GEK111712 Control & Protection
Short Description
GEK111712 Control & Protection...
Description
GEK 111712 June 2007
g
GE Energy
Control and Protection Articles DLN2.6+ Gas Fuel System
These instructions do not purport to cover all details or variations in equipment nor to provide for every possible contingency to be met in connection with installation, operation or maintenance. Should further information be desired or should particular problems arise which are not covered sufficiently for the purchaser's purposes the matter should be referred to the GE Company. © 2007 General Electric Company
GEK 111712
Control and Protection Articles DLN2.6+ Gas Fuel System
The below will be found throughout this publication. It is important that the significance of each is thoroughly understood by those using this document. The definitions are as follows: NOTE Highlights an essential element of a procedure to assure correctness. CAUTION Indicates a potentially hazardous situation, which, if not avoided, could result in minor or moderate injury or equipment damage.
WARNING INDICATES A POTENTIALLY HAZARDOUS SITUATION, WHICH, IF NOT AVOIDED, COULD RESULT IN DEATH OR SERIOUS INJURY
***DANGER*** INDICATES AN IMMINENTLY HAZARDOUS SITUATION, WHICH, IF NOT AVOIDED WILL RESULT IN DEATH OR SERIOUS INJURY.
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TABLE OF CONTENTS I. GENERAL ..................................................................................................................................................... 4 II. EQUIPMENT................................................................................................................................................. 4 A. Gas Flow Measurement (96FM-1)............................................................................................................ 4 B. Gas Fuel Auxiliary Stop Valve (VS4-1) ................................................................................................... 4 C. Stop/Speed Ratio Valve (VSR-1).............................................................................................................. 4 D. Gas Fuel Control Valves (VGC-1, VGC-2, VGC-3, VGC-4)................................................................... 5 E. Gas Purge Block Valves (VA13-1, VA13-2, VA13-20, VA13-21, VA13-3, VA13-4, VA13-23, VA1224) ............................................................................................................................................................. 5 F. Gas Purge Pressure Ratio Monitoring (96GN-1, 96GN-2, 96GN-3, 96GN-4) ......................................... 5 III. OPERATION ................................................................................................................................................. 5 A. General ...................................................................................................................................................... 5 B. Pre-start Conditions................................................................................................................................... 7 C. Startup and Loading Operation ................................................................................................................. 8 IV. CONTROL ................................................................................................................................................... 12 A. P2 Pressure Control................................................................................................................................. 12 B. Gas Flow and Split Control..................................................................................................................... 15 C. Gas Fuel Temperature Compensation ..................................................................................................... 16 D. Gas Purge Control ................................................................................................................................... 17 V. PROTECTION ............................................................................................................................................ 17 A. P2 Pressure Control Protection and SRV Position Control Protection ................................................... 18 B. Fuel Temperature and Modified Wobbe Index Protection...................................................................... 18 C. Independent Flow Path Purge Protection ................................................................................................ 19
LIST OF FIGURES Figure 1. DLN2.6+ Fuel Nozzle Arrangement....................................................................................................... 6 Figure 2. Gas Fuel Leak Test Function Block Diagram....................................................................................... 10 Figure 3. SRV P2 Pressure Control Algorithm Diagram ..................................................................................... 13 Figure 4. SRV Speed Ratio Valve Control Schematic ......................................................................................... 14 Figure 5. GCV Control Algorithm Diagram ........................................................................................................ 15 Figure 6. GCV Control Schematic ....................................................................................................................... 16
LIST OF TABLES Table 1. DLN2.6+ Mode Staging Diagram............................................................................................................ 7 Table 2. Protective levels and actions for the Gas Fuel System – Gas Pressure .................................................. 18 Table 3. Independent Flow Path Purge Protection ............................................................................................... 20
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Control and Protection Articles DLN2.6+ Gas Fuel System
I. GENERAL The Gas Fuel System's function is to provide accurate and repeatable gas fuel flow, and flow split control for a multi-stream fuel injection system DLN2.6+. It also provides safe and reliable isolation from the gas fuel source. This unit use preheated Gas Fuel @ 365 deg F. The DLN2.6+ control hardware and the control system are described in this document. See Appendix 1. Simplified Schematic of the Gas Fuel System and Gas Purge System. The Safer case ID for this System is EP-S-767. II. EQUIPMENT The Gas Fuel System consists of a gas flow metering tube, a gas fuel section of the accessory module (or a separate gas fuel module for single shaft units), and the gas turbine on-base equipment. This section houses both the gas fuel system, and gas fuel purge system. A brief description of the system’s overall major components is described below. See Gas Fuel System Articles for further details. A. Gas Flow Measurement (96FM-1) A mass flow transmitter, 96FM-1 and Gas fuel meter tube / orifice (MG2-1) are used to measure natural gas flow. The device, 96FM-1 operates on the principle of measuring delta pressure across the gas fuel meter orifice, static gas pressure and gas temperature. These measured parameters establish the mass flow when the flow area ratio (beta) is known (constant). Gas Fuel Flow is only used for monitoring, and display in the gas turbine controls. B. Gas Fuel Auxiliary Stop Valve (VS4-1) The Auxiliary Stop Valve is a butterfly type isolation valve that provides the required ANSI Class VI shutoff of the gas fuel supply. This valve is only required for units with performance heating of the gas fuel. For units without performance heating, the Stop/Speed Ratio Valve (SRV) provides the required ANSI Class VI shutoff capability and the Auxiliary Stop Valve is not required. This valve is pneumatically actuated with a spring close actuator design for fail-safe operation. When the system is pressurized or not tripped, the solenoid operated pilot valve (20VS4-1) directs instrument air to the actuator of the aux. stop valve. The valve actuator acts against the valve spring causing the valve to open. During a trip event the solenoid valve (20VS4-1) is de-energized which vents the pressurized air in the Stop Valve actuator to atmosphere. The compressed spring causes the Fuel Gas Auxiliary Stop valve to close. This valve opens during the gas turbine ignition sequence when the turbine is started. C. Stop/Speed Ratio Valve (VSR-1) The SRV is a V-notch type ball valve that serves two functions. First it operates as the primary stop valve, making it an integral part of the protection system. The SRV is tripped closed by the hydraulic trip system via the directional trip relay VH5-1. This valve is hydraulically actuated with a spring close actuator for fail-safe operation. An emergency trip or flame out on a normal shutdown will trip the valve to its closed position, isolating gas fuel to the turbine. Closing the SRV can be achieved in two ways: dumping the hydraulic trip oil to the SRV, or driving the SRV closed electrically using the servo valve 90SR-1 with the control systems SRV position control loop. The other function of the SRV is pressure regulation. The control system uses the SRV to regulate the pressure (P2) upstream of the Gas Control Valves. This function is described in further detail in Section IV - Control.
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D. Gas Fuel Control Valves (VGC-1, VGC-2, VGC-3, VGC-4) There are four independent Gas Control Valves (GCV’s) in the DLN2.6+ system. The GCV’s are angle body plug valves actuated by hydraulic cylinders with a spring close actuator for fail-safe operation. The actuator design is single acting. The plugs in the GCV’s are contoured to provide a proportional flow area in relation to valve stroke. The GCV’s use a skirted valve disc and venturi seat to obtain high pressure recovery. This high pressure recovery design achieves critical pressure operation at substantially lower valve pressure drops. The result is that the flow through the GCV’s is independent of the pressure drop across the valves and is a function of valve inlet pressure (P2) and valve area only. The valves are positioned by the control system to maintain a percentage of the total fuel to each of the fuel circuits. This fuel split is a function of DLN operating mode, and the combustion reference temperature. E. Gas Purge Block Valves (VA13-1, VA13-2, VA13-20, VA13-21, VA13-3, VA13-4, VA13-23, VA12-24) The gas purge block valves are a V-notch type ball valve. These valves are arranged in a double block and bleed configuration. The actuator design for each valve is a pneumatically operated rackand-pinion with a fail close spring. The valves are driven open and closed using a pneumatically operated pilot valve (VA36-1, VA36-2, VA36-20, VA36-21, VA36-3, VA36-4, VA36-23, VA3624). These valves provide for rapid exhaust of instrument air from the actuators of the VA13 valves. The VA36 valves are driven by a solenoid operated pilot valve (20PG-1, 20PG-2, 20PG-20, 20PG21, 20PG-3, 20PG-4, 20PG-23, 20PG-24) for actuation. A metering needle valve is provided on the inlet pressure side of the pilot valves (VA36) to control the opening rate of each VA13 valve, except for VA13-2. This valve, VA13-2, is equipped with an I/P controller, used to provide for a variable slew rate and positioning. The opening rate of the purge valves must be slow in order to minimize transients. Excessive megawatt and temperature transients are caused by too rapid purging of gas fuel from the fuel manifolds into the combustion system. The purge system is designed to minimize this effect. Each valve has two limit switches that indicate open and closed valve position and are used for monitoring and diagnostics in the control system. F. Gas Purge Pressure Ratio Monitoring (96GN-1, 96GN-2, 96GN-3, 96GN-4) When a gas circuit is being purged, a minimum gas purge pressure ratio must be maintained to ensure positive airflow across all the fuel nozzles. This pressure ratio is sufficient to overcome any combustion can-to-can pressure variation. The differential pressure transmitters measure the gas manifold pressure relative to compressor discharge pressure. These pressures are used for monitoring and alarm in the control system. III. OPERATION A. General Gas fuel flow is controlled with the Auxiliary Stop Valve, SRV, Diffusion GCV (D5) and the Premix GCV’s (PM1, PM3 and PM2). The SRV and the GCV’s work in conjunction to regulate the total fuel flow delivered to the gas turbine. The GCV’s control the desired fuel flow in response to a control system fuel command, Fuel Stroke Reference (FSR). The response of the fuel flow to the GCV position commands is made linear by maintaining a predetermined pressure upstream of the GCV’s (P2). The GCV’s upstream P2 pressure is controlled by modulating the SRV as a function of turbine percent speed (TNH), and feedback FPG2 from the P2 pressure transducers (96FG-2A, 96FG-2B, 96FG-2C). The DLN2.6+ Gas Fuel System has four fuel manifolds (D5, PM1, PM3 & PM2). These are independent fuel circuits each having an individual GCV for controlling gas fuel delivery. Each
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Control and Protection Articles DLN2.6+ Gas Fuel System
combustion chamber has six fuel nozzles arranged in a five-around-one configuration. The PM1 nozzle is in the center. The five outer nozzles have a diffusion passage and a premix passage (PM3 or PM2). The center nozzle only receives premix fuel flow. Refer to Figure 1 for the DLN2.6+ Nozzle Arrangement. Circuit D5 PM1 PM3 PM2
Description Outer nozzle diffusion passage Center nozzle premix passage Premix passage on 3 outer nozzles Premix passage on 2 outer nozzles
Each of these four gas fuel circuits requires continuous independent control. Thus, each has a separate manifold and control valve.
D5 PM3 D5 PM3 D5 PM2
PM1 D5 PM2 D5 PM3
Figure 1. DLN2.6+ Fuel Nozzle Arrangement Each fuel circuit requires a certain percentage of the total fuel. The percentage of fuel to each circuit is a function of Combustion Reference Temperature (TTRF1) and DLN operating mode. There are six steady state DLN modes of operation: Diffusion, Sub Piloted Premix, Piloted Premix, Sub Premix, Premix and Extended Piloted Premix (not a normal operating mode). There are also two transient load rejection modes. All fuel is directed to the PM1 nozzle during load rejections from Premix or Sub-Premix modes. Load rejections from all other modes will be to Sub-Piloted Premix mode. The steady state DLN operating modes are a function of Combustion Reference Temperature (TTRF1). Refer to Table 1 DLN2.6+ Mode Staging Diagram. This method of staging the fuel requires individual control valves (GCV-1, GCV-2, GCV-3, and GCV-4) for each fuel circuit. In order to optimize flow split accuracy, the control valves are operated with a critical pressure ratio. A P2 pressure value, or control valve supply pressure, is preprogrammed into the control scheme in order to maintain this critical pressure ratio throughout the operating range. This control method ensures pressure fluctuations downstream of the gas valves does not impact flow accuracy. During certain DLN modes of operation, some circuits will have no fuel flow scheduled. A means of purging these stagnant circuits is required to prevent condensate from accumulating, and to minimize the potential for auto-ignition. The gas fuel that remains in a circuit after fuel flow is commanded off is purged into the combustion chambers when the purge is commanded on. A connection to the purge air system is located just downstream of the gas control valves. This ensures that the entire length of pipe and manifold is purged.
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Table 1. DLN2.6+ Mode Staging Diagram
MODE Diffusion (P)
Sub Piloted Premix (L) Piloted Premix (H) Sub Premix (M) Premix (B)
OPERATIONAL RANGE Ignition to TTRF=1500
CIRCUITS FUELED D5
95% speed to TTRF=2000 TTRF=1500 to 2300
D5 + PM1
CIRCUITS PURGED None below 95% Speed PM1+PM2+PM3 above 95% Speed PM3 + PM2
D5 + PM1 + PM3
PM2
TTRF=1900 to 2300 PM1 + PM3 TTRF=2150 to Base PM1 + PM3 + PM2 Load Extended Piloted TTRF=2150 to Base D5 + PM1 + PM3 + Premix (A) Load PM2 Load Rejection (I) Transient from PM1 Premix or Sub-Premix Modes Load Rejection (P or Transient from all D5 or D5 + PM1 (see L) other Modes Note 1)
D5 + PM2 D5 None Premix: D5 Sub-Premix: D5+PM2 None
NOTES 1. Load Rejection to normal Full Speed No Load (FSNL) mode, which will be either Diffusion Mode (D5 fueled) or Sub Piloted Premix Mode (D5 + PM1 fueled). 2. The unit will be load limited in each mode (except Premix and Ext. Piloted Premix) at the upper end of the design ranges defined in this table. B. Pre-start Conditions Prior to unit start, the following start conditions must be satisfied for the Gas Fuel and Gas Purge System. 1. Gas supply pressure within limits. 2. Gas module ventilation pressure normal. 3. Servo currents within limits. 4. P2 pressure feedback signal within limits. 5. Control valves track commands within limits. 6. Valve position feedback within limits. 7. All GCV’s closed.
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Control and Protection Articles DLN2.6+ Gas Fuel System
8. Auxiliary Stop Valve and SRV cavity pressure normal. 9. No gas purge valve position faults. During the start period the above conditions are continuously checked by the control system If any condition is not satisfied the unit will not be allowed to start. Prior to starting the gas fuel system, the following systems must be operating: • Gas fuel compressors, when pipe line supply pressure is insufficient • Instrument air system Pre-Ignition P2 Pressure High Start Inhibit The Pre-Ignition P2 Pressure High protection sequencing makes sure that the P2 cavity is clear of any pressure with the vent valve open, just prior to light-off. If the P2 pressure (FPG2) exceeds a specified increasing pressure for a determined amount of seconds prior to firing permissive (L2TVZ), a pre-ignition trip or Start Inhibit (L4PRETX) and alarm will occur. C. Startup and Loading Operation Gas Purge Valve Test At the very beginning of startup the gas purge valves are tested automatically by the controls to verify that slew times are within specification. All valves are cycled open and closed with the resultant time to complete each operation for each valve being measured. An alarm signal is generated for any valve that does not cycle within the allotted time frame. Alarm signals are generated for both open and close time violations for each valve. The turbine will not start cranking if any valve fails to pass the test. Once the gas purge valve test is complete, the gas turbine starts to crank. Gas Fuel Leak Test The Gas Leak Test will test the SRV and GCV’s for high leakage rates upon startup and shutdown by monitoring the pressure in the P2 cavity. The tests will take place when the turbine starts purging speed for the startup test and just after the turbine is shutdown for the shutdown test. Once either of these enable commands have been met, the test will start. The Gas Leak Test is composed of four (4) steps. Refer to Figure 2 Gas Leak Test Function Block Diagram. Test A: Test A will monitor the leakage across the SRV. The Auxiliary Stop Valve will be commanded open, the Fuel Gas Vent Solenoid valve will be commanded closed (20VG-1), and the Fuel Gas Trip Solenoid will be opened (20FG-1) to allow the trip system piping to fill up with oil for the next phase of testing. If the leakage across the SRV is excessive and the P2 cavity pressure rises above K86GLTA pressure in K86GLT1 seconds, turbine startup will be inhibited and the machine will shutdown. Open SRV: Once Test A has been passed, the SRV will be commanded open by forcing the SRV pressure offset to a large pressure value for K86GLT2 seconds to ensure that the P2 cavity has been pressurized to full line (supply) pressure. The SRV and the Aux Stop Valve will
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then be commanded closed again and Test B will start. When K86GLT2 times out, the P1 pressure and P2 pressure will be latched and compared to each other. If the difference between the two pressures is greater than K96FG_DIFF an alarm will notify the controller. Test B: Test B will monitor the P2 pressure and make sure that the GCV’s or Vent Valve is not leaking excessively. If the P2 pressure drops below more than K86GLTB psi after K86GLT3 seconds, the turbine will start inhibit and latch in an alarm. If Test B passes after K86GLT3 seconds, the vent will open and the pressure will drain out of the P2 cavity. If there is no Aux Stop Valve, the test is over, and the turbine will proceed with normal operation. Relieve Pressure: If the Aux Stop Valve is required for the system, the test will wait 5 seconds to ensure that all valves have been returned to normal state. The SRV will then be opened again to relieve any pressure that has been built up between that Aux Stop Valve and the SRV during the tests. Once this pressure has been drained, the SRV will close and the test will be completed. NOTE If the SRV does not return to its normal sealed position when the test times out, the P2 Pre-Ignition Trip will alarm and inhibit startup.
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Control and Protection Articles DLN2.6+ Gas Fuel System
P2 Pressure P1 pressure
Test B Limit Test B Fail Test A Fail Test A Limit if Aux Stop Valve Option
P2 pressure with vent open
time K86GLT1
K86GLT2
K86GLT3
K86GLT4 K86GLT5 K86GLT6
Test A
vent valve 20VG-1
open SRV
Test B
relieve pressure
open closed
aux stop vlv open VS4-1 closed SRV
GCV(s) FG Trip Solenoid 20FG-1
open closed open closed open closed
Figure 2. Gas Fuel Leak Test Function Block Diagram
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Control and Protection Articles DLN2.6+ Gas Fuel System
Valve Test GSRV FPG2 x-mitters GCV Venting
Condition
Aux Stop Vlv
Unit Start and Open Normal Shutdown * Unit Start and Open Normal Shutdown * Unit Start and Closed Normal Shutdown * Unit Start and Closed Normal Shutdown *
GEK 111712
SRV
Vent Vlv
GCVs
Pass Criteria
Test Time
Action
Closed
Closed
Closed
P2 < 100 PSIG
30 sec
Startup Lockout
Open
Closed
Closed
1 sec
Alarm
Closed
Closed
Closed
Closed
Open
Closed
ABS (P1-P2) < 20 PSI P2 > P1 – 150 PSI P2 < 6 psi
30 sec After vent timer
Startup Lockout Startup Lockout
Post Ignition P2 Pressure High and Low Trip After firing and before warm-up complete, the P2 pressure is monitored to ensure that it is within a predetermined pressure range. The desired unit P2 pressure during warm-up is calculated to achieve the unit pressure range. If the pressure is lower or higher than a specified tolerance for a given time frame, the unit will alarm and trip. Start The gas turbine is started in D5 Diffusion mode. When the unit reaches firing speed, the SRV (VSR1) and the D5 Diffusion GCV (VGC-1) servo controls are enabled. The P2 cavity vent valve (VA1315) closes. The SRV opens to control P2 pressure to its setpoint, and the D5 GCV (VGC-1) opens to the ignition setpoint. After flame is established and the turbine warm-up cycle is complete, the unit accelerates to Full Speed No Load (FSNL) in Diffusion mode. At 95% speed, the unit transfers to Sub Piloted Premix mode (SPPM). The control code also has the capability of delaying the Diffusion to SPPM transfer up to TTRF1 levels of 1500 degrees F. At the SPPM transfer point, the PM1 GCV (VGC-2) is enabled open to the Prefill position. After Prefill is complete, the PM1 GCV (VGC-2) is commanded to a target fuel split. The purge system will be enabled 60 seconds after the unit reaches 95% speed. Once the purge system is enabled in SPPM mode, the PM3 purge valves will open. After an 11 second delay, the PM2 purge valves will open, admitting compressor discharge air into the gas manifolds. If Diffusion mode is utilized above 95% speed, the purge on sequence will be PM1, then PM3, then PM2 (with 11 second delays). At rated speed, the unit is synchronized and the breaker is closed. The unit load increases towards full load. The fuel system remains in Sub Piloted Premix mode, until the first combustion reference temperature (TTRF1) switch point. At this switch point, the fuel system transfers to Piloted Premix (PPM) mode. This first switch point occurs between TTRF1=1500 and TTRF1=2000. At the Piloted Premix (PPM) switch point, the PM3 circuit purge valves (VA13-3, VA13-4) are commanded closed. When these valves are confirmed closed, the PM3 GCV (VGC-3) is enabled open to the Prefill position. After Prefill is complete, the PM3 GCV (VGC-3) is commanded to a target fuel split. The fuel split schedule for each DLN mode is a function of combustion reference temperature (TTRF1). The unit is now in Piloted Premix (PPM) Steady State mode. At the second combustion reference temperature (TTRF1) switch point, the fuel system transfers to Sub Premix mode. This switch point occurs between TTRF1=1900 and TTRF1=2300. The D5 GCV (VGC-1) is commanded closed and confirmed closed and the D5 circuit purge valves (VA13-1, VA13-2) are commanded open, admitting compressor discharge air into the gas manifold. The unit is now in Sub Premix (SPM) Steady State mode.
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At the third combustion reference temperature (TTRF1) switch point, the fuel system transfers to Premix mode. This switch point occurs between TTRF1=2150 and TTRF1=2300. The PM2 circuit purge valves (VA13-23, VA13-24)) are commanded closed. Once confirmed closed, the PM2 GCV (VGC-4) is enabled open to the Prefill position. After Prefill is complete, the PM2 GCV (VGC-4) is commanded to a target fuel split. The unit is now in Premix (PM) Steady State mode. The unit continues to operate in Premix mode up to Base load. A backup mode (Extended Piloted Premix) is also available for testing, commissioning and fault accommodation. This mode provides fuel to all four fuel circuits. Ext.PPM mode can be enabled by a pushbutton on the HMI screen. If this option is selected, the unit will load using SPPM, PPM, and Ext.PPM modes, and will not use SPM or PM modes. Ext.PPM mode is also used if a D5 purge fault occurs while in Premix mode (see section V). The shutdown sequence of the fuel system is the reverse of the startup and otherwise similar. During shutdown the unit transfers to SPM, PPM, SPPM and then to Diffusion mode, down to flame out at approximately 17% speed. At approximately 94% turbine speed the gas purge valves are closed. After flame out the hydraulic trip system is de-energized and the SRV and GCV’s close. The P2 cavity vent valve opens. IV. CONTROL A. P2 Pressure Control The SRV (VSR-1) modulates to control gas supply pressure (P2) to the independent GCV’s. The median selected value FPG2 of the triple redundant P2 pressure transducers (96FG-2A, 96FG-2B, 96FG-2C) is used in the Proportional + Integral controller. The P2 pressure setpoint (FPGR) is a function of unit speed (TNH). The output of the proportional + integral controller is a valve position reference (FRCROUT). The position command to the SRV is switched negative to ensure fast and positive shutoff in the event of a unit trip, or following flame out on shutdown. This negative position command, (FPKGSD) saturates the servo amplifier current in the closed direction. FRCROUT is the position reference to a servo amplifier. Refer to Figure3 SRV P2 Pressure Control Algorithm Diagram.
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Application Code Control Algorithm Speed (TNH) P2 Pressure Fault Logic (Not Tracking)
96FG-2A
96FG-2C
P2 Reference = Speed(TNH) * Gain (FPKGNG) + Offset (FPKGNO)
FSGR FAGR
(Not Tracking)
GFPRGR
Signal Space Inputs
96FG-2B
Signal Space Inputs
SRV Control Fault Logic
MED Select
FPG2
Proportional + Integral Control
True
Signal Space Output FRCROUT
FRCROUT
False
Shutdown Command
FPKGSD
Protection Permissives
L3GRV
Figure 3. SRV P2 Pressure Control Algorithm Diagram The P2 pressure setpoint is lower at low speed, in order to provide better flow control at low fuel flow. Thus, the GCV does not have to reduce full line pressure, allowing the GCV to open further, providing for better low flow control. The P2 pressure increases linearly to the 100% rated speed pressure setpoint. The pressure downstream of the GCV’s (P3) at maximum fuel flow is used to determine the rated speed pressure setpoint. The downstream pressure (P3) is a function of gas turbine compressor pressure ratio, total flow, flow split settings and fuel nozzle size. The rated speed pressure setpoint must be sufficient to maintain critical pressure drop across the GCV’s at max fuel flow. The controlled P2 pressure and the critical pressure drop design of the gas control valves ensures that the percentage valve stroke is proportional to percentage fuel flow. The control of the SRV is accomplished by using an inner and outer loop. The SRV’s position control loop is the inner control loop. The pressure control loop is the outer control loop. Three LVDT’s sense SRV stem position and their outputs are returned to each channel of the control system. The error between the position command FRCROUT and the position feedback FSGR then becomes the input to the servo amplifier. The servo amplifier drives the servo valve in the direction required to decrease the position error. A null current bias is applied to the amplified signal in order to overcome the fail safe servo spring bias. Refer to Figure 4 SRV Control Schematic.
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14
TBAI
m_amps
Press = f (ma)
Firmware
p r e s s
IO_CAL
VAIC VAIC VAIC
Valves
to Gas Control
P2 Pressure Feedback x3
A/D
96FG2A
96FG2B
96FG-2C
96FG-2B
96FG-2A
Signal Space Inputs
96FG2C
see figure 3
Application Code Control Algorithm
FRCROUT
Signal Space Output
FAGR
Signal Space Inputs FSGR
+ Σ
-
FSGR
Figure 4. SRV Speed Ratio Valve Control Schematic 96SR-3
96SR-1 96SR-2
VSR-1
p o s i t i o n
from Aux Stop
VH5-1
+ Σ +
Voted Current -FAGR
Volts Median Selected
FSGR = f (VOLTS)
IO_CAL
Firmware
VSVO VSVO VSVO
Current_Bias (Null)
Current_Gain
from Hyd Trip Oil
from Hyd Oil
LVDT Position Feedback x3
SRV Servo Valve 90SR-1
Drain
D/A
A/D
TSVO
GEK 111712 Control and Protection Articles DLN2.6+ Gas Fuel System
Control and Protection Articles DLN2.6+ Gas Fuel System
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B. Gas Flow and Split Control The gas control valves (VGC-1, VGC-2, VGC-3, VGC-4) modulate to control fuel flow and split between the four fuel circuits. The percentage of fuel to each circuit is a function of Combustion Reference Temperature, and DLN operating mode. The control system’s fuel command, FSR, is the percentage of maximum fuel flow required by the control system to maintain either speed/load, or another setpoint. The combined flow references used to position the four independent control valves is proportional to FSR. FSR is divided into two parts, which make up the fuel split setpoint, FSR1 and FSR2. FSR1 is the percentage of maximum fuel flow required from the Liquid Fuel System, and FSR2 is the percentage of maximum fuel flow required from the Gas Fuel System. FX1 is the fuelsplit command. A fuel-split command FX1 of 1.0 is equal to 100 % liquid fuel FSR1, and a fuel-split command FX1 of 0.0 is equal to 100% gas fuel FSR2. For a gas only machine FSR2 is always equal to FSR. FSR2 is again divided into four parts for the four independent control valves. FQRG1, FQRG2, FQRG3, and FQRG4 are the percentage of FSR2 to be sent to the D5 Diffusion gas fuel nozzles, the PM1 Premix center gas fuel nozzles, PM3Premix outer gas fuel nozzles, and PM2 Premix outer gas fuel nozzles. FSRG1OUT, FSRG2OUT, FSRG3OUT, and FSRG4OUT are the final output signals to the position loop regulators after the shutdown position select logic. The position commands to each valve are switched negative to ensure fast and positive shutoff in the event of a unit trip, or following flame out on shutdown. This negative position command, FSKSHUT saturates the servo amplifier current in the closed direction. FSRG1OUT is the position reference to a servo amplifier, which drives the coils of the D5 Diffusion GCV. FSRG2OUT is the position reference to a servo amplifier, which drives the coils of the PM1 Premix GCV. FSRG3OUT is the position reference to a servo amplifier, which drives the coils of the PM3 Premix GCV. FSRG4OUT is the position reference to a servo amplifier, which drives the coils of the PM2 Premix GCV. Refer to Figure 5 GCV Control Algorithm Diagram. Application Code Control Algorithm Signal Space Inputs
Combustion Ref Temp (TTRF)
DLN Mode Selection Logic
Σ
(Not Tracking)
Rate Limit
True
FSR2
FSR
FSRGnOUT
Signal Space Output FSRGnOUT
False
-
FSR
FSGn FAGn
DLN Split Schedule
+
MIN Select
GCV Control Fault Logic
FSR1
FX1 Fuel Split Command
Shutdown Command
FSKSHUT
Protection Permissives
L3GnX
Figure 5. GCV Control Algorithm Diagram
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Control and Protection Articles DLN2.6+ Gas Fuel System
Three LVDT’s sense the GCV’s stem position and their outputs are returned to each channel of the control system. The error between the position command and the position feedback then becomes the input to the servo amplifiers. The servo amplifiers drive the servo valves in the direction required to decrease the position error. A null current bias is applied to the amplified signal in order to overcome the fail safe servo spring bias. Refer to Figure 6 GCV Control Schematic.
VSVO VSVO
TSVO
VSVO Firmware
IO_CAL Signal Space Inputs FSGn
Application Code Control Algorithm
FAGn FSGn
p o s i t i o n
LVDT Position Feedback
FSGn = f (VOLTS)
A/D
x3
Volts
Median Selected
see figure 5 Signal Space Output
Voted Current - FAGn
-
+ Σ +
+ Σ
FSRGnOUT
Current_Gain
D/A
Current_Bias (Null)
VGC-n
to Gas Manifolds
from SRV
SRV Servo Valve 65GC-n
from Hyd Oil
96GC-n 96GC-m 96GC-o
VGC-n 65GC-n 96GC-m 96GC-n 96GC-o FSG-n FAG-n FSRGnOUT
GCV Insturmentation and Control Signals VGC-1 VGC-2 VGC-3 VGC-4 65GC-1 65GC-2 65GC-3 65GC-4 96GC-1 96GC-4 96GC-7 96GC-10 96GC-2 96GC-5 96GC-8 96GC-11 96GC-3 96GC-6 96GC-9 96GC-12 FSG-1 FSG-2 FSG-3 FSG-4 FAG-1 FAG-2 FAG-3 FAG-4 FSRG1OUT FSRG2OUT FSRG3OUT FSRG4OUT
Figure 6. GCV Control Schematic C. Gas Fuel Temperature Compensation Redundant gas temperature thermocouples (FTG-1A, -1B, -2A, -2B) measure supply temperature at the auxiliary stop valve inlet. The median selected value is used in the controls to compensate for varying gas supply. In order to maintain consistent fuel flow during open loop fuel control, a correction factor is applied to the control valves fuel command. This correction factor is a function of the fuel temperature deviation from nominal design temperature. All open loop fuel control setpoints are set for the nominal design temperature. The correction factor increases or decreases the fuel stroke command in order to provide the target mass flow with changing fuel temperature.
16
Control and Protection Articles DLN2.6+ Gas Fuel System
GEK 111712
D. Gas Purge Control Purge air flow is controlled by purge valves (VA13-nn) arranged in a double block and bleed configuration. The purge system is designed in an independent flow path scheme. The diffusion purge flow path is controlled by two purge valves (VA13-1 and VA13-2). The PM1 purge flow path is controlled by two valves (VA13-20 and VA13-21). The PM3 purge flow path is controlled by two valves (VA13-3 and VA13-4). The PM2 purge flow path is controlled by two valves (VA13-23 and VA13-24). Solenoid valves (20PG-nn) pneumatically actuate the purge valves open or closed. The purge valve actuation system is designed to control the rate at which fuel is purged into the combustors. A manual needle valve provides a means of adjusting the opening rate for all of the purge valves, except for VA13-2. An I/P controller is used for controlling the position of VA13-2 (gas-side Diffusion purge valve). The needle valves for the air-side purge valves are set to open the purge valves quickly, and the needle valves for the gas-side purge valves are set to open the purge valves at a slower rate. The I/P controller on VA13-2 provides additional flexibility for valve opening rates and allows for steady state positioning at mid-stroke levels. A quick exhaust valve (VA36-nn) is utilized for rapid closure of the purge valves. Refer to gas fuel purge schematic (MLI-0477) for settings. The control system energizes the solenoid valves in order to open the purge valves. When a solenoid valve is energized, pilot air is supplied to the quick exhaust valve (VA36-nn) which opens, allowing air to flow to the purge valve (VA13-nn) actuator. The purge valves open at a rate set by the metering needle valve (or VA13-2 I/P controller) in the air supply line of the VA36 valve, and allow purge air to flow to the gas manifold. A purge valve test is performed automatically by the controls at the beginning of machine startup to verify that valve open/close slew times are within specification. Refer to Startup and Shutdown Control & Protection Article for further information. The purge valves provide different functions depending on the mode of operation. While operating in purge mode, they provide the flow control to the gas manifolds. While operating in blocking mode these valves provide a double block and bleed isolation of fuel from the purge system. An inter-cavity vent valve (20VG-nn) is located between the two purge valves which provides a block and bleed system. In the event that fuel leaks past the gas purge valves in either direction and becomes too excessive for the vent valve to bleed off, pressure switches will sense the cavity pressure, and an appropriate action will be executed as described in Section V Protection. V. PROTECTION The following key describes the protective actions initiated by the control system for the gas fuel and gas fuel purge systems. Key:
A SI LO RB FRB PIT SD STP TP OFF PBR
= Alarm = Start Inhibit = Lockout Loading & DLN Mode Transfers = Normal load runback until condition clears = Fast load runback until condition clears = Pre-ignition Trip (No Trip after Warmup complete) = Unit Shutdown = Soft trip = Trip = System is turned off = Push button reset
17
GEK 111712
Control and Protection Articles DLN2.6+ Gas Fuel System
A. P2 Pressure Control Protection and SRV Position Control Protection Alarms and protective actions are initiated by the control system to protect the gas turbine and combustion hardware from a loss of fuel control. The following table illustrates these protective features. In addition to P2 pressure protection, valve position fault protection is provided in the control system. If the SRV is not tracking position commands, it will result in DLN split errors, loss of load control, and potential trips. The SRV has triple redundant position feedback transducers (LVDT’s). Table 2. Protective levels and actions for the Gas Fuel System – Gas Pressure Test
Action
SRV Not Tracking
A, TP, PBR
Pre-Ignition P2 Check
A, SI, TP, PBR
Post Ignition A, TP, PBR P2 Check
Running P2 Check
A, RB
Bottle Test
A, SI, TP, PBR
Low fuel pressure
A, SI, RM
Description Alarm if the SRV valve actual position is not following specified position reference until warm-up is complete. Trip if the SRV valve actual position is not following specified position reference until warm-up is complete. If the P2 pressure exceeds specifications for predetermined seconds before firing permissive is reached. If the P2 Pressure exceeds specifications for predetermined seconds between firing permissive and warmup complete, alarm and trip the unit. If the P2 pressure falls below specified pressure for predetermined seconds between firing permissive and warmup complete, alarm and trip the unit. If the P2 pressure falls below specified limit for predetermined seconds, alarm and run the unit back to safe mode. If the P2 pressure rises above specified pressure during test A, start inhibit. If the P2 pressure dips below specified pressure during test B, start inhibit. If the P1 pressure and P2 pressure transmitters do not read within specified differential pressure of each other, alarm the controller. Low fuel module inlet pressure (P1) or gas control valve inlet pressure (P2) results in an alarm and unload to SPPM mode for gas-only units. Dual fuel units will alarm and initiate a transfer to liquid fuel operation.
B. Fuel Temperature and Modified Wobbe Index Protection In order to protect the gas fuel system hardware, triple sensors at the gas module inlet monitor fuel temperature. High fuel temperature readings will result in the following actions: • High temperature: Alarm • High-High temperature: Shutdown performance heater • High-High-High temperature: Shutdown turbine In order to protect against combustor damage, the unit is not allowed to operate above a specified TTRF1 level (currently set at 1900 F) if fuel properties are outside of the design Modified Wobbe
18
Control and Protection Articles DLN2.6+ Gas Fuel System
GEK 111712
Index (MWI) by more than 5%. MWI is calculated based on site-specific fuel properties and fuel temperature. If the control detects MWI is outside of limits, operation will be limited to the specified TTRF1 level. If the unit is already operating above the specified TTRF1 level when the MWI fault occurs, the control system will unload the unit. C. Independent Flow Path Purge Protection Alarms and protective actions are initiated by the control system to protect the purge system and combustion hardware from loss of purge or failure to block gas fuel from entering the purge system. Failure to purge fuel from unused fuel circuits poses a risk of auto-ignition, formation of condensate in the piping or fuel nozzle back flow. A loss of fuel blocking can result in a hazardous condition where gas fuel is present in the purge system, with the potential for fuel ignition. Multiple sensors are used in the purge fault detection strategy, including open and closed limit switches on each valve, as well as purge cavity pressure switches. A low pressure while purging indicates a loss of purge fault and a high pressure while blocking indicates a loss of blocking fault. The controls utilizes all of these sensors in the protective strategy in order to provide single fault tolerant protection. The following table illustrates these protective actions. Refer to Table 7 Independent Flow Path Purge Protection for more information.
19
GEK 111712
Control and Protection Articles DLN2.6+ Gas Fuel System
Table 3. Independent Flow Path Purge Protection EVENT Loss of Purge (LOP)
Low D5 Purge Air Temperature
FAULT
SETPOINT
• Single limit switch indicating out of position or a single pressure switch indicating low • Both limit switches out of position on a single valve (double fault) OR • Voted low pressure OR • Combination of two: valve limit switches or limit switch & pressure fault
< 50 psig
• D5 temperature sensors (median-select) indicate low temperature with D5 purge commanded on. • D5 temperature sensors (median-select) indicate low temperature with D5 purge commanded on.
< 270 deg. F for 10 minutes.
ACTION Alarm
< 50 psig
< 240 deg. F for 15 minutes
• Alarm. • Lockout Purge to faulted circuit. • D5 Purge Fault: Lockout PM and SPM modes (immediate transfer to Ext.PPM or PPM) • PM1, PM3 or PM2 Fault: Lockout mode transfers to DLN modes that will add fuel to faulted circuit if the fault persists for 15 minutes. Alarm • Alarm. • Lockout D5 purge. • Lockout PM and and SPM modes (immediate transfer to Ext.PPM or PPM)
> 50 psig • Single limit switch Alarm, Pre-Ignition Trip. indicating out of position or a single pressure switch indicating high > 50 psig Shutdown • Limit switch double fault on a single valve OR • Combination of two: valve limit switches or limit switch & pressure fault > 50 psig Trip • Voted high pressure OR • Both limit switches on a single valve out of position plus any other single valve limit switch or pressure fault OR • Three single faults in a row (VA13-1 limit switch fault; pressure switch fault; VA13-2 limit switch fault) < 0.94 Ratio Alarm • Low purge pressure sensed Low purge at manifold pressure ratio Consult the Control Specification for detailed adjustments and settings of the gas fuel and gas fuel purge systems. Loss of Blocking (LOB)
Consult Device Summary for detailed device settings and calibration.
20
To Vent
NO
NC
NO
1
2 3
96FG -1E
1
2 3
96FG -1F
P1 - Inlet Pressure to VSR-1
96FGD -1
3
Rev 3 BMG December 17, 2004
Gas Fuel Supply
FG1
YStrainer
FTG2A,2B
FTG1A,1B
NO
96FG -1D
1
2
NC
Trip oil
Purge Air
33VS4-1,2
VS4-1
20VS4-1
Instrument Air PC
Bleed to Vent Line
3
96FG -2A
ML0477
VSR-1
96SR-1,2,3
NC
VH5-1
90SR-1
Hyd Oil
3 1
2 3
96FG -2C
P2 - Intermediate Pressure VSR-1 to VGC-1,2,3,4
1
2
96FG -2B
For Reference Only - See MLI 0422 for Customer Specific Schematic
Bleed to Vent Line
Top Level Summary
1
2 NO
VA13-15
Vent
VGC-1
Bleed to Vent Line
96GC-10,11,12
VGC-4
VH5-2
65GC-1
Hydraulic Oil
Trip oil
VH5-5
65GC-4
Pitch
Trip oil
Hydraulic Oil
VH5-4
65GC-3
Hydraulic Oil
VH5-3
65GC-2
Pitch
Pitch
Trip oil
Hydraulic Oil
Pitch
Trip oil
Bleed to Vent Line
96GC-7,8,9
VGC-3
Bleed to Vent Line
96GC-4,5,6
VGC-2
Bleed to Vent Line
96GC-1,2,3
NO
NO
NO
NO
NO
P3 - Discharge Pressure from VGC-1,2,3,4
dP
Temporary Strainer
dP
Temporary Strainer
dP
Temporary Strainer
dP
Temporary Strainer
D5
Pigtails #1
Low Point Drain
PM 2
Pigtails #1
Low Point Drain
PM 3
Pigtails #1
Low Point Drain
PM1
Pigtails #1
Low Point Drain
P4 - Inlet Pressure to Endcover
CPD
96GN-4
Typ for 18 comb cans
CPD
96GN-3
Typ for 18 comb cans
CPD
96GN-2
Typ for 18 comb cans
Inner Outer
CPD
96GN-1
Typ for 18 comb cans
GAS TURBINE COMPARTMENT (MLO962)
Power Generation Engineering
GAS FUEL MODULE (MLA0160)
Instrument Air PC
20VG-1
g 9FA DLN2.6+ Advanced Combustor Gas Fuel System
Control and Protection Articles DLN2.6+ Gas Fuel System GEK 111712
APPENDIX FIGURE A1 – SIMPLIFIED GAS FUEL SYSTEM SCHEMATIC
21
22
RMD 07 April, 2004
From Purge Air (CDC)
From Instr. Air (Plant)
EX
33PG-1,2
FC
IA for Actuators (Not Shown, see above)
IA for Actuators (Not Shown, see above)
NC
20PG-3 VA36-3 VA13-3 Needle Valve
FC
20PG-23 VA36-23 VA13-23 Needle Valve
33PG-5,6
FC
NC
VA13-8
NC
NO
FO
NC
NO NC
NO NC
PC
Needle Valve
NC
NO NC
NO NC
PC
63PG-4A 63PG-4B 63PG-4C
NO
20VG-5 VA13-14
To Vent
FC
FC
33PG-21,22
FC
20PG-24 VA36-24 VA13-24 Needle Valve
33PG-7,8
FC
20PG-4 VA36-4 VA13-4 Needle Valve
33PG-17,18
VA13-21
EX
NC
VGC-4
NC
VGC-3
NC
VGC-2
33PG-3, 4, 15, 16(spare)
VA13-2
VA36-2
20PG-2
65EP-G1P
I/P
EX
VA36-21
VTA
20PG-21
63PG-2A 63PG-2B 63PG-2C
NO
20VG-3 VA13-12
VTA
VPR54-22
To Vent
VA13-13
NC
PC
To Vent
63PG-3B 63PG-3C
NO
20VG-4
63PG-3A
NO
NC
NO
FO
PC
To Vent
63PG-1B 63PG-1C
VTA
33PG-19,20
33PG-9,10
FC
VA13-20
VA36-20
NC
NO
20VG-2
VTA
For Reference Only - See MLI 0477 & 0417 for Customer Specific Schematics
63PG-1A
NO
VA13-1
VA36-1
20PG-1
20PG-20
Needle Valve
Needle Valve
EX
VTA
VTA
Top Level Summary
NC
VGC-1
NO
NO
NO
NO
GAS FUEL MODULE (MLA0160)
g9FA DLN2.6+ Advanced Combustor Gas Fuel Purge System
dP
Temporary Strainer
dP
Temporary Strainer
dP
Temporary Strainer
dP
Temporary Strainer
D5
Low Point Drain
PM2
Low Point Drain
PM3
Low Point Drain
PM1
Low Point Drain
TE TE TE PT- PT- PTG1-1A G1-2A G1-3A TE TE TE PT- PT- PTG1-IB G1-2B G1-3B
GAS TURBINE COMPARTMENT (MLO962)
96GN-4
CPD
96GN-3
CPD
96GN-2
CPD
96GN-1
CPD
Power Generation Engineering
GEK 111712 Control and Protection Articles DLN2.6+ Gas Fuel System
APPENDIX FIGURE A2 – SIMPLIFIED GAS FUEL PURGE SYSTEM SCHEMATIC
Control and Protection Articles DLN2.6+ Gas Fuel System
GEK 111712
THIS PAGE INTENTIONALLY LEFT BLANK.
23
GEK 111712
Control and Protection Articles DLN2.6+ Gas Fuel System
g
GE Energy General Electric Company www.gepower.com
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