Abstract:
A dual-fluid cooling spray device for use with a steam turbine. The steam turbine includes a casing, a plurality of rotating blades and a plurality of stationary vanes. The dual fluid spray device includes a nozzle adjacent to one row of rotating blades and/or stationary vanes, and a dual fluid housing assembly penetrating the casing and coupled to the nozzle. Both a steam pipe and a water pipe are coupled to the dual-fluid housing assembly.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a device for cooling the last rows of rotating blades and stationary vanes of a steam turbine and, more specifically, to a device that combines water and steam to deliver a dual-fluid having a fine droplet size to the last rows of rotating blades and stationary vanes of a steam turbine. 
     2. Background Information 
     Steam turbines are well known in the prior art. Such turbines include a casing which houses rows of stationary vanes and rotating blades. Compressed working steam expands while passing through the vanes and blades, causing the blades to rotate. The blades cause a shaft, which is coupled to a generator, to rotate, thus allowing power to be generated. 
     With advances in steam turbine design, turbine blades now have large enough diameters and rotate at a sufficient speed that windage heating during a shut down of the turbine creates temperatures which approach the operating limits of the blades and vanes. Particularly during shut down of the turbine, the normal flow of working steam is effectively terminated by closure of the valves admitting steam into the turbine. Any fluid, e.g., residual steam in the turbine, tends to remain within the turbine and/or the exhaust region. That is, any fluid within the turbine and/or the exhaust region does not move significantly upstream or downstream from the turbine and/or the exhaust region. These conditions are characterized by a strong recirculation and a backflow from the exhaust region through the last stage of the turbine. As the blades rotate at high speeds, e.g., near normal operating r.p.m., the recirculating fluid which is trapped in the exhaust region is heated due to friction. Heat from the fluid is transferred to the blades and vanes. Such heating can cause the blade and vane temperature to rise to above 600° F. Allowing the blades to reach these temperatures reduces the margin between material strength (which is temperature dependent) and operating stresses (which are speed dependent). During a start-up of the turbine, the flow through the turbine is reduced to about 3 to 5% of normal flow. Under these conditions, windage heating and recirculation occurs, but is not as severe. 
     To maintain the margin between strength and stress limits of the blades, cooling devices are used to reduce the temperature within the turbine and/or the exhaust region during start-up and shutdown sequences. Prior art cooling devices for steam turbines include mechanisms which inject water droplets into the flow path. These water droplets typically have a Sauter mean droplet size of 300-400 microns. One disadvantage of such devices is that the larger droplets require a disproportionately long time to complete the evaporation process, thus reducing cooling effectiveness. A second disadvantage is that the larger water droplets cause erosion damage over time as the droplets impact on the rotating blades. 
     Therefore, there is a need for a cooling device for the exhaust region of a steam turbine that produces a very fine cooling spray that will provide effective evaporative cooling but which will not cause erosion damage to the rotating blades. 
     There is a further need for a cooling device that is adaptable for use with existing steam turbines. 
     SUMMARY OF THE INVENTION 
     These needs and others are met by the present invention which provides a cooling device which uses a dual-fluid cooling spray. The cooling device includes a nozzle in the exhaust region of a steam turbine. The nozzle is coupled to both a water source and a steam source. By combining the water and steam into a dual-fluid spray, micro-droplets having a Sauter mean droplet size of 30 microns can be produced. 
     The dual-fluid cooling device includes at least one of nozzle located within a turbine&#39;s casing and positioned upstream of the last stationary vane, or after the last rotating blade. The nozzle may be located after the last rotating blade as recirculation of the exhaust flow will draw the dual-fluid into the blade path. 
     The cooling device is structured to create micro-droplets having a Sauter mean droplet size of 30 microns and which are, typically, between 1 and 150 microns in diameter. The cooling device creates such fine sized droplets by mixing water supplied at about 110 p.s.i, and dry steam at a minimum temperature about 50-100° F. above the saturation temperature at about 110 p.s.i. absolute. The mixing of the water and steam occurs external to the nozzle exit plane, that is, the dual fluid is mixed immediately as the steam and water exit the nozzle. This produces a dual fluid spray having a droplet size between 1 micron and 150 microns. The dual fluid spray is ejected from a nozzle at a pressure of about 0.5 p.s.i.a. to 5 p.s.i.a, corresponding to the turbine exhaust pressure. The temperature of the mixed-out dual fluid spray will be at or above the saturation temperature depending on the dispersion of the spray and the temperature of the surrounding fluid. Micro-droplets evaporate more rapidly than the droplets of the prior art and produce a greater cooling effect. Micro-droplets are not large enough to cause significant erosion of the blades. 
     In another embodiment, a plurality of nozzles are provided within a turbine&#39;s casing and positioned upstream of the last stationary vane, or after the last rotating blade. The plurality of nozzles preferably includes eight nozzles approximately evenly spaced around the casing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a cross-sectional partial view of a turbine incorporating a cooling device. 
     FIG. 2 is a cross-sectional view of a nozzle according to the present invention. 
     FIG. 2A is a bottom view of the nozzle taken along line A—A in FIG.  2 . 
     FIG. 3 is an axial view of a turbine incorporating a plurality of cooling devices. 
     FIG. 4 is a cross-sectional partial view of another embodiment of the cooling device. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A steam turbine  10  which includes a cooling device  30  according to the present invention is shown in FIG.  1 . The turbine includes an elongated casing  12  having an inner side  13 , a plurality of stationary vanes  14  disposed within a plane, or row  15 , a plurality of rotating blades  16  disposed within a plane, or row  17 . The rows of vanes  15  are attached to the casing  12  and the rows of blades  17  are attached to a central shaft  19  that extends along the longitudinal axis of the casing  12 . The turbine also includes an annular channel  18  that is generally bounded by casing  12  on the outside and a casing  21  on the inside defining an exhaust flow path for the working steam. The channel  18  has a flow direction beginning with a narrow, upstream side  20  and ending with a wider downstream side  22 . A turbine  10  may have more than one row of vanes  15  and blades  17 . Groups of vanes and blades are referred to as a stage, i.e. the first row of vanes  15  plus the first row of blades  17  is the first stage of the turbine  10 . 
     To reduce the affects of windage heating, at least one cooling spray device  30  is supported by the casing  12  and is disposed proximal to the rows of vanes or blades  15 ,  17 . The cooling spray device  30  includes at least one nozzle  32 , at least one dual-fluid housing assembly  34 , a steam pipe  38  and a water pipe  40 . The nozzle  32  is in fluid communication with the nozzle housing assembly  34 . The housing assembly  34  penetrates casing  12  and is in fluid communication with the nozzle  32 . 
     Nozzle housing assembly  34  has two-inlet ports, a first, water inlet port  51  and a second, steam inlet port  52 . The water inlet port  51  is in fluid communication with a water pipe  40 . The steam inlet port  52  is in fluid communication with a steam pipe  38 . As shown in FIG. 2, the nozzle housing assembly  34  includes two chambers, a water chamber  35  and a steam chamber  36 . Water chamber  35  is in fluid communication with water inlet port  51 . Steam chamber  36  is in fluid communication with steam inlet port  52 . A hollow member  37 , which is in fluid communication with water chamber  35  and nozzle  32  extends, through steam chamber  36 . Steam chamber  36  is also in fluid communication with nozzle  32 . As shown in FIG. 2A, a plurality of support members  38  may brace hollow member  37 . 
     As shown in FIG. 3, water pipe  40  is in fluid communication with a water source  60 , such as a reservoir (not shown) or condensate pump discharge. Steam pipe  38  is in fluid communication with a steam source  62  such as a steam generator (not shown) or steam from a steam gland letdown station. In the nozzle housing assembly  34 , the ratio of water to steam is 2 to 3 by weight. Water enters the housing assembly  34  at a temperature generally in the range from about 80° F. to 160° F. (27 to 71° C.), and more preferably at about 150° F. (66° C.). Water enters the housing assembly  34  at a pressure generally in the range from about 60 to 200 p.s.i.a.(4 to 14 bar), and, more preferably, about 110 p.s.i.a. (7.6 bar). Steam enters the nozzle housing assembly  34  at a minimum temperature about 50° F. to 100° F. (28 to 56° C.) above the saturation temperature, at about 60 to 300 p.s.i.a. (4 to 21 bar), and, more preferably, about 110 p.s.i.a. (7.6 bar). Typically, the steam will be at a temperature between about 400° F. (205° C.) and 740° F. (393° C.). When the steam is at about 110 p.s.i.a. (7.6 bar), the steam temperature is about 335° F. (168° C.). 
     The nozzle  32  is structured to provide a dual fluid spray having a droplet size between 1 micron and 150 microns, and, more preferably, having a Sauter mean droplet size of about 30 microns. The dual fluid water and steam components are ejected from the nozzle at pressures ranging from about 0.5 to 5.0 p.s.i.a. (0.03 to 0.35 bar). The temperature of the dual fluid spray is about near the saturation temperature which varies depending on the ejection pressure. The nozzle  32  may be structured to provide the dual fluid spay within about a 25° cone directed in a direction parallel to the rows of vanes  15  and rows of blades  17 . At the preferred ejection pressure, the spray will should have sufficient momentum to reach the inner casing  21  opposite the nozzle  32 . 
     As shown in FIG. 1, the nozzle  32  may be mounted downstream of the last blade  16  and be either flush with or recessed behind the inner side  13  of casing  12 . Alternatively, as shown in FIG. 4, the housing assembly  34  may include an elongated section  39  which spaces nozzle  32  away from casing inner side  13 . Elongated Section  39  may be needed to direct the dual-fluid spray beyond internal structures integral to casing  12 . Also, the nozzle  32  may be positioned as shown between the rows of vanes  15  and rows of blades  17 . 
     As shown in FIG. 3, a plurality of cooling spray devices  30  may be spaced around casing  12  to eject the dual fluid spray evenly throughout channel  18 . The optimal spacing of nozzles  32  may be determined by flow field analysis using computational fluid dynamic methods such as those employed by the programs Fluent, by Fluent Inc. or TascFlow by AEA Technology Engineering Software Inc. Using nozzles  32  structured to provide a dual fluid spray in a 25° cone directed in a direction generally parallel to the rows of vanes  15  and rows of blades  17 , it is preferred to have eight nozzles  32  generally within a plane and evenly spaced about the circumference of casing  12 . The equal spacing between the nozzles  32  may be altered due to various structures either on or within the casing  12 . 
     While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of invention which is to be given the full breadth of the claims appended and any and all equivalents thereof.