Patent Publication Number: US-8113764-B2

Title: Steam turbine and a method of determining leakage within a steam turbine

Description:
BACKGROUND OF THE INVENTION 
     The present invention is directed to a power plant and, more particularly, to a system and method of determining leakage within a steam turbine. 
     Most steam turbines having opposing high pressure (HP) and intermediate pressure (IP) sections running at a hot reheat temperature in excess of 1050° F. (566° C.) require an external cooling system in order to maintain acceptable first reheat stage stress levels. As a result of an interaction between the cooling system and internal leakages between HP and IP sections, it is difficult to determine an amount steam leaking between the HP and IP sections. More specifically, in operation, a running clearance exists between a shaft interconnecting the HP and IP sections and a packing assembly that provides a seal about the shaft. The running clearance allows high pressure, high temperature steam to leak from the HP section, along the shaft, to the IP section. The high pressure, high temperature steam leakage affects an overall efficiency of the steam turbine. That is, as steam leakage increases, steam turbine performance decreases. 
     There have been numerous attempts to determine the amount of leakage in order to adjust the running clearance and packing geometry for enhanced steam turbine performance. At present, an inference method is employed to calculate the amount of leakage. The inference test relies upon measuring an effect on an exit portion of the IP section resulting from changes made to parameters at an inlet portion of the HP section. In essence, the inference method measures an indirect parameter in order to determine enthalpy changes in the exit portion of the IP section to estimate the amount of steam leaking along the shaft. Employing an indirect measurement to determine an amount of leakage results in a solution that is, at best, one step above a guess. Determining the amount of leakage will enable engineers to adjust the running clearance and packing geometry between the shaft and the packing assembly to create added efficiencies in steam turbine operation. Without knowing, within some level of certainty, the amount of high temperature, high pressure steam leaking along the shaft, adjusting the running clearance and packing geometry to enhance steam turbine performance will remain a time consuming, high cost, and inexact trial and error process. 
     BRIEF DESCRIPTION OF THE INVENTION 
     A steam turbine constructed in accordance with exemplary embodiments of the present invention includes a first turbine section having a flow of high temperature steam, a second turbine section and a shaft operatively connecting the first turbine section and the second turbine section. The steam turbine further includes a packing assembly positioned about the shaft. The packing assembly limits an amount of the flow of high pressure steam passing along the shaft from the first turbine section to the second turbine section. A first conduit is fluidly connected to the packing assembly. The first conduit is configured to introduce a flow of low temperature, low pressure steam to the packing assembly. A second conduit is also fluidly connected to the packing assembly downstream from the first turbine section and upstream from the first conduit. The second conduit receives a portion of the high temperature, high pressure steam passing into the packing assembly from the first turbine section. A valve is fluidly connected to the second conduit. The valve is configured to be selectively operated so as to allow the high temperature, high pressure steam to mix with the low pressure, low temperature steam in the second conduit. 
     Exemplary embodiments of the present invention also include a method of determining a leakage within a steam turbine having first and second opposing turbine sections connected by a shaft surrounded by a packing assembly. The first turbine section leaks high temperature high pressure steam along the shaft within the packing assembly. The steam turbine includes a first and second conduits connected to the packing assembly with the second conduit being positioned between the first conduit and the first turbine section. The method includes guiding the high temperature, high pressure steam through the second conduit, and introducing a low temperature, low pressure steam into the first conduit. The low temperature, low pressure steam is passed along the shaft toward the second conduit. The method further requires operating a valve fluidly connected to the second conduit, and mixing the high temperature, high pressure steam and the low temperature, low pressure steam in the second conduit to form a combined steam flow. At least one parameter of the combined steam flow is measured, and the valve is adjusted until the at least one parameter of the combined steam flow drops relative to a corresponding parameter of the high temperature, high pressure steam flow. An amount of high temperature, high pressure steam leaking from the first turbine section along the shaft toward the second turbine section is calculated based on the combined steam flow. 
     Additional features and advantages are realized through the techniques of exemplary embodiments of the present invention. Other exemplary embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features thereof, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic represent of a steam turbine having opposed high pressure (HP) and intermediate pressure (IP) turbines constructed in accordance with exemplary embodiments of the present invention; 
         FIG. 2  is a block diagram of a system for determining leakage between the HP and IP turbines; and 
         FIG. 3  is a flow diagram illustrating a method of determining leakage between the HP and IP turbines of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to  FIG. 1 , a steam turbine which, in accordance with an exemplary embodiment of the present invention, is shown as part of a combined cycle steam turbine (CCP) is generally indicated at  2 . Steam turbine  2  includes a first or high pressure (HP) turbine section  4  operatively connected to an opposing second or intermediate pressure (IP) turbine section  6  by a shaft  8 . A mid packing assembly  10  extends about shaft  8 . Mid packing assembly  10  includes a plurality of packing rings (not shown) that provide a seal about shaft  8 . Steam turbine  2  also includes a first conduit  14  fluidly connected to packing assembly  10 . First conduit  14  includes a first end section  16 , fluidly connected to packing assembly  10 , which extends to a second end section  17  through an intermediate section  18 . In accordance with the exemplary embodiment shown, second end section  17  connects to an IP bowl section (not separately labeled) of second turbine section  6 . Steam turbine  2  further includes a second conduit  24  having a first end portion  26  fluidly connected to packing assembly  10 , that extends to a second end portion  27  through an intermediate portion  28 . Second end portion  27  connects to a condenser unit  30  in the exemplary embodiment shown. However, it should be understood that second end portion  27  could connect to any lower pressure unit associated with steam turbine  2 . Second conduit  24  is shown to include a pressure sensor  40  for sensing a pressure parameter of steam in second conduit  24 , a temperature sensor  42  for sensing a temperature parameter of steam in conduit  24 , a flow meter  44  for sensing a flow parameter of steam in second conduit  24  and a valve  48 . In the exemplary embodiment shown, valve  48  is electrically operated to control a flow of steam passing through second conduit  24 . However, it should be understood that valve  48  can also be manually operated. 
     First turbine section  4  receives a flow of high temperature/high pressure (ht/hp) steam  54  from a heat recovery steam generator (HRSG)  56 . HT/HP steam  54  has a temperature of about 1050° F. and a pressure of approximately 2000 psia. During operation, a portion of ht/hp steam  54  flows along shaft  8  within packing assembly  10  towards second turbine section  6 . HT/HP steam  54  entering second turbine section  6  impacts an overall efficiency of steam turbine  2 . Towards that end, it is desirable to control leakage about shaft  8 . 
     In order to determine the amount of leakage within packing assembly  10 , steam turbine  2  includes a leakage measuring system  60  illustrated in  FIG. 2 . Leakage measuring system  60  includes a controller  104  operatively connected to pressure sensor  40 , temperature sensor  42 , flow meter  44  and valve  48 . As will be discussed more fully below, a flow of low temperature/low pressure (lt/lp) steam  164  is introduced into first conduit  14 . lt should be understood that the term “low temperature/low pressure steam” refers to steam at a temperature and pressure that is lower than the high temperature/high pressure steam in first turbine section  4 . Leakage measuring system  60  selectively opens valve  48  allowing ht/hp steam  54  within packing assembly  10  to mix with lt/lp steam  164  to form a combined homogenous steam flow  174  in second conduit  24 . Controller  104  determines the amount of leakage of ht/hp steam based on parameters of at least the combined flow. 
     Reference will now be made to  FIG. 3  in describing a method  200  of determining an amount of ht/hp steam leaking into packing assembly  10 . Initially, ht/hp steam  54  is caused to flow from first turbine section  4  along packing assembly  10  towards second turbine section  6  as indicated in block  202 . The ht/hp steam  54  originates with the operation of steam turbine  2 . Once steam turbine  2  has reached operational levels, valve  48  is opened as indicated in block  204 . As pressure drops within packing assembly  10  lt/lp steam  164  having a known temperature and a known pressure begins to flow toward second conduit  24  as indicated in block  206 . Controller  104  monitors temperature and pressure of steam passing though second conduit  24 . Valve  48  continues to be opened causing a pressure drop in second conduit  24 . The pressure of ht/hp steam  54  continues to fall until lt/lp steam  164  enters second conduit  24  to form combined steam flow  174 . Once a parameter, e.g., temperature, of the combined flow begins to drop toward the predetermined temperature as sensed by temperature sensor  42 , controller  104  queries flow meter  44  for a flow rate of combined flow  174  as indicated in block  210 . Based on the formula provided below, controller  104  then and calculates an amount of ht/hp steam  54  leaking into packing assembly  10  as indicated in block  212 . At this point, determining an effective hot running clearance or gap between packing assembly  10  and shaft  8  can be calculated in block  214 .
 
 Q=kAη 
 
     Where: k=flow coefficient base on packing type
         A=flow path cross sectional area   η=f(Pressure and packing geometry)       

     At this point it should be appreciated that the present invention provides a system and method of determining steam leakage in a steam turbine using known values instead of inferred parameters. The use of known values increases measurement accuracy allowing engineers to establish an effective running clearance between the shaft and packing assembly to enhance operation of the steam turbine. It should also be appreciated that while the low temperature/low pressure steam is described as emanating from an IP bowl section of the IP turbine, various other sources of lt/lp steam having known temperatures and pressures can be employed. Finally, it should be appreciated that the temperatures and pressures described above are for exemplary purposes and can vary within the scope of exemplary embodiments of the present invention. 
     In general, 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 exemplary embodiments of the present invention 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 language of the claims.