Abstract:
The invention relates to a sealing segment for sealing between a stationary component and a rotating component. The sealing segment having a passage that extends so as to enable pressure balancing between radial ends of the sealing segment so by providing a seal that can ensure a minimum seal distance is maintained without the need for the complexities of actuators or other mechanical devices.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to seal segment for rotary machines such as steam turbines, gas turbines, aircraft engines, and compressors and more specifically to retractable seal segments. 
       BACKGROUND INFORMATION 
       [0002]    Rotary machines such as steam and gas turbines used for power generation and mechanical drive applications, aircraft engines used for propulsion, and compressors used for pressurization, are generally large machines consisting of multiple turbine and compressor stages. In such machines, pressurised fluid flowing through the turbine and/or compressor stages passes through a series of stationary and rotary components. In a typical steam turbine, the stationary components may be a machine casing while the rotary component may be a rotor. Annular seals mounted on the stationary components are used to control leakage of fluid along the path between the stationary and rotary components. In fact, the efficiency of the turbine is directly dependent on the ability of these seals to prevent such leakage. These seals can be radial or axial in orientation, and can be one of several kinds such as labyrinth packing seals, leaf seals, abradable seals, compliant plate seals, etc. Radial seals are often segmented for assembly reasons and/or for displacement in the radial direction. While radial segmented labyrinth seals have proved to be quite reliable in steam turbines, their performance degrades over time as a result of transient events in which the stationary and rotary components interfere, rubbing the labyrinth teeth into a “mushroom” profile and opening the seal clearance. 
         [0003]    One means of reducing the negative effects of rubs or contact during transient events has been to employ the variable clearance “positive-pressure” (VCPP) arrangement, in which springs are used to hold the seal segments open at a large running clearance under the no- or low-flow transient conditions, when such rubbing is most likely to occur. During steady-state conditions, when the machine is typically operating at a higher load with higher fluid pressures, the ambient pressure around the seal segment overcomes the spring force acting to close the rings to a close running clearance. Examples of known variable clearance positive-pressure (VCPP) labyrinth seals may be found in U.S. Pat. Nos. 6,695,316; 6,022,027; 5,810,365; 5,603,510; 5,002,288; and 4,443,311. 
         [0004]    However, the variable clearance positive-pressure arrangement employs segmented seals that respond solely to the machine load. Once the machine reaches a design load, the packing ring segments close and remain closed until the machine load, and therefore the fluid pressure inside the machine, drops adequately. Thermal transients may persist, however, even after the design load has been reached. Therefore, it is ideally desired that the seal segments remain open until the thermal transients subside. Furthermore, the VCPP seals are susceptible to rubbing in case of rotor vibrations during steady-state operation, when the seal segments are forced closed by the ambient fluid pressure. In such circumstances, the current VCCP arrangement is not effective in avoiding rubs since it is a passive method for positioning the seal segments. It would be desirable to provide an “actively controlled” seal positioning arrangement in which the seal segments are held open not just during no- or low-flow conditions, which correspond to the start-up and shut-down transients, but that a minimum seal distance can be maintained in dependent of load and transient effects. 
         [0005]    So-called “Smart Seals” employ high force capacity pneumatic actuators to provide a radially outward force to “actively” open the seal segments under any machine operating condition. The need to preserve actuator life, however, requires pressure balancing that is achieved by means of pressure control systems. Also, the actuators must be externally pressurized to overcome the fluid ambient pressure, which necessitates an external high-pressure gas supply system. Examples of “Smart Seals” configurations may be found in U.S. Pat. Nos. 6,786,487; 6,655,696; 6,572,115 and 6,502,823. 
       SUMMARY 
       [0006]    A seal segment is disclosed that can ensure a minimum seal distance is maintained without the need for the complexities of actuators or other mechanical devices. 
         [0007]    The disclosure is based on the general idea of a bypass around the first seal elements of a retractable seal that enable a bypass flow to create a pressure cushion is in the seal gap as the seal closes, thereby providing a self-adjustable minimum seal gap control system. 
         [0008]    One general aspect includes a sealing system for sealing between a stationary component and a rotating component, the sealing system includes a stationary component and a sealing segment, which is retractable located in the stationary component so as form a flow region between the stationary component and the sealing segment, the sealing segment has an inner arcuate surface with an upstream end and a downstream end orthogonal to a curvature of the inner surface. The sealing segment also includes a plurality of sealing elements arranged in a plurality of rows that are arranged between on the inner arcuate surface between the upstream end and the downstream end and extend in a direction of the curvature of the inner arcuate surface. A passage extends through the sealing segment having a first opening in the inner arcuate surface between the plurality of rows of the sealing elements such that at least one of the plurality of sealing rows is arranged between the first opening and the upstream end and at least one of the plurality of sealing rows is arranged between the first opening and the downstream end, thereby forming a bypass flow path, which includes at least a portion of the flow region, extending from an upstream end of the sealing segment and through the passage so by enabling the formation of fluid cushion that creates retracting force on the sealing segment towards the stationary components. 
         [0009]    Further aspects may include one or more of the following features. A sealing system where the passage has a second opening located between the upstream end and the downstream end so as to enable a flow of fluid to bypass the at least one plurality of sealing rows arranged between the opening and upstream end. A sealing system where the first opening and the second opening are arranged such that a line drawn between the first opening and the second opening is orthogonal to the inner arcuate surface. A sealing system where the passage has a second opening located towards the upstream end so as to enable a flow of fluid to bypass the at least one plurality of sealing rows arranged between the opening and upstream end. A sealing segment where the passage is cylindrical in shape. A sealing system where the passage is configured and arranged to direct fluid flow passing through the first opening perpendicular to the inner arcuate surface. A sealing system further comprising a biasing means for retracting the seal segment away from a direction of facing of the inner arcuate surface. 
         [0010]    It is a further object of the invention to overcome or at least ameliorate the disadvantages and shortcomings of the prior art or provide a useful alternative. 
         [0011]    Other aspects and advantages of the present disclosure will become apparent from the following description, taken in connection with the accompanying drawings which by way of example illustrate exemplary embodiments of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    By way of example, an embodiment of the present disclosure is described more fully hereinafter with reference to the accompanying drawings, in which: 
           [0013]      FIG. 1  is a schematic of a preferred embodiment of the disclosure 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Exemplary embodiments of the present disclosure are now described with references to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth to provide a thorough understanding of the disclosure. However, the present disclosure may be practiced without these specific details, and is not limited to the exemplary embodiment disclosed herein. 
         [0015]    An exemplary embodiment of a retractable sealing segment  20  is shown in  FIG. 1  that provides a seal between a stationary component  10  and a rotating component  15 . The sealing segment  20  comprises common features of a sealing segment  20  including an inner arcuate surface  22  that has an upstream end  24  at a higher pressure region, and a downstream end  26 . These ends  24 ,  26  are orthogonal to a curvature of the inner arcuate surface  22 . A plurality of sealing elements  40  are arranged in a plurality of sealing rows  42  that are arranged between on the inner arcuate surface  22  between the upstream end  24  and the downstream end  26  and extend in the direction of the curvature  28  of the inner arcuate surface  22 . In this way the seal elements  40  provide a sealing between the upstream end  24  of the inner arcuate surface  22  and the downstream end  26  of the inner arcuate surface  22 . 
         [0016]    The sealing segment  20  is a retractable seal defined by the fact that it may retract towards the stationary component  10  away from the rotating component  15 . The relative movement of the sealing segment  20  enables the formation of a flow region  29  that enables a flow from an upstream end of the sealing segment  20  to the back of the seal segment  20 . Flow may be prevented from further continuing behind the seal segment  20  to the downstream end of the seal segment  20  by a contact point  21  located between the sealing segment  20  and the stationary component  10  acting as a seal. The sealing at the contact point  21  may be enhanced by, for example, axial thrust exerted on the seal segment  20  as a result of a pressure drop formed between the upstream and downstream ends of the sealing segment  20 . 
         [0017]    In an exemplary embodiment shown in  FIG. 1 , the sealing segment  20  further includes a biasing mean  44  such as a spring, leaver or the like, that biases the inner arcuate surface  22  away from the direction of facing of the inner arcuate surface  22 . The purpose of the spring is to provide a means to open the seal to counter-act closing pressure and gravitational force exerted on the sealing segment  20  during start up and low load. In an exemplary embodiment shown in  FIG. 1  the biasing means  44  is located between the stationary component  10  and the sealing segment  20 . In a not shown exemplary embodiment, the biasing means  44  is located between adjacent sealing segments  20  on circumferentially facing surfaces of the sealing segments  20 . 
         [0018]    In an exemplary embodiment shown in  FIG. 1 , the sealing segment  20  further includes a passage  30  that extends through the sealing segment  20 . The passage  30  has a first opening  32  and second opening  34 . 
         [0019]    The first opening  32  opening in the inner arcuate surface  22  between the plurality of sealing rows  42  of the sealing elements  40 , such that at least one of the plurality of sealing rows  42  is arranged between the first opening  32  and the upstream end  24  of the inner arcuate surface  22  and at least one of the plurality of sealing rows  42 , is arranged between the first opening  32  and the downstream end  26  of the inner arcuate surface  22 . 
         [0020]    The second opening  34  is located on a surface of the sealing segment  20  other than the inner arcuate surface  22 , preferably either between the upstream end  24  and the downstream end  26  of the sealing segment  20  as shown in  FIG. 1  or at a point towards the upstream end  24 . That is, at a point between the mid-point of the upstream end  24  and the downstream end  26  and the upstream end  24 . The purpose of the passage is to create a bypass around the sealing element  40  located between the first opening  32  and the upstream end  24  that utilises leakage flow that may pass around the back of the seal, via the flow region  29 . In this way, when the sealing segment  20  is fully open, corresponding to a wide seal gap  46 , the flow passed the seal is low, due to a low pressure drop and as a result there is a low flow around the back of the seal and down through the passage  30 . In this arrangement, the passage  30  has only a negligible influence on the sealing segment  20  performance. As the seal gap  46  closes, for example by pressure loading on the sealing segment  20 , a cavity  47  is formed in the region of the first opening  32  by the inner arcuate surface  22 , sealing elements  40  and the rotating component  15 . The closing of the seal gap  46  further results in an increase in pressure drop across the downstream sealing elements  40  resulting in a proportionally larger flow through the passage  30  into the sealing cavity  47  and a pressure force equalisation between the radial top and radial bottom, i.e. the inner arcuate surface  22 , of the seal segment  20  which counter balances the pressure acting on top of the seal with pressure within the sealing cavity  47  acting on the inner arcuate surface  22  in the opposite direction. Any further increase in the pressure acting on top of the seal is unable to overcome the sealing opening force of the biasing means  44 . In this way the passage  30  acts as a self-controlling means to prevent complete closure of the sealing segment  20 . 
         [0021]    In this position the flow through the passage  30  is a higher proportion of the flow through the gap  46  due to the area of the passage  30  being a larger fraction of the area between the seals  40  and the rotor  15 . The pressure in the cavity  47  will also be higher. Pressure in cavity  47  will tend towards the pressure at an upstream end  24  with increasing movement of the seal  20  towards the rotor  15 . The sealing segment  20  will continue to move inwards until a radial force balance is achieved due to pressure acting on the arcuate surface  22  and pressure acting on the outer arcuate surface of the seal element  20 . Further closure of the gap  46  will result in pressure in the cavity  47  increasing and the seal segment  20  moving away from the rotor  15  until a new radial force balance is achieved due to pressure acting on the arcuate surface  22  and pressure acting on the outer arcuate surface of seal element  20 . In this way the passage  30  acts as a self-controlling means to prevent complete closure of the sealing segment  20 . 
         [0022]    The pressure in the seal gap  46  caused by the bypass flow through the passage  30  is a function of the pressure drop over the top of the sealing segment  20  and through the passage  30  and therefore is a function of the location of the second opening  34  and the cross sectional area of the passage  30 . An optimal seal gap  46  is therefore achieved by adapting these parameters to the particular operating conditions of an installation. 
         [0023]    In particular for retrofit solutions, if may be advantageous to ensure that the passage  30  forms a straight path as this enables simple configuration by drilling of the sealing segment  20 . 
         [0024]    In an exemplary embodiment shown in  FIG. 1 , the passage  30  is configured and arranged to direct fluid flow passing through the first opening  32  perpendicular to the inner arcuate surface  22 . This reduces the risk of instability of the sealing segment  20  resulting from flow through the passage  30  into the seal gap  46 . 
         [0025]    Although the disclosure has been herein shown and described in what is conceived to be the most practical exemplary embodiment, the present disclosure can be embodied in other specific forms. For example, although one from of retractable springs have be shown in the Figures, any suitable spring arrangement may use. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather that the foregoing description and all changes that come within the meaning and range and equivalences thereof are intended to be embraced therein. 
       REFERENCE NUMBERS 
       [0000]    
       
           10  stationary component 
           15  rotating component 
           20  sealing segment 
           21  contact point 
           22  inner arcuate surface 
           24  upstream end (of the inner arcuate surface) 
           26  downstream end (of the inner arcuate surface) 
           28  direction of curvature (of the inner arcuate surface) 
           29  flow region 
           30  passage 
           31  bypass flow 
           32  first opening (of the passage) 
           34  second opening (of the passage) 
           40  sealing element 
           42  row 
           44  biasing means 
           46  seal gap 
           47  sealing cavity