis  60 , the angled surfaces may be rounded in profile for even smoother flow through the port(s)  46 , etc. An additional relieved area, called ramp  58 , further smooths the path for fluid flow at the lower end  54  of each port  46 . 
         [0037]    The surface of the ramp  58  shown in  FIGS. 3A and 3C  may be a flat or curved feature that is substantially parallel with the centerline or axis  60  of the valve cage  30  and, because of the conical outer shape of the valve cage  30  in the illustrated embodiment, forms an angle  52  of approximately 7° with the outer surface of the valve cage  30 . This angle  52  may typically vary from about 5° to 10° depending on the particular dimensions of the valve cage, but may be subject to other angles beyond this relatively small range in alternative embodiments. Persons skilled in the art will recognize that a variety of modifications to this port profile may be made to accommodate particular circumstances of manufacturing or application in the field, without departing substantially from the purpose of the profile shown in  FIGS. 3A and 3C . The essential concept is to relieve the passage through which fluids are to flow by removing sharp angles, etc. to provide a smooth, obstruction-free passage. As a result, the plunger descends more rapidly and more predictably than conventional plunger designs. 
         [0038]    Continuing with  FIG. 3A , the port  46  is also oriented at a small angle relative to the length of the bypass plunger  10 . To illustrate, the length of the port  46  forms an angle of approximately 15° with respect to the axis  60  if the position of the port  46  is projected on to the plane of the centerline or axis  60  of the bypass plunger  10 . Thus, this angle may be substantially in alignment with a helical path around the body or wall  32  of the valve cage  30 . Orienting a port  46  in this way will cause the plunger  10  to rotate or spin as it descends within the well casing because the fluid flow through the angled port  46  exerts a torque on the plunger  10 . Further, to balance the effect of the helical orientation of the port  46 , the port  46  is preferably disposed at two, three, or four locations around the valve cage  30  and separated at uniform radial intervals around the body  32  of the valve cage  30 . The use of two or more ports  46  spaced at uniform intervals around the body  32  of the valve cage  30  also facilitates the passage of fluid through the plunger as it descends through the well tubing.  FIG. 3B  depicts a view of the lower end of the valve cage  30  to show the appearance of the valve cage  30  with three of the helically-oriented ports  46  disposed at even intervals around the body  32  of the valve cage  30 . The benefits of the helical orientation of the several, evenly separated ports  46  is to facilitate rotation of the bypass plunger  10  and provide a smooth descent and uniform wear of the bypass plunger  10 , thus extending its useful life through many gas lift cycles. 
         [0039]    The combination of the helical orientation of the ports  46 , preferably disposed at several uniform radial positions around the body of the valve cage  30 , each having the relieved ends  54 ,  56 ,  58 , provides a rotary gas lift plunger that outperforms known bypass plungers by providing smoother, faster descent along with more uniform wear and extended life in the field.  FIG. 4  provides a perspective view of a bypass valve cage  30  showing the appearance of two of the ports  46  when disposed at three evenly separated positions—120° apart—around the body  32  of the valve cage  30 . 
         [0040]      FIGS. 5 ,  6 , and  7  illustrate perspective views of one embodiment of a clutch assembly  70  used in the rotary bypass plunger  10  according to the present invention. In  FIG. 5  the clutch assembly  70  includes a split bobbin  72  that surrounds the valve stem  102 . The split bobbin  72  is held in place by a tension band  76  that is placed around the two segments  72 A,  72 B of the split bobbin  72 , and within the space defined by the first and second rims  82 ,  84  of the bobbin segments  72 A,  72 B, thus clamping the bobbin segments  72 A,  72 B against the outer surface of the valve stem  102 . The bobbin segments  72 A,  72 B are identical in this illustrated embodiment, each one resembling a semicircle except for being slightly shortened from a full 180° by the gap  78 , which may be provided by making a 0.063 to 0.125 inch saw cut, for example, through the diameter of a single formed circular bobbin  72 . In other embodiments, the bobbin may be split into three or more segments, although two segments are adequate for this purpose and somewhat simpler to manufacture and handle during assembly. The split bobbin  70  illustrated in  FIG. 5  is shown with the segments  72 A and  72 B separated by the amount of the gap  78  even though the bobbin  70  is not installed on a valve stem  102 . When installed on the valve stem  102 , the gap  78  may typically be reduced under the effect of the tension band  76 . 
         [0041]    Continuing with  FIG. 5 , the tension band  76  is made of a resilient material and is configured to tightly press the bobbin segments  72 A,  72 B against the outer surface  104  of the valve stem  102 . In the present embodiment the inside diameter  86  of each half  72 A,  72 B of the split bobbin  72  is the substantially the same as the outside diameter of the valve stem  102  but is formed as slightly less than a full semicircle because of the small gap  78  provided between the proximate ends of the split bobbin  72  when it is in place around the valve stem  102 . This enables the inner surface of the bobbin halves  72 A,  72 B to fully contact the valve stem  102  to provide maximum friction to resist the movement of the valve stem  102  through the clutch assembly  70  except when the plunger  10  contacts the bottom of the well bore during a gas lift operation. 
         [0042]    Also depicted in  FIG. 5  is an additional feature of the split bobbin  72 , the series of grooves  80  formed on the inner surfaces of the split bobbin  72 . These grooves, preferably uniformly disposed around the circumference of the bobbin segments  72 A,  72 B, provide passages for fluids to flush particles of sand away from the contact area of the bobbin  72  with the outer surface of the valve stem  102 . The grooves  80  may be formed by machining or swaging, for example. In the illustrated example, four such grooves  80  are formed in each bobbin segment  72 A,  72 B, although the number may be varied, generally between two and six grooves  80  in each segment may be practical. However, the greater the number of grooves in the split bobbin  72 , the more the grooves  80  will be limited to trapping most grains rather than allowing them to be flushed out of the clutch assembly  70 . 
         [0043]      FIG. 6  illustrates a perspective view of a resilient tension band  76  for use in the clutch assembly  70  embodiment depicted in  FIG. 4 . The tension band  76 , which is formed as a ring having an inside diameter  90  about the same as or slightly smaller than the outer diameter of the central portion of the assembled split bobbin  72 A/ 72 B and an outside diameter  92  slightly less than the outer diameter of the rims  82 ,  84  of the split bobbin  72 A/ 72 B, which in turn is only slightly less than the inner bore  50  of the valve cage  30  just below the internal ridge  38 . The tension band  76  preferably has a width  94  dimensioned to fill the full width between the first and second rims  82 ,  84  of the split bobbin  72 A/ 72 B. It can further be seen that the resilient tension band  76 , which has a rectangular cross section to fit within the rims  82 ,  84  of the split bobbin  72 , acts to form a very compact clutch assembly  70 . This configuration exerts a constant clamping force around the valve stem  102 . It has been found that the clamping force exerted by the elastomer tension band  76  does not diminish significantly over a great many gas lift cycles. 
         [0044]    Moreover, the synthetic rubber material used in the tension band  76  is essentially impervious to the corrosive effects of most of the materials in the fluids found in oil and gas wells. These properties are unlike the use of small diameter coil springs, for example, which, being made of metal, are susceptible to such corrosion. Such corrosion requires additional maintenance—and down time—to replace and restore the tension of the springs or other metal components used to provide the necessary tension in the clutch  70 . The tension band  76  is preferably fabricated of a synthetic rubber material having a durometer of between 60 and 90 on the Shore “A” Scale. This requirement provides for sufficient tension when the tension band  76  is stretched over the rims  82 ,  84  of the split bobbin  72  to secure the clutch assembly  70  around the valve stem  102 . In the embodiments described herein, the clutch assembly  70  is designed to resist a linear pull on the valve stem  102  of approximately 2.8 to 3.6 lb. in this example, although adjustments to the tension may generally vary from 1.0 to 6.0 lb. in other examples but are not so limited because some applications mat require the clutch to satisfy clamping forces beyond this range. The performance of the clutch assembly  70  is also dependent on the finish applied to the valve stem  62 , as will be described with  FIG. 7 . 
         [0045]    Suitable materials for the tension band  76  for the clutch assembly  70  include neoprene and buna-N, respectively polychloroprene and acrylonitrile butadiene. An alternative is hydrogenated nitrile rubber. Another example, preferred for the present invention, is a fluoroelastomer such as a fluoronated hydrocarbon better known as Viton®, a registered trademark of the E. I. DuPont de Nemours and Company or its affiliates of Wilmington, Del., USA. In particular, the preferred material will have a Shore A durometer of 60 to 90, and for most applications a Shore durometer of 75 on the A scale has been found to work the best. 
         [0046]      FIG. 7  illustrates a perspective view of the assembly  100  of a bypass valve stem  102  and clutch assembly  70  for use in the embodiment of  FIGS. 1 through 6  of the present invention.  FIG. 7  also includes the details of the finish required on the surface  104  of the stem portion of the valve stem  102  that provides a surface roughness between 500 and 550 micro inches. This figure of 500 to 550 microinches describes the tolerance in the surface finish between the peak and valley portions of the roughened surface. In the illustrated embodiment the roughness of the surface  104  of valve stem  102  may be provided by a shallow continuous groove inscribed helically along the outer surface  104  of the portion of the valve stem  102  that is disposed within the clutch assembly  70 . The net effect of the clamping force provided by the tension band  76  combined with the surface roughness provided by the inscribed grooves  104  is to resist a pull on the lower end  108  of the valve stem  102  within the range of one to six lb. In one preferred embodiment the level of pull is set within the range of 2.8 to 3.6 lb. This surface roughness  104  thus forms an integral component of the friction effect of the clutch assembly  70  when it is installed on the valve stem  102 , improving its effectiveness and consistency. 
         [0047]      FIGS. 8 and 9  depict an alternate embodiment  130  of the clutch assembly  70  that is shown in  FIGS. 5 and 6 . Clutch assembly  130  may be used interchangeably with clutch assembly  70 . The clutch assembly  70  uses a single tension band  76 , whereas the clutch assembly  130  uses two tension bands and a split bobbin assembly  132  comprised of segments  132 A/ 132 B that has an additional rim  142  surrounding the bobbin.  FIG. 8  thus illustrates a clutch assembly  130  that includes a split bobbin  132  that surrounds the valve stem  102 . The split bobbin  132  is held in place by a pair of tension bands  134 / 136  that are placed around the two segments  132 A,  132 B of the split bobbin  132 , and within the space defined by the first and second rims  140  and  142 , and  144  and  142  of the bobbin segments  132 A,  132 B, thus clamping the bobbin segments  132 A,  132 B against the outer surface of the valve stem  102 . The bobbin segments  132 A,  132 B are identical in this illustrated embodiment, each one resembling a semicircle except for being slightly shortened from a full 180° by the gap  146 , which may be provided by making a 0.063 to 0.125 inch saw cut, for example, through the diameter of a single formed circular bobbin  132 . 
         [0048]    In other embodiments, the bobbin may be lengthened to cover a greater portion of the valve stem  102 . Further, the bobbin may be split into three or more segments (not shown), although two segments are adequate for this purpose and somewhat simpler to manufacture and handle during assembly. The split bobbin  130  illustrated in  FIG. 8  is shown with the segments  132 A and  132 B separated by the amount of the gap  146  even though the bobbin  130  is not installed on a valve stem  102 . When installed on the valve stem  102 , the gap  146  may typically be reduced under the effect of the pair of tension bands  134  and  136  used together. In other similar embodiments, the number of tension bands such as the tension bands  134 ,  136  may exceed two, an intermediate rim or rimes such as the rim  142  may or may not be used or needed, and the bobbin  132  may be split into more than two segments. In some embodiments the tension bands may simply be ordinary O-rings, such as those that are made of Viton®, as described herein above, which may be selected for size, thickness, or durometer to enable adjustment of the clamping force of the clutch assembly. Two or more such O-rings may be used to provide a particular adjustment to the tension—weaker or stringer—exerted on the bobbin segments of the clutch assembly. 
         [0049]    Continuing with  FIG. 8 , the tension bands  134 ,  136  may be made of a resilient material and is configured to tightly press the bobbin segments  132 A,  132 B against the outer surface  104  of the valve stem  102 . In the present embodiment the inside diameter  138  of each half  132 A,  132 B of the split bobbin  132  is the substantially the same as the outside diameter of the valve stem  102  but is formed as slightly less than a full semicircle because of the small gap  146  provided between the proximate ends of the split bobbin  132  when it is in place around the valve stem  102 . This enables the inner surface of the bobbin halves  132 A,  132 B to fully contact the valve stem  102  to provide maximum friction to resist the movement of the valve stem  102  through the clutch assembly  130  except when the plunger  10  contacts the bottom of the well bore during a gas lift operation. 
         [0050]    Also depicted in  FIG. 8  is an additional feature of the split bobbin  132 , the series of grooves  150  formed on the inner surfaces of the split bobbin  132 . These grooves, preferably uniformly disposed around the circumference of the bobbin segments  132 A,  132 B, provide passages for fluids to flush particles of sand away from the contact area of the bobbin  132  with the outer surface of the valve stem  102 . The grooves  150  may be formed by machining or swaging, for example. In the illustrated example, four such grooves  150  are formed in each bobbin segment  132 A,  132 B, although the number may be varied, generally between two and six grooves  150  in each segment may be practical. However, the greater the number of grooves in the split bobbin  132 , the more the grooves  150  will be limited to trapping most grains rather than allowing them to be flushed out of the clutch assembly  130 . 
         [0051]      FIG. 9  illustrates a perspective view of a pair of resilient tension bands  134 ,  136  for use in the clutch assembly  130  embodiment depicted in  FIG. 8 . The use of two or more tension bands instead of one may be preferred in some applications. For example, when it is necessary to provide a clutch assembly such as clutch assembly  70  or  130  to increase the effective clamping surface area against the valve stem  102 , the split bobbin may be lengthened along the longitudinal axis to accommodate additional tension bands. In the example illustrated in  FIGS. 8 and 9 , the tension bands  134 ,  136 , may each be formed as a ring having an inside diameter  120  about the same as or slightly smaller than the outer diameter of the central portion of the assembled split bobbin  132 A/ 132 B and an outside diameter  122  approximately the same (as shown in  FIGS. 7 and 8 ) slightly less than the outer diameter of the rims  140 ,  142 ,  144  of the split bobbin  132 A/ 132 B, which in turn may only be slightly less than the inner bore  50  of the valve cage  30  just below the internal ridge  38 . The tension bands  134 ,  136  preferably each have a width  124 ,  126  dimensioned to fill the width between the first and second rims  140 ,  142  and  142 ,  144  respectively of the split bobbin  132 A/ 132 B. It can further be seen that the resilient tension bands  134 ,  136 , which may have a rectangular cross section to fit within the respective rims  140 ,  142 ,  144  of the split bobbin  132 , act to form a very compact clutch assembly  130 . Alternately, the intermediate rim  142  may be deleted and a pair of tension bands placed side-by-side around the split bobbin as indicated by the dashed line  128  encircling the tension band  76  depicted in  FIG. 7 . Either of these configurations exerts a constant clamping force around the valve stem  102 . It has been found that the clamping force exerted by the elastomer tension bands  134 ,  136  do not diminish significantly over a great many gas lift cycles. 
         [0052]    The materials suitable for the tension bands  134 ,  136  in  FIGS. 8 and 9 , or other embodiments thereof, are as described in  FIG. 5  herein above. That is, the tension bands  134 ,  136  are preferably fabricated of a synthetic rubber material having a durometer of between 60 and 90 on the Shore “A” Scale. This requirement provides for sufficient tension when the tension bands  134 ,  136  are stretched over the rims  140 ,  142 ,  144  of the split bobbin  132  to secure the clutch assembly  130  around the valve stem  102 . In the embodiments described herein, the clutch assembly  130  is designed to resist a linear pull on the valve stem  102  of approximately 2.8 to 3.6 lb. in this example. Adjustments to the tension may generally vary from 1.0 to 6.0 lb. in other examples but are not so limited because some applications may require the clutch to satisfy clamping forces beyond this range as mentioned herein. The performance of the clutch assemblies  70 ,  130  are also dependent on the finish applied to the valve stem  102 , as previously described with  FIG. 7 . 
         [0053]    Returning now to  FIGS. 1 and 2 , the bypass valve assembly  14  may be assembled by first installing the valve stem  102  into the larger end of the valve cage  30  until it seats against the internal ridge  38  within the bore of the valve cage  30 . The valve cage may then be screwed onto the lower end of the plunger body  12  and secured with a set screw in the threaded hole  40 . Next, the clutch assembly  70  is installed over the lower end  108  of the valve stem  102  until it is seated against the opposite side of the internal ridge  38  within the valve cage  30 , followed by threading the end cap  34  into the lower end of the valve cage  30  to secure the clutch assembly  70  within the valve cage  30 . The end cap  34  may be tightened to a specified torque with the aid of a spanner wrench (not shown as it does not form part of the invention) inserted into the socket  38 , and secured using a set screw installed in the threaded hole  42 . 
         [0054]    While the invention has been shown in only one of its forms, it is not thus limited but is susceptible to various changes and modifications without departing from the spirit thereof.