Abstract:
An improved, unattended, liquid pumping device for oil and gas wells featuring a bellows controlled flow valve that opens and closes at preset pressures. Additionally a well head receiver design that releases shut in production gas below the pumping device and provides a positive pressure differential across the pumping device prior to valve opening.

Description:
PRIORITY CLAIM 
   This application claims priority to U.S. Application Ser. No. 60/282,398, filed Apr. 6, 2001, which is hereby incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The invention relates to an autonomous pressure actuated liquid pump for use in gas and oil wells. In particular, it relates to liquid lift pumps in which well pressure, acting against an internal bellows or diaphragm, causes an internal flow control valve to open or close thereby releasing or shutting-in well gas flow. 
   BACKGROUND 
   The economic viability of marginal petroleum wells depends on the well&#39;s product flow and pressure capacity and the rate at which undesirable liquids (i.e., brine) infiltrate the well casing. A number of patents have been issued over the past 50 years addressing oil and gas well swabbing devices that offer the potential for unattended self actuation in an operating well environment. None of these inventions have proven to be operationally acceptable. 
   One known device is an airlift system, which features a cylindrical pumping device through which the fuel product flows. Flow in the annular passage between the cylindrical device and the well casing or tube walls is eliminated by closing off this area with flexible friction cups (rubber like material) or other mechanical means. A valve controls the flow of liquid and gas through the cylindrical pump. When the valve is closed the well is effectively shut-in. In other words, when the valve is closed the cylindrical pumping device seals the well closed. The resulting pressure build-up below the pump lifts the pump and the liquid above it to the surface. The flow capacity and shut-in pressure capability of the well must be sufficient to accomplish the lift. Upon reaching the well head, the control valve in the pump, is mechanically forced open to release the shut in pressure. The fluid below the pump then flows through the pump and out of the well. 
   Two basic approaches have been used in prior devices to close the flow control valve after the cylindrical pump has reached the desired location in the well. In some applications the flow control valve is forced closed by the impact of the pump striking a fixed stand located in the well. Situations develop operationally, however, where the fluid above the stand rises to a level that is too high for the subsequent shut-in pressure of the well, and the lift can not be accomplished. When this occurs, the well continues to be shut in until the cylindrical pumping device is mechanically retrieved from the bottom of the well. 
   In other applications, such as in the device disclosed in U.S. Pat. No. 4,986,727 of Blanton, the valve is closed when the well pressure is sufficient to overcome the resistance of a pressurized bellows in the device. This well pressure (set point pressure) is composed of both the flow pressure (also referred to as back pressure or casing pressure) and the hydrostatic pressure resulting from the column of liquid above the control valve. 
   Because the pumping device does not sense liquid level but is pressure activated, the control valve closes whenever the pressure at the valve reaches the set point pressure. Also, regardless of the pressure level in the well, the pumping device will descend into the well whenever the pressure differential across it (top to bottom) decrease to less than about 10 PSI. When the control valve is mechanically forced open, at the well head, the pressure differential across the swabbing device approaches zero. 
   Field experience indicates that when the control valve is forced open the unattended pumping device routinely drops down the well while the well was still flowing at very high pressure following shut-in. This has resulted in erratic operation, partial fluid lifts, valve cycling, and dry device lifts that have caused damage to the pumping device as well as to the supporting equipment at the well head. To prevent this, it was found necessary to hold the pumping device at the well head until the casing pressure dissipated to a normal operating level. This has required the design of automated latching devices to restrain the pumping device at the surface, using either maintenance personnel or timing devices to activate a release when the casing pressure is reduced to an acceptable working level. These solutions have added to the cost and complexity of the installations. 
   SUMMARY OF THE INVENTION 
   This invention provides a self-actuating solution to overcome the shortcomings of the prior art devices. First, the pump is operable to retrieve a preset amount of liquid instead of trying to lift all of the liquid in the well. Second, by eliminating the flow of the exhausting fuel products through the pumping device when it is at the surface, the present invention reduces or eliminates the need for forcing the control valve open when the pumping device is at the well head. Third, the physical relationship of the control valve seating area to the bellows effective cross sectional area is designed to maintain the control valve in a closed position at the well head, until the well pressure dissipates to an acceptable level for continuing liquid pumping operations. Also, the invention may include a well head receiver that both cushions the pump against shock on its upward travel, and suspends the pump until the pressure is reduced. This invention reduces or eliminates the operational need for electricity, radio communications, timers, or extra maintenance support at remote well sites and provides self activated well pumping that will accommodate variations in service line pressure and well liquidification rates. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
       FIG. 1  is a view of a well-pumping system according to the present invention; 
       FIG. 2  is a cross sectional view of the pumping device of the well-pumping system illustrated in  FIG. 1 ; 
       FIG. 3  is an enlarged cross-sectional view of a biasing element of the pumping device illustrated in  FIG. 2 ; 
       FIG. 4  is an enlarged cross-sectional view of a biasing element of the pumping device illustrated in  FIG. 2 , illustrating the biasing element in a retracted position; 
       FIG. 5  is a side elevational view of the biasing element illustrated in  FIG. 3 ; 
       FIG. 6  is a graphic presentation relating the down well valve closing pressure to the bellows initial charge pressure and the relevant physical parameters of the bellows and valve assembly; and 
       FIG. 7  is a graphic presentation relating the valve opening pressure to the valve closing pressure and to the pressure above the valve at the time of opening. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention operates to eject fluid from a well by sealing off the well. In wells, often the gas pressure in the well is insufficient to eject fluid from the well. By lowering a device into the fluid in a well, and sealing the well, the gas pressure builds up because the gas below the device can not diffuse upwardly through the fluid. The buildup of gas pressure is sufficient to propel a column of fluid in the well above the device and eject the fluid from the well. 
   Referring now to  FIGS. 1 and 2 , a system for pumping liquid out of a well is illustrated. The system includes a pump  10  that forms a fluid-tight seal with the wall of a well  5 . In  FIG. 2  the pump  10  is illustrated down in the well. In  FIG. 1 , the pump  10  is illustrated at the top of the well. 
   To pump fluid out of the well, the pump  10  is lowered into the well  5 , so that it sinks into fluid in the well. Once the fluid pressure in the well above the pump exceeds a threshold, the pump  10  seals-off the well. In doing so, the device seals in the gas in the well, causing the fluid pressure below the pump to build up. The fluid pressure below the pump then drives the pump upwardly along with the fluid above the pump. As the pump  10  is driven upwardly, the fluid above the pump is discharged through discharge lines  70 ,  80  connected to the well  5 . When the pump reaches the top of the well, the gas pressure below the pump drives the pump into a receiver  60  that maintains the pump above the lower discharge  80  line while gas from the well flows through the line. When the flow of gas from the well diminishes, the pump  10  is lowered again to pump more fluid out of the well. 
   The raising and lowering of the pump  10  is controlled automatically in response to the fluid pressure in the well. Specifically, the pump  10  includes a valve  20  that controls the flow of fluid through the pump. A biasing element  30  controls the operation of the valve  20 . More specifically, the biasing element  30  biases the valve  20  into an open position. When the valve  20  is open, the pump  10  descends into the well, and fluid flows through the pump. The rate of descent is limited by the friction between the pump and the well walls and flow restrictions through the pump. When the pump reaches the liquid level in the well, it continues to descend, but at a reduced rate. 
   When the pressure differential across the pump  10  exceeds a threshold (closing threshold) related to the biasing force of the biasing element, the valve  20  automatically closes so that fluid can no longer flow through the pump. As described above, the fluid pressure in the well builds up and then drives the pump upwardly. At the top of the well  5  the pump  10  is displaced into a receiver assembly  60  that maintains the pump. While the pump is maintained in the receiver, the gas pressure in the well dissipates as gas flows through the lower discharge line  80 . When the fluid pressure across the pump drops below a threshold (opening threshold), the biasing element  30  automatically opens the valve  20  and the pump  10  descends again into the well. In this way, the pump automatically descends and ascends within the well to pump fluid from the well. 
   Referring now to  FIG. 2 , the details of the pump  10  will be described in greater detail. The pump  10  includes an elongated substantially hollow cylindrical housing  12 . A lower housing  15  is fixedly attached to the lower end of the cylindrical housing  12 . An end cap  25  closes the lower end of the lower housing. Preferably, the lower end cap  25  is releasably connected with the lower housing  15 . In the present instance the lower end cap  25  is threadedly connected to the lower housing. A plurality of holes in the lower end cap  25  form inlet ports  26 , so that fluid can flow into the pump  10  through the inlet ports  26  when the pump descends into the well. 
   A top cap  50  is attached to the upper end of the housing  12 . The top  50  has a central bore providing a fluid path. The lower end of the top cap  50  is attached to the upper end of the housing  12 . Preferably the top cap  50  is releasably connected to the housing; and in the present instance, the top cap  50  has external threads that mate with internal threads in the housing  12  to attach the top cap to the housing. 
   The upper end of the top cap  50  is generally open, and preferably includes an internally threaded portion for mounting a stem  18 . The stem  18  is an elongated solid shaft that cooperates with the catcher latch  62  to hold the pump  10  at the top of the well, as discussed further below. The stem  18  preferably has an externally threaded portion cooperable with the top cap  50  to releasably attach the stem to the top cap. In this way, the stem threads into the top cap thereby sealing the upper end of the top cap. 
   As shown in  FIG. 2 , a plurality of holes through the sides of the top cap  50  provide outlet ports  52 . In this way, fluid flowing through the pump  10  flows through the top cap  50  and out the outlet ports  52 . 
   A plurality of sealing elements or cups  40 ,  41  disposed around the housing provide a fluid-tight seal between the housing and the inner wall of the well  5 . The cups  40 ,  41  are disposed between the inlet ports  26  at the bottom of the pump  10  and the outlet ports  52  at the top of the pump. The cups  40 ,  41  are elastomeric elements having a central bore. The cups  40 ,  41  are spaced apart axially from one another by a spacer  45 . The spacer  45  is an elongated cylindrical collar having an internal diameter slightly larger than the external diameter of the housing. 
   The cups  40 ,  41  and spacer  45  are captured on the housing between a locking ring  55  and a lip that is the formed by the top edge of the lower housing  15 . Specifically, an internal annular shoulder of the lower cup  41  abuts both the top edge of the lower housing, and the locking ring  55  threaded onto the top cap  50  engages the top edge of the upper cup  40 . 
   The locking ring  55  is a threaded collar or nut that cooperates with external threads on the top cap  50 . In this way, the locking ring  55  is operable to tighten down or compress the cups  40 ,  41 . Since the cups  40 ,  41  are formed of elastomeric material, preferably a metal washer is disposed between the locking ring  55  and the upper cup  40 . The metal interface between the locking ring and the washer facilitates turning the ring to tighten down on the cups  40 ,  41 . 
   During use, the cups may wear and need to be replaced. Accordingly, preferably the pump  10  is configured so that the cups  40 ,  41  can be readily removed and replaced without disassembling the pump. Therefore, in the present instance the top cap  50  is preferably configured so that it need not be removed to replace the cups. Specifically, preferably the exterior diameter of the top cap  50  is small enough to allow the cups to slide over the top cap. In particular, preferably the external diameter of the top cap  50  is approximately the same as, or less than, the external diameter of the housing. 
   Configured in this way, the cups  40 ,  41  can be replaced as follows. The locking ring  55  is unscrewed from the top cap  50  and removed along with the washer  57 . The cups  40 ,  41  and spacer  45  are then slid off the housing and over the top cap. A new lower cup is then slid over the top cap and down over the housing until it engages the top edge of the lower housing  15 . The spacer  45  is then slid over the top cap  50  and housing until it abuts the top edge of the new lower cup. A new upper cup is then slid over the top cap and down over the housing until it engages the top edge of the spacer. The washer  57  is then placed over the top cap  50  on top of the upper cup. Finally, the locking ring  55  is threaded onto the top cap and tightened down with the upper cup. 
   Referring to  FIG. 2 , the valve  20  controls the flow of fluid through the housing  12 . In  FIG. 2 , the valve  20  and biasing element  30  are shown in elevation. The valve  20  comprises a valve element  22  that cooperates with a valve seat  23  to form a fluid-tight seal. Preferably, the valve element  22  is formed of an elastomeric material. The valve seat  23  is preferably a tapered annular surface formed in the interior wall of the lower housing. 
   When the valve is closed, fluid does not flow through the pump. In addition, since the cups  40 ,  41  provide a fluid-tight seal between the housing  12  and the wall of the well  5 , fluid does not flow around the pump. Accordingly, when the valve  20  is closed, the pump  10  operates as a seal, sealing the well closed. This allows a pressure differential to build up across the tool. Specifically, when the valve is closed, the pressure below the cups increases relative to the pressure above the cups. 
   The biasing element  30  biases the valve  20  toward an open position in which fluid can flow through the pump through the inlet and outlet ports  26 ,  52 . Preferably, the biasing element  30  is fixed relative to the housing  12 , so that the biasing element is not displaceable relative to the housing. However, in some applications it may be desirable to allow the biasing element to be displaced relative to the housing. In the present instance, the biasing element  30  is fixed in place between the upper cap  50  and a retaining ring  47 , as discussed further below. 
   The biasing element  30  can be formed of one of a number of elements for providing a biasing force against the valve  20 . For instance, the biasing element could comprise a compression spring operable to bias the valve open. However, preferably, the biasing element  30  comprises a pressurized bellows, as discussed further below. 
   Referring to  FIGS. 3-5 , the details of the biasing element  30  will be discussed in greater detail. The biasing element  30  comprises a housing  32 , and a hollow canister  34  in which bellows  35  are disposed. The housing comprises an enlarged head formed by a plurality of radially extending tabs  33 . Referring to  FIG. 2 , in which the biasing element is illustrated in elevation, the tabs  33  are captured between the top cap  50  and the retaining ring  47  to attach the biasing element to the housing  12 . The retaining ring  47  is a cylindrical ring fixedly connected to the interior wall of the housing, such as by welding or pressfit. The retaining ring  47  forms a shoulder against which the tabs  33  of the biasing element  30  rests. The lower edge of the top cap  50  engages the tabs  33 , so that when the top cap is threaded onto the housing  12 , the tabs  33  are captured between the retaining ring  47  and the top cap. As shown in  FIGS. 3 and 5 , the tabs  33  are circumferentially spaced apart, so that fluid can flow between the tabs and the retaining ring. 
   The bellows  35  are operable to expand and contract vertically within the canister. The bellows canister  34  is substantially cylindrical and is fixedly attached to the lower end of the housing  32 , circumscribing the bellows  35 . A plurality of vent holes are formed in the side walls of the canister  34 . The lower end of the canister is generally open, having an annular flange extending radially inwardly to form a lip. The opening is configured to cooperate with the exterior surface of a connecting block  36 . The connecting block  36  is attached to the lower end of the bellows  35  so that the connecting block is displaced vertically when the bellows expand or contract. 
   The top end of the connecting block  36  flares outwardly forming a flange having a diameter substantially similar to the interior of the canister  34 . In this way, the flange forms a sliding fit with the interior of the canister  34 . The lip formed at the lower end of the cannister operates as a stop that cooperates with the flared head of the connecting block to prevent the connecting block  36  from being completely displaced out of the cannister. 
   An elongated rod  37  is connected with the lower end of the connecting block  36 . Preferably, the rod  37  is integrally formed with the connecting block  36 , as shown in  FIGS. 3 and 4 . The lower end of the rod  37  has a reduced diameter tip and an internally threaded bore. As shown in  FIG. 2 , the valve element  22  is mounted on the reduced diameter tip of the rod  37  by a bolt that is threaded into the tip of the rod. In this way, the valve element  22  is connected with the bellows, so that the valve element is displaced vertically when the bellows expand or contract. 
   In  FIG. 3 , the bellows  35  is illustrated in an extended position, which corresponds to the valve being opened as illustrated in FIG.  2 . In  FIG. 4 , the bellows is illustrated in a contracted position, which corresponds to the valve being closed so that the valve element  22  seals against the valve seat  23 . 
   Referring to  FIG. 2 , preferably, an alignment ring  49  for supporting and aligning the rod  37  is disposed in the lower housing  15 . The alignment ring has a central bore corresponding to the external diameter of the rod  37 , forming a sliding fit between the rod and the central bore of the ring. The alignment ring  49  also includes a plurality of holes so that fluid can flow through the alignment ring when the valve  20  is open. 
   The bias of the biasing element  30  is controlled in part by the fluid pressure within the bellows  35 . As shown in  FIGS. 3-5 , a cavity is formed within the bellows  35 . An air-fill valve  31  attached to the housing  32  of the biasing element controls the flow of fluid into the bellows  35 . In this way, the bellows can be charged by filling the bellows with pressurized air through the air-fill valve  33 . As the bellows are filled with pressurized air, the bellows expand outwardly, displacing the connecting block  36  and attached valve element  22  downwardly. The airfill valve  31  can be accessed without disassembling the pump  10 , by simply unscrewing the stem  18  from the top cap  50 . 
   The bellows  35  compresses in response to hydrostatic pressure on the bellows when the pump is in the liquid in the well. As the bellows compresses, the valve  20  closes. The stroke of the valve element  22  between the opened position and the closed position corresponds to the compression of the bellows from the charged length to the compressed length when the valve  20  is closed. The biasing element  30  is configured to reduce the volume of the bellows cavity, thereby increasing the bellows compression ratio, as described further below. 
   The force of the biasing element  30  opposing the fluid pressure on the bellows is also influenced by the weight of the valve element  22  and connecting rod  37 , as well as the effective cross-sectional area of the bellows. Referring to  FIG. 6 , the relationship between the valve closing pressure and the bellows charge pressure is illustrated. The slope of this line is equal to the compression ratio of the bellows (the charged volume/the compressed volume) corrected for the bellows temperature change (operating temperature (absolute)/charge temperature (absolute)). The Y axis intercept of this line is the sum of the bellows bias force and the weight of the valve element  22  and connecting rod  37  divided by the effective cross-sectional area of the bellows. It is to be noted that, to a first order, valve geometry does not affect the valve closing pressure. 
   The dashed line on  FIG. 6  represents the same design, with the exception that the valve  20  is positioned initially 10% closer to the valve seat  23  (a 10% reduction in bellows stroke). If the charged volume for the bellows cavity is fixed, reducing the bellows stroke 10% increases the volume of the compressed volume for the bellows, thereby decreasing the bellows compression ratio. As can be seen by  FIG. 6  decreasing the bellows compression ratio decreases the sensitivity of the valve closing pressure to the bellows pressure. In other words, for an increased bellows compression ratio, a change in bellows pressure causes a greater change in the valve closing pressure relative to a lower bellows compression ratio. Accordingly, preferably the bellows compression ratio is greater than approximately 1, and more preferably is between 1.1 and 2.2. 
   Referring to  FIG. 1 , the pumping device  10  is illustrated at the well head. An upper discharge line  70  and lower discharge line  80  are connected to the well  5  for receiving the fluid from the well. The upper discharge line  70  extends between the well  5  and the lower discharge line  80 . Preferably, the lower discharge line  80  is approximately twice as large in diameter as the upper discharge line  70 . The opening from the well  5  to the upper discharge line  70  is vertically spaced along the well from the opening to the lower discharge line  80  a distance that is greater than the distance from the point that the lower cup  41  seals with the well to the point that the upper cup  40  seals with the well. In this way, when the device  10  is at the top of the well the lower cup  41  seals against the well at a point above the opening to the lower discharge line  80 , and the upper cup  40  seals against the well at a point below the opening to the upper discharge line  70 , as shown in FIG.  1 . 
   The cups  40 ,  41  may catch on the openings to the upper and lower discharge lines  70 ,  80  when the cups pass over either of the openings. Over time, this may accelerate the wear on the cups leading to reduced life of the cups. Therefore, preferably, covers  65 ,  85  cover the openings to the discharge lines  70 ,  80 . The covers  65 ,  85  are perforated to allow fluid to readily flow from the well into the discharge lines. The covers create a smoother surface along the wall of the well  5 , reducing the wear between the cups  40 ,  41  and the well at the discharge lines. 
   A check valve  75  is disposed along the upper return line. The check valve  75  is configured to allow higher pressure fluid in the upper discharge line  70  to flow into the lower discharge line  80  and to impede fluid flow from the lower discharge line up into the upper discharge line. In this way, preferably the fluid in the upper discharge line remains at a higher pressure than the fluid in the lower discharge line to prevent fluid from flowing from the lower discharge line into the upper discharge line and into the well above the device  10 . In addition, an upper shut-off valve  72  is provided on the upper discharge line  70  to shut-off the upper discharge line, and a lower discharge valve  82  is provided to shut-off the lower discharge line. The shut-off valves  72 ,  82  may be any one of a number of types of valves, such as a ball valve. 
   The receiver assembly  60  is disposed at the top of the well head is configured to receive the pump  10  and retain the pump at the top of the well. Specifically, the receiver assembly  60  includes a catcher latch  62  for engaging the stem  18  of the device. The stem  18  includes at least one circumferential groove that cooperates with the latch  62  to hold the device in the receiver  60 . The latch  62  is spring loaded radially inwardly, so that as the pump is displaced upwardly into the receiver, the latch rides over the surface of the stem until it engages the circumferential groove on the stem. In this way, the latch  62  mechanically couples with the stem to hold the pump in the receiver so that the pump will not descend into the well even after the valve reopens. The top of the receiver assembly  60  is preferably attached to the well head by a coupling, such as a hammer union  64 . Therefore, the device  10  can be removed from the well for service by catching the device in the receiver  60  with the latch  62  and then uncoupling the hammer union to remove the top of the receiver. 
   To lift the pump so that the latch  62  catches the pump, the lower discharge line is shut-off by the shut-off valve  82 . As shown in  FIG. 1 , during normal use, the fluid pressure in the well below the pump suspends the pump so that the lower cup  41  is just above the lower discharge line. By shutting the lower shut-off valve  82 , the fluid pressure in the well pushes the pump further up until the lower cup  41  is just above the upper discharge line  70 . In this position, the stem is in the receiver far enough for the latch  62  to engage the groove in the stem  18 . 
     FIG. 1  illustrates the pumping device at the well head just below the receiver. As the pump is rising, but before it reaches the lower discharge line  80 , the liquid carried to the surface above the pump is discharged through the lower discharge line. As the amount of liquid above the pump decreases, the positive pressure differential across the pump increases, thereby accelerating the pump into the receiver assembly  60 . The gas and liquid remaining above the friction cups  40 ,  41 , cushion the pump as the pump enters the receiver. The gas and liquid remaining above the pump in the receiver exhaust through the upper discharge line  70 , which is isolated from the lower discharge line by a check valve  75 . The check valve  75  prevents fluid flow from the lower discharge line  80  back into the receiver  60  above the pump. 
   When the friction cups  40 ,  41  pass above the lower discharge line  80 , the shut-in well gas pressure discharges into the lower discharge line. The flow control valve  20  in the pump remains in the closed position as the shut-in pressure dissipates. The check valve  75  provides separation between the pressure above the pump (i.e. above the friction cups  40 ,  41 ) and the dissipating shut-in pressure in the lower discharge line  80 , thereby maintaining a positive pressure differential across the pump  10 . The gas pressure in the well is sufficient to support the pump to maintain it in the receiver until the valve  20  opens. As the fluid pressure in the well decreases below the preset pressure differential across the pump (from the high shut-in pressure), the flow control valve  20  opens. When the valve is opened, the pressure differential across the pump approaches zero and the pump descends into the well for additional liquid pumping. 
   The pressure differential between the valve opening pressure and the valve closing pressure corresponds to the amount of liquid that the pump  10  can pump out of the well. The greater the pressure differential, the greater the amount of liquid can be pumped out in a single pump stroke. Therefore, it is desirable to maximize the pressure differential between the valve opening pressure and the valve closing pressure. 
     FIG. 7  illustrates the relationship between control valve  20  opening pressure and control valve closing pressure as a function of the pressure above the control valve (i.e., above the friction cups  40 ,  41 ) at the time of control valve opening. The value at which the curve intercepts the left axis (Popen/Pclose) is a function (to a first order) of the ratio of the bellows effective cross sectional area to the cross sectional area of the valve seat  23 . It is independent of bellows charge pressure, spring forces, bellows stroke, etc. For this reason a large valve seating area and a small bellows cross sectional area will provide improved (lower) control valve opening pressures. The dashed line illustrates a 10% reduction in valve ( 5 ) cross-sectional seating diameter. Accordingly, preferably the cross-sectional area of the bellows  35  is less than the cross-sectional area of the valve seat  23 . More specifically, preferably the ratio of the cross-sectional area of the bellows divided by the cross-sectional area of the valve seat is within the range of 0.15 to 0.5. 
   The amount of liquid that will be lifted to the surface on subsequent pumping runs is determined from the selected control valve closing pressure (for a given design implementation, this is controlled by the bellows charging pressure and the bellows stroke) reduced by the valve opening pressure (controlled by the valve cross sectional seating area, which can be adjusted by stroke or shim washers) but increased by the decrease in casing pressure during the downward transit of the pump. The resulting pressure is related to the height of the column of liquid to be pumped by the density of the liquid (typically for salt water about 100 feet of liquid in a 4 inch casing per each 44 PSIG). 
   It will be recognized by those skilled in the art that changes or modifications can be made to the above-described embodiments without department from the broad inventive concept of the invention. It should therefore be understood that this invention is not limited to the particular embodiments described herein but is intended to include all changes and modifications that are within the scope and spirit of the invention as set forth in the following claims.