7FA Gas Turbine DLN 2.6 Gas Fuel Control System.pdf

7FA Gas Turbine – DLN 2.6 Gas Fuel Control System Table of Contents Topic Page DLN 2.6 GAS FUEL CONTROL SYSTEM .............................................................................. 2 Chamber Arrangement............................................................................................................ 4 Gas Fuel Operation ..................................................................................................................... 4 Combustion Reference Temperature....................................................................................... 5 DLN-2.6 Inlet Guide Vane Operation ..................................................................................... 5 Flame Detection ...................................................................................................................... 6 Ignition System ........................................................................................................................ 6 Contingency Operation ............................................................................................................... 7 Unit Trip.................................................................................................................................. 7 False Start ............................................................................................................................... 7 DLN-2.6 Display Messages ........................................................................................................ 7 Fuel Control System ................................................................................................................... 8 Auxiliary Stop Valve ................................................................................................................ 10 Stop/Speed Ratio Valve ............................................................................................................ 11 Gas Control Valves ................................................................................................................... 14 Gas Fuel Vent Solenoid Valve 20VG ....................................................................................... 17 SUMMARY ................................................................................................................................. 17 Table of Figures Figure Page Figure 1: DLN 2.6 Fuel Nozzle Arrangement ............................................................................... 3 Figure 2: DLN 2.6 Gas Fuel System .............................................................................................. 4 Figure 3: DLN 2.6 Fuel Modes for Startup and Shutdown............................................................ 5 Figure 4: Flame Detectors .............................................................................................................. 6 Figure 5: Spark Plugs ..................................................................................................................... 7 Figure 6: Electro-hydraulic Servo Valve Internals ........................................................................ 8 Figure 7: Electro-hydraulic Servo Valve and Motor ..................................................................... 9 Figure 8: Fuel Gas Compartment ................................................................................................. 10 Figure 9: Auxiliary Stop Valve .................................................................................................... 11 Figure 10: Stop Speed Ratio Valve .............................................................................................. 12 Figure 11: Speed Ratio/Stop Valve Control Schematic ............................................................... 13 Figure 12: Gas Control Valve ...................................................................................................... 14 Figure 13: Gas Fuel Control Schematic ....................................................................................... 16

7FA Gas Turbine – DLN 2.6 Gas Fuel Control System

DLN 2.6 Gas Fuel Control System System Description Refer to Drawing 133E3891. The dry low NOx 2.6 (DLN-2.6) control system regulates the distribution of fuel delivered to a multi-nozzle, total premix combustor arrangement. The fuel flow distribution to each combustion chamber fuel nozzle assembly is calculated to maintain unit load and fuel split for optimal turbine emissions. In a Dry Low NOx 2.6 (DLN-2.6) combustion system, there are four gas fuel system manifolds: Premix 1 (PM1), Premix 2 (PM2), Premix 3 (PM3), and Quaternary (Q). The PM1 gas fuel delivery system consists of one diffusion type fuel nozzle for each combustion chamber. The PM2 gas fuel delivery system consists of two premix type fuel nozzles for each combustion chamber. The Quaternary gas fuel delivery system consists of injection pegs located in each combustion casing. The PM3 gas fuel delivery system consists of three premix type fuel nozzles for each combustion chamber. The GCVs regulate the percentage of the total fuel flow delivered to each of the gas fuel system manifolds. The DLN 2.6 combustion system consists of six fuel nozzles per combustion can, each operating as a fully premixed combustor; five located radially, one located in the center. The center nozzle, identified as PM 1, (PreMix 1), two outer nozzles located adjacent to the crossfire tubes, identified as PM2, (PreMix 2), and the remaining three outer nozzles, identified as PM3, (PreMix 3). Another fuel passage, located in the airflow upstream of the premix nozzles, circumferentially around the combustion can, is identified as the quaternary fuel pegs (Figure 1).


Figure 1: DLN 2.6 Fuel Nozzle Arrangement The Stop/Speed Ratio Valve (SRV) and the Gas Control Valves (GCVs) work in conjunction to regulate the total fuel flow delivered to the gas turbine. This arrangement uses four separate GCVs to control the distribution of the fuel flow to a multi-nozzle combustion system (Figure 2). The GCVs control the desired fuel flow in response to a control system fuel command, Fuel Stroke Reference (FSR). The response of the fuel flow to GCVs’ commands is made predictable by maintaining a predetermined pressure upstream of the GCVs. The GCVs’ upstream pressure, P2, is controlled by modulating the SRV based on turbine speed as a percentage of full speed, TNH, and feedback from the P2 pressure transducers, 96FG-2A, B, and C.


Figure 2: DLN 2.6 Gas Fuel System Chamber Arrangement The 7F machine employs 14 combustors. There are two spark plugs in cans 2 and 3, and four flame detectors in cans 11, 12, 13, and 14 with crossfire tubes connecting adjacent combustors. Each combustor consists of a six-nozzle/end cover assembly, forward and aft combustion casings, flow sleeve assembly, multi-nozzle cap assembly, liner assembly, and transition piece assembly. A quaternary nozzle arrangement penetrates the circumference of the combustion can, porting fuel to casing injection pegs located radially around the casing. Gas Fuel Operation The DLN 2.6 fuel system operation is fully automated, sequencing the combustion system through a number of staging modes before reaching full load. Figure 3 represents typical operation sequence, from firing to full load fuel flow staging and a typical shutdown fuel staging sequence from full load to unit flame out at part speed. The fuel flow to the six fuel nozzles and quaternary pegs are controlled by the GCV’s.


Figure 3: DLN 2.6 Fuel Modes for Startup and Shutdown As shown, the primary controlling parameter for fuel staging is the calculated combustion reference temperature (TTRF), which is discussed later. Other DLN 2.6 operation influencing parameters available to the operator are the selection of IGV temperature control ON or OFF', and the selection of inlet bleed heat ON or OFF. To achieve maximum exhaust temperature, as well as an expanded load range for optimal emission, IGV temperature control should be selected ON, and inlet bleed heat should be selected ON. DLN 2.6 operational mode is displayed on the main display, as well as the DLN display. Operational mode is defined as “the sum of the nozzles being delivered fuel;” therefore, if PM1 and PM3 are fueled, the unit is in Mode 4. Likewise, if PM2 and PM3 are fueled, the unit is in Mode 5. When the quaternary passages are fueled, a Q is added to the mode number. Combustion Reference Temperature The combustion reference temperature signal (TTRF) is a calculation in the DLN-2.6 control software. This calculated temperature represents a reference for combustor mode sequencing and fuel split scheduling, but not unit load control. It should be noted that TTRF is not a true indication of actual machine firing temperature, only a reference for DLN 2.6 mode transition sequencing. A careful checkout of the combustion reference temperature during initial commissioning is required.

DLN-2.6 Inlet Guide Vane Operation

7FA Gas Turbine – DLN 2.6 Gas Fuel Control System The DLN-2.6 combustor emission performance is sensitive to changes in fuel to air ratio. The combustor was designed according to the airflow regulation scheme used with inlet guide vane (IGV) temperature control. Optimal combustor operation is crucially dependent upon operating along the predetermined temperature control scheme. Controlled fuel scheduling depends on the state of IGV temperature control. IGV temperature control ON can also be referred to as combined cycle operation while IGV temperature control OFF is referred to as simple cycle operation. Flame Detection Reliable detection of the flame location in the DLN-2.6 system is critical to controlling the combustion process and protecting the gas turbine hardware. Four flame detectors mounted in combustion chambers 11, 12, 13, and 14 (Figure 4) detect flame in all modes of operation. The signals from these flame detectors are processed in control logic and used for various control and protection functions.

Figure 4: Flame Detectors Ignition System Two spark plugs, located in combustion chambers 2 and 3 (Figure 5), are used to ignite fuel flow at firing speed, on startup only. Flame is propagated to those combustion chambers without spark plugs through crossfire tubes connecting adjacent combustion chambers around the gas turbine.


Figure 5: Spark Plugs Contingency Operation Unit Trip In case of a unit trip, the gas fuel system is shut down by deactivating the dump valves on the SRV and GCV's. This allows the hydraulic fluid, opens the valve, to be ported to drain, while fluid is ported from the hydraulic supply to close the valve, assisted by spring force. False Start During, a false start - flame is not established in the four monitored combustion chambers after 10 seconds - the stop ratio valve (SRV) and gas control valves (GCV’s) are shut and the unit is run through a second unit purge cycle. At the end of this purge cycle, fuel is admitted and firing is again attempted. If the second attempt is unsuccessful in maintaining flame, the unit is tripped and the SRV and GCV’s close. DLN-2.6 Display Messages The following display messages will appear on the Mark VI control panel CRT to inform the operator of the current combustion mode of operation: 

Mode 1 (or M1)



Mode 2 (or M2)



Mode 3 (or M3)



Mode 4 (or M4)

7FA Gas Turbine – DLN 2.6 Gas Fuel Control System 

Mode 5 (or M5)



Mode 5Q (or M5Q)



Mode 6Q (or M6Q)

Fuel Control System The gas turbine fuel control system changes fuel flow to the combustors in response to the fuel stroke reference signal (FSR). FSR actually consists of two separate signals added together, FSR1 - the called for liquid fuel flow and FSR2 - the called for gas fuel flow; normally, FSR1 + FSR2 = FSR. Standard fuel systems are designed for operation with liquid fuel and/or gas fuel. It starts with the servo drive system, where the setpoint is compared with the feedback signal and converted to a valve position. It also describes how the FSR from the control systems is conditioned and sent as a setpoint to the servo system. The heart of the fuel system is a three-coil electro-hydraulic servo valve (servo) as shown in Figure 6. The servo valve is the interface between the electrical and mechanical systems and controls the direction and rate of motion of a hydraulic actuator based its input current.

Figure 6: Electro-hydraulic Servo Valve Internals

7FA Gas Turbine – DLN 2.6 Gas Fuel Control System The servo valve contains three electrically isolated coils on the torque motor (Figure 7). Each coil is connected to one of the three controllers providing redundancy should one of the controllers or coils fail. A null bias spring positions the servo so the actuator will go to the fail safe position should ALL power and/or control signals be lost.

Figure 7: Electro-hydraulic Servo Valve and Motor

7FA Gas Turbine – DLN 2.6 Gas Fuel Control System Gas system components are located in the gas compartment that is part of the accessory module (Figure 8).

Figure 8: Fuel Gas Compartment Auxiliary Stop Valve The auxiliary stop valve (Figure 9) provides additional shutoff of the fuel gas flow when required by either normal operation or emergency conditions. The auxiliary stop valve is pneumatically actuated valve that is spring biased to the closed position. The auxiliary stop valve has an integral trip relay valve 20VS4 that is located between the pneumatic supply air and the valve actuator. When 20VS4 is energized, pneumatic pressure will force the auxiliary stop valve to the open position by overcoming the spring bias. Upon a shut down or trip, 20VS4 is de-energized and the pneumatic pressure supply is shut off to the valve actuator, the remaining pneumatic pressure in the valve actuator is released through a quick exhaust port in 20VS4, and the valve close.


Figure 9: Auxiliary Stop Valve Stop/Speed Ratio Valve The SRV (Figure 10) serves two functions. First, it operates as a stop valve, making it an integral part of the protection system. An emergency trip or normal shutdown trips the valve to its closed position, preventing gas fuel flow to the turbine. The SRV can be closed two ways: 1) dumping the hydraulic oil from the SRV's hydraulic actuator cylinder or 2) driving the SRV closed electrically using the control system's SRV position control loop. The control system also uses the SRV as a pressure control valve to regulate the pressure, P2, upstream of the GCVs. While the SRVs position control loop is considered an inner control loop, the pressure control loop is considered an outer control loop (Figure 11).


Figure 10: Stop Speed Ratio Valve


Figure 11: Speed Ratio/Stop Valve Control Schematic The control system computes a P2 pressure command, FPRGOUT which is a linear function of TNH. Three pressure transducers sense the inter-valve pressure, P2. Each channel of the control system computes its own FPRGOUT and each is wired to a single pressure transducer. The pressure transducers are used to determine the error between desired P2 pressure, FPRGOUT and actual P2 pressure. The resulting error is scaled through an integration algorithm using the current gas FSR command, FSR2, to compute a valve position command. Two LVDTs sense SRV stem position and their outputs are returned to each channel of the control system. The largest feedback signal is selected to determine the error between desired SRV valve position command and actual valve position. The error then becomes the input to the servo amplifier, which drives the servo valve in the direction required to decrease the position error. The SRV is closed automatically on flame failure, failure to ignite on startup, or actuation of the fire detection equipment. Following a unit trip, the master protective and ignition permissive circuits are used to prohibit starting until the conditions are acceptable.

7FA Gas Turbine – DLN 2.6 Gas Fuel Control System In the event of an emergency trip or normal shutdown, a negative P2 pressure is commanded by FPRGOUT to drive the SRV servo valve into negative saturation and quickly close the SRV. However, dumping hydraulic fluid from the SRV actuator cylinder allows the SRV return spring to close the valve well before the servo valve can empty the cylinder.

Gas Control Valves The GCVs (Figure 12) are actuated by hydraulic cylinders moving against spring loaded valve plugs. Three coil servo valves are driven by electrical signals from the control system to regulate the hydraulic fluid in the actuator cylinders. Redundant sensors in the form of Linear Variable Differential Transformers (LVDTs) mounted on each valve provide the control system with valve position feedback for closed loop position control.

Figure 12: Gas Control Valve The plugs in the GCVs are contoured to provide the proper flow area relative to valve stroke. The combined position of the control valves is intended to be proportional to FSR. The GCVs use a skirted valve disc and venturi seat for adequate pressure recovery. High-pressure recovery, occurs at valve pressure ratios substantially less than the critical pressure ratio. As a result, the flow through the GCVs is independent of the pressure drop across the valves and is a function of valve inlet pressure, P2, and valve area only. The control system's fuel command, FSR, is the percentage of maximum fuel flow required by the control system to maintain speed, load, or another setpoint. FSR is broken down into two parts that make up the fuel split setpoint, FSR1 and FSR2. 

FSR1: The percentage of maximum fuel flow required from the Liquid Fuel System



FSR2: The percentage of maximum fuel flow required from the Gas Fuel System

7FA Gas Turbine – DLN 2.6 Gas Fuel Control System o FSR2 is four parts: FSRPM1, FSRPM2, FSRPM3, and FSRQT  

FSRPM1: the percentage of FSR2 controlling the GCV1 gas fuel valve FSRPM2: the percentage of FSR2 to be directed to the GCV2 gas fuel valves, and so on.

FSRPM1 is used as a reference to a servo amplifier that drives the coils of GCV #1. FSRPM2 is used to drive the coils of GCV #2, and so on (Figure 13). Each processor of the control system computes its own FSR2, FSRPM 1, 2, 3, and FSRQT, and each processor drives one of the three servo valve coils. The GCVs' position control loops function similarly to the SRV's position control loop. The SRV and GCVs are equipped with hydraulically-actuated, spring-return dump valves. The dump valves are held in their normal operating state by a supply of hydraulic oil referred to as trip oil. The trip oil system is triple redundant to ensure that no single device failure can disturb the operation of the power generating unit.


Figure 13: Gas Fuel Control Schematic

7FA Gas Turbine – DLN 2.6 Gas Fuel Control System Gas Fuel Vent Solenoid Valve This solenoid valve vents the volume between the stop/speed ratio valve and the gas control valves when the solenoid is de-energized. The solenoid is energized and the vent valve closed when the master control protection circuit is energized and the turbine is above the cooldown slow roll speed. It will be closed and remain closed during gas fuel operation. The vent is open when the turbine is shut down because the stop/speed ratio and gas control valves have metal plugs and metal seats and therefore, are not leak tight. The vent insures that during the shutdown period, fuel gas pressure will not build up between the stop/speed ratio and gas control valves, and that no fuel gas will leak past the closed gas control valve to collect in the combustors or exhaust. If the vent valve fails during normal operation, the SRV continues to maintain constant pressure, P2. This is accomplished by opening further, making up any lost flow through the vent valve.

Summary

What you have learned…

This chapter described the operation of the DLN-2.6 Gas Fuel Control System.

7FA Gas Turbine DLN 2.6 Gas Fuel Control System.pdf

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