Patent Publication Number: US-2018030991-A1

Title: Methods for repairing or restoring impeller seals of a centrifugal compressor

Description:
TECHNICAL FIELD 
     This present disclosure relates generally to centrifugal compressors, and more particularly, to methods for repairing seal teeth on centrifugal compressor impellers. 
     BACKGROUND 
     In centrifugal compressors, labyrinth seals are commonly utilized to help minimize leakage in the gas flow path. The seals generally include teeth that rotate relative to a surrounding seal land, and are typically worn out by time in service during the relative rotation, or are damaged by foreign particles, both of which may result in increased leakage. As a result of the increased leakage, repairing or restoring the impeller is required, as it is more economical than replacement. During the repair process, certain impeller dimensions, e.g., the area of the discharge opening, the inner bore diameter, and the seal diameter, with close tolerances must be maintained during the repair process. Maintaining these impeller dimensions is critical for assembly and performance of the impeller during operation. Common repair methods with arc welding techniques include building weld overlays on an impeller eye of impeller, and later reshaping the weld overlays, via a machining process, into a shape of the impeller teeth. Unfortunately, these common repair methods causes a change in the impeller dimensions as heat is introduced into the impeller bore, which leads to a distortion of the impeller bore. Because of the distortion, the impeller bore requires welding, which leads to additional downtime of the compressor due to the additional welding procedure, and an increase in the amount of weld needed to complete the weld operation. 
     Therefore, there remains a need for an improved method which eliminates distortion, e.g., of the impeller bore when repairing the seal teeth, and reduces downtime of the compressor and decrease the amount of weld needed for the repair process. 
     SUMMARY 
     In one exemplary embodiment, a method for repairing or restoring a tooth for an impeller seal is provided. The method includes the step of inspecting the impeller seal tooth and determining what portions of the impeller seal tooth to repair or restore. 
     The method also includes the step of removing the worn portions of the impeller seal tooth, i.e., the portions determined as requiring repair. After removing the worn portions, the method includes the step of forming a weld into a tapered shape that corresponds with a shape of a fully repaired/restored seal tooth. The weld is formed via a pulsed laser beam welding process (LBW), e.g., by introducing a minimal heat onto a weldable surface of a base of the seal tooth to be repaired. By introducing a minimal heat, via the LBW process, the impeller bore does not require further welding as a result of forming the weld in the tapered shape. 
     The method further includes the step of smoothing the tapered shape weld into a final repaired seal tooth shape, and subsequently, testing the repaired seal tooth via a balance and speed testing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
         FIG. 1  is a sectional schematic cross-sectional view of a multi-staged centrifugal compressor, in accordance with the disclosure provided herein; 
         FIG. 2  is a sectional side view of seal teeth on an impeller in a compressor stage of the compressor of  FIG. 1 , in accordance with the disclosure provided herein; 
         FIG. 3A  is a schematic cross-sectional view of a partially damaged tooth of the impeller, in accordance with the disclosure provided herein; 
         FIG. 3B  is a second schematic cross-sectional view of a damaged tooth, in accordance with the disclosure provided herein; 
         FIG. 4  is a flowchart for an embodiment of a method for repairing/restoring an impeller seal tooth, in accordance with the disclosure provided herein. 
     
    
    
     DETAILED DESCRIPTION 
     The components and materials described hereinafter as making up the various embodiments are intended to be illustrative and not restrictive. Many suitable components and materials that would perform the same or a similar function as the materials described herein are intended to be embraced within the scope of embodiments of the present invention. 
     Referring now to the drawings wherein the showings are for purposes of illustrating embodiments of the subject matter herein only and not for limiting the same,  FIG. 1  illustrates a sectional schematic cross-sectional view of a multi-staged centrifugal compressor  10  comprised of one or more compression stages  20 . Each compression stage  20  may include one or more impellers  30  ( FIG. 2 ), which may be attached to a shaft and are the rotating wheels directing a gas flow. 
     With continued reference to  FIGS. 1 and 2 , each impeller  30  may include an impeller seal, also referred to as a labyrinth seal  31 , on an inlet side (eye) of the impeller  30 . The labyrinth seal  31  may include at least one seal tooth  32  or a plurality of seal teeth  32 , which in operation, e.g., seals against an abradable surface  23  of a ring  22  while preserving a mechanical contact in between. In one exemplary embodiment, the abradable surface  23  may be a surface of a stationary abradable insert ring  23  which may sit a steel backing ring  22  that holds the abradable material in place. 
     With continued reference to the figures and now  FIG. 4 , a flowchart for an exemplary embodiment of a method  1000  of repairing an impeller seal tooth  32  is provided. It should be appreciated that a full tooth or a portion of a tooth may be repaired via the methods described herein. Additionally or alternatively, multiple seal teeth may be repaired one after another or simultaneously. 
     The repair restore process may proceed as follows: at least one or more teeth  32  of the impeller seal  31  may be inspected to determine if any portions  33  ( FIG. 3A, 3B ) of the tooth  32  is worn or damaged, thus requiring repair ( 1005 ). In this step, a visual or other known nondestructive inspection method may be performed to determine the condition of the seal tooth  32  and to determine what worn portions  33  of the tooth  32  should be repaired. For example, as shown in the embodiment of the tooth  32  in  FIG. 3A , an inspection  1005  of the impeller seal  30  may identify that only a small portion  33  of the tooth  32  is damaged and requires repair, as compared to the embodiment of  FIG. 3B , where an inspection  1005  may identify that a larger portion  33  than the portion of  FIG. 3A  is worn and requires repair. It should be appreciated that this larger portion  33  may affect (cover) a majority or all of the seal tooth  32 , thus requiring that a majority of the seal tooth  32  be repaired/restored. 
     Upon determining what damaged/worn portions  33  require repair, the method  1000  includes the step of removing the worn portions  33  of the tooth  32  ( 1010 ). In one embodiment, a machining process (also called pre-machining) may be used to remove the worn portions  33  of the tooth  32  down to a weldable surface, shoulder, or reference point  34  of the tooth  32 . It should be appreciated that pre-machining the worn portion  33  may continue until the weldable surface  34  of the tooth  32  is achieved. 
     With continued reference to the figures, in embodiments where a partial repair of the tooth  32  is desired ( FIG. 3A ), i.e., where the worn portions  33  merely covers a smaller area, the tooth  32  may be machined to a surface of the tooth  32  devoid of any damaged portions  33  and adapted for receiving a weld buildup  40  thereon. 
     Additionally or alternatively, where a full repair of the tooth  32  is desired ( FIG. 3B ), the worn portions  33  and the tooth  32  may be machined down to a base  34  of the worn tooth  32 , with the base  34  providing the weldable surface  34  for receiving the weld buildup  40 . After machining the worn portion  33  to the weldable surface  34 , the machined tooth  32  or weldable surface  34  may be inspected ( 1015 ). It should be appreciated that the inspecting steps described in the method  1000 , e.g., step  1015 , may be optional performed, or may be applied at various points throughout the method  1000  without departing from the scope of the invention disclosed. 
     With continue reference to the figures, the method  1000  may include forming a weld onto the weldable surface  34  into a shape corresponding to a shape of a restored seal tooth ( 1020 ). In this step, instead of applying general layers of a weld as taught by the common methods of repair, the weld is applied or built up, via a pulsed laser beam welding (LBW) process, into a tapered or substantially tapered shape, e.g., from the weld surface  34  to a peak point  36  as determined by a restored seal tooth  32 . 
     In one embodiment, the peak point  36  may be the point at the height of the seal tooth  32  from the base or root, e.g., where the seal tooth  32  in operations engages the abradable insert ring  23 . As used herein, a tapered shape tooth may be a tooth shape where the lower portion (i.e., base) is generally wider than its upper portion (i.e., tip). This tapered shape is exemplified in  FIG. 3 , which illustrates the weld  40  being generally wider at or near the root of the tooth  32 , i.e., the base of the tooth, and gradually decreasing in width as the weld  40  approaches the peak point  36 , i.e., the tip of the tooth. 
     It should be appreciated that less material (weld) may be used for forming the weld into the tapered shape as compared to traditional methods, which requires a plurality of weld overlays be performed onto or across the seal teeth base for later machining the seal teeth shape out of the weld overlays. Additionally, it should be further appreciated that forming the tapered shape, via the LBW process, applies less heat to the impeller than the traditional repair methods, which reduces the likelihood of distorting the impeller dimensions, e.g., the impeller bore. 
     With continued reference to  FIG. 4 , the method  1000  may further include the step of smoothing the tapered shape of the seal tooth  32  into the restored seal tooth  32 , e.g., into a seal tooth  32  ready for final inspection and use. In one embodiment, the smoothing step may include inspecting the tapered shape weld buildup  40  ( 1025 ); heat treating the tapered shape weld or portions thereof ( 1030 ); and machining the weld or portions into the restored seal tooth ( 1040 ). As previously stated, additionally or alternatively, further inspection steps may be implemented through the process, e.g., an inspection after heat treating the weld ( 1035 ) and after a final machining of the tooth  32  into the restored tooth ( 1045 ). 
     After machining the tapered shape weld into the restored seal tooth  32 , the restored seal tooth  32  may be tested ( 1050 ). In this step, testing of the restored seal tooth  32  may include a balance and/or speed testing, e.g., a low-speed balance testing and an over-speed testing of the restored seal tooth  32 . Upon completion of either testing or both, a further inspection ( 1055 ) may be conducted prior to returning the repaired impeller  30  to operation. 
     By using the above method(s), the need to weld the impeller bore in the prior-art methods to correct distortions is eliminated, as the amount of heat applied in the new method is reduced. Additionally, the amount of material for forming the repaired seal tooth in the new method is reduced by forming the weld into a shape that corresponds to a restored seal tooth. 
     The new method saves weld by eliminating the need to apply multiple weld overlays across the seal teeth as taught in the traditional arc welding process, and then reshaping the teeth only after solidification, as the new method forms the weld in the shape of a restored seal tooth, thus allowing an individual tooth to be restored. 
     While specific embodiments have been described in detail, those with ordinary skill in the art will appreciate that various modifications and alternative to those details could be developed in light of the overall teachings of the disclosure. For example, elements described in association with different embodiments may be combined. Accordingly, the particular arrangements disclosed are meant to be illustrative only and should not be construed as limiting the scope of the claims or disclosure, which are to be given the full breadth of the appended claims, and any and all equivalents thereof. It should be noted that the terms “comprising”, “including”, and “having”, are open-ended and does not exclude other elements or steps and the use of articles “a” or “an” does not exclude a plurality. Additionally, the steps of various methods disclosed herein are not required to be performed in the particular order recited, unless otherwise expressly stated.