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Gas turbine control and protection
Gas turbine control and protection Introduction - total train control and protection objectives Start-up and shutdown sequencing Gas turbine control Protection systems
Introduction - total train control and protection objectives At this point, we have covered the design and operation of a gas turbine. We will now discuss the gas turbine driven total train control and protection. The total train control and protection objectives for a gas turbine are identical to those of a steam turbine. However, since a gas turbine operates at much higher temperatures than a steam turbine, an additional control function is required. This function is known as gas turbine sequencing. A sequencer automatically operates the gas turbine from a permissive to start up to the normal operating speed. In doing so, the sequencer will assure that all support functions, fiiel purge functions, combustor firing functions and acceleration to normal speed is achieved in a safe and reliable manner. In addition, the sequence system also controls the deceleration of the gas turbine and operation of support systems during a normal shutdown. As mentioned above, the basic control function is similar to a steam turbine. This is true however, since the fuel for a gas turbine is a combination of air/fuel mixture and since there are alternatives (liquid, gas or dual fuel) we will focus our attention on the various fuel system configurations and functions in this section. Finally, the protection system will be discussed in detail with emphasis
439
Principles of Rotating Equipment
placed upon additional engine protection other than the over speed bolt which is similar to steam turbine operation. Namely, hot gas path temperature override function will be discussed as well as gas turbine air compressor protection. In addition we will also present a gas turbine control system brief history, which will detail the various types of control systems that are installed in the field. Gas turbine control and protection objectives are similar to that of a steam turbine. However, due to the significantly higher operating temperature, certain additional features must be incorporated. Figure 30.1 presents these features. Gas turbine control and protection additional features In terms of control and protection, a gas turbine system is the same as a steam turbine system. Due to a gas turbine's characteristics, the following features must be added: • Automated start and stop (sequencing) • Hot gas path excessive termperature governor override • Air compressor operating point override
Figure 30.1 Gas turbine control and protection additional features
The design objective of the sequencing and temperature override systems are shown in Figure 30.2. Gas turbine sequencing and temperature override Regardless of the type of gas turbine, start-up and shutdown must be sequenced (automated) to: • Assure all auxiliaries are operative • Gas path is urged • Differential thermal effects are minimized • Air compressor operates in the stable range In addition, the hot gas path temperature must be limited by continuously inputing the hot gas path average temperature to the governor
Figure 30.2 Gas turbine sequencing and temperature override
440
Gas Turbine Control it Protection
In addition, it is necessary to assure that the compressor operating point is optimized (as high an operating efficiency as possible). Figure 30.3 shows one such method - adjustable compressor stator valve control. Air compressor operating point control
voLOKE rum
Since air compressor inlet pressure and gas m.w. are constant, compressor discharge pressure (C.D.P.) is usually selected as the key parameter for determining the air compressor operating point.
Figure 30.3 Air compressor operating point control (Courtesy of Elliott Co.)
Gas turbine control system effectiveness and reliability has made great advances since the early years of gas turbine design. Since gas turbines and their control systems are production items, custom design control features have been minimized. Essentially, there are four (4) generations of control system design beginning in the 1950s. Figure 30.4 outlines a history of these systems.
441
Principles of Rotating Equipment
Gas turbine control system history Years
Sequencing
Control
Protection
1950-1970
relays
mechanical/hydraulic solid state components (analog type)
relays
1970-1980
solid state
integrated circuits (analog type)
solid state and relays
1980-1990
microprocessor redundancy
micro-processor (digital type)
microprocessor
1990-
microprocessor
micro-processor redundancy fault tolerance self diagnostics
independent microprocessor dual voting
Figure 30.4 Gas turbine control system history
Figure 5 presents an overiew of gas turbine control system input, set point and output functions. We will cover each of the following sequencing, control and protection functions in detail.
OUTPUTS
INPUTS ROTOR SPEED(S) FUEL FLOW/ VALVE PROTECTION HOT GAS PATHTEMP(S) COMP.DISCH. PRESSURE SUPPORT SYSTEM • SIGNALS
AUX.SYS. START
FUEL CONTROL
STARTER CRANK
COMBUSTION AIR
PURGE CYCLE
CONTROL
RAMPED ACCEL
HOT GAS PATH OVERRIDE
NOMINAL SPEED
EMISSIONS CONTROL
NORMAL SHUT DOWN RAMP
CONDmON MONITORING
COOL DOWN STOP
PROTECTION
SET POINTS (PROCESS VARIABLE(S) OR SPEED)
Figure 30.5 Gas turbine control system functions overview
442
' TO FUEL SYSTEM ^-^
TRIP ACTION TRIP ACTION
MANUAL TRIP
INTERNAL SIGNALS (LUBE, CONTROL)
TO ENGINE CONTROL SERVOS
Gas Turbine Control a Protection
Start-up and shutdown sequencing The objectives of the gas turbine start and stop sequencing system are noted in Figure 30.6. Gas turbine start-stop sequencing system Sequenced (automated) start-up and shutdown are required to: • Prevent engine fires or explosions • Minimize thermal effects • Eliminate damage caused by vibration when passing through natural frequencies (critical speeds) • Eliminate damage caused by air compressor stall (surge) Figure 30.6 Gas turbine start-stop sequencing system
A generic gas turbine starting sequence outline and a parameter travel are presented in Figures 30.7 and 30.8. Gas turbine starting sequence^ Step 1. 2. 3. 4. 5. 6. 7. 8.
9.
10.
description enable start start acknowledged support systems started2 starter engagement/crank speed attained engine purge cycle igniter activation, fuel system activation lite ofP warm up idle, starter disengagement ramped accelertion to normal speed (considers thermal effects, critical speeds) warm up normal speed (no load) guide vanes, nozzles, etc. in auto position load engine
default parameter all permissives clear any permissive default all support system parameters clear starter operating/crank speed cycle timer set point flame detectors combustion temp or speed change rate warm up timer set point/starter disengagement vibration, exhaust temp, etc.
all trip signals
all trip signals
1. Generic procedure to demonstrate typical sequencer steps. 2. Some applications will have to have manual support system start. 3. All engine guide vanes, variable nozzles set at no load position. Figure 30.7 Gas turbine starting sequence
443
1
Principles of Rotating Equipment Percent 100 F
Approximate Time - l\^inutes Figure 30.8 Typical gas turbine start characteristics - start-up unloaded (Courtesy of General Electric Company)
Facts concerning start sequences for industrial and aero-derivative type gas turbines are presented in Figure 30.9.
Start sequence facts The start-up sequence times vary with gas turbine type and manufacturer typical values • Heavy duty - 10-20 minutes • Aeroderivative - 2-5 minutes Normal shutdowns are sequenced to allow uniform cool down of components Cool down is accomplished by low speed operation
Figure 30.9 Start sequence facts
444
Gas Turbine Control £t Protection
Gas turbine control Like steam turbines, the speed control (governor system) is the heart of the control system. It performs the identical function of'cruise control' in your vehicle. Refer to Figure 30.10 for gas turbine control system objectives. Gas turbine control objectives Meet load requirements Maintain optimum heat rate (firing temperature and efficiency)
Figure 30.10 Gas turbine control objectives
Input parameters are noted in Figure 30.11. Gas turbine control system inputs In order to meet control objectives, inputs to control system are: • •
Power turbine rotor speed Air compressor • Rotor(s) speed! • Compressor discharge pressure (CDP) • Hot gas path temperatures
^if multi-shaft turbine
Figure 30.11 Gas turbine control system inputs
A typical control schematic for a gas turbine drive is shown in Figure 30.12.
445
Principles of Rotating Equipment PRCXJESSGAS
£
-WWWV-gh—^=^—--I PROCESS GAS
Ky •AIR
^y NPT
NGG
Ij^" i
FUEL
11. SET POINT
OUTPUT H
CONTROLLER
OUTPUT
Figure 30.12 A gas turbine driven turbo-compressor (Courtesy of M E Crane Consultant)
Control system output for this system is discussed in Figure 30.13. Gas turbine control system outputs In order to control gas turbine output power, • Vapor energy/mass - AH • Mass rate - M Must be controlled by: • Fuel rate control (AH) • Airflow rate (if furnished) (M) • Gas generator speed • Adjustable stator vanes • Power turbine variable nozzles (if furnished) (M)
Figure 30.13 Gas turbine control system outputs
As discussed in the previous chapter, there are many available fuel options. Figure 30.14 presents fuel options and facts concerning fuel specific energy and specifications.
446
Gas Turbine Control ft Protection Gas turbine fuel systems facts •
Available fuel options: • Gas (most frequently used) • Liquid • Dual (gas or liquid)
•
The lower the fuel heating value, btu/scf the greater required fuel system capacity (valve size, line size, etc.)
•
Fuel characteristics must be accurately specified in terms of: • Type • Composition • Moisture content • Cleanliness • Viscosity (liquid fuels)
•
All fuels must be treated as hazardous materials and require: • Proper electrical area classifications • Purging of all fuel areas prior to firing • Draining of liquids to safe locations • Tight shutoff valves isolating the fuel system from the engine
•
A one quart slug of a liquid in a gas fuel system will generate a 10 m.w. load spike if ingested in one second!
Therefore: • Appropriate gas fuel superheats must be continuously maintained (minimum of 50°F superheat) • Possible heat tracing of lines and valves required in gas fuel systems • Liquid knock out facilities
Figure 30.14 Gas turbine fuel systems facts
Refer to Figure 30.15 for typical gas turbine fuel alternatives.
447
Principles of Rotating Equipment
Typical gas turbine fuels Types of liquid fuels
Types of fuel gases
• • • •
Conventional liquid fuels Distillate Crude oils Residuals
• • • •
•
Less conventional liquid fuels • Jet fuels • Kerosene
•
Unconventional liquid fuels • Naptha • Natural gas liquids • Natural gasolines • Process residuals
•
natural gas ipg's propane butane
refinery gases • high hydrogen content • coal-delivered gases
Figure 30.15 Typical gas turbine fuels (Courtesy of General Electric Company)
The remainder of this section concerning gas turbine control will cover the design requirements for the fuel control system. Figure 30.16 defines the system design requirements.
Fuel system general design requirements The fuel system must be designed for the maximum fuel pressure requirement Maximum fuel pressure requirement is a direct function of air compressor discharge pressure
VOXJQME FLOW
448
crH/iooo
Gas Turbine Control ft Protection
•
Since compressor discharge pressure will vary significantly • Air compressor operating point must be referenced to fuel systems • Fuel flow requirement will vary significantly • Start-up • No load operation • Full load • Fuel heating values can vary • Fuel viscosity (liquid fuels) can vary • A different start-up fuel may be required
All of the preceding requirements impact • Fuel control valve sizing and arrangement • Fuel orifice sizing Pump and flow divider sizing (liquid systems) • Fuel skid minimum and maximum supply pressures
Figure 30.16 Fuel system general design requirements
Specific fiiel system requirements concern tlie selection of the stop (shut-off) and control valve. Guidelines are presented in Figure 30.17. Gas turbine fuel system specific requirements to combusters Fuel
Control 1 1 Trip system | 1 system
Speed ratio control system
Fuel gas Control valve
•
Positive fast acting stop valve
•
Large range of accurate control
Options: • • •
Stop speed ratio valve
Filter
Split range control valves Two stage - pressure control and flow control
Minimum fuel superheat requirement (50°F)
Figure 30.17 Gas turbine fuel system specific requirements
449
Principles of Rotating Equipment Figure 30.18 shows a typical gas fiiel control system. Gas fuel control system
Strainer Supply
1
Stop/ Ratio Valve
/
^TP~^
96FG Pres XDCR
C^
1
^
iir
ID , \
Dual LVDT's
Hydraulic Trip Relay
V"
Ring Manifold
T^
1^^
or Hydrauli Iraulic ^ Cylinder
Gas Control Valve
Dual LVDT's /•
Trip Oil
N^
y^
Three3 Coil Servo Valves
Combustor
V ^
Hydraulic Cylinder
A^
Operation: Control system regulates ratio valve and gas control valve to keep both valves in desired operation range, during start-up, normal operation and transient load changes.
Figure 30.18 Gas fuel control system (Reprinted from paper GER 3648A with permission of General Electric)
Liquid fuel system requirements are defined in Figure 30.19 below.
450
Gas Turbine Control ft Protection
Liquid fuel system requirennents Fuel quantity control link
Shut down system (trip) r^ Bypass I valve
Fuel control
T
To combusters
o
Flow divider Filter
Typical burner
Fuel pump
From fuel tank Stop valve
Filter
Speed } - J dector Atomizing air if necessary
Positive fast acting stop valve Minimum fuel skid supply pressure (prevent pump cavitation) Pump type compatible with fuel viscosity (centrifugal pumps for low viscosity) Large range of accurate controls (options) • Use of bypass valve • Variable speed flow divider (individual P.D. pump to each nozzle) Figure 30.19 Liquid fuel system requirements (Courtesy of General Electric Company)
Figures 30.20 and 30.21 contain typical fuel system schematics for normal and low viscosity liquid fuels.
451
Principles of Rotating Equipment
Normal viscosity fuel schematic (diesel, etc.)
INDIVIDUAL CHECK VALVES
T
-CZT)
(TYPICAL) I FUEL • NOZZLES
n SPEED
H±D
PICKUPS ON GEAR
FUEL STROKE REFERENCE
Operation: Control system regulates fuel flow through positive displacement fuel dividers to each nozzle, speed pickup on flow divider positions bypass valve to regulate required flow to nozzles (flow divider)
Figure 30.20 Normal viscosity fuel schematic (diesel, etc.) (Reprinted from article GER 3648A with permission of General Electric Company)
452
Gas Turbine Control Et Protection
Low viscosity fuel system schematics AP, Pos
-*" Turbine Controt
"t-
Startup Orino0
Pan«i
'^sv
Startup Manifold Valv*
Startup System Not Required t( Using Separate Fuel for Startup
sv ^
Alt«rnat« Roctrc.
Stop Valve
Operation: Since liquid fuel is low viscosity, positive displacement (gear pumps) cannot be used. A high speed centrifugal pump (sundyne or equal) and individual nozzle flow orifices are used. An in-line control and pump bypass valve are used to supply the required pressure to the nozzle orifices.
Figure 30.21 Low viscosity fuel system schematics (Reprinted from article GER 3648A with permission of General Electric Company)
There are also dual fuel control systems. Facts concerning dual fuel systems are shown in Figure 30.22.
Gas turbine control dual fuel systems
• Provide option of operation on liquid or gas • Liquids usually back up fuel • Can transfer or line without trip Figure 30.22 Gas turbine control dual fuel systems
453
Principles of Rotating Equipment Protection systems The design of the gas turbine protection system incorporates the features of steam turbine protection systems plus hot gas path over temperature control. These facts are presented in Figure 30.23.
Gas turbine protection systems In addition to overspeed protection of each rotor, a gas turbine protection system also shuts the fuel valve on: • • •
Hot gas path overtemperature High compressor discharge pressure Train external trips • Lube oil low pressure • Control oil low pressure • Vibration • Manual trip • Etc.
Figure 30.23 Gas turbine protection systems
454
Introduction - total train control and protection objectives At this point, we have covered the design and operation of a gas turbine. We will now discuss the gas turbine driven total train control and protection. The total train control and protection objectives for a gas turbine are identical to those of a steam turbine. However, since a gas turbine operates at much higher temperatures than a steam turbine, an additional control function is required. This function is known as gas turbine sequencing. A sequencer automatically operates the gas turbine from a permissive to start up to the normal operating speed. In doing so, the sequencer will assure that all support functions, fiiel purge functions, combustor firing functions and acceleration to normal speed is achieved in a safe and reliable manner. In addition, the sequence system also controls the deceleration of the gas turbine and operation of support systems during a normal shutdown. As mentioned above, the basic control function is similar to a steam turbine. This is true however, since the fuel for a gas turbine is a combination of air/fuel mixture and since there are alternatives (liquid, gas or dual fuel) we will focus our attention on the various fuel system configurations and functions in this section. Finally, the protection system will be discussed in detail with emphasis
439
Principles of Rotating Equipment
placed upon additional engine protection other than the over speed bolt which is similar to steam turbine operation. Namely, hot gas path temperature override function will be discussed as well as gas turbine air compressor protection. In addition we will also present a gas turbine control system brief history, which will detail the various types of control systems that are installed in the field. Gas turbine control and protection objectives are similar to that of a steam turbine. However, due to the significantly higher operating temperature, certain additional features must be incorporated. Figure 30.1 presents these features. Gas turbine control and protection additional features In terms of control and protection, a gas turbine system is the same as a steam turbine system. Due to a gas turbine's characteristics, the following features must be added: • Automated start and stop (sequencing) • Hot gas path excessive termperature governor override • Air compressor operating point override
Figure 30.1 Gas turbine control and protection additional features
The design objective of the sequencing and temperature override systems are shown in Figure 30.2. Gas turbine sequencing and temperature override Regardless of the type of gas turbine, start-up and shutdown must be sequenced (automated) to: • Assure all auxiliaries are operative • Gas path is urged • Differential thermal effects are minimized • Air compressor operates in the stable range In addition, the hot gas path temperature must be limited by continuously inputing the hot gas path average temperature to the governor
Figure 30.2 Gas turbine sequencing and temperature override
440
Gas Turbine Control it Protection
In addition, it is necessary to assure that the compressor operating point is optimized (as high an operating efficiency as possible). Figure 30.3 shows one such method - adjustable compressor stator valve control. Air compressor operating point control
voLOKE rum
Since air compressor inlet pressure and gas m.w. are constant, compressor discharge pressure (C.D.P.) is usually selected as the key parameter for determining the air compressor operating point.
Figure 30.3 Air compressor operating point control (Courtesy of Elliott Co.)
Gas turbine control system effectiveness and reliability has made great advances since the early years of gas turbine design. Since gas turbines and their control systems are production items, custom design control features have been minimized. Essentially, there are four (4) generations of control system design beginning in the 1950s. Figure 30.4 outlines a history of these systems.
441
Principles of Rotating Equipment
Gas turbine control system history Years
Sequencing
Control
Protection
1950-1970
relays
mechanical/hydraulic solid state components (analog type)
relays
1970-1980
solid state
integrated circuits (analog type)
solid state and relays
1980-1990
microprocessor redundancy
micro-processor (digital type)
microprocessor
1990-
microprocessor
micro-processor redundancy fault tolerance self diagnostics
independent microprocessor dual voting
Figure 30.4 Gas turbine control system history
Figure 5 presents an overiew of gas turbine control system input, set point and output functions. We will cover each of the following sequencing, control and protection functions in detail.
OUTPUTS
INPUTS ROTOR SPEED(S) FUEL FLOW/ VALVE PROTECTION HOT GAS PATHTEMP(S) COMP.DISCH. PRESSURE SUPPORT SYSTEM • SIGNALS
AUX.SYS. START
FUEL CONTROL
STARTER CRANK
COMBUSTION AIR
PURGE CYCLE
CONTROL
RAMPED ACCEL
HOT GAS PATH OVERRIDE
NOMINAL SPEED
EMISSIONS CONTROL
NORMAL SHUT DOWN RAMP
CONDmON MONITORING
COOL DOWN STOP
PROTECTION
SET POINTS (PROCESS VARIABLE(S) OR SPEED)
Figure 30.5 Gas turbine control system functions overview
442
' TO FUEL SYSTEM ^-^
TRIP ACTION TRIP ACTION
MANUAL TRIP
INTERNAL SIGNALS (LUBE, CONTROL)
TO ENGINE CONTROL SERVOS
Gas Turbine Control a Protection
Start-up and shutdown sequencing The objectives of the gas turbine start and stop sequencing system are noted in Figure 30.6. Gas turbine start-stop sequencing system Sequenced (automated) start-up and shutdown are required to: • Prevent engine fires or explosions • Minimize thermal effects • Eliminate damage caused by vibration when passing through natural frequencies (critical speeds) • Eliminate damage caused by air compressor stall (surge) Figure 30.6 Gas turbine start-stop sequencing system
A generic gas turbine starting sequence outline and a parameter travel are presented in Figures 30.7 and 30.8. Gas turbine starting sequence^ Step 1. 2. 3. 4. 5. 6. 7. 8.
9.
10.
description enable start start acknowledged support systems started2 starter engagement/crank speed attained engine purge cycle igniter activation, fuel system activation lite ofP warm up idle, starter disengagement ramped accelertion to normal speed (considers thermal effects, critical speeds) warm up normal speed (no load) guide vanes, nozzles, etc. in auto position load engine
default parameter all permissives clear any permissive default all support system parameters clear starter operating/crank speed cycle timer set point flame detectors combustion temp or speed change rate warm up timer set point/starter disengagement vibration, exhaust temp, etc.
all trip signals
all trip signals
1. Generic procedure to demonstrate typical sequencer steps. 2. Some applications will have to have manual support system start. 3. All engine guide vanes, variable nozzles set at no load position. Figure 30.7 Gas turbine starting sequence
443
1
Principles of Rotating Equipment Percent 100 F
Approximate Time - l\^inutes Figure 30.8 Typical gas turbine start characteristics - start-up unloaded (Courtesy of General Electric Company)
Facts concerning start sequences for industrial and aero-derivative type gas turbines are presented in Figure 30.9.
Start sequence facts The start-up sequence times vary with gas turbine type and manufacturer typical values • Heavy duty - 10-20 minutes • Aeroderivative - 2-5 minutes Normal shutdowns are sequenced to allow uniform cool down of components Cool down is accomplished by low speed operation
Figure 30.9 Start sequence facts
444
Gas Turbine Control £t Protection
Gas turbine control Like steam turbines, the speed control (governor system) is the heart of the control system. It performs the identical function of'cruise control' in your vehicle. Refer to Figure 30.10 for gas turbine control system objectives. Gas turbine control objectives Meet load requirements Maintain optimum heat rate (firing temperature and efficiency)
Figure 30.10 Gas turbine control objectives
Input parameters are noted in Figure 30.11. Gas turbine control system inputs In order to meet control objectives, inputs to control system are: • •
Power turbine rotor speed Air compressor • Rotor(s) speed! • Compressor discharge pressure (CDP) • Hot gas path temperatures
^if multi-shaft turbine
Figure 30.11 Gas turbine control system inputs
A typical control schematic for a gas turbine drive is shown in Figure 30.12.
445
Principles of Rotating Equipment PRCXJESSGAS
£
-WWWV-gh—^=^—--I PROCESS GAS
Ky •AIR
^y NPT
NGG
Ij^" i
FUEL
11. SET POINT
OUTPUT H
CONTROLLER
OUTPUT
Figure 30.12 A gas turbine driven turbo-compressor (Courtesy of M E Crane Consultant)
Control system output for this system is discussed in Figure 30.13. Gas turbine control system outputs In order to control gas turbine output power, • Vapor energy/mass - AH • Mass rate - M Must be controlled by: • Fuel rate control (AH) • Airflow rate (if furnished) (M) • Gas generator speed • Adjustable stator vanes • Power turbine variable nozzles (if furnished) (M)
Figure 30.13 Gas turbine control system outputs
As discussed in the previous chapter, there are many available fuel options. Figure 30.14 presents fuel options and facts concerning fuel specific energy and specifications.
446
Gas Turbine Control ft Protection Gas turbine fuel systems facts •
Available fuel options: • Gas (most frequently used) • Liquid • Dual (gas or liquid)
•
The lower the fuel heating value, btu/scf the greater required fuel system capacity (valve size, line size, etc.)
•
Fuel characteristics must be accurately specified in terms of: • Type • Composition • Moisture content • Cleanliness • Viscosity (liquid fuels)
•
All fuels must be treated as hazardous materials and require: • Proper electrical area classifications • Purging of all fuel areas prior to firing • Draining of liquids to safe locations • Tight shutoff valves isolating the fuel system from the engine
•
A one quart slug of a liquid in a gas fuel system will generate a 10 m.w. load spike if ingested in one second!
Therefore: • Appropriate gas fuel superheats must be continuously maintained (minimum of 50°F superheat) • Possible heat tracing of lines and valves required in gas fuel systems • Liquid knock out facilities
Figure 30.14 Gas turbine fuel systems facts
Refer to Figure 30.15 for typical gas turbine fuel alternatives.
447
Principles of Rotating Equipment
Typical gas turbine fuels Types of liquid fuels
Types of fuel gases
• • • •
Conventional liquid fuels Distillate Crude oils Residuals
• • • •
•
Less conventional liquid fuels • Jet fuels • Kerosene
•
Unconventional liquid fuels • Naptha • Natural gas liquids • Natural gasolines • Process residuals
•
natural gas ipg's propane butane
refinery gases • high hydrogen content • coal-delivered gases
Figure 30.15 Typical gas turbine fuels (Courtesy of General Electric Company)
The remainder of this section concerning gas turbine control will cover the design requirements for the fuel control system. Figure 30.16 defines the system design requirements.
Fuel system general design requirements The fuel system must be designed for the maximum fuel pressure requirement Maximum fuel pressure requirement is a direct function of air compressor discharge pressure
VOXJQME FLOW
448
crH/iooo
Gas Turbine Control ft Protection
•
Since compressor discharge pressure will vary significantly • Air compressor operating point must be referenced to fuel systems • Fuel flow requirement will vary significantly • Start-up • No load operation • Full load • Fuel heating values can vary • Fuel viscosity (liquid fuels) can vary • A different start-up fuel may be required
All of the preceding requirements impact • Fuel control valve sizing and arrangement • Fuel orifice sizing Pump and flow divider sizing (liquid systems) • Fuel skid minimum and maximum supply pressures
Figure 30.16 Fuel system general design requirements
Specific fiiel system requirements concern tlie selection of the stop (shut-off) and control valve. Guidelines are presented in Figure 30.17. Gas turbine fuel system specific requirements to combusters Fuel
Control 1 1 Trip system | 1 system
Speed ratio control system
Fuel gas Control valve
•
Positive fast acting stop valve
•
Large range of accurate control
Options: • • •
Stop speed ratio valve
Filter
Split range control valves Two stage - pressure control and flow control
Minimum fuel superheat requirement (50°F)
Figure 30.17 Gas turbine fuel system specific requirements
449
Principles of Rotating Equipment Figure 30.18 shows a typical gas fiiel control system. Gas fuel control system
Strainer Supply
1
Stop/ Ratio Valve
/
^TP~^
96FG Pres XDCR
C^
1
^
iir
ID , \
Dual LVDT's
Hydraulic Trip Relay
V"
Ring Manifold
T^
1^^
or Hydrauli Iraulic ^ Cylinder
Gas Control Valve
Dual LVDT's /•
Trip Oil
N^
y^
Three3 Coil Servo Valves
Combustor
V ^
Hydraulic Cylinder
A^
Operation: Control system regulates ratio valve and gas control valve to keep both valves in desired operation range, during start-up, normal operation and transient load changes.
Figure 30.18 Gas fuel control system (Reprinted from paper GER 3648A with permission of General Electric)
Liquid fuel system requirements are defined in Figure 30.19 below.
450
Gas Turbine Control ft Protection
Liquid fuel system requirennents Fuel quantity control link
Shut down system (trip) r^ Bypass I valve
Fuel control
T
To combusters
o
Flow divider Filter
Typical burner
Fuel pump
From fuel tank Stop valve
Filter
Speed } - J dector Atomizing air if necessary
Positive fast acting stop valve Minimum fuel skid supply pressure (prevent pump cavitation) Pump type compatible with fuel viscosity (centrifugal pumps for low viscosity) Large range of accurate controls (options) • Use of bypass valve • Variable speed flow divider (individual P.D. pump to each nozzle) Figure 30.19 Liquid fuel system requirements (Courtesy of General Electric Company)
Figures 30.20 and 30.21 contain typical fuel system schematics for normal and low viscosity liquid fuels.
451
Principles of Rotating Equipment
Normal viscosity fuel schematic (diesel, etc.)
INDIVIDUAL CHECK VALVES
T
-CZT)
(TYPICAL) I FUEL • NOZZLES
n SPEED
H±D
PICKUPS ON GEAR
FUEL STROKE REFERENCE
Operation: Control system regulates fuel flow through positive displacement fuel dividers to each nozzle, speed pickup on flow divider positions bypass valve to regulate required flow to nozzles (flow divider)
Figure 30.20 Normal viscosity fuel schematic (diesel, etc.) (Reprinted from article GER 3648A with permission of General Electric Company)
452
Gas Turbine Control Et Protection
Low viscosity fuel system schematics AP, Pos
-*" Turbine Controt
"t-
Startup Orino0
Pan«i
'^sv
Startup Manifold Valv*
Startup System Not Required t( Using Separate Fuel for Startup
sv ^
Alt«rnat« Roctrc.
Stop Valve
Operation: Since liquid fuel is low viscosity, positive displacement (gear pumps) cannot be used. A high speed centrifugal pump (sundyne or equal) and individual nozzle flow orifices are used. An in-line control and pump bypass valve are used to supply the required pressure to the nozzle orifices.
Figure 30.21 Low viscosity fuel system schematics (Reprinted from article GER 3648A with permission of General Electric Company)
There are also dual fuel control systems. Facts concerning dual fuel systems are shown in Figure 30.22.
Gas turbine control dual fuel systems
• Provide option of operation on liquid or gas • Liquids usually back up fuel • Can transfer or line without trip Figure 30.22 Gas turbine control dual fuel systems
453
Principles of Rotating Equipment Protection systems The design of the gas turbine protection system incorporates the features of steam turbine protection systems plus hot gas path over temperature control. These facts are presented in Figure 30.23.
Gas turbine protection systems In addition to overspeed protection of each rotor, a gas turbine protection system also shuts the fuel valve on: • • •
Hot gas path overtemperature High compressor discharge pressure Train external trips • Lube oil low pressure • Control oil low pressure • Vibration • Manual trip • Etc.
Figure 30.23 Gas turbine protection systems
454