Abstract:
A method for operating an energy system includes calculating an auto-ignition temperature of a fuel in use with the energy system, storing the auto-ignition temperature in a system memory, unloading a gas turbine associated with the energy system to a pre-determined range of operations, controlling a temperature of the exhaust flow discharged from the gas turbine, opening an exhaust bypass damper to a pre-determined position to enable a pre-determined volume of air to enter an exhaust flow path defined within the energy system, and releasing the energy system for normal operation after a pre-determined amount of time has elapsed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to combined cycle power systems, such as those used in a power plant, and more specifically, to methods and systems for completing a hot purge of a heat recovery steam generator (HRSG). 
         [0002]    At least some known combined cycle power systems include at least one gas turbine which powers a generator used to supply electrical power to a power grid. Exhaust from at least some known gas turbines is supplied to an HRSG that extracts heat from the exhaust to generate steam for use in other processes such as, but not limited to, driving a steam turbine and/or providing hot steam for heating or desalinization. Power generated by a steam turbine also drives an electrical generator that provides additional electrical power to the power grid. 
         [0003]    Combined cycle power systems, that include a bypass damper in the exhaust flowpath, are often designed such that the bypass damper can be used to facilitate transitioning a gas turbine&#39;s exhaust path from simple cycle to combined cycle, and back, while under load. The primary regulatory guidance governing this equipment is NFPA 85 and ISO 21789. The practice of transitioning between modes is expressly permitted if the system startup includes a cold purge of both the exhaust flow path through the gas turbine and the HRSG. 
         [0004]    However, more system operators are demanding operating scenarios in which a post-operational purge is conducted before placing equipment in the exhaust path into service, such as an HRSG. If a transition from simple cycle to combined cycle operation is required, it may be desirable to do so without shutting down the gas turbine. Such a practice, known as “hot purging” an exhaust path, is recognized in NFPA 85, but with strict limits. For example, the operation may only occur when the gas turbine exhaust temperatures are at least 100° F. below the lowest auto-ignition temperature (AIT) of any fuels or fuel mixes that may be introduced into either the gas turbine or any downstream firing equipment. 
         [0005]    In addition, the International Organization for Standardization (ISO) has developed ISO 21789, which is a worldwide gas turbine safety standard. This standard permits gas turbine exhaust gases to be used for purging provided that they are proven, and controlled, to be less than 80% of the AIT of any flammable gases or vapors that may be present. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0006]    In one aspect, some configurations of the present invention provide a method for operating an energy system. The method includes calculating an auto-ignition temperature of a fuel in use with the energy system and storing the auto-ignition temperature in a system memory. A gas turbine associated with the energy system is then unloaded to a pre-determined range of operations and the temperature of the exhaust flow discharged from the gas turbine is reduced. An exhaust bypass damper is then opened to a pre-determined position to enable a pre-determined volume of air to enter an exhaust flow path defined within the energy system. The energy system is then released for normal operation after a pre-determined amount of time has elapsed. 
         [0007]    In another aspect, some configurations of the present invention provide a control system for use in operating an energy system. The control system includes a turbine control system and a control system processor configured to determine an auto-ignition temperature of the fuel in use with the energy system. 
         [0008]    In a further aspect of the invention, some configurations of the present invention provide a method for controlling a hot purge of a heat recovery steam generator (HRSG). The HRSG is part of a combined-cycle power system including a control system processor and a system memory. The method includes calculating an auto-ignition temperature of a fuel in use with the energy system and storing the auto-ignition temperature in the system memory, unloading a gas turbine associated with the energy system to a hot purge range of operations, and controlling the operation of a plurality of inlet guide vanes of the gas turbine to control a temperature of the gas turbine exhaust. When the exhaust has reached a proper temperature, an exhaust bypass damper is opened to a hot purge position to enable a pre-determined volume of air to enter the heat recovery steam generator. The power system is released for normal operation after a pre-determined amount of time has elapsed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a block diagram of an exemplary control system that may be used to hot purge a heat recovery steam generator (HRSG) used in a combined cycle power system; and 
           [0010]      FIG. 2  is a flow chart illustrating an exemplary method used to initiate and control a hot purge of an HRSG, such as may be used with the control system shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]    According to regulations, such as NFPA 85, a heat recovery steam generator (HRSG) must be purged of any hazardous gases before it can be put into operation. Using conventional procedures, this generally requires 20-30 minutes of purge time and such actions may only be accomplished in a combined-cycle power system when the associated gas turbine is offline and in an unfired condition. As such, using known purge procedures may increase the startup time of the system, and does not allow the system to produce power or support other processes until the purge cycle is complete and the gas turbine can be brought online. 
         [0012]      FIG. 1  is a block diagram of an exemplary control system  100  that may be used with a combined-cycle power system, such as those used in a power plant, that includes a gas turbine  102 , an exhaust bypass damper  104 , and a heat recovery steam generator (HRSG)  106 . In the exemplary embodiment, HRSG  106  is coupled to gas turbine  102  and to other processes (not shown). Further, in the exemplary embodiment, control system  100  includes a turbine control system  110  and a control system processor  112 . 
         [0013]    In the exemplary embodiment, turbine control system  110  is coupled to a gas turbine  102 , a plurality of inlet guide vanes (IGVs)  108 , an exhaust bypass damper  104 , and a turbine control system processor  114 . Turbine control system processor  114  is configured to monitor and adjust the position of the IGVs  108 . When the IGVs  108  are open, air flow into gas turbine  102  will increase to facilitate reducing the temperature of the gas turbine exhaust gas. Similarly, when the IGVs  108  are closed, the temperature of the gas turbine exhaust gas increases. Turbine control system processor  114  is also configured to monitor and adjust the load of gas turbine  102 . Moreover, turbine control system processor  114  is configured to monitor and adjust the position of exhaust bypass damper  104 . Sensors (not shown) coupled to gas turbine  102 , IGVs  108 , and exhaust bypass damper  104  are also electrically coupled to turbine control system processor  114 . These sensors communicate data to turbine control system processor  114  such as, but not limited to, the load of gas turbine  102 , the temperature of the gas turbine exhaust, the relative position of the IGVs  108 , and the relative position of exhaust bypass damper  104 . When exhaust bypass damper  104  is open, gas turbine exhaust bypasses HRSG  106 , and air enters HRSG  106 , purging HRSG  106  of gas turbine exhaust and/or other potentially hazardous gases and vapors. 
         [0014]    In the exemplary embodiment, control system processor  112  is electrically coupled to turbine control system processor  114 . Turbine control system processor  114  calculates the auto-ignition temperature (AIT) of the fuel in use in the power system, and stores the calculated AIT in a system memory (not shown). Turbine control system processor  114  compares the gas turbine exhaust temperature to the AIT of the fuel. As mentioned above, NFPA 85 requires that the gas turbine exhaust be at least 100° F. below the AIT of the fuel in use. Additionally, ISO 21789 requires that the gas turbine exhaust temperature be less than 80% of the AIT, measured in degrees Celsius, of any flammable gases or vapors that may be present. Further, control system processor  112  is configured to enable the turbine control system processor  114  to control the loading and unloading of gas turbine  102 , the relative position of the IGVs  108 , and the relative position of exhaust bypass damper  104 . 
         [0015]      FIG. 2  is a flow chart of an exemplary method  200  that may be used to initiate and control a hot purge of a heat recovery steam generator, such as HRSG  106  (shown in  FIG. 1 ). The exemplary method  200  is based on a comparison of the temperature of the exhaust gas from a gas turbine, such as gas turbine  102  (shown in  FIG. 1 ) with a pre-determined auto-ignition temperature (AIT) of the fuel in use in a combined-cycle power system. The AIT is a function of the constituents of the fuel and is calculated  202  based on algorithms published by NFPA and ISO. The AIT is then stored  204  in a system memory of a control system, such as control system  100  (shown in  FIG. 1 ). 
         [0016]    In the exemplary embodiment, and when the gas turbine is at load, the associated inlet guide vanes, such as IGVs  108  (shown in  FIG. 1 ) are opened, and an associated exhaust bypass damper, such as exhaust bypass damper  104  (shown in  FIG. 1 ) closed, the system operator initiates  206  the hot purge of HRSG  106  via control system  100 . 
         [0017]    Once the hot purge has been initiated  206 , in the exemplary embodiment, control system processor  112  (shown in  FIG. 1 ) communicates to turbine control system processor  114  (shown in  FIG. 1 ) that a hot purge of HRSG  106  has been initiated. The gas turbine  102  is unloaded  208  to a pre-determined operating level within a hot purge region. The hot purge region of operation of gas turbine  102  is within a range of a zone of lower operating loads such as, but not limited to, approximately 10-20% of the nominal load. When the unloading  208  of gas turbine  102  is complete, turbine control system processor  114  then modulates or repositions  210  the IGVs  108  to facilitate reducing the gas turbine exhaust temperature to the required temperature range. As the temperature of the gas turbine exhaust is reduced, turbine control system processor  114  receives data representative of the exhaust temperature and the gas turbine exhaust temperature is compared to the stored  204  AIT. 
         [0018]    In the exemplary embodiment, when the gas turbine exhaust gas temperature reaches a pre-determined level, per NFPA 85 or ISO 21789, exhaust bypass damper  104  is opened  212  to a hot purge position. Repositioning exhaust bypass damper  104  allows air to enter HRSG  106 , thus purging and exchanging  214  the contents of HRSG  106 . Exhaust bypass damper  104  remains open to the hot purge position until a pre-determined volume of air is exchanged within HRSG  106 . The time period that exhaust bypass damper  104  remains open is determined by the total volume of air required to be exchanged, per NFPA 85 or ISO 21789, and the available volume of HRSG  106 . 
         [0019]    When the required volume of air has been exchanged, gas turbine  102  is released  216  for normal operations. At such time, to resume normal operations, exhaust bypass damper  104  is moved to a full open position, thus substantially full operation of the HRSG  106  and associated processes. The IGVs  108  are then repositioned to a normal operating position, and the load of gas turbine  102  is then increased to its normal operating region. 
         [0020]    The above-described methods and apparatus facilitate improving power system startup time and operability. Calculating the AIT of the gas turbine fuel and comparing the gas turbine exhaust temperature with the AIT, allows the gas turbine to be kept online in a simple cycle mode while the HRSG is purged of exhaust gases and other potentially hazardous fumes and/or vapors. The ability to keep the gas turbine online facilitates a faster power system startup time and allows the power system to continue to produce power during the purge. 
         [0021]    Exemplary embodiments of methods and apparatus that facilitate a hot purge of an HRSG are described above in detail. The methods and apparatus are not limited to the specific embodiments described herein, but rather, components of the methods and apparatus may be utilized independently and separately from the other components described herein. For example, the calculation of the AIT for the fuel used in the power system may also be completed and/or used in combination with other industrial plant or component design and monitoring systems and methods, and is not limited to practice with only power systems as described herein. Rather, the present invention can be implemented and utilized in connection with many other component or plant designs and monitoring applications. 
         [0022]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.