g
GE Aircraft Engines
__________________________Marine & Industrial Control DynamicsLM2500 AND LM6000
DRY LOW EMISSIONS
CONTROL OVERVIEW
Prepared by : Peter Harrison
27th March, 1997
‘Even-b800d\Data\Users’[S:]’\General\Common\Dle_fld\dleovr.doc
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Dry Low Emissions Control Overview
Table of Contents
1. OVERVIEW
6
1.1. Dry Low Emissions Combustor 6
1.2. Dry Low Emissions Control Components 10
1.3. Fuel System 12
1.3.1. Four Valve System 12
1.3.2. Three Valve System 13
1.4. Bleed System 14
1.5. Fuel Control 15
1.6. Flame Temperature Control 15
1.6.1. Bulk Flame Temperature 15
1.6.2. A and C Ring Flame Temperature 17
2. FLAME TEMPERATURE ALGORITHM
19
3. FUEL PROPERTIES
25
3.1. Effect of fuel properties 25
3.2. Ring fuel nozzle scalars 27
3.3. Calorimeter and Chromatograph Operation 27
4. STARTING
28
4.1. Start Sequence 28
4.2. Control Operation 29
5. OPERATION AT IDLE
36
6. OPERATION WITHIN A COMBUSTOR CONFIGURATION WINDOW
38
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8. COMBUSTOR STAGING
43
8.1. General 43 8.2. Starting 44 8.3. LM6000 Core Idle-Sync-Idle 448.4. Idle and Above Operation 44
8.5. LM6000 BC to AB zone avoidance 46
8.6. LM6000 BRNUL upper limit 46
8.7. ABC to AB stage down - LM2500 46
8.8. ABC to AB stage down - LM6000 47
8.9. Load drop/ overspeed 48
TABLE OF FIGURES
FIGURE 1.1 DLE COMBUSTOR 6
FIGURE 1.2 26-VALVE FUEL MANIFOLD 7
FIGURE 1.3 STAGING VALVE LAYOUT 7
FIGURE 1.4 COMBUSTOR CONFIGURATIONS 8
FIGURE 1.5 DLE ENGINE WITH COMBUSTOR STAGING VERSUS NON-DLE ENGINE 8
FIGURE 1.6 DLE ENGINE WITH COMBUSTOR STAGING AND BLEED MODULATION 9
FIGURE 1.7 DLE BLEED VALVE 11
FIGURE 1.8 FOUR VALVE FUEL SYSTEM 12
FIGURE 1.9 THREE VALVE FUEL SYSTEM 13
FIGURE 1.10 EFFECT OF COMBUSTOR CONFIGURATION AND COMPRESSOR BLEED 14
FIGURE 1.11 BULK FLAME TEMPERATURE WINDOW 16
FIGURE 1.12 SPECIFICATION LM6000 RING AND BULK FLAME TEMPERATURE SCHEDULES 17
FIGURE 2.1 LM2500 DLE FLAME TEMPERATURE ALGORITHM 19
FIGURE 2.2 FLAME TEMPERATURE ALGORITHM/AIRFLOW CONTROL INTERFACE 21
FIGURE 2.3 LM2500 FLAME TEMPERATURE SENSITIVITY 22
FIGURE 2.4 LM2500 FLAME TEMPERATURE SENSITIVITY DUE TO PS3 VARIATION 23
FIGURE 2.5 LM2500 FLAME TEMPERATURE SENSITIVITY DUE TO WFAGMV VARIATION 23
FIGURE 2.6 LM2500 FLAME TEMPERATURE SENSITIVITY DUE TO LHV 23
FIGURE 2.7 LM2500 FLAME TEMPERATURE SENSITIVITY DUE TO SG 24
FIGURE 4.1A TYPICAL LM2500 DLE START CHARACTERISTICS 32
FIGURE 4.2A TYPICAL LM6000 DLE START CHARACTERISTICS 34
FIGURE 6.1 LM2500 BLEED SEQUENCE 39
FIGURE 6.2 LM6000 BLEED SEQUENCE 40
FIGURE 6.3 LM6000 VBV BLEED MODULATION 40
FIGURE 7.1 RING FUEL FLOW DEMANDS 41
FIGURE 8.1 COMBUSTOR STAGING DURING LOAD ACCELS 45
FIGURE 8.2 COMBUSTOR STAGING DURING LOAD DECELS 45
FIGURE 9.1 ACOUSTICS/BLOWOUT AVOIDANCE LOGIC 49
FIGURE 9.2 BLOWOUT DETECTION ALGORITHM WF/PS3 ERROR CALCULATION 50
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List of Tables
TABLE 1.1 ADDITIONAL LM2500 CONTROL COMPONENTS FOR DLE APPLICATION 10
TABLE 1.2 ADDITIONAL LM6000 CONTROL COMPONENTS FOR DLE APPLICATION 10
TABLE 1.3 FUEL CONTROL REGULATORS 15
TABLE 4.1 START-RUN SEQUENCER OUTPUTS 28
TABLE 4.2 LM2500 START-RUN ENGINE MODES AND CONTROL ACTIONS DURING START TO IDLE 29
TABLE 5.1 LM2500 TYPICAL CORE IDLE PARAMETERS 37
TABLE 5.2 LM6000 TYPICAL CORE IDLE PARAMETERS 38
TABLE 7.1 DISPLAYED FLAME TEMPERATURES 43
TABLE 8.1 STAGING CONTROL PARAMETERS 44
TABLE 8.2 LM6000 BRNUL T3 SWITCH POINTS 46
TABLE 9.1 ABAL CORRECTIVE ACTION 51
TABLE 9.2 ABAL SPECIAL FEATURES 52
1.
Overview
1.1.
Dry Low Emissions Combustor
The LM2500 and LM6000 Dry Low Emissions (DLE) gas turbines employ a triple annular combustor. Figure 1.1 shows the basic combustor configuration.
A B C
End view showing the 75 cups Cross section of 3 cup assembly
Figure 1.1 DLE combustor
Gas fuel is introduced into the combustor via 75 air/gas premixers packaged in 30 externally removable and replaceable modules. The premixers produce a very uniformly mixed lean fuel/air mixture. Half of these modules have two premixers and the other half have three. The 75 premixers, or cups, as they are often referred to for the DLE, are arranged in three rings or domes. The middle ring is referred to as the pilot or the B ring and has 30 cups. The pilot ring is always fueled. The inner ring is referred to as the C ring and has 15 cups, whereas the outer ring, which is referred to as the A ring, like the pilot has 30 cups. Unlike the pilot ring, fuel to the cups in the inner and outer rings has to be turned on and off by means of staging valves. This is because of the limited flame temperature (or fuel-air ratio) range over which the combustor can operate. The flame temperature range is limited by thermal stress limits on the high side and lean blowout on the low side. The minimum bulk or average flame temperature for an LM6000 ranges from approx. 3300 deg F at no load sync idle to approx. 2900 deg F at maximum power, whereas the maximum bulk or average flame temperature ranges from approx. 3450 deg F at no load sync idle to approx. 3000 deg F at maximum power. With such a limited flame temperature operating range, it is necessary to “stage” the combustor, i.e. it is necessary to turn sections of the combustor “on” and “off”. In the current design, 15 staging valves supply the inner ring, one cup per staging valve, and 10 staging valves supply the outer ring, three cups per staging valve. One additional staging valve, as described later, is used to control the fuel flow level to what was originally referred to as an enhanced lean blowout (ELBO) circuit, that is connected to 15 of the 30 pilot cups. This brings the total number of staging valves to 26. The staging valves are
mounted on the fuel manifold assembly as illustrated in Figures 1.2 and 1.3.
A B
C
AFT LOOKING FORWARD
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7 Figure 1.2 26-valve fuel manifold
outer inner ELBO Forward looking AFT 26 25 24 23 22 Ignitor 21 20 19 Boroscope 18 17 16 15 14 13 12 11 10 9 8 Ignitor 7 6 Boroscope 5 4 3 2 1
Figure 1.3 Staging Valve Layout
Section of LM6000 PB showing staging valves mounted on fuel manifold Staging
In the near future, as part of a cost reduction initiative, the LM2500 DLE will change to a new fuel manifold and staging valve configuration which uses fewer staging valves, 5 for the inner and 5 for the outer, which, when the ELBO staging valve is added, brings the total to 11. The two configurations are often referred to as the 26-valve and 11-valve (or 5/5/1) systems respectively. The staging valves allow different fueling configurations for the combustor, ranging from B-only for starting and idle operation, to fueling of all three rings (ABC) for operation at high power. As mentioned earlier, different combustor configurations are required to keep the combustor flame temperature within limits. The different combustor configurations are shown in Figure 1.4
(Starting only for LM2500) Figure 1.4 Combustor configurations
Figure 1.5 shows the operating line of an LM6000 DLE engine employing combustor staging, compared with a conventional non-DLE engine.
1600 1800 2000 2200 2400 2600 .0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 GENERATOR MEGAWATTS
LM6000 DLE ENGINE ZERO BLEED versus NON-DLE ENGINE
BC/2
BC
AB
ABC
NON DLE ENGINE MIN TFLAME
MAX TFLAME
Figure 1.5 DLE engine with combustor staging versus non-DLE engine
It is clear from Figure 1.5 that there is a limited operating power range for each combustor configuration. Operating at a higher power than intended in a given combustor configuration means exceeding the max allowable average flame temperature and can result in extensive damage to the combustor. Attempting to
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there are “gaps” between each configuration, i.e. power regions in which the DLE engine could not run. This is overcome by using compressor bleed as illustrated in Figure 1.6.
1600 1800 2000 2200 2400 2600 .0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 GENERATOR MEGAWATTS
LM6000 DLE Combustor Operating Modes
BC/2
BC
AB
ABC
Typical Mode Maps
at T2=47F
Based on OrangeCo MA XB LEE D BLEED NOBLE EDFigure 1.6 DLE engine with combustor staging and bleed modulation
Another important requirement for the triple annular DLE combustor is the ability to independently vary the combustor flame temperature of each ring. This is achieved by individual control of the total fuel flow to each ring.
1.2.
Dry Low Emissions Control Components
The DLE application for both the LM2500 and LM6000 requires additional control components over and above those required for their non DLE counterparts. These additional control components are listed in Tables 1.1 and 1.2 .
Description Type Used for
Engine inlet temp (T2) One dual element RTD Flame temperature control
Compressor exit temp. (T3) One dual element TC Flame temperature control
Flame detector Two UV detectors Combustor lightoff detection
Acoustic sensor (PX36) Two piezoelectric charge Flame temperature trim
sensors
Staging valves Twenty six solenoid operated Combustor staging
valves with switch position f/b
Eighth stage (ST8) bleed valve One hydraulically operated Flame temperature control
valve with LVDT dual f/b
Compressor discharge (CDP) One hydraulically operated Flame temperature control
bleed valve valve with LVDT dual f/b
Table 1.1 Additional LM2500 control components for DLE application
Description Type Used for
Compressor exit temp. (T3) Two dual element TC’s Flame temperature control
(new location for DLE) Power management
Compressor exit press (PS3) Two transducers Flame temperature control
(new location for DLE) IGV scheduling
Power management Stall detection
Acoustic sensor (PX36) Two piezoelectric charge Flame temperature trim
sensors
Staging valves Twenty six solenoid operated Combustor staging
valves with switch position f/b
Eighth stage (ST8) bleed valve One hydraulically operated Flame temperature control
valve with LVDT dual f/b
Compressor discharge (CDP) One hydraulically operated Flame temperature control
bleed valve valve with LVDT dual f/b
Table 1.2 Additional LM6000 control components for DLE application
The pressure and temperature sensors are conventional and are described in more detail in the Control System Specifications (M50TF3740 and M50TF3731 for the LM2500 and LM6000 respectively) and the Installation Design Manuals (MID-IDM-2500-10 and MID-IDM-6000-3 for the LM2500 and LM6000 respectively). Note that the LM6000 T3 sensor and PS3 pressure tap locations for the DLE engine are
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piezoelectric charge devices similar to vibration monitoring accelerometers but are used to sense dynamic pressures in the combustor. A purchaser-supplied monitoring system is used in conjunction with the pressure transducers to provide a signal to the control system. The staging valves mounted on the gas manifold are electrically activated and are de-energized open. There are two suppliers for the staging valves, and although valves from the two suppliers look different, they can be intermixed. The eighth stage and compressor discharge bleed valves are located off-engine and each comprise a torque motor servo valve, actuator, LVDT, and air valve. The LM2500 uses 4.0 inch diameter air valves for both eighth stage bleed and compressor discharge bleed. The LM6000 on the other hand uses a 2.5 inch diameter air valve for compressor discharge bleed and a 6.0 inch diameter air valve for eighth stage bleed. A bleed valve assembly is shown in Figure 1.7.
Figure 1.7 DLE bleed valve
Airflow
LVDT Torquemotor connector Hyd supply and return1.3.
Fuel System
For the DLE application a three-ring high accuracy fuel system, with associated sensors, is required. Two different fuel system configurations are currently in service. The first DLE gas turbines used a four valve system. More recent units have a three valve system.
1.3.1. Four Valve System
The four valve fuel system comprises a single main, or total, metering valve and three trim, or delta P regulator, valves, as illustrated in Figure 1.8. The system is described in more detail in Section 7.
PGAS Measurement FUEL SHUT-OFF (Provided by Packager) GP1, TFUEL Measurement
Outer Manifold Trim Valve
Staging Valve Staging Valve Staging Valve A M a n ifo ld B M a n ifo ld C M a n ifo ld
Ten sets per engine
Cup Cup Cup Cup Cup Every other Pilot Cup Thirty per engine
Fifteen per engine
Fifteen sets per engine
(Outer Ring) (Pilot Ring) (Lean Blow-out Enhancement) (Inner Ring) LM2500 Fuel System Schematic
Four Valve Trim Fuel System
Pilot Manifold Trim Valve
Outer Manifold Trim Valve Fuel Metering Valve
GP2 Measurement Outer Manifold Pressure (GP3O) Pilot Manifold Pressure (GP3P) Inner Manifold Pressure (GP3I)
Figure 1.8 Four valve fuel system
The main metering valve is positioned in response to a total fuel flow demand, whereas the trim valves are positioned in response to delta P demands. As described later, the delta P demands are calculated in the control based upon the relative fuel flows required in each ring.
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1.3.2. Three Valve System
The three valve fuel system is more straightforward in that it employs three metering valves that are independently positioned in direct response to the fuel flows required in each ring. The three valve fuel system requires two orifices to be connected, one between the pilot manifold and the outer manifold, the other between the pilot manifold and the inner manifold. These orifices limit the manifold pressure build up in a non flowing ring. This reduces the initial fuel flow pulse, and therefore flame temperature, when a ring is first fueled (i.e. the first staging valve is opened). The three valve fuel system configuration is shown in Figure 1.9 and described in more detail in Section 7.
( ) ( ) ( ) PGAS Measurement FUEL SHUT-OFF (Provided by Packager) TFUEL Measurement
FUEL SYSTEM PACKAGER GE GP1O -Outer Inlet Pressure GP2O - Outer Manifold Pressure Outer Metering Valve Inner Metering Valve Pilot Metering Valve GP1P- Pilot Inlet Pressure GP2P - Pilot Manifold Pressure GP1I- Inner Inlet Pressure GP2I - Pilot Manifold Pressure Pressure Relief Orifice Optional I/F Optional I/F Optional I/F
Ten sets per engine
Cup Cup Cup Cup Cup Every other Pilot Cup Thirty per engine
Fifteen per engine
Fifteen sets per engine
(Outer Ring) (Pilot Ring) (Lean Blow-out Enhancement) (Inner Ring)
LM2500 Fuel System Schematic Three Metering Valve System
Staging Valve Staging Valve Staging Valve A M a n ifo ld B M a n ifo ld C M a n ifo ld
1.4.
Bleed System
As mentioned earlier, in order to limit the variation in combustor flame temperature, the combustor configuration is changed from B-only for starting and idle operation to ABC for high power operation. However, this alone is not sufficient to keep the combustor flame temperature between the blowout and thermal stress limits. Changing combustor configuration changes the local fuel-air ratio in each cup by changing the fuel flow to each cup. Another way to change a cup fuel-air ratio is by varying compressor bleed in order to change the combustor airflow. By changing combustor configuration and modulating compressor bleed, local fuel/air ratio, and hence flame temperature, can be kept within limits across the entire power range, as illustrated in Figure 1.10.
Figure 1.10 Effect of combustor configuration and compressor bleed
For the DLE gas turbine, two bleed valves are added (eighth stage compressor bleed and compressor discharge bleed). For the LM6000 an existing variable bleed valve (VBV) is also used to provide additional bleed air modulation.
INCREASINGBLEED
Staging Transition Points B BC/2 mode BC AB ABC Min Tflame Min Bleed Max Tflame Max Bleed POWER
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LM2500 LM6000
REGULATOR
Power turbine speed 1 Power turbine speed
Gas generator speed 2 Core speed
Gas generator decel speed rate 3 Core decel speed rate Gas generator accel speed rate 4 Core accel speed rate
Min fuel flow 5 Min fuel flow
Max turbine temperature 6 Max turbine temperature
Max gas generator speed 7 Max compressor discharge press.
Max fuel flow 8 Max compressor discharge temp.
May be used by control vendors 9 Max core speed
for application-specific purposes 10 Max fuel flow
1.5.
Fuel Control
Fuel control, in the context of the DLE control system, refers to that part of the digital control system that determines the total combustor fuel flow demand (WF36DMD). The total fuel flow demand is subsequently split into three ring fuel flow demands based upon individual ring (A and C) combustor flame temperature demands. The DLE fuel control is very similar to previous non DLE (single annular combustor) LM2500 and LM6000 fuel control systems. The only real change is the addition of maximum and minimum fuel flow limits that correspond to maximum and minimum bulk flame temperature limits. These limits are
encountered primarily during starting, decelerations in B-only configuration, operation at maximum power in ABC configuration and briefly during rapid transients.
The fuel control comprises a set of regulators and fuel flow limiters, that through a series of min/max selects, often referred to as the priority selection logic, output a single fuel flow demand (WF36DMD). Regulators adjust fuel flow to regulate an engine variable (power turbine speed, gas generator speed etc.), whereas fuel flow limiters directly apply upper or lower fuel flow limits to the fuel flow demand (min fuel flow, max fuel flow). Only one regulator or fuel flow limiter can be in control at any time. The regulators and fuel flow limiters for the LM2500 and LM6000 are listed in Table 1.3. The “REGULATOR” parameter, as shown in this table, is available in the control and can be monitored to determine which regulator (1 thru 8 for the LM2500 or 1 thru 10 for the LM6000) is active at any time.
Table 1.3 Fuel Control Regulators
1.6.
Flame Temperature Control
The original DLE control strategy, proposed for the LM6000, provided control of the average or bulk flame temperature. During development engine testing it became clear that potentially damaging high dynamic pressures in the 300 Hz to 700 Hz frequency range could occur with the DLE combustor. To avoid these high dynamic pressures, often referred to as combustor acoustics, and also ensure each fueled ring remained lit, it became necessary to control the flame temperature independently in each of
the rings. The net result was a strategy that independently controlled the A ring and C ring flame temperatures, as well as the bulk flame temperature.
1.6.1. Bulk Flame Temperature
For the bulk flame temperature for each combustor configuration a flame temperature window is defined as illustrated in Figure 1.11.
“STAGING UP” TRANSITION
POINT
MIN BLEED
MAX BULK FLAME TEMP/FUEL FLOW LIMIT
INCREASING TFLAME INCREASING BLEED “STAGING DOWN” TRANSITION POINT MAX BLEED AIRFLOW CONTROL REGULATION
Figure 1.11 Bulk flame temperature window
The upper boundary (TFLMAX) in general indirectly determines the maximum pilot flame temperature and hence also the maximum NOX level, whereas the lower boundary (TFLMIN) in general indirectly sets the pilot lean operating line. The bulk maximum and minimum flame temperatures (TFLMAX and TFLMIN) are scheduled in the control as a function of combustor configuration and T3. The left-hand or low power boundary is defined by the maximum compressor bleed capability, and the right-hand or high power boundary corresponds to zero bleed. As power varies the control adjusts bleed so that the bulk flame temperature is maintained at a demanded level between the min and max limits, until either the maximum or zero bleed limit is reached. This control concept is used for both the LM2500 and LM6000. For the LM2500 the bulk flame temperature is maintained at the “50% level” (i.e. mid way between the min and max limits), whereas for the LM6000 the bulk flame temperature is maintained at the “50% level” for operation in the B, BC/2, and BC configurations, but is reduced to the “25% level” (i.e. closer to the min limit), whenever possible, in the AB and ABC configurations.
During power increases, bleed is progressively decreased, until zero bleed is reached, whereupon bulk flame temperature increases toward the maximum limit. Just before the maximum limit is reached, unless already in the ABC configuration, staging to the next combustor configuration “up” is initiated. During power decreases, the bulk flame temperature is maintained at the demanded level until maximum bleed is
reached, whereupon the bulk flame temperature decreases toward the minimum limit. Just before the minimum limit is reached, unless already in the B configuration, staging to the next configuration “down” is initiated.
MIN BULK FLAME TEMP/FUEL FLOW LIMIT
POWER,T3 T4
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1.6.2. A and C Ring Flame Temperature
As mentioned earlier, to avoid high combustor dynamic pressures etc., it became necessary to control the flame temperature independently in the A and C rings. Unlike the bulk flame temperature, which is controlled between maximum and minimum limits, the A and C ring flame temperatures always track reference schedules. The ring reference schedules are, like the bulk flame temperature min and max schedules, programmed in the control as a function of combustor configuration and T3. Figure 1.12 shows a typical set of ring and bulk flame temperature schedules for a LM6000.
2600 2700 2800 2900 3000 3100 3200 3300 3400 3500 0 100 200 300 400 500 600 T3 (deg F) T fl a m e (d e g F ) TFLAME MAX TFLAME MIN BC/2 Mode 2600 2700 2800 2900 3000 3100 3200 3300 3400 3500 200 300 400 500 600 700 800 T3 (deg F) T fl a m e (d e g F ) TFLAME MAX TFLAME MIN TFLAME INNER
Figure 1.12a Specification LM6000 ring and bulk flame temperature schedules - B and BC/2 modes
B C M o de 2800 2900 3000 3100 3200 3300 3400 3500 3600 3700 400 500 600 700 800 900 1000 T3 (deg F ) T fl a m e (d e g F ) TFLA M E M A X TFLA M E M IN T FLA M E IN N E R A B M o de 2600 2700 2800 2900 3000 3100 3200 3300 3400 3500 600 700 800 900 1000 1100 1200 T3 (deg F) T fl a m e (d e g F ) TF LA M E M AX TF LA M E M IN TFLA M E O U TE R A B C M o de 2600 2700 2800 2900 3000 3100 3200 3300 3400 3500 600 700 800 900 1000 1100 1200 T 3 (d eg F ) T fl a m e (d e g F ) TFLA M E IN N E R TFLA M E M A X TFLA M E M IN TFLA M E O U TE R
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2.
Flame Temperature Algorithm
Because the combustor flame temperature cannot be measured directly in a reliable and accurate manner, it is estimated based on fuel flow demands and a “physics” based calculation of combustor airflow. The algorithm comprises four main sections, as illustrated in Figure 2.1.
Figure 2.1 LM2500 DLE flame temperature algorithm
to staging control and airflow control (lm6000 T3 BRNDMD P2SEL T2SEL P3SEL T3SEL WB3Q WB26Q NGGSEL NGGDOT LHVSEL WF36DMD AIRFLOW CALCULATION BULK TFLAME TO FUEL FLOW
OUTER & INNER RING TFLAME TO FUEL FLOW FUEL FLOW TO BULK TFLAME TFLMAX TFLMIN TFLOREF TFLIREF WF36DMD
}
}
F_TFLCYC to airflow control (lm2500 only) to fuel flow split demands WFMX WFMN WFOREFABC WFIREFABC SWCOMBI SWCOMBO SWCOMB F_PFL F_H3 F_WA36Engine inlet pressure Engine inlet temperature Comp. discharge pressure Comp.discharge temp CDP bleed flow Stage 8 bleed flow Gas gen speed Gas gen speed rate Fuel lower heating value Total fuel flow demand
}
air flow splits Control Display TFLAMEPCT = F_TFLCYC - TFLMIN TFLMAX - TFLMIN *100Engine sensor and control information is used to calculate the combustor airflow (F_WA36) based on an assumed HP turbine flow function. Once combustor airflow is known, then the combustor fuel flows (WFMX and WFMN for the bulk limits, and WFOREFABC and WFIREFABC for the ring demands) can be
calculated for the given scheduled flame temperatures (bulk TFLMAX and TFLMIN, and ring TFLOREF and TFLIREF respectively), and also bulk flame temperature (F_TFLCYC) can be calculated based on the current bulk or total fuel flow demand (WF36DMD). Note that the LM2500 uses the bulk flame temperature F_TFLCYC as feedback to the airflow control, whereas in the LM6000 the bulk flame temperature is for monitor purposes only and feedback to the airflow control effectively is derived from WFMX and WFMN. The differences between the LM2500 and LM6000 flame temperature algorithm/airflow control interface are illustrated in Figure 2.2.
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Figure 2.2 Flame temperature algorithm/airflow control interface TFLERR
F_TFLCYC
[from Tflame algorithms]
{
WFMN WFMX
WF/PS3 DEMAND [from Tflame alg.]
+
[from fuel control] WF36DMD
DWB36
[total bleed demand PS3EST
[from sensor compensation]
-+ N D P + I regulator PS3ERR PS3DMD [from fuel control] WF36DMDLM6000 IMPLEMENTATION:
-+ RegulatorP+I DWB36[total bleed demand]
LM2500 IMPLEMENTATION:
-N D P + I regulator[from Tflame algorithm] “WFMID”
Position in Tflame window [0.5 - 0.25] 1.0 PS3EST PS3ERR
LM6000 EQUIVALENT TO:
TFLDMDPrimary flame temperature influences include combustor fuel flow, fuel lower heating value (LHV), compressor discharge temperature (T3) and compressor discharge pressure (PS3). Note that errors in flame temperature due to fuel flow are a result of errors in the fuel metering system (i.e. differences between actual and demanded fuel flows). The accuracy, or more importantly, the consistency of the calculated flame temperature is of course influenced by all of the algorithm inputs. Sensitivity studies performed during the design and development of the LM2500 and LM6000 control systems illustrate the relative importance of all algorithm inputs. Errors in these inputs can cause unpredictable or erratic behavior of the overall system. Figure 2.3 provides a chart summarizing the influence of all control variables on flame temperature for the LM2500. This chart shows the average variation in bulk and ring flame temperatures for specific perturbations in each of the control variables. The perturbation magnitudes chosen are based on control specification accuracies. This chart clearly shows that compressor discharge pressure (PS3), fuel flow (WFAGMV) and lower heating value (LHV) have the biggest effect on flame temperatures. Figures 2.4, 2.5, and 2.6 show the sensitivity to these parameters for each combustor configuration. In the field, errors in SG as well as LHV are often encountered. Figure 2.7 shows the sensitivity to SG error. It should be noted that although variable stator vanes (VSV) appear to have a large effect on bulk flame temperature, this only occurs when the airflow is commanding the bleeds fully closed, and under these circumstances bulk flame temperature is not being regulated by the control.
Tflame Sensitivity 0 5 10 15 20 25 30 35 cdpsel (0.707%) Fpv(wfagmv) (1.0%) gp3isel (0.75 psi) gp3osel (0.75 psi) gp3psel (0.75 psi) K(wfagmv) (1.0%) lhvsel (1.0%) nggsel (10 rpm) p2sel (1.003 psi) ps3sel (3.162 psi) sgsel (1.0 %) st8sel (0.707%) swcmbi (1.0%) swcmbo (1.0%) swcmbp (1.0%) t2sel (1.414 degF) t3sel (6.5 degF) tfuelsel (2.83 degF) wb26q wb3q (5.0%) wf36innz (1.0%) wf36otnz (1.0%) wf36plnz (1.0%) wfagmv (3.0%) VSV (3 deg) P a ra m e te r
Tflame Variation (deg F)
Bulk Inner Outer
Figure 2.3 LM2500 Flame temperature sensitivity
PS3 Effect on Tflame
3.162 psia Change in PS3
0 20 40 60 80 100 120 140 160 IDLE NO HI LO NO HI LO NO HI LO NO deg F Bulk Inner Outer High “indicated” PS3 = low “indicated” TflameGE Proprietary Information - Technical Export License TSU/OTS March 27th, 1997
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Figure 2.4 LM2500 Flame temperature sensitivity due to PS3 variation
Metering Valve Fuel Flow Effect on Tflame 2.0% Change in WFAGMV for WF>2700 pph 3.0% Change in WFAGMV for WF<2700 pph
0 10 20 30 40 50 60 70 IDLE B NO B HI BC LO BC NO BC HI AB LO AB NO AB HI ABC LO ABC NO ABC d e g F Bulk Inner Outer
Figure 2.5 LM2500 Flame temperature sensitivity due to WFAGMV variation
LHV Effect on Tflame
1.0% Change in LHV
-35 -30 -25 -20 -15 -10 -5 0 IDLE B NO B HI BC LO BC NO BC HI AB LO AB NO AB HI ABC LO ABC NO ABC d e g F Bulk Inner OuterFigure 2.6 LM2500 Flame temperature sensitivity due to LHV
High “indicated” fuel flow = high “indicated” Tflame
Specific Gravity Effect on Tflame
1.0% Change in SG
-20
-15
-10
-5
0
5
10
IDLE B NO B HI BC LO BC NO BC HI AB LO AB NO AB HI ABC LO ABC NO ABC d e g F Bulk Inner Oute rFigure 2.7 LM2500 Flame Temperature sensitivity due to SG
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3.
Fuel Properties
3.1.
Effect of fuel properties
As already mentioned fuel lower heating value (LHV) directly affects the relationship between fuel mass flow and flame temperature. A 1% variation in LHV has the same effect on flame temperature as a 1% variation in fuel mass flow, i.e. it is Btu/hr that is important as far as flame temperature is concerned.
Other fuel properties affect fuel mass flow metering. The DLE application requires precise control of metered fuel flow, which in turn means precise characterization of individual metering valves. A gas metering valve is generally characterized by the following equations:
P1 Inlet absolute pressure [lbf/in2] P2 Discharge absolute pressure [lbf/in2] T1 Inlet fuel temperature [deg R] k Ratio of specific heats Cp/Cv [-]
Sg Specific gravity [-]
Ae Metering valve effective area [in2]
Wf Fuel mass flow [lbm/hr]
unchoked or choked flow
At idle and low power conditions for choked flow conditions, i.e. when < the above equation simplifies to: Pr = max P2 P1 , -k k-1 * Sg T1 * 3953.73 Ae P1* * * 1/2 Wf = unchoked choked P2 P1 2 1+k k/(k-1) 1/2 2 1+k (k+1)/k k/(k-1) 2/k Pr Pr
choked flow
In order to achieve the required fuel metering accuracy the fuel system supplier has to characterize each main metering valve in a four valve fuel system and the three metering valves in a three valve fuel system. The control has to be programmed with specific characterization tables for each installation.
Units are often a source of confusion, particularly those for LHV. In the GE control specification LHV has units of Btu/lbm. Often when working at field installations LHV is calculated in different units and requires conversion for use with GE control algorithms. The following conversions can be used under these circumstances:
1 joule = 0.7376 lb-ft = 1 N-m 1 Kj = 737.60 lb-ft
1 Btu = 778.169 lbf-ft
scf = standard cubic feet at 60 deg F and 14.696 psia scm = standard cubic meter at 60 deg F and 14.696 psia 1 scm = 35.3198 scf
ncm = normal cubic metre (standard for European compressor industry) at 0 deg C and 14.696 psia
Sg = specific gravity of gas
Wa = weight density of air at 60 deg F and 14.696 psia = 0.0764 (lbm/scf) Wa = weight density of air at 0 deg C and 14.696 psia = 0.0807 (lbm/ncf) Wg = weight density of gas = Sg * Wa
2 1+k (k+1)/(k-1) Sg k T1* 2797.52* Ae P1* * Wf = *
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3.2.
Ring fuel nozzle scalars
Gross errors in fuel properties affecting fuel metering accuracy, if not so large as to result in unstable control operation, may be found by monitoring the total ring fuel flow scalar WFNOZTOTFF for a 4-valve fuel system or the individual ring fuel flow scalars WFNOZOTRFF, WFNOZPILFF and WFNOZINRFF for a 3-valve fuel system. These variables are calculated in the control as the ratio of the demanded fuel flow to the estimated fuel nozzle flow. The estimated fuel nozzle flows are calculated based on assumed fuel nozzle flow functions. With a perfect metering system and exact fuel nozzle flow functions these scalars would be = 1.0. Values in the range 0.9 to 1.1 can be considered typical.
Once errors in fuel metering accuracy have been ruled out, by monitoring the fuel nozzle flow scalars as described above, gross errors in LHV if not so large as to result in unstable control operation may be found by comparing bulk flame temperature F_TFLCYC with the measured gas turbine temperature T54SEL or T48SEL
3.3.
Calorimeter and Chromatograph Operation
The DLE engine has a requirement for lower heating value and specific gravity inputs to schedule fuel flow accurately. The fuel system supplier might also have requirements to have other gas property inputs. Two types of commercially available instruments which provide some of these inputs are calorimeters and chromatographs.
A calorimeter takes gas from the pipeline and air and burns them under closely controlled conditions. The resulting exhaust gas temperature is measured by a precise sensing system. A controller regulates changes in the quantity of combustion and cooling air to the burner. This serves to maintain the exhaust gas at a constant temperature. So, if the heating value of the gas is changing, the required air flow will be modified by the controller to maintain constant exhaust gas temperature. By measuring the change in airflow, fuel gas properties can be determined. Typical outputs from a calorimeter are specific gravity, calorific value, and the Wobbe index. From the calorific value and specific gravity, the lower heating value can be calculated. Calorimeters do not provide a gas constituent breakdown - i.e. mole percent methane, mole percent O2, etc. Since calorimeters are continuously burning gas from the pipeline, they respond quickly to gas property changes. The time for calorimeters to respond to changes in gas properties is usually less than twenty seconds. Calorimeters would be used when changes in gas properties occur frequently.
A chromatograph provides a breakdown of the gas constituents. From the breakdown, the lower heating value and specific gravity can be calculated. A chromatograph typically consists of the following
components:
1. A carrier gas system for transporting the component through the column at a constant flow rate.
2. A chromatographic column for separating the sample into individual components.
3. A detector for detecting the components in the gas.
Gas from the pipeline is injected into the chromatograph. Inside the chromatograph, the pipeline gas is mixed with a carrier gas such as helium. The carrier gas takes the pipeline gas and flows it through the chromatographic column and the detector. The carrier gas needs to be inert to avoid interaction with the pipeline gas. The chromatographic column consists of around 30 feet of quarter inch diameter tubing. The column acts as a barrier to the gas flow. As the carrier and pipeline gas mix flow through the column, the gas constituents are broken down. The lighter gas constituents are able to move through the column more easily than the heavier ones. Thus, they are the first to exit the column. So as the gas sample exits the column, it is grouped by gas constituents according to their molecular weight, the lighter constituents exiting first. The sample then goes to the detector. Here, the type and percentage of each constituent is
(used in GE test cells) is called thermal conductivity. Here the gas sample from the column is passed through a form of Wheatstone Bridge where the four arms of the bridge are heated to a precisely controlled, high temperature. When the constituents pass by one pair of the bridge elements, the wires lose heat which translates to a bridge resistance change. The amount of resistance change determines the type and
percentage of each constituent. From the constituent type and percentage, the lower heating value, specific gravity, Wobbe index, ratio of specific heats, and compressibility can be calculated using a few formulae and the molecular weights of the constituents. The typical processing time for a chromatograph is around 5 minutes. Chromatographs should be used only where the gas properties are not changing rapidly or where knowing the other gas property information (ratio of specific heats, compressibility) is required.
4.
Starting
4.1.
Start Sequence
A description of the overall start sequence for the LM2500 and LM6000 can be found in Section 20.0 of the respective Installation Design Manual.
Outputs from the start-run sequencer are shown in Table 4.1. ENGMODE - Engine mode condition
STGVLVOPEN - Enable normal control of staging valves LM2500
Z_OP_STRTR - Operate starter LM2500
Z_OPEN_STRTR LM6000
Z_IGNDMD - Turn ignitor(s) on LM2500
Z_IGNITORDMD LM6000
Z_VENTDMD - Open gas vent valve(s) LM2500
Z_VENT1DMD LM6000
Z_VENT2DMD LM6000
FUELON - Command fuel on LM2500
Z_FSOV1DMD - Open shut off valve 1 Z_FSOV2DMD - Open shut off valve 2 Table 4.1 Start-Run Sequencer Outputs
Start-run engine modes, and control actions during start to idle, are similar for the LM2500 and LM6000 and are described in detail in Section 20.0 of the respective Installation Design Manual and are summarized for the LM2500 in Table 4.2.
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ENGMODE CONTROL ACTION
0 PRESTART
Engine and control prestart checks
2 CRANK
Check gas supply pressure above minimum start pressure PGAS>200
3 START
Enable normal control of staging valves STGVLVOPEN = False
Turn on starter Z_OP_STRTR = True
Turn off starter and STOP if NGGSEL does not reach 2000 rpm
within 20 sec Z_OPSTRTR = False
Open vent valve Z_VENTDMD = True
Purge for 2 minutes (purge time and speed set by site requirements)
Close vent valve Z_VENTDMD = False
4 IGNITE
Turn ignitor on Z_IGNDMD = True
After 2 sec open shutoff valves Z_FSOV1DMD = True Z_FSOV2DMD = True FUELON = True Close shutoff valves, turn ignitor off and perform purge if light off LITEOFF = True? not detected within 10 sec
Turn ignitor off 10 sec after light off Z_IGNDMD = False
5 RUN1
ESHUTDOWN if starter cutout speed not reached NGGSEL>=4500? within 90 sec
Turn starter off when starter cutout speed reached Z_OP_STRTR = False
6 RUN2
ESHUTDOWN if idle not achieved within 2 minutes NGGSEL>=NGGIDL?
8 RUN3
Optional 5 minute idle warm -up
9 RUNNING
ESHUTDOWN if “check power turbine rotation at idle” required and NPTSEL has not reached 350 rpm
If “check power turbine rotation at idle” not required then raise NGG speed ref. as required by application
DECEL TO IDLE if NPTSEL has not reached 350 rpm within 60 sec and ESHUTDOWN if NPTSEL has not reached 350 rpm within a further 5 minutes
10 SHUTDOWN
11 PURGE
12 ESHUTDOWN
13 STOP
Table 4.2 LM2500 Start-run engine modes and control actions during start to idle
4.2.
Control Operation
Control operation for starting to core-idle is similar for the LM2500 and LM6000. There are two phases. The first phase, which applies to the majority of the start, until the core approaches idle speed, is performed with the airflow control disabled and no eighth stage or compressor discharge bleed. The bulk Tflame schedules, during this initial phase have no effect. Instead a start fuel control calculates upper and lower bulk (or total) fuel flow limits (WFMAX = WFMAXSI and WFLBO = WFLBOSI respectively) based on independent max. and min. equivalence ratio schedules. These schedules were originally intended to correspond to combustor thermal stress and lean blowout limits but were ultimately adjusted during the LM6000 development engine testing to provide reliable starts (blowout-free) with acceptable combustor acoustic levels. Note that for the LM6000, the VBV’s throughout a start, are scheduled just as for their non DLE counterparts, i.e fully open (100%) once LP rotor speed reaches 1250 rpm. As far as the overall fuel control is concerned, during the first this initial start phase, in addition to the upper and lower fuel flow limits, two other fuel flow regulators/limiters can come into play, i.e. a core speed acceleration rate regulator and a max. WF/PS3 accel schedule limit. The WF/PS3 accel schedule limit exists in both the LM2500 and LM6000 controls, and is based on their non DLE counterparts, but in general is encountered only on the LM2500. The schedules when originally developed for the non DLE engines were intended to provide compressor stall protection. The WF/PS3 accel schedule limit is “merged” with the start fuel control upper limit WFMAXSI through a Min select to form the final WFMAX upper limit. By virtue of the fuel control priority selection logic, the WFMAX upper limit will always override the WFLBO lower limit, which means that it is possible for the WF/PS3 accel schedule limit to override both the upper and lower start fuel control fuel flow limits. A leaking or badly calibrated PS3 pressure transducer, resulting in a low sensed pressure, could in turn result in the WF/PS3 accel schedule inadvertently lowering the final fuel flow and producing a hung or aborted (flame out) start. Note that although the start fuel control upper and lower fuel flow limits are, like the “idle-and-above” Tflame algorithm limits, a function of T3 and PS3, because of accuracy concerns in the start region the T3 and PS3 are from internal model estimates, rather than the sensors. Therefore, although errors in sensed PS3 will affect the WF/PS3 max. fuel flow limit, errors in neither PS3 nor T3 sensed values will affect the start fuel control upper and lower fuel flow limits. LHV is an input to the start fuel control and so errors in LHV will affect the start fuel control upper and lower limits!
When the second phase of the start to core-idle is entered the complete airflow control/bulk flame temperature control strategy is enabled, and the upper and lower fuel flow limits come from the Tflame algorithm as opposed to the start fuel control. Transition from the first phase to the second phase is strictly a function of core speed. At a specific core speed (N25SEL = N25SIATV = N25SI + N25SIJA = 6300 rpm for the LM6000 and NGGSEL=NGGSI =4900rpm for the LM2500) the airflow control is enabled and as the core approaches that same specific speed the fuel flow upper and lower limits transition from the start fuel control limits to the Tflame algorithm limits. This occurs over the core speed range of 6200 to 6300 rpm for the LM6000 and 4800 to 4900 rpm for the LM2500.
Other DLE-specific control actions occur during the first phase of the start. When the IGNITE mode of the start is entered, in addition to the opening of the shutoff valves and the energizing of the ignitor, in order to ignite the fuel, the outer staging valve(s) that supplies fuel to the three combustor cups alongside the energized ignitor(s) is opened. At this point all of the inner staging valves are closed. The outer staging valve(s) is open for the complete ten seconds of the IGNITE mode. Note that both the LM2500 and LM6000 have provision for two ignitor locations. The staging control logic will open staging valve #22 and/or #9 depending upon whether ignition demands IGN1DMD and/or IGN2DMD are set during the IGNITE mode. When either or both of these staging valves are open during the IGNITE mode, the outer ring fuel flow is determined just as it is for operation above idle in AB or ABC mode, i.e. as described earlier, the outer fuel flow WFOREFABC is calculated in the Tflame algorithm based on a scheduled ring flame temperature TFLOREF. This fuel flow represents the outer fuel flow per staging valve and, depending upon the number of “ignition” outer staging valves open (one or two), is translated into a total outer ring fuel flow demand (WFOREF). Being that the outer fuel flow is derived from the Tflame algorithm, it will be influenced by errors in any Tflame inputs, in particular PS3 and T3. So, during the IGNITE mode, a bulk or total fuel flow is demanded (WF36DMD) , and from this is subtracted the outer ring fuel demand (WFOREF) to give a resultant pilot ring fuel flow demand (WFPREF). The inner ring is not fueled during the IGNITE mode, only the pilot ring and three or six of the outer ring combustor cups (one or two of the outer staging valves). As the start progresses, part of the inner combustor ring can also be fueled. The logic that determines this is fairly straightforward and functions as follows: If the demanded fuel flow (WF36DMD), under the influence of the core speed accel rate regulator, in attempting to track the core speed accel rate schedule, is forced onto the start fuel control upper limit (WFMAXSI) for more than three seconds then the combustor
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the number of cups from thirty plot cups to thirty pilot cups plus eight inner cups, and the upper and lower fuel flow limits will increase accordingly. This allows WF36DMD to increase and thereby “speed-up” the start. The inner ring fuel flow is determined just like the outer ring, i.e. in the same manner as it is for above-idle operation in BC/2 (LM6000 only), BC or ABC mode.
In summary:
All three rings can be fueled during a start with total fuel flow being determined by the core rate regulator and limited by Tflame max. and min. fuel flow limits. Errors in fuel properties (SG, fuel temp. and Cp/Cv) affect mass flow metering accuracy and errors in LHV affect max. and min. fuel flow limits - problems with either of these can hang-up a start or prevent a lite-off completely - it’s important to recognize that a 5 % error in fuel flow can mean approximately a 150 deg F error in Tflame! Remember that max fuel flow has an overriding WF/PS3 limit and that PS3 sensor calibration or leaks resulting in low PS3 can cause the WF/PS3 accel limit to hang-up a start. Also remember that PS3 and T3 sensors do not affect bulk Tflame limits in the initial phase of the start before the airflow/Tflame control is enabled, but they do affect the outer ignition fuel flow/Tflame and the inner fuel flow/Tflame if staging to BC/2 occurs. The outer ignition fuel flow is essential for lite-off to occur - the appropriate staging valve must be opened, i.e. the one that fuels the cups alongside the energized ignitor. At a core speed of 4900 rpm for the LM2500 and 6300 rpm for the LM6000 the airflow/Tflame control is enabled and the fuel control fuel flow/Tflame limits come from the Tflame algorithm. At this point the bulk Tflame min. and max. schedules become effective. Fuel metering or fuel property errors can at this transition point result in a blowout!
Typical start characteristics for both the LM2500 and LM6000 are shown in Fig 4.1 and 4.2 respectively.
0 400 800 1200 1600 0 20 40 60 80 100 120 140 Time - sec W F 3 6 D M D -p p h 0 2000 4000 6000 8000 0 20 40 60 80 100 120 140 Time - sec N G G S E L -rp m WF36DMD NGGSEL
0 500 1000 1500 2000 2500 3000 0 20 40 60 80 100 120 140 Time - sec N P T S E L -rp m
Figure 4.1a Typical LM2500 DLE start characteristics
0 200 400 600 800 1000 0 20 40 60 80 100 120 140 Time - sec T 5 4 S E L -d e g F . 0 2 4 6 8 10 0 20 40 60 80 100 120 140 Time - sec B R N D M D T54 BRNDMD NPTSEL
GE Proprietary Information - Technical Export License TSU/OTS March 27th, 1997 33 0 1 2 3 4 0 20 40 60 80 100 120 140 Time - sec D W B 3 6 P C T -%
Figure 4.1b Typical LM2500 DLE start characteristics
0 1 2 3 4 5 0 20 40 60 80 100 120 140 Time - sec P X 3 6 S E L -p s i p e a k to p e a k
Figure 4.1c Typical LM2500 DLE start characteristics
PX36SEL DWB36PCT
N25SEL 0 2000 4000 6000 8000 0 20 40 60 80 100 120 Time - sec N 2 5 S E L -rp m WF36DMD 0 500 1000 1500 2000 0 20 40 60 80 100 120 Time - sec W F 3 6 D M D -p p h
GE Proprietary Information - Technical Export License TSU/OTS March 27th, 1997 35 T48SEL 0 200 400 600 800 1000 1200 0 20 40 60 80 100 120 Time - sec T 4 8 S E L -d e g F . BRNDMD 0 2 4 6 8 10 0 20 40 60 80 100 120 Time - sec B R N D M D N2ROTOR 0 500 1000 1500 2000 2500 0 20 40 60 80 100 120 Time - Sec N 2 R O T O R -rp m
DWB36PCT 0 20 40 60 80 100 0 20 40 60 80 100 120 Time - sec D W B 3 6 P C T -% PX36SEL 0 1 2 3 4 5 0 20 40 60 80 100 120 Time - sec P X 3 6 S E L -p s i p e a k to p e a k
Figure 4.2c Typical LM6000 DLE start characteristics
5.
Operation at Idle
5.1 Core Idle
Control operation at core-idle is very similar for both the LM2500 and LM6000. The combustor operates in the pilot-only (B) mode (BRNDMD = 0) and the total fuel flow ( = pilot fuel flow) is adjusted by the fuel control core speed regulator (REGULATOR = 2) to set the core speed at the core-idle reference setting. For the LM2500, core idle speed is set to a nominal physical speed setting of NGGFLOOR = 6800 rpm. For the LM6000, core idle speed varies as a function of T2, decreasing as T2 increases ( 7819.3 rpm at 0 deg F, 7678.0 rpm at 48 deg F, 7409 rpm at 80 deg F). The airflow control is active and adjusts bleed in order to regulate bulk flame temperature. Both the eighth stage and compressor bleed may be used depending on the level of bleed required. For the LM6000 the VBV, depending on T2, may be scheduled fully open irrespective of the level of bleed required. The bleed sequencing is described in more detail in section 6.0. Typical characteristics to expect at core idle, for key parameters are given in Tables 5.1 and 5.2 for the LM2500 and LM6000 respectively,
ENGINE FACTORY TEST
GE Proprietary Information - Technical Export License TSU/OTS March 27th, 1997 37 LHVSEL (BTU/lbm) 20696 BRNDMD (-) 0 NGGSEL (rpm) 6800 WF36DMD (lbm/hr) 1462 TFLMIN (deg F) 2723 TFLMAX (deg F) 3586 TFLCYC (deg F) 3158 DWB36PCT (%) 59 T3SEL (deg F) 350 T54SEL (deg F) 794
PX36SEL (psi peak-peak) 0.5
T2 (deg F) 59 76.7 57 LHVSEL 20400 - -BRNDMD 0 0 0 N25SEL (rpm) 7617 7442 7717 WF36DMD (pph) 2479 1972 1977 T3SEL (deg F) 508 443 511 TFLMIN (deg F) 2900 3000 2700 TFLMAX (deg F) 3400 3500 3200 TFLCYCS (deg F) 3205 3273 2966 DWB36PCT (%) 0 68 53 T48SEL (deg F) 918 848 848 PX36SEL (psi p-p) - 0.34 0.58
Table 5.2 LM6000 Typical core idle parameters
5.2 LM6000 Core Idle to Sync Idle Transition
The original staging logic design assumed that there may not be “overlap” between core idle and sync idle, i.e. as accelerating from core idle to sync idle it may not be possible to transition directly from zero bleed B mode to high bleed BC/2, if required, and stay within the bulk Tflame limits. Therefore the staging logic was developed to provide partial staging when transitioning from core idle to sync idle. The logic functions as follows - as the gas turbine is accelerated from core idle and zero bleed/max. bulk Tflame is reached one inner staging valve is opened (BRNDMD is incremented) which results in bulk Tflame reducing. This process is repeated every time zero bleed/max. bulk Tflame is reached until BC/2 configuration is reached ( 8 inner staging valves open - BRNDMD = 8). At that point staging logic strategy changes to the scheme that is used from synch idle to max power as described in Section 8.
6.
Operation within a combustor configuration window
6.1 Flame temperature control
As previously described in Section 1.6, a combustor configuration window is defined in terms of bulk flame temperature upper and lower boundaries and bleed upper and lower boundaries. The airflow control adjusts bleed in order to regulate the bulk flame temperature, until either max or min. bleed is reached. Bleed decreases to min. as power is increased. Further increases in power at that point will cause the bulk flame temperature to increase toward the max. upper limit. Conversely, bleed increases to max. as power is decreased, and once max bleed is reached further decreases in power will cause the bulk flame temperature to decrease toward the min. lower limit. This was illustrated in Figure 1.8.
Note that in other than pilot-only (B) mode, the respective ring flame temperatures (outer (A) and/or inner (C) ) are also being controlled. Unlike the bulk flame temperature, which is regulated by varying bleed, ring flame temperatures are controlled by varying the fuel flow split between the fueled rings. Unlike the bulk flame temperature, ring flame temperatures continue to be controlled when min. or max. bleed limits are reached. This means that as power is increased when the bleed is at the min. limit, the ring flame
GE SIMULATION ENGINE FACTORY TEST 9/20/94 ENGINE 109-208 SILKEBORG SITE 4/23/96
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schedule and the ring flame temperature schedule(s) at this min. bleed condition therefore determine how hot the pilot ring will get. Raising the bulk max flame temperature schedule or lowering the ring flame temperature schedule(s) at min. bleed will raise the pilot ring flame temperature. Similar but opposite effects occur at max. bleed.
In summary, rasing or lowering bulk flame temperature at constant power results in increasing or decreasing bleed, and increasing or decreasing pilot flame temperature with very little change in the inner and/or outer flame temperatures; whereas raising or lowering either the inner or outer flame temperature at constant power results in very little change in bleed and bulk flame temperature, but results in decreasing or increasing pilot flame temperature.
6.2 Bleed Sequencing
Varying bleed between min. and max. levels involves use of eighth stage compressor bleed (ST8) and compressor discharge bleed (CDP). In addition, for the LM6000 the VBV’s are modulated between min. and max. schedule limits. The bleeds are operated sequentially. The airflow control/bulk flame temperature regulator outputs a total bleed flow demand (DWB36) which can vary between zero and a maximum
allowable limit (DWB36MAX). The total bleed demand is generally monitored as a percentage of max (DWB36PCT) and varies between 0 and 100%. The total bleed demand is translated into bleed valve position demands. The LM2500 bleed sequence is simpler than the LM6000. The LM2500 uses 0 to 12% CDP(percentage of core airflow W2) followed by 0 to 3% ST8. The LM6000 uses 0 to 2% ST8 (percentage of core airflow W25), followed by min. to max. VBV, followed by 2 to 10% ST8 and finally 0 to 3.5% CDP. The sequencing is illustrated in Figure 6.1 and 6.2 for the LM2500 and LM6000 respectively. In practice, the ST8 bleed on the LM2500 has proved to be very ineffective. Operation in the LM2500 ST8 modulating region tends to be very unstable with ST8 bleed valve scheduling either min (“off”) or max (“on”) bleed. VBVs are used on the LM6000 to provide additional bleed modulation as illustrated in Fig 6.3.
DWB36 TFLERR Stage 8 Bleed CDP Bleed P+I Regulator CDP 0-12% ST8 0-3% Control Display DWB36PCT= DWB36 DWB36MAX *100
DWB36MAX upper limit
Zero lower limit
VBV DMD ST8 BLD VLV DMD CDP BLD VLV DMD P+I regulator ST8 0 ->2% VBV min->max ST8 2 ->10% CDP 0 ->3.5% PS3ERR Total Bleed Demand DWB36 Control Display DWB36PCT= DWB36 DWB36MAX *100
DWB36MAX upper limit
Zero lower limit
Figure 6.2 LM6000 bleed sequence
` SUMP PRESSURIZATION LIMIT MAX BOOSTER STALL MARGIN LIMIT MIN INCREASING BLEED VBV [%] N25R2 [ RPM] Limits vary as a function of T2also
Figure 6.3 LM6000 VBV bleed modulation
7.
Fuel Metering
7.1 Fuel System Demands
As described in Section 1.3 there are two different fuel system configurations currently in service on the LM2500 and LM6000. The first DLE gas turbines used a four valve system, whereas more recent units have a three valve system. Schematics of the two systems are provided in Section 1.3. Both systems
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calculated directly from the flame temperature algorithm outputs WFOREFABC and WFIREFABC. WFOREFABC and WFIREFABC are the outer and inner demanded fuel flows per staging valve, and are multiplied by the respective number of staging-valves-open variables OTREST and INREST to determine the total outer and inner ring fuel flow demands WFOREF and WFIREF. These final inner and outer ring fuel flow demands are subtracted from the total fuel flow demand WF36DMD to provide the pilot ring fuel flow demand WFPREF. These three demands represent the required fuel flows at the combustor.
TFLAME A REF
TFLAME C REF
TOTAL FUEL FLOW DEMAND WF36DMD WFPREF
WFOREF WFOREFABC WFIREFABC WFIREF OTREST Number of A StagingValves Open INREST Number of A StagingValves Open EXTENSION OF TFLAME BULK ALGORITHM
FUEL FLOW PER STAGING VALVE
FUEL FLOW PER RING TFLAME TO FUEL FLOW CONVERSION TFLAME TO FUEL FLOW CONVERSION TO FUEL METERING SYSTEM + + -
-Figure 7.1 Ring fuel flow demands
The subsequent logic that is used to translate the three combustor fuel flow demands into final fuel system demands varies significantly between the three valve and four valve systems, but in both cases employs a model-based gas volume dynamics compensation scheme.. The three valve system is more
straightforward, but places heavier demands on the control processor because independent gas volume dynamics compensation is provided for each of the three rings (the four valve system has a single overall gas volume dynamics compensation algorithm).
With the three valve scheme each of the three combustor fuel flow demands are input to a gas volume dynamic compensator and the outputs represent the metering valve fuel flow demands WFOTRDMD, WFPLTDMD and WFINRDMD that are passed to the fuel system supplier’s fuel metering system. For the four valve system the logic is not so straightforward. Referring back to the schematic of the four valve system provided in Section 1.3, one can see a main metering valve downstream of which are three trim or delta P regulator valves. The main metering valve controls the total combustor fuel flow, and the three trim valves vary the fuel flow split between each combustor ring. The total combustor fuel flow demand WF36DMD is input to a single gas volume dynamics compensator, the output of which represents the main metering valve fuel flow demand WFMVDMD that is passed to the fuel supplier’s fuel metering system. The individual ring fuel flows are controlled by varying the pressure drop across each of the trim valves. The pilot trim valve delta P follows a predefined schedule that is a function of total fuel flow demand. Using assumed flow functions for each of the three combustor staging valve/premixer fuel circuits the inner and outer trim valve demanded delta P’s relative to the pilot are calculated based on the the three
combustor fuel flow demands WFIREF, WFPREF and WFIREF as illustrated in Figure 7.2. The resultant outputs from this logic are the three trim valve delta P demands DP2P3ODMD, DP2P3PDMD and
DP2P3IDMD that are passed to the fuel supplier’s metering system. The actual pressure drops across the trim valves are measured using pressure taps located on the gas manifold (GP3OSEL, GP3PSEL and GP3ISEL) and located upstream of the trim valves (GP2SEL).
(GP2 - GP3O) DMD = (GP2 - GP3P) - (GP3OREF-GP3REF) VOL DYN COMP TFLAME TO WF CALC TFLAME TO WF CALC PILOT FUEL NOZZ FF (total) OUTER FUEL NOZZ FF (per stg vlv) INNER FUEL NOZZ FF (per stg vlv) PILOT DELTA P SCHED WF36DMD WFIREFABC WFOREFABC WFPREF TFLIREF TFLOREF WF36DMD WFMVDMD WFPLTDMD WFOTRDMD WFINRDMD WFMVDMD / WF36DMD WFPREF WFOREFABC WFIREFABC OTREST INREST + +
-+ + -+ + DP2P3ODMD DP2P3IDMD DP2P3PDMD (GP2-GP3P) GP3IREF GP3OREF GP3PREFFigure 7.2 Trim valve delta P demands
7.2 Monitor Fuel Flows and flame temperature
Included in the control, for monitor purpose only, are estimated ring fuel flow calculations. Fuel flow at the combustor is calculated for each ring using the measured pressure ratio across the staging valve and premixer fuel circuit. (GP2X/PS3 for a 3-valve fuel system and GP3X/PS3 for a 4-valve fuel system - where X=O, P or I) together with other relevant parameters that include the temperature of the gas fuel and the compressor discharge temperature (T3). The outputs from this calculation are estimated “raw” fuel flows for each ring - WFOTR, WFPIL, and WFINR. These “raw” fuel flows are estimates based on assumed flow functions for each staging valve + premixer fuel circuit. These “raw” fuel flows are corrected using the ring fuel flow scalars WFNOZTOTFF (for a 4-valve fuel system) and WFNOZOTRFF, WFNOZPILFF and WFNOZINRFF (for a 3-valve fuel system), that were described in section 3, to provide a best estimate of each ring fuel flow - WFOTRM, WFPILM, and WFINRM. From these ring fuel flows estimated ring flame temperatures are calculated - F_TFLODF, F_TFLPDF, and F_TFLIDF. These appear on the control display with the leading “F_” prefix omitted and with an “S” suffix added to indicate that these are smoothed (with respect to time) variables. The various flame temperatures that generally appear on the control display are summarized in Table 7.1.
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TFLMIN Min bulk Tflame demand (deg F)
TFLDMD Bulk Tflame demand (deg F)
TFLCYCS Estimated Actual Bulk Tflame (smoothed) (deg F) TFLAMEPCT TFLCYCS relative to TFLMIN and TFLMAX
0% when TFLCYCS = TFLMIN, 100% when
TFLCYCS = TFLMAX (%)
TFLOREF Outer Tflame demand (deg F)
TFLIREF Inner Tflame demand (deg F)
TFLODFS Estimated actual outer Tflame (smoothed) (deg F) TFLPDFS Estimated actual pilot Tflame (smoothed) (deg F) TFLIDFS Estimated actual inner Tflame (smoothed) (deg F) Table 7.1 Displayed flame temperatures
8.
Combustor Staging
8.1.
General
The combustor staging logic controls the opening and closing of the 10 outer (A) and 15 inner (C) staging valves, as well as the single enhanced lean blowout (ELBO) staging valve. The inner and outer staging valves are opened and closed in accordance with the required combustor configuration. As described previously, there are five combustor configurations, viz B, BC/2 (starting only for the LM2500), BC, AB and ABC. Transitioning from one combustor configuration to another involves increasing or decreasing bleed in conjunction with opening and closing of staging valves. Because of the finite response of the airflow (bleed) control and because of the small combustor flame temperature windows it is not possible to switch
immediately from one combustor configuration to another. Therefore, a series of intermediate, or partial, staging configurations are required when going from one steady-state, or permanent combustor
configuration to another. In the control there are two key variables BRNREQ and BRNDMD that specify the steady-state combustor configuration target (BRNREQ) and the current combustor configuration demand (BRNDMD). BRNDMD can assume any integer value between 0 and 40, whereas BRNREQ can only assume the values 0, 8, 15, 25, 40 that correspond to the permanent combustor configurations B thru ABC respectively. BRNDMD is translated through look-up tables in the control into inner and outer staging valve commands (INRCMDID and OTRCMDID). This information is summmarized in Table 8.1. INRCMDID and OTRCMDID specify the inner and outer staging valve patterns. For each value of INRCMDID and
OTRCMDID, specific inner and outer staging valves are opened. The staging patterns are different for the LM2500 and LM6000 and are defined in the Output Signal Processing section of the Control System Specifications M50TF3740 and M50TF3731 respectively.
B 0 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 B+C/2 8 8 8 9 9 10 10 11 11 12 12 13 13 14 14 B+C 15 15 15 16 13 1 1 17 12 2 2 18 10 3 3 19 9 4 4 20 7 5 5 21 6 6 6 22 4 7 7 23 3 8 8 24 1 9 9 A+B 25 25 0 10 10 26 1 10 10 27 2 10 10 28 3 10 10 29 4 10 10 30 5 10 10 31 6 10 10
BURNER BRNREQ BRNDMD INRDMD OTRDMD OTRCMDID
CONFIG. INRCMDID OTRSTSOP
INRSTSOP
BRNREQ - Burner Config. Steady-State Target
BRNDMD - Burner Config. Demand
INRDMD - No. Inner Staging Vlv. Open Demand
INRCMDID - Inner Staging Vlv. Pattern ID INRSTSOP - No. Inner Staging Vlv. Open Feedback OTRDMD - No. Outer Staging Vlv. Open Demand
OTRSTSOP - No. Outer Staging Vlv. Open
Feedback
OTRCMDID - Outer Staging Vlv. Pattern ID OUTER STAGING VALVE IGNITION CONTROL IF (BRNDMD < 16) THEN
TABLE II
Z_IGN1DMD Z_IGN2DMD OTRDMD OTRCMDID
OTRSTSOP
F F 0 0
T F 1 11
F T 1 12
FOR BRNDMD < 16 SEE IGNITION CONTROL
