7fa gas turbine dln 2.6 gas fuel control system.pdf

8/18/2019 7FA Gas Turbine DLN 2.6 Gas Fuel Control System.pdf

http://slidepdf.com/reader/full/7fa-gas-turbine-dln-26-gas-fuel-control-systempdf 1/17

7FA Gas Turbine – DLN 2.6 Gas Fuel Control System

Table of Contents

Topic PageDLN 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 FiguresFigure PageFigure 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




DLN 2.6 Gas Fuel Control System

System Description

Refer to Drawing 133E3891 . The dry low NOx 2.6 (DLN-2.6) control system regulatesthe distribution of fuel delivered to a multi-nozzle, total premix combustor arrangement.The fuel flow distribution to each combustion chamber fuel nozzle assembly is calculatedto 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 systemmanifolds: Premix 1 (PM1), Premix 2 (PM2), Premix 3 (PM3), and Quaternary (Q). ThePM1 gas fuel delivery system consists of one diffusion type fuel nozzle for eachcombustion chamber. The PM2 gas fuel delivery system consists of two premix type fuelnozzles for each combustion chamber. The Quaternary gas fuel delivery system consistsof injection pegs located in each combustion casing. The PM3 gas fuel delivery systemconsists of three premix type fuel nozzles for each combustion chamber. The GCVsregulate the percentage of the total fuel flow delivered to each of the gas fuel systemmanifolds.

The DLN 2.6 combustion system consists of six fuel nozzles per combustion can, eachoperating 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 tothe 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 ofthe premix nozzles, circumferentially around the combustion can, is identified as thequaternary 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 inconjunction 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-nozzlecombustion system (Figure 2 ). The GCVs control the desired fuel flow in response to acontrol system fuel command, Fuel Stroke Reference (FSR). The response of the fuelflow to GCVs’ commands is made predictable by maintaining a predetermined pressureupstream of the GCVs. The GCVs’ upstream pressure, P2, is controlled by modulatingthe SRV based on turbine speed as a percentage of full speed, TNH, and feedback fromthe 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, andfour flame detectors in cans 11, 12, 13, and 14 with crossfire tubes connecting adjacentcombustors. Each combustor consists of a six-nozzle/end cover assembly, forward andaft combustion casings, flow sleeve assembly, multi-nozzle cap assembly, liner assembly,

and transition piece assembly. A quaternary nozzle arrangement penetrates thecircumference of the combustion can, porting fuel to casing injection pegs locatedradially around the casing.

Gas Fuel Operation

The DLN 2.6 fuel system operation is fully automated, sequencing the combustionsystem through a number of staging modes before reaching full load. Figure 3 representstypical operation sequence, from firing to full load fuel flow staging and a typicalshutdown fuel staging sequence from full load to unit flame out at part speed. The fuelflow to the six fuel nozzles and quaternary pegs are controlled by the GC V’s.




Figure 3: DLN 2.6 Fuel Modes for Startup and Shutdown

As shown, the primary controlling parameter for fuel staging is the calculatedcombustion reference temperature (TTRF), which is discussed later. Other DLN 2.6operation influencing parameters available to the operator are the selection of IGVtemperature control ON or OFF ', and the selection of inlet bleed heat ON or OFF . Toachieve maximum exhaust temperature, as well as an expanded load range for optimalemission, IGV temperature control should be selected ON , and inlet bleed heat should beselected 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 modenumber.

Combustion Reference Temperature

The combustion reference temperature signal (TTRF) is a calculation in the DLN-2.6control software. This calculated temperature represents a reference for combustor mode

sequencing and fuel split scheduling, but not unit load control. It should be noted thatTTRF is not a true indication of actual machine firing temperature, only a reference forDLN 2.6 mode transition sequencing. A careful checkout of the combustion referencetemperature during initial commissioning is required.

DLN-2.6 Inlet Guide Vane Operation




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 inletguide vane (IGV) temperature control. Optimal combustor operation is cruciallydependent upon operating along the predetermined temperature control scheme.Controlled fuel scheduling depends on the state of IGV temperature control. IGVtemperature control ON can also be referred to as combined cycle operation while IGVtemperature 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 controllingthe combustion process and protecting the gas turbine hardware. Four flame detectorsmounted in combustion chambers 11, 12, 13, and 14 (Figure 4 ) detect flame in all modesof operation. The signals from these flame detectors are processed in control logic andused 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 ignitefuel flow at firing speed, on startup only. Flame is propagated to those combustionchambers without spark plugs through crossfire tubes connecting adjacent combustionchambers 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 onthe SRV and GCV's. This allows the hydraulic fluid, opens the valve, to be ported todrain, while fluid is ported from the hydraulic supply to close the valve, assisted byspring force.

False Start

During, a false start - flame is not established in the four monitored combustion chambersafter 10 seconds - the stop ratio valve (SRV) and gas control valves (GCV’s) are shut andthe unit is run through a second unit purge cycle. At the end of this purge cycle, fuel isadmitted and firing is again attempted. If the second attempt is unsuccessful inmaintaining 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 informthe operator of the current combustion mode of operation:

Mode 1 (or M1)

Mode 2 (or M2)

Mode 3 (or M3)

Mode 4 (or M4)




Mode 5 (or M5)

Mode 5Q (or M5Q)

Mode 6Q (or M6Q)

Fuel Control SystemThe gas turbine fuel control system changes fuel flow to the combustors in response tothe fuel stroke reference signal (FSR). FSR actually consists of two separate signalsadded together, FSR1 - the called for liquid fuel flow and FSR2 - the called for gas fuelflow; normally, FSR1 + FSR2 = FSR. Standard fuel systems are designed for operationwith liquid fuel and/or gas fuel.

It starts with the servo drive system, where the setpoint is compared with the feedbacksignal and converted to a valve position. It also describes how the FSR from the controlsystems 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 shownin Figure 6 . The servo valve is the interface between the electrical and mechanicalsystems and controls the direction and rate of motion of a hydraulic actuator based itsinput current.

Figure 6: Electro-hydraulic Servo Valve Internals




The servo valve contains three electrically isolated coils on the torque motor (Figure 7 ).Each coil is connected to one of the three controllers <RST> providing redundancyshould one of the controllers or coils fail. A null bias spring positions the servo so theactuator will go to the fail safe position should ALL power and/or control signals be lost.

Figure 7: Electro-hydraulic Servo Valve and Motor




Gas system components are located in the gas compartment that is part of the accessorymodule (Figure 8 ).

Figure 8: Fuel Gas Compartment

Auxiliary Stop Valve

The auxiliary stop valve (Figure 9 ) provides additional shutoff of the fuel gas flow whenrequired by either normal operation or emergency conditions. The auxiliary stop valve is

pneumatically actuated valve that is spring biased to the closed position. The auxiliarystop valve has an integral trip relay valve 20VS4 that is located between the pneumaticsupply air and the valve actuator. When 20VS4 is energized, pneumatic pressure willforce the auxiliary stop valve to the open position by overcoming the spring bias. Upon ashut down or trip, 20VS4 is de-energized and the pneumatic pressure supply is shut off tothe valve actuator, the remaining pneumatic pressure in the valve actuator is releasedthrough 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 itan integral part of the protection system. An emergency trip or normal shutdown trips thevalve to its closed position, preventing gas fuel flow to the turbine. The SRV can beclosed two ways: 1) dumping the hydraulic oil from the SRV's hydraulic actuatorcylinder or 2) driving the SRV closed electrically using the control system's SRV positioncontrol loop. The control system also uses the SRV as a pressure control valve toregulate the pressure, P 2, upstream of the GCVs. While the SRVs position control loop isconsidered an inner control loop, the pressure control loop is considered an outer controlloop (Figure 11 ).




Figure 10: Stop Speed Ratio Valve




Figure 11: Speed Ratio/Stop Valve Control Schematic

The control system computes a P 2 pressure command, FPRGOUT which is a linear functionof TNH. Three pressure transducers sense the inter-valve pressure, P 2. Each channel of thecontrol system computes its own FPRGOUT and each is wired to a single pressuretransducer. The pressure transducers are used to determine the error between desired P 2

pressure, FPRGOUT and actual P 2 pressure. The resulting error is scaled through anintegration 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 toeach 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 errorthen becomes the input to the servo amplifier, which drives the servo valve in the directionrequired to decrease the position error.

The SRV is closed automatically on flame failure, failure to ignite on startup, or actuation ofthe fire detection equipment. Following a unit trip, the master protective and ignition

permissive circuits are used to prohibit starting until the conditions are acceptable.




In the event of an emergency trip or normal shutdown, a negative P 2 pressure is commanded by FPRGOUT to drive the SRV servo valve into negative saturation and quickly close theSRV. However, dumping hydraulic fluid from the SRV actuator cylinder allows the SRVreturn spring to close the valve well before the servo valve can empty the cylinder.

Gas Control ValvesThe GCVs (Figure 12 ) are actuated by hydraulic cylinders moving against spring loadedvalve plugs. Three coil servo valves are driven by electrical signals from the controlsystem to regulate the hydraulic fluid in the actuator cylinders. Redundant sensors in theform 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 valvestroke. The combined position of the control valves is intended to be proportional toFSR. 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 flowrequired 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 FuelSystem

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




o FSR2 is four parts: FSRPM1, FSRPM2, FSRPM3, and FSRQT

FSRPM1: the percentage of FSR2 controlling the GCV1 gas fuelvalve

FSRPM2: the percentage of FSR2 to be directed to the GCV2 gasfuel 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, andFSRQT, 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 dumpvalves. The dump valves are held in their normal operating state by a supply of hydraulicoil referred to as trip oil . The trip oil system is triple redundant to ensure that no singledevice 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.pdf

Documents