Abstract:
An opposed-flow steam turbine having an HP section and an IP section connected by a shaft, with mid-span packing surrounding the shaft in a region between the HP and IP sections; and a steam conduit extending from the mid-span packing and through a shell of the turbine; the steam conduit incorporating a pressure tap for directly and continuously measuring pressure in the mid-span packing during operation of the steam turbine.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a diagnostic tool used to determine degradation of turbine components by pressure measurements in the mid-span packing region between HP and IP sections of the turbine. 
     Often when conducting a performance validation test on a steam turbine, the flow between the High Pressure (HP) and Intermediate Pressure (IP) sections through the mid-span packing is unknown because it cannot be measured directly. The methods used to determine this flow are very time consuming, require significant cooperation from the customer and their unit operators, and are only completed on units with precision contractual tests, or units that are the subject of characterization tests. Because of these constraints, an assumed value of this flow is used in the majority of performance analyses. 
     Some units have a provision for performing a blowdown test. To carry out the test, a port is provided through the packing head and shell, with an attached pipe containing an isolation valve and a test section for attaching instrumentation used for measuring temperature, pressure and flow. During normal operation when no test is undertaken, however, the isolation valve is closed, and the test instrumentation is removed. 
     There remains a need, therefore, for a simple and relatively inexpensive technique for continuously measuring pressure at the mid-span packing region between the HP and IP sections of a steam turbine, so that the gathered pressure can be used as an ongoing diagnostic tool for determining/identifying degradation of various turbine components. 
     BRIEF SUMMARY OF THE INVENTION 
     In a first exemplary but nonlimiting embodiment, the invention relates to an opposed-flow steam turbine having an HP section and an IP section connected by a shaft, with mid-span packing surrounding the shaft in a region between the HP section and the IP section; and a steam conduit extending from the mid-span packing and through a shell of the turbine; the steam conduit incorporating a pressure tap for directly and continuously measuring pressure in the mid-span packing during operation of the steam turbine. 
     In another exemplary but nonlimiting embodiment, the invention relates to an opposed-flow steam turbine having an HP section and an IP section connected by a shaft, with mid-span packing surrounding the shaft in a region between the HP section and the IP section; and wherein a steam conduit extends from the mid-span packing and through a shell of the turbine and connects to a condenser, the steam conduit incorporating an isolation valve and a blowdown orifice upstream of the condenser; and a pressure tap attached to the steam conduit for directly and continuously measuring steam pressure in the mid-span packing, the pressure tap located externally of the mid-span packing and upstream of the isolation valve. 
     In still another exemplary but nonlimiting embodiment, the invention relates to a method of operating an opposed-flow steam turbine having an HP section and an IP section connected by a shaft, with mid-span packing surrounding the shaft in a region between the HP section and the IP section, the method comprising providing a steam conduit extending from the mid-span packing and through a shell of the turbine; mounting a pressure tap in the steam conduit; and measuring steam pressure in the mid-span packing directly and substantially continuously during operation of the opposed flow steam turbine. 
     The invention will now be described in connection with the drawings identified below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic diagram of opposed-flow, HP and IP sections of a steam turbine configured for blowdown testing, but modified in accordance with a first exemplary but nonlimiting embodiment of the invention; and 
         FIG. 2  is a simplified schematic diagram of opposed-flow, HP and IP sections of a steam turbine that is not configured for blowdown testing, but modified in accordance with a second exemplary but nonlimiting embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With initial reference to  FIG. 1 , a steam turbine in accordance with a first exemplary but nonlimiting embodiment of the invention is generally indicated at  10 . The steam turbine  10  includes a first or high pressure (HP) turbine section  12  operatively connected to an opposing second or intermediate pressure (IP) turbine section  14  by a shaft or rotor  16 . Mid-span packing assembly (or simply, mid-span packing)  18  extends about the shaft  16  and may include a plurality of packing rings (not shown but conventional in nature) that prevent or minimize steam leakage about and along the shaft  16 . 
     High pressure steam is emitted to the turbine or HP bowl  12  by means of conduit  20  while spent steam is routed to a cold reheater via line  22 . High reheat steam is supplied to the IP bowl  14  via conduit  24 , with spent steam exiting line  26 . During operation, a portion of the high temperature/high pressure steam flows along the shaft  16  within the mid-span packing assembly  18 , toward the IP section  14 . Steam entering the turbine section  14  impacts the overall efficiency of the turbine  10  and thus, it is desirable to control leakage about and along the shaft  16  through the mid-span packing. 
     In turbine configurations as shown in  FIG. 1 , provision is made for a blowdown test, a hole is provided through the packing head and shell, with a pipe or conduit  28  attached, incorporating an isolation or blowdown valve  30  and blowdown orifice as shown in  FIG. 1 . A test section is identified downstream of the valve  30  where pressure, temperature and flow measurements are taken. During normal turbine operation, the valve  30  is closed. When a blowdown test is required, the necessary instrumentation is added in the test section and valve  30  is opened, drawing steam from both turbine sections  12  and  13  into the conduit  28 . Typically after a blowdown test, the data-gathering instrumentation is removed and the blowdown valve  30  closed while normal turbine operation continues. 
     In accordance with an exemplary but nonlimiting embodiment of this invention, a pressure tap or sensor  32  is located in the conduit  28  upstream of the blowdown or isolation valve  30 . During normal turbine operation and with the isolation or blowdown valve  30  closed, the pressure tap or sensor  32  will record the pressure within the mid-span packing  18 , with any leakage steam flowing passed the mid-span packing in one direction along the rotor, from the HP turbine section  12  to the IP turbine section  14 . 
     The direct pressure measurements, taken over sustained periods of time while the turbine is in operation, provide a reliable diagnostic tool. For example, an indication of the state of the packing within the mid-span packing  18  may be obtained in various ways. Specifically, the measured pressure at the time of the test can be compared to the design pressure to guide an assumption about the amount of N 2  flow; the measured pressure during an N 2  inference test can be used to ensure that the test itself is not affecting the sealing surfaces of the turbine; the measured pressure ratio between the HP section  12  and the mid-span packing  18  over time can be used to monitor changes in the seal clearances in the packing  18 ; or a constant measured pressure during a time period with a change in IP section efficiency could indicate internal damage, that may be opening other leakage flow paths between the HP and IP sections. 
     Thus, the present arrangement can help diagnose performance shortfalls on new units as well as indicate degradation on in-service units. Validation teams can use these pressure readings to conduct more accurate analyses; design teams can use the data to verify their assumptions; and the commercial team may use the data to remedy any performance shortfalls and to guarantee as well as to identify any areas in an existing unit that may be suited for an upgrade. 
       FIG. 2  illustrates a similar arrangement but where no blowdown provision has been incorporated into the turbine. Here, the pressure tap or sensor  32  can be applied directly at the mid-span packing assembly  18  to achieve the same result as provided in the arrangement as  FIG. 1 . 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.