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

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

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32 7 10 10

33 8 10 10

34 9 10 10

35 10 10 10

36 11 10 10

37 12 10 10

38 13 10 10

39 14 10 10

A+B+C 40 40 15 10 10

Table 8.1 Staging control parameters

Staging control is similar for the LM2500 amd LM6000, and is described in the following paragraphs for the various phases of engine operation.

8.2. Starting

In B mode at fuel-on. Outer staging valves #9 and/or #22 alongside the ignitor(s) are opened during the IGNITE mode. If the max sub-idle Tflame fuel flow limit is encountered during a start for > 3 seconds then the combustor is staged from B to BC/2 (BRNDMD increments from 0 to 8) and remains in BC/2 mode until (as described in section 4.2) the airflow control is enabled as the core speed reaches a sub-idle switch setting (4900 rpm for the LM2500 and 6300 rpm for the LM6000). When the airflow control is enabled the LM2500 staging control switches into the idle and above mode and the LM6000 switches into the core idle-sync-idle transition mode.

8.3. LM6000 Core Idle-Sync-Idle

Because for the LM600, as described in section 5.2, when transitioning between core-idle and sync idle there is not necessarily overlap between the B and BC/2 modes, partial staging is allowed. At core-idle the LM6000 operates in B mode. As the core is slowly accelerated until no-load synch-idle is reached, the combustor progressively stages from B to BC/2 (BRNDMD slowly increments from 0 to 8 and usually overshoots until it finally settles at 8). It is possible depending upon T2 and the bulk flame temperature schedules for synch-idle to be acheived in B mode. Transitioning from synch-idle back to core-idle results in the combustor progressively staging (if not already in B mode) from BC/2 to B mode.

8.4. Idle and Above Operation

Staging between configurations occurs at extreme corners of a combustor operating window. When accelerating, as illustrated in Figure 8.1, staging is initiated at max. bulk Tflame, min. bleed. When this occurs BRNREQ switches immediately to the new configuration value (8 (LM6000 only) , 15, 25 or 40) and BRNDMD increments to the BRNREQ value. INRCMDID and OTRCMDID follow BRNDMD and inner and/or outer staging valves are progressively opened or closed.

BC

ABC

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45 Figure 8.1 Combustor staging during load accels

Similar actions occur when decelerating except, as illustrated in Figure 8.2, staging is initiated at min. bulk Tflame, max. bleed.

Figure 8.2 Combustor staging during load decels BC/2 mode

(lm6000 only)

AB

Staging Transition Points B

BC/2 mode (lm6000 only)

BC

AB

ABC

ABC to AB stage down logic can cause

“early” stage down

POWER

8.5. LM6000 BC to AB zone avoidance

The LM6000 includes zone avoidance logic which has two functions, both associated with the BC to AB region. The first function is intended to overcome lack of overlap between BC and AB modes, whereas the second function is intended to avoid high pilot flame temperatures (and therefore Nox) typically seen in the BC mode. Although not strictly part of the staging control, the zone avoidance logic does indirectly force staging from BC to AB. The logic, which produces a megawatt demand bias, is only active in grid mode and has no effect in isochronous (island) mode.

The first function which is activated when a BC to AB transition is initiated (BRNREQ>15) adds in a nominal 1 megawatt bias (ZAMWBIASJ) that ramps up at a nominal rate of 0.07 megawatt/sec (ZAMWRATEJ).

The second function is activated in BC mode (BRNREQ=15) if T3 reaches a nominal threshold of 760 deg F (T3MAXBCJ) and starts ramping in a megawatt bias at a nominal rate of 0.1 megawatt/sec (ZAT3RTUPJ) until the BC to AB transition is initiated (BRNREQ>15). Once the BC to AB transition is initiated, the megawatt bias is ramped out at a nominal rate of

-0.1 megawatt/sec (ZAT3RTDNJ). Note that this second function has to be enabled by setting the control adjustment ZAT3ENAJ = TRUE. The nominal setting for ZAT3ENAJ is specified = FALSE.

8.6. LM6000 BRNUL upper limit

The LM6000 staging logic includes an upper limit BRNUL that is applied to BRNREQ and BRNDMD.

BRNUL is switched 8 to 15 to 25 to 40 as a function of T3SEL. This logic was added to limit the severe effects of partial blowouts that were experienced during early LM6000 engine tests. When a partial blowout occurs the fuel control power turbine speed will increase the demanded fuel flow WF36DMD to compensate for the unburned fuel. When WF36DMD increases, the bulk Tflame/airflow regulator decreases bleed to maintain bulk Tflame. Without the BRNUL limit logic, this could result in the control reaching max bulk Tflame and min bleed and then staging to the next configuration. This situation could repeat until the ABC mode is reached, even if, for the current delivered power, the combustor should have been in B or BC/2 mode! However there are some pitfalls with the BRNUL limit logic. The T3 switch point settings may not be consistent with the bulk Tflame schedules which could result in BRNUL not allowing BRNREQ and

BRNDMD to “switch up” on an accel, or could result in BRNREQ and BRNDMD “switching down”

prematurely! Therefore, if erratic staging occurs, along the lines just described, it may necessary to adjust the T3 switch points. The nominal T3 switch point settings are defined in Table 8.2.

BRNUL switching T3 switch parameter T3 nominal switch setting (deg F)

8 to 15 T3BCJA 665.0

15 to 25 T3ABJA 783.0

25 to 40 T3ABCJA 874.0

Table 8.2 LM6000 BRNUL T3 switch points

8.7. ABC to AB stage down - LM2500

The maximum power of the LM2500 DLE is set by either T54 or gas generator speed. For T2’s greater than 10 deg F, the T54 limit of 1535 deg F will limit the maximum power. Unfortunately, for a DLE engine, maximum power is usually not the place where the highest T54 is reached. For any gas turbine engine, the exhaust gas temperature will increase as compressor bleed is increased. This also applies for the DLE

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47

bleed side, to have a higher T54 than the right side of the window, i.e. the low bleed side of the window. If the T54 limit is reached with the total bleed level (DWB36PCT) greater than 0%, the engine will start to decel due to the T54 fuel control regulator reducing fuel flow demand. Keeping in mind the bulk flame temperature window of section 1.6.1, deceling means moving from right to left across the window. As the engine moves from right to left, the total bleed levels increases. As the total bleed level increases, T54 increases. This causes the engine to decel even further. This turns into an unrecoverable cycle. One way to avoid this situation is to keep the bulk flame temperature as low as possible in ABC mode. The field mapping procedure contains the instructions on how to do this. Additionally, the control logic has been modified with two special features to help this situation. First, the control logic will automatically stage down to AB mode if T54 is within 5 degrees of the limit and the total bleed level is greater than 70%. This has been named the ABC to AB stage down logic. Once in AB mode, the control logic locks out staging back to ABC until the calculated flame temperature percentage (TFLAMEPCT) is less than 80%. This interlock prevents the engine from cycling back and forth between AB and ABC modes. Second, the T54 limit schedule has been modified to reflect the trend of high T54s at high bleed. The T54 limit schedule consists of three parts. One part is the maximum power, zero bleed limit of 1535. Another part raises the limit with increasing bleed to 1550 deg F. The final part of the T54 limit schedule raises the T54 limit by 50 deg F only when transitioning from AB to ABC modes. After 20 seconds of operation in ABC mode, this 50 degree bias drops out.

In summary, field engine operation to date have shown a potential for hitting the T54 limit in ABC mode prior to reaching zero bleed. This problem can be eliminated by reducing the bulk schedules in ABC mode. To give added margin, the control logic contains the following features to prevent this undesired situation:

1. ABC to AB stage down logic

2. Modified T54 limit schedule - raise T54 limit by 15 degrees at higher bleed levels and bump T54 limit by 50 degrees when staging into ABC mode

8.8. ABC to AB stage down - LM6000

The LM6000 has similar characteristics to the LM2500 in the ABC mode, although in the case of the LM6000, T48 and corrected (to station 25) core speed (N25R) increasing with increasing bleed (decreasing power) is of concern.

For the LM6000, ABC to AB stage down is initiated if the following conditions persist for > 2 seconds: In ABC or AB-ABC transition region (BRNDMD>25) and max core speed regulator (REGULATOR=9) or max T48 regulator (REGULATOR=6) or “throttle” core speed regulator (REGULATOR=2) is encountered and bleed (DWB36PCT) is > 30 %. Once an ABC to AB stage down has occurred then, like the LM2500, to prevent cycling back and forth between AB and ABC modes, staging back to ABC is inhibited, in the case of the LM6000, until bleed level (DWB36PCT) is increased above 30% in AB mode or until staging down to BC mode (BRNDMD<25) has occurred.

The nominal reference settings for max corrected core speed, max physical core speed and max T48 are 9525 rpm, 10711 rpm and 1590 deg F respectively. The max T48 setting, as for the LM2500, is raised, in the case of the LM6000 to 1650 deg F when staging from AB to ABC, and remains raised until the bulk Tflame drops from the 50% to 25% level in ABC mode.

Also for the LM6000, high temperatures at the gas turbine exhaust in the ABC high bleed region have proven to be of concern in customers’ installations (can result in exceeding boiler temperature limitations etc.). Therefore the LM6000 stage down logic includes an “external” customer forced stage down input (Z_ABC2AB to force stage down, Z_ABUNLCK to unlatch stage down).

8.9. Load drop/ overspeed

Both the LM2500 and the LM6000 include additional staging logic to optimize speed holding and load drop capability for power generation isochronous operation. This logic anticipates the need to change combustor configuration before the normal combustor window switching conditions are satisfied (min or max bulk Tflame and max or min bleed), and as a result can minimize the variations in power turbine speed when large changes in load occur. For example load drops on an LM2500 can cause the staging control to switch from ABC to B mode almost instantaneously.