Patent Application
20140052426
The subject matter disclosed herein relates to reducing thermal stresses in a heat recovery steam generator (HRSG).
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.
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.
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.
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.
These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.
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:
The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.
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.
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.
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.
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.
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.
