GE Power Systems

GE Power Systems

Design Considerationsfor Heated Gas Fuel

D.M. EricksonS.A. DayR. DoyleGE Power SystemsGreenville, SC

GER-4189B

g

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Gas Fuel Performance Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Gas Compressor Heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

General System Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1

Gas Fuel Cleanliness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Gas Fuel Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Gas Fuel Supply Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Gas Fuel Supply Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Combustion Specific System Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

DLN-1 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

DLN-2.0 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

DLN-2+ Requirements (PG9351FA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

DLN-2+ FB Requirements (PG7251FB & PG9371FB) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

DLN-2.6 Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

DLN-2.5H Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Typical GE Gas Fuel Heating System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

System Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Design Considerations for Heated Gas Fuel

GE Power Systems ■ GER-4189B ■ (03/03) i


GE Power Systems ■ GER-4189B ■ (03/03) ii

Introduction

Gas Fuel Performance HeatingAs the need for higher efficiency power plantsincreases, a growing number of combined-cyclepower plants are incorporating performancegas fuel heating as a means of improving overallplant efficiency. This heating, typically increas-ing fuel temperatures in the range of365°F/185°C, improves gas turbine efficiencyby reducing the amount of fuel needed toachieve desired firing temperatures. For fuelheating to be a viable method of performanceenhancement, feedwater has to be extractedfrom the heat recovery steam generator(HRSG) at an optimum location. Boiler feed-water leaving the intermediate pressure econo-mizer is commonly used. Using gas-fired, oil-fired or electric heaters for performance gasfuel heating will not result in a power plantthermal efficiency improvement.

Proper design and operation of the Gas FuelHeating System is critical in insuring reliableoperation of the gas turbine. Improper selec-tion of components, controls configurationand/or overall system layout could result inhardware damage, impact plant availability andcreate hazardous conditions for plant person-nel. This paper addresses the critical design cri-teria that should be considered during thedesign and construction of these systems.

Also included in this paper is the design of a"typical" GE Gas Fuel Heating System. This sys-tem has been developed taking into considera-tion the system requirements defined within.

Gas Compressor HeatingGas compressors may be needed to meet speci-fied minimum gas supply pressures levels. Theuse of a compressor adds heat to the gas andraises its operational temperature. The temper-

ature level of the gas at the exit of the gas com-pressor is a function of its inlet conditions. Thistemperature may vary from site to site andshould be evaluated against any combustionspecific requirements defined in this docu-ment.

General System RequirementsThe following section identifies general systemrequirements that apply to all gas fuel heatingsystems. These requirements, in addition tothose described in the Combustion SpecificSystem Requirements section shall be followedduring the design and development of the sys-tem.

Gas Fuel CleanlinessGas fuel supplied to the gas turbine shall meetthe particulate requirements as specified in thelatest revision of GEI-41040, "ProcessSpecification –– Gas Fuels for Combustion inHeavy Duty Gas Turbines," (Reference 1). If thecomponents in the Gas Fuel Heating System areconstructed of materials susceptible to corro-sion, a method of final filtration upstream ofthe gas turbine interface is required. Particulatecarryover greater than that identified in GEI-41040 can plug fuel nozzle passages, erode com-bustion hardware and gas valve internals andcause damage to first stage turbine nozzles. Thenew gas piping system must be properly cleanedprior to initial gas turbine operation. Additionaldesign considerations related to gas fuel clean-liness may be found in GER-3942, Gas FuelClean-Up System Design Considerations for GEHeavy Duty Gas Turbines," (Reference 2).

Gas Fuel QualityAs defined in GEI-41040, the fuel delivered tothe gas turbine must be liquid free and containa specified level of superheat above the higherof the hydrocarbon or moisture dewpoints.


GE Power Systems ■ GER-4189B ■ (03/03) 1

Saturated fuels, or fuels containing superheatlevels less than specified, can result in the for-mation of liquids as the gas expands and coolsacross the gas turbine control valves. Theamount of superheat provides margin to com-pensate for temperature decrease due to pres-sure reduction, and is directly related to incom-ing gas supply pressure. (Note: Within thisdocument, gas fuel heating strictly for dewpointconsiderations is still considered to be in a"cold" state. Heating for performance purposesis considered "heated" fuel.)

The design of the Gas Fuel Heating System shallprevent carryover of moisture or water to thegas turbine in the event of a heat exchangertube failure. Water entrained in the gas cancombine with hydrocarbons causing the forma-tion of solid hydrocarbons or hydrates. Thesehydrates, when injected into the combustionsystem, can lead to operability problems,including increased exhaust emissions andmechanical hardware damage. Proper meansof turbine protection, including heat exchang-er leak detection, shall be provided.

Gas Fuel Supply PressureGas being supplied to the gas turbine interfacepoint (customer connection FG1) shall meetthe minimum gas fuel supply pressure require-ments as defined in the proposal documenta-tion. These minimum pressure requirementsare established to insure proper gas fuel flowcontrollability and to maintain required pres-sure ratios across the combustion fuel nozzles.The Gas Fuel Heating System shall be designedto insure that these requirements are met dur-ing all modes of operation over the entireambient temperature range.

The design of the Gas Fuel Heating System shallinsure that the design pressure of the gas tur-bine gas fuel system is not exceeded.

Overpressure protection, as required by appli-cable codes and standards, shall be furnished.In addition to minimum and maximum pres-sures, the gas turbine is also sensitive to gas fuelpressure variations. Sudden drops in supplypressure may destabilize gas pressure and flowcontrol. Sudden increases in supply pressuremay potentially trip the turbine due to a hightemperature condition. Limitations on pressurefluctuations are defined in the gas turbine pro-posal documentation.

Gas Fuel Supply TemperatureThe Gas Fuel Heating System shall be designedto produce the desired gas fuel temperature atthe interface with the gas turbine equipment.Guaranteed performance is based on thedesign fuel temperature at the inlet to the gasturbine gas fuel module (FG1). The gas fuelheating and supply systems shall compensatefor heat losses through the system.Compensation shall include but not be limitedto elevated heater outlet temperatures, use ofpiping and equipment insulation, and mini-mization of piping length from heater outlet toturbine inlet.

The Gas Fuel Heating System shall be designedto support specified gas fuel temperature set-points required by the gas turbine. These set-points include high and low temperaturealarms, gas turbine controls permissives, andgas turbine controls functions. These setpointsare derived by GE Gas Turbine Engineeringand are based on operability requirementsand/or design limitations of components with-in the gas turbine gas fuel system.

During specified cold and hot gas fuel turbineoperating modes, the Gas Fuel Heating Systemshall attain and maintain the fuel at a tempera-ture that corresponds to a Modified WobbeIndex (MWI) within ±5% of the target value.



The Modified Wobbe Index is a calculatedmeasurement of volumetric energy content offuel and is directly related to the fuel tempera-ture and lower heating value (LHV). TheModified Wobbe Index is derived as follows:

MWI = Tg • SG

Where:

MWI = Modified Wobbe Index (temperature corrected)

LHV = Lower Heating Value of Fuel(BTU/SCF)

Tg = Absolute Temperature (°R)

SG = Specific Gravity of fuel relative to airat ISO Conditions

The ±5% Modified Wobbe Index range insuresthat the fuel nozzle pressure ratios are main-tained within their required limits. If gas fuelconstituents and heating value are consistent,the 5% tolerance can be based strictly on tem-perature variation. If the heating value of thefuel varies, as is the case when multiple gas sup-pliers are used, heating value and specific grav-ity must be considered when evaluating theallowable temperature variation to support the5% Modified Wobbe Index limit.

For the use of gas fuels having a significant vari-ation in composition or heating value, a per-manent gas chromatograph shall be furnishedin the plant’s main gas supply line. LHV andspecific gravity readings from the gas chro-matograph are used to regulate the amount offuel heating so that the ±5% Modified WobbeIndex requirement is satisfied. This controlfunction shall be performed automatically bythe plant control system.

Consideration shall be made to the location ofthe gas chromatograph relative to the inlet ofthe gas fuel module and the time delay frominstrument reading to fuel gas control.

Combustion Specific SystemRequirementsThe GE Gas Turbine product line incorporatesthe use of both Dry Low NOx (DLN) and Non-Dry Low NOx (conventional) combustiondesigns. Currently, there are five different DLNconfigurations offered by GE: DLN-1, DLN-2.0,DLN-2+, DLN-2.6 and DLN-2.5H. Each com-bustion design is applied to one or more gasturbine models. These designs have differenthardware configurations and operabilityschemes and in turn have certain contrastinggas fuel heating requirements. Performancetype gas fuel heating is not normally applied toconventional combustion systems, and thus willnot be addressed in this document. Table 1identifies the combustion designs that areapplied to the various turbine models. This sec-tion will detail the system design and operabil-ity requirements that apply to the specific DLNcombustion design.

DLN-1 RequirementsOn gas turbines that utilize DLN-1 combustiondesigns, the Gas Fuel Heating System and sup-porting control system shall be designed to pro-vide either cold or heated fuel as based on thegas turbine’s requirements.

The gas turbine control system will provide apermissive signal indicating when heated orunheated fuel is required. The plant controlsystem shall use this signal to initiate gas fuelheating on start-up and to cease gas fuel heatingon shutdown. For DLN-1 combustion designs,the fuel shall be in a cold state from ignition



LHV

(Primary combustion mode) through Lean-Lean and into Secondary Premix combustionmode. The fuel can be heated only afterPremix steady state is achieved. The gas can behot or cold in Premix Mode, but must be coldin Primary, Lean-Lean or Extended Lean-LeanMode. The gas must be cold prior to transfer-ring out of Premix Mode.

During a “hot gas restart,” the DLN-1 combus-tion system has the ability to be fired on the hotfuel contained in the fuel supply system. Activeheating of the fuel shall not be re-establisheduntil the combustion system reaches PremixSteady State Mode.

DLN-2.0 RequirementsOn gas turbines that utilize DLN-2.0 combus-tion designs, the Gas Fuel Heating System and

supporting control system shall be designed toprovide either cold fuel or heated fuel as basedon the gas turbine’s requirements.

DLN-2.0 combustion systems are designed tooperate on both unheated and heated fuels atignition as well as Primary and Lean-LeanModes. While in Premix Transfer, PilotedPremix, and Premix Modes, the system isdesigned to operate on heated fuels only.Permissives configured within the gas turbinecontrols permit or prevent changes in combus-tion mode until the gas is heated sufficiently inorder to satisfy the Modified Wobbe Indexrequirements. Thermocouples located directlyupstream of the gas turbine’s Stop Speed RatioValve initiate this permissive.

During turbine shutdown, gas fuel heating shallbe disabled only after transferring out ofPremix Mode.

DLN-2+ Requirements (PG9351FA)On gas turbines that utilize DLN-2+ combustiondesigns, the Gas Fuel Heating System and plantcontrols shall be designed to provide eithercold or heated fuel as based on the gas turbine’srequirements.

DLN-2+ combustion systems are designed tooperate on heated or unheated fuel inDiffusion and Sub-piloted Premix Mode.Diffusion and Sub-piloted Premix Mode opera-tion consists of ignition, acceleration to FullSpeed No Load, and up to approximately 10%load. (See Figure 1.) During Piloted PremixMode operation, from approximately 10% loadto 25% load, the gas fuel temperature can behot or cold. However, the gas must satisfy theModified Wobbe Index hot temperature limits,in Piloted Premix Mode, from approximately25% to 50% load. During Premix Mode opera-tion, the gas temperature must be sufficient tosatisfy the Modified Wobbe Index limit. In


Table 1. Combustion design to turbine modelcross reference


Combustion ApplicableDesign Turbine Models

DLN-1 PG5271RPG5371PPG6541BPG6561BPG6571BPG6581BPG7111EAPG7121EAPG9171E

DLN-2.0 PG6101FAPG7221FAPG7231FAPG9311FAPG9331FA

DLN-2+ PG9351FAPG7251FBPG9371FB

DLN-2.6 PG7231FAPG7241FAPG9231EC

DLN-2.5H PG7371HPG9441H

addition, Extended Piloted Premix Mode, from50% load to Baseload, requires the gas to meetthe Modified Wobbe Index hot limit.Permissives set within the gas turbine controls,prevent a transfer into the appropriate PilotedPremix load or Premix Mode, during loading,until the required temperature is attained.

Thermocouples located directly upstream ofthe gas turbine’s Stop Speed Ratio Valve initiatethis permissive.

During turbine shutdown, gas fuel heatingshall be ceased only after transferring out of

Piloted Premix Mode at approximately 25%load.

DLN-2+ FB Requirements(PG7251FB & PG9371FB)

On gas turbines that utilize the DLN-2+ FB com-bustion system design, the Gas Fuel HeatingSystem and plant controls shall be designed toprovide either cold or heated fuel as based onthe gas turbine’s requirements. (See Figure 2.)

DLN-2+ FB combustion systems are designed tooperate on heated or unheated fuel in


Figure 1. DLN-2+ (PG9351) fuel heating operational requirements

Figure 2. DLN-2+ FB (PG7251FB & PG9371FB) fuel heating operational requirements


SP

EE

D

Diffusion and Sub-piloted Premix Mode.Diffusion and Sub-piloted Premix Mode opera-tion consists of ignition, acceleration to FullSpeed No Load, and up to approximately 10%load. (See Figure 2.) During Piloted PremixMode operation, from approximately 15% loadto 20% load, the gas fuel temperature can behot or cold. However, the gas must satisfy theModified Wobbe Index hot temperature limits,in Piloted Premix Mode, from approximately20% to 40% load. During Premix Mode opera-tion, the gas temperature must be sufficient tosatisfy the Modified Wobbe Index limit. In addi-tion, Extended Piloted Premix Mode, from40% load to Baseload, requires the gas to meetthe Modified Wobbe Index hot limit.

Permissives set within the gas turbine controlsprevent operation in Premix Mode or in PilotedPremix Mode until the required fuel tempera-ture is attained. Thermocouples located direct-ly upstream of the gas turbine’s Stop/SpeedRatio Valve initiate this permissive.

During turbine shutdown, gas fuel heating shallbe ceased only after reducing load below 20%.

DLN-2.6 RequirementsOn gas turbines that use DLN-2.6 combustiondesigns, the Gas Fuel Heating System and plantcontrols shall provide either cold or heated fuelas based on the gas turbine’s requirements. (SeeFigure 3.)

DLN-2.6 combustion systems are designed tooperate on heated or unheated fuel in Modes 1,2 and 3. Heated fuel operation in Modes 1, 2and 3 is permitted, but not recommended, fornormal operation. The gas must be heated tosatisfy the Modified Wobbe Index hot tempera-ture limits prior to transferring to combustionMode 4, at approximately 25% load.Thermocouples located directly upstream ofthe gas turbine’s Stop Speed Ratio Valve initiatea permissive to transfer into Mode 4.

Fuel temperature must be maintained withinthe hot gas temperature limits at all modes


Figure 3. DLN-2.6 fuel heating operational requirements


SP

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D

above Mode 3 (approximately 25% load) dur-ing both unit operation and shutdown. Duringturbine shutdown, gas fuel heating shall beceased only after transferring out of Mode 4and into Mode 3. The gas fuel temperature isrecommended, but not required, to be less than120°F/49°C before transferring from Mode 3to Mode 1.

DLN-2.5H RequirementsOn gas turbines that operate with DLN-2.5Hcombustion designs, the Gas Fuel HeatingSystem and plant controls shall provide eithercold or heated fuel based on the gas turbine’srequirements.

The DLN-2.5H combustion systems aredesigned to operate on both unheated andheated fuels at ignition through DiffusionMode and into Piloted Premixed Mode. Thegas must be heated in order to satisfy theModified Wobbe Index hot gas temperaturelimits prior to transferring to Premixed Mode.

Typical GE Gas Fuel Heating SystemThe following section details the mechanicaldesign and operational features of the typicalGE Gas Fuel Heating System. The design intentof this system is to produce gas fuel that meetsall requirements previously specified in this doc-ument. In addition to supporting heated fuel tothe gas turbine, the typical system pro-vides safe-guards that prevent gas fuel from entering theHRSG system. This commonly ignored condi-tion can occur when a tube leak is present dur-ing gas turbine operation or unit shutdown.

This typical design is provided as a reference tothe customer. Deviations from this design maybe acceptable, providing that the requirementsof the gas turbine are met.

System DescriptionFigure 4 identifies the equipment, instrumenta-

tion and piping configuration of the typical GasFuel Heating System. This system, as described,was initially applied to the MS9001H combinedcycle power plant, which used intermediatepressure feedwater as the medium for fuel heat-ing. The design criteria utilized during thedevelopment of this system shall be followedduring the detailed design of all gas turbine gasfuel heating systems that utilize feedwater orsteam as the heating medium. Job specific gasheating systems may deviate from this designbased on gas conditions and interfacing bal-ance of plant systems.

Design Criteria

The standard Gas Fuel Heating System designmeets the following design criteria:

■ Provide heated fuel that meets theModified Wobbe Index requirement ofthe gas turbine’s combustion system.

■ Prevent water from being admitted to thegas turbine combustion system following aheat exchanger tube leak or rupture.

■ Provide early indication of heatexchanger tube failure.

■ Prevent gas fuel from entering the feed-water system following a heat exchang-er tube failure.

■ Remove gas entrained particulate asspecified in the latest revision of GEI-41040, Process Specification –– GasFuels for Combustion in Heavy DutyGas Turbines (Reference 1).

■ Provide overpressure protection to thegas turbine Gas Fuel Heating Systempiping and components.

■ Ensure water pressure is higher than gaspressure during gas turbine operationand shutdown.

Heater Leak Detection Protection Philosophy

The heat exchanger leak detection scheme



shall incorporate three levels of alarms or auto-mated control. These three levels have beenestablished to prevent the admission of waterinto the gas turbine while preventing inadver-tent trips or load decreases due to failure of asingle sensing instrument. (See Figure 5.)

The heater leak detection controls have beenestablished to provide early detection of a heatexchanger leak and to mitigate the effects ofboth the leak on the gas turbine and the bal-ance of plant systems.

Each gas fuel heater shell is furnished with a lowpoint drain pot. The two drain pots house aseries of level switches used in the tube leakdetection controls. The lower heat exchanger

drain pot is furnished with a single high levelswitch and three triple-redundant high-highlevel switches. A drain pot will open upon acti-vation of the corresponding high level switch.

When two out of the three high-high levelswitches are activated, the feedwater to andfrom the heat exchanger will isolate. Thisaction will quickly reduce the temperature ofthe gas fuel and initiate a transfer of the gas tur-bine to a cold mode of operation. Specificallyfor the typical Gas Fuel Heating System, thedetails of the three levels are as follows:

Level 1. At a minimum, a single sensing instru-ment (i.e., level switch) is implemented toalarm and evacuate the heating medium from


Figure 4. Typical Gas Fuel Heating System Flow Diagram


GASSUPPLY

FV

PDT

TE

PDT

PT

TE TEGAS TURBINEGAS MODULE

IP ECONOMIZEROUTLET

LSH

LSHHA

LSHHC

LSHHB

LSH

LG

LV

COALESCINGFILTER SKID

LSH LSH

TE PT

PT TE

LSH

FV

FV

LSHHA

LSHHC

LSHHB

FV

LSH

GAS FUELHEATER SKID

FUEL GASSCRUBBER SKID

DRAIN TANK

PIPILG LG

ATMOSPHERICCONDENSATE TANKFV TCVTE

TE

FA

FV

FV

FV

FV

PDSH

LV

LEVEL 1 LEAK DETECTOR *

LEVEL 3LEAK DETECTOR *

LEVEL 2 LEAK DETECTOR *

* Leak Detection Levels Refer to LevelDefinition in this document only

FV

ELECTRIC STARTUPSUPERHEATER

TE

the gas stream/liquid collection sump followinga tube leak/rupture. This provides initial indi-cation that the heat exchanger tube leak/rup-ture is present.

Level 2. At a minimum, triple redundant sensinginstrumentation is implemented and set at alevel higher than that of Level 1. Output fromthese signals shall alarm and automatically iso-late the heating medium from the gas stream,(i.e., isolating the feedwater from the heatexchanger). This provides secondary indicationthat the heat exchanger leak/tube rupture ispresent and that action taken based on Level 1has failed. Automatic isolation of the heatingmedium from the gas stream will initiate atransfer of the gas turbine to a cold mode ofcombustion operation and/or lower turbineload.

Level 3. At a minimum, triple redundant sensinginstrumentation is implemented and set at alevel higher than that of Level 2. Output fromthese signals shall be integrated into the cus-tomer’s master trip signal. This provides a finallevel of indication/mitigation following a rup-ture/leak event. Activation of these level switch-

es prevents water from being admitted to thecombustion system by either isolating the gassupply or tripping the gas turbine.

System Flowpath

As the incoming gas fuel supply enters the plantfacility, it first passes through one of two 100%coalescing filters. These filters are required toremove both liquids and particulate from thecustomer’s gas supply. The filters may not berequired if similar equipment is installed up-stream by the gas supplier. Liquids collecting inthe Coalescing Filter Sump are automaticallydrained into the common Drain Tank. A differ-ential pressure switch installed across the filtersmonitors pressure differential and alarms whencleaning or cartridge replacement is required.

Downstream of the Coalescing Filter, the gasfuel supply enters the Electric StartupSuperheater. This startup heater is requiredwhen the gas supply does not meet the mini-mum superheat requirement.

The electric heater is turned off and thenbypassed at the point when the PerformanceGas Fuel Heater is capable of maintaining gas



Figure 5. Heat exchanger leak detection control scheme

temperatures above the minimum superheatrequirement.

As fuel exits the superheater, it enters thePerformance Gas Fuel Heater. This systemincorporates a stacked two-shell heater arrange-ment with the gas on the shell side and thefeedwater on the tube side. Each of the heatexchanger shells is furnished with low point col-lection sumps. These sumps house level instru-mentation that provide early indication of aheat exchanger tube leak or rupture and auto-matically control the sump drain valves.

Activation of a single high level switch indicatesdetection of a Level 1 leak, while triple redun-dant high-high level switches indicate a Level 2leak (See Figure 5.) A full bypass/bypass valve isprovided around the Gas Fuel Heater to allowfor certain modes of operation when the heatexchanger is not in service. Dependent on com-bustion type and frame size, these “cold”modes of operation may be load and/or emis-sions limited. (Refer to the CombustionSpecific Requirements.)

The gas fuel exiting the Gas Fuel Heater Skidenters the gas fuel scrubber. This “dry” scrubberperforms two functions in that it a) providesthe final level of particulate filtration upstreamof the gas turbine, and b) removes gas-entrained water droplets present as the resultof a minor tube leak (i.e., pinhole). Two levelsof instrumentation within the scrubber moni-tor for the presence of liquids. A high levelswitch will generate an alarm and automatical-ly open the scrubber drain valve that drains col-lected fluids to the drain tank. Two out of threehigh-high level switches indicate detection of aLevel 3 leak, thus initiating a signal to trip thegas turbine. (See Figure 5.)

Downstream of the gas fuel scrubber, the gasfuel supply enters the gas fuel metering tube.

The metering tube houses a flow orifice, two dif-ferential pressure transducers, three tempera-ture elements and a pressure transducer. Thegas turbine control systems read signals provid-ed by these instruments to calculate a pressureand temperature-compensated fuel flow.

The typical Gas Fuel Heating System uses inter-mediate pressure feedwater as the heatingmedium. The feedwater enters the Gas FuelHeater Skid and passes through a double block-and- bleed valve arrangement to the tube sideof the heat exchanger. These automated block-and- bleed valves prevent gas from backflowinginto the feedwater systems during unit shut-down if a tube leak is present. A similar three-valve block-and-bleed configuration is providedat the heat exchanger feedwater outlet. The gastemperature control valve is located directlydownstream of the second isolation valve.

Component Description

The following section provides a detaileddescription of the hardware components withinthe typical Gas Fuel Heating System. Unit spe-cific components may differ based on incom-ing gas conditions, heating requirements andover-all plant configuration. The componentout-line drawings may differ depending on theequipment supplier:

Coalescing Filter Skid — The Coalescing FilterSkid is designed to protect the downstream gasfuel system against the entry of both liquidphase fuel and particulate contaminants. (SeeFigure 6.) At rated flow, the efficiency of the fil-ter is 100% for solid and liquid particulate larg-er than 0.3 microns at rated flow. This skid isnot designed to remove large quantities (i.e.,“slugs”) of liquids.

The skid, as shown, consists of two 100% gasflow coalescing filters. Each filter is designed



for performing maintenance without removingthe gas turbine from service. Peaking units mayuse a simplex arrangement, where the filter canbe cleaned or maintained during unit downtime.

Each filter house contains a liquid collectionsump. The sump is furnished with a drainsystem that automatically removes liquids fromthe vessel. A high level switch is providedto monitor the sump level. (See CoalescingFilter Skid Controls.) (Note: If large quantitiesof gas entrained liquids are expected, a scrub-ber may be required upstream of the coalesc-ing filter.)

Electric Startup Superheater — The ElectricStartup Superheater is needed at ignition whenthe fuel supply does not meet the minimumrequired superheat level as defined in GEI-41040. (See Figure 7.) The heater’s capacity issized to provide this temperature rise for

fuel flows up to the point where the perform-ance heater can maintain the temperature.The heater’s capacity will not maintain thesuper-heat level at fuel flows in excess of thisvalue.

The heater is an industrial unit designed fornatural gas application. A Silicon ControlledRectifier (SCR) controls the heater. The SCRcontroller maintains a constant differentialacross the heater and over the entire range ofgas fuel flows where superheating is necessary.(Note: Non-electric heat exchanger designs,i.e., gas-fired or oil-fired, may be used for thisapplication. The startup superheater requires aheat source available at gas turbine ignition.)

Gas Fuel Performance Heater Skid — The GasFuel Performance Heater Skid consists of twostacked shell and tube heat exchangers inseries, gas and water side isolation valves, ventand drain valves, and instrumentation required



Figure 6. Standard Coalescing Filter Skid

to support the operation of the gas fuel heater.(See Figure 8.) The heat exchangers are singlepass, fixed tubesheet type, and include expan-sion bellows on the shell. The heat exchangersare mounted on a common base. The heatexchanger is designed for the intermediatepressure feedwater to flow within the tubes andthe lower pressure gas fuel to flow through theshell.

With water pressure being higher than gas pres-sure, this configuration insures that gas will notenter the feedwater system following tube leakor rupture. The design of the system incorpo-rates various safeguards designed to preventwater entering the gas from being admitted tothe gas turbine combustion system.

Each heat exchanger is furnished with a drainpot at one end of the shell. These drain potshouse level instrumentation that provide earlyindication of tube leak/rupture prior to andduring gas turbine operation.

The physical configuration of the heat exchang-er has the gas inlet at the side of the first stageheat exchanger and the outlet at the top of the

second stage heat exchanger. The nozzles ori-ented in this manner prevent water from col-lecting in the inlet or outlet piping following atube rupture event.

Each heat exchanger is furnished with a flowrestrictive orifice plate located at the inlet andoutlet tube sheets of each shell. This orificeplate controls the amount of water that exits asa result of catastrophic tube rupture. This de-sign is required to both minimize the effect onthe feedwater system and to limit the quantity ofwater entering the gas stream. The downstreamorifices are non-concentric with the tubes toallow draining during shutdown. The Gas FuelHeater is sized to accommodate temperaturedownstream of the heat exchanger and will beable to supply the desired temperature for alloperating conditions.

It may be necessary to provide an automated by-pass system around the Gas Fuel Heater inorder to satisfy the Combustion SpecificRequirements defined in this document. Theneed for this by-pass will depend highly on theactual heater system applied to a unit.



Figure 7. Standard Electric Startup Superheater

Gas Fuel Scrubber — The Gas Fuel Scrubberprovides the final level of filtration directlyupstream of the turbine. (See Figure 9.) Thescrubber also removes water droplets from thegas stream following the event of a heater tubeleak or rupture. For particulate 8 microns orlarger, removal is 100% efficient at the designflow rate. The performance of the scrubberinsures that the outlet gas will contain no morethan 0.10 gallons of entrained liquid per mil-lion standard cubic feet of gas, at the rated flow.The scrubber is furnished with an automaticdrain system that discharges to the Drain Tank.

The Gas Fuel Scrubber is a vertical, multi-cyclone, high-efficiency dry-type separator. Thescrubber vessel is manufactured of carbon steeland is designed to satisfy the requirements ofSection VIII of the ASME Boiler and PressureVessel Code (Reference 3). The outlet flange ofthe scrubber serves as the carbon-to-stainlesssteel interface point for the Gas Fuel HeatingSystem. In other words, the piping and valvesbetween the scrubber and gas turbine connec-tion shall be stainless steel.

Drain Tank — The Drain Tank is an atmospher-ic horizontal tank constructed of carbon steel.The Drain Tank collects and stores liquids dis-charged from the Coalescing Filter Skid, thePerformance Heater drain pots, and the GasFuel Scrubber. The vents from the performanceheater also discharge to the Drain Tank. Due tothe potential for collecting both gaseous andliquid hydrocarbons, a flame arrestor is mount-ed on the Drain Tank vent. The tank is mount-ed within a containment dike in order to pro-tect the environment from hazardous dis-charges.

The Drain Tank is furnished with a local levelgauge and a high level switch. Manual drainingof the tank is required when the level reaches aspecified setpoint. If excessive amounts of liq-uids collect in the drain tank, they should beanalyzed and their origins determined.

System Controls

This section provides a detailed description ofthe controls hardware and software associatedwith the typical Gas Fuel Heating System. Unit



Figure 8. Standard Gas Fuel Performance Heater Skid

specific controls may deviate from the followingdescriptions based upon the specific plant con-figuration.

Coalescing Filter Skid Controls — Each of the twofull capacity coalescing filters is furnished witha level controller and integral drain valve. Thecontroller maintains a minimal level in thesump by continuously opening and closing thedrain valve. Collected liquids are discharged tothe Drain Tank. A single high level switch mon-itors sump level. An alarm within the plant’scontrol system will initiate upon activation ofthis switch. Each filter is also furnished with alocal level gage.

A high differential pressure switch monitors thepressure drop across the coalescing filter that isin use. Activation of this switch generates analarm in the plant controls indicating that aswitch over to the clean filter is required. Thegas outlet of each filter is furnished with a localpressure gage.

Electric Startup Superheater Controls — TheElectric Startup Superheater controls are con-

figured to achieve the desired gas fuel temper-ature at the heater outlet based on the temper-ature differential across the heater. The con-trols are set to maintain a constant differentialtemperature with a maximum temperaturelimit.

The constant differential is the differencebetween the minimum supply gas temperatureand the minimum superheat level abovethe fuel’s dewpoint. All control functions areper-formed locally by a dedicated SCR con-troller.

Gas Fuel Heater Skid Controls — The gas temper-ature controls regulate and monitor tempera-ture of the gas fuel supply to the turbine.Temperature elements and transmitters are fur-nished at the gas side and waterside inlets tothe gas fuel heater and on the gas side outlet.Signals provided by these instruments are sentto the control system. These signals are used tomodulate the flow control valve located at thewaterside outlet of the heater in order to attainthe desired gas fuel temperature.



Figure 9. Standard Gas Fuel Scrubber

SummaryThis publication was developed to (a) identifythe requirements of the gas turbine with respectto the gas fuel heating systems, and (b) providea descriptive overview of GE’s standard Gas FuelHeating System. This standard system has beendeveloped to meet these requirements, whileinsuring safe and reliable gas turbine andpower plant operation.

Due to the nature of this system, it is imperativethat the detailed system incorporates means ofpersonnel protection. This includes, but is notlimited to, the discharge direction of pressuresafety relief valves, the inclusion of personnel

protection insulation and the prevention of gasfuel from entering and “hiding” in the plant’ssteam and feedwater system.

References

1. “Process Specification –– Fuel Gases forCombustion in Heavy Duty Gas Turbines,”GE Power Systems, GEI-41040.

2. “Gas Fuel Clean-Up System DesignConsiderations for GE Heavy Duty GasTurbines,” GE Power Systems, GER-3942.

3. Section VIII, ASME Boiler and PressureVessel Code.



List of FiguresFigure 1. DLN-2+ (PG9351) fuel heating operational requirements

Figure 2. DLN-2+ FB (PG7251FB and PG9371FB) fuel heating operational requirements

Figure 3. DLN-2.6 fuel heating operational requirements

Figure 4. Typical Gas Fuel Heating System Flow Diagram

Figure 5. Heat exchanger leak detection control scheme

Figure 6. Standard Coalescing Filter Skid

Figure 7. Standard Electric Startup Superheater

Figure 8. Standard Gas Fuel Performance Heater Skid

Figure 9. Standard Gas Fuel Scrubber

List of TablesTable 1. Combustion design to turbine model cross reference



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