Abstract:
An integrally-geared centrifugal compressor has a drive mechanism and a gear box operatively engaged with the drive mechanism. The gear box includes a main input gear driven by the drive mechanism and a pinion shaft driven by the main input gear. The pinion shaft is driven at a higher rotational speed (generally above the first critical speed) compared to the main input gear. The compressor further includes at least one process stage for compressing a fluid medium. The at least one process stage has at least one centrifugal compressor with at least one impeller operatively engaged with the pinion shaft. At least one thrust bearing having a bearing surface is provided in operative engagement with the pinion shaft. The bearing surface is made from a non-metallic material, such as a poly-ether-ether-ketone (PEEK) or equivalent material.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present disclosure relates generally to a thrust bearing for use on a compressor. More particularly, the present disclosure relates to a thrust bearing having a non-metallic pad material for use on an integrally-geared centrifugal compressor. 
         [0003]    2. Description of the Related Art 
         [0004]    Compressors are commonly used in a variety of industries. Certain industrial processes often require a delivery of high volumes of a fluid medium, such as air or a chemical gas, at high pressures, typically ranging from 25 to 600 psig. In order to meet these industrial demands, a variety of centrifugal compressors have been developed. These compressors may have one or more compressor stages. In a multi-stage compressor, the fluid medium is compressed to a first pressure in a first stage which is then fed to one or more additional stages, where it is compressed to a pressure higher than the first pressure. 
         [0005]    Integrally-geared centrifugal compressors are often used to compress the fluid medium in a variety of industrial processes. The integrally-geared centrifugal compressor has a large bull gear that is driven by a driver, such as a motor or a turbine. The bull gear is geared with a smaller pinion shaft, such that the pinion shaft is driven at a higher speed than the bull gear. The pinion shaft has at least one impeller provided in a casing volute. The impeller is rotated synchronously with the pinion shaft to draw low-pressure fluid medium into the housing and increase the pressure of the fluid medium as it drawn from the housing inlet to the housing outlet. 
         [0006]    The pinion shaft is typically supported by a bearing assembly that supports the radial and axial forces exerted on the pinion shaft. 
         [0007]    The bearing assembly may include a thrust bearing that includes a bearing surface for forces acting in an axial direction of the pinion shaft. The thrust bearing provides an interface between the rotating pinion shaft and the casing volute of the compressor. In one embodiment, a rider ring is provided proximate to the bull gear such that the thrust loads on the pinion shaft are transferred to the much larger bull gear. Rider rings have thrust limits and are limited in terms of industrial application. In other embodiments, it is common to employ one or more pads at the interface with the pinion shaft. For example, the one or more pads have a tapering pad surface that creates a tapered oil film that is in contact with the pinion shaft. The remaining surface of the tapering pad is inclined such that lubricant may be channeled into it. In operation, a lubricant film is formed due to a buildup of hydrostatic pressure generated between the rotating pinion shaft and the stationary pad to define a self-acting thrust load support surface. Tapering land bearings are best suited for compressors that operate under specific, constant speed conditions. In another embodiment, the one or more pads are pivotally movable relative to the surface of the pinion shaft. Each pad can tilt individually to generate a self-sustaining hydrodynamic film during bearing operation. 
         [0008]    In embodiments where the bearing has a thrust pad, the pad surface is typically formed from a metallic material, such as Babbit, coated steel, or bronze. The type of the one or more pads defines the load capacity, mechanical losses, and lubrication requirements of the thrust bearing. These characteristics directly affect the overall compressor mechanical efficiency. As the rotational speeds of the compressor increase, several unique challenges arise in relation to the design of the thrust bearings. In high speed operations, where the pinion shaft operates at a speed in the range of 15,000 to 70,000 rpm (generally above the first critical speed), the thrust bearing has a running clearance for setting axial position of the impeller relative to the stationary housing. This imposes high lubrication requirements in order to prevent direct contact between the pad surface and the rotating pinion shaft. Additionally, in order to support the thrust loads during high speed operation, the pads must have a large bearing area, which increases the mechanical loss of the thrust bearing. Furthermore, the metallic material of the thrust pads is subject to temperature limitations, which further increases the lubrication requirements. Additionally, static electricity can be built up within the metal housing of the compressor. The metallic material on the thrust pad surface can cause a discharge of static (DC) or AC electricity on the thrust face. This electrical discharge can cause damage to the surface of the thrust face, thereby requiring frequent servicing or replacement. 
       SUMMARY OF THE INVENTION 
       [0009]    Accordingly, in view of the disadvantages of the existing thrust bearings for integrally-geared centrifugal compressors, an improved bearing that overcomes these disadvantages is desired. In one embodiment of the present invention, an integrally-geared compressor may have a drive mechanism and a gear box operatively engaged with the drive mechanism. The gear box may include a main input gear driven by the drive mechanism and a pinion shaft driven by the main input gear. The pinion shaft may be driven at a higher rotational speed compared to the main input gear. The compressor may further include at least one process stage for compressing a fluid medium. The at least one process stage may have at least one centrifugal compressor with at least one impeller operatively engaged with the pinion shaft. At least one thrust bearing may have a bearing surface and is provided in operative engagement with the pinion shaft. The bearing surface may be made from a non-metallic material, such as a poly-ether-ether-ketone (PEEK) material. 
         [0010]    In another embodiment, the bearing surface may have an array of tiltable pads arranged in a circular configuration. The array of pads may have a bottom substrate with the non-metallic material deposited on the bottom substrate. The thrust bearing may have a substantially annular shape with a central opening for receiving the pinion shaft. The central opening may include one or more journal pads. At least one lubrication passage may be provided for delivering lubricant to the bearing surface. The thrust bearing may further include an outer retainer that extends around a concentrically-arranged carrier, wherein the bearing surface is disposed on the carrier. 
         [0011]    In accordance with a further embodiment, a thrust bearing for a multi-stage, integrally-. geared compressor may have an outer retainer having a substantially annular shape and a carrier concentrically-arranged within the outer retainer. The carrier may have a substantially annular shape with a central opening. One or more journal pads may be disposed in the central opening, where the one or more journal pads are configured for operatively engaging a pinion shaft of the compressor. The bearing may further have an array of pads in a circular arrangement on the carrier. The array of pads defines a bearing surface, which may be made from a non-metallic material. The non-metallic material may be a PEEK or an equivalent material. The array of pads may further include a bottom substrate with the non-metallic material deposited on the bottom substrate. At least one lubrication passage may be provided for delivering lubricant to the bearing surface. Additionally, at least one sensor for measuring a performance characteristic of the bearing may optionally be provided. The at least one sensor may be a temperature sensor or a shaft motion sensor. 
         [0012]    In a further embodiment, a drive assembly for a multi-stage, integrally-geared compressor may have a drive mechanism and a gear box operatively engaged with the drive mechanism. The gear box may include a main input gear driven by the drive mechanism and a pinion shaft driven by the main input gear. The pinion shaft may be driven at a higher rotational speed compared to the main input gear. At least one thrust bearing may have a bearing surface, which is in operative engagement with the pinion shaft. The bearing surface may be made from a non-metallic material, such as a PEEK material. The bearing surface may have an array of tiltable pads arranged in a circular configuration. The array of pads may have a bottom substrate with the non-metallic material deposited on the bottom substrate. The thrust bearing may have a substantially annular shape with a central opening for receiving the pinion shaft. At least one lubrication passage may be provided for delivering lubricant to the bearing surface. 
         [0013]    These and other features and characteristics of the thrust bearing for a compressor, as well as the methods of operation and functions of the related elements of structures and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. As used in the specification and the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a perspective view of a packaged compressor assembly in accordance with one embodiment of the present invention; 
           [0015]      FIG. 2  is a perspective view of a multi-stage, integrally-geared centrifugal compressor shown in  FIG. 1 ; 
           [0016]      FIG. 3  is an enlarged view of Detail A shown in  FIG. 2 ; 
           [0017]      FIG. 4  is a perspective view of a pinion shaft and thrust bearing assembly shown in  FIG. 3 ; and 
           [0018]      FIG. 5  is a perspective view of a thrust bearing shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    For purposes of the description hereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “lateral”, “longitudinal”, and derivatives thereof shall relate to the invention as it is oriented in the drawing figures. However, it is to be understood that the invention may assume alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the invention. Hence, specific dimensions and other physical characteristics related to the embodiments disclosed herein are not to be considered as limiting.  100181  Referring to  FIG. 1 , a compressor assembly  10  is illustrated. The compressor assembly  10  is a multi-stage, integrally-geared centrifugal compressor assembly configured for pressurizing a fluid medium, such as air or an industrial gas, for use in an industrial process. While a multi-stage compressor assembly  10  is illustrated in  FIG. 1 , one of ordinary skill in the art will appreciate that, depending on a desired output pressure and volume, a single-stage compressor assembly may be contemplated. In another embodiment, a multi-stage compressor with two or more high-speed pinions is also contemplated. 
         [0020]    The compressor assembly  10  includes a first process stage  12  in the form of a first centrifugal compressor  14  and second process stage  16  in the form of a second centrifugal compressor  18 . The first and second compressors  14 ,  18  are driven by a drive mechanism  20 . In one embodiment, the drive mechanism  20  is an electric motor that is operated at a substantially constant rotational speed of 1450 to 3600 rpm. The drive mechanism  20  drives the first and second compressors  14 ,  18  by way of a geared connection, as will be described in detail hereinafter. While the drive mechanism  20  shown in  FIG. 1  is embodied as an electric motor, other embodiments of the drive mechanism  20  are not precluded. For example, the drive mechanism  20  may be embodied as a turbine, such as a steam or a gas turbine. 
         [0021]    As the drive mechanism  20  drives the first compressor  14 , the fluid medium is drawn into the first compressor  14  through the intake  22 . A throttle valve  24  controls the volume of the fluid medium passing through the intake  22  and entering the first compressor  14 . The fluid medium is compressed in the first compressor  14  to a first pressure, which is higher than the intake pressure of the fluid medium. The compressed fluid medium discharged from the first compressor  14  to a first heat exchanger  26 , which reduces the temperature of the fluid medium before it is introduced into the second compressor  18 . The second compressor  18  increases the pressure of the fluid medium to a second pressure, which is desirably higher than the first pressure. In order to reduce the temperature of fluid medium after it is compressed by the second compressor  18 , the fluid medium is discharged to a second heat exchanger  28 . One or more additional process stages (not shown) may be incorporated to further increase the pressure of the fluid medium. The pressurized fluid medium is discharged through an outlet  30 . 
         [0022]    With continued reference to  FIG. 1 , the compressor assembly  10  serves to compress the fluid medium to a pressure higher than the intake pressure. For example, in the first process stage  12 , the fluid medium may be compressed to a pressure of −5 to 50+ psia. The fluid medium heated by compression in the first process stage  12  is cooled in the first heat exchanger  26 . In the second process stage  16 , the fluid medium is compressed to a pressure of up to 150 psia, for example, (unless used as a booster, where the pressure may exceed this value) before being cooled in the second heat exchanger  28  and discharged through the outlet  30 . The pressures and volumes described hereinabove are exemplary only and may be varied to suit a desired industrial application. 
         [0023]    The operation of the compressor assembly  10  may be monitored and controlled by a control panel  32 . The control panel  32  may have a display  34  configured for displaying the operating characteristics of the compressor assembly  10 . For example, the display  34  may indicate the operating speed of the compressor assembly  10 , and pressure and volume flow output of the first and second process stages  12 ,  16 . The display  34  may also show any warnings indicative of an abnormal running condition of the compressor assembly  10 . A plurality of controls  36 , such as buttons or knobs, are provided to control the operation of the compressor assembly  10 . 
         [0024]    With reference to  FIG. 2 , a compressor  11  having a first compressor  14 , a second compressor  18 , and a third stage  95  is illustrated. The compressor  11  has a housing  38  for enclosing the internal components of the first and second compressors  14 ,  18 . The housing  38  also receives a main drive gear that is driven by the drive mechanism (shown in  FIG. 1 ) by way of a drive shaft  40 . The drive shaft  40  receives the power input from the drive mechanism  20  (shown in  FIG. 1 ) and transfers the power input to the first and second compressors  14 ,  18  by way of the main drive gear, as will be described hereinafter. 
         [0025]    The housing  38  is divided into a plurality of separate chambers, which are isolated from each other in a pressure-tight manner. As will be described in detail hereinafter, the housing  38  has a gear chamber  42  for enclosing the drivetrain of the compressor  11 . Each of the first and second compressors  14 ,  18  has a casing volute  44  that defines the pressure chamber therein. The gear chamber  42  is separated in a pressure-tight manner from the casing volute  44  of the first and second compressors  14 ,  18 , such that substantially minimal compressed or uncompressed fluid medium enters the gear chamber  42 . 
         [0026]    With reference to  FIG. 3 , an enlarged view of Detail A shown in  FIG. 2  is illustrated.  FIG. 3  illustrates an enlarged view of a partial cross-section of the second compressor  18 . While the following disclosure will be focused on describing the operation of the second compressor  18 , it is to be understood that the first compressor  14  operates in an identical manner and a detailed description of the operating principle of the first compressor  14  is omitted for brevity. As shown in  FIG. 3 , the gear chamber  42  encloses a main input gear, hereinafter referred to as a bull gear  46 . The bull gear  46  is driven directly by the drive shaft  40  (shown in  FIG. 2 ) that receives the power input from the drive mechanism  20  (shown in  FIG. 1 ). In this manner, the bull gear  46  is driven at a same input speed as the drive mechanism  20  (i.e., &lt;3600 rpm). The bull gear  46  is in a geared connection with a pinion gear  48  of a pinion shaft  50  that extends along a central axis of the second compressor  18 . For example, the bull gear  46  and the pinion shaft  50  may have a helical gear connection. In another embodiment, the gear connection between the bull gear  46  and the pinion shaft  50  may be realized by way of a spur gear connection. Because the pinion shaft  50  has a substantially smaller diameter compared to the bull gear  46  (on the order of 7:1 to 23:1), the pinion shaft  50  is driven at a rotational speed that is significantly higher than the rotational speed of the bull gear  46 . In one exemplary and non-limiting embodiment, for a bull gear  46  rotational speed of 3600 rpm and a gear ratio between the bull gear  46  to the pinion shaft  50  of 10.96:1, the resulting rotational speed of the pinion shaft  50  is 39,456 rpm. 
         [0027]    The pinion shaft  50  has a low-pressure impeller  52  disposed at one end and a high-pressure impeller  54  disposed at the opposite end. The low- and high-pressure impellers  52 ,  54  are enclosed inside the casing volute  44 . Pressurized fluid medium from the first process stage  12  (shown in  FIG. 1 ) is acted on by the low-pressure impeller  52 , which, in combination with a diffuser section  65  pressurizes the fluid medium before it is acted on by the high-pressure impeller  54 . 
         [0028]    With continued reference to  FIG. 3 , the pinion shaft  50  is supported within the gear chamber  42  of the housing  38  by a pair of journal bearings  56  that support the vertical loads on the pinion shaft  50 . Thrust loads imposed on the pinion shaft  50  are handled by a pair of thrust bearings  58 . Each thrust bearing  58  is received within a bearing housing  60  on the housing  38 . The thrust forces imposed on the thrust bearing  58  are transferred to the housing  38  by way of the bearing housing  60 . A pair of seals  63  is provided between the thrust bearings  58  and the low- and high-pressure impellers  52 ,  54  to separate the gear chamber  42  from the casing volute  44  in a pressure-tight manner. The seals  63  prevent the compressed fluid medium from entering the gear chamber  42 , while simultaneously preventing the lubricant from the gear chamber  42  from entering the casing volute  44 . 
         [0029]    With reference to  FIG. 4 , an enlarged view of the thrust bearing  58  located proximate to the high-pressure impeller  54  is illustrated. The thrust bearing  58  is arranged such that its bearing surface  62  is located opposite a thrust collar  64  of the pinion shaft  50 . In operation, lubricant is injected at annulus  60  to aid in the formation of a hydrodynamic film at the interface between the bearing surface  62  of the thrust bearing  58  and the thrust collar  64  of the pinion shaft  50 . Specifically, the rotation of the thrust collar  64  of the pinion shaft  50  sucks the lubricant to the interface between the bearing surface  62  of the thrust bearing  58  and the thrust collar  64 . 
         [0030]    Turning to  FIG. 5 , the combination journal/thrust bearing  58 , hereinafter referred to as bearing  58 , is shown removed from the compressor  11 . The bearing  58  has a generally annular shape with a central opening  68  configured for receiving the pinion shaft  50 . The central opening  68  includes a plurality of journal pads  70  segmented around a sidewall of the central opening  68 . The journal pads  70  extend radially inward from the sidewall of the central opening  68 . Adjacent journal pads  70  are separated by a gap  72  having a lubricant passage  74  for introducing the lubricating lubricant to the journal pads  70 . The interaction of the lubricant with the journal pads  70  achieves the desired hydrodynamic effect for supporting the pinion shaft  50  on a thin film of a mineral and/or synthetic lubricant. While  FIG. 5  illustrates three journal pads  70 , it is to be appreciated that more or fewer journal pads  70  may be provided. For example, the bearing  58  may have 3 to 8 journal pad segments. 
         [0031]    With continued reference to  FIG. 5 , the annular shape of the bearing  58  is defined by an outer retainer  76  that extends around a carrier  78 . As illustrated in  FIG. 5 , the retainer  76  and the carrier  78  are arranged concentrically, with the carrier  78  being recessed within the retainer  76 . The retainer  76  and the carrier  78  are rotationally connected to each other. The retainer  76  is secured in the bearing housing  60  of the gear chamber  42  (shown in  FIG. 4 ). For example, the retainer  76  may be secured by inserting a lip  80  extending around the outer circumference of the retainer  76  into the bearing housing  60 , as shown in  FIG. 4 . 
         [0032]    An array of pads  82  is disposed in a circumferential arrangement around the carrier  78 . The pads  82  define the bearing surface  62  of the bearing  58 . The pads  82  have a generally trapezoidal shape, with parallel bases spaced apart in a radial direction. While  FIG. 5  illustrates the pads  82  as having a trapezoidal shape, it is to be understood that any other shape may be used to meet the requirements of the bearing  58 . The pads  82  respectively reside in pad seats  84  arranged circumferentially within the carrier  78 . Each pad  82  includes a pair of locking tabs  86  that capture the pad  82  therebetween. Oil then flows from the area of the journal pads  70  to the bearing surface  62  of the pads  82 . 
         [0033]    A lower portion  88  of the pad  82  proximate to the carrier  78  is supported on a tilt member  90  that extends radially from an outer circumference of the carrier  78  to an inner circumference of the carrier  78 . Each pad  82  is positioned such that a middle part of its lower portion  88  rests on the tilt member  90 . In this manner, the tilt member  90  defines a tilt axis that allows the pad  82  to tilt on either side of the tilt member  90 . Each pad  82  may be tilted individually in order to optimize its position during operation such that lubricant may enter the space between the upper surface of the pad  82  and the pinion shaft  50 . Additionally, the pads  82  can tilt on their axes as the pinion shaft  50  goes through critical speeds during operation. This eliminates edge loading typically found on fixed pad systems. 
         [0034]    With continued reference to  FIG. 5 , an upper portion  92  of each pad  82  is substantially planar and is formed from a non-metallic material, such as a PEEK material or an equivalent. The lower portion  88  of the pad  82  may be formed from metal to define a substrate for depositing the PEEK material on the upper portion  92 . Alternatively, the entire pad  82  may be formed from the PEEK material. PEEK material is a high performance polymer suitable for high-temperature applications where thermal properties are critical for performance. The PEEK material offers superior thermal and wear resistance characteristics compared to conventional pad materials, such as Babbit. Specifically, a non-metallic material, such as PEEK, has a higher melting temperature compared to Babbit (450° F. for PEEK compared to 350° F. for Babbit), which enables higher loads while maintaining softness. This allows the bearing pads having a non-metallic material, such as PEEK, to absorb more contaminants compared to their Babbit counterparts. 
         [0035]    The non-metallic materials, such as the PEEK material and PEEK-like materials, have a higher specific load capacity compared to conventional bearing pad materials of equal load area, which allows for a reduction in surface area of the pads  82  compared to conventional bearings. Because the surface area of the pads is directly correlative to frictional losses during operation, pads  82  having the PEEK material can realize a significant reduction in frictional losses due to a reduction in the surface area of the bearing  58 . For example, a non-metallic thrust bearing can reduce mechanical losses by up to 25%. 
         [0036]    Another advantage of using PEEK material on the pads  82  is the reduction in lubrication requirements. Because the pads  82  are made with a reduced surface area compared to conventional bearing pads, less lubricant is required to achieve full hydrodynamic lubrication. In this manner, a more compact lubrication system with smaller lubricant pumps, valves, and filters can be used. 
         [0037]    The pads  82  may further have one or more additional layers or coatings thereon (not shown), which are dependent on the application and needs for the bearing  58 . By interacting with the lubricant, the pads  82  achieve the desired hydrodynamic effect for carrying the thrust load imposed by the pinion shaft  50 . 
         [0038]    Because PEEK is a non-metallic material, the pads  82  act as an electrically-insulating layer between the bearing  58  and the pinion shaft  50 . Thus, any static charges that may build up in the pinion shaft  50  during operation of the compressor  11  are not discharged to the bearing  58 . In this manner, the use of PEEK material avoids damage to the bearing  58  as a result of electrical discharge. Optionally, the bearing  58  may be provided with a metal brush grounding ring (not shown) to completely insulate the compressor  11  from electrical discharge. The bearing  58  may also be equipped with one or more sensors (not shown), such as load cells, temperature, and position sensors, to measure various running conditions of the bearing  58 . 
         [0039]    While various embodiments of the thrust bearing for a compressor were provided in the foregoing description, those skilled in the art may make modifications and alterations to these embodiments without departing from the scope and spirit of the invention. For example, it is to be understood that this disclosure contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment. Accordingly, the foregoing description is intended to be illustrative rather than restrictive. The invention described hereinabove is defined by the appended claims and all changes to the invention that fall within the meaning and the range of equivalency of the claims are to be embraced within their scope.