Abstract:
Rotor and stator assemblies that utilize magnetic bearings for supporting the rotor shaft during operation can be suitably used in corrosive environments, such as sour gas. The rotor and stator assemblies include NACE compliant magnetic bearing arrangements for sour gas applications. One embodiment includes a stator assembly that comprises a stator sleeve formed of a magnetic material, a sleeve extender coaxial to the stator sleeve formed of a non-magnetic material fixedly attached to each end of the stator sleeve, wherein a point of attachment is heat treated, and a wall formed of the non-magnetic material fixedly attached to the sleeve extender configured to hermetically house a stator and form the encapsulated stator assembly.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of and claims the benefit of U.S. patent application Ser. No. 11/934,396 entitled Encapsulated Stator Assembly and Process For Making, filed on Nov. 2, 2007 which in turn claims the benefit of U.S. Provisional Patent Application No. 60/905,710, filed on Mar. 8, 2007, and entitled Magnetic Bearings For Use In Corrosive Environments. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This disclosure relates to rotor and stator assemblies that utilize magnetic bearings and can be used in corrosive environments and processes of assembling the magnetic bearings. The rotor and stator assemblies can be used in turboexpanders, pumps, compressors, electric motors and generators, and similar turbo-machinery for the oil and gas industry. 
         [0003]    A turboexpander is an apparatus that reduces the pressure of a feed gas stream. In so doing, useful work may be extracted during the pressure reduction. Furthermore, an effluent stream may also be produced from the turboexpander. This effluent stream may then be passed through a separator or a distillation column to separate the effluent into a heavy liquid stream. Turboexpanders utilize rotating equipment, which is relatively expensive and typically includes a radial inflow turbine rotor mounted within a housing having a radial inlet and an axial outlet. The turbine rotor is rotatably mounted within bearings through a shaft fixed to the rotor. Such turboexpanders may be used with a wide variety of different gas streams for such things as air separation, natural gas processing and transmission, recovery of pressure letdown energy from an expansion process, thermal energy recovery from the waste heat of associated processes, and the like. Compressors can be associated with turboexpanders as a means to derive work or simply dissipate energy from the turboexpander. 
         [0004]    There are three primary types of bearings that may be used to support the rotor shaft in turbomachinery such as the turboexpander or compressor noted above. The various types of bearings include magnetic bearings, roller-element bearings, and fluid-film bearings. A magnetic bearing positions and supports a moving shaft using electromagnetic forces. The shaft may be spinning (rotation) or reciprocating (linear translation). In contrast, fluid-film and roller-element bearings are in direct contact with the rotor shaft and typically require a fluid based lubricant, such as oil. 
         [0005]    Magnetic bearings provide superior performance over fluid film bearings and roller-element bearings. Magnetic bearings generally have lower drag losses, higher stiffness and damping properties, and moderate load capacity. In addition, unlike other types of bearings, magnetic bearings do not require lubrication, thus eliminating oil, valves, pumps, filters, coolers, and the like, that add complexity and includes the risk of process contamination. 
         [0006]    In a typical magnetic bearing arrangement for rotor and stator assemblies, a stator comprising a plurality of electromagnetic coils surrounds a rotor shaft formed of a ferromagnetic material. Each of the electromagnetic coils, referred to as magnetic radial bearings because they radially surround the rotor, produce a magnetic field that tends to attract the rotor shaft. The rotor shaft assembly is supported by these active magnetic radial bearings inside the stator at appropriate positions about the rotor shaft. By varying the amount of current in the coils of a particular magnet, the attractive forces may be controlled so that the rotor remains centered between the magnets. Sensors in the stator surround the rotor and measure the deviation of the rotor from the centered position. A digital processor uses the signals from the sensors to determine how to adjust the currents in the magnets to center the rotor between the magnets. The cycle of detecting the shaft position, processing the data, and adjusting the currents in the coils, can occur at a rate of up to 25,000 times per second. Because the rotor “floats” in space without contact with the magnets, there is no need for lubrication of any kind. 
         [0007]    Anti-friction bearings as well as seals may be installed at each end of the rotor shaft to support the shaft when the magnetic bearings are not energized. This avoids any contact between the rotor shaft and the stator&#39;s radial magnetic bearings. These auxiliary or “back-up” bearings are generally dry, lubricated, and remain unloaded during normal operation. 
         [0008]    In the oil and gas industry, the rotor and stator assemblies can operate in a process gas, which can also serve as a cooling agent. The process gas typically is natural gas at pressures of about 10 bar to about 200 bar. Unfortunately, natural gas can have a high degree of contaminants. These contaminants can include corrosive agents such as hydrogen sulfide (H 2 S), water, CO 2 , oil, and others. In the worst case, the combination of water and H 2 S leads to what is called wet sour gas, a more corrosive gas. Magnetic bearings typically require cooling so as to maintain an acceptable temperature in the bearing components. Utilizing the process gas directly as the coolant provides a significant advantage in enabling a seal-less system, which eliminates the need for buffer gases (which are not generally available in upstream oil and gas applications) and enhancing safety and operability of the turbo-machinery installed. However, the cooling of the magnetic bearing assembly, and hence its use, in a process gas environment that contains the above contaminants poses a significant risk to the vulnerable components of the magnetic bearing. 
         [0009]    The National Association of Corrosion Engineers (NACE) Standard MR0175, “Sulfide Stress Corrosion Cracking Resistant Metallic Materials for Oil Field Equipment” is a widely used standard in the oil and gas industry that specifies the proper materials, heat treat conditions, and hardness levels required to provide good service life of machinery used in sour gas environments. A NACE compliant material or component is substantially resistant to corrosion such as may occur upon exposure of a non-NACE compliant material to sour gas and/or wet sour gas. For example, NACE compliant welds generally require a post-weld heat treatment process to relieve any weld stresses that would normally contribute to the susceptibility for corrosion. Currently, there are no magnetic bearing systems used in the oil and gas industry that are fully NACE compliant. 
         [0010]    NACE compliance is desirable because the rotor shaft assembly includes several components that could be exposed to a sour gas environment during operation. These include, among others, the rotor shaft itself, the magnetic rotor laminations about the rotor shaft, and the rotor-landing sleeves. As an example of the sensitivity to corrosive agents, it has been found that if the rotor laminations are exposed to wet sour gas they typically fail due to hydrogen embrittlement and stress related corrosion cracking. Stress related corrosion cracking is an issue since the magnetic rotor laminations are typically manufactured as punchings that are shrunk-fit onto the rotor shaft. During operation at working speeds, these components experience relatively high mechanical stresses due to the shrink-fit stresses and radial forces imparted thereon. 
         [0011]    Another drawback of current magnetic bearing systems used in rotor and stator assemblies relates to the steel alloys typically used in the construction of the rotor shaft and/or rotor laminations. The selection of steel compositions that are most resistant to sour gas generally have poor magnetic properties. Because of this, high electromagnetic losses on the rotor shaft occur resulting in heat loads exceeding 1.00 W/cm 2  (6.45 W/in 2 ). The exposure to the high temperatures from the heat loads can lower resistance of the steels to sour gas corrosion. Increasing the size of the components to minimize the heat loads is not practical in view of the costs, and foot prints associated with the larger components. 
         [0012]    In addition to the rotor shaft and laminations, the rotor shaft assembly typically includes a rotor landing sleeve shrunk-fit onto each end of the rotor shaft. This landing sleeve engages an inner race of a roller-element backup bearing in the event of a rotor landing, during which the magnetic bearing fails and the backup bearing has to support the rotor during the subsequent shut-down procedure. Currently, the rotor landing sleeve is formed of a material that is not NACE compliant and is therefore subject to corrosion in a sour gas environment. 
         [0013]    The magnetic bearing stator is a stationary component that provides the source of the magnetic field for levitating the rotor assembly. An air gap separates the stator from the rotor shaft. In order to maximize the magnetic field strength and the levitation force this air gap is made as small as possible while still meeting mechanical clearance requirements between the rotor shaft and the stator. The gap size is typically on the order of millimeter fractions. If the gap is increased, the coils in the stator require more current to levitate the rotor, or the diameter or axial length of the stator has to be increased, all of which increase the overall stator size. If the stator size is limited and cannot be increased, then the levitation force is reduced if the air gap is larger than required by mechanical clearances. 
         [0014]    Current stators are either encapsulated or non-encapsulated. In the case of encapsulated stators, a stator “can” protects the stator components from the process environment. Current stator cans are generally comprised of two concentric tubes of the same material joined at the ends. This tubular can section is located in the gap between the stator and the rotor shaft. If the can material is non-magnetic then it adds an additional magnetic gap on top of the required mechanical clearance, which reduces bearing capacity. In order to maintain bearing capacity, the material of the tubular can section can be selected to be magnetic. 
         [0015]    In current practice, the stator can sections are assembled from magnetic NACE compliant alloys (typical examples are chromium-nickel alloys with a 15-18 wt % chromium 3-5 wt % nickel and 3-5 wt % copper content such as 17-4 precipitation hardened (PH) stainless steel) and are welded together. The welds would normally require a post-weld heat treatment at temperatures in excess of 600° C. in order to be fully NACE compliant. However, due to the temperature limits of the encapsulated electric stator components and the method of current manufacture, no heat treatment is possible. Therefore, the welds are not currently NACE compliant and are subject to corrosion and failure such as from exposure to sour gas. Moreover, some components of the stator, such as sensors, as well as power and instrumentation wires, cannot be encapsulated and are exposed to the process gas environment. 
         [0016]    Referring now to prior art  FIG. 1 , there is shown an exemplary turbo expander-compressor system generally designated by reference numeral  10  that includes a rotor and stator assembly having multiple magnetic bearings for supporting a rotor shaft. The system  10  includes a turbo expander  12  and compressor  14  at opposite ends of a housing  16  that encloses multiple magnetic bearings  18  for supporting rotor shaft  20 . 
         [0017]    Each magnetic bearing  18  includes a stator  22  disposed about the rotor shaft  20 . The stator  22  includes stator poles, stator laminations, stator windings (not shown) arranged to provide the magnetic field. Fixed on the rotor shaft  20  are rotor laminations  24 , each rotor lamination aligned with and disposed in magnetic communication with each stator  22 . When appropriately energized, the stator  22  is effective to attract the rotor lamination  24  so as to provide levitation and radial placement of the rotor shaft  20 . The illustrated system  10  further includes additional axial magnetic bearings  26  and  28  so as to align the rotor shaft  20  in an axial direction by acting against a magnetic rotor thrust disk  30 . Roller-element backup bearings  32  are disposed at about each end of the rotor shaft and positioned to engage a rotor landing sleeve  34  disposed on the rotor shaft  16  when the magnetic bearings fail or when system  10  is in an off state. When the system  10  is configured to accommodate axial or thrust loads, the width of the sleeve  34  is increased to accommodate any axial movement. 
         [0018]    The backup bearings  32  are typically made of roller-element bearings. In such bearings, the inner and outer races require steel alloys of high hardness (typically in excess of HRC 40 (Rockwell C-Scale Hardness)) to accomplish low wear and long bearing life. However, in steel alloys, the properties of high hardness and corrosion resistance are contradicting requirements. As a result, current races are made of high-hardness steel alloys that do not meet NACE corrosion requirements. 
         [0019]    The system  10  further includes a plurality of sensors represented by  36  as well as power and instrumentation wires  38  in electrical communication with controller units (not shown). The sensors  36  are typically employed to sense the axial and radial discontinuities on the rotor shaft  20  such that radial and axial displacement along the shaft can be monitored via the controller unit so as to produce a desirable magnetic levitation force on the rotor shaft  20 . 
         [0020]    Prior art  FIG. 2  illustrates a partial cross-sectional view of an exemplary rotor and stator assembly  50 . The rotor and stator assembly  50  includes a rotor shaft assembly  52  that includes rotor laminations  54  attached to a rotor shaft  56 . An encapsulated stator assembly  60  surrounds the rotor shaft assembly  50  and includes a stator frame  62 , magnetic stator laminations  64  wrapped in conductive windings  66 , and a stator sleeve  68 . The stator sleeve  68  generally has a thickness ranging from 0.05 to 5.0 millimeters (mm). The encapsulated stator assembly  60  includes a hermetically sealed can defined by walls  70  and the stator sleeve  68 , the walls  70  having a thickness of about one centimeter. The can is formed from multiple sections that are welded at various interfaces  72 . These welds are not NACE compliant. Other stator components not shown are stator slots, poles, sensors, and power and instrumentation wires. An air gap  80  separates the rotor shaft assembly  52  from the stator assembly  60 . In operation, the rotor shaft  56  levitates in a magnetic field produced by the stator assembly  60 . 
         [0021]    Given the increasing use of rotor and stator assembly that utilize magnetic bearing systems in corrosive environments, a growing need exists to overcome the above-described deficiencies of current magnetic bearings. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0022]    Disclosed herein are corrosion resistant stator assemblies and processes for fabricating the same. In one embodiment, a stator assembly comprises a stator sleeve formed of a magnetic material; a sleeve extender coaxial to the stator sleeve formed of a non-magnetic material fixedly attached to each end of the stator sleeve, wherein a point of attachment is heat treated; and a wall formed of the non-magnetic material fixedly attached to the sleeve extender configured to hermetically house a stator and form the encapsulated stator assembly. 
         [0023]    In another embodiment, the stator assembly comprises a stator sleeve; magnetic stator laminations wrapped in conductive windings in magnetic communication with the sleeve; and a barrier layer formed on the stator sleeve, the magnetic stator laminations, and combinations thereof. 
         [0024]    A process for forming an encapsulated stator assembly comprises welding a stator sleeve extender formed of a non-magnetic material to a stator sleeve formed of a magnetic material and subsequently heat-treating the welded stator sleeve extender and the stator sleeve at a temperature effective to relieve weld stress; attaching stator electromagnetic components to the stator sleeve; and welding a housing formed of the non-magnetic material to the stator sleeve extender, wherein the housing is configured to encapsulate and hermetically seal the stator electromagnetic components. 
         [0025]    The features and advantages of the components and processes disclosed herein may be more readily understood by reference to the following drawings and detailed description, and the examples included therein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    The figures below, wherein like elements are numbered alike, are for illustrative purposes. 
           [0027]      FIG. 1  is a prior art schematic of a magnetic bearing system illustrating a magnetic bearing rotor assembly and stator used for example, in an expander-compressor. 
           [0028]      FIG. 2  is a prior art schematic of an encapsulated stator showing the stator can with NACE non-compliant welds, arranged relative to a rotor assembly. 
           [0029]      FIG. 3  is a schematic showing a rotor assembly coated with a polymer barrier layer. 
           [0030]      FIG. 4  is a schematic showing the steps of building a stator can with NACE compliant welds. 
           [0031]      FIG. 5  is a schematic of the roller-element backup bearing disposed relative to a rotor shaft and rotor landing sleeve. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    The present disclosure provides rotor and stator assemblies that include magnetic bearings and processes for assembling the magnetic bearings that are suitable for use in corrosive environments. The magnetic bearing assemblies can be made to be fully NACE compliant as may be desired for some applications. For example, NACE compliant rotor shaft assemblies were achieved by coating the magnetic steel rotor shaft and rotor laminations with a barrier film. For magnetic bearing systems employing an encapsulated stator assembly, NACE compliant stator cans were achieved using a combination of magnetic and non-magnetic materials for the encapsulation, that when welded together required heat treatment only in joints between different materials. Similarly, rotor landing sleeves, inner and outer races of backup bearings, as well as power and instrumentation wires can be made NACE compliant by the use of specific materials, which will be described in greater detail below. 
         [0033]    A turboexpander is used as an illustrative example, but the magnetic bearings for corrosive environments disclosed herein are useful in axial bearings and other implementations of magnetic bearings; for example, pumps, compressors, motors, generators, and other turbomachinery. 
         [0034]      FIG. 3  illustrates one embodiment for rendering the rotor assembly of magnetic bearings suitable for use in corrosive environments, such as in sour gas and wet sour gas environments. The rotor shaft assembly  100  includes a rotor shaft  102 , rotor laminations  104  disposed about the shaft, and rotor landing sleeve  108 . A barrier layer  106  is shown disposed on all of the exposed surfaces of the rotor shaft assembly. In an optional embodiment, the barrier layer is formed on selected surfaces of the rotor shaft assembly. For example, the barrier layer could be formed on selected areas of the rotor assembly most prone to corrosion. These include selected areas of the rotor shaft, the rotor laminations, or the punchings used to collectively form the rotor laminations. In one embodiment, the barrier layer is applied to rotors comprising laminations made from iron-silicon (FeSi) that are known to have no or only a low corrosion resistance. NACE compliant alloys such as 17-4 PH stainless steel generally do not require the polymeric surface coating because they are inherently corrosion resistant. 
         [0035]    Optionally, a primer coat can be applied prior to application of the barrier layer. The particular thickness of the primer layer will depend on the type of barrier material selected but in general should be selected to be effective for use in the particular environment in which the magnetic bearing is disposed. It is well within the ordinary skill of those in the art to optimize the thickness of the layer based on the polymer composition and the intended application. 
         [0036]    Suitable materials for forming the barrier layer  106  for protecting the rotor shaft assembly  100  in corrosive environments include, but are not intended to be limited to, various fully (i.e., perfluorinated) and partially fluorinated polymers. Suitable fully fluorinated polymers include polytetrafluoroethylene (PTFE), and perfluoroalkoxy-tetrafluoroethylene copolymer (PFA), fluorinated ethylene-propylene copolymer (FEP) and the like. PFA is a copolymer of tetrafluoroethylene [CF 2 ═CF 2 ] with a perfluoralkyl vinyl ether [F(CF 2 ) n CF 2 OCF═CF 2 ]. The resultant polymer contains the carbon-fluorine backbone chain typical of PTFE with perfluoroalkoxy side chains. One particular form of PFA suitable for the barrier layer is tetrafluoroethylene-perfluoromethylvinylether copolymer (MFA). Partially fluorinated polymers include ethylene-chlorotrifluoroethylene copolymer (ECTFE), ethylene-tetrafluoroethylene copolymer (ETFE) and polyvinylidene fluoride (PVDF). 
         [0037]    Combinations of fluoropolymers sold under the tradenames Xylan™ by Whitford Corporation, and Teflon™ and Teflon-S™ by Dupont are also useful barrier layer materials. Xylan™ coatings comprise in part PTFE, PFA, and FEP. Teflon™ coatings comprise in part PTFE, PFA, FEP, and ETFE fluorocarbon resins. Teflon-S™ is another related family of fluorocarbon coatings containing binding resins, which provide increased hardness and abrasion resistance or other desirable properties. 
         [0038]    Other organic materials useful in forming the barrier layers include powdered epoxies, filled epoxies, filled silicones, and filled PPS (polyphenylene sulfide). Representative thermosetting epoxy powder coatings include, but are not intended to be limited to, Scotchkote™ 134 and Scotchkote™ 6258 from 3M Corporation. 
         [0039]    Scotchkote™ 134 fusion bonded epoxy coating (FBEC) is a one part, heat-curable, thermosetting epoxy coating comprising in part di(4-hydroxyphenol) isopropylidene diglycidyl ether-di(4-hydroxyphenol) isopropylidene copolymer. Scotchkote™ 6258 fusion bonded epoxy coating (FBEC) is a one part, heat-curable, thermosetting epoxy coating comprising in part a mixture of di(4-hydroxyphenol)isopropylidene diglcycidyl ether-di(4-hydroxyphenol)isopropylidene copolymer, and epichlorohydrin-o-cresol-formaldehyde polymer. Scotchkote™ 134 and Scotchkote™ 6258 are applied as a dry powder optionally over a 25.4 micrometer (1 mil) phenolic primer coat and heat cured to a thickness of 254 to 381 micrometers (10 to 15 mil) at a temperature of 150° C. to 250° C. for up to 30 minutes. 
         [0040]    Still other materials useful for forming the barrier layer  106  in  FIG. 3  include conversion coatings of oxides, phosphates, and chromates, and more specifically, conversion materials sold under the trade names Sermalon™, Sermaloy™, Sermagard™ and Sermatel™ by Sermatech. 
         [0041]    The Sermalon™ coating system comprises an aluminum-filled chromate/phosphate bond coat, an intermediate high temperature polymeric inhibitive coating, and a PTFE impregnated topcoat. Coating thickness ranges from 100 to 150 micrometers. SermaLoy™ is an intermetallic nickel aluminide with a silicon-enriched outer layer. Sermatel™ is a family of inorganic coatings that bond to metal creating a metal-ceramic composite. Sermagard™ is a water based aluminized coating with ceramic binder. 
         [0042]    Thicknesses of the polymer barrier layer  106  can range from 2 micrometers to 600 micrometers (0.079 mil to 23.6 mil). 
         [0043]    The polymer barrier layer  106  can be applied to the substrate (i.e., on all or selected surfaces of rotor assembly) in the form of a liquid dispersion or a powder, optionally over a primer layer. Liquid dispersions, comprising polymeric material in a water or solvent suspension, can be applied in a spray and bake coating process in which the liquid dispersion is sprayed onto the substrate for subsequent heating above the melting temperature of the polymeric material contained in the dispersion. Known methods of applying polymeric material in powdered form include spraying of the powder onto the substrate using an electrostatic gun, electrostatic fluidized bed, or a flocking gun, for example. In another example, the powder can be sprayed onto a substrate that has been heated above the melt temperature of the polymeric material to form a coating, also referred to as thermal spraying. It is also known to apply coatings in a process known as “rotolining” in which the substrate and powder is heated, in an oven for example, above the melt temperature of the polymeric material while the substrate is rotated to form a seamless coating on the substrate. 
         [0044]    As previously discussed, the barrier layer  106  is applied to at least one exposed selected surface of the rotor shaft assembly  100 , which can include one or more surfaces defined by the rotor laminations  104 , the rotor shaft  102 , the rotor landing sleeve  108 , other rotor assembly surfaces or the fully assembled rotor  100 . The purpose is to encapsulate portions of or the entire rotor assembly in a protective coating that inhibits corrosion, such as may occur upon exposure to sour gas. 
         [0045]    The components of the rotor shaft assembly are typically formed of magnetic steel. In one embodiment, the rotor laminations are made of iron-silicon (FeSi) material and the polymeric barrier coating is disposed thereon. 
         [0046]    In another embodiment, the rotor laminations are clad with a barrier layer comprising a hydrogen resistant nickel based alloy comprising 40-90 wt % (weight percent) nickel based on the total weight of the nickel based alloy. Herein, “X-Y wt %” means “X wt % to Y wt %” where X and Y are numbers. In particular, the hydrogen resistant nickel based alloy is HASTELLOY® C22® from Haynes International, comprising about 56 wt % nickel, about 2.5 wt % cobalt, about 22 wt % chromium, about 13 wt % molybdenum, about 3 wt % tungsten, about 3 wt % iron, about 0.5 wt % manganese, about 0.08 wt % silicon, about 0.35 wt % vanadium and about 0.010 wt % carbon based on total weight of the nickel based alloy. 
         [0047]    In another embodiment, the rotor shaft is formed of a magnetic steel of type 17-4PH stainless steel alloy, a precipitation hardened martensitic stainless steel comprising 10-20 wt % chromium based on total weight of the precipitation hardened martensitic stainless steel, and further comprising copper and niobium additions. More specifically the precipitation hardened martensitic stainless steel comprises about 16.5 wt % chromium, about 4.5 wt % nickel, about 3.3 wt % copper and about 0.3 wt % niobium based on total weight of the precipitation hardened martensitic stainless steel. The use of the magnetic steel permits construction of a rotor shaft assembly having compact dimensions. The polymeric barrier layer or the optional HASTELLOY® C22® coating on the rotor laminations provides for additional resistance to corrosion such as from exposure to sour gas. However, the usage of sour gas resistant alloys such as the type 17-4PH alloy impacts the magnetic properties of the rotor compared to, for example, iron-silicon alloys (FeSi), thus increasing the electromagnetic losses. This poses a significant challenge particularly during ambient air testing of the assembled machine as required by the American Petroleum Institute. Ambient air has significantly lower pressure and therefore lower cooling capacity than a pressurized process gas. In addition, its thermal and transport properties are inferior to many process gases, further reducing its cooling capacity compared to pressurized process gas. One way to compensate for this is to increase the rotor size so as to increase the exposed area, thus reducing the rotor surface heat flux and increasing the cooling capability. However, this reduces the attractiveness of the magnetic bearing in the intended application. If the rotor dimensions are not increased, the resulting rotor could have a rotor surface heat flux in excess of 1 W/cm 2  (6.45 W/in 2 ). If tested in ambient air, this can easily result in excessive heat rise beyond the laminated rotor insulation material capabilities. All of these disadvantages can be avoided by testing the assembled machine in air or other gases (such as Nitrogen) at a pressure elevated enough and/or at temperature lowered enough to maintain an acceptable temperature of the bearing components. The exact combination of needed pressure and temperature is design dependent and requires knowledge of the expected rotor losses at test conditions to be properly selected. Alloys other than the 17-4PH alloy such as PERMALLOY™ of Western Electric Company and MOLY PERMALLOY™ alloy from Allegheny Ludlum Corporation, low-carbon martensitic stainless steels, or similar materials, can also be used to fabricate the rotor laminations. PERMALLOY™ and MOLY PERMALLOY™ comprise about 80 wt % nickel, about 14 wt % iron, about 4.8 wt % molybdenum, about 0.5 wt % manganese, and about 0.3 wt % silicon based on total weight of the alloy. Low carbon martensitic stainless steels comprise about 11.5-17.0 wt % chromium, about 3.5-6.0 wt % nickel, and no more than 0.060 wt % carbon based on total weight of the low carbon martensitic stainless steel. 
         [0048]    In another embodiment, the rotor landing sleeve  108  as shown in  FIG. 3  is formed of a cobalt based superalloy steel comprising 40-70 wt % cobalt based on total weight of the cobalt based superalloy steel. The use of cobalt based superalloy steels advantageously makes the rotor landing sleeve NACE compliant. More specifically, suitable cobalt based superalloy steels include, but are not intended to be limited to, cobalt based superalloy steels sold by Haynes International Corp. under the trade names ULTIMET®, comprising about 54 wt % cobalt, about 26 wt % chromium, about 9 wt % nickel, about 5 wt % molybdenum, about 3 wt % iron, about 2 wt % tungsten, about 0.8 wt % manganese, about 0.3 wt % silicon, about 0.8 wt % nitrogen, and about 0.06 wt % carbon based on the total weight of the cobalt based superalloy steel. Other suitable cobalt based superalloy steels include HAYNES™ 6B, comprising about 51 wt % cobalt, about 10 wt % nickel, about 20 wt % chromium, about 15 wt % tungsten, about 3 wt % iron, about 1.5 wt % manganese, about 0.4 wt % silicon, and about 0.10 wt % carbon based on total weight of the cobalt based superalloy steel, and chrome coatings sold by Armoloy Corporation under the trade name Armoloy®. ULTIMET® and HAYNES™ 6B alloys comprise primarily cobalt, chromium, and nickel. These cobalt based superalloys exhibit outstanding tribological characteristics that are necessary to prevent damage to the rotor shaft surface during a magnetic bearing failure when the rotor shaft is dropped onto the roller-element backup bearings, while at the same time meeting corrosion resistance requirements. In addition, there are nickel-cobalt based alloys (such as the MP35N alloy) that can be work hardened and aged to increase their hardness and thus strength and still remain NACE compliant. 
         [0049]      FIG. 5  shows a general schematic of a roller-element backup bearing  200  comprising inner races  208  and outer races  206  relative to rotor shaft  202  and landing sleeve  204 . In another embodiment, the inner and outer races of the roller-element backup bearing are made of a martensitic nitrogen stainless steel comprising 10-20 wt % chromium and 0.1-1.0 wt % nitrogen based on total weight of the martensitic nitrogen stainless steel. Typical compositions are about 0.25 to 0.35 wt % carbon, about 0.35 to 0.45 wt % nitrogen, about 0.5 to 0.6 wt % silicon, about 14.5 to 15.5 wt % chromium, and about 0.95 to 1.05 wt % molybdenum based on the total weight of the composition. These martensitic nitrogen stainless steels are commercially available from the Barden Corporation as Cronidur-30™ or SKF Bearings USA as VC444. These martensitic nitrogen stainless steels are available in hardnesses sufficiently high for the application in roller-element backup bearing races (HRC of higher than 55) and also provide excellent corrosion resistance. 
         [0050]    In yet another embodiment, the various stator components can be protected from corrosive gas environments by applying a barrier material to selected surfaces. These include the stator can surfaces, power and instrumentation wires, stator sensors, and stator sleeve. This is advantageous for non-encapsulated stator assemblies. 
         [0051]    In another embodiment, test methods disclosed herein permit testing a compact magnetic bearing with a rotor surface heat flux in excess of 1 W/cm 2  (6.45 W/in2) in a factory environment prior to installation on site. This entails operating the bearing in the factory in a pressurized atmosphere of air or other inert gas as opposed to methane or natural gas used at an oil production site. The air or the other inert gas is pre-cooled by chillers or heat exchangers, or is optionally a cryogenic fluid that expands to a selected temperature and pressure prior to being supplied to the magnetic bearing. The temperature of the atmosphere ranges from −260° C. to 40° C. The atmosphere is pressurized to at least 2 bar to increase its heat removal capability while maintaining the rotor temperature within engineering limitations. 
         [0052]    As previously discussed, the rotor and stator assembly can include an encapsulated stator assembly, also referred to herein as a stator can. In one embodiment, the stator can is constructed with NACE compliant materials and welds using a combination of magnetic and non-magnetic steel alloys. Magnetic steel alloys are placed in areas of the stator can where the magnetic steel provides an electro-magnetic advantage, e.g., the stator sleeve. Non-magnetic steel (such as Inconel) has better corrosion resistance and does not require post-weld heat treatment and therefore it is placed in areas where magnetic steel properties are not required. 
         [0053]    In one embodiment, the magnetic steel alloy of the encapsulated stator comprises a precipitation hardened martensitic stainless steel comprising 10-20 wt % chromium based on total weight of the precipitation hardened martensitic stainless steel. More specifically, the precipitation hardened martensitic stainless steel comprises about 16.5 wt % chromium, about 4.5 wt % nickel, about 3.3 wt % copper, and about 0.3 wt % niobium based on total weight of the precipitation hardened martensitic stainless steel. 
         [0054]    In one embodiment, the non-magnetic material of the encapsulated stator comprises a nickel based alloy comprising 40-70% nickel based on total weight of the nickel based alloy. More specifically, the nickel based alloy comprises about 58 wt % nickel, about 21.5 wt % chromium, about 9 wt % molybdenum, and about 5 wt % iron based on total weight of the nickel based alloy. 
         [0055]      FIG. 4  schematically illustrates a process for fabricating a NACE compliant stator can. The process  150  includes welding non-magnetic stator sleeve extender portions  152  to a stator sleeve  154  at interface  156 . By forming a composite of the sleeve without any stator components disposed thereon, a NACE compliant weld can be formed by exposing the welded composite to post-weld heat treatment that ensures low hardness (below HRC 33) of the weld area and all heat affected zones. The welds are formed by any welding process in the art that allows post-weld heat treatment such that the weld stresses resulting from the welding of dissimilar materials are relieved and that a hardness of less than HRC 33 is accomplished. Exemplary welding processes include autogenous electron beam and electron-beam with filler, laser weld, TIG weld, MIG weld, arc weld, torch weld and combinations comprising at least one of the foregoing processes. By way of example, the stator sleeve extender sections  152  can comprise a non-magnetic superalloy steel welded to each end of the stator sleeve  154  that comprises a type 17-4PH magnetic steel. More specifically, the non-magnetic superalloy steel can comprise a nickel based alloy comprising 40-70% nickel based on total weight of the nickel based alloy. Even more specifically, the nickel based alloy can comprise Inconel 625® commercially available from Inco Alloys International, comprising about 58 wt % nickel, about 21.5 wt % chromium, and about 9 wt % molybdenum, and about 5 wt % iron. The resulting unit is then heat-treated to form the NACE compliant welds at interface  156 . 
         [0056]    A suitable post-weld heat-treatment process is a double age hardening process as per NACE MR0175 to one of the following heat cycles: 1.) solution anneal at 1040±14° C. and air cool or liquid quench to below 32° C.; followed by a first precipitation-hardening cycle at 620±14° C. for a minimum of 4 hours at temperature and air cool or liquid quench to below 32° C.; and followed by a second precipitation-hardening cycle 620±14° C. for a minimum of 4 hours at temperature and air cool or liquid quench to below 32° C.; or 2.) solution anneal at 1040±14° C. and air cool or liquid quench to below 32° C.; followed by a first precipitation-hardening cycle at 760±14° C. for a minimum of 4 hours at temperature and air cool or liquid quench to below 32° C.; followed by a second precipitation-hardening cycle 620±14° C. for a minimum of 2 hours at temperature and air cool or liquid quench to below 32° C. 
         [0057]    Next, the stator components such as a stator frame  160  comprising magnetic stator laminations  158  wrapped in conductive windings  162  are attached. The remaining stator can sections  164  are then welded at interfaces  166  to complete the stator can. The can sections  164  are formed of the same or similar non-magnetic steel previously used, such as the Inconel™ 625 superalloy steel noted above. Because similar materials are welded, the welds at the interfaces  166  are NACE compliant and do not need a post-weld heat treatment. Thus, a NACE compliant encapsulated stator can be assembled without subjecting the internal stator electric components to damaging levels of heat. 
         [0058]    Next, the power and instrumentation wires are attached to the stator components. To provide maximum corrosion protection, the external power and instrumentation wires can be made NACE compliant, wherein the wires comprise a wire sleeve comprising a non-magnetic corrosion-resistant alloy surrounding an electrically conductive material. An example of such a NACE compliant wire is the use of NACE compliant materials such as Inconel alloys as a wire sleeve material. The wire sleeve encapsulates the electrical conductor which is insulated with, for example, ceramics such as magnesium oxide (MgO) which provide excellent electric insulation under pressurized conditions 
         [0059]    The following examples fall within the scope of, and serve to exemplify, the more generally described methods set forth above. The examples are presented for illustrative purposes only, and are not intended to limit the scope of the invention. 
       Example 1 
       [0060]    In this example, individual metal samples were powder coated with Scotchkote™ 6258 thermosetting epoxy as a barrier coating, and heat cured to a thickness of 300 micrometers and 327 micrometers. The part was preheated to a temperature of 150° C. to 246° C. before applying the powder. The powder was then cured at 177° C. for 30 minutes. These samples were tested in autoclaves with process gas to determine the suitability of the coatings in sour gas environment. A series of tests were performed in which the level of hydrogen sulfide in natural gas was varied from 6,000 parts per million (ppm) to 20,000 ppm and the level of moisture was varied from 50 ppm water to saturation. The samples were also exposed to varying temperatures from 30° C. to 130° C. 
         [0061]    No evidence of corrosion was observed in the samples that were exposed to hydrogen sulfide, and water at temperatures below 79° C. 
       Example 2 
       [0062]    In this example, small scale rotors (order of magnitude of 2 to 3 inch outer diameter) were powder coated with Scotchkote™ 134. The rotors were preheated to a temperature of 150° C. to 246° C. before the powder was applied. The powder was then cured at 177° C. for 30 minutes to a thickness of 300 micrometers to 327 micrometers. These samples were also tested in autoclaves with process gas to determine the suitability of the coatings in sour gas environment. 
         [0063]    The samples showed no evidence of corrosion when exposed to high levels of hydrogen sulfide (6000 to 20,000 ppm), water (50 parts per million (ppm) to saturation) and 80° C. 
       Example 3 
       [0064]    In this example, two full-size production rotors were coated with Sermalon™ at a thickness of 178 micrometers to 406 micrometers (7 mil to 16 mil). They were tested in the field under production conditions and passed. These production rotors were installed at site and the coating withstood the corrosive operating gas environment for in excess of 2,000 hours and prevented sour gas attack of the underlying metal components. The samples showed no evidence of corrosion. 
       Example 4 
       [0065]    In this example, NACE environmental tests were performed on samples of Cronidur 30 representative of backup bearing races. The material passed standard 720 hr proof ring tests per NACE TM0177 Solution A at stress levels representative of backup bearing races without signs of corrosion. 
       Example 5 
       [0066]    In this example, NACE environmental tests were performed on samples of Haynes 6-B representative of backup bearing landing sleeves. The material passed standard 720 hour proof ring tests per NACE TM0177 Solution A at stress levels representative of backup bearing landing sleeves without signs of corrosion. 
       Example 6 
       [0067]    In this example, NACE environmental tests were performed on weld samples of Inconel 625 and 17-4 PH representative of the stator can welds. The material passed standard 720 hour proof ring tests per NACE TM0177 modified Solution A at stress levels representative of stator cans without signs of corrosion in the weld. 
         [0068]    The combination of the various embodiments described above provide for a magnetic bearing having superior resistance to corrosive elements such as may be encountered in a sour gas environment. 
         [0069]    The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. The endpoints of all ranges directed to the same characteristic or component are independently combinable and inclusive of the recited endpoint. 
         [0070]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.