Systems and methods for pre-warming a heat recovery steam generator and associated steam lines

Embodiments of the invention can provide systems and methods for pre-warming a heat recovery steam generator and associated steam lines. According to one embodiment, a method for pre-warming a heat recovery steam generator can be provided. The method can include providing heating steam from a steam source. The heating steam is directed from the steam source to a superheater so that at least a portion of the superheater can be warmed. Once exiting the superheater, the heating steam can be further directed from the superheater to at least one bypass line and maintained in the bypass line until the bypass line attains a predefined temperature or pressure. Furthermore, the method can include directing, after the bypass line attains a predefined temperature or pressure, at least a portion of the heating steam from the bypass line to a reheater so that the reheater can be warmed.

FIELD OF THE INVENTION

The invention relates to a combined cycle power plant, and more specifically to systems and methods for providing systems and methods for pre-warming a heat recovery steam generator and associated steam lines.

BACKGROUND OF THE INVENTION

A combined cycle power plant can use a combination of a gas turbine and a steam turbine to produce electrical power. In a combined cycle power plant, a gas turbine cycle can be operatively combined with a steam turbine cycle by way of a heat recovery steam generator (“HRSG”).

In a combined cycle power plant, the gas turbine cycle can be referred to as a topping cycle, and the steam turbine cycle can be referred to as a steam bottoming cycle. Since the steam turbine or bottoming cycle is driven by heat from the exhaust of the gas turbine or topping cycle, the HRSG does not, in some instances, become fully operational until the gas turbine or topping cycle has increased the steam turbine or bottoming cycle to a suitable temperature.

For example, during start-up of the gas turbine or topping cycle, there is a relatively rapid increase in the flow rate of the hot gas exhaust from the gas turbine as the turbine accelerates to operating speed. At this point, the temperature of the exhaust gas gradually increases as the firing temperature of the gas turbine is increased and managed at a suitable level to produce a desired power output.

Although the hot exhaust gas from the gas turbine typically flows through the HRSG during the gas turbine start-up, a considerable period of time can elapse before an initially cold HRSG is capable of generating steam at sufficient pressure and temperature. In conventional systems, the gas turbine or topping cycle was kept at a relatively low load until the temperature of the HRSG increased to a level where the HRSG could generate steam at a desired pressure and temperature. By maintaining the topping cycle at a low load for an extended period of time, the steam passing to the steam turbine could be controlled at a temperature and pressure that would reduce stresses on the cold steam turbine metal and component parts. When the topping cycle was not maintained at a low load for this warm-up phase, the steam turbine or bottoming cycle was subjected to stresses that reduced its operational life. Parts fatigue, casing and shaft distortions, and physical deterioration of the HRSG system's seals and blades are just some examples of the damage these stresses can cause. In contrast, operating the gas turbine or topping cycle at a relatively low load can reduce these stresses and corresponding damage. Doing so, however, can reduce the combined cycle system's overall power output, leads to inefficiency, and increases emissions.

Consequently, there is a need for systems and methods for pre-warming a HRSG and associated steam lines. Furthermore, there is a need for systems and methods for pre-warming a HRSG and associated steam lines in multiple state conditions, such as cold, warm, and hot conditions.

BRIEF DESCRIPTION OF THE INVENTION

Embodiments of the invention can address some or all of the needs described above. Certain embodiments of the invention are directed generally to systems and methods for pre-warming a heat recovery steam generator (“HRSG”) and associated steam lines. Certain other embodiments of the invention are directed to systems and methods for pre-warming a superheater, a reheater, multiple stages of a steam turbine, and/or a high pressure evaporator and drum. According to one embodiment, a method for pre-warming a HRSG can be provided. The method can include providing heating steam from a steam source. The method can also include directing the heating steam from the steam source to a superheater to warm at least a portion of the superheater. In addition, the method can include directing at least a portion of the heating steam from the superheater to at least one bypass line and maintaining it in the bypass line until a predefined temperature or pressure is attained. Furthermore, the method can include directing at least a portion of the heating steam, once the bypass line attains a predefined temperature or pressure, from the bypass line to a reheater so that the reheater can be warmed.

According to another embodiment of the invention, a system for pre-warming a HRSG can be provided. The system can include a steam source for providing heating steam. The system can also include at least one steam line connected to the steam source and a superheater for directing heating steam from the steam source to the superheater. The system can further include at least one steam line connected to the superheater and at least one bypass line for directing at least a portion of the heating steam from the heating steam to the at least one bypass line. The system can also include a controller connected to the at least one bypass line for maintaining at least a portion of the heating steam in the at least one bypass line at a predefined temperature or pressure. Moreover, the system can include at least one steam line connected to the bypass line and a reheater for directing at least a portion of the heating steam at the predefined temperature or pressure from the at least one bypass line to the reheater.

According to yet another embodiment of the invention, a method for pre-warming a HRSG can be provided. The method can include providing heating steam from a steam source. The method can also include directing the heating steam from the steam source to a superheater to warm at least a portion of the superheater. The method can further include directing at least a portion of the heating steam from the steam source to a high pressure section of a steam turbine to warm at least a portion of the high pressure section of the steam turbine. In addition, the method can include directing at least a portion of the heating steam from the superheater to a reheater, and from the reheater to an intermediate section of the steam turbine. Finally, the heating steam can be directed from the intermediate pressure section of the steam turbine to a low pressure section of the steam turbine to warm the low pressure section of the steam turbine.

Other embodiments and aspects of the invention will become apparent from the following description taken in conjunction with the following drawings.

DETAILED DESCRIPTION OF THE INVENTION

The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which example embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the example embodiments set forth herein; rather, these embodiments are provided so that this disclosure will convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

FIG. 1illustrates a conventional heat recovery steam generator (“HRSG”) system100known in the prior art. In the system100shown, steam is cycled through a series of steam turbine stages, including a high pressure stage105, an intermediate pressure stage110, and a low pressure stage115. A condenser120is connected to low pressure stage115and high pressure stage105to collect condensation. Valve121connects between the condenser120and high pressure stage105to control pressure between these two components.

Superheated steam can be provided by a high pressure superheater125, which directs superheated steam to the high pressure stage105via main steam line130. Drain126is available for draining condensate between superheater125and main steam line130. Between main steam line130and high pressure superheater125, valve129can control pressure and steam flow between these two components. Valve132between main steam line130and high pressure stage105likewise can control pressure and steam flow between these two components. Drain131is available to drain condensate from main steam line130.

High pressure superheater125can receive steam from a high pressure evaporator and drum135. High pressure evaporator and drum135can convert feedwater into steam using hot exhaust gas dispelled by the gas turbine or topping cycle. High pressure superheater125can also rely on the gas turbine exhaust gas to convert at least a portion of the steam from high pressure evaporator and drum135to superheated steam. In system100, feedwater control is provided by valve136.

When superheated steam is directed to high pressure stage105from main steam line130, the superheated steam is converted to mechanical energy by way of an associated shaft. Rotation of the shaft can be used to drive a load, such as an electrical generator. When exiting high pressure stage105, the steam has lost energy due to the energy conversion. The steam can be reheated before being fed to the intermediate pressure stage110. As a result, once the exiting steam passes through non-return valve133, the steam can enter intermediate pressure steam line134, which is connected to reheat steam line140. Drain136is present for removing condensate from intermediate pressure steam line134, and valve138controls pressure and steam flow between intermediate pressure steam line134and reheat steam line140.

Reheat steam line140is connected to reheater145. Reheater145can increase the temperature of the steam and provides the reheated steam to hot reheat steam line150, which connects reheater145with intermediate pressure stage110. Valve147and valve152can control pressure and steam flow between these components, and drains146and151are available for removing condensate from the system100. As steam exits intermediate pressure stage110, the steam enters the low pressure stage115, and as the steam travels through these two stages110,115, the associated heat energy is again converted into mechanical energy by way of an associated shaft. Upon exiting the low pressure stage115, some or all of the residual steam and water, which is the byproduct of the steam conversion, can be collected in condenser120.

Also present in system100is high pressure steam bypass line160, which connects the outlet of high pressure superheater125to reheat steam line140, and is operable to direct steam from superheater125to reheat steam line140. High pressure steam bypass valve165is operable to control the corresponding steam pressure and steam flow in high pressure steam bypass line160. Hot reheat steam bypass line170connects the outlet of reheater145to condenser120and is operable to direct steam between them. Steam flow and pressure between reheater145and condenser120can be managed with reheat bypass valve175. At valve180is an inlet for intermediate pressure steam, and at joints185,190, and195another HRSG system can be integrated with system100.

Because the steam turbine or bottoming cycle derives mechanical energy from steam and superheated steam, the bottoming cycle components and associated steam lines operate at an extremely high temperature. When not operating, though, the temperatures of these components and steam lines may fall outside a certain operating range. When the system100has not been operated for an extended period of time, such as 48 hours or more, it is said to be in a “cold” thermo-state condition. If the HRSG system is then turned on immediately such that superheated steam is fed through the cold system to generate power, physical stresses on the components and steam lines will result.

FIG. 2illustrates an exemplary system200according to one embodiment of the invention. Similar to system100, system200includes multiple steam turbine stages designed to produce mechanical energy when steam is introduced to them with different temperatures and different pressures. These turbine stages can include a high pressure stage105, an intermediate pressure stage110, and a low pressure stage115. Connected to low pressure stage115and high pressure stage105is a condenser120. Also present in system200is high pressure superheater125, high pressure evaporator and drum135, reheater145, and associated steam lines and valves for connecting these component parts. For example, like system100, main steam line130connects high pressure superheater125to high pressure stage105of the steam turbine or bottoming cycle. Main steam line130is operable to direct steam from superheater125to high pressure stage105. Hot reheat steam line150connects intermediate pressure stage110of the bottoming cycle to reheater145, and is operable to direct steam from reheater145to hot reheat steam line150. Dispersed among the turbine stages, lines, and components are valves132,152, and non-return valve133for controlling steam pressure and steam flow, and drains126,131,139,146, and151for removing condensate from the system. When condensate is removed, it can be collected in a storage tank or channeled to condenser120for reuse.

Also like system100, system200includes high pressure steam bypass line160, connecting the outlet of high pressure superheater125to reheat steam line140, and hot reheat steam bypass line170, connecting the outlet of reheater145to condenser120. Among these elements are similar valves for controlling steam flow, including high pressure steam bypass valve165and reheat bypass valve175.

Unlike system100, though, system200includes at least one steam source205, which is operable to provide heating steam to system200for pre-warming high pressure superheater125, high pressure evaporator and drum135, reheater145, and their associated steam lines. A steam source205can be, for example, a boiler, a steam generator, a pressurized water heater, plant steam, or another system for producing steam. Using the heating steam from steam source205, the bottoming cycle of a HRSG system can be warmed independently of the gas turbine or topping cycle's operation. This independence allows the topping cycle to be operated at any load, including a load equivalent to its maximum continuous power output rating. Moreover, this independence provides the HRSG system a relatively fast start-up capability since the HRSG system can be warmed independently of the gas turbine or topping cycle, and the HRSG system does so while preserving or otherwise minimizing adverse effects on the steam turbine cycle system's operational life as described more fully below.

Other differences between system100and system200can include any number of steam lines, valves, and associated valve controls for managing the warming steam through the various components of the steam turbine or bottoming cycle. For example, in system200, the outlet of high pressure superheater125is connected to steam line220, which through isolation valve129, is connected to main steam line130. Steam line220is operable to direct steam from the outlet of superheater125to main steam line130. Steam line220is further operable to direct steam from the outlet of superheater125to output steam line230when bypass valve235is opened. Controller240can maintain pressure and steam flow in output steam line230and in high pressure steam bypass line160through steam bypass valve165and bypass valve235. In the exemplary embodiment, output steam line230can be connected to condenser120, a storage tank, the atmosphere, or another suitable system for collecting steam.

One may recognize the applicability of embodiments of the invention to other environments, contexts, and applications. One will appreciate that components of the system200shown in and described with respect toFIG. 2are provided by way of example only. Numerous other operating environments, system architectures, and device configurations are possible. Accordingly, embodiments of the invention should not be construed as being limited to any particular operating environment, system architecture, or device configuration.

FIG. 3illustrates an exemplary method300for pre-warming a HRSG system and its associated steam lines. The method begins at block305where heating steam is provided by a steam source and continues at block310by directing the heating steam to a superheater. One embodiment for implementing this exemplary method is exemplary system200shown inFIG. 2. For example, when the bottoming cycle of system200is below normal operational temperature, heating steam is provided from steam source205and is admitted into the high pressure superheater125through valve206to warm up the superheater tubes. In doing so, superheater125can be warmed.

Method300continues at block315where heating steam is directed from the superheater to a main bypass line connected between the superheater and a reheater. Block320maintains the heating steam in the main bypass line until a predefined temperature or pressure is attained. In one embodiment, these blocks are implemented by system200with superheater125, high pressure steam bypass line160, steam line220, and steam bypass valve165. More specifically, after heating steam travels through superheater125, it is discharged into high pressure steam bypass line160and steam line220so that these lines can be warmed. Steam bypass valve165and valve235open and close to let the warming steam into high pressure steam bypass line160and output steam line230. Steam bypass valve165and valve235can work in conjunction with each other to control the pressure in the respective associated steam lines during the warming period. In one embodiment, each of the valves165and235can have predefined or otherwise programmed pressure set points, which can be calculated based at least in part on any number of conditions associated with the system200, such as the initial pressure in high pressure steam bypass line160, and so on.

In the embodiment shown inFIG. 2, controller240can control both steam bypass valve165and valve235, but it should be understood that while only one controller is illustrated in association with only these two valves, any number of controllers and valves in any number of configurations can be used to control the flow of heating steam and to pre-warm the HRSG system and any associated steam lines.

In the exemplary method300at block320, heating steam can be maintained in the bypass line until a predefined temperature or pressure is attained. The method300continues at block325where heating steam is directed to a reheater after the temperature and pressure in the bypass line have reached the appropriate thresholds. In exemplary system200, some or all of these blocks can be implemented by high pressure steam bypass line160, high pressure steam bypass valve165, reheat steam line140, and reheater145. Once the warming steam enters high pressure steam bypass line160and steam line220, these lines will begin to warm to a predefined pressure or temperature, and once reaching the appropriate pressure or temperature, high pressure bypass valve165begins to open to allow the heating steam to exit high pressure steam bypass line160and to enter reheat steam line140and reheater145. After entering reheater145, the heating steam will warm the reheater tubes.

Method300concludes at block330where the heating steam is directed from the reheater to a condenser for collection and reuse. Exemplary system200provides one embodiment for directing heating steam from the reheater to a condenser for collection and reuse when isolation valves129,147, and138are closed. In this configuration, main steam line130, steam turbine stages105,110, and115, and intermediate pressure steam line134are isolated from the heating steam. The heating steam will thus exit reheater145when isolation valve147is closed, and flow through hot reheat steam bypass line170. Hot reheat bypass valve175opens to allow heating steam into hot reheat steam bypass line170and thereafter controls pressure in the line during the warming period. As is understood in the art, the corresponding set point of the pressure control is based on the conditions in the system, such as the line pressure in the hot reheat steam pipe. Once this set point is reached, though, the heating steam travels through reheat steam bypass line170and to condenser120for collection.

In another exemplary embodiment of a method for pre-warming a HRSG, rather than direct the heating steam from the reheater to a condenser, the heating steam can be used to further warm other steam lines that may be present in the steam turbine or bottoming cycle.

For example, and in reference to exemplary system200, isolation valves129and147can be opened so that heating steam can enter and warm main steam line130and hot reheat steam line150. In one embodiment, isolation valve129opens gradually so that warming steam will be introduced to main steam line130slowly. In this way, abrupt changes in pressure in main steam line130can be avoided. In another embodiment, steam flow in main steam line130is not so controlled, but is either open or closed. Other embodiments for steam control are also available and should be well recognized in the art.

In another exemplary embodiment of a method for pre-warming a HRSG, the heating steam is used to warm the steam turbines in addition to the steam lines and components. For example, in exemplary system200, valves132and152can be closed so that steam turbine stages105,110, and115are isolated from the heating steam. Condensate is then collected in drains131,146, and151for reuse. Alternatively, valves132,152, and isolation valve138can be opened so that steam turbine stages105,110, and115can be warmed along with intermediate pressure steam line134.

As will be appreciated in the art, embodiments for warming these components can vary according to any number of factors. One such factor, for example, is the availability of heating steam. When the quantity of available heating steam is sufficient to warm the entire system, one embodiment can warm some or all of the entire steam turbine or bottoming cycle at once. When the quantity of heating steam is insufficient to warm the entire system at once, though, another embodiment can warm the bottoming cycle in stages. In exemplary system200, such a multi-phased warming cycle can be accomplished through the opening and closing of valves132,152, and isolation valves129and138, or by the closing and opening of other valves so described above and illustrated herein, so that components of the bottoming cycle are warmed in stages.

In another exemplary method, in addition to providing heating steam from a steam source, directing it to a superheater and bypass line, maintaining it in the bypass line until a predefined temperature or pressure is attained, and directing it to a reheater, the heating steam can be further directed to a high pressure section of a steam turbine or bottoming cycle so that the high pressure section can be warmed. One implementation of this operation can be described with reference to exemplary system200.

In system200, valve208can be opened so that high pressure stage105can be warmed. Controller275is present for controlling valve208so that it opens and closes according to the operation desired. Once so opened, heating steam can flow from steam source205through high pressure stage105against the operable direction of the turbine. In one embodiment, valve132is closed and the heating steam is collected in condenser120. In another embodiment, valve132is opened so that high pressure stage105and main steam line130are warmed simultaneously.

It will be appreciated that as the heating steam can be further directed to the high pressure stage after warming other components of the bottoming cycle, it can also be directed only to this section. That is, in yet another exemplary method for pre-warming a HRSG, heating steam is directed to a high pressure section of a steam turbine to warm the high pressure section of the steam turbine from a steam source. In exemplary system200, such a method could be implemented by closing valves129,206, and207, and opening valve208. With this configuration, heating steam can flow from steam source205to high pressure stage105and collected in condenser120.

In another exemplary method for pre-warming a HRSG system, heating steam can be directed so that it warms the high pressure section of the steam turbine and the reheater. In exemplary system200, such a method can be implemented by closing valves206,207, and165and opening valve208and valve138. With this configuration, heating steam can flow from steam source205to high pressure stage105and through non-return valve133. From there, the heating steam will travel through intermediate pressure steam line134through valve138to reheat steam line140and reheater145. In one embodiment, the heating steam is directed to warm intermediate pressure stage110. In another embodiment, the heating steam is collected in condenser120.

In still another embodiment of a method for pre-warming a HRSG system, heating steam is directed from a steam source to a high pressure evaporator and drum so that the high pressure evaporator and drum can be warmed. In one embodiment, heating steam is directed to the high pressure evaporator and drum only after other components are warmed. In another embodiment, heating steam is directed to the high pressure evaporator and drum before being directed to other components. In still yet another embodiment, heating steam is directed only to the high pressure evaporator and drum so that the high pressure evaporator and drum is warmed in isolation of other components of the bottoming cycle. Again, the preference for one embodiment over another may hinge on whether the quantity of available heating steam is sufficient to warm the entire system or only a part.

System200presents an exemplary system for implementing embodiments of this method. To illustrate, the high pressure evaporator and drum135can be warmed after other components by opening valve207only after those components reach the appropriate temperature. Thus, the functioning of valve207can be a function of system temperature, pressure, and/or steam flow from source205. On the other hand, high pressure evaporator and drum135can be warmed first by closing valve206and opening valve207so that the heating steam can be directed through high pressure evaporator and drum135before warming superheater125. Controller240is present for controlling valves207and206according to these exemplary methods. Although not illustrated, high pressure evaporator and drum135can also be warmed in isolation of other components using a suitable valve arrangement that would isolate it from other components in the steam turbine or bottoming cycle. Such valve arrangements are well known within the art.

Thus, the above described methods and systems provide relatively fast start-up capability for HRSG systems because they enable the steam turbine or bottoming cycle to be brought up to operational temperature when the gas turbine or topping cycle is operating at any load. More specifically, they significantly mitigate the stress issues induced by fast start-ups in HRSG systems when those HRSG systems are not at the proper operational temperature for power generation.

Using the disclosed systems and methods, combined cycle power plants can have relatively fast start-up capability no matter their thermo-state condition since the bottoming cycle can be warmed independently of the gas turbine or topping cycle. Once warmed, the steam turbine or bottoming cycle can be operated at a level corresponding to the level of the gas turbine or topping cycle with reduced stresses in the metal parts and components of what would otherwise be a cold steam turbine. As a result, the operational lives of the bottoming cycle's parts and components are preserved or otherwise extended.

This independent warming of the steam turbine or bottoming cycle allows the bottoming cycle to operate at an increased load with reduced stresses in the HRSG and the steam turbines that otherwise would result. In addition, fuel for operating the gas turbine cycle is not wasted as it would be if the gas turbine cycle were required to operate at a low load while the steam turbine or bottoming cycle warmed to its normal operating temperature. With this reduction in wasted fuel, there is also a reduction in emissions.

While the systems and methods described herein make reference to only a single gas turbine and a HRSG system using only a three pressure turbine system, one can appreciate that systems using multiple gas turbines, steam turbines, and HRSG systems can be adapted to employ the systems and methods disclosed and to capitalize on certain aspects disclosed herein.

Many modifications and other embodiments of the inventions set forth herein will also come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Thus, it will be appreciated by those of ordinary skill in the art that the invention may be embodied in many forms and should not be limited to the embodiments described above. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.