Patent Publication Number: US-8523533-B1

Title: Constant horsepower regenerative assist for a hydraulic rod pumping unit

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to pump units for oil wells, and in particular to a hydraulic pumping unit having a regenerative assist. 
     BACKGROUND OF THE INVENTION 
     Hydraulic pumping units have been provided for pumping fluids from subterranean wells, such as oil wells. The pumping units have hydraulic power units and controls for the hydraulic power units. The hydraulic power units have an electric motor or a gas motor which powers a positive displacement pump to force hydraulic fluid into a hydraulic ram. The ram is stroked to an extended position to lift sucker rods within a well and provide a pump stroke. The ram lifts the weight of the sucker rods and the weight of the well fluids being lifted with the sucker rods. When the ram reaches the top of the pump stroke, the hydraulic fluid is released from within the ram at a controlled rate to lower the weight of the sucker rods into a downward position, ready for a subsequent pump stroke. The hydraulic fluid is released from the ram and returns to a fluid reservoir. Potential energy of the weight of the lifted sucker rods is released and not recovered when the hydraulic fluid is released from within the ram and returns directly to the fluid reservoir without being used to perform work. 
     Hydraulic assists are commonly used in hydraulic well pumping units to assist in supporting the weight of the sucker rods. Hydraulic accumulators are used in conjunction with one or more secondary hydraulic rams which are connected to primary hydraulic rams to provide an upward support force. The hydraulic accumulators are provided by containers having hydraulic fluids and nitrogen pre-charges ranging from one to several thousand pounds per square inch. Although the volumes of the containers are constant, the volume of the nitrogen charge region of the containers will vary depending upon the position of the ram piston rod during a stroke. At the top of an up-stroke of the ram, the nitrogen charge region of a connected accumulator will have the largest volume, with the nitrogen having expanded to push hydraulic fluid from within the accumulator and into the secondary rams. At the bottom of a down-stroke the nitrogen charge region will be at its smallest volume, compressed by hydraulic fluid being pushed from the secondary rams back into the accumulator. According to Boyle&#39;s Law, the pressure in the charge region is proportional to the inverse of the volume of the charge region, and thus the pressure will increase during the up-stroke and decrease during the up stroke. This results in variations in the amount of sucker rod weight supported by the secondary hydraulic rams during each stroke of the ram pumping unit. 
     Drive motors for hydraulic pumps are sized to provide sufficient power for operating at maximum loads. Thus, motors for powering hydraulic pumps for prior art accumulator assisted pumping units are sized for lifting the sucker rod loads when the minimum load lifting assist is provided by the accumulator and the secondary ram. Larger variations in accumulator pressure and volume between the top of the up-stroke and the bottom of the down-stroke have resulted larger motors being required to power the hydraulic pump connected to the primary ram than would be required if the volume and pressure of the nitrogen charge section were subject to smaller variations. Large motors will burn more fuel or use more electricity than smaller motors. Several prior art accumulator containers may be coupled together to increase the volume of the nitrogen charge region in attempts to reduce variations in pressure between top of the up-stroke and the bottom of the down-stroke. This has resulted in a large number of accumulator containers being present at well heads, also resulting in increasing the number of hydraulic connections which may be subject to failure. 
     SUMMARY OF THE INVENTION 
     An assist for a hydraulic rod pumping unit is disclosed which does not make use of secondary hydraulic rams, and which provides both downstroke energy recovery and a constant horsepower assist using smaller accumulator sizes than used in the prior art. Two variable displacement, positive displacement pumps are coupled to a single drive motor. The first pump is connected between a hydraulic fluid reservoir and a hydraulic ram for the pumping unit. The accumulator pump is connected between the hydraulic fluid reservoir and an accumulator chamber, which preferably has a nitrogen pre-charge region. The ram and accumulator pumps are connected to a control unit which automatically controls the displacement of each of the pumps and selectively determines whether each of the pumps are operable as a hydraulic motor or a hydraulic pump. Preferably, the ram and accumulator pumps are variable displacement, open loop piston, hydraulic pumps which are modified for operating in a reverse flow direction, such that the hydraulic fluid may pass from the hydraulic ram, back into the pump discharge port, through the pump, through the pump suction port and into a fluid reservoir with the drive shaft for both of the hydraulic pumps and the rotor, or drive shaft, of the drive motor turning in the same angular direction as that for pumping the hydraulic fluid into the ram. Reversing the flow direction of the hydraulic fluid through the pumps selectively uses respective ones of the pumps as hydraulic motors which provides power for turning the other pump. 
     A control unit determines actuation of the pumps for either pumping fluids or providing a hydraulic motor for turning the other pump, in combination with the power output by the drive motor. The control unit includes a microprocessor which controls hydraulic motor displacement for each pump with feedback from pump/motor displacement, pressure transducer and speed sensor. During the up stroke of the well head pumping unit, the accumulator pump is operated as a motor driven by the charge on the accumulator and the control unit increases motor displacement proportional to the pressure decrease in the accumulator charge to maintain a constant output torque or HP to assist the pump even if drive shaft speeds change During the down stroke of the well head pumping unit, the accumulator motor is operated as a pump that charges the accumulator and the hydraulic ram pump is operated as a motor driven by the down stroke rod load that drives the accumulator pump with displacement, pressure and speed feedback that decreases pump displacement proportional to the pressure increase to maintain a constant HP during re-charging of the accumulator. This results in recovery of the potential energy stored by lifting the weight of the sucker rod assembly during the ram up stroke being recovered by passing the hydraulic fluid from the ram through the ram pump in the reverse flow direction, and actuating the ram pump to act as a motor and assist the drive motor in driving the accumulator pump. The power assist provided by using the accumulator pump as a motor results in reducing the size requirements for the drive motor to power the ram pump to drive the hydraulic ram for moving the weight of the sucker rods and associated well fluids. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying Drawings in which  FIGS. 1 through 3  show various aspects for a hydraulic rod pumping unit having a constant horsepower regenerative assist, as set forth below: 
         FIG. 1  is a schematic diagram depicting a side elevation view of the hydraulic rod pumping unit during an up stroke; 
         FIG. 2  is a schematic diagram depicting a side elevation view of the hydraulic rod pumping unit during a downstroke; 
         FIG. 3  is a partial top view of the hydraulic rod pumping unit showing three hydraulic rams used in the unit; and 
         FIG. 4  is a longitudinal section view of a variable volume piston pump which is operable in both conventional flow and reverse flow directions with the motor shaft continuously moving in the direction for pumping fluid. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1 and 2  are a schematic diagram depicting a side elevation view of a hydraulic rod pumping unit  12  having a constant horsepower regenerative assist.  FIG. 1  shows the pumping unit in an up stroke, and  FIG. 2  shows the pumping unit in a down stroke. The pumping unit  12  is preferably a long stroke type pumping unit with heavy lift capabilities for pumping fluids from a well. The ram pumping unit  12  preferably has three single acting hydraulic rams  26 , a sucker rod assembly  10 , and a hydraulic power unit  14 .  FIG. 3  is a partial top view of the hydraulic rod pumping unit  12  and shows the three hydraulic rams  26  connected together by a plate  32  to which the piston rods  30  are rigidly connected. A polished rod  8  is suspended from the plate  32  by a polished rod clamp  50 , and extends through a stuffing box  6  for passing into a well head  4  and connecting to sucker rods  10  of a downhole well pump for lifting fluids from the well. 
     Each of the hydraulic rams  26  has a guide  28  and a rod  30  which reciprocate within a cylinder  42 . Preferably, the rod  30  provides the piston element within each of the hydraulic rams  26 , and the guide  28  does not seal but rather centers the end of the rod  30  and provides bearings within the cylinder  42 . The only hydraulic connection between the power unit  14  and the ram  26  is a single high pressure hose  48  which connects to a manifold plate  52 , which ports fluid between each of the rams  26  and the hose  48 . The hydraulic power unit  14  includes a drive motor  16 , two variable volume piston pumps  18  and  20 , a fluid reservoir  22 , a hydraulic accumulator  24 , and a control unit  44 . The drive motor  16  may be an electric motor, or a diesel, gasoline or natural gas powered engine. The control unit  44  preferably includes a motor control center and a microprocessor based variable speed pump down system. The hydraulic accumulator  24  preferably is of a conventional type having a nitrogen charge region which varies in volume with pressure. The pump down system monitors the polished rod load and position to make appropriate speed adjustments to optimize production from the well while keeping operational costs at a minimum. The ram pump  18  and the accumulator pump  20  preferably each have a pump control unit  46  mounted directly to respective ones of the associated pumps housings. Valves  96  and  98  are provided for preventing hydraulic fluid from draining from the hydraulic rams  26  and the accumulator  24 , respectively, when the drive motor  16  is not running. 
     The control unit  44  and the two pump control units  46  are provided for controlling operation of the pump  18  and the pump  20 . The control unit  44  is preferably a microprocessor-based controller which is provided sensor inputs for calculating the stroke position of the piston rod  30  of the ram  26 , and the polished rod load. The polished rod load is calculated from the measured hydraulic pressure and the weight of the sucker rods  10  at the well head  4 . The control unit  44  will feed control signals to the pump control units  46 , to vary the flow rate through respective ones of the pump  18  and the pump  20 . The pump control units  46  are integral pump controllers which are preferably provided by microprocessor-based units that are mounted directly to respective ones of the pumps  18  and  20 , such as such a Model 04EH Proportional Electrohydraulic Pressure and Flow Control available from Yuken Kogyo Co., Ltd. of Kanagawa, Japan, the manufacturer of the pumps  18  and  20  of the preferred embodiment. The Yuken Model 04EH pump controller includes a swash plate angle sensor and a pump pressure sensor, and provides control of each of the swash plate angles C and D (shown in  FIG. 3 ) to separately control the pressure outputs and the flow rates of the hydraulic fluid through respective ones of the pumps  18  and  20 . 
       FIG. 4  is a longitudinal section view of the variable volume piston pump used for both the pump  18  and the pump  20 . The pump is operable in both a conventional flow direction mode and a reverse flow direction mode, with a drive shaft  56  of the pump  18  and the rotor of the drive motor  16  continuously turning in the same angular direction for both flow directions. The pump  18  has a pump housing  54  within which is the drive shaft  56  is rotatably mounted. The pump drive shaft  56  is connected to the rotor of the drive motor  16  (shown in  FIG. 1 ), in conventional fashion. A cylinder block  58  is mounted to the drive shaft  56 , in fixed relation to the drive shaft  54  for rotating with the drive shaft  56 . Preferably, a portion of the outer surface of the drive shaft  56  is splined for mating with splines in an interior bore of the cylinder block  58  to secure the drive shaft  56  and the cylinder block  58  in fixed relation. The cylinder block  58  has an inward end and an outward end. The inward end of the cylinder block  58  has a plurality of cylinders  60  formed therein, preferably aligned to extend in parallel, and spaced equal distances around and parallel to a centrally disposed, longitudinal axis  90  of the drive shaft  56 . The drive shaft  56  and the cylinder block  58  rotate about the axis  90 . Pistons  62  are slidably mounted within respective ones of the cylinders  60 , and have outer ends which are disposed outward from the cylinders for engaging retainers  62 . The retainers  62  secure the outer ends of the pistons  62  against the surface of a swash plate  66 . The outward end of the cylinder block  58  is ported with fluid flow ports for passing hydraulic fluid from within the cylinders  60 , through the outward end of the cylinder block  58 . A port plate  76  is mounted in fixed relation within the pump housing  54 , and engages the outward, ported end of the cylinder block  58 . The port plate  76  has a first fluid flow port  78  and a second fluid flow port  80 , with the first flow port  78  and the second flow port  80  connected to the pump suction port  82  and the pump discharge port  84 . The suction port  82  and the discharge port  84  are defined according to conventional operation of the pumps  18  and  20 , in moving hydraulic fluid from the fluid reservoir  22  and into the hydraulic ram  26 . The pistons  62 , the cylinders  60  and the cylinder block  58  rotate with a pump drive shaft  56 , with the outer ends of the pistons  62  engaging the swash plate  66  and the ported end of the cylinder block  58  engaging the port plate  76 . 
     The swash plate  66  is mounted to a yoke or a cradle  68 , preferably in fixed relation to the cradle  68 , with the swash plate  66  and the cradle  68  pivotally secured within the motor housing  54  for angularly moving about an axis which is perpendicular to the longitudinal axis  90  of the drive shaft  56 . A bias piston  70  is mounted in the pump housing  54  to provide a spring member, or bias means, which presses against one side of the cradle  68  and urges the swash plate  66  into position to provide a maximum fluid displacement for the pump  18  when the pump  18  is operated in conventional flow direction mode to pump the hydraulic fluid from the fluid reservoir  22  into the hydraulic ram  26 . A control piston  72  is mounted in the pump housing  54  on an opposite side of the pump drive shaft  56  from the bias piston  70  for pushing against the cradle  68  to move the cradle  68  and the swash plate  66  against the biasing force of the bias piston  70 , minimizing fluid displacement for the pump  18 , when the pump  18  operated in the conventional flow direction mode to pump the hydraulic fluid from the reservoir  22  into the hydraulic ram  26 . 
     The swash plate  66  preferably has a planar face defining a plane  86  through which extends the central longitudinal axis  90  of the pump drive shaft  56 . A centerline  88  defines a neutral position for the swash plate plane  86 , with the centerline  88  is preferably defined for the pump  18  as being perpendicular to the longitudinal axis  90  of the drive shaft  56 . When the swash plate  66  is disposed in the neutral position, the stroke length for the pistons  62  will be zero and the pump  18  will have zero displacement since the pistons  62  are not moving within the cylinder block  58 , as the cylinder block  58  is rotating with the drive shaft longitudinal axis  90 . When the swash plate  66  is in the zero stroke position, with an angle C between the swash plate plane  86  and the centerline  88  equal to zero, the pump  18  is said to be operating at center and fluid will not be moved. The angle C between the centerline  88  and the plane  80  of the swash plate  66  determines the displacement for the pump  18 . Stroking the control piston moves the cradle  68  and the swash plate  66  from the neutral position, in which the plane  86  the swash plate  66  is aligned with the centerline  88 , to a position in which the angle C is greater than zero for operating the pump  18  in the conventional flow mode to provide hydraulic fluid to the ram  26 . The larger the angle C relative to the centerline  88 , the larger the displacement of the pump  18  and the larger the volume of fluid moved by the pump  18  for a given speed and operating conditions. 
     If the plane  86  of the swash plate  66  is moved across the centerline  88  to an angle D, the pump swash plate  66  is defined herein to have moved across center for operating the pumps  18  and  20  over center as a hydraulic motor in the reverse flow mode. When the swash plate  66  is moved across center, the pumps  18  and  20  will no longer move fluid from the fluid reservoir  22  to respective ones of the hydraulic ram  26  and the accumulator  24 , but instead will move the hydraulic fluid in the reverse flow direction, either from the hydraulic ram  26  to the fluid reservoir  22  or from the accumulator  24  to the fluid reservoir  22 , for the same angular direction of rotation of the pump drive shafts  38 ,  40  and the rotor for the drive motor  16  as that for pumping hydraulic fluid into the hydraulic ram  26  or the accumulator  24 . With fluid flow through the pump  18  reversed, the pressure of the hydraulic fluid in the hydraulic ram  26  may be released to turn the pump  18  as a hydraulic motor, which applies mechanical power to the drive shafts  38  and  40  connecting between the pumps  18  and  20 , and the drive motor  16 . Similarly, with fluid flow through the pump  20  reversed, the pressure of the hydraulic fluid in the accumulator may be released to turn the pump  20  as a hydraulic motor, which applies mechanical power to the drive shafts  38  and  40  connecting between the pumps  18  and  20 , and the drive motor  16 . 
     A position sensor  36  is provided for sensing the stroke position of the rod  30  within the cylinder  42  of the ram  26 . The position sensor  36  is preferably provided by a proximity sensor which detects a switch actuator  34  to detect when the ram  26  is at a known position, such as at the bottom of the downstroke as shown in  FIG. 1 . The control unit  44  is operable to reset a calculated position to a known reference position which is determined when the sensor  36  detects the ram switch actuator  34 . Then, the control unit  44  calculates the position of the piston rod  30  within the cylinder  42  by counting the stroke of pump  18  and angle of swash plate  66  within the pump  18 , taking into account the volume of the rod  30  inserted into the cylinder  42  during the up stroke. The piston rod  30  acts as the piston element in each of the hydraulic rams  26 , such that the cross-sectional area of the piston rod  30  times the length of the stroke of the rod  30  provides the volume of hydraulic fluid displaced during the stroke length. The angle of the swash plate  66  provides the displacement of the pump  18 . The rpm at which the pump  18  is turned is known by either the synchronous speed of an electric motor, if an electric motor is used, which is most often 1800 rpm, or the speed set by the governor for a diesel or gas engine. The calculated stroke position is reset to a reference position near the bottom of the downstroke for the ram  26 . From the known angular speed and measured angle of the swash plate  66  for selected time intervals, the controller  44  calculates the total flow of hydraulic fluid through the ram pump  18  from the time the piston rod  30  is a the known reference position as detected by the proximity sensor  36 , and then determines the stroke for the piston rod according to the cross-sectional area of the piston rod  30 . 
     During operation of the pumping unit  12 , the load or weight of the piston rod  30  and the sucker rods  10  provide potential energy created by being lifted with hydraulic pressure applied to the hydraulic ram  26 . The potential energy is recaptured by passing the hydraulic fluid from the ram  26  through the hydraulic pump  18 , with the swash plate  66  for the pump  18  disposed over center such that the pump  18  acts as a hydraulic motor to apply power to the pump  20 . The control unit  44  positions the swash plate  66  at the angle D from the centerline  88 , such that the hydraulic pump  18  recaptures the potential energy stored by the raised sucker rods and powers the pump  20  to store energy in the hydraulic accumulator  24 . Then, during the up-stroke the potential energy stored in the accumulator  24  is recaptured by passing the hydraulic fluid from the accumulator  24  through the hydraulic pump  20 , with the swash plate  66  for the pump  20  disposed over center such that the pump  20  acts as a hydraulic motor to apply power to the pump  20 . The potential energy from the accumulator  23  is applied to the drive shafts  38  and  40  to assist the drive motor  24  in powering the pump  18  to power the ram  26  during the up stroke. 
     The control unit  44  will analyze data from both pressure on the hydraulic rams  26 , and from the calculated the position of the piston rod  30 , and will adjust the position of the swash plates  66  in each of the respective pumps  18  and  20  to control the motor displacement. This controls the rate of the oil metered from respective ones of the hydraulic ram  26  and the accumulator  24 , thus controlling the down-stroke speed of the ram  26 , the pump  18  and the pump  20 , which provides a counterbalance for the weight of the sucker rod assembly  10  and may be operated to provide a constant horsepower assist for the drive motor  16 . Increasing the displacement increases the speed and decreasing the displacement decreases the speed for the pump  18  and the pump  20 , controlling the horsepower assist during an up stroke of the ram  26 . During up-stroke of the hydraulic ram  26 , the drive motor  16  is operated to move the hydraulic fluid through the pump  18 , from the suction port  82  to the discharge port  84  and to the ram  26 . The up-stroke speed of the pump  18  is controlled manually or is controlled automatically by a microprocessor-based control unit  44 . During the downstroke of the hydraulic ram  26 , the pump  18  is stroked over center by moving the swash plate  66  over center, and the hydraulic fluid will flow from the ram  26  into the port  84 , through the pump  18  and then out the port  82  and into the reservoir  22 , with the pump  18  acting as a hydraulic motor to drive the drive the pump  20 , which assisted in providing provided power to the pump  18  for the up-stroke. During the downstroke, the pump  20  will similarly provide power to assist turning the pump  18 , with the control unit  44  controlling the angle of the swash plate  66  in the pump  20  and thus rate at which hydraulic fluid is released from the accumulator  24  and power is applied to the drive shafts  38  and  40 . 
     The load on the piston rod  30  at various linear positions as calculated by the controller  44  and detection of the down bottom of stroke position by the proximity sensor  36  are also analyzed by the control unit  44  to automatically provide selected up-stroke and downstroke speeds, and acceleration and deceleration rates within each stroke, for optimum performance in pumping fluids from the well head  4 . Should the well begin to pump down, the up-stroke and the downstroke speeds may be adjusted to maintain a constant fluid level within the well. The control unit  44  monitors key data and provides warnings of impending failure, including automatically stopping the pump from operating before a catastrophic failure. The load on the piston rod  30 , or the polished rod load for the sucker rods  10  at the well head  4 , is preferably determined by measuring hydraulic pressure in the hydraulic rams  26 . Sensors may are also preferably provided to allow the control unit  44  to also monitor the speed of the pump drive shafts  38  and  40  and the rotor for the drive motor  16 . 
     The hydraulic pump  18  is a variable displacement pump which is commercially available and requires modification for operation according to the present invention. Pump  18  is commercially available from Yuken Kogyo Co., Ltd. of Kanagawa, Japan, such as the Yuken model A series pumps. Other commercially available pumps may be modified for operating over center, in the reverse flow direction, such as a PD Series pump or a Gold Cup series pumps available from Parker Hannifin HPD, formerly Denison Hydraulics, Inc., of Marysville, Ohio, USA. The Gold cup series pump which uses a hydraulic vane chamber actuator for position a swash plate rather than the control piston of the Yuken model A series pump. The hydraulic vane chamber is preferably powered by a smaller hydraulic control pump connected to the drive shaft of the pumps  18  and  20 , rather than being powered by the pumps  18  and  20 . Hydraulic fluid is passed on either side of a moveable vane disposed in the vane chamber to move the vane within the chamber, and the vane is mechanically linked to a swash plate to move to swash plate to a desired position. In other embodiments, other type of actuators may be used to control the position of a swash plate relative to the centerline, such as pneumatic controls, electric switching, electric servomotor, and the like. The modifications for the pumps required for enabling operation according to the present invention are directed toward enabling the swash plates for the respective pumps to move over center, that is over the centerline, so that the pump may be operated over center in the review flow direction mode. The commercially available pumps were designed for use without the respective swash plates going over center, that is, they were designed and manufactured for operating in conventional flow direction modes and not for switching during use to operate in the reverse flow direction mode. Typical modifications include shortening sleeves for control pistons and power pistons, and the like. Internal hydraulic speed controls are also typically bypassed to allow operation over center. For the Denison Gold Cup series pumps, pump control manifolds may be changed to use manifolds from other pumps to allow operation of the pump over center. Closed loop pumps and systems may also be used, with such pumps modified to operate over center, in the reverse flow direction. 
     The hydraulic pumping unit having a constant horsepower regenerative assist provides advantages over the prior art. The pumping unit comprises a single acting hydraulic ram, without secondary rams provided for assist in lifting the sucker rod string. During a downstroke, the pumping unit provides for regeneration and recapture of energy used during the up-stroke. The sucker rod load is used during the downstroke to power a ram pump which a controller has actuated to act as a hydraulic motor and provide useable energy for driving a accumulator pump to charge an accumulator. During the up-stroke the controller actuates the accumulator pump to act as a motor and fluid released from the accumulator provides power for assisting the drive motor in powering the ram pump to raise the ram and lift the sucker rod string. Preferably, controller operates the pumps to determine the rate at which fluids flows from the ram and through the pump, such as by selectively positioning the swash plates for each of the hydraulic pumps to determine a counterbalance flow rate at which hydraulic fluid flows from the ram back into the ram pump and is returned to a reservoir, and the counterbalance flow rate at which the hydraulic fluid flows form the accumulator back into the accumulator pump and is returned to the reservoir. In other embodiments, valving may be utilized to control flow, or a combination of valving and pump controls. 
     Although the preferred embodiment has been described in detail, it should be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.