Patent Publication Number: US-8978243-B2

Title: Methods and systems for oil free low voltage conduits

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate generally to methods and systems and, more particularly, to mechanisms and techniques for electrically connecting various internal parts of a turbomachinery to an external connection. 
     2. Description of the Related Art 
     During the past years, the importance of turbomachines in various industries has increased. A turbomachine can be a compressor, expander, turbine, pump, etc. or a combination thereof. Turbomachines are used in engines, turbines, power generation, cryogenic applications, oil and gas, petrochemical applications, etc. Thus, there is a need for improving the efficiency and reliability of turbomachines. 
     A known turbomachine often used in industry includes a compressor driven by an electrical motor. Such a turbomachine may be employed, for example, for recovering methane, natural gas, and/or liquefied natural gas (LNG). The recovery of such gasses would reduce emissions and reduce flare operations during the loading of LNG onto ships. Other uses of this kind of turbomachine are known in the art and not discussed here. 
     An example of such a turbomachine is shown in  FIG. 1 . The turbomachine  2  includes an electrical motor  4  connected to a compressor  6 . The connection between the two machine shafts can be achieved by a mechanical joint  8 . The motor&#39;s external casing  10  may be attached to the compressor&#39;s external casing  12  by, for example, bolts  14 . The compressor  6  may include one or more impellers  16  attached to a compressor shaft  18 . The compressor shaft  18  is configured to rotate around a longitudinal axis X. The rotation of the compressor shaft  18  is enhanced by using magnetic bearings  20  and  22  at both ends of the compressor shaft  18 . 
     However, the magnetic bearings  20  and  22  require a supply of electrical power in order to function. The electrical power is supplied to the magnetic bearings  20  and  22  via cables  24  and  26 . Cable  24  connects to the magnetic bearing  20  while cable  26  connects to the magnetic bearing  22 . Cable  24  is provided with a head  28  that is configured to mate with a corresponding head  30  of an external electrical cable  32 . Cable  26  connects in a similar way to an external cable  34 . Cables  24  and  26  are exposed to the media that is processed by the compressor. This media may be corrosive and is likely to have a high pressure and temperature. Thus, specific precautions need to be taken for protecting the cables. Cables  24  and  26  may be attached to an internal wall of the compressor casing  12 . The same is true for the motor  4 , in which cables  36  and  38  connect magnetic bearings  40  of the motor  4  to an outside power source. 
     The cables  24  and  26  are representative of conventional low voltage conduits for delivering electricity to the magnetic bearings  20  and  22 . These conventional conduits are typically constructed using metallic conduits which contain electrical cables. These conduits are then filled with oil to provide both electrical insulation and to provide additional resistance to external pressures which often exist in the various working environments for the turbomachine  2 . The electrical cables  24  and  26  can reside in metallic conduits, which can be flexible or rigid. An example, as shown in  FIG. 2 , of a flexible metallic conduit is a corrugated pipe  42 . The corrugated pipe has a small thickness of sheet metal, which may be a stainless steel. Low voltage electrical connections are typically attached to each end of the corrugated pipe by welding. The corrugated pipe is then typically surrounded by a bridle  44 , an example of which is shown in  FIG. 3 , of metal that assists in protecting the corrugated pipe from damage during assembly and operation. An example of a conventional rigid conduit is a rigid pipe which contains the electrical cables and has an electrical connector on each end of the pipe. These conventional electrical cables can generally operate in conditions of up to 125° C. and 140 bar. These conventional conduits have various considerations for use as will now be described. 
     Oil filled corrugated pipes  42  typically need to support external pressure applied upon them while maintaining some flexibility. This tradeoff results in thin walls to reduce stress when bending, while attempting to provide support against externally applied pressure. Handling and fabrication of the oil filled corrugated pipes is also challenging due to the small wall thickness of these corrugated pipes as well as the need to be correctly filled so as to remove the presence of gas which may generate conduit restriction when under an external gas pressure. Also the thermal gradient needs to be considered since the oil expansion from heating can also generate undesirable mechanical stress on the corrugated pipe. Additionally, for flexible pipes, which contain electrical cables, the environment within the turbomachine  2 , for example, an acid or sour gas presence, may also cause failure (or premature replacement requirements) for the thin walled, flexible, corrugated pipes  42 . 
     For rigid pipes, routing and assembly within the turbomachine  2  is generally not optimal due to the lack of flexibility of the pipes. Accordingly, it would be desirable to have other methods and systems for routing electrical cables within a turbomachine. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an exemplary embodiment, a turbomachine is provided. The turbomachine includes: a compressor having a compressor shaft configured to rotate; at least two magnetic bearings provided at opposite ends of the compressor shaft and configured to support the compressor shaft; a motor having a motor shaft configured to be connected to the compressor shaft, and a first electrical cable configured to connect a first magnetic bearing to a first external connection, wherein the first electrical cable has a first end, a second end, an internal core section and an outer sheath section with the internal core section and the outer sheath section extending from the first end to the second end. Additionally, there is a first connector configured to connect the first end of the first electrical cable to the first magnetic bearing, wherein the first connector is welded or brazed to the first end of the first electrical cable to both the internal core section and the outer sheath section; and a second connector configured to connect the second end of the first electrical cable to the first external connection, wherein the second connector is welded or brazed to the second end of the second electrical cable to both the internal core section and the outer sheath section. 
     According to another exemplary embodiment, a method for electrically connecting magnetic bearings in a turbomachine to external connectors is provided. The method includes: welding or brazing a first connector to a first end of a first electrical cable, wherein the first connector is welded or brazed to both an internal core section and an outer sheath section of the first end of the first electrical cable; and welding or brazing a second connector to a second end of the first electrical cable, wherein the second connector is welded or brazed to both the internal core section and the outer sheath section of the second end of the first electrical cable. 
     According to another embodiment, a turbomachine is provided. The turbomachine comprises: a rotor with a rotor shaft configured to rotate; at least a first magnetic bearing and a second magnetic bearing arranged and configured to support the rotor shaft. A first electrical cable is configured to connect the first magnetic bearing to a first external connection, wherein the first electrical cable has a first end, a second end, an internal core section and an outer sheath section with the internal core section and the outer sheath section extending from the first end to the second end. A first connector is configured to connect the first end of the first electrical cable to the first magnetic bearing, wherein the first connector is welded or brazed to the first end of the first electrical cable to both the internal core section and the outer sheath section. A second connector is configured to connect the second end of the first electrical cable to the first external connection, wherein the second connector is welded or brazed to the second end of the second electrical cable to both the internal core section and the outer sheath section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings illustrate exemplary embodiments, wherein: 
         FIG. 1  depicts a turbomachine; 
         FIG. 2  illustrates a corrugated pipe; 
         FIG. 3  shows a bridle for a corrugated pipe; 
         FIG. 4  illustrates a turbomachine according to an exemplary embodiment; 
         FIG. 5  shows a cross section of an electrical cable according to an exemplary embodiment; 
         FIGS. 6 and 7  show weld positions for attaching a connector to an electrical cable according to an exemplary embodiment; 
         FIG. 8  illustrates an electrical cable with belts according to an exemplary embodiment; 
         FIG. 9  shows multiple conductive cores in an electrical cable according to an exemplary embodiment; 
         FIG. 10  shows a possible routing for an electrical cable according to an exemplary embodiment; and 
         FIG. 11  shows a flowchart of a method according to exemplary embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. Additionally, the drawings are not necessarily drawn to scale. Also, the following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     According to exemplary embodiments, oil free electrical conduits can provide low voltage to magnetic bearings in turbomachines, for example, compressor, expander, turbine, pump, etc. or a combination of them, in ways which avoid or minimize some or all of the issues described for conventional electrical conduits in the Background section. Additionally, some exemplary embodiments, can realize cost savings over conventional conduits by, for example, providing a longer working life at higher pressures and temperatures for the electrical conduits. 
     Prior to describing various exemplary embodiments associated with the oil free (or fluid free), low voltage conduits, an example of such an exemplary turbomachine  46  as shown in  FIG. 4  is now described. This exemplary turbomachine  46  can have similarities to the turbomachine  2  shown in  FIG. 1 , except that, at a minimum, the conduits for providing the voltage to the magnetic bearings are different (as well as other inter-related parts as are described below) as compared to the conduits used in a conventional turbomachine  2 . According to an exemplary embodiment, the turbomachine  46  includes an electrical motor  48  connected to a compressor  50 . A compressor shaft  52  and a motor shaft  54  are connected and configured to rotate around a longitudinal axis X. The rotation of the compressor shaft  52  is enhanced by using magnetic bearings  56  and  58  at both ends of the compressor shaft  52 . 
     According to exemplary embodiments, electrical power is supplied to the magnetic bearings  56  and  58  via electrical cables  60  and  62 . Electrical cable  60  connects to the magnetic bearing  56  while electrical cable  62  connects to the magnetic bearing  58 . Electrical cable  60  can be provided with a connector  64  on one end to mate with an external connection  66  and another connector  68  to mate with the magnetic bearing  56 . Electrical cable  62  can be provided with a connector  70  on one end to mate with an external connection  72  and another connector  74  to mate with the magnetic bearing  58 . These electric cables  60  and  62  are exposed to the media that is processed by the compressor  50 , which may be corrosive, under a high pressure and/or at an elevated temperature, for example, 500° C. and 220 bar (which may occur on a suction side of the turbomachine  46 ) or up to 700 bar (which may occur on a discharge side of a turbomachine  46 ). However, according to alternative exemplary embodiments, the electrical cables  60  and  62 , magnetic bearings  56  and  58 , connectors  64  and  68 , and methods of connection described herein could be used with other temperature and pressure combinations, that is, it is expected that as higher pressures and temperatures are used in turbomachinery (temperatures over 500° C. and pressures over 700 bar) exemplary embodiments described herein can be generally scaled as needed for use in those environments. 
     According to alternative exemplary embodiments, various modifications to the turbomachine  46  can also support exemplary embodiments which are described in more detail below. Modifications to turbomachine  46  can include having a compressor and a motor having a single, one-piece shaft (or rigidly connected shafts). For this case, at the interface between the compressor and the motor, a shared bearing can be used, leading to a configuration with a total of three magnetic bearings for this turbomachine. Alternatively, as more elements are added to a turbomachine which use one or more shafts, the quantity of magnetic bearings used can change and be significantly increased, for example, two magnetic bearings per shaft per component of the turbomachine which uses a shaft in an environment that includes the process fluid. It is to be understood that the quantity of magnetic bearings used herein is not to be considered limiting. Additionally, turbomachines can include more or fewer parts and components. Alternatively, a turbomachine can be described more generically which can still make use of exemplary embodiments described herein. For example, a turbomachine can alternatively be described to include a rotor with a rotor shaft which rotates and includes magnetic bearings, electrical cables and connectors as described in exemplary embodiments described herein. 
     According to an exemplary embodiment, electrical cables  60  and  62  can be flexible while having better chemical properties for withstanding corrosive properties as well as having better mechanical properties for operating at elevated temperatures and pressures that can exist within turbomachine  46  as compared to, for example, conventional cables  24  and  26  within a conventional turbomachine  2  as shown in  FIG. 1 . An example of such an exemplary electrical cable  60  (or  62 ) is shown in  FIG. 5 . According to an exemplary embodiment, electrical cable  60  can include a conductive core  76 , for example, copper, a copper alloy or other acceptable conductor for operating in this environment, with a cladding  78 , for example, an austenitic stainless steel cladding. The cladding  78  is surrounded by an insulator  80 , for example, a magnesium or aluminum oxide powder, which is then surrounded by an outer sheath  82 , for example, IN625 or an austenitic stainless steel. The insulator  80  prevents electrical leakage from the conductive core  76  to the outer sheath  82  while also providing support for use under high pressures allowing for little or no deformation of the electric cable  60 . 
     According to exemplary embodiments, electrical cables  60  and  62  do not require internal oil fill (or other fluid fill). The diameter of the electrical cables  60  and  62  may be approximately 6 mm, with a length varying depending upon the size of the turbomachine  46  and how the electrical cables are routed within the turbomachine  46 . Also it is possible to form out the electrical cables  60  and  62  before assembly by, for example, using tooling to fit and make the desired routing within the turbomachine  46 . Additionally, electrical cables  60  and  62  may each represent a plurality of electrical cables for delivering the low voltage to the magnetic bearings  56  and  58 . 
     According to exemplary embodiments, connectors  64 ,  68 ,  70  and  74  can be welded to each end of the electrical cables  60  and  62 . An example of a connector  64 ,  68 ,  70  and  74  that can be welded or brazed to an end of an electrical cable  60  or  62  is now described with respect to  FIGS. 6 and 7 .  FIG. 6  shows the electrical cable  60  and the connector  64 . Reference point  84  shows a position where the outer sheath  82  can be fully circumferential welded or brazed to the connector  64 . Area A, which shows an expanded view of a second weld location, is shown in an expanded view in  FIG. 7 . 
     According to exemplary embodiments,  FIG. 7  shows where the cladding  78  can be welded or brazed to a connector pin  88  of the connector  64  at reference point  86 . For both of the attachments, if brazing is used, a brazing powder should be selected that avoids or minimizes any negative interactions, for example, undesirable thermal expansion or damage to the insulator  80 , between the electrical cable  60  and the connector  64 , as well as, between the various sections of the electrical cable  60 . Similar methods for attaching a connector  68 ,  70  and  74  can be used for the other end of electrical cable  60 , both ends of electrical cable  62  and other electrical cables used. 
     According to exemplary embodiments, a plurality of electrical cables  60  with a similar shape can be used to supply a low voltage from the external connection  66  to the magnetic bearing  56  as shown in  FIG. 8 . Also, each connector  64  and  66  can be attached to multiple electric cables  60 . As shown in  FIG. 8 , one or more belts  90  can be used to keep the electrical cables  60  generally together and to provide some dampening for them from the vibrations that can be generated when the turbomachine  46  is in operation. The electrical cables  60  can also be supported by and/or attached to the casing of the turbomachine  46  to further reduce the possibility of damage from vibrations. According to an alternative exemplary embodiment, a plurality of conductive cores can reside within a single electrical cable as shown in  FIG. 9 . More specifically,  FIG. 9  shows a single electric cable  98  with multiple conductive cores  92 , an insulation section  94  and an outer sheath  96 . The materials used for these sections are similar to those described above for a single conductive core in a single electrical cable. Also, each conductive core  92  can be surrounded by a cladding. 
     According to an exemplary embodiment,  FIG. 10  shows a routing of the electrical cable  60  from the magnetic bearing  56  to the external connection  66 . A length for this routing can be, for example, one meter. However, other routings and lengths of the electrical cable  60  within the turbomachine  46  can be used as desired. Additionally, while exemplary embodiments described herein have generally described electrical cables providing low voltage to magnetic bearings in a compressor, which can be a part of the turbomachine, these examples are not to be considered limiting to only the compressor. Instead, these exemplary embodiments can also be applied to electric cables providing electricity to magnetic bearings in other components of turbomachines as desired. 
     Utilizing the above-described exemplary systems according to exemplary embodiments, a method for electrically connecting magnetic bearings in a turbomachine to external connectors is shown in the flowchart of  FIG. 11 . The method for electrically connecting magnetic bearings in a turbomachine to external connectors includes: a step  100  of welding or brazing a first connector to a first end of a first electrical cable, wherein the first connector is welded or brazed to both an internal core section and an outer sheath section of the first end of the first electrical cable and a step  102  of welding or brazing a second connector to a second end of the first electrical cable, wherein the second connector is welded or brazed to both the internal core section and the outer sheath section of the second end of the first electrical cable. According to an alternative exemplary embodiment, the connectors  64 ,  68 ,  70  and  72  may be welded to the cladding  78  of the internal core section  76 . 
     According to at least one of the exemplary embodiments described herein, various advantages can be provided in the environment of turbomachines. For example, according to exemplary embodiments flexible electrical cables described herein can have better chemical properties for withstanding corrosive properties associated with various process fluids, as well as having better mechanical properties for operating at elevated temperatures and pressures then currently used electrical cables in conventional turbomachines. For another example, exemplary embodiments described herein can be used for other magnetic bearings which operate in a process fluid. The exemplary electrical cable  60  can have a smaller size and thickness with acceptable flexibility as compared with various conventional rigid pipe solutions. Additionally, significant cost reductions can be realized by implementing exemplary embodiments described herein, for example, the electrical cable  60  could cost as much as twenty times less than the cost of similarly used conventional electrical cables in a turbomachine. 
     The above-described exemplary embodiments are intended to be illustrative in all respects, rather than restrictive, of the present invention. Thus, embodiments of the present invention is capable of many variations in detailed implementation that can be derived from the description contained herein by a person skilled in the art. For examples, other types of connectors (other than connectors  64 ,  68 ,  70  and  74 ) could be used for connecting the electrical cables  60  and  62  to the magnetic bearings  56  and  58  as well as the external connections  66  and  72  while maintaining all of the desired electrical, mechanical and chemical properties desired for use within the turbomachine  46 . All such variations and modifications are considered to be within the scope and spirit of the present invention as defined by the following claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the invention unless explicitly described as such. Also, as used herein, the article “a” is intended to include one or more items. 
     This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter 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.