Abstract:
A turbine of a gas turbine engine has an air riding seal that forms a seal between a rotor and a stator of the turbine, the air riding seal including an annular piston movable in an axial direction under the influence of a pressure on one side with a pressure acting on an opposite side that self-balances the air riding seal during the steady state condition of the engine and lifts off the seal during engine transients.

Description:
GOVERNMENT LICENSE RIGHTS 
     This invention was made with Government support under contract number DE-SC0008218 awarded by Department of Energy. The Government has certain rights in the invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to a turbo machine, and more specifically to a gas turbine engine with an air riding seal formed between a rotor and a stator of a turbine in which the air riding seal is self-balancing. 
     Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98 
     In a gas turbine engine, such as a large frame heavy-duty industrial gas turbine (IGT) engine, a hot gas stream generated in a combustor is passed through a turbine to produce mechanical work. The turbine includes one or more rows or stages of stator vanes and rotor blades that react with the hot gas stream in a progressively decreasing temperature. The efficiency of the turbine—and therefore the engine—can be increased by passing a higher temperature gas stream into the turbine. However, the turbine inlet temperature is limited to the material properties of the turbine, especially the first stage vanes and blades, and an amount of cooling capability for these first stage airfoils. 
     A seal is required between a stator and a rotor of a turbine in order to prevent a main stream gas flow from leaking into a rim cavity where the rotor disk surfaces can be affected by high thermal loads. One effective seal is an air riding or seal disclosed in U.S. Pat. No. 8,066,473 issued to Aho, J R. on Nov. 29, 2011 and entitled FLOATING AIR SEAL FOR A TURBINE, the entire contents being incorporated herein by reference. The Aho, J R. air riding seal provides a very effective seal with a minimal of wear for the high temperature environment for which the seal is used. 
     One problem with the Aho, J R. air riding seal is during engine transients such as when the gas turbine engine is shut down or started up. During these transient phases, the pressure balance equilibrium across the seal is perturbed causing the potential for the seal to contact the rotor creating wear and premature failure. The present invention provides a means to ensure the seal doesn&#39;t contact the rotor under any circumstances as well as the ability to control the activation of the seal. 
     BRIEF SUMMARY OF THE INVENTION 
     A turbine of a gas turbine engine includes an air riding seal formed by an annular piston movable in an axial direction that forms a seal between a rotor and a stator of the turbine. The annular piston includes a radial extending labyrinth tooth that separates a first pressure chamber from a second pressure chamber on which an air pressure acts to move the annular piston in the axial direction. A central passage formed in the annular piston connects the first pressure chamber to a cushion chamber that forms the seal between the rotor surface of the annular piston to move the annular piston toward the rotor surface. A radial orifice connects the second pressure chamber to the central passage to move the annular piston away from the rotor surface. 
     The air riding seal can include a pressure supply valve with an open and closed position that can supply pressure to the second pressure chamber that will move the annular piston away from the rotor surface. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows a cross section view of a self-balancing air riding seal of a first embodiment of the present invention in a start-up position. 
         FIG. 2  shows a cross section view of a self-balancing air riding seal of the first embodiment of the present invention in a shut-down position. 
         FIG. 3  shows a cross section view of a self-balancing air riding seal of the first embodiment of the present invention in a steady-state position. 
         FIG. 4  shows a cross section view of a self-balancing air riding seal of a second embodiment of the present invention in a passive design. 
         FIG. 5  shows a cross section view of a self-balancing air riding seal in a third embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is an improvement in the floating or air riding annular seal of the air riding seal in U.S. Pat. No. 8,066,473 issued to Aho, J R. on Nov. 29, 2011 and entitled FLOATING AIR SEAL FOR A TURBINE in which the air riding seal is a self-balancing air riding seal. The air riding seal of the present invention is intended for use in a turbine of an industrial gas turbine engine to provide a seal between a rotor and a stator of the engine. During an engine transient such as start-up or shut-down of the engine, the air riding seal would lose pressure and thus make contact with the rotation surface and prematurely wear out the seal surface. Thus, during these transient phases, the self-balancing air riding seal will lift off of the rotor surface so as to minimize of even eliminate any rubbing and thus wear of the seal. 
       FIG. 1  shows a self-balancing air riding seal of the present invention with a rotor surface  11 , a stator  12  with an annular piston  13  that moves in an axial direction to form the air riding seal, a cushion chamber  14  in which pressurized air forms an air cushion that allows for the annular piston  13  to ride on a thin film of air over the rotor surface  11  that forms the seal, and seals  16  that seal the annular piston  13  within an annular chamber in which the annular piston moves axially. 
     A pressure switching valve supplies high pressure air (P) to a backside surface of the seal in chamber  18 .  FIG. 1  is the start-up position in which the annular piston  13  is off from the rotor surface  11 . The high pressure from chamber  18  passes through an orifice or passage  17  and  15  into the annular cushion chamber  14  to produce a film cushion of air for the air riding seal. Air pressure from chamber  18  also flows in chamber  19  through passages  17  and  15  and an orifice  20 . Pressure in chamber  18  is greater than pressure in chamber  19  and thus the annular piston  13  is pushed toward the rotor surface to close the seal. As the seal closes, the orifice  20  is blocked off which causes the pressure in chamber  19  to increase to a value near to the pressure in chamber  18  and thus a balancing force across the lab seal tooth arm  9  is produced, effectively reducing the closing force. 
     Air pressure in  FIG. 1  is supplied to chamber  18  to a backside surface of the annular piston seal  13  to push the annular piston seal  13  toward the rotor surface  11 . The axial passage  17  in combination of the labyrinth seal  9  results in the pressure in chamber  18  to be greater than the pressure in chamber  19 . This pressure differential results in the seal  14  being closed. As the seal closes, the orifice  20  is blocked and causes the pressure in chamber  19  to increase due to movement of the annular piston  13  toward the left in  FIG. 1 , and thus the forces across the lab seal  9  are balanced, effectively reducing the closing force. 
       FIG. 2  shows a shutdown position of the air riding seal of the present invention. The pressure is applied to the secondary cavity in chamber  19  which forces the seal to move away from the rotor surface  11 . The pressure applied to chamber  19  will increase as long as the orifice  20  is still blocked. As the orifice  20  opens, the pressure from chamber  19  will flow through passages  15  and  17  and into the chamber  18  until the forces are equal against the seal  13 . 
       FIG. 3  shows a steady state position for the air riding seal. As the seal approaches the rotor  11 , the pressure in the cushion chamber  14  increases to produce a hydrostatic pressure balance. Area and pressure ratios can be designed to provide a desired steady state operating clearance between the rotor  11  and the piston  13 . 
       FIG. 4  shows a second embodiment of the air riding seal with self-balancing in which a primary source of pressure (P) is supplied continuously to chamber  18 , while chamber  19  is supplied selectively through a valve. The same pressure source can be used for both chambers  18  and  19 , or a lower pressure supplied to chamber  19  as long as the friction force is overcome such that the seal moves. In one position of the valve, the pressure source (P) is applied to chamber  19 . In the second position of the valve, the chamber  19  is closed off. 
       FIG. 5  shows a third embodiment of the self-balancing air rind seal of the present invention. The  FIG. 5  embodiment includes a rotor  21 , an annular air cushion chamber  24  formed on a side of a annular piston head  27  attached or integral to an annular piston  23 , seals  26 , stator casing  22 , a central passage w 5  within the annular piston  23 , a spring shaft  30  and a spring stop  29  on an open end, and a spring  28  over the shaft  30  to bias the annular piston  23  in one direction. The annular piston  23  and the annular piston head  27  can be one piece or two pieces bonded together. The annular piston  23  forms a seal between a high pressure and a low pressure across the air cushion  24  formed between the rotor surface  21  and the annular piston head  27 . 
     During start-up and shut-down of the turbine, the bias force of the spring  28  will pull the seal away from the rotor surface  21 . Pressure at  31  plus pressure at  32  plus pressure at  33  equals the pressure at  34 . Then the total force equals negative pressure of the spring  28 . At the critical pressure, the opening and closing forces are balanced. Pressure at  31  is greater than pressure at  34 . The total force is thus zero. Above the critical pressure, the closing forces are greater than the opening forces, and the seal moves toward the rotor  21 . The total force is greater than zero. At a steady-state operation, as the seal approaches the rotor the hydrostatic pressure increases and the seal balances to the desired operating clearance. 
     The geometry of the seal can be designed such that the critical pressure is achieved at a desired operating point (delta P) by adjusting the area ratios. The bias force of the spring can be achieved by a spring, a magnet, or other similar mechanism.