lm2500_fuelctrl

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Fuel Control Requirements

Fuel Scheduling at Steady State Operation

For steady state operation, the fuel control provides the correct fuel flow for liquid fuel, natural gas, ordual fuel operation. As described in Fuels, Water, Steam, there are various fuel systems dependent onfuel type and the use of water/steam for NO suppression or steam for power augmentation. Themaximum fuel flow is limited by various operational limits, which are described in Table 1. Theminimum fuel flow is set by the gas generator idle speed, which has a minimum value of 6800 rpm. Asingle fuel selection is required for operation below gas generator idle speed.

For dual fuel operation, the 7LM2500-PE/PH model gas turbines are designed to operate with no limiton natural gas/liquid fuel ratios. The gas turbine can be started on either natural gas or on liquid fuel, butnot on a combination of natural gas/liquid fuel. In case one fuel is cutoff, the minimum fuel flow for

each fuel should be selected so that the governor action will prevent a decrease in gas generator speedbelow 6800 rpm.

The required fuel pressure at the engine manifold is defined in Fuel for specific fuel, water, and steamsystems.

Starting Fuel Flow and Sub-idle Operation

Starting fuel flow limits have been established for both gaseous and liquid fuels and are defined onFigure 1. The minimum fuel flow shall be introduced at 1200 + 100 rpm gas generator speed for liquidfuel simultaneously with ignition while accelerating on the starter whose acceleration torque is defined

in Starting. For gas fuel, the minimum fuel flow should be introduced at 1700 + 100 rpm gas generatorspeed simultaneously with ignition while accelerating on the starter. Continued acceleration to gasgenerator idle shall be controlled by the combination of the fuel flow schedule defined by Figure 1 andthe starter torque defined in Stating until starter cutout speed is achieved.

Total acceleration time will depend on the volume of the fuel supply lines, reference Fuels andOperation.

Starting can be achieved following a high speed purge cycle commonly used on gas fueled installationsprovided that the starting flow is controlled using the sub-idle acceleration flow schedule defined byFigure 1.

Figure 1. LM2500-PE/PH Sub-idle Acceleration Fuel Schedules (Sea Level).

Starting Fuel Flow andSub-idle Operation

Transient Fuel Flow WF versus P3 ScheduledN/dt Schedules (All

Models)Effect of Deviation

from SchedulesTemperature Control

(T5.4)

x

Page 1 of 11Controls - Fuel Scheduling

19/11/2005http://inside-aep.ps.ge.com/insideaep/aep/iad/engine_prod_delv/idms/lm2500/11fuelsche...


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Transient Fuel Flow

During normal operation the fuel to the gas turbine will be controlled by a power turbine speed/loadgovernor. For normal operation, gradual load changes are strongly recommended (2 to 3 minute rampfrom idle to max power). This is also true for load reductions (2 to 3 minute ramp from max power to

idle).

Sudden load changes, e.g. load rejection or step load increases, require much faster transient responsefrom the engine and will drive the engine in the direction of stall, turbine over temperature or flameout.Accordingly, control of fuel during these transients and also during startup operation, requires additionalcontrol functions and schedules independent of the speed governor, power turbine control, and steadystate operating limits. The acceleration and deceleration schedules presented in this section have beenestablished for the LM2500 gas turbine to control operation during startups and rapid load changes andmaintain adequate margins to avoid stall, turbine over temperature and flameout.




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NOTE: Also, a gas generator speed (NG) governor is often used to control the gas generator idle speedto the power turbine speed/load minimum setting.

Two acceptable methods for limiting fuel flow on transients are discussed in this section, namely:

l. Accel/decel schedules in the form of fuel flow (WF) versus compressor discharge pressure (PS3).

2 Accel/decel schedules in the form of rate of change of gas generator rotor speed (dNG/dT) as afunction of corrected gas generator rotor speed (NG).

Details of the Purchaserstransient control methods should be reviewed with GE for all newapplications.

WF versus P Schedule

The WF vs PS method has been used extensively in control of aircraft engines and on some LM2500models without steam injection.

Typically, startup is on a WF versus PS acceleration schedule as shown in Figure 1 . Transition is madearound gas turbine idle, to NG gas generator speed governor, and a further transition at synchronous no-load condition to power turbine control basis.

The transient fuel flow requirements and limitations for the Model 7LM2500-PE-MG/NG, on a heatconsumption basis, are presented in Figures 2 through 4 . These figures provide specific data at inlettemperature distortion levels of 0%, 3%, and 6%, and at ambient temperatures of -65, 59 and l30F atsea level. Distortion level definition is provided in MID-TD-2500-l.

The transient fuel flow requirements described in Figures 2 through 4 are based on sea level altitude.Altitude corrections to the Purchasersacceleration and deceleration schedules may be made by

adjusting the schedules as follows:

Schedule at altitude = Schedule at sea level x

where:

= ratio of site to sea level atomospheric pressure

These curves are applicable to either gaseous or liquid fuels by using the appropriate lower heating valuefor the fuel to be used. The format presented includes the engine maximum operating line, the MFS(maximum fuel schedule) limit, and a line indicating the threshold of lean blowout. The Purchasersacceleration schedule, giving proper consideration to the pertinent application tolerances (controlaccuracy, heating value variation, gas supply pressure, temperature variation, etc.), should be

appropriately located between the maximum operating line and the MFS maximum limit.

Acceleration fuel rates greater than those shown may result in over temperature and possible stall of thegas generator. Acceleration fuel rates lower than those shown may result in hung starts in the belowidle speed range. Above the idle speed point, low acceleration fuel rates will not harm the gas generatorbut will result in slower acceleration times. The fuel or heat consumption rates shown on the curves willgive starting times to 5000 rpm idle of 60 to 90 seconds, and accelerations from idle to maximum powerof approximately 15 seconds. For normal accelerations from idle to power settings, the changes shouldbe made more slowly to improve the time between hot section repair intervals and the life of the gasturbine. A time of 2 to 3 minutes from idle to load/power is reasonable.

3

3

3




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The deceleration schedule should be set to the threshold of lean blowout as described in Figures 2through 4 for the Model LM2500-PE, subject to any constraints or limitations imposed by the systemdynamics for a specific application, i.e., power generation or mechanical drive. The threshold of leanblowout is defined as the nominal minimum deceleration schedule to preclude flameout during a rapidfuel flow reduction. Richer deceleration schedules may be utilized; however, they may cause overspeedof the power turbine upon the instantaneous rejection of 100% load. Normal decelerations frommaximum power to idle should be made on the governor, and should take 2 to 3 minutes. This will

improve the time between hot section maintenance and the life of the engine.

A deceleration schedule based on the threshold of lean blowout will result in little possibility offlameout under normal transient operating conditions, and should be suitable for most applications.

For some specific applications, however, notably in power generation, it may be necessary to establish aleaner deceleration schedule based on a determination of the actual engine system blowoutcharacteristics and limits in order to prevent an excessive overspeed condition of the power turbine andconnected equipment during the transient immediately following an instantaneous load rejection.

Since actual blowout is dependent upon a number of variables including the specific fuel characteristics,

fuel composition, fuel temperature, fuel control system slew rate, etc. the actual lean blowout limit mustbe determined using the contract control system and actual site fuel. For reference it should be noted thatengine health and condition are not affected by blowout in this low fuel/air ratio area of operation.

During transient operation, when the fuel flow is being controlled by either the acceleration ordeceleration fuel schedule, the transient sensor response error from nominal schedule fuel flow shall notbe greater than that of an equivalent first order lag having a time constant of 0.050 seconds for all ratesof change of PS3 (dPs3/dt) within the limits of 400 psi/sec (increasing) and 1000 psi/sec (decreasing).

Figure 2. LM2500 Acceleration and Deceleration Fuel Flow Requirements at Sea Level, -65F InletTemperature.




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Figure 3. LM2500 Acceleration and Deceleration Fuel Flow Requirements at Sea Level, 59F InletTemperature.




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Figure 4. LM2500 Acceleration and Deceleration Fuel Flow Requirements at Sea Level, 130F InletTemperature.




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dN/dt Schedules (All Models)

The dNG/dT versus NG method has been used extensively in control of industrial gas turbines, andprovides more flexibility to accommodate changes in parameters such as fuel heating value and steaminjection rates. This method also is compatible with changes in gas turbine flow function, allowing oneset of schedules to be used on both PE and PH models without modification.

Rate of change (dNG/dT) acceleration and deceleration schedules are used to limit fuel flow duringrapid load change transients.

Figures 5 and 6 show the acceleration and deceleration schedules as a function of corrected gasgenerator speed (NGR).

These schedules may be used to limit transient operation for all models with and without steaminjection.

Figure 5. LM2500-PE/PH Maximum Acceleration Schedule.




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Figure 6. LM2500-PE/PH Deceleration Schedule.




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Effect of Deviation from Schedules

The deceleration schedule is set to preclude flameout during rapid fuel flow reduction. Richerdeceleration schedules may be used but will result in increased over speed of the power turbine oninstantaneous full load rejections.

Acceleration schedules shown are set to prevent over temperature or stall of the gas turbine on rapidload increases. The schedules shown will typically yield starting times to gas turbine idle (NG = 6800rpm) of approximately 60 to 90 seconds. Leaner schedules will result in longer times to start. If theschedule is too lean, hung starts will result.

Acceleration of the gas turbine from idle to maximum power at the maximum acceleration will takeapproximately l5 seconds. (As discussed previously, during normal operation, load changes should bemade slowly, taking 2 to 3 minutes from idle to maximum power. Gradual load changes are beneficial tohot section life).

Temperature Control, Low Pressure Turbine Inlet (T5.4)

T5.4 Limit

The control system must limit the power turbine inlet gas temperature, both during transient and steadystate operation, to the maximum level established in the applicable Contract Documents and/or as




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established in Table 1, LM2500 Operating Limits and PurchaserInterlocks. The T54 Field ControlLimit listed in Table 1 is the basic field-operation control parameter and is the value to be used in thecontrol system design. An alarm value is given only to provide margin so that the alarm will notannunciate when operating at the control limit. The Field Trip value is considered to be a safety limitabove which the engine should never be operated.

T5.4 control limits for PE and PH model series gas turbines should be set in accordance with the

application contract documents, and/or Table 1.

The air flow in the starting range is relatively low, 0 to 9 pps, and therefore, the thermocouple responsein this area is slow. Consequently the use of this parameter for scheduling starting fuel flow is notrecommended.

T5.4 Measurement

The T5.4 system provides eleven individual probe outputs which must be electronically processed toobtain an average indicated T5.4 for use in the control system. There are a variety of approaches whichcan be used to calculate an unbiased indicated average T5.4 for control purposes. These include:

1. Arithmetic average of non-rejected thermocouples.

2. Median reading of all eleven thermocouples.

3. Special hybrid algorithms used in multiplex computer control systems.

Regardless of approach, control action, as prescribed in Table 1, should be taken in the event ofexcessive number of thermocouple rejections.

Thermocouple Rejection Criteria for Average Calculation

An advantage of the individual readout systems is that they facilitate recognition of excessively lowreadings, such as those caused by faulty thermocouples. These readings, if not rejected, will bias theaverage temperature indication and result in over firing of the gas turbine.

If indicated T5.4 temperature is obtained by averaging individual thermocouple outputs, the followingrules should be used for rejecting a thermocouple output from the average.

1. Reject any thermocouple with complete loss of signal.

2. Reject any thermocouple reading 200F below the average. Control on average of the remaining

thermocouples.

Median Temperature for Control

A characteristic of median temperature for control is that median temperature still provides an unbiasedtemperature indication even when small numbers of faulty thermocouples are not rejected.

These systems should, however, still detect faulty thermocouples and take control actions specified inTable 1.




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Temperature Spread Monitoring

Individual readout control thermocouple harnesses also facilitate the monitoring of temperature spreadsfor diagnostic and maintenance purposes.

Recommended limits on Tmax- Tmin are given in Table 1. Only obviously faulty thermocouples shouldbe rejected from this calculation, since low reading thermocouples can be an indication of a fuel flowdistribution problem or a gas path problem.

While continued operation is allowed with faulty thermocouples, as previously discussed, meaningfulcondition monitoring with spread measurements requires all thermocouples be operational. Any faultythermocouples should be replaced at earliest opportunity.
