Abstract:
A system and method control condensate formation within headers of a heat recovery steam generator (HRSG) in a power generation facility. The system includes measurement devices to measure first parameters including pressure at respective first locations within the power generation facility. A model estimates second parameters at second locations within the power generation facility. A prediction model outputs a prediction of time of condensate formation on each of the headers based on the first parameters and the second parameters, and a controller controls the condensate formation based on the prediction.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to reducing thermal stresses in a heat recovery steam generator (HRSG). 
         [0002]    In a combined cycle power plant, waste heat from a gas turbine is used by a heat recovery steam generator (HRSG) to generate steam for operation of the steam turbine. Because of the large thickness of the HRSG header pipes, a temperature gradient can form across the thickness of a header pipe when condensate forms on the inside of the headers, cooling the inner metal while the middle and outer metals remain hotter. This temperature gradient creates thermal stresses that reduce the life of the headers. Prior art HRSG systems have tried to control the temperature gradient by opening the drains and vents of the HRSG using rule-based or timed control. However, these prior controls are not effective. Thus, systems and methods to monitor and control condensate formation to mitigate the temperature gradient effect would be appreciated in the power generation industry. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    According to an aspect of the invention, a system to control condensate formation within headers of a heat recovery steam generator (HRSG) in a power generation facility includes measurement devices configured to measure first parameters including pressure at respective first locations within the power generation facility; a model configured to estimate second parameters at second locations within the power generation facility; a prediction model configured to output a prediction of time of condensate formation on each of the headers based on the first parameters and the second parameters; and a controller configured to control the condensate formation based on the prediction. 
         [0004]    According to another aspect of the invention, a method to control condensate formation within headers of a heat recovery steam generator (HRSG) in a power generation facility includes measuring parameters including steam temperature at respective first locations within the power generation facility; estimating thermodynamic characteristics within the headers of the HRSG; predicting time of condensate formation for each header of the HRSG based on the parameters and the thermodynamic characteristics obtained from the measuring and the estimating; and controlling the condensate formation to prevent or reduce the condensate formation predicted by the predicting. 
         [0005]    According to yet another aspect of the invention, a computer-readable medium stores instructions that, when processed by a processor, cause the processor to implement a method to control condensate formation within headers of a heat recovery steam generator (HRSG) in a power generation facility. The method includes receiving first parameters including steam temperature measured at respective first locations within the power generation facility; estimating thermodynamic characteristics within the headers of the HRSG; predicting time of condensate formation for each header of the HRSG based on the parameters and the thermodynamic characteristics obtained from the measuring and the estimating; and controlling the condensate formation to prevent or reduce the condensate formation predicted by the predicting. 
         [0006]    These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0008]      FIG. 1  is a block diagram of exemplary components of a combined cycle power plant; 
           [0009]      FIG. 2  depicts aspects of an exemplary bottom header of the HRSG; 
           [0010]      FIG. 3  depicts aspects of an exemplary top header of the HRSG; 
           [0011]      FIG. 4  is a block diagram of a control system to control condensate formation according to embodiments of the invention; and 
           [0012]      FIG. 5  depicts processes involved in controlling condensate formation according to embodiments of the invention. 
       
    
    
       [0013]    The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0014]      FIG. 1  is a block diagram of exemplary components of a combined cycle power plant  100 . The combined cycle power plant  100  includes at least a gas turbine  110  and a steam turbine  130 . A heat recovery steam generator (HRSG)  120  uses waste heat of the gas turbine  110  to generate steam that it supplies to the steam turbine  130 . The HRSG  120  includes headers  210  ( FIGS. 2 and 3 ) that act as main collection/distribution channels for steam and water. 
         [0015]      FIG. 2  depicts aspects of an exemplary bottom header  210  of the HRSG  120 . The bottom header  210  is fed by tubes  220  and includes a drain  230  to drain water that collects in the bottom header  210 . In alternate embodiments, the drain  230  may be part of one or more tubes  220  rather than included on the header  210 . A header  210  is comprised of thick metal and can be thought of as having an inner-wall  211 , a mid-wall  213 , and an outer-wall  215 . It is due to this thickness of the metal comprising the headers  210  that a temperature gradient can form across the inner-wall  211 , mid-wall  213 , and outer-wall  215  when (cooler) condensate forms within a header  210  against the inner-wall  211  while the outer-wall  215  remains extremely hot. The temperature gradient has the adverse effect of causing thermal stress in the metal of the headers  210  and reducing their fatigue life. 
         [0016]      FIG. 3  depicts aspects of an exemplary top header  210  of the HRSG  120 . The top header  210  supplies tubes  220  and includes a vent  240  for steam. In alternate embodiments, the vent  240  may be part of one or more tubes  220  rather than included on the header  210 . The drains  230  and vents  240  of the headers  210  are important for controlling condensate and for efficient operation of the HRSG  120 . The drains  230  are important for draining water within the headers  210  and preventing corrosion of the metal comprising the headers  210 . The drains  230  also serve to clear out water from areas of the HRSG  120  that should only have dry steam. For example, draining of water and condensate avoids water hammer events during startups. The vents  240  release steam or displace air trapped at startup. The drains  230  and vents  240  are operated at various times during startup, shutdown, and in transient states. As noted above, in prior art systems, the operation of drains  230  and vents  240  has not been based on a prognostic determination of condensate formation. 
         [0017]      FIG. 4  is a block diagram of a control system  400  to control condensate formation according to embodiments of the invention. The measurement devices  410  include measurement devices and sensors that collect data from different parts of the combined cycle power plant  100 . These measurement devices  410  collect information that may include steam temperatures, gas temperatures, HRSG  120  pressure, and turbine speed over time. The processor  420  may represent one or more processors and memory devices that work together to process the information obtained by the measurement devices  410  and generated by the models  430 ,  440 ,  450  to instruct the controller  460  on how to operate drains  230  and vents  240  as needed to control condensate formation. The processor  420  determines the operating mode based on information from the measurement devices  410 . Based on the mode determination, condensate formation may need to be controlled. That is, the control system  400  uses the output of the models  430 ,  440 ,  450  to control condensate formation when the processor  420  determines that the operating mode is the startup mode. 
         [0018]    Each of the models  430 ,  440 ,  450  may include one or more processors and memory devices themselves. Although shown and discussed separately for ease of understanding, some or many of the components of the control system  400  may be implemented by a collection of processors and memory devices. The thermodynamic model  430  provides metal temperatures of each of the headers  210  of the HRSG  120 . The thermodynamic model  430  estimates parameters for locations within the combined cycle power plant  100  (e.g., HRSG  120 ) that do not include measurement devices  410 . Temperature and pressure information is only available at a few locations in the HRSG  120  (e.g., at final super heater outlet), but there are several headers in the HRSG  120  without any sensors. The thermodynamic model  430  is used to estimate steam properties at those locations. The temperature and pressure values are used by the condensate formation prediction model  440  to predict when condensate will form in each header  210 . The information provided by the condensate formation prediction model  440  may be used to control the drains  230  and vents  240  or to otherwise control condensate formation in different ways, each of which is detailed below. The controller  460  outputs the control signal from the control system  400  to carry out each form of control. The controller may additionally use information from the virtual sensor model  450 . The virtual sensor model  450  uses information from the measurement devices  410  and from the thermodynamic model  430  to estimate metal temperature of each of the headers  210  at the inner-wall  211 , mid-wall  213 , and outer wall  215  and pressure within the headers  210 . Thus, while the thermodynamic model  430  generates temperature and pressure values that are used by the condensate formation prediction model  440  to predict when condensate will form, the virtual sensor model  450  quantifies the effect of that condensate formation for the controller  460  (quantifies the thermal gradient) to enhance the decision-making process of the controller  460  regarding when and how much to control condensate formation. 
         [0019]    In one embodiment, the control system  400  prevents condensate formation altogether. This is accomplished by reducing the duration of the purge cycle of the gas turbine  110  as needed based on the predicted time of formation of condensate by the condensate formation prediction model  440 . That is, the purge cycle is controlled to end prior to the time when condensate is predicted to begin forming. The reduction in the duration of the purge cycle may be accomplished with a control signal output by the controller  460  to the gas turbine  110  controller. The reduction in the duration of the purge cycle has the effect of less cool air being drawn into the combined cycle power plant  100  that could condense trapped steam. This scenario takes advantage of the prognostic features of the condensate formation prediction model  440 . 
         [0020]    In alternate embodiments, the control system  400  controller  460  outputs one or more control signals to operate drains  230  and vents  240  of the headers  210  to mitigate condensate formation. One of these embodiments involves monitoring the metal temperature of the headers  210  as estimated by the thermodynamic model  430 . Any of the temperatures (inner-wall  211 , mid-wall  213 , or outer-wall  215 ) may be used. When the rate of change of a difference in the temperature exceeds a threshold value (e.g., a value that indicates the onset of condensate formation), the controller  460  controls the drains  230  and vents  240  to reduce or eliminate the condensate and, thereby, control the temperature gradient across the metal of the headers  210 . Another embodiment involves monitoring the surface metal temperature of one or more headers  210  using a thermocouple (one per header), which may be, for example, one of the measurement devices  410 . When the temperature of the one or more monitored headers changes at a rate that exceeds a threshold value (e.g., a value that indicates the onset of condensate formation), the controller  460  controls the drains  230  and vents  240  to reduce or eliminate the condensate and, thereby, control the temperature gradient across the metal of the headers  210 . 
         [0021]      FIG. 5  depicts processes  500  involved in controlling condensate formation according to embodiments of the invention. The processes  500  include measuring parameters at  510  at different locations within the combined cycle power plant  100 . The parameters may include steam temperature, gas temperature, HRSG  120  pressure, mass flow rate of exhaust gas, and turbine speed recorded by various measurement devices  410 . Processing the parameters at  520  includes determining a mode of operation and, specifically, determining whether the operating mode is the startup mode during which condensate formation must be controlled. Estimating thermodynamic characteristics at  530  includes using a thermodynamic model  430  to estimate parameters for locations within the combined cycle power plant  100  (e.g., HRSG  120 ) that do not include measurement devices  410 . Estimating thermal gradient at  540  includes using the virtual sensor model  450  and using the measured parameters and estimated parameters (at block  530 ) to estimate the metal temperature of each of the headers  210  at the inner-wall  211 , mid-wall  213 , and outer wall  215  and pressure within the headers  210 . The processes  500  include predicting condensate formation with the condensate formation prediction model  440  at  550 . Predicting condensate formation includes predicting when condensate will form in each header  210  using output from the thermodynamic model  430  at the condensate formation prediction model  440 . At  560 , controlling condensate formation may be by one of the embodiments discussed above or by an alternate embodiment that prevents or mitigates condensate formation based on the predicted time of condensate formation. Controlling condensate formation with the controller  460  includes using the predicted time of condensate formation (based on the thermodynamic model  430  and condensate formation prediction model  440 ) and may additionally include using the virtual sensor model  450  to better determine control by having a quantitative measure of the effect of condensation. 
         [0022]    As noted previously, each of the models  430 ,  440  may be implemented by one or more processors and one or more memory devices. In addition, the processor  420  and controller  450  may be implemented by the same or additional processors and memory devices. The control system  400  has the technical effect of preventing or reducing condensate formation and, thereby, reducing the thermal stresses caused by high temperature gradients across the metal of the headers  210 . 
         [0023]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.