Patent Publication Number: US-7708534-B2

Title: Pressure equalizer in thrust chamber electrical submersible pump assembly having dual pressure barriers

Description:
FIELD OF THE INVENTION 
   This invention relates in general to submersible well pump assemblies, and in particular to a pressure equalizer that reduces the pressure differential between lubricant in the motor and the exterior hydrostatic pressure in the well bore. 
   BACKGROUND OF THE INVENTION 
   Electrical submersible pumps are used to convey large volumes of fluid from wells typically for hydrocarbon production. Normally an electrical submersible pump assembly will comprise a rotary pump and a downhole electrical motor. The rotary pump may be a centrifugal pump made up of a large number of centrifugal stages of impellers and diffusers. Alternately, a progressing cavity pump may be utilized in some circumstances. 
   The motor is filled with a dielectric motor oil or lubricant. The motor oil expands when the temperature rises, which normally occurs when lowering a pump into a well. Also, heat of the motor during operation causes the temperature to rise. The expansion of the oil could exceed the volume capacity of the motor, causing a leak. To avoid this occurrence, a seal section is connected between the motor and the pump. The seal section has an inlet for admitting well fluid and a flexible barrier to separate the well fluid from the lubricant and equalize the pressure between the lubricant contained in the motor and the well bore fluid. The seal section has a vent that allows the motor to vent excess oil into the well environment if the volume of oil increases beyond the volume capacity of the assembly. 
   Also, commonly the seal section will have a thrust bearing to take thrust load from the pump above. Conventional seal sections thus have four basic functions: a) equalizing pressure between the well bore and inside the motor; b) providing a reservoir for the motor oil; c) compensating for the expansion of oil due to temperature increase; and d) taking the thrust load from the pump above. 
   One problem with existing seal sections is that if a leak occurs, well fluid will enter the motor and cause an electrical short, thus destroying the equipment. To avoid this occurrence, in some cases, several seal sections are coupled together, with each operating independently of the other. In that arrangement, if the top seal section should leak and fail, the underlying seal sections will continue to protect the motor. Redundant seal sections are costly, however, and only add an additional amount of time before failure eventually occurs. Often, if the top seal section fails, vibration and leakage will also cause failure out of the other seal sections in fairly short order. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a sectional, partly schematic view illustrating a pump assembly constructed in accordance with this invention. 
       FIG. 2A  is a sectional view of a thrust chamber employed in the pump assembly of  FIG. 1 . 
       FIG. 2B  is a sectional view of the top of the motor of the assembly of  FIG. 1 . 
       FIG. 2C  is a sectional view of the bottom of the motor of  FIG. 1 , and also showing an equalizing chamber. 
       FIG. 3  is a sectional view similar to  FIG. 2A , but shown in a different sectional plane. 
       FIG. 4  is a sectional view of the equalizing chamber of  FIG. 1 , shown along the line  4 - 4  of  FIG. 2C . 
       FIG. 5  is a side elevational view of an alternate embodiment of a pressure equalizing barrier. 
       FIG. 6  is another side elevational view of the barrier of  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a well is illustrated as having a casing  11 . A string of production tubing  13  is lowered into casing  11 . A rotary pump  15  is attached to the lower end of tubing  13  for delivering well fluid up tubing  13 . Pump  15  is typically a centrifugal pump having a large number of stages, each stage having an impeller and a diffuser. Alternately, pump  15  can be other types, such as a progressive cavity pump. Pump  15  has an intake  17  that draws well fluid into pump  15 . 
   A thrust unit  19  is connected to the lower end of pump  15 . An electrical motor  21  is secured to the lower end of thrust unit  19 . Motor  21  is normally a three-phase electrical motor supplied with power from a power cable  23  extending down from the surface. A pressure equalizing assembly  24  is secured to the lower end of motor  21 . 
   Referring to  FIG. 2A , thrust unit  19  has a housing  25 . A conventional thrust bearing assembly for absorbing thrust from pump  15  ( FIG. 1 ) is contained within housing  25 . The thrust bearing assembly will normally have a rotating bearing member or runner  27  and a stationary downthrust base  29 . Since upthrust can occur with pump  15  ( FIG. 1 ), the thrust bearing assembly may also have an upthrust base  31  mounted above thrust runner  27 . Thrust runner  27  is mounted to a thrust chamber shaft  33  for rotation therewith. 
   Thrust chamber shaft  33  is coupled to the lower end of pump shaft  35 . The lower end of thrust chamber shaft  33  is coupled to motor shaft  37 . A connector  39  between pump shaft  35  and thrust chamber shaft  33  transmits torque and compression and optionally tension. A connector  41  similarly serves to transmit torque, compression and optionally tension between motor shaft  37  and thrust chamber shaft  33 . 
   Thrust unit  19  has a head section  43  that secures to the upper end of housing  25 . Also, a base section  45  secures to the lower end of thrust unit housing  25 . An upper seal assembly  47  forms a seal around thrust chamber shaft  33  to prevent the entry of well fluid from cavity  53  into thrust unit housing  25 . Similarly, a lower seal assembly  49  is located within base section  45  for sealing around thrust chamber shaft  33 . A set of bushings  51  within head section  43  and within base section  45  provide radial stability for shaft  33  but do not form seals. 
   Upper cavity  53  within thrust unit head section  43  will be filled with well fluid during operation. A lower cavity  55  surrounds lower seal assembly  49 . The various spaces between upper and lower seals  47 ,  49  may be considered part of a thrust chamber  57  containing thrust chamber lubricant  58 . Thrust chamber lubricant  58  serves to lubricate the bearing components  27 ,  29 ,  31  and  51 . Thrust unit  19  has a fill port  59  for filling thrust chamber  57  with thrust chamber lubricant  58 . The filling may be conventional, optionally using a vacuum pump to first evacuate air. Additionally, a port having a check valve  63  allows excess thrust chamber lubricant  58  to be expelled to the exterior during filling and during operation. In this example, check valve  63  is positioned at the upper end of thrust chamber  57  close to upper seal assembly  47 . 
     FIG. 3 , which shows thrust unit  19  taken from a different sectional plane than  FIG. 2A , illustrates a passage  65  that communicates thrust chamber lubricant  58  from the thrust bearing area to a point just above lower seal  49 . Thrust chamber shaft  33  has a passage  67  that extends axially upward within shaft  33  from the bottom of shaft  33  to a point a short distance above lower seal  49 . Cross-drilled ports  69  extend through shaft  33  near the upper end of passage  67  just above lower seal  49 . Passage  65  communicates thrust chamber lubricant  58  to shaft axial passage  67  via ports  69 . An axial passage  71  extends through lower connector  41  for communication with thrust chamber shaft passage  67 . Connector  41  has seals  73  that prevent any leakage of thrust chamber lubricant  58  to cavity  55 . 
   Cavity  55  is filled with a motor lubricant  75 , as illustrated in  FIG. 2A . Motor lubricant  75  is a dielectric oil that is contained within motor  21  ( FIG. 1 ). Motor lubricant  75  and thrust chamber lubricant  58  may be the same type of lubricant. Alternately, they may have different properties for their different functions. A motor shaft passage  77  extends axially through motor shaft  37  from the lower end to the upper end. Thrust chamber lubricant  58  is able to migrate downward and upward through motor shaft passage  77 . 
   Referring to  FIG. 2B , a motor head  79  is shown. Head  79  has bushings  81  for radially stabilizing motor shaft  37 . Motor head  79  may also optionally have a motor thrust bearing  83  to absorb downthrust on motor shaft  37  due to the weight of the rotor and shaft  37  of motor  21 . A fill port  85  is shown in  FIG. 2B  for use in filling motor  21  with motor lubricant  75 . The filling is handled conventionally. Fill port  85  joins a passage  87  that extends from the chamber containing motor thrust bearing  83 . Motor lubricant  75  located within passage  87  communicates into thrust unit lower cavity  55  ( FIG. 2A ), but is sealed from thrust chamber lubricant  58  by seals  73 . 
   Referring still to  FIG. 2B , motor  21  has a housing  89 , the lower portion of which is shown in  FIG. 2C . Housing  89  contains electrical components of motor  21 , such as the rotor and the stator, which includes windings. Referring to  FIG. 2C , an adaptor  91  secures to the lower end of motor housing  89 , such as by threads as shown. Adapter  91  has a tubular neck  93  extending upward. A set of radial bushings  95  provides support for the lower end of motor shaft  37 . Adapter  91  has a central cavity  97  into which the lower end of motor shaft  37  extends. Cavity  97  will be filled with thrust chamber lubricant  58  via passage  77  in motor shaft  37 . A lip seal  99  seals around the lower portion of motor shaft  37  to prevent thrust chamber lubricant  58  from leaking past bushings  95  into motor housing  89 . 
   In this example, pressure equalizer  24  is a separate unit attached to the lower end of motor housing  89 , but it could be incorporated within motor housing  89 . Equalizer  24  includes a housing  101  that has an inlet port  103  for admitting well fluid. In this embodiment, inlet port  103  is located on the bottom of housing  101 , but it could be located elsewhere. The equalizing components include an outer flexible barrier  105  located within housing  101 . Outer barrier  105  in this example is a thin-walled metal container. Outer barrier  105  has a bottom  107  having a fill port that receives a plug  109 . Outer barrier  105  has a rim  111  on its upper end that joins outer barrier  105  to adapter  91 . Rim  111  is retained by a collar  113  that has a shoulder on which rim  111  rests and internal threads that secure to external threads on adapter  91 . 
   An inner barrier  115  is located within outer barrier  105  in this example. Inner barrier  115  has the same configuration but a smaller diameter as well as length. Inner barrier  115  also has a closed bottom  117 . Bumpers  118  may be located on the bottoms  107 ,  117  to avoid vibration damage. In this example, outer and inner barriers  105 ,  115  are generally elliptical in shape between the upper and lower ends. This shape facilitates the walls of outer and inner barriers  105 ,  115  flexing radially. Barriers  105 ,  115  collapse and expand radially in response to pressure changes. 
   Inner barrier  115  is retained by an inner barrier adapter  119  at its upper end. Inner barrier adapter  119  is secured into the lower end of cavity  97  of adapter  91 . Seals on the exterior of inner barrier adapter  119  prevent thrust bearing lubricant  58  from leaking around the sides of inner barrier adapter  119  into inner barrier  115 . A passage  121  extends from cavity  97  through adapter  91 , rim  111  and into outer barrier  105  at a point between inner barrier  115  and outer barrier  105 . A plug  123  blocks passage  121  from the exterior. 
   A motor lubricant passage  125  extends from the interior of motor housing  89  downward in through adapter  119  to the interior of inner barrier  115 . A fill port  127  communicates with motor lubricant passage  125  for filling motor housing  89  with motor lubricant  75 . During filling, fill port  127  is used in combination with another port at the upper end of motor  21 , such as port  85  ( FIG. 2B ) to determine when the spaces for motor lubricant  75  are full. A vent passage  131  extends from the lower end to the upper end of adapter  119  in communication with cavity  97 . Vent port  131  contains one or more check valves  133  that allow the upward flow of motor lubricant  75  if the pressure differential is sufficient to open check valves  133 , Check valves  133 , however, will not allow any flow of thrust chamber lubricant  58  downward through vent port  131 . 
   In operation, motor  21  and inner barrier  115  will be filled with motor lubricant  75 . Thrust unit  19  and outer barrier  105  will be filled with thrust chamber lubricant  58 . Thrust chamber lubricant  58  will also occupy passage  121 , cavity  97  and motor shaft passage  77  ( FIG. 2C ). Motor lubricant  75  will be located in motor  21  and thrust unit cavity  55  ( FIG. 2A ). The operator lowers the pump assembly into the well on tubing  13 . The well temperature will cause motor lubricant  75  and thrust chamber lubricant  58  to expand. When the operator begins supplying power to motor  21 , heat from the motor further increases the temperature of the pump assembly, and causes more expansion of thrust chamber lubricant  58  and motor lubricant  75 . When the lubricant spaces are full, continued thermal expansion increases the pressure differential of thrust chamber lubricant  58  and motor lubricant  75  over the wellbore pressure. When the pressure differential reaches a selected level, excess motor lubricant  75  will vent through port  131  ( FIG. 2C ) into cavity  97 , thus commingling with thrust chamber lubricant  58 . Excess thrust chamber lubricant  58  will also vent, not only to accommodate the additional motor lubricant  75  that was vented into cavity  97 , but also because of the expansion of thrust chamber lubricant  58 . Thrust chamber lubricant  58  vents to the wellbore through check valve  63  shown in  FIG. 2A . 
   The hydrostatic pressure of the well fluid will be communicated to thrust chamber lubricant  58  and motor lubricant  75  via port  103  ( FIG. 2C ). The well fluid will locate on the exterior of outer barrier  105 . The pressure of the well fluid will act on the flexible outer barrier  105  to increase the pressure of thrust chamber lubricant  58  to approximately that of the wellbore. The increased pressure in outer barrier  105  acts on inner barrier  115  in a similar manner, causing the pressure in inner barrier  115  to increase. During operation, the internal pressures of thrust chamber lubricant  58  and motor lubricant  75  will be approximately the same and substantially equal to the hydrostatic pressure of the well fluid in the wellbore. 
   When motor  21  is turned off, motor  21  and thrust unit  19  will cool, allowing the lubricants  58  and  75  to shrink in volume. The original volume of lubricant in both the thrust unit  19  and motor  21  is less now because some was vented during the initial startup. The decrease in volume of lubricants  58 ,  75  could cause a vacuum to occur inside motor  21  and thrust unit  19 . If a vacuum were allowed to persist, well fluid could be pulled past the O-rings and mechanical seals  47 ,  49 , which could contaminate motor  21 . The flexibility and elliptical shape, however, of the inner and outer barriers  115 ,  105  prevent this potential problem from occurring. A vacuum produced during cool-down causes inner and outer barriers  105 ,  115  to collapse to a lesser volume that accounts for the amount of lubricant  75 ,  58  previously expelled to the wellbore. This collapsing will re-equalize the negative pressure differential. When motor  21  is started again, barriers  115 ,  105  will expand again as lubricants  75  and  58  expand. In most cases, motor  21  and thrust unit  19  will resume a previous operating temperature, therefore no additional lubricant will be discharged through the check valves. 
     FIGS. 5 and 6  illustrate an alternate embodiment for at least one or both of the thin-walled metal barriers  105 ,  115 . In this embodiment, the barrier comprises a flexible bag  135 , which is constructed from a strong engineering fabric, such as Kevlar or woven carbon fiber. Bag  135  is impervious, which is achieved by impregnation of the fabric with high temperature elastomeric materials, such as fluoroelastomers or fluoropolymers. These compounds penetrate and embed within the internal fibers of the fabric, rather than being separate layers or coatings over the fabric. The impermeability of the fabric at high temperatures is retained regardless of any decreased mechanical properties of the impervious material used for impregnation. The strength of bag  135  is provided by the fabric, while the impervious properties are conferred by the infused fluoroelastomer or fluoropolymer compounds. 
   Bag  135  has a mouth that will clamp to outer rim  111  ( FIG. 2C ) and a closed lower end  143 , which may be a seam as shown in  FIG. 6  or a spherical or flat bottom. Bag  135  could be located within the metal-walled outer barrier  105  ( FIG. 2C ), or bag  135  could be located within an elastomeric and fabric bag of similar construction. Also, bag  135  could be utilized alone, without another bag, if one chose to use the same lubricant in thrust unit  19  ( FIG. 1 ) and motor  21  and to allow the lubricant to commingle throughout thrust unit  19  and motor  21 . 
   The invention has significant advantages. If the thrust unit should leak, the thrust bearings and radial bushings will continue to operate in well fluid. Additionally, since the thrust chamber lubricant is completely isolated from entry into the spaces for the motor lubricant, the motor will not be contaminated even if the thrust unit develops a leak. This assembly will function at extreme temperatures and is only limited by the capabilities of the lubricant and the insulation of the electrical motor. 
   While the invention has been shown in only two of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes without departing from the scope of the invention. For example, rather than thin wall metal barriers and elastomeric/fabric bladders, bellows with accordion sidewalls could be employed.