gas turbine Gas Fuel System

GEK 116281 Novem ber 2008 2008

g

GE Energy

Gas Fuel System 9FA DLN 2.6+

Th ese ese in structi ons do do not purpor t to cover cover all detail s or vari ations in equi pment nor to provi de for every every possible possible contingency to be met met in connection connection wi th i nstall nstall ation, operation operation or mai ntenance. ntenance. Should fur ther in formation be de desir ed or should parti cular probl ems ari se whi ch are not covered covered suf suf fi ciently f or th e purchaser' purchaser' s pur poses poses the matter shoul shoul d be referr ed to the GE Company. © General Electric Company, 2008. GE Proprietary Information. All Rights Reserved.

GEK 116281 116281

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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© General Electric Company, 2008. GE Proprietary Information. All Rights Reserved.

Gas Fuel System

GEK 116281

TABLE OF CONTENTS I. II.

III.

IV.

V.

GENERAL .................................................................................................................................................. 5 EQUIPMENT ............................................................................................................................................. 5 A. Gas Flow Measurement (Coriolis Meter-96FM-4/MG2-4) ................................................................... 5 B. Safety Shut off System........................................................................................................................... 5 C. Gas Strainer............................................................................................................................................ 5 D. Gas Fuel Auxiliary Stop Valve (VS4-1) ................................................................................................ 6 E. Stop/Speed Ratio Valve (VSR-1) .......................................................................................................... 6 F. Gas Fuel Control Valves (VGC-1, VGC-2, VGC-3, VGC-4) ............................................................... 7 G. Gas Fuel Vent Valve (VA13-15) ........................................................................................................... 7 H. Thermocouples (FTG - 1A, 1B & FTG-2A, 2B) ................................................................................... 7 I. Pressure Gauges ..................................................................................................................................... 8 J. Pressure Transducers (96FG-1A, 1B, 1C) ............................................................................................. 8 K. Calibration of Fuel Gas Pressure Transducers, 96FG-2A, 2B, 2C ........................................................ 8 L. Gas Purge Pressure Ratio Monitoring (96GN-1, 96GN-2, 96GN-3, 96GN-4).............. ........................ 8 OPERATION.............................................................................................................................................. 8 A. General................................................................................................................................................... 8 B. Pre-start Conditions ............................................................................................................................. 11 C. Startup & Loading Operation.................... ........................................................................................... 11 CONTROL................................................................................................................................................ 14 A. P2 Pressure Control...................... ........................................................................................................ 14 B. Gas Flow and Split Control...................... ............................................................................................ 17 C. Gas Fuel Temperature Compensation.................................................................................................. 19 D. Gas Purge Control................................................................................................................................ 19 PROTECTION ......................................................................................................................................... 20 A. P2 Pressure Control Protection and SRV Position Control Protection ................................................ 20 B. Independent Flow Path Purge Protection............................................................................................. 21

LIST OF FIGURES Figure 1. DLN2.6+ Fuel Nozzle Arrangement........................ ............................................................................... 9 Figure 2. Startup and Shutdown Mode Sequence ................................................................................................ 10 Figure 3. Gas Fuel Leak Test Function Block Diagram....................................................................................... 13 Figure 4. SRV P2 Pressure Control Algorithm Diagram ..................................................................................... 15 Figure 5. SRV Stop/Speed Ratio Valve Control Schematic..................... ............................................................ 16 Figure 6. GCV Control Algorithm Diagram ........................................................................................................ 17 Figure 7. GCV Control Schematic ....................................................................................................................... 18

LIST OF TABLES Table 1. DLN Mode Staging Diagram ................................................................................................................. 10 Table 2. Protective levels and actions for the Gas Fuel System – Gas Pressure .................................................. 20 Table 3. Independent Flow Path Purge Protection ............................................................................................... 21


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Gas Fuel System

APPENDIX Figure A1. – Simplified Gas Fuel System Schematic .......................................................................................... 22 Figure A2. – Simplified Gas Fuel Purge System Schematic................................................................................ 23

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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 DLN 2.6+. It also provides safe and reliable isolation from the gas fuel source. The DLN 2.6+e 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. II. EQUIPMENT

The Gas Fuel System consists of a gas flow meter, fuel gas compartment of the accessory module/fuel gas module and the gas turbine on-base equipment. The fuel gas compartment of the accessory module/ fuel gas module houses both the gas fuel system and gas fuel purge system. A brief description of the system’s overall major components is given below. See Gas Fuel System Articles for further details. A. Gas Flow Measurement (Coriolis Meter-96FM-4/MG2-4)

Coriolis flow meters measure mass flow by taking advantage of the Coriolis Effect; the inertial effects that arise as a fluid flows through a tube are directly proportional to the mass flow of the fluid. In a Coriolis flow meter, vibration is induced in the process-fluid-filled flow tube(s), then the mass flow rate is captured by measuring the difference in the phase of vibration between one end of the flow tube and the other. B. Safety Shut off System

The Safety Shut off System is comprised of two separate valves, a Stop Valve (VS4-4) and a Vent Valve (VA13-18). These valves are located outdoors, upstream of the Fuel Gas Compartment/Module (FG1). They stop and vent (to atmosphere) the fuel gas to the GT in the event of Fire Detection, Hazardous Gas Detection, and E Stop activation. Each valve is pneumatically actuated via a 3-way solenoid valve, 20VS4-4 Stop Valve and 20VG-8 Vent valve. Each valve has limit switches to indicate open and closed position; 33VS4-4 / 33VS4-5 Stop Valve and 33VG-8 / 33VG-9 Vent Valve. C. Gas Strainer

A strainer is provided in the fuel gas supply line upstream of the stop/speed ratio valve. The gas strainer is a last chance strainer. The purpose of the strainer is to stop foreign objects or materials that may prevent the gas turbine from operating/shutting down safely. The strainer is not designed for continual fuel gas clean up. The strainer utilizes a differential pressure transducer 96FGD-1 or switch 63FGD-1 along with a differential pressure gauge installed across the strainer to monitor blockage. The pressure transducer or switch initiates an alarm to the turbine control panel if the gas differential pressure increases beyond a specified setting. During operation, the strainer differential pressure should be closely monitored. If an alarm is registered, the strainer should be inspected and if required cleaned. All strainer contamination shall be considered abnormal and the source of contamination shall be verified. All strainer maintenance shall be performed in accordance with the manufacturer’s recommendations. Each strainer is equipped with a lockable vent valve that vents to FG2, see MLI 0422 for additional information on location of strainer vent valve and instructions for venting the fuel gas to a safe area. Prior to strainer maintenance, the upstream and downstream pressure shall be zero. To determine the downstream pressure, utilize the pressure gauge FG-1 and pressure transducers 96FG-1A, -1B & -1C. The upstream pressure can be verified by using the differential pressure gauge FG-3 and differential pressure transmitter 96FGD-1 or switch 63FGD-1. When using the differential pressure © General Electric Company, 2008. GE Proprietary Information. All Rights Reserved.

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gauge/transmitter/switch to determine that the upstream pressure is zero, the downstream pressure shall be zero; the sensing lines to the differential devices shall be open; and the pressure equalizer valve shall be closed. The fuel gas strainer may be one of the following types Y Strainer, Duplex Strainer, or Multi Element Strainer. All three designs utilize a metal mesh filtration element. 1. Y Type Strainer The Y Type Strainer is basket type strainer; it is designed as a single unit with one strainer basket. 2. Duplex Strainer The duplex strainer is a basket type strainer; it is designed as a single unit with two separate baskets and a flow transfer valve. The transfer valve is designed to allow fuel gas to flow through only one basket at a time. If a high differential pressure is observed, the transfer valve allows for the transfer to a clean basket. 3. Multi-Element Strainer The Multi-Element Strainer is designed as a single unit with multiple elements arranged in parallel to the fuel gas flow path, each element is fed with fuel gas simultaneously. In order to perform maintenance on the Multi-Element strainer, the unit must be removed from the fuel gas piping. Therefore it may be necessary to place temporary pipe supports in the fuel gas pipe spools upstream and downstream of the unit. D. Gas Fuel Auxiliary Stop Valve (VS4-1)

The Auxiliary Stop Valve is an ANSI Class VI butterfly type control valve. 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 actuator to atmosphere. The compressed spring causes the valve to close. This valve acts as a backup stop valve for the Stop/Speed Ratio Valve. This valve opens during the gas turbine ignition sequence when the turbine is started. E. Stop/Speed Ratio Valve (VSR-1)

The Stop/Speed Ratio Valve 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 system’s SRV position control loop. The SRV also functions as a pressure-regulating valve. The control system uses the SRV to regulate the pressure (P2) upstream of the GCV’s. This function is described in further detail in Section IV Control.

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F. Gas Fuel Control Valves (VGC-1, VGC-2, VGC-3, VGC-4)

There are four independent gas control valves in the DLN2.6+ system. The gas control valves (GCV’s) are 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 ratios. 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 passages. This fuel split is a function of DLN operating mode, and the combustion reference temperature. Gas Purge Block Valves (VA13-1, VA13-2, VA13-3, VA13-4, VA13-20, VA13-21, VA13-23, VA13-24). 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, VA36-3, VA36-4, VA36-20, VA36-21, 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-3, 20PG-4, 20PG-20, 20PG-21, 20PG-23, 20PG-24) for actuation. A needle-metering valve is provided on the inlet pressure side of the pilot valves (20PG) to control the opening rate of each VA13 valve, except for VA13-2. This valve VA13-2, is equipped with an I/P (Current to Pneumatic) 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, protection and diagnostics in the Gas Turbine control system. G. Gas Fuel Vent Valve (VA13-15)

This solenoid operated valve vents the volume between the stop/speed ratio valve and the gas control valves when the solenoid (20VG-1) 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 valve 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 valve and gas control valves, and that no fuel gas will leak past the closed gas control valves to collect in the combustors or exhaust. If the vent valve fails during normal operation, the SRV will continue to maintain constant pressure, P2. This is accomplished by opening further, making up any lost flow through the vent valve. H. Thermocouples (FTG - 1A, 1B & FTG-2A, 2B)

Upstream of Stop/Speed Ratio Valve (SRV) are two Dual element thermocouples FTG-1A/1B, FTG2A/2B which sense the temperature and send an electrical signal back to the controller.


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GEK 116281

I.

Gas Fuel System

Pressure Gauges

One differential pressure gauge (PDI-FG-3) measures the pressure differential between the inlet and outlet of the gas strainer. Three pressure gauges with hand valves are installed in the fuel gas supply line. The upstream pressure gauge (PI-FG-1) measures the pressure of the gas entering the stop/speed ratio valve; the intermediate pressure gauge (PI-FG-2) measures P2 pressure ahead of the gas control valves; and the downstream gauges (PI-D5, PI-PM1, PI-PM2, PI-PM3) measure the pressure of the gas leaving the gas control valves. J. Pressure Transducers (96FG-1A, 1B, 1C)

Three pressure transducers are installed in the fuel gas piping upstream of the Stop/Speed ratio valve and are used to initiate an alarm when the fuel pressure becomes too high or low. K. Calibration of Fuel Gas Pressure Transducers, 96FG-2A, 2B, 2C

The fuel gas pressure transducer, 96FG, is a pressure transducer with a dc voltage output directly proportional to pressure input in psig. It incorporates solid state circuits and an amplifier in the transducer case. A diode is connected across the output of the transducer. This prevents any possibility of a spurious signal driving the transducer amplifier negative, out of its normal operating range. The transducer is normally factory adjusted and calibrated; however, the calibration must be verified in the field and if necessary returned to the orginal manufacture for recalibration. L. Gas Purge Pressure Ratio Monitoring (96GN-1, 96GN-2, 96GN-3, 96GN-4)

When a gas passage 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 gas fuel Auxiliary Stop Valve, Stop/Speed Ratio Valve, D5 (Diffusion), PM1, PM2, and PM3 Gas Control Valves. The Stop/Speed Ratio Valve (SRV) and the Gas Control Valves (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 GCV position command is made linear by maintaining a predetermined pressure upstream of the GCV’s. 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, -2B, -2C). The 2.6+ Dry Low Nox Combustion System has four fuel passage manifolds (D5, PM1, PM2, & PM3). All four of these passages (D5, PM1, PM2, and PM3) are independent fuel passages each having an individual gas control valve (GCV) for controlling gas fuel delivery. Each combustion chamber has six DLN2.6+ fuel nozzles arranged in a circular configuration with one in the middle. The D5 gas fuel delivery system consists of five diffusion type fuel nozzles for each combustion chamber. The PM2 and PM3 gas fuel delivery systems consist of five fuel nozzles for each combustion chamber. The PM1 gas fuel delivery system consists of one fuel nozzle for each combustion chamber. See figure 1 for the DLN2.6+ Nozzle Arrangement.

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Each fuel passage requires a certain percentage of the total fuel. The percentage of fuel to each passage is a function of Combustion Reference Temperature (TTRF), and DLN operating mode. The start up mode of operation is D or Diffusion. There are four steady state DLN modes of operation: 1D, 3, 6.2, & 6.3. There is also a transient load rejection mode (mode 1) where all the fuel is diverted to the premix nozzles PM1 and PM2. The steady state DLN operating modes are a function of Combustion Reference Temperature. See Table 1 DLN Mode Staging Diagram. See Figure 2 Startup and Shutdown Sequences.

Figure 1. DLN2.6+ Fuel Nozzle Arrangement

This method of staging the fuel requires individual control valves (GCV-1, GCV-2, GCV-3, and GCV-4) for each fuel passage. In order to simplify the fuel flow and split control, 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 provides a linear relationship between control valve stroke and gas flow. During certain DLN modes of operation, some passages will have no fuel flow scheduled. A means of purging these stagnant passages is required to prevent condensate from accumulating, and to minimize the potential for auto-ignition. The gas fuel that remains in a passage 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. DLN Mode Staging Diagram MODE

PASSAGES FUELED

PASSAGES PURGED

D

D5

PM1 + PM2 +PM3

1D

D5 + PM1

PM2 + PM3

3

PM1 + PM2

D5 + PM3

6.2

PM1 + PM2 + PM3

D5

6.3

PM1 + PM2 + PM3

D5

3D

D5 + PM1 + PM2

PM3

6D

D5 + PM1 + PM2 + PM3

NONE

1

PM1 + PM2

D5 + PM3

Figure 2. Startup and Shutdown Mode Sequence

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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 fuel supply pressure at FG1 within limits. 2. Gas fuel module/compartment ventilation pressure normal. 3. SRV and GCV 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. 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, several systems must be operating: ●

Gas fuel compressors, when pipe line supply pressure is insufficient

●

Gas fuel electric startup heater

●

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 while the vent valve is open, just prior to light-off. If the P2 pressure (FPG2) exceeds a specified pressure prior to firing permissive (L2TVZ), a pre-ignition trip or Start Inhibit (L4PRETX) and alarm will occur. C. Startup & 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 close with the resultant time to complete each operation for each valve being measured. An alarm signal generates 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.


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Gas Fuel Leak Test

The Gas Leak Test will test the SRV, GCV’s and Aux stop valve (If provided) 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. See Figure 3 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 Gas fuel Vent valve will be commanded shut (VA13-15), and the Gas fuel 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 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 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 the 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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Figure 3. Gas Fuel Leak Test Function Block Diagram


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Valve Test

Condition

SRV

Gas Fuel System

Aux SRV Vent Vlv GCV’s Stop Vlv Open Closed Closed Closed

Pass Criteria

Test Time

Unit Start and P2 < 100 PSIG 30 sec Normal Shutdown * FPG2 Unit Start and Open Open Closed Closed ABS (P1-P2) < 20 1 sec x-mitters Normal Shutdown * PSI GCV Unit Start and Closed Closed Closed Closed P2 > P1 – 150 PSI 30 sec Normal Shutdown * Venting Unit Start and Closed Closed Open Closed P2 < 6 psi After Normal Shutdown * Vent timer

Action

Startup Lockout Alarm 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 pre-determined pressure window. The desired unit P2 pressure during warm-up is calculated to achieve the unit pressure window. If the pressure is lower or higher then a specified tolerance for a given time frame, the unit will alarm and trip. IV.

CONTROL A. P2 Pressure Control

The SRV (VSR-1) modulates to control gas supply pressure (P2), to the independent Gas control valves. 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 set-point (FPGR) is a function of Gas Turbine 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 close direction. FRCROUT is the position reference to a servo amplifier. See Figure 4 SRV P2 Pressure Control Algorithm Diagram.

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Figure 4. SRV P2 Pressure Control Algorithm Diagram

The P2 pressure set-point is lower at low speed, in order to provide better flow control at low fuel flow. Thus, the gas control valve (VGC) 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 set-point. The pressure downstream of the gas control valves (P3), at maximum fuel flow is used to determine the rated speed P2 pressure set-point. The downstream pressure (P3) is a function of gas turbine compressor pressure ratio and fuel nozzle size. The rated speed pressure set point must be sufficient to maintain critical pressure drop across the control valves at max flow. The controlled P2 pressure and the critical pressure drop design of the gas control valves ensure 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. The Three nos. of LVDTs sense SRV position and their outputs are returned to Gas Turbine control system. The error between the position command FRCROUT and the position feedback 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. See Figure 5 SRV Control Schematic


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c i t a m e h c S l o r t n o C e v l a V o i t a R d e e p S / p o t S V R S . 5 e r u g i F

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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 passages. The percentage of fuel to each passage 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 set-point. 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 set-point, 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 fuel split 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, PM3 Premix outer gas fuel nozzles, and PM2 Premix 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 close 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. See Figure 6 GCV Control Algorithm Diagram.

Figure 6. GCV Control Algorithm Diagram © General Electric Company, 2008. GE Proprietary Information. All Rights Reserved.

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Three LVDTs 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. See Figure 7 GCV Control Schematic.

VGC-n 65GC-n 96GC-m 96GC-n 96GC-o FSG-n FAG-n FSRGnOUT

GCV Instrumentation and Control Signals VGC-1 VGC-2 VGC-3 65GC-1 65GC-2 65GC-3 96GC-1 96GC-4 96GC-7 96GC-2 96GC-5 96GC-8 96GC-3 96GC-6 96GC-9 FSG-1 FSG-2 FSG-3 FAG-1 FAG-2 FAG-3 FSRG1OUT FSRG2OUT FSRG3OUT

VGC-4 65GC-4 96GC-10 96GC-11 96GC-12 FSG-4 FAG-4 FSRG4OUT

Figure 7. GCV Control Schematic

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C. Gas Fuel Temperature Compensation

Triple redundant gas temperature thermocouples (FTG-1A, -1B, -2A) 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 set points 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. 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 operates independently from the rest of the purge system, 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. This control method does not allow for intermediate positioning of the purge valves. A manual needle valve (I/P controller for VA13-2 only) provides a means of adjusting the opening rate, and the quick exhaust valve (VA36-nn) controls the closing rate of the purge valves. Refer to gas fuel purge schematic (MLI-0477) for details. 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 in the air supply line of the 20PG 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 admit purge air to the fuel passage. While operating in blocking mode, these valves provide a double block and bleed isolation of fuel from the purge air 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 become 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.


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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 Warm-up complete) = Unit Shutdown = Soft trip = Trip = System is turned off = Push button reset

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 (LVDTs). Table 2. Protective levels and actions for the Gas Fuel System – Gas Pressure Test

Action

Description

SRV Not Tracking

A, TP, PBR

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.

Pre-Ignition P2 Check

A, SI, TP, PBR

If the P2 pressure exceeds specifications for predetermined seconds before firing permissive is reached, alarm and inhibit start.

Post Ignition P2 Check

A, TP, PBR

If the P2 Pressure exceeds specifications for predetermined seconds between firing permissive and warm-up complete, alarm and trip the unit. If the P2 pressure falls below specified pressure for predetermined seconds between firing permissive and warm-up complete, alarm and trip the unit.

Running P2 Check

A, RB

If the P2 pressure falls below specified limit for predetermined seconds, alarm and run the unit back to safe mode.

Bottle Test

A, SI, TP, PBR

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.

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GEK 116281

B. 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, failure to purge idle gas from fuel passages, or failure to block gas fuel from entering the purge system. Failure to purge fuel from unused passages poses a risk of auto-ignition. A loss of purge will result in the formation of condensate in manifolds and piping. 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. There are 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 utilize all of these sensors in the protective strategy in order to provide single fault tolerant protection. The following table illustrates these protective actions. Table 3. Independent Flow Path Purge Protection EVE NT

Loss of Purge (LOP)

CONDITION

A

●

B

●

● ●

Loss of Blocking (LOB)

A

●

B

●

●

C

● ●

●

Low purge pressure ratio

A

●

FAULT

SET-POINT

ACTION

Single limit switch indicating out of position or a single pressure switch indicating low

< 50 psig

A

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

A, LO, FRB, STP

Single limit switch indicating out of position or a single pressure switch indicating high

> 50 psig

A, PIT (valve sensors ONLY)

Limit switch double fault on a single valve OR Combination of two: valve limit switches or limit switch & pressure fault

> 50 psig

SD

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)

> 50 psig

TP

< 0.95 Ratio

A

Low purge pressure sensed at D5 manifold

Consult the Control Specification for detailed adjustments and settings of the gas fuel and gas fuel purge systems. Consult the Device Summary for detailed device settings and calibration. © General Electric Company, 2008. GE Proprietary Information. All Rights Reserved.

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APPENDIX

c i t a m e h c S m e t s y S l e u F s a G d e i f i l p m i S – . 1 A e r u g i F

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GEK 116281

c i t a m e h c S m e t s y S e g r u P l e u F s a G d e i f i l p m i S – . 2 A e r u g i F


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g

GE Energ y General Electric Company www.gepower.com

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gas turbine Gas Fuel System

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