Abstract:
A method using an inverse three-way valve model with feed-forward fuel flow control is provided for controlling liquid fuel flow in a turbine power generation system to achieve a bump-less driven watts (dwatt) power output during fuel mode transitions between passive mode and active mode operations of a three-way check valve that delivers liquid fuel to the turbine combustor nozzles. The method utilizes an inverse fluid flow model for a three-way check valve which is based upon a valve position surrogate for the three-way check valve to develop a calculated estimate of a fuel spike/dwatt oscillation likely to occur during mode transitions of the three-way check valve and to produce a feed-forward control used to modulate a fuel path bypass valve within the turbine fuel supply circulation system that provides the liquid fuel to the three-way valve during transfers of valve operation between passive and active mode operations.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to fuel delivery systems for gas turbine engines and more specifically to an inverse fuel model and method for implementing liquid fuel flow control in a gas turbine to achieve a nearly bump-less driven watts (dwatt) power output during fuel mode transitions between passive and active modes of operation of a three-way check valve which delivers liquid fuel to the turbine combustor. 
         [0002]    A gas turbine engine includes a compressor, combustor and turbine. Compressed air is delivered by the compressor to the combustor in which fuel is mixed with the air and combusted. Hot combustion gases turn the turbine that drives the compressor and generates work from the gas turbine engine. The combustor is formed of combustion cans typically arranged in an annular array between the compressor and turbine. Fuel to the combustor flows through pipes and valves that meter the fuel to the combustion cans. The valves are used to control fuel flow and to ensure that fuel flows equally to each of the combustion cans. 
         [0003]    Industrial gas turbines are often capable of alternatively running on liquid and gaseous fuels, e.g., natural gas. These gas turbines have fuel supply systems for both liquid and gas fuels. The gas turbines generally do not burn both gas and liquid fuels at the same time. Rather, when the gas turbine burns liquid fuel, the gas fuel supply is turned off. Similarly, when the gas turbine burns gaseous fuel, the liquid fuel supply is turned off. Fuel transfers occur during the operation of the gas turbine as the fuel supply is switched from liquid fuel to gaseous fuel, and vice versa. 
         [0004]    Gas turbines that burn both liquid and gaseous fuel require a liquid fuel purge system to clear the fuel nozzles in the combustors of liquid fuel. The liquid fuel supply system is generally turned off when a gas turbine operates on gaseous fuel. When the liquid fuel system is turned off, the purge system operates to flush out any remaining liquid fuel from the nozzles of the combustor and provide continuous cooling airflow to the nozzles. 
         [0005]      FIG. 1  is a simplified schematic diagram of an exemplary gas turbine having liquid and gas fuel systems.  FIG. 1  shows schematically a gas turbine power generation system  100  having liquid fuel system  102  and a liquid fuel purge system  104 . The gas turbine is also capable of running on a gas, such as natural gas, and includes a gaseous fuel system  106 . Other major components of the gas turbine include a main compressor  108 , a combustor  110 , a turbine  112  and a system controller  114 . The power output of the gas turbine  112  is a rotating turbine shaft  116 , which may be coupled to a generator  130  that produces electric power. 
         [0006]    In the exemplary industrial gas turbine shown, the combustor may be an annular array of combustion chambers, i.e., combustion cans  118 , each of which has a liquid fuel nozzle  120  and a gas fuel nozzle  122 . The combustor may alternatively be an annular chamber. Combustion is initiated within the combustion cans at points slightly downstream of the nozzles. Air from the compressor  108  flows around and through the combustion cans  118  to provide oxygen for combustion. Moreover, water injection nozzles  124  are arranged within the combustor  110  to add excess mass flow to the hot combustion gases and to cool the combustion cans  118 . The air for the liquid fuel system purge may be provided from the compressor  108 , boosted by a purge air compressor (not shown) and controlled by other elements of the system (not shown). When the gas turbine power generation system  100  operates on natural gas (or other gaseous fuel), the liquid fuel purge system  104  blows compressed air into the combustion cans  118  through the liquid fuel nozzles  120  of the liquid fuel  102  system to purge liquid fuel and provide a flow of continuous cooling air to the liquid fuel nozzles  120 . 
         [0007]      FIG. 2  is a simplified diagram of a gas turbine engine with an existing liquid fuel system. Liquid fuel is provided to the liquid fuel system  200  from a liquid fuel source  205 . The liquid fuel system  200  includes a flow path to a flow divider  230  through a low pressure filter  210 , a fuel pump  215 , a bypass control valve  220 , and a stop valve  225 . Pressure relief valve  235 , bypass control valve  220  and stop valve  225  serve to recirculate liquid fuel to the upstream side of the low pressure filter  210  and regulate flow to flow divider  230  and fuel delivery to three-way check valve(s)  245 . The flow divider  230  divides liquid fuel flow into a plurality of liquid fuel flow paths leading to one or more three-way check valve(s)  245  which feed fuel to individual combustion cans  270  of the turbine. 
         [0008]    The turbine system controller  114  provides control signals to the fuel pump and each of the various valves to regulate and control fuel flow that is provided to the combustors in response to a fuel reference demand for a given power output. Conventionally, the controller  114  may include, among other things, an output control signal for initiating a predetermined liquid fuel prefill flow rate through the liquid fuel system, an output control signal for controlling transitions of a fuel delivery three-way valve  245  between purge air delivery and liquid fuel operation, and an output control signal for controlling a fuel bypass control valve  220  for regulating fuel flow to a fuel flow divider  230  and a turbine combustor can. The controller  114  may also accept input signals from various turbine system sensors and incorporate a hardware processor for implementing an algorithm to generate appropriate control signals based on sensor inputs and measured system parameters such as a Driven Megawatts power output. 
         [0009]    Each liquid fuel flow path downstream of the flow divider includes a combustor fuel delivery three-way check (endcover) valve  245  (three-way valve) and a distribution valve  260  before entering a combustor combustion can  270 . Three-way valve  245  permits flow to the combustion can nozzles from the liquid fuel flow path (described above) or air flow from a liquid fuel purge air system  280 . Three-way valve  245  is designed to selectably allow fuel flow to the combustor nozzles  120  from a liquid fuel supply system while preventing backflow of fuel into the liquid fuel purge air system or to allow purge air to the combustor nozzles  120  while preventing backflow of purge air into the liquid fuel system upstream of the three-way valve. By preventing purge air from entering the liquid fuel system, the air-fuel interfaces with the fuel supply are minimized. 
         [0010]    When gas (gaseous) fuel is supplying the turbine, the three-way valve  245  is positioned to block liquid fuel flow and allow purge air to pass for cooling the fuel nozzles in the combustor. This purge must be shut off when liquid fuel is turned on. 
         [0011]    The three-way valve  245  has passive and active operational modes. During the active mode, three-way valve  245  is controlled by external forces, such as a “Pilot” (instrument) air pressure applied by the turbine system controller  114 . In passive mode, the three-way valve is controlled by the pressure of the liquid fuel. The passive mode is used to switch the three-way valve between purge air flow and purge liquid fuel flow. The active mode is applied to hold the three-way valve in a liquid fuel ON flow setting during high fuel-flow conditions. The active mode is not used to switch the three-way valve from fuel flow to purge air, or vice versa. Three-way valve  245  is biased to purge air flow, if there is insufficient fuel pressure present to operate the valve. The three-way valve  245  (operating in the passive mode) automatically switches to pass fuel to the combustor fuel nozzles when the fuel pressure increases. The increase in fuel pressure itself is the actuating force that switches the three-way valve from applying purge air to applying liquid fuel flow to the combustor. 
       BRIEF DESCRIPTION AND SUMMARY OF THE INVENTION 
       [0012]    Conventionally, a three-way valve used to deliver liquid fuel to the combustor of a liquid/gas fuel turbine engine is transferred (transitioned) from a “passive mode” operation to “active mode” operation at a predetermined load point during startup and from active mode to passive mode during shutdown of turbine operation. During this transition, a fuel spike and an oscillation is often observed in the generated driven watts power output (dwatt). Such fuel spikes and/or power output oscillations, in addition to being undesirable in the delivered output power, are indicative of a turbine operating condition which is potentially detrimental to turbine components. Accordingly, there is a need and desire to eliminate such fuel spikes and dwatt power output oscillations that occur during the transitions between the passive and active operational modes of the three-way valve fuel delivery operation in a liquid/gas fuel turbine. 
         [0013]    The description of embodiments disclosed herein generally relate to a fuel delivery flow control method and, more particularly, to an “inverse” fuel flow model used for controlling the liquid fuel delivery flow to a combustor in a gas turbine power generation system so as to achieve a “bump-less” driven watts (dwatt) power output during fuel mode transfers/transitions between passive mode and active mode operation of the three-way valve(s) used for delivering fuel to turbine combustor nozzles. An “inverse” three-way valve fuel flow model is developed based on a valve position surrogate for the three-way valve and pressure difference in fuel across the three-way valve that occurs during transitioning of the three-way valve between operational modes. A fuel flow spike estimation which is developed from inverse valve model is then used to produce valve spool position control signals for controlling a liquid fuel supply system bypass valve during the mode transitions. The valve spool position setting of the bypass valve effectively determines how much liquid fuel is recirculated back to a fuel supply source and how much and at what rate liquid fuel is provided to the combustor fuel delivery three-way valve. The model-based control signals are provided to the bypass valve in a preemptive “feed-forward” manner during the three-way valve mode transfer. This “feed forward” approach to controlling the bypass valve effectively anticipates and prevents or at least significantly reduces fuel spikes and the resultant dwatt power output spike or oscillation that occurs as a result of an operating mode transfer. 
         [0014]    In a non-limiting exemplary implementation, an inverse valve model equation is used as an operation model for a spring-loaded three-way valve that delivers fuel to the turbine combustor. A fuel flow/dwatt power output spike estimation is made based on the inverse valve model and used to provide a feed-forward fuel flow control signal, which is utilized to control the operation of a fuel flow bypass valve in the gas turbine fuel flow supply system. For example, a valve modeling equation is first determined (using conventional valve modeling technique) which estimates the operation of at least one of the three-way valves in the fuel lines providing liquid fuel to the combustor cans of the gas turbine engine. Based on this estimated valve position, an estimate of a possible spike in fuel flow, and consequentially in dwatt output, that can occur during transfer of the three-way valve between operational modes is obtained. Then, an “inverse” three-way valve model is developed as an inverse of the valve modeling equation for the three-way valve. Based upon a measurement of the differential pressure across the three-way valve, this inverse valve model then functions as a position surrogate to provide an estimate of the three-way valve (spool) position to at least a certain predetermined degree of accuracy. A fuel spike estimate produced by the inverse valve model is then used as a feed-forward bias to manage a fuel flow control loop set point for operating bypass valve  220 . 
         [0015]    A tuning algorithm for the three-way valve inverse model may also be initially run to calibrate the valve model at the time of startup (or commissioning) of the turbine using appropriate design data available from the valve manufacturer/vendor for the particular three-way valve(s) used in the turbine. 
         [0016]    Although the embodiments described herein provide an example of use in a gas turbine power generation system, it is also contemplated that the method and principles described herein are applicable to use in any system dependent upon a fluid flow process (e.g., power plant or any other chemical industry process) where there may occur a sudden change in fluid flow resistance (e.g., due to a sudden opening or closing of either controlled or uncontrolled components like valves or other variable area devices) which cause undesired oscillations/variations in the process parameters like flow, pressure, temperature, concentration of species etc. Using the methodology disclosed herein, the undesired variations can be predicted and a feed forward controller mechanism may be used to reduce or avoid the undesired oscillations/variations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    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: 
           [0018]      FIG. 1  is a simplified schematic diagram of an exemplary gas turbine having liquid and gas fuel systems; 
           [0019]      FIG. 2  is a simplified diagram of a gas turbine engine with an existing liquid fuel system; 
           [0020]      FIG. 3  is a signal flow functional diagram of input and output data signals of the inverse three-way valve model and fuel bypass valve controller implemented by the turbine system controller to provide feed-forward control of the bypass valve position; 
           [0021]      FIG. 4  is a process flow chart for implementing the inverse three-way valve model and generating the feed-forward signal for controlling the fuel bypass valve position; and 
           [0022]      FIG. 5  is a diagram of liquid fuel and purge air pressures at the fuel delivery three-way valve. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    The turbine system controller  114  may include a computer processor or comparable circuitry (not explicitly depicted) for executing software and/or other programmed instructions for performing calculations and implementing an inverse three-way valve model. The controller  114  also including appropriate conventional hardware/software for performing and operating as a bypass valve controller for providing feed-forward control signals to create a set-point and control the operating position (valve spool position) of the fuel bypass valve  220 . 
         [0024]      FIG. 3  illustrates example signal flow paths  300  of input and output data signals for the inverse three-way valve model  301  and fuel bypass valve controller  303  implemented by the turbine system controller  114  to provide a feed-forward control of the bypass valve. 
         [0025]    As an example embodiment, the three-way valve model  301  is implemented as software configured to be executed by a computer processor (not shown in  FIG. 1 ) of the turbine controller  114  which accepts input data or signals indicative of specifically monitored turbine system operating parameters and conditions including the existing purge air pressure and the liquid fuel pressure measured both upstream and downstream of fuel delivery three-way valve  245 . Such input signals may be obtained, for example, from sensors located at or within appropriate components and positions within turbine system  100 . Based on the liquid fuel pressure data and the purge air pressure data inputs, the three-way valve model  301  (described in greater detail below with reference to  FIG. 4 ) provides a fuel flow spike estimation output and may also be used to provide valve position analytic data specific to a three-way valve  245 . The fuel spike estimation is used to augment a fuel flow rate feedback signal/data  302  obtained from three-way valve  245  to produce augmented flow feedback signal/data. This augmented flow feedback signal/data is provided to a Bypass Valve Controller  303 , which may be a part of turbine control system  114 . The Bypass Valve Controller  303  then generates the feed-forward control signal for modulating the valve operating position of fuel bypass valve  220  based on a fuel flow reference data/signal and the augmented flow feedback signal/data to produce signals for controlling the position of bypass valve  220  in the turbine liquid fuel supply system. 
         [0026]      FIG. 4  illustrates an example process flow chart  400  for implementing the inverse three-way valve model  301  and generating the feed-forward signal for controlling and modulating the fuel bypass valve  220  operating position. Initially, in block  401 , an estimate of the valve stroke, ST, of at least one of the turbine combustor fuel delivery three-way valves  245  is determined as a function of a measured pressure differential between a purge air pressure for the valve and a liquid fuel pressure that initiates a mode transfer process (i.e., a transition from passive mode operation of the valve to active mode operation or vice versa). An estimate of the fluid flow resistance, CV, across three-way valve  245  is then determined, at block  403 , as a function of the valve stroke estimate. Next, as shown at block  405 , an estimate of the fluid fuel flow, W E , through three-way valve  245  is determined as a function of the estimated fluid flow resistance and a measured pressure difference existing between upstream and downstream sides of the three-way valve. Then, as indicated in block  407 , a fuel flow spike estimate, W s , of the fuel flow spike that is likely to occur as a result of the transfer of the valve between modes is determined as being a function of the difference in the determined estimate of fluid flow and a known or predetermined measured steady state flow value for the three-way valve. Based on the determined estimated fuel flow spike, inverse three-way valve model  301  provides an inverse of that estimated fuel flow spike as an output. 
         [0027]    At this stage, the inverse fuel flow spike estimation produced by valve model  301  is augment a fuel flow feedback signal provided to Bypass Valve Controller  303 . Conventionally, a valve controller such as is configured to calculate an error value between a desired set point for the valve and a measured process variable. This measured process variable is provided as feedback signal input to the controller and the controller attempts to minimize the error over time by adjustment of a control variable for the process according to a predetermined mathematical control law. In this case, for example, Bypass Valve Controller  303  is provided with a fuel flow feedback signal from three-way check valve  245  that is augmented by the inverse fuel flow spike estimation and which is then used by the controller to adjust the position of the bypass valve  220  according to a predetermined conventional control law. 
         [0028]    As indicated at block  409  of  FIG. 4 , an augmented fuel flow rate feedback is produced as a function of both the fuel spike estimation and the current fuel flow rate feedback data/signal obtained at three-way valve  245 . Next, as indicated at block  411 , a predetermined control law is used by Bypass Valve Controller  303  to calculate a position command in response to the augmented fuel flow rate feedback data and a current fuel flow rate reference signal. This position command is sent to the fuel supply system bypass valve  220  and sets or modulates the current operating position (spool position) of the bypass valve to affect the fuel rate/amount provided to three-way valve  245 . Since the bypass valve  220  position command produced by Bypass Valve Controller  303  is developed based upon an inverse of a three-way valve operational model for three-way valve  245 , it can effectively counteract or at least mitigate a fuel flow spike and the disturbances that are likely to occur in the dwatt power output (or other relevant monitored system parameters) during the three-way valve&#39;s transference between operational modes. 
         [0029]    As illustrated at the left side portion of  FIG. 4 , a valve model tuning algorithm  412  may be used initially or whenever needed to calibrate three-way valve inverse model  301  (e.g., at time of startup or commissioning of the turbine). The tuning algorithm  412  is configured to periodically check the steady state error between the calculated fuel flow estimate and the measured fuel flow through three-way valve  245 . No tuning of the model is required or performed if the steady state error is found to be within a predetermined threshold (that threshold being based, for example, on specifications and operational parameter data obtainable from a valve manufacturer/vendor of the particular three-way valve(s) used in the turbine). If the error is above the threshold, then slight tuning (e.g., incremental changes) of various model parameter values, such as the estimated valve stroke and/or the estimated flow resistance and/or the estimated fluid flow through the valve, is performed by tuning algorithm  412 . 
         [0030]      FIG. 5  illustrates example liquid fluid and purge air pressures measured at the three-way valve which are indicated in the  FIG. 4  process flow for implementing the inverse three-way valve model  301 . P 1  represents the pressure of the liquid fuel from flow divider  230  at three-way valve  245 , P 2  represents the pressure of the liquid fuel just down-stream of three-way valve  245  and P A  represents the pressure of purge air at three-way valve  245 . At block  401  of  FIG. 4 , the three-way valve stroke, ST, may be calculated, for example, in accordance with Equation 1 below: 
         [0000]    
       
         
           
             
               
                 
                   ST 
                   = 
                   
                     
                       
                         P 
                         1 
                       
                       - 
                       
                         P 
                         A 
                       
                       - 
                       
                         P 
                         Lift 
                       
                     
                     
                       
                         P 
                         max 
                       
                       - 
                       
                         P 
                         Lift 
                       
                     
                   
                 
               
               
                 
                   Equ 
                   . 
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0031]    where P max  and P Lift  are conventional operational pressure parameters for the three-way valve which are typically specified by the valve manufacturer. 
         [0032]    At block  403  of  FIG. 4 , a flow resistance, CV, across three-way valve  245  may be calculated, for example, in accordance with Equation 2 below: 
         [0000]        CV=f ( ST )  Equ. 2
 
         [0033]    where CV is typically specified as a function of valve stroke ST by the manufacturer of the three-way valve. 
         [0034]    At block  405  of  FIG. 4 , an estimate of fluid flow, W E , through three-way valve  245  may be calculated, for example, in accordance with Equation 3 below: 
         [0000]        W   E   =CV *SQRT( P   1   −P   2 )  Equ. 3
 
         [0035]    At block  407  of  FIG. 4 , the estimated spike, W S , in fluid flow may be calculated, for example, in accordance with Equation 4 below: 
         [0000]        W   S   =W   E   −W   measured   Equ. 4
 
         [0036]    where W measured  is fluid flow measured just upstream of the three-way valve (for example, after flow divider  230  in the system of  FIG. 2 ). 
         [0037]    Finally, at blocks  409 - 411  of  FIG. 4 , a position command for providing feed-forward control of a bypass valve in the fluid flow system (for example, a new position command for fuel bypass valve  220  in the  FIG. 2  system) may be determined as a function of the estimated spike, W S , and various gain control values used for the bypass valve controller  303 , as indicated by Equation 5 below: 
         [0000]      Bypass Valve Position= f ( W   s   ,KP,KI )  Equ. 5
 
         [0038]    where W S  is the calculated estimated spike, and KP and KI are user settable Proportional and Integral gain control values for the bypass valve controller. 
         [0039]    While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.