Patent Publication Number: US-2020284428-A1

Title: System and method for accomodating thermal displacement in a power generation plant

Description:
TECHNICAL FIELD 
     Embodiments of the invention relate generally to power generation and, more particularly, to a system and method for accommodating thermal displacement in a power generation plant. 
     DISCUSSION OF ART 
     The generation of electrical energy by means of coal-fired power generation plants is being constantly improved in terms of reduced environmental impact of coal combustion and an increase in the efficiencies of the plants. These improvements are largely driven by an increased focus on reducing the emissions associated with the combustion of coal, in an order to mitigate climate impacts. 
     The thermodynamic efficiency in a steam turbine-based power plant depends at least in part on the temperature and pressure of the steam, specifically, the higher the temperature and pressure of the steam, the greater the potential efficiency. Consequently, there has been a move in recent years to employ even higher steam temperatures and pressures than in the past. This trend has led to the development of new classes of power plants including a so-called “ultra-supercritical power plant” (which uses steam cycles at temperatures greater than about 600° C. and steam pressures in excess of 240 bar), and the advanced ultra-supercritical power plant (which uses steam cycles at even greater temperatures, in the range of about 700° C. to about 760° C.). 
     Maximum steam temperatures are limited, however, by the physical properties of the materials used to form components, such as the boiler steam pipes, that are exposed to the high-temperature steam. Typically, such materials lose strength as the temperature to which they are exposed rises. Historically, the development of stronger steels and alloys have enabled the use of higher steam temperatures in boilers—from subcritical steam temperatures to supercritical steam temperatures, and more recently, ultra-supercritical and advanced ultra-supercritical. To withstand the high pressure and temperature conditions that define these known classes of power plants, specialist alloys (e.g., nickel alloys) are required, especially for the superheater and for the piping that carries or conveys the steam from the steam generator to the turbine(s) (including the main steam piping and hot reheat piping). 
     In a typical power plant, particularly ultra-supercritical and advanced ultra-supercritical power plants, the steam generator (boiler) can often be in excess of 100 meters tall, and the main steam outlet and hot reheat outlet are typically located in a top portion of the boiler (i.e., well above ground level). Relatively long lengths of main steam piping and hot reheat piping are therefore necessary to connect the main steam outlet headers and hot reheat outlet headers at the top portion of the boiler to the turbines (e.g., the high-pressure turbine and intermediate pressure turbine, respectively), which are typically located at ground level. These long lengths of piping for such critical piping systems can contribute substantially to the cost of the power plant, particularly where much more expensive nickel-based alloys are utilized due to the increasingly higher steam temperatures. 
     Accordingly, recent developments in power plant design have also focused on ways of minimizing material costs for these critical piping systems. For example, existing solutions have involved elevating the turbines above ground level to bring the turbine inlets closer to the main steam outlet (and main steam outlet headers) and hot reheat outlet (and hot reheat outlet headers). By shortening the distance between these connection points, shorter lengths of piping could theoretically be utilized, thereby decreasing material costs. 
     Bringing the connection points closer together to thereby shorten the main steam piping and hot reheat piping, however, can present other challenges when trying to accommodate higher steam temperatures during system operation. In particular, at such high operating temperatures, the boiler, outlet headers, turbines and piping will expand (referred to herein as “thermal displacement” or “thermal expansion”). The rigid piping typically used in boiler applications are unable to accommodate thermal displacement, which can lead to the overstressing of system components. To avoid overstressing, it is therefore typically necessary to arrange “expansion loops” in serial fluid communication with the main steam and hot reheat piping runs. Such expansion loops are typically fabricated from standard pipes and elbows, having the same wall thickness and inside diameter of the pipes of the boiler piping system and are conventionally employed to absorb temperature expansion and contraction in steel pipes. However, the use of such expansion loops results in the need for additional pipe length (e.g., to form the expansion loops) and material costs. 
     In view of the above, there is therefore a need for a main steam and hot reheat piping arrangement whereby the length of such piping can be significantly reduced as compared to conventional power plant systems to decrease material costs, while also accommodating thermal expansion of boiler components during system operation to avoid overstressing. 
     BRIEF DESCRIPTION 
     A system for a power generation plant is provided. The system includes a boiler having a superheater, a first header fluidly coupled to an outlet of the superheater and being configured to receive steam from the superheater. A turbine is positioned generally adjacent to the outlet of the superheater, and a main steam piping system extends from the first header to the turbine and is arranged to direct a flow of the steam from the first header to the turbine. The system also includes a first flexible portion upstream from the first header, fluidly coupled between the first header and the boiler. 
     In another embodiment, a system for a power generation plant is provided. The system includes a boiler having a superheater and a reheater. A main steam outlet header is fluidly coupled to an outlet of the superheater and is arranged to receive steam from the superheater. A hot reheat outlet header is fluidly coupled to an outlet of the reheater and is arranged to receive steam from the reheater. The system further includes a high-pressure turbine adjacent to the outlet of the superheater, and an intermediate-pressure turbine adjacent to the outlet of the reheater. A main steam piping system extends from the main steam outlet header to the high-pressure turbine and is configured to direct a flow of the steam from the main steam piping system to the high-pressure turbine. A hot reheat piping system extends from the hot reheat outlet header to the intermediate-pressure turbine and is configured to direct a flow of the steam from the hot reheat piping system to the intermediate-pressure turbine. The system includes a first flexible portion upstream from the main steam outlet header which is operative to flex with respect to the main steam outlet header and the boiler in response to a thermal displacement of the boiler; and the system further includes a second flexible portion upstream from the hot reheat outlet header which is operative to flex with respect to the hot reheat outlet header and the boiler in response to the thermal displacement of the boiler. 
    
    
     
       DRAWINGS 
       The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below: 
         FIG. 1  is a schematic illustration of a coal-fired power generation plant, according to an embodiment of the invention. 
         FIG. 2  is a perspective view of a main steam piping arrangement and hot reheat piping arrangement of the coal-fired power generation plant of  FIG. 1 , according to an embodiment of the invention. 
         FIG. 3  is a detailed perspective view of the main steam piping arrangement of  FIG. 2 . 
         FIG. 4  is a detailed perspective view of the hot reheat piping arrangement of  FIG. 2 . 
         FIG. 5  is a cross-sectional view of the main steam piping arrangement and hot reheat piping arrangement of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     Reference will be made below in detail to exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference characters used throughout the drawings refer to the same or like parts. While embodiments of the invention are suitable for coal-fired power plants, embodiments of the invention may also be suitable for any type of power generation facility where thermal displacements must be accommodated. 
     As used herein, “operatively coupled” refers to a connection, which may be direct or indirect. The connection is not necessarily a mechanical attachment. As used herein, “fluidly coupled” or “fluid communication” refers to an arrangement of two or more features such that the features are connected in such a way as to permit the flow of fluid between the features and permits fluid transfer. 
     Embodiments of the invention relate to a system and method for accommodating thermal displacements of components in a power generation plant. The system includes a boiler having a superheater, a first header fluidly coupled to an outlet of the superheater and being configured to receive steam from the superheater, a turbine elevated to a location generally adjacent to the outlet of the superheater, and a main steam piping system extending from the first header to the turbine and being configured to direct a flow of the steam from the first header to the turbine. The system further includes a flexible portion upstream from the first header allowing located between the first header and the boiler and operative to flex in response to to thermal displacement of the boiler. In an embodiment, the flexible portion may comprise a plurality of flexible or bendable tubes that operatively couple corresponding tubes of the superheater in fluid communication with the first header. 
     Referring to  FIG. 1 , a coal-fired power generation system/plant  10  according to an embodiment of the invention is illustrated. The system  10  includes a coal fired pressurized boiler  12  to which lead a feedwater line  14  and a steam line  16  are input. The boiler  12  may take any configuration known in the art and includes, among other things, a reheater  13 , an evaporator, and a superheater  15  for heating steam at high pressure. The reheater, evaporator and superheater are formed as tube bundles (i.e., each inherently comprising a plurality of individual tubes) within the boiler  12  and each have an inlet and an outlet, as known in the art. As illustrated in  FIG. 1 , the outlet of the reheater  13  is fluidly coupled to a hot reheat outlet header  21  positioned outside the boiler body on a front side thereof, while the outlet of the superheater  15  is fluidly coupled to a main steam outlet header  19  which is likewise positioned outside the boiler body on a front side thereof. Main steam piping  18  and hot reheat piping  20  (also referred to herein as “main steam pipe system” and “hot reheat pipe system”, respectively) lead from the main steam outlet header  19  and the hot reheat outlet header  21 , respectively, to a high-temperature high-pressure steam turbine HP 1  and a high-temperature intermediate-pressure steam turbine IP 1 , respectively. A further piping system  22  feeds exhaust gas from boiler  12  to a high-pressure gas turbine HPT. The three turbines HPT, HP 1  and IP 1 , together with a high-pressure compressor HPC and a generator G 1 , on rotor  25 , form a turbo train which is arranged vertically and parallel with boiler  12 . 
     The high-temperature steam turbines HP 1 , IP 1  and high-pressure gas turbine HPT operate at high temperatures since their components are manufactured from nickel-based or other specialty alloy materials. Main steam piping system  18 , hot reheat piping system  20 , and piping system  22 , are also manufactured from nickel-based or other specialty alloy materials. These piping systems  18 ,  20 ,  22  each comprise a plurality of pipes that may be run over a minimal length due to the vertical arrangement of the turbines parallel with the boiler. Their minimal length provides a significant cost saving in view of the high costs of the nickel-based or other specialty alloy materials. In an embodiment, rather than being arranged vertically, the turbines may simply be elevated from the ground level so as to be in close association with the steam outlets of the boiler (namely, the outlets of the headers from which the turbines are fed). 
     In operation, feedwater is fed via line  14  into boiler  12 , and is there heated in the evaporator and superheater to a temperature in excess of, for example, 700° C. at a pressure of 350 bar and, more particularly, to a temperature between about 700° C. and 820° C. at a pressure of between about 350 bar and 425 bar. From the boiler, the superheated steam is fed to the main steam piping system  18  via the header  19 , and to high temperature high pressure steam turbine HP 1 , where the steam is expanded. The expanded steam, which still has a temperature in excess of at least 600° C., for example, is then fed via a line  24  to a conventional high-pressure steam turbine HP 2 . Together with a conventional intermediate-pressure steam turbine IP 2 , a conventional low-pressure steam turbine LP 2  and a second generator G 2 , this forms a conventional turbo train arranged on a second rotor  26 . 
     The steam expanded in conventional high-pressure steam turbine HP 2  is returned via a line  16  (e.g., cold reheat piping) to boiler  12 , where it is again heated in the reheater  13 . This reheated steam is fed to the hot reheat piping system  20  via the header  21 , into high temperature intermediate pressure steam turbine IP 1 . The steam expanded in IP 1  is fed via a line  28  to conventional intermediate-pressure steam turbine IP 2  and there further expanded and further expanded in the series connected low-pressure steam turbine LP 2 . Finally, the steam is fed to a condensation and feedwater heating facility  50 . 
     Piping system  22  feeds exhaust gas from boiler  12  via a high-temperature filter  30  to high pressure gas turbine HPT, in which the exhaust gas is expanded. The expanded exhaust gas is then fed to a series connected controllable low-pressure gas turbine LPT arranged on a separate rotor. The exhaust gas is further expanded therein and then fed to a selective catalytic reducer SCR in order to reduce nitrogen oxides. The exhaust gas may then be fed to a series of exhaust gas heated feedwater heaters (not shown). 
     Turning now to  FIGS. 2-5 , detailed views of the main steam piping system  18  and hot reheat piping system  20  is shown. As illustrated therein, the turbine (e.g., high-pressure high temperature steam turbine HP 1 ) is mounted on a platform that is elevated such that the turbine inlet is generally adjacent to the main steam outlet header  19 . Similarly, the intermediate-pressure turbine IP 1  may be elevated so that the hot reheat inlet to the turbine IP 1  is generally adjacent to the hot reheat outlet header  21 . As shown therein, the main steam outlet header  19  and hot reheat outlet header  21  are therefore positioned on the front side of the boiler such that the outlets of the headers and the inlets of the turbine are in close proximity to one another. 
     As illustrated in  FIG. 2 , the turbines may be configured to rotate about a substantially horizontal axis  40 , although the vertical arrangement described above may also be utilized without departing from the broader aspects of the invention. In an embodiment, the turbine may be rotated  90  degrees relative to the conventional arrangement (i.e., rotation axis  40  in the direction of boiler, as shown in  FIG. 2 ) to further position the inlet of the turbine closer to the boiler  12 , and to achieve a symmetrical arrangement. In any implementation, the turbines are disposed elevated with respect to the ground so that the length of the main steam piping  18  and hot reheat piping  20  (which fluidly connects the main steam outlet header  19  and hot reheat outlet header  21  to the turbine) can be minimized. In particular, by elevating the turbine and rotating the turbine ninety degrees so that the rotation axis extends towards the front face of the boiler, the lengths of the main steam piping  18  and hot reheat piping can be reduced to an almost negligible length, so that the investment in such steam pipes is greatly reduced. This is in contrast to conventional systems where long lengths of piping are typically necessary to connect the steam outlets near the top of the boiler to the steam turbines located generally at ground level. 
     In an embodiment, the main steam piping system  18  and hot reheat piping system  20  may be outfitted with main steam bypass valves  42  and hot reheat bypass valves  44  to selectively control the flow of steam therethrough, as shown in  FIGS. 2-4 . As discussed above, as shown in  FIGS. 2-5 , the main steam outlet header  19  and hot reheat outlet header  21  are separated from the boiler, and fluidly coupled to the heat exchanger tubing of the superheater  15  and reheater  13 , respectively, within the boiler (which absorb heat from the combustion gases passing through the boiler to generate steam) to receive steam and exhaust gasses therefrom. For example, in an embodiment, the system  100  includes header connecting tube arrays  46  and  48  that fluidly couple the tube bundles of the superheater  15  and the tube bundles of the reheater  13 , respectively, to the main steam outlet header  19  and hot reheat outlet header  21 , respectively. In an embodiment, the connecting tube arrays  46 ,  48  each comprise plurality of relatively thin-walled, small diameter tubes  45 ,  47  (in contrast to the thick-walled, relatively large diameter pipes of the main steam piping  18  and hot reheat piping  20 ). The tubes  45 ,  47  of the connecting tube arrays  45 ,  48  may be arranged to comprise respective horizontal tube portions  52  in fluid communication with respective vertical tube portions  54 . In embodiments, the respective horizontal tube portions  52  of the tubes  45 ,  47  will be elongate, defining a generally horizontal longitudinal axis therethrough; and the respective vertical tube portions  54  of tubes  45 ,  47  will be elongate and define a generally vertical longitudinal axis therethrough. The tubes  45 ,  47  of the tube arrays  46 ,  48  will have thinner walls and smaller diameters than the headers  19 ,  21  and the piping  18 ,  20 ,  22 , and therefore the tubes  45 ,  47  of the tube arrays  46 ,  48  will be more readily flexible in response to heating and cooling of the system  10  than the headers  19 ,  21  and the piping  18 ,  20 ,  22 . 
     In an embodiment, the connecting tubes  45 ,  47  of the connecting tube arrays  46 ,  48  are arranged extending from, and in fluid communication with, corresponding heat exchanger tubes of the superheater  15  and reheater  13 , for example by welding or mechanically coupling, and may generally have the same wall thickness and diameter as the heat exchanger tubes of the superheater and reheater. 
     As best illustrated in  FIG. 5 , the tubes  45 ,  47  of the connecting tube arrays  46 ,  48  may each include a generally horizontal tube portion  52  and a generally vertical tube portion  54 . As will be described in more detail herein, the generally horizontal tube portion  52  and the generally vertical tube portion  54  cooperatively define at least one flexible coupling tube portion  56  of system  10 . With further reference to  FIG. 5 , the positions of the main steam outlet headers  19  and hot reheat outlet headers  21  are illustrated in relation to a conventional position of the main steam outlet headers  19  and hot reheat outlet headers  21 , which are indicated by reference numerals  60  and  70 , respectively. As discussed above, this arrangement allows for relatively short lengths to be used for the main steam piping  18  and hot reheat piping  20 , while at the same time accommodating, or allowing for, thermal expansion during boiler operation, as discussed hereinafter. 
     In particular, to compensate for the horizontal and vertical thermal displacement from the boiler and the thermal displacement from the turbine and critical piping, the headers  19 ,  21  are located outside the main steel structure of the boiler  12 , and are thus essentially ‘decoupled’ or separated from the boiler  12 . The connecting tube arrays  46 ,  48  extend from the heat exchanger tube bundles within the boiler  12  (e.g., the superheater  15  and reheater  13 , respectively), and are fluidly connected to the headers  19 ,  21  outside the main structure of the boiler  12 . Because the connecting tube arrays  46 ,  48  comprise relatively thin-walled tubes having small diameters (e.g., about 1.5 inches in one embodiment), these tubes  45 ,  47  are operative to more readily elastically bend and flex than typical piping having thicker walls and larger diameters that is conventionally fluidly coupled to the heat exchanger tube bundles. 
     In particular, the horizontal header connecting tube portions  52  of the connecting tube arrays  46 ,  48  allow for thermal expansion of the boiler in the vertical direction, as they are able to deflect upwardly and downwardly as the boiler expands and contracts in the vertical direction. Similarly, the vertical header connecting tube portions  54  of the connecting tube arrays  46 ,  48  allow for thermal expansion of the boiler in the horizontal direction and for thermal expansions from the turbine(s), piping  18 ,  20 , and outlet headers  19 ,  21 , as they are able to deflect laterally as these components expand and contract laterally. In an embodiment, the outlet headers  19 ,  21  may be supported on z-stops, but they are otherwise able to expand horizontally towards the boiler due to the thermal expansion of the turbine and piping  18 ,  20 . 
     In contrast to existing designs where the main steam outlet header and hot reheat outlet header are essentially coupled to the boiler, e.g., through a substantially rigid connection with the enclosing walls of the boiler, the main steam outlet header  19  and/or hot reheat outlet header  21  are moved outside of the main enclosing walls of the boiler  12  and fixedly integrated with the main steam piping system  18  and hot reheat piping  20  system, respectively. 
     As noted hereinabove, while it was previously necessary to accommodate thermal displacement in the piping systems coupled to the heat exchanger tube bundles by arranging expansion loops or like structures between the outlet headers and the turbine (i.e., within the main steam and hot reheating piping downstream from the headers), the embodiments described herein eliminate this need. By disposing the outlet headers  19 ,  21  outside of the boiler  12  structure, and fluidly coupling the flexible tube arrays  46 ,  48  between the outlet headers  19 ,  21  and the boiler  12  (i.e., upstream of the headers), the small diameter tubes  45 ,  47  of the tube arrays  46 ,  48  defining the at least one flexible coupling tube portion  56 , flexibly accommodate thermal displacement events. In an embodiment, the system  10  includes a first flexible coupling tube portion  56  disposed upstream from the first header  19  is fluidly coupled between the first header  19  and the boiler  12  operative to flex relative the first header  19  and the boiler  12  in response to a thermal displacement of the boiler. In an embodiment the system  10  may include a second flexible coupling tube portion  56  upstream from the second header  21  operative to flex relative the second header  21  and the boiler  12  in response to a thermal displacement of the boiler. The flexure provided by relatively small diameter tubes  45 ,  47  of the tube arrays  46 ,  48  accommodate thermal expansion and contraction of system components, minimizing or preventing the buildup of stresses in the piping  18 ,  20  system. 
     By disposing the flexible coupling tube portion  56  defined by the tube arrays  46 ,  48  upstream from the headers  19 ,  21  (again, by locating the headers outside the boiler  12  and by using the plurality of narrow thin-walled tubes to provide flexibility during thermal expansion), no expansion loops or like structures that would be typically necessary to allow for flexure are needed in the thicker-walled main steam piping system and hot reheat piping system, thereby allowing the lengths of piping for such systems to be greatly minimized. As discussed above, this can significantly reduce the costs of manufacture of a power plant as a whole, particularly where very expensive, specialty alloys (e.g., nickel based alloys) are used for critical piping systems. In addition to substantial cost savings, using short piping lengths for the main steam and hot reheat piping also results in faster manufacturing and reduced assembly time. 
     The configuration of the system of the invention, namely, the particular arrangement of the main steam piping system, hot reheat piping system, outlet headers and connecting tube arrays discussed above, allows the headers  19 ,  21 , critical piping systems and turbines to move/expand relative to the boiler  12 , and vice versa. As set forth in detail above, this configuration arranges at least one flexible coupling tube portion  56  of the system located upstream from the headers  19 ,  21  (between the superheater/reheater and the headers  19 ,  21 , indicated generally by flexible coupling tube portion  56  in  FIG. 5 ) allowing the main steam piping system  18  and hot reheat piping system  20  downstream from the header to be constructed in short lengths (which are not prone to bending or flexing during thermal expansion due to the large diameter and wall thickness thereof). This ability to construct the main steam piping system and hot reheat piping system from relatively short lengths of material results in substantial material cost savings, driving down the cost of facility construction as a whole. This is in contrast to existing systems where the headers are essentially fixedly and rigidly connected to the boiler itself, such that only the turbine and critical piping were permitted to move/expand relative to the boiler and outlet headers and where expansion loops or longer lengths of critical piping are necessary to provide the necessary flexibility downstream from the headers to allow for thermal expansion of system components. 
     While the header connecting tubes have been described herein as being able to bend or flex to provide flexibility to the system to accommodate relative movement between the boiler and headers as a result of their relatively thin-walled and/or smaller diameter construction as compared to the relatively thick-walled and/or larger diameter main steam and hot reheat piping, it is contemplated that the ability to provide relative movement between the boiler and headers may be achieved by other means as well. For example, a greater flexing or bending ability may be provided in the header connecting tubes as compared to the main steam and hot reheat piping by varying the parameters of the header connecting tubes as compared to the main steam and hot reheat piping. Varying the parameters may include for example, providing the header connecting tubes with thinner walls, smaller diameter, different material selection, or other different material properties that facilitate bending as compared to the main steam and hot reheat piping. 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. 
     This written description uses examples to disclose several embodiments of the invention, including the best mode, and also to enable one of ordinary skill in the art to practice the embodiments of 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 one of ordinary skill in the art. Such other examples are intended to be within the scope of the claims 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 languages of the claims.