Abstract:
Well fluid extraction apparatus includes a resiliently downwardly biased discharge head and a canister lowerable from the discharge head into the well to receive fluid therefrom, and then liftable into telescoped engagement with the discharge head. A specially configured seal structure between the discharge head and canister resiliently exerts a downward releasing force on the raised canister to facilitate its subsequent separation from the discharge head. An embodiment of the discharge head is pressure balanced to prevent gas pressure in the well from driving it upwardly. Leveling structure is provided for operatively connecting the apparatus to the vertically sloped upper end of a well casing and vertically leveling the connected apparatus.

Description:
BACKGROUND OF THE INVENTION 
   The present invention generally relates to apparatus useable in conjunction with a subterranean well and, in representatively illustrated embodiments thereof, more particularly provides portable apparatus for extracting fluids, such as oil, gas and water, from a subterranean well. 
   Disclosed in Applicant&#39;s U.S. Pat. No. 7,007,751, the full disclosure of which is hereby incorporated herein by reference, is an apparatus for extracting oil or other fluids from a well. The apparatus is utilizes a fluid retrieval canister which is lowered on a cable into a well to receive well fluid therefrom. Once it receives fluid from the well, the canister is raised by the cable into sealed telescoping engagement with a vertically movable discharge head portion of the overall apparatus and compressed air is forced into the raised canister, via the discharge head, to discharge the received fluid therefrom for transfer to a storage container. The canister is then lowered into the well and the retrieval process is repeated. 
   While this well fluid extraction apparatus is generally well suited for its intended purpose, in testing of the apparatus it has been discovered that it is in need of improvement in three areas. First, over time the seal structure which sealingly engages the raised canister with the discharge head can become coated with operational contaminants and thereafter tend to cause the canister to hang up in the discharge head and undesirably resist release therefrom for re-lowering of the canister into the well. Second, when the gas pressure in the well is above a certain magnitude (representatively, about 10 psig) such pressure exerts an undesirably high upward force on the upwardly movable discharge head. Third, the apparatus is less than totally satisfactory in wells having vertically tilted upper casing portions to which the apparatus is connected because the connected apparatus is also in a vertically tilted orientation and thus subjects the cable to accelerated wear and stress. 
   As can be seen from the foregoing, it would be desirable to provide an improved well fluid extraction apparatus, of the general type described, in which these three operational problems are eliminated or at least substantially reduced. It is to this goal that the present invention is primarily directed. 
   SUMMARY OF THE INVENTION 
   In carrying out principles of the present invention, in accordance with representatively illustrated embodiments thereof, specially designed apparatus is provided for extracting fluid, such as oil, from a subterranean well having an upper casing end portion which typically projects upwardly from the ground. 
   The apparatus basically comprises a vessel for receiving fluid from the well, the vessel extending along an axis, and a discharge head structure connectable to an upper portion of the well and useable to remove well fluid from the vessel. A system is provided which is selectively operable to axially lower the vessel into the well, representatively on a cable, to a first position to receive well fluid, and lift the vessel from its first position to a second position in which the discharge head structure and the vessel are forcibly and sealingly engaged for removal of the received well fluid using the discharge head structure. 
   In one embodiment of the invention, an air compressor is used to force compressed air through the discharge head and into the vessel engaged therewith to force retrieved oil outwardly from the vessel for collection in a suitable retrieval container. In another embodiment of the invention, the inlet of a pump, representatively a peristaltic pump, is connected to the vessel interior, via the discharge head, and is used to draw the retrieved oil out of the container and pump it to the retrieval container. 
   According to an aspect of the invention, when the vessel is lifted to its second position an upper end portion of the vessel is telescopingly received within the discharge head. A specially designed seal structure creates a peripheral seal between the telescoped vessel and discharge head and has a resilient portion which is compressed in a manner causing the resilient portion to exert both radial and axially upwardly directed forces on the discharge head. The axial force facilitates the release of the vessel portion received in the discharge head when it is desired to re-lower the vessel into the well to retrieve another load of oil. Such axial release force exerted by the seal structure helps to overcome a tendency of the vessel to “hang up” in the discharge head due to operational deposits and debris on the seal structure. Representatively, the seal structure includes at least one O-ring seal carried by an upper end nose portion of the vessel which is telescopingly received within the discharge head when the vessel is raised to its second position, and an annular, convexly curved interior ledge corner within the discharge head which sealingly contacts and deforms the O-ring seal at a downwardly and radially inwardly sloped angle. 
   According to another aspect of the invention, the discharge head is upwardly movable against a spring biasing force, and the well fluid extraction apparatus is provided with a pressure balancing structure operative to prevent well pressure below the discharge head from exerting an appreciable net upward force on the discharge head. Representatively, the discharge head structure includes a hollow, stationary cylinder, and a piston slidingly and sealingly received in the cylinder for vertical reciprocation relative thereto. The piston has a radially central portion projecting downwardly from said cylinder and adapted to telescopingly engage the vessel, and an annular flange projecting radially outwardly from the radially central portion and being slidably and sealingly received within the interior of the cylinder. The pressure balancing structure includes the piston and a pressure transfer passage for communicating well pressure below the discharge head structure with an interior portion of the cylinder above the annular flange therein. The resulting downward pressure force on the flange offsets the upward pressure force on the balance of the piston. The flange has a peripheral seal slidingly engaging an interior side portion of the cylinder. Preferably, the pressure balancing structure further includes a pressure relief passage for venting to atmosphere pressure leaking downwardly past this peripheral seal. 
   According to a further aspect of the invention, the well fluid extraction apparatus is provided with connection and leveling structure for operatively securing the well fluid extraction apparatus to the upper casing end portion in a sealed, selectively variable pivotal orientation relative to a horizontal axis to maintain the discharge head axis in a precisely vertical orientation even though the axis of the upper casing end portion is tilted away from vertical. Representatively the connection and leveling structure includes a connection flange having a tubular central body portion threadable onto the upper casing end, and a leveling flange anchored to a tubular member in which the discharge head is sealingly disposed. An annular, flat chamfered sealing surface is formed on the underside of the leveling flange and is slidingly and sealingly engageable with a radiused annular surface formed on the upper end of the tubular central body portion of the connection flange and sloping downwardly and radially outwardly. When the curved and chamfered annular surfaces engage each other, the well fluid extraction apparatus may be tilted as necessary about a horizontal axis to bring the discharge head axis to a precisely vertical orientation, despite a tilting away from vertical of the casing upper end portion axis, and locked in this precisely vertical orientation by simply tightening bolts extending through aligned peripheral portions of the connection and leveling flanges. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified, somewhat schematic cross-sectional view through a portable well fluid extraction apparatus embodying principles of the invention; 
       FIG. 2  is an enlarged scale cross-sectional view through a retrieval canister/discharge head portion of the apparatus; 
       FIG. 3  is a simplified, somewhat schematic cross-sectional view through an alternate embodiment of the portable well fluid extraction apparatus; and 
       FIG. 4  is an enlarged scale cross-sectional view through a retrieval canister/discharge head portion of the  FIG. 3  apparatus. 
   

   DETAILED DESCRIPTION 
   Referring initially to  FIGS. 1 and 2 , in a first embodiment thereof the present invention provides a portable well fluid extraction apparatus  10  (see  FIG. 1 ) for extracting fluid, such as oil, from a subterranean well representatively having an upper casing end portion  12  (see  FIG. 2 ) projecting upwardly from the ground  14 . The portable apparatus  10 , which may be easily transported to and from the well site in the bed of a pickup truck, includes a housing  16  in which a reel  18  is supported for selectively driven rotation by an electric motor  20  connected to the reel  18  through a gearbox  22 . An electrically driven air compressor  24  having a discharge line  26  connected to its outlet is supported within a bottom portion of the housing  16  and, like the electric motor  20 , is controlled by various conventional electrical components (not shown) within an electrical control box  28  in the housing  16 . 
   A drainage pan  30  having a bottom side wall  31  horizontally extends along an interior bottom portion of the pan for collecting fluid which may drip from various of the components within the housing  16 , the bottom side wall  31  of the pan  30  having a suitable drain fitting  32  connected thereto. A horizontal base structure  34 , having a top plate portion  36 , is anchored to the housing  16  directly beneath the pan  30 , and the open upper end of a vertical pipe member  38  is welded to the underside of the top plate portion  36 . The side wall portion of the pipe member  38  has a smaller transverse gas discharge pipe  40  welded thereto and a schematically depicted pressure relief valve  42  installed therein. An annular leveling flange  44  is coaxially welded to the bottom end of the pipe member  38  and has, on its bottom side, an annular radially inner surface portion  46  (see  FIG. 2 ) having, around its periphery, a flat chamfered configuration. 
   Disposed on the top side of the bottom pan wall  31  and overlying the open upper end of the vertical pipe member  38  is a rectangular discharge plate  48 . Four vertical bolts  50  (only one of which is visible in  FIGS. 1 and 2 ) slidably extend downwardly through corner openings in the plate  48  and are threaded at their lower ends into the base structure top plate portion  36 . Coiled compression springs  52  coaxially encircle and are captively retained on the bolts  50  and serve to resiliently bias the discharge plate  48  downwardly into contact with the top side of the bottom pan wall  31  and yieldingly permit the discharge plate  48  to be moved upwardly away from the pan wall  31 . 
   A hollow, generally tubularly configured discharge head structure  54  is coaxially and slidably received in the interior of the vertical pipe member  38  and is sealed therein by an O-ring seal  56  interiorly carried by the pipe member  38 . The upper end of the discharge head  54  is bolted to the bottom side of the discharge plate  48  so that the discharge head  54  is vertically movable with the plate  48 . As can be seen in  FIG. 2 , the plate  48  acts as a vertical stop to prevent further downward movement of the discharge head  54  from its  FIG. 2  lower limit position in which the bottom side of the plate  48  downwardly abuts the bottom side wall  31  of the pan  30 . 
   A well fluid retrieval vessel, in the form of a vertically elongated hollow cylindrical canister  58  having a reduced diameter radially stepped cylindrical upper end nose portion  60 , underlies the discharge head  54 . Extending vertically through the interior of the canister  58  is a fluid delivery tube  62  having an open lower end  64  upwardly adjacent the bottom interior end surface  66  of the canister  58 . The open upper end of the tube  62  communicates with a vertical discharge passage  68  extending through the nose  60  and opening horizontally outwardly through its side surface. For purposes later described herein, nose  60  also has formed vertically therethrough fill and vent passages  70 , 72  which communicate at their lower ends with the interior of the canister  58  and have upper end portions that open outwardly through the side surface of the nose  60 . 
   An upper end portion of the nose  60  receives and is locked to the lower end of a raising and lowering cable  74  by a connector structure  76  surrounding the lower cable end and threaded into the open upper end of the nose  60 . From its juncture with the main seal structure  76 , the cable  74  slidably extends upwardly through a vertically elongated tubular packing box  78  with vertically alternating annular wiper seals  80  and spacing structures  82  disposed therein. As best illustrated in  FIG. 2 , a lower end of the packing box  78  is captively retained between facing annular horizontal ledges  84 , 86  respectively formed on the discharge plate  48  and the central passage  88  extending vertically through the discharge head  54 . The lower end of the packing box  78  carries an O-ring seal  90  which exteriorly seals the lower packing box end within an upper end portion of the central discharge head passage  88 . 
   From the top end of the packing box  78 , the cable extends upwardly around a rotatable pulley  91  (see  FIG. 1 ) mounted on a horizontal arm portion  92  of a generally inverted L-shaped support post member  94 , disposed within the apparatus housing  16 , and then winds around the motor-driven reel  18 . For purposes later described herein, an axially movable, vertically extending rod linkage  96  is anchored at a lower end thereof to the discharge plate  48 , and rotatably connected at an upper end thereof to a pivotally supported limit switch arm  98  (see  FIG. 1 ). 
   With the important exceptions noted below, the well fluid extraction apparatus  10  depicted in  FIGS. 1 and 2  operates in a manner similar to the operation of the well fluid extraction apparatus embodiment shown in FIG. 3A of my copending U.S. application Ser. No. 10/443,353, with the following being a brief description of the apparatus  10  in the present application. For a more complete operational description, reference is hereby made to such copending application. 
   Basically, to retrieve well fluid from the well of which the illustrated upper casing end portion  12  is a part, the extraction apparatus is coupled to the upper casing end  12  as later described herein, and the motor  20  is operated to lower the canister  58  from its illustrated upper limit (or “docked”) position downwardly through the casing into the well until the canister  58  is immersed in oil. The oil then flows into the canister interior through its fill passage  70 , with the filling of the canister  58  with oil being facilitated by the canister vent passage  72 . After the canister  58  is filled with oil, the motor  20  is operated to lift the oil-filled canister  58  back to its illustrated upper limit position in which its nose portion  60  is telescopingly received in the central discharge head passage  88  and sealed therein in a novel manner subsequently described herein. 
   With primary reference now to  FIG. 2 , then the nose portion  60  of the canister  58  is pulled by the cable  74  into the central discharge head passage  88 , the upward cable pull drives the discharge head  54  and the discharge plate  48  upwardly away from their  FIG. 2  normal position against the downward biasing force of the bolt springs  52 . In response to such upward movement of the discharge head  54  and discharge plate  48 , the rod linkage  96  is also axially moved upwardly which in turn pivots the limit switch arm  98  (see  FIG. 1 ) in a counterclockwise direction to thereby cause a limit switch (not shown) in the control box  28  to de-energize the motor  20  and terminate driven rotation of the reel  18 . 
   With the oil-filled canister  58  brought back to this upper limit position, a first vertical flow passage  100  in the discharge head  54  communicates the canister fill and vent passages  70 , 72  with the compressor discharge line  26 , and a second vertical flow passage  102  in the discharge head  54  communicates the canister discharge passage  68  with an oil flow line  104  coupled to a suitable extracted oil-receiving container (not illustrated). The compressor  24  is then energized to force compressed air  106  through the compressor discharge line  26 , downwardly through the discharge head passage  100  and into the interior of the canister  58  via its fill and vent passages  70 , 72 . Compressed air entering the canister interior sequentially drives the oil  108  therein upwardly through the canister fluid delivery tube  62 , through the canister and discharge passages  68  and  102 , and outwardly through the oil flow line  104  and into the extracted oil-receiving container to which it is connected. The canister&#39;s above-described lowering, raising and oil extraction cycle is then repeated as required. 
   According to a key aspect of the present invention, specially designed cooperating seal structures are incorporated into the discharge head  54  and the canister nose  60  and uniquely facilitate the downward release of the canister  58  from the discharge head  54 , so that the canister  58  can be re-lowered into the well, even if operational deposits on the facing telescoped discharge head and canister nose would otherwise tend to cause the canister nose  60  to “hang up” within the discharge head  54 . 
   As can be seen in  FIG. 2 , the cylindrical canister nose  60  has, along its vertical length, a radially stepped configuration in which each successively higher axial section of the nose  60  has a smaller diameter than the preceding axial section. Representatively, there are three such radial dimension reductions on the illustrated nose  60 . The lower three axial sections of the nose  60  have annular grooves formed therein which respectively carry a lower resilient O-ring seal  110 , a vertically intermediate O-ring seal  112 , and an upper O-ring seal  114 . When the canister nose  60  is telescopingly received in the discharge head  54  as illustrated in  FIG. 2 , the seals  110 , 112 , 114  are respectively contacted and resiliently deformed by rounded annular corner portions  116 , 118 , 120  of the indicated interior annular ledge portions of the discharge head  54 . These rounded annular corner portions  116 , 118 , 120  engage their associated O-ring seals  110 , 112 , 114  at downwardly and radially inwardly inclined angles, representatively of about 45 degrees, to form annular point contact seals therewith. 
   Because the O-ring seals  110 , 112 , 114  are carried on the canister nose  60  instead of being carried within the interior of the discharge head  54 , both the nose  60  and the discharge head  54  may conveniently be of one piece constructions. Additionally, and quite importantly, because the interior rounded corner portions  116 , 118 , 120  of the discharge head  54  contact the O-ring seals  110 , 112 , 114  at downwardly and radially inwardly inclined angles, the deformed O-ring seals exert not only radially outwardly directed resilient sealing forces on the discharge head  54 , but exert resilient downwardly directed forces on the canister nose  60 . When it is desired to lower the raised canister  58  back into the well, and the upward cable force is lessened, this downwardly directed seal force exerted on the canister  58  helps to downwardly release it from the discharge head  54  and avoid canister “hang ups” therein which might otherwise occur due to operational deposits on the O-ring seals. 
   Because a cable is being used to lower and raise the canister  58  into and out of the well, it is desirable that the central axis of the cylindrical discharge head structure  54 , when connected to the upper end of the well casing  12 , be as precisely vertical as possible to avoid chafing the cable as it is being raised and lowered. However, it is often the case that the axis of the upper casing end  12  is substantially tilted away from vertical. To essentially eliminate this potential extraction apparatus mounting problem, the present invention provides a specially constructed mounting and leveling apparatus  122  which will now be described in conjunction with  FIG. 2 . 
   The mounting and leveling apparatus  122  includes the previously described annular leveling flange  44 , and an annular mounting flange  124  that coaxially circumscribes an internally threaded annular mounting collar or tubular central body portion  126  having, around its upper end, an annular convexly radiused surface  128  which slopes downwardly and radially outwardly (representatively at a 45 degree angle) around its periphery. The radiused surface  128  is preferably a segment of a spherical surface centered on the central vertical axis of the flanges  44  and  124 . 
   To operatively mount the extraction apparatus  10  on the upper casing end portion  12 , the collar  126  is first threaded onto the upper casing end portion  12  as shown. Next, the annular flat chamfered surface  46  of the leveling flange  44  is placed atop the radiused surface  128 , with the surface  46  being tangent to the surface  128 , and bolts  130  are extended downwardly through aligned holes  132 , 134  in the flanges  44 , 124  and threaded into nuts  136 . By appropriately rocking the apparatus  10  about a horizontal axis, to bring the axis of the discharge head  54  to a precisely vertical orientation, and then tightening the bolts  130 , the axis of the discharge head  54  may be maintained in a precisely vertical orientation despite the fact that the axis of the upper casing end portion  12  is tilted away from vertical. The forcible engagement between the flat chamfered annular surface  46  and the radiused surface  128  creates an annular seal area therebetween which prevents gas outflow from the well through the at the juncture between the surfaces  46  and  128 . While the surface  46  is preferably a flat chamfered surface, it could alternatively be an annular edge contact surface, or a rounded annular contact surface, and still create the desired seal with the surface  128 . 
   By virtue of the various sealing elements described above, pressurized gas  138  within the well may be produced, by flowing it outwardly through the gas discharge pipe  40  to a suitable receiving container (not shown), at the same time that the apparatus  10  is extracting oil  108  from the well. With the apparatus  10  shown in  FIGS. 1 and 2 , it is not necessary to vent the gas  138  to atmosphere during the described oil extraction process. 
   Instead of using the air compressor  24  to blow oil  108  out of the canister  58  when it is docked to the discharge head  54 , a pump such as, for example, the peristaltic pump  140  illustrated in phantom in  FIG. 2 , may be utilized to extract oil  108  from the canister  58 . To do this, the lower O-ring seal  110  is removed, the inlet to the discharge head flow passage  100  is suitably blocked, and the oil flow line  104  is coupled to the inlet of the pump  140  as indicated in phantom in  FIG. 2 . With the canister  58  docked to the discharge head  54 , the pump  140  is started to draw the oil  108  in the canister  58  into the pump  140  for discharge therefrom into a suitable receiving container (not shown). During operation of the pump  140 , gas  138  within the well is permitted to be drawn into the canister  58 , through its fill and vent passages  70  and  72  as illustrated in phantom in  FIG. 2 , via the seal area vacated by the removal of the lower O-ring seal  110 . 
   An alternate embodiment  10   a  of the well fluid extraction apparatus  10  previously described in conjunction with  FIGS. 1 and 2  is illustrated in  FIGS. 3 and 4 . To facilitate the ready comparison between apparatus  10  and apparatus  10   a , components in apparatus  10   a  similar to those in apparatus  10  have been given identical reference numerals to which the subscripts “a” have been added. 
   As can be seen in  FIG. 3 , in the extraction apparatus  10   a , the air compressor  24  (see  FIG. 1 ) is not used, but instead is replaced by a pump, representatively a peristaltic pump  142 . Pump  142  has a discharge line  144  through which extracted oil is delivered to a suitable receiving container (not shown), and an inlet line  146 . Moreover, as will be subsequently described herein, the discharge head structure  54   a  is provided with a unique “pressure balanced” configuration, and the configuration of the canister  58   a  is slightly altered. 
   In the previously described well fluid extraction apparatus  10 , during use of the apparatus pressurized gas within the well exerts an upward pressure force on the underside of the vertically movable discharge head  54  (see  FIG. 2 ). This upward pressure force is resisted by the bolt springs  52 . When the gas pressure within the well is 10 psig or below, the springs  52  are able to hold the discharge head  54  in its lower limit position. However, when the gas pressure within the well is substantially greater than 10 psig, this excess pressure is relieved by the previously mentioned relief valve  42 . As will now be described, the pressure balanced discharge head structure  54   a  uniquely utilizes the well gas pressure to substantially prevent any net upward well pressure force from being imposed on the discharge head  54   a.    
   The discharge head  54   a  includes a tubular cylinder structure  148  having an open upper end bordered by an inturned annular flange  150 , and an open lower end to which an annular end plate  152  is coaxially and removably secured. The upper end of the cylinder structure  148  is anchored to a stationary plate  154  that underlies the vertically movable discharge plate  48   a.    
   A hollow tubular pressure balancing piston  156  slidably mounted in the cylinder  148  for vertical reciprocating motion relative thereto. Piston  156  has a tubular upper end portion  158 , a tubular lower end portion  160 , and a vertically intermediate outwardly projecting annular flange portion  162  having a top side annular surface area. The upper piston portion  158  is slidingly and sealingly engaged within the cylinder  148  by an O-ring seal  164  carried by the upper cylinder flange  150 , and the upper end of the upper piston portion  158  extends through an opening in the plate  154  and is anchored to the discharge plate  48   a  for vertical movement therewith. An O-ring seal  166  carried by the piston flange  162  slidingly and sealingly engages the interior side surface of the cylinder  148 , and an O-ring seal  168  carried by the annular end plate  152  slidably and sealingly engages the lower piston end portion  160 . 
   Positioned vertically between the piston flange  162  and the annular end plate  152  is an annular vent clearance space  170  communicating with a vertical vent passage  172  extending upwardly through the upper piston end portion  158  and opening outwardly through a hole  174  in the stationary plate  154 . A vertical pressure transfer passage  174  extends upwardly through the bottom end of the piston  148  and communicates with the interior of the cylinder  148 , and an oil transfer passage  176  extends upwardly through the interior of the piston  148  and communicates at its upper end with the pump inlet line  146 . 
   The canister  58   a  has fill and vent passages  70   a  and  72   a , but is not provided with the lowermost O-ring seal  110  incorporated into the previously described extraction apparatus  10 . The two O-ring seals  112   a , 114   a  carried by the nose  60   a  are contacted by the corresponding annular interior corner portions  118   a , 120   a  of the piston portion  148  of the discharge head structure  54   a . When the canister nose  60   a  is telescopingly and sealingly received within the piston portion  148  of the discharge head structure  54   a , the discharge passage  68   a  within the nose  60   a  communicates with the oil transfer passage  176  within the piston  148 . To retrieve oil  108  contained in the docked canister  58   a  the pump  142  (see  FIG. 3 ) is started to sequentially draw oil  108  upwardly through the canister tube  62   a , the nose and piston passages  68   a  and  176 , through the pump inlet line  146 , and then through the pump  142  and its discharge line  144  into the receiving container (not shown). 
   As oil  108  is being extracted from the docked canister  58   a , gas  138  is permitted to be drawn into the canister interior, via the canister&#39;s fill and vent passages  70   a  and  72   a , to prevent a vacuum from being created within the canister  58   a . At the same time, gas  138  is being produced from the well via the gas discharge pipe  40   a.    
   Due to the specially designed pressure balanced construction of the discharge head structure  54   a , the gas pressure within the well does not exert an appreciable net upward force on the discharge head structure  54   a  which would tend to compress the bolt springs  52   a . This is due to the fact that well gas pressure from below the discharge head structure  54   a  is transmitted upwardly into the interior of the cylinder  148 , to exert a pressure force on the top side surface of the piston flange  162 , via the pressure transfer passage  174  in the piston  148 . 
   This downwardly directed pressure force on the piston flange  162  tends to drive the piston  156 , and thus the discharge plate  48   a , downwardly toward their lower limit positions shown in  FIG. 4  to thereby prevent gas pressure within the well from exerting compressive force on the bolt springs  52   a . To assure that such well gas pressure exerts no appreciable net upward force on the discharge head structure  54   a , the annular surface area of the top side of the piston flange  162  is sized to be equal to or greater than the area of a circle having a diameter equal to the outer diameter of the lower end portion  160  of the piston  156 . 
   The vent passages  170 , 172  serve to prevent gas pressure leaking downwardly past the piston flange seal  166  from exerting an upward pressure force on the piston flange  162 . In the event that such seal leakage occurs, pressurized gas from above the piston flange  162  that passes downwardly past the seal  166  is simply vented to atmosphere (via the passages  170 , 172  and the plate hole  174 ) without exerting an upwardly directed pressure force on the piston flange  162 . 
   The foregoing detailed description is to be clearly understood as being given by way of illustration and example only, the spirit and scope of the present invention being limited solely by the appended claims.