Patent Application
20100077970
The present invention relates generally to model-based predictive control of temperatures. More specifically, the invention relates to model-based predictive temperature control of steam in relation to inter-stage attemperation, which may be used in heat recovery steam generation (HRSG) systems in combined cycle power generation applications.
HRSG systems may produce steam with very high exhaust temperatures. In particular, HRSG systems may include superheaters through which steam may be superheated before being used by a steam turbine. If the exhaust steam from the superheaters reaches high enough temperatures, the steam turbine, as well as other equipment downstream of the HRSG, may be adversely affected. For instance, high cyclic thermal stress in the steam piping and steam turbine may eventually lead to shortened life cycles. Conventional control systems have been devised to help monitor and control the temperature of exhaust steam from HRSG systems. Unfortunately, these control systems often allow temperature overshoot during transient periods where, for instance, inlet temperatures into the superheaters increase rapidly. In addition, these control systems often require a great deal of tuning and re-tuning.
Systems and methods for controlling exhaust steam temperatures from a finishing superheater are provided. In certain embodiments, the system includes a controller which includes control logic for predicting an exhaust temperature of steam from the finishing superheater using model-based predictive techniques (e.g., based on thermodynamic calculations). Based on the predicted exhaust temperature of steam, the control logic may use feed-forward control techniques to control the operation of an inter-stage attemperation system upstream of the finishing superheater. The control logic may determine if attemperation is desired based on whether the predicted exhaust temperature of steam from the finishing superheater exceeds a set point temperature as well as whether the inlet temperature of steam into the finishing superheater drops below a set point temperature of steam. The attemperation system may include a characterizing function to linearize the valve operation controlled by the control logic to inject cooled, high-pressure feedwater into the steam upstream of the finishing superheater, which may, in turn, control the exhaust temperature of steam from the finishing superheater. The disclosed embodiments may also be applied to any systems where an outlet temperature of a fluid from a heat transfer device may be controlled.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Any examples of operating parameters are not exclusive of other parameters of the disclosed embodiments.
As discussed in detail below, temperature control systems may be employed which are configured to predict the exhaust temperature of steam from a finishing superheater using model-based predictive techniques. In the disclosed embodiments, the temperature control systems may use the predicted value for exhaust temperature in conjunction with feed-forward control to control the operation of an inter-stage attemperation system upstream of the finishing superheater, thereby controlling the exhaust temperature from the finishing superheater. In particular, embodiments of the model-based predictive temperature control may determine if attemperation is desired based on whether the predicted exhaust temperature of steam from the finishing superheater exceeds a set point temperature as well as whether the inlet temperature of steam into the finishing superheater approaches or is less than the saturation temperature of steam.
The system 10 may also include a multi-stage heat recovery steam generator (HRSG) 30. The components of the HRSG 30 in the illustrated embodiment are a simplified depiction of the HRSG 30 and are not intended to be limiting. Rather, the illustrated HRSG 30 is shown to convey the general operation of such HRSG systems. Heated exhaust gas 32 from the gas turbine 12 may be transported into the HRSG 30 and used to heat steam used to power the steam turbine 20. Exhaust from the low-pressure stage 24 of the steam turbine 20 may be directed into a condenser 34. Condensate from the condenser 34 may, in turn, be directed into a low-pressure section of the HRSG 30 with the aid of a condensate pump 36.
The condensate may then flow through a low-pressure economizer 38 (LPECON), which may be used to heat the condensate. From the low-pressure economizer 38, the condensate may be directed into a low-pressure drum 40. From the low-pressure drum 40, the condensate may be drawn into a low-pressure evaporator 42 (LPEVAP), which may return steam to the low-pressure drum 40. The steam from the low-pressure drum 40 may be sent to the low-pressure stage 24 of the steam turbine 20. Condensate from the low-pressure drum 40 may be pumped into an intermediate-pressure economizer 44 (IPECON) by an intermediate-pressure boiler feed pump 46. From the intermediate-pressure economizer 44, the condensate may be directed into an intermediate-pressure drum 48. From the intermediate-pressure drum 48, the condensate may be drawn into an intermediate-pressure evaporator 50 (IPEVAP), which may return steam to the intermediate-pressure drum 48. The steam from the intermediate-pressure drum 48 may be sent to the intermediate-pressure stage 26 of the steam turbine 20. Condensate from the low-pressure drum 40 may also be pumped into a high-pressure economizer 52 (HPECON) by a high-pressure boiler feed pump 54. Again, the connections between the economizers, evaporators, and the steam turbine may vary across implementations as the illustrated embodiment is merely illustrative of the general operation of an HRSG system.
Finally, condensate from the high-pressure economizer 52 may be directed into a high-pressure drum 56. From the high-pressure drum 56, the condensate may be drawn into a high-pressure evaporator 58 (HPEVAP), which may return steam to the high-pressure drum 56. Steam exiting the high-pressure drum 56 may be directed into a primary high-pressure superheater 60 and a finishing high-pressure superheater 62, where the steam is superheated and eventually sent to the high-pressure stage 28 of the steam turbine 20. Exhaust from the high-pressure stage 28 of the steam turbine 20 may, in turn, be directed into the intermediate-pressure stage 26 of the steam turbine 20, and exhaust from the intermediate-pressure stage 26 of the steam turbine may be directed into the low-pressure stage 24 of the steam turbine 20. In certain embodiments, a primary and secondary re-heater may also be used with the primary high-pressure superheater 60 and the finishing high-pressure superheater 62.
As mentioned above, embodiments of inter-stage attemperation may be used in conjunction with HRSG systems applied in combined cycle power generation applications as illustrated in
In HRSG systems, it may be desirable to maintain a pre-determined design steam temperature, generated by the HRSG 30, to maintain the efficiency of the processes and the life of the steam turbine 20 and associated equipment. In the event of excessive steam temperatures, beyond the control of the inter-stage attemperation system, the gas turbine 12 exhaust temperature into the HRSG 30 may be reduced to avoid high stress in the downstream steam turbine 20 and associated equipment. In some cases, due to excessive temperatures, control measures may trip (i.e., temporarily cause to be bypassed or otherwise cease operation) the gas turbine 12 and/or steam turbine 20. This may result in a loss of power generation which may, in turn, impair plant revenues and operability. Inadequately controlled steam temperatures may also lead to high cyclic thermal stress in the steam piping and steam turbine 20, affecting their useful life.
However, superheater and re-heater inter-stage attemperation may be used to achieve robust temperature control of the steam leaving the HRSG 30. An inter-stage attemperator 64 may be located in between the primary high-pressure superheater 60 and the finishing high-pressure superheater 62. The inter-stage attemperator 64 enables more robust control of the exhaust temperature of steam from the finishing high-pressure superheater 62. An inter-stage attemperator controller 66 may include controller logic for more precisely controlling the steam exhaust temperature from the finishing high-pressure superheater 62. Again, although not illustrated in
As discussed below, embodiments of the attemperator control logic may include a model-based predictive temperature control scheme to overcome drawbacks of other techniques. Thus, a short discussion of another technique will precede a detailed discussion of the model-based techniques. In particular, a non-model-based technique (i.e., to be replaced with a model-based technique) may consist of a control structure where an outer loop creates a set point temperature T3 for steam entering the finishing high-pressure superheater 62 based on a difference between a desired and actual steam temperature T4 exiting the finishing high-pressure superheater 62. An outer loop proportional-integral-derivative (PID) controller may establish the set point temperature for an inner loop PID controller. The inner loop of the control logic may drive the control valve 68 based on the difference between the actual and set point temperature to suitably reduce the steam temperature T3 before it enters the finishing high-pressure superheater 62. Unfortunately, this non-model-based technique may not always work to control steam temperature overshoots during transient changes in the gas turbine 12 output, as shown in
Regarding the overshoot problem with the non-model-based technique as illustrated in
In general, the exhaust temperature overshoot problem may be more pronounced as the gas temperature approaches an isotherm (corner point). This condition may be characterized by low steam flow and higher gas temperature. For a fixed gas turbine operation, the exhaust temperature overshoot depends not only on the controller logic but also on the temperature set point profile. In the non-model-based technique, this temperature set point may be chosen such that the attemperation control valve 68 may open even before the steam reaches the required design temperature. Then, the attemperation control valve 68 may be ramped up to the specified design value to avoid overshoot at the corner point. Insisting on a temperature set point profile where the set point reaches the design steam temperature before the gas temperature reaches the isotherm represents an aggressive schedule and may likely lead to the exhaust temperature overshoot problem. Conversely, as discussed below, embodiments of model-based predictive temperature control may use the gas temperature information to design the temperature set point profile such that the design steam temperature may be reached at or after the gas temperature reaches the isotherm. Such temperature set point profile may therefore result in fewer temperature overshoots. As illustrated by arrow 74, the disclosed model-based predictive temperature control may reduce or eliminate the overshoot problem discussed above in association with the non-model-based technique.
In order to protect the steam turbine 20 and associated piping, valving, and other equipment, certain control design parameters may be imposed on the performance of the controller logic of the inter-stage attemperator controller 66. For instance, one design parameter may be to maintain the exhaust steam temperature T4 at the set point temperature Tsp (e.g., 1050° F.) whenever possible. However, whenever the exhaust steam temperature T4 overshoots the set point temperature Tsp, certain “swing clause” values may be used to determine allowable average temperature Tavg values which may be allowed for the exhaust steam temperature T4. For instance, Tavg may not be allowed to stay above the set point temperature Tsp plus 15° F. continuously. In addition, Tavg may not be allowed to stay above the set point temperature Tsp plus 25° F. for more than 400 hours over a 12-month period. Furthermore, Tavg may not be allowed to stay above the set point temperature Tsp plus 50° F. for more than 80 hours over a 12-month period and also may not be allowed to stay above the set point temperature Tsp plus 50° F. for 15 minutes in a row. These “swing clause” values are merely illustrative and are not intended to be limiting. They are merely shown to illustrate the type of control design constraints which may be used by the controller logic of the inter-stage attemperator controller 66. Other control design constraints on the controller logic may include physical limitations such as allowing feedwater spray to be introduced by the control valve 68 only when the spray may be vaporized and absorbed into the steam flow.
At step 80, relevant inlet variables into the finishing high-pressure superheater 62, including the inlet temperature Tin of steam, may be determined. These variables may also include, but are not limited to, the inlet temperature of gas, the inlet pressure of steam and gas, the inlet flow rate of steam and gas, the steam specific heat, the equivalent heat transfer coefficient, the equivalent heat transfer area, and so forth. These variables may be used in step 82 to predict the outlet temperature Tout of steam from the finishing high-pressure superheater 62 using model-based predictive techniques, as described in greater detail below. The model-based techniques may include various methods for predicting the outlet temperature Tout of steam. For instance, one method for creating a prediction model may be to use past performance data regarding the finishing high-pressure superheater 62 to correlate how the outlet temperature Tout of steam from the finishing high-pressure superheater 62 changes based on empirical inlet data variables (e.g., using data-driven input/output maps). Another method may be to employ thermodynamic principles to create a prediction model based on mathematical equations which may approximate how the finishing high-pressure superheater 62 actually functions as a heat transfer device.
The disclosed embodiments employ model-based predictive techniques with a feed-forward controller loop in parallel with a proportional-integral (PI) controller loop. In particular, whereas non-model-based PID controllers may allow for control based only on actual data prior to the control time period, the model-based techniques enable forward-looking prediction of what the expected outlet temperature Tout temperature from the finishing high-pressure superheater 62 may be based on inlet conditions during the control time period. The predicted value of outlet temperature Tout may be used with a feed-forward controller loop in parallel with a PI controller loop to more precisely determine an adjusted set point temperature Tsp value for an inner loop, which may actually be responsible for controlling the attemperation process. The PI in the outer loop may be used to compensate for differences which may exist between measured values and the predicted model values due to the fact that the predictive model may inherently differ slightly from the reality of the physical system. Therefore, control logic using the disclosed embodiments may allow for a more robust, forward-looking control structure for the attemperation process.
In general, the method 76 may include two separate decision steps regarding whether the attemperation process should be performed. For instance, at step 84, a determination may be made of whether the outlet temperature Tout predicted by the model-based techniques is greater than the set point temperature Tsp established in step 78. If the predicted outlet temperature Tout is greater than the set point temperature Tsp, then attemperation may be warranted and the method 76 may continue to the second decision at step 86. Otherwise, attemperation may be unnecessary for the current time period and the method 76 may proceed back to step 80 to re-evaluate the situation for a subsequent time period.
In other words, the predictive model is configured to provide a feed-forward signal capable of setting the set point for a controller in the loop. However, a feed-back signal from the finishing high-pressure superheater 62 outlet temperature may provide the final trimming of the outlet temperature. The predictive model-based control approximates the spray flow to the needed value rapidly while the temperature trimming provides small adjustments to remove any inaccuracies in the predictive model.
At step 86, a determination may be made of whether the inlet temperature Tin into the finishing high-pressure superheater 62 is greater than the saturation temperature Tsat of steam plus some pre-determined safety value Δ. This step may be desirable to ensure that the steam stays well above the saturation temperature Tsat of steam. This determination may be made using steam tables and the inlet pressure Pin of the steam. If the inlet temperature Tin of steam is greater than Tsat+Δ, then attemperation may be warranted and the method 76 may continue to step 88. However, if the inlet temperature Tin of steam is already currently less than Tsat+Δ, then attemperation may be bypassed and the method 76 may proceed back to step 80 to re-evaluate the situation for a subsequent time period. This control step is essentially an override of the spray attemperation to prevent water impingement on the tubes of the finishing high-pressure superheater 62, which would result in higher than normal stresses in the tubes.
Therefore, even if it is determined in step 84 that attemperation may be desirable in order to keep the outlet temperature Tout of steam under the set point temperature Tsp, attemperation may be bypassed in order to maintain the steam temperature sufficiently above the saturation point. In other words, the outlet temperature Tout of steam may be allowed to temporarily rise above the set point temperature Tsp. From a control standpoint, the decision between proceeding with attemperation because the predicted outlet temperature Tout of steam is greater than the set point temperature Tsp and not proceeding because the inlet temperature Tin of steam is not greater than Tsat+Δ may be implemented using an override controller. It should be noted that the decisions shown in steps 84 and 86 may be somewhat simplified compared to actual control logic which may be implemented. However, steps 84 and 86 are merely meant to be illustrative of the type of multiple-part decision which may be implemented in controlling exhaust steam temperatures using the disclosed embodiments of model-based predictive temperature control.
Finally, at step 88, the process of attemperation may be performed upstream of the inlet into the finishing high-pressure superheater 62 in order to reduce the inlet temperature Tin of steam such that the outlet temperature Tout of steam may likewise be reduced. As discussed above with respect to
Using the disclosed embodiments of model-based temperature control, a linearization function block may be provided in the control loop to make the loop gain fairly constant. This approach may allow for simplified tuning (e.g., requiring tuning only at one load) and consistent loop response over the load range. Linearization of the control valve response in this manner may also prove particularly useful when operating a large plant with heavy equipment since it may be difficult to gain access to the equipment in order to check the tuning of the loops.
Therefore, an advantageous component of the method 76 for controlling exhaust steam temperatures illustrated in
where Ue is the equivalent heat transfer coefficient across the finishing high-pressure superheater 62, Ae is the equivalent heat transfer area across the finishing high-pressure superheater 62, Ws is the steam flow rate, and Cps is the steam specific heat. Therefore, in general, this steady state first principle model may only take into account variables at the finishing high-pressure superheater 62 to predict the exhaust temperature Ts4 of steam from the finishing high-pressure superheater 62.
In contrast, in another embodiment, a second equation may be employed which uses the primary high-pressure superheater 60 as a surrogate. This surrogate model may be of the form:
where PHPSH stands for primary high-pressure superheater 60 and FHPSH stands for finishing high-pressure superheater 62. Both of these models may function adequately for controlling the exhaust steam temperature. In addition, various other appropriate models (e.g. transient, as opposed to steady state, models) may be envisioned and utilized as these two models are merely illustrative. However, the steady state first principle model may prove better suited for use with the feed-forward controller design of the disclosed embodiments. Similar modeling may be utilized in the event that a primary and/or secondary re-heater (not shown) are used in conjunction with the inter-stage attemperator 64. Furthermore, similar modeling may also be utilized in the event that a secondary superheater is used in conjunction with the primary and finishing superheaters.
These parallel control paths 98, 100 may be referred to as an outer loop 102 while the other three illustrated parallel paths 104, 106, 108 may be referred to as an inner loop 110. However, the exact control elements and control paths may vary among implementations as the illustrated control elements and paths are merely intended to be illustrative of the disclosed embodiments. For example, in certain embodiments, the feed-forward controller 92 and outer loop PI controller 96 may be replaced by a single model predictive control algorithm. In addition, in other embodiments, the cascading structure of the outer loop 102 and the inner loop 110 may be replaced by a single model predictive controller which may directly manipulate the control valve 68 and inter-stage attemperator 64.
As discussed above with respect to
As shown in
Moreover, while the disclosed embodiments may be specifically suited for inter-stage attemperation of steam, they may also be used in other similar application such as food and liquor processing plants. Indeed, as discussed above, the disclosed embodiments may be utilized in many other scenarios other than the control of exhaust steam temperatures. For instance, the disclosed embodiments may be used in virtually any system where a fluid is to be heated, or cooled for that matter, using a heat transfer device. Whenever it may be important to control the outlet temperature of the fluid from the heat transfer device, the disclosed embodiments may utilize model-based predictive techniques to predict the outlet temperature based on inlet conditions into the heat transfer device. Then, using the predicted outlet temperature with the disclosed embodiments, attemperation of the inlet temperature into the heat transfer device may be performed to ensure that the actual outlet temperature from the heat transfer device stays within an acceptable range (e.g., below a set point temperature or above a saturation temperature). Furthermore, control of the model-based prediction and attemperation process may be performed using the feed-forward techniques as described above. Therefore, the disclosed embodiments may be applied to a wide range of applications where fluids may be heated or cooled by heat transfer devices.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
