Abstract:
A bearing assembly for a centrifugal pump is configured to accommodate both forward and reverse axial thrust, and to be lubricated by process fluid. A pump incorporating a dual acting bearing assembly that is lubricated by process fluid and that is located between the final two impeller stages of the pump. A desalination system comprises a pump with an inlet for receiving seawater at a first end, an outlet for discharging high pressure seawater at a second end, one or more impellers coupled to a rotatable shaft between the first end and the second end for increasing a pressure of the seawater, and a bearing assembly lubricated by the seawater for absorbing axial thrust in both directions along the shaft.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates generally to thrust bearings for centrifugal pumps. In particular, the present disclosure relates to process fluid-lubricated thrust bearings for accommodating both forward and reverse axial thrust in centrifugal pumps. 
       BACKGROUND 
       [0002]    Centrifugal pumps comprise rotating elements within a pump casing that increase the pressure of fluids traveling from an inlet port to a discharge port. One or more impellers are mounted on a shaft within the pump casing and increase the pressure on the fluid as the fluid travels through the casing. A motor drives the shaft to provide the rotating movement. 
         [0003]    Some types of centrifugal pumps, which may be referred to as barrel pumps, use a plurality of impeller stages mounted on a common shaft. The impellers face the same direction and successively increase the pressure of process fluid as it flows through the stages. 
         [0004]    Various forces act on the impeller shaft during operation of the pump. Forces acting along a direction parallel to the shaft&#39;s axis of rotation may be referred to as axial forces, and forces acting perpendicular to the axis may be referred to as radial forces. A long shaft must typically be radially supported at intermediate points along its length to prevent excessive sagging or curvature. Many pumps utilize bushings that closely fit around the shaft are mounted between the impellers to counteract radial forces and to maintain the desired radial position of the shaft. 
         [0005]    Axial thrust also acts on the impellers during operation. The axial thrust is additive for each impeller, and very strong axial forces may develop along the shaft, depending on the number of stages. Each stage comprises an impeller (i.e. a rotating element coupled to be driven by the shaft) and a diffuser (i.e., a stationary element within the casing to promote smooth flow of fluid). All of the stages are housed in the casing. An axial force generated by the difference between the low pressure at the inlet of the pump and the high pressure at the outlet of the pump can also act on the shaft in the same direction as the axial thrust generated by each impeller. Axial forces in this direction may be referred to as active loads. 
         [0006]    Pumps used in some systems, such as for example reverse osmosis systems, can also be subjected to axial force in the opposite direction, which is sometimes referred to as a “reverse load”. Such reverse load can occur, for example, when the pump is turned off, if the pressure downstream of the pump drops rapidly, or in other situations. For example, reverse loads can occur if a pump is started accidentally or improperly (e.g., with a discharge valve fully open), or if a check valve downstream from the pump in a reverse osmosis system is malfunctioning. 
         [0007]      FIG. 1  shows an example of a basic desalination system  10  according to the prior art. Low pressure untreated water  11 , which may for example be sea water, is provided to the inlet port of a high pressure pump  12 . The pump  12  increases the pressure of the water to provide high pressure untreated water  13  to a reverse osmosis (RO) unit  14 . In large scale desalination systems, the pressure at the outlet port of the pump  12  may be several hundred psi or more greater than the pressure at the inlet port of the pump, so as to overcome the osmotic pressure of salt in sea water and maximize the output of treated water. The RO unit  14  outputs treated water  15  with a relatively low salt concentration and condensate  16  with a relatively high salt concentration. During normal operation, the shaft of the pump  12  is subjected to an active load in the direction indicated by arrow  17 . The shaft of the pump  12  may also be periodically subjected to a reverse load in the direction indicated by arrow  18 . The pump  12  typically has a bearing assembly (not shown) for accommodating active loads, and a pressure bleed-off circuit  19  that provides the discharge pressure at or near the outlet to a balancing drum or disk closer to the inlet of the pump in order for accommodating reverse loads. 
         [0008]    The condensate  16 , which exits the RO unit  14  with a relatively high pressure, optionally passes through an energy recovery device (ERD)  21 , and exits through a valve  22  as low pressure condensate  23 . The ERD  21  uses the pressure of the high pressure condensate  16  to increase the pressure of a secondary stream of untreated water  11 A, which exits the ERD  21  as high pressure untreated water  24 , which joins with the high pressure untreated water  13  from the pump  12  and is provided to the RO unit  14 . A circulation pump  25  may be provided to circulate the high pressure untreated water  24 . Such a system may be more susceptible to reverse loading because of the pressure of the high pressure untreated water  24 . 
         [0009]    U.S. Pat. No. 6,309,174 discloses a thrust bearing for multistage centrifugal pumps that has a balance disk coupled to the pump shaft within a bearing cavity separated from a discharge chamber of the pump by a sidewall. Throttle ports are provided through the sidewall, which are configured to control the flow rate of process fluid therethough as the balance disk moves axially to allow pressure to build up and be relieved from a pressure cavity between the balance disk and the sidewall to control the axial position of the balance disk. Sealing lands on the sidewall are provided to take up axial loads not accommodated by the fluid pressure on the balance disk. Such a bearing has only limited capacity to take up reverse loads, which are only accommodated by the fluid pressure on the balance disk. 
         [0010]    The inventor has determined a need for pumps with improved bearing assemblies. 
       SUMMARY 
       [0011]    This specification will describe a water bearing assembly adapted to resist both forward and reverse thrust. This bearing assembly may be used, for example, in a high pressure turbine/barrel pump. In this context, the bearing assembly provides an alternative to oil lubricated pumps, which require more maintenance than water bearings. The inventor has observed that prior art systems utilizing oil lubricated thrust bearings typically require a bearing frame assembly to retain the lubrication oil, and can be expensive to maintain. Further, the oil must generally be replaced regularly, due to contamination such as for example by dust and/or condensation of environmental moisture. Sometimes the bearing frame assembly must also be replaced. Dual acting bearing assemblies according to certain embodiments are lubricated by the process fluid flowing in the pump, and advantageously avoid problems associated with oil lubricated bearings. 
         [0012]    This specification also describes a desalination system that uses a pump with the dual acting water bearing system. This desalination system does not depend on a pressure bleed-off circuit to protect the pump from reverse loads. Optionally, the pressure bleed-off circuit might even be removed. The inventor has observed that bleed-off circuits can deteriorate over time (for example the reaction time of the circuit may increase due to clogging caused by water contamination, etc.), causing damage to a conventional pump. The bleed-off lines have to be chemically cleaned periodically involving maintenance costs and lost productivity due to downtime. If left unattended, blockage in the bleed-off lines accumulates over time until the pump fails unexpectedly. The inventor believes that a dual acting bearing assembly as described herein could increase the reliability of a desalination system. 
         [0013]    Although the reverse thrust is less than the forward thrust, the bearing system may be symmetrical. In this way, there is no chance of the bearing being installed in a reverse orientation that would lead to pump failure. 
         [0014]    Further aspects and details of example embodiments are set forth below. 
     
    
     
       DRAWINGS 
         [0015]    The following figures set forth embodiments in which like reference numerals denote like parts. Embodiments are illustrated by way of example and not by way of limitation in the accompanying figures. 
           [0016]      FIG. 1  shows an example of a desalination system with a high pressure pump and a reverse osmosis unit according to the prior art. 
           [0017]      FIG. 1A  shows an example of desalination system with a high pressure pump and a reverse osmosis unit according to one embodiment of the invention. 
           [0018]      FIG. 2  is a block diagram of an example high pressure pump according to one embodiment of the invention. 
           [0019]      FIG. 3  is an axial view of an example thrust bearing for the pump of  FIG. 3 . 
           [0020]      FIG. 4  is a sectional view taken along line A-A of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    For simplicity and clarity of illustration, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. Numerous details are set forth to provide an understanding of the examples described herein. The examples may be practiced without these details. In other instances, well-known methods, procedures, and components are not described in detail to avoid obscuring the examples described. The description is not to be considered as limited to the scope of the examples described herein. 
         [0022]      FIG. 1A  shows a desalination system  20  according to one embodiment of the invention. The desalination system  20  of  FIG. 1A  is similar to the prior art system  10  discussed above, except that the system  20  of  FIG. 1A  has a high pressure pump  100  according to one embodiment of the present invention which comprises a dual acting bearing assembly. A pressure bleed-off circuit  19  is thus not required in the system  20  of  FIG. 1A . 
         [0023]    As described further below, the bearing assembly of the pump  100  is preferably lubricated by the process fluid (e.g. seawater) passing therethrough. By utilizing the process fluid to provide lubrication, the need for a separate lubricant such as oil or the like is avoided. 
         [0024]      FIG. 2  schematically illustrates a centrifugal pump  100  according to one embodiment. The pump  100  receives low pressure process fluid  101  at an inlet port at or near one end of a casing  109  and discharges high pressure process fluid  102  from a discharge port at or near the other end of the casing  109 . A motor  104  rotates a shaft  106  running through the pump  100 . A plurality of impeller stages  108  successively increase the pressure of the process fluid. Each impeller stage comprises an impeller coupled to be driven by the shaft  106 , and a diffuser held stationary with respect to the casing  109 . A thrust bearing assembly  110  is positioned between the final two impeller stages  108 . Details of an example implementation of the thrust bearing assembly are discussed below with reference to  FIGS. 3 and 4 . 
         [0025]    Thrust bearing assembly  110  is housed within a casing  109  of the pump, and located downstream from of a next to last impeller stage  108 A and upstream from a last impeller stage  108 B of the pump. Portions of a diffuser  108 D and an impeller  1081  of each of the next to last and last impeller stages  108 A and  108 B are shown in  FIG. 4 . The bearing assembly  110  comprises a thrust collar  112  mounted on the shaft  106 , and retained within a central cavity of a bearing housing  120 . The bearing housing  120  is held stationary with respect to the casing  109 , and may be sandwiched between the diffusers  108 D of the next to last and last impeller stages  108 A and  108 B. In the illustrated embodiment, the bearing assembly  120  has a first side  120 A and a second side  120 B, which may be separate pieces. 
         [0026]    A plurality of flow ports  122  are formed in the bearing housing  120  around a radial periphery of the central cavity (i.e., the flow ports  122  are positioned around the outside of the bearing housing, between the central cavity and the casing  109 ). Each of the flow ports  122  provides a main flow path  124  to allow process fluid to pass through the bearing assembly  110 . The combined cross-sectional area of the flow ports  122  is sufficient to permit process fluid to flow smoothly between the final two stages of the pump  100 . 
         [0027]    An annular cavity  123  is also formed in the bearing housing  120 . The annular cavity  123  is positioned around the radial periphery of the central cavity, and provides fluid communication between the flow ports  122  and the central cavity. 
         [0028]    Because the bearing assembly  110  is between the final two stages  108 A and  108 B of the pump  100 , the pressure of the fluid in the flow ports  122  is relatively high during operation of the pump  100 . This high pressure assists in maintaining a lubricating film for the bearing assembly  110 , as discussed further below. Also, positioning the bearing assembly  110  between the final two stages  108 A and  108 B of the pump  100  facilitates installation and maintenance of the bearing assembly  110 , since only the last impeller stage  108 B would need to be removed or disassembled to access the bearing assembly  110 . 
         [0029]    A plurality of first side lubrication ports  126 A provide lubrication flow paths  128 A for process fluid from the flow ports  122  to enter the central cavity from the first side. A plurality of second side lubrication ports  126 B provide lubrication flow paths  128 B for process fluid from the flow ports  122  to enter the central cavity from the second side. Process fluid also enters the central cavity from the annular cavity  123 , as indicated by additional lubrication flow paths  129 A and  129 B. First and second side lubrication nozzles  130 A and  130 B are provided on the first and second sides  120 A and  120 B of the bearing housing  120  where the lubrication ports  126 A and  126 B meet the central cavity. 
         [0030]    First and second side thrust pads  132 A and  132 B are provided on the first and second sides  120 A and  1208  of the bearing housing  120  facing toward the central cavity. The thrust pads  132 A and  132 B preferably comprise a friction-reducing material, such as for example a polyether ether ketone (PEEK) polymer or the like. In some embodiments, the thrust pads  132 A and  132 B may comprise solid blocks of a blend of bearing grade PEEK polymer, or other friction reducing material. In other embodiments, the thrust pads  132 A and  132 B may comprise metallic bases with a layer of friction reducing material sintered or otherwise deposited on the faces of the thrust pads  132 A and  132 B facing toward the thrust collar  112 . 
         [0031]    The spacing between the first side thrust pads  132 A and the second side thrust pads  1328  is slightly larger than an axial thickness of the thrust collar  112 , such that the pads  132 A and  132 B constrain axial movement of the thrust collar  112  in either direction. Thus both active and reverse loading on the shaft  106  is accommodated. 
         [0032]    The lubrication nozzles  130 A and  130 B direct process fluid from the lubrication flow paths  128 A and  1288  toward the interface between the thrust collar  112  and the respective thrust pads  132 A and  132 B, such that thin film of process fluid is maintained between the thrust collar  112  and the pads  132 A and  132 B. Process fluid from the annular cavity  123  also reaches the interface between the thrust collar  112  and the thrust pads  132 A and  132 B, as indicated by the additional lubrication flow paths  129 A and  129 B. The process fluid provides lubrication and also cooling at the interface between the thrust collar  112  and the pads  132 A and  132 B. 
         [0033]    The particular details of construction of the bearing assembly may vary in a number of ways. For example, the flow ports and the lubrication ports could have different shapes and locations than those shown in the drawings. In some embodiments, a different number of pads could be located on either side of the thrust collar. In some embodiments, two separate thrust collars or other annular rotatable elements could be provided, one for accommodating force in each direction. In some embodiments, separate bearings may be provided for accommodating force in each direction. In some embodiments, instead of the whole bearing assembly being located in the same part of the pump, a bearing for accommodating force in one direction could be in one part of the pump and a bearing for accommodating force in the other direction could be located in another part of the pump. 
         [0034]    The example embodiment shown in the figures has a number of advantages. For example, it is a single unit, making economical use of a single collar. Both the forward and reverse thrust pads and the collar are located in a part of the pump likely to have sufficient water pressure to maintain a film of lubricating water when required. In some embodiments, the bearing assembly is symmetrical so as to avoid reverse assembly errors and provide a single part that can be specified with a single max pressure in either direction, as opposed to being custom made for other applications with a different ratio of forward to reverse thrust. 
         [0035]    Bearing assemblies according to certain embodiments of the invention advantageously avoid the need for external lubrication or cooling systems. Further, due to the placement of the bearing assembly between the final two stages of the pump, the bearing assembly is always flooded with process fluid even during startup or sudden stops of the pump. Such bearing assemblies may be particularly advantageous when applied in high pressure pumps used in reverse osmosis systems for desalination of seawater. 
         [0036]    The present disclosure may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive.