Patent Application
20120151921
The present invention relates generally to turbomachines and more particularly to a method for enhancing the operational flexibility of a steam turbine during a startup phase.
Steam turbines are commonly used in power plants, heat generation systems, marine propulsion systems, and other heat and power applications. Steam turbines typically include at least one section that operates within a pre-determined pressure range. This may include: a high-pressure (HP) section; and a reheat or intermediate pressure (IP) section. The rotating elements housed within these sections are commonly mounted on an axial shaft. Generally, control valves and intercept valves control steam flow through the HP and the IP sections, respectively.
The normal operation of a steam turbine includes three distinct phases; which are startup, loading, and shutdown. The startup phase may be considered the operational phase beginning in which the rotating elements begin to roll until steam is flowing through all sections. Generally, the startup phase does not end at a specific load. The loading phase may be considered the operational phase in which the quantity of steam entering the sections is increased until the output of the steam turbine is approximately a desired load; such as, but not limiting of, the rated load. The shutdown phase may be considered the operational phase in which the steam turbine load is reduced, and steam flow into each section is gradually stopped and the rotor, upon which the rotating elements are mounted, is slowed to a turning gear speed.
For steam turbines equipped with cascade steam bypass systems, the startup phase begins with steam admission to the IP section using intercept valves. Subsequently, steam is admitted to the HP section. This process of admitting steam into the HP section, which completes the startup process, is generally referred to as forward flow transfer. Typically, steam flow through the HP and IP sections is balanced during forward flow transfer. The amount of steam flow is typically dependent on the operating Reheat (RH) pressure. This balanced flow transfer technique may, however, introduce a few issues.
This technique does not consider all of the physical parameters that may affect steam turbine operation during the startup phase. For example, during a hot start, a larger amount of HP steam flow is typically required to prevent high HP Section exhaust temperature. In order to balance the HP and IP steam flow after transfer, the IP steam flow prior to transfer should also be correspondingly higher. However, the increased IP steam flow prior to transfer may increase the axial thrust on the IP section, as insufficient steam may flow through the HP section to balance the higher IP steam flow. If, on the other hand, the IP steam flow prior to transfer is reduced to lessen the axial thrust load, the HP steam flow upon transfer will be lower than desired; and result in undesirably high HP section exhaust temperature. Rotor stress is another physical parameter that may be considered during forward flow transfer. In cold starts, very high HP steam flow may cause undesirable rotor stress. Therefore, multiple physical parameters that can affect the steam turbine should be considered before permitting steam flow into the IP and HP sections.
Some start-up strategies attempt to satisfy one or two of these physical parameter constraints. For example, but not limiting of, one strategy attempts to reduce rotor stresses introduced during the start-up process. This technique, however, does not address high HP section exhaust temperature caused by low steam flow into the HP section during forward flow transfer. Other starting strategies do not address all limiting physical parameters of existing steam flow balancing hot-start strategies, such as, but not limiting of, high HP section exhaust temperature.
These issues reduce the operational flexibility, require larger mechanical components, and potentially reduce the net-output delivered by the steam turbine during the startup phase. Therefore, there is a desire for a method for increasing the operational flexibility of the steam turbine during the startup phase.
In accordance with an embodiment of the present invention, a method of unbalancing steam flow entering a turbomachine during a startup process, the method comprising: providing a turbomachine comprising at least a first section and a second section, and a rotor partially disposed within the first section and the second section; providing a first valve configured for controlling steam flow into the first section; and a second valve configured for controlling steam flow into the second section; determining whether the turbomachine is operating in a startup phase; which begins when steam causes the rotor to rotate until steam is flowing through the first section and the second section; determining an allowable turbine operating space (ATOS), wherein ATOS incorporates data on a least one of the following: steam flow through each section, an axial thrust limit of each section, and an HP section exhaust windage limit to approximate operational boundaries for each section of the turbomachine; determining an allowable range within ATOS of a physical parameter associated with the startup phase; modulating the first valve to control steam flow into the first section, wherein the modulation is partially limited, by the allowable range of the physical parameter; modulating the second valve to allow steam flow into the second section, wherein the modulation is partially limited by the allowable range of the physical parameter; and wherein ATOS, in real time, expands operational boundaries of the first section and the second section, and allows unbalanced steam flow between the first section and the second section of the turbomachine during the startup phase.
In accordance with an alternate embodiment of the present invention, the method of independently apportioning steam flow between sections of a steam turbine during a startup process, the method comprising: providing a power plant comprising a steam turbine, wherein the steam turbine comprises a HP section, an IP section, and a rotor partially disposed within the HP and IP sections; providing a first valve configured for controlling steam flow entering the HP section; and a second valve configured for controlling steam flow entering the IP section; determining whether the steam turbine is operating in a startup phase; determining an allowable turbine operating space (ATOS), wherein ATOS incorporates data on a least one of the following: steam flow through each section, an axial thrust limit of each section, and an HP Section exhaust windage limit to approximate operational boundaries for each section of the turbomachine; determining an allowable range within ATOS of a physical parameter associated with at least one of the first section or the second section; generating a range of valve strokes for the first and second valves based on the allowable range of the physical parameter; modulating the first valve to allow steam flow into the HP section, wherein the modulation limits the range of valve strokes for the first valve; and modulating the second valve to allow steam flow into the IP section, wherein the modulation limits the range of valve strokes for the second valve; and wherein the physical parameter allows apportioning steam flow into the HP and the IP sections, during the startup phase of the steam turbine.
The present invention has the technical effect of expanding the operational flexibility of a steam turbine during a startup phase. As the steam turbine operates, the present invention determines the Allowable Turbine Operating Space (ATOS) of each turbine section. Next, the present invention may adjust the steam entering each turbine section based on ATOS. Here, the quantity steam flow entering each turbine section is not dependent on the quantity of steam flow entering another turbine section.
The following detailed description of preferred embodiments refers to the accompanying drawings, which illustrate specific embodiments of the invention. Other embodiments having different structures and operations do not depart from the scope of the present invention.
Certain terminology may be used herein for the convenience of the reader only and is not to be taken as a limitation on the scope of the invention. For example, words such as “upper”, “lower”, “left”, “right”, “front”, “rear”, “top”, “bottom”, “horizontal”, “vertical”, “upstream”, “downstream”, “fore”, “aft”, and the like; merely describe the configuration shown in the Figures. Indeed, the element or elements of an embodiment of the present invention may be oriented in any direction and the terminology, therefore, should be understood as encompassing such variations unless specified otherwise.
Detailed example embodiments are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments. Example embodiments may, however, be embodied in many alternate forms, and should not be construed as limited to only the embodiments set forth herein.
Accordingly, while example embodiments are capable of various modifications and alternative forms, embodiments thereof are illustrated by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit example embodiments to the particular forms disclosed, but to the contrary, example embodiments are to cover all modifications, equivalents, and alternatives falling within the scope of example embodiments.
It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments. As used herein, the term “and/or” includes any, and all, combinations of one or more of the associated listed items.
The terminology used herein is for describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises”, “comprising”, “includes” and/or “including”, when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
The present invention may be applied to a variety of steam turbines, or the like. An embodiment of the present invention may be applied to either a single steam turbine or a plurality of steam turbines. Although the following discussion relates to a steam turbine having an opposed flow configuration and a cascade steam bypass system, embodiments of the present invention are not limited to that configuration. Embodiments of the present invention may apply to other configurations that are not opposed flow and/or not equipped with a cascade steam bypass system.
Referring now to figures, where the various numbers represent like elements through the several views,
As illustrated in
The steam turbine 102 may include a first valve 116 and a second valve 118 for controlling the steam flow entering the first section 110 and the second section 112, respectively. In various embodiments of the present invention, the first valve 116 and the second valve 118 may be a control valve 116 and an intercept valve 118 for controlling the steam flow entering the HP section 110 and the IP section 112, respectively.
The cascade steam bypass system 120 may function during the startup phase of the steam turbine 102. As illustrated in
After the startup phase is substantially complete, steam exiting from the HP section 110 may flow through the reheater unit 104 where the temperature of the steam is raised before flowing into the IP section 112. Subsequently, the steam from the reheater unit 104, may flow in to the IP section 112 via the intercept valve 118, and the LP section 114, as illustrated in
As illustrated by line 202, the known flow strategy seeks to balance the steam flow between the sections 110, 112. This typically involves maintaining equal steam flow through the HP section 112 and the IP section 114 after the startup phase is complete. The line 202 connecting the points A, B, and C represent the variation of the steam flow through the HP section 112 with the steam flow through the IP section 114 during the loading process of the steam turbine 102. Line 202 may be considered the natural pressure line; which indicates equal or balanced flow through the HP and IP sections 110, 112.
The following should be considered when reviewing the FIGS and corresponding discussion on ATOS. All figures should be considered non-limiting examples that may be associated with certain steam turbine 102 configurations. Furthermore, the numerical ranges on each figure are for illustrative purposes only. ATOS should be considered a region within which a steam turbine 102 may operate. Each ATOS boundary, discussed and illustrated below, should not be considered a fixed or limiting boundary. ATOS, and its associated boundaries should be considered a changing and dynamic operating environment. This environment is determined, in part, by the configuration, operational phase, boundary conditions and mechanical components and design of the steam turbine 102. Other directions, shapes, sizes, magnitudes, and sizes of ATOS and its boundaries, not illustrated in the figures, do not fall outside of the nature and scope of embodiments of the present invention. Therefore, the direction, magnitude, shape, and size of ATOS and its boundaries, as illustrated in the figures, are merely illustrations of non-limiting examples.
The X-axis illustrates steam flow through the HP section 110. The left Y-axis illustrates steam flow through the IP section 112 and the right Y-axis illustrates a reheat pressure. The natural pressure line 202, passing through the points A, B, and C illustrates the balanced flow strategy, as previously discussed.
The axial thrust lines 1-2 and 3-4 are a function of steam flow through the opposing HP and IP sections 110, 112. Lines 1-2 and 3-4 may represent the allowable flow imbalance that a specific steam turbine 102 may tolerate before experiencing an undesirably high axial thrust load. The actual shape and associated values of these lines depend, inter alia, on the thermodynamic design of each section 110, 112 and the size of the associated thrust bearing. Advanced steam turbine designs may increase the axial thrust force and limit the allowable flow imbalance, reducing ATOS 302. Similarly, increasing the thrust bearing size may allow greater flow imbalance and increase ATOS 302.
The HP section Exhaust Windage Line, line 5-6, may be a function of the minimum HP flow required to prevent undesirably high temperatures at the latter stages of the HP section 110; as a function of the RH pressure and HP inlet steam temperature. Higher RH pressure may drive higher pressure at the HP section exhaust. This may decrease the pressure ratio through the HP section 110, for a given flow and a given inlet steam temperature. This may also increase the HP section exhaust temperature. Similarly, higher HP inlet steam temperature may also increase the HP section exhaust steam temperature, for a given steam flow at a given RH pressure.
During the operation of some steam turbines 102, the HP section exhaust temperature may approach material-specific limiting values when the RH pressure reaches a higher than desired condition with high inlet steam temperature. However, as the steam turbine 102 operates at reduced inlet steam temperatures, the likelihood of high HP section exhaust temperature is lessened even with high RH pressure. Here, the enthalpy of HP inlet steam reduces significantly with reduced temperature. Therefore, the HP section windage considerations may be limiting in certain conditions, such as, but not limiting of, when the HP inlet steam temperature is high.
As discussed, lines 1-2, 3-4, and 5-6 are boundaries that may define ATOS 302 at a given operational condition. These lines are dynamic in nature. Embodiments of the present invention may determine, in real time, ATOS 302; and allow greater operational flexibility. In practical terms, each ATOS boundary may be considered a physical parameter that defines ATOS 302 of a specific steam turbine 102. The physical parameter may include, but is not limiting to: axial thrust, rotor stress, steam temperature, steam pressure, HP section exhaust windage limit, minimum HP flow for forward flow transfer (MINF), or maximum HP flow for forward flow transfer (MAXF).
As illustrated in
In an embodiment of the present invention, ATOS allows for the decoupling of the steam flow through the HP section 110, and the IP section 112 during the startup phase. This may allow for increased steam flow and operational flexibility during the startup phase. In an embodiment of the present invention, as illustrated in
The method 400 may control the first valve 116 and the second valve 118 for controlling steam flow through the first section 110 and the second section 112 respectively. In various embodiments of the present invention, the first valve 116 and the second valve 118 may be the control valve 116 and the intercept valve 118 that control steam flow through the HP section 110 and the IP section 112 respectively, as previously discussed.
In step 410, the method 400 may determine which operating phase of the steam turbine 102. As discussed, the steam turbine 102 normally operates in the three distinct, yet overlapping, phases; startup, loading, and shutdown. As discussed, the startup phase may begin when the rotor 115 begins to roll until steam is flowing through all sections 110, 112. Generally, the startup phase does not end at a specific load Embodiments of the present invention may be function during the startup phase.
In step 420, the method 400 may determine whether the steam turbine 102 is operating in the startup phase. Here, the method 400 may receive operating data or operational data from a control system 106 that operates the steam turbine 102. This data may include, but is not limited to, positions of the valves 116, 118. If the steam turbine 102 is operating in the startup phase then the method 400 may proceed to step 430; otherwise, the method 400 may revert to step 410.
In step 430, the method 400 may determine the current ATOS 302. Here, the method 400 may receive current data related to the ATOS boundaries, as described. The method 400 may receive data on the physical parameter associated with the ATOS boundaries. This data may be compared to the allowable or the preferred limits and the boundaries. For example, but not limiting of, an ATOS boundary may include an axial thrust. Here, the method 400 may determine the current axial thrust and allowable axial thrust for the current operating conditions.
In an alternate embodiment of the present invention, the method 400 may incorporate a transfer function, algorithm, or the like to calculate, or otherwise determine ATOS 302.
In step 440, the method 400 may determine an allowable range of a physical parameter associated with at least one of the first section 110 of the steam turbine 102. The physical parameter may include, but is not limiting to, an operational and/or physical constraints. These constraints may include, but are not limited to: axial thrust, rotor stress, steam temperature, steam pressure, HP section exhaust windage limit, MINF, or MAXF. The method 400 may then generate a range of valve strokes for the first valve 116 based on the allowable range of the physical parameter.
In step 450, the method 400 may modulate the first valve 116 to allow steam flow into the first section 110 of the steam turbine 102. The method 400 may modulate the first valve 116 based on the allowable range of the physical parameter.
In step 460, the method 400 may determine an allowable range of a physical parameter associated with at least one of the second section 112 of the steam turbine 102. The physical parameter may include, but is not limiting to, an operational and/or physical constraints. These constraints may include, but are not limited to: axial thrust, rotor stress, steam temperature, steam pressure, HP section exhaust windage limit, MINF, or MAXF. The method 400 may then generate a range of valve strokes for the second valve 118 based on the allowable range of the physical parameter.
In step 470, the method 400 may modulate the second valve 118 to allow steam flow into the second section 112 of the steam turbine 102. The method 400 may modulate the second valve 118 based on the allowable range of the physical parameter.
Embodiments of the present invention allow for real time determination of a change in the physical parameters that bound ATOS 302. Therefore, after steps 450 and 470 are completed, the method 400 may revert to step 410.
Similar to
In use, an embodiment of the present invention provides a new startup phase methodology for the steam turbine 102; which may include multiple stages. In an embodiment of the present invention, each stage may be based, at least in part, on a current ATOS boundary.
As discussed, the numerical ranges discussed and illustrated on
The following provides a non-limiting example of an embodiment of the present invention, in use during a startup phase. In an embodiment of the present invention, the startup process of the steam turbine 102 may include two stages. First from S1-S3a; which includes initiating steam flow into the IP section 114, via the second valve 118. In an embodiment of the present invention, initial steam flow through the IP section 114 may be about 20%.
Next, from S3a-S3, which may include the first valve 116 initiating steam flow into the HP section 112. Here, steam flow through the IP section 114 may be increased to the current operational range of the IP section 114. In an embodiment of the present invention, steam flow through the IP section 114 may be increased to approximately 42%, and steam flow through the HP section 112 may be increased to approximately 40%. As illustrated by comparing
Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art appreciate that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown and that the invention has other applications in other environments. This application is intended to cover any adaptations or variations of the present invention. The following claims are in no way intended to limit the scope of the invention to the specific embodiments described herein.
This application is related to commonly-assigned U.S. patent application, ______ [GE Docket 238811], filed ______; U.S. patent application, ______ [GE Docket 238815], filed ______; and U.S. patent application, ______ [GE Docket 238821], filed ______.
