GEK 106832 March 1998 Replaces CMBMTR00
GE Power Systems Gas Turbine
Combustion Monitor Function
These instructions do not purport to cover all details or variations in equipment nor to provide for every possible contingency to be met in connection with installation, operation or maintenance. Should further information be desired or should particular problems arise which are not covered sufficiently for the purchaser’s purposes the matter should be referred to the GE Company. © 1998 GENERAL ELECTRIC COMPANY
GEK 106832
Combustion Monitos Function
I. INTRODUCTION
Monitoring of the exhaust thermocouples to detect combustion problems is performed by the SPEEDTRONIC™ software coupled with solid state analog devices for interfacing with the primary controls and protective devices. For operating instructions, calibration, PROM constant listing and operational check procedures for a specific gas turbine, refer to the Control Specifications. Refer to the software Master Sequence Document for details of algorithm implementation and identification of selectable display lists and PROM constants. II. COMBUSTION MONITORING FUNCTION
The primary function of the combustion monitor is to reduce the likelihood of extended damage to the gas turbine if the combustion system deteriorates. The monitor does this by examining the temperature control system exhaust temperature thermocouples and compressor discharge temperature thermocouples. From changes that may occur in the pattern of the thermocouple readings, warning and protective signals are generated by the combustion monitor and sent to the gas turbine control panel. This means of detecting abnormalities in the combustion system is effective only to the extent there is incomplete mixing as the gases pass through the turbine, and an uneven inlet pattern will cause an uneven exhaust pattern. The uneven inlet pattern could be caused by a rupture in a transition piece, loss of fuel or flame in a combustor, or other combustion malfunctions. The usefulness and reliability of the combustion monitor depends on the condition of the exhaust thermocouples. It is important that each of the thermocouples is in good working condition. III. COMBUSTION MONITORING FUNCTION SOFTWARE
The controllers contain a series of programs written to perform the monitoring task (see Combustion Monitoring Schematic Figure 1). The main monitor program is written to analyze the thermocouple readings and make appropriate decisions. Several different algorithms have been developed for doing this depending on the turbine model series, and the type of thermocouples used. The significant program constants used with each algorithm are specified in the Control Specification for each gas turbine. IV. THERMOCOUPLES SCAN AND COLD JUNCTION COMPENSATION
The exhaust thermocouple values, read and sorted in the exhaust temperature feedback algorithm, are used by the combustion monitor. From each set of readings, the median exhaust temperature, the allowable spread, and two sets of spread ratios are calculated and compared with specific exhaust temperature limits to determine whether alarm and trip conditions have been exceeded. V. ACTUAL SPREADS
Three spreads are calculated from the exhaust thermocouple readings: SPREAD #1 (TTXSP1)--The difference between the highest and the lowest thermocouple reading; SPREAD #2 (TTXSP2)--The difference between the highest and the 2nd lowest thermocouple reading; and, SPREAD #3 (TTXSP3)--The difference between the highest and the 3rd lowest thermocouple readings.
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GEK 106832
VI. ALLOWABLE SPREAD
The allowable spread is the sum of two values: a nominal allowable spread and a bias. The nominal allowable spread is the steady state spread limit. It varies, typically, between 30 and 125 degrees F as a function of average exhaust temperature and compressor discharge temperature. The bias is an adder to the nominal allowable spread which accounts for the temporary increase in actual spreads that occur during transient operational periods. It is a temperature value which varies between 0 and, typically, 200 degrees F. During steady state operation, the bias value is 0 degrees F. When a transient operational condition occurs, such as a rapid change in load, the bias value steps to 200 degrees F. It remains at that value until, typically, 2 minutes after the transient condition ends. Its value then decays exponentially to 0 degrees F on a 2 minute time constant. Refer to Figure 6. The transient conditions which cause the bias to step to 200 degrees F are: 1) fuel transfer, 2) turbine startup and shutdown, 3) load changes produced by a governor RAISE or LOWER signal, and 4) load changes produced by a rapid change in FSR. VII. SPREAD TESTS
The combustion monitor algorithm uses the acutal spreads, the allowable spread, and adjacency tests to determine if an actual combustion problem exists. The algorithm logic is summarized by the Venn diagram shown in Figure 2. VIII. DISPLAY MESSAGES
The various monitor outputs to the control panel cause alarm message display as well as appropriate control action. The combustion monitor outputs are given below. A. Exhaust Thermocouple Trouble Alarm (L30SPTA)
If any thermocouple value causes the largest spread to exceed a constant (usually 5 times the allowable speed) a thermocouple alarm (L30SPTA) is produced. If this alarm persists for four seconds, the alarm will latch and the alarm message “EXHAUST THERMOCOUPLE TROUBLE” will be displayed and remain on until acknowledged and reset. B. Combustion Trouble Alarm (L30SPA)
A combustion alarm can occur if a thermocouple value causes the largest spread to exceed a constant (usually the allowable spread). If this alarm persists for three seconds, the alarm will latch and the “COMBUSTION TROUBLE” message will be displayed and remain on until it acknowledged and reset. C. High Exhaust Temp Spread Trip (L30SPT)
A high exhaust temperature spread trip can occur if a combustion trouble exists and the second largest spread exceeds 0.8 times the allowable spread and the first and second lowest thermocouples are adjacent or if an exhaust thermocouple trouble exists and the second largest spread exceeds 0.8 times the
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GEK 106832
Combustion Monitos Function
allowable spread and the second and third lowest thermocouples are adjacent or if the third spread exceeds 0.8 times the allowable spread or if a combustion trouble exists and a controller failure occurs. If any of these conditions exist for 9 seconds, the trip will latch and “HIGH EXHAUST TEMP SPREAD TRIP” message will be displayed. The turbine will be tripped through the master protective circuit. The alarm and trip signals will be displayed until they are acknowledged and reset. D. Monitor Enable (L83SPM)
The protective function of the monitor is enabled by signals from the control panel. The purpose of the “enable” signal (L83SPM) is to prevent false action during startup and normal shutdown transient conditions. When the monitor is not enabled, no new protective actions are taken. To troubleshoot the monitor when the gas turbine is not running, the “enable” signal must be simulated, i.e., logic signal L14HS, and L4 inputs must be “1” ’s. IX. TROUBLESHOOTING WITH THE COMBUSTION MONITOR FUNCTION A. General
The two main sources of alarm and trip signals being generated by the combustion monitor are failed thermocouples and combustion system deterioration. Other causes include poor fuel distribution due to plugged or worn fuel nozzles, and flameout due to water injection (if used). The tests for combustion alarm and trip action have been designed to minimize signals due to failed thermocouples but it is impossible to always isolate this cause. The best protection against shutdowns due to failed thermocouples is good thermocouple maintenance. Replace failed thermocouples promptly during normal downtime so that the likelihood of operation with more than one failed thermocouple is low. The TC TROUBLE ALARM is intended to call attention to failed thermocouples so that they will be promptly replaced. The TC ALARM will also be activated by the early stages of some combustion problems. The COMBUSTION TROUBLE ALARM is intended as an early warning of an actual combustion malfunction. DO NOT OPERATE THE GAS TURBINE FOR A PROLONGED TIME PERIOD WHEN THE COMBUSTION ALARM HAS BEEN ACTIVATED. If black smoke is seen in the exhaust, trip the machine immediately. If there is no black smoke in the exhaust, any period of operation with this alarm should be used to diagnose the trouble, as failed thermocouples are much easier to detect when hot than at an ambient temperature. In addition, the fuel oil nozzle pressure can be read during operation to help diagnose poor fuel distribution. (See Combustion Alarm - Corrective Action.) If the gas turbine has been tripped by the monitor, it will be necessary to determine the cause. Figures 3 through 5 are troubleshooting flowcharts to help in that determination. Á
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Only the TC Failure Alarm occurs
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Combustion Monitos Function
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Combustion Alarms occurs (with or without TC failure alarm) Á
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Combustion Alarm and Trip occur (with or without TC failure alarm) and gas turbine has tripped Á
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GEK 106832
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If TC Alarm, Trip and Combustion Alarm occur and the gas turbine continues to run, monitor malfunction has occurred. Attempt to reset monitor. Á
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All possible problems cannot be foreseen and initiative and judgment may be required to troubleshoot. X. THERMOCOUPLE CHECKING
The monitor display and the printer, if available, can be used to check thermocouple readings at any time. The combustion monitor will continue to protect the gas turbine while the thermocouple readings are being displayed and printed. Thermocouple malfunction may be detected by observing the monitor display. An open thermocouple will indicate a value of -40°F. A thermocouple may be intermittently open if the display is erratic or cyclic. With the turbine coasting down and the exhaust metal still hot, but the exhaust air cooler, a thermocouple that reads significantly higher than the others is probably shorted in its junction box. XI. THERMOCOUPLE RESISTANCE CHECK
With the turbine not running, disconnect each thermocouple in turn at the termination board and measure the thermocouple resistance using a good quality ohmmeter. Measure each thermocouple in each polarity and average the two results to avoid errors from the thermoelectric action. Record all readings, determine the average, and the deviation of each reading from the average. Any thermocouple with a deviation of more than 15 ohms from the average reading should be considered defective. XII. INTERPRETING FAILED THERMOCOUPLE PATTERNS A. Automatic Trip
If an automatic trip has occurred and a thermocouple has failed, the failed thermocouple may have caused the trip. If not, assume a possible combustion or associated problem. Proceed in accordance with Figure 5
CAUTION
Follow the restart instructions carefully. It is impossible to be sure there is no combustion problem from the tests referred to above. In case of doubt or if failed thermocouple patterns are not found, it is recommended that the General Electric Company Field Service Representative be consulted.
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GEK 106832 XIII. COMBUSTION TROUBLE MODES TO BE CONSIDERED A. Combustor
1. Failed Liner (Cracked or Burned) 2. Failed Transition Piece (Cracked or Burned) 3. Collapsed Liner 4. Hot Crossfire Tubes B. Fuel System
1. Break in Liquid Fuel Line 2. Break in Gas Fuel Line 3. Plugged Check Valve 4. Check Valve Stuck Open/Closed 5. Liquid Fuel in Gas Manifold 6. Stuck Flow Divider 7. Failed Fuel Pump C. Fuel Nozzle
1. Plugged Fuel Nozzle (Liquid or Gas) 2. Unscrewed Fuel Nozzle 3. Fuel Nozzle Erosion 4. Red Hot Fuel Nozzle D. Atomizing Air System
1. Break in Atomizing Air Line 2. Faulty Purge System 3. Atomizing Air Compressor Failure 4. Plugged Atomizing Air Passage at Manifold or Nozzle E. Pressure Vessel Integrity
1. Cracked Combustor Casing
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Combustion Monitos Function
Combustion Monitos Function
GEK 106832
2. Blown Gasket 3. Damaged Crossfire Tube Piping 4. Cracked or Blown Sight Port 5. Leakage at Flame Detector or Spark Plug F. First Stage Nozzle
1. Burned Out First Stage Nozzle 2. Plugged First Stage Nozzle XIV. CORRECTIVE ACTION REQUIREMENTS A. Combustion Alarm - Corrective Action
In the event of a Combustion Alarm, it is most likely that a condition exists within the turbine that, if left alone, could lead to serious combustor or turbine damage. Precautionary measures and troubleshooting procedures should be initiated immediately to locate and correct the problem. Proceed in accordance with Figure 4 and the following:
WARNING Keep personnel away from the vicinity of the turbine and accessory compartments until the problem has been identified.
1. Look for abnormal smoke from the exhaust. Trip the turbine if abnormal smoke is observed.
CAUTION
During the troubleshooting period, watch the display of temperature spread closely and trip the turbine in the event of a continuous or sudden increase. 2. Read and record the exhaust thermocouple temperatures using the display and the printer. 3. Record the “spread” displayed. 4. Proceed with a normal turbine shutdown. 5. If faulty exhaust thermocouples have been discovered, replace the thermocouples. Proceed with the trouble shooting investigation to locate the problem. Perform the following steps:
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GEK 106832
Combustion Monitos Function
6. Inspect the turbine for signs of leaks or damage. 7. If no evidence is discovered in (6), start the turbine in the crank position (unfired). Inspect the turbine compartment for leaks or damage. 8. If no evidence is discovered in (7), fire the turbine and hold in a warm-up condition (do not permit acceleration). Watch for an abnormal smoke condition from the exhaust. TRIP THE TURBINE IF ABNORMAL SMOKE IS OBSERVED. This fired condition, while the turbine is at low pressure, will permit personnel to read and record the individual fuel nozzle pressure readings in the accessory compartment on liquid fuel fired gas turbines. 9. If an abnormal condition is observed, trip the turbine and take corrective action. 10. If no abnormal condition can be discovered at this point, the turbine should be shut down and a combustion inspection performed. FURTHER ATTEMPTS AT DIAGNOSIS BY OPERATING THE MACHINE ARE NOT RECOMMENDED. 11. Preliminary disassembly of the combustor end cover will permit inspection of the combustion liner, a portion of the transition piece, the fuel nozzles and the crossfire tubes. The extent of the inspection must be dictated by the nature of the problem. 12. The problem area discovered during the combustion inspection should be corrected and the turbine should be returned to normal operation. B. Turbine Trip - Corrective Action (See Figure 5)
In the event of an emergency turbine trip due to the following conditions. 1. Abnormal smoke condition at any time. 2. Combustion Alarm. Thermocouples show combustion alarm pattern, and the thermocouples have not failed. 3. Thermocouple Alarm, Combustion Alarm and Trip. Trip pattern, and thermocouples have not failed. 4. Combustion Alarm and Trip. Trip pattern exists and thermocouples have not failed. Proceed with the following action: 1. It is likely that an abnormal condition exists within the turbine and corrective action should be initiated. 2. Inspect the turbine compartment hardware for signs of leaks or damage. 3. Perform a combustion inspection. Inspect all combustor hardware including the transition piece for signs of damage. Replace the damaged hardware and return the turbine to normal operation.
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Combustion Monitos Function
GEK 106832
C. Automatic Trip - Corrective Action
In the event of an automatic trip due to COMBUSTION ALARM and TRIP (THERMOCOUPLE TROUBLE ALARM or not) and investigation shows failed thermocouples could have caused the trip, proceed with the following action. 1. Replace the failed thermocouples. 2. Open the turbine compartment doors and keep all personnel away from line-of-sight of the turbine compartment. 3. Proceed with a normal start. If an abnormal smoke condition is observed from the exhaust stack, trip the turbine and proceed with a complete combustion inspection of the liner, transition pieces, crossfire tubes and fuel nozzles. 4. If no further alarms are noted by FSNL (full speed, no load), proceed with a normal loading procedure and observe the temperature spread reading and exhaust smoke. 5. Any further trips due to the combustion monitor indicates an unsatisfactory condition, and a complete combustion system inspection should be performed. 6. Otherwise resume normal operation. XV. WATER INJECTION (OPTIONAL)
During water injection, if one or more combustors are extinguished by the water, the combustion alarm function will shut off the water and the COMBUSTION ALARM will be displayed. The Trip function will trip the gas turbine if the combustors do not refire within a short time period. If the combustion alarm function operates, and the water injection is turned off, reset the monitor. If the alarm does not recur, restart the water injection as desired, using the normal procedures. If the alarm does recur proceed with troubleshooting procedures in accordance with Figure 4. If a trip occurs during water injection operation, proceed with troubleshooting procedures in accordance with Figure 5.
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GEK 106832
Combustion Monitos Function
Combustion Monitor Algorithm
CTDA( ) TTKSPL1 Max. TTKSPL2 Min. TTXC
Median Select Calculate Allowable Apread
TTXSPL Median Select
TTKSPL5 Max. TTKSPL7 Min.
L83SPMB
Calculate Bias Values
Constants
A B
TTXD2 Calculate Actual Spreads
A B A B A B
A
B
A
B
A
B
A
B
L60SP2 L60SP3
Figure 1. Combustion Monitoring Function Algorithm (Schematic)
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L60SP1
L60SP4
Combustion Monitos Function
GE K 106832
S2
Also Trip If:
Venn Diagram
5allow
S1 Sallow
> K1
Trip If S1 & S2 Or S2 & S3 Are Adjacent Typical Trip if S1 & S2 Are Adjacent K3
Monitor Alarm K1
Communications Failure
K1 = 1.0 K2 = 5.0 K3 = 0.8
TC Alarm K2
S1 Sallow
Figure 2. Exhaust Temperature Spread Limits
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GEK 106832
Combustion Monitos Function
TC Failure Alarm (Only)
TC = Thermocouple
Check For Smoke
Yes
Black Smoke
No
Initiate Emergency Trip
– Display and Record Thermocouple Readings and Median Exhaust Temperature
(See Turbine Trip Or Shutdown/Corrective Action)
– Record Spread 1 and Spread 2
No
Defective Thermocouples
Yes
– Record ALW Sprd, TCD-1 and TCD-2 – Record Alarm Messages Note: Printout Above If Printer is Available * TCD = Compressor Discharge Temperature
Possible Causes: – Intermittent Thermocouple – Transient Gas Turbine Operation – Full System Trouble – Incipient Combustion Trouble
Reset Alarms Clear Annunciator
– Replace Failed Thermocouples At First Opportunity. (Operation With Failed Thermocouples Raises Risk Of False Trip)
Continue Normal Operation
Figure 3.
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Combustion Monitos Function
GEK 106832
Combustion Alarm (With or Without TC Failure Alarm)
**WARNING** Keep Personnel Away From the Vicinity of the Turbine and Accessory Compartments Until the Problem Has Been Identified.
TC = Thermocouple
– Display and Record Thermocouple Readings and Median Exhaust Temperature
Check For Smoke
– Record Spread 1 and Spread 2 Yes
Black Smoke?
No
– Record ALW Sprd, TCD–1 and TCD–2 – Record Alarm Messages Note: Printout Above If Printer is Available
Initiate Emergency Trip
See Combustion Alarm/ Corrective Action
See Turbine Trip Corrective Action
* TCD = Compressor Discharge Temperature
Figure 4.
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GEK 106832
Combustion Monitos Function
Combustion Alarm and Trip (With or Without TC Failure Alarm)
TC = Thermocouple
Automatic Trip
– Display and Record Thermocouple Readings and Median Exhaust Temperature – Record Spread 1 and Spread 2 – Record ALW Sprd, TCD-1 and TCD-2 – Record Alarm Messages Note: Printout Above If Printer is Available * TCD = Compressor Discharge Temperature Yes
No
Any TC Failed
(See Automatic Trip)
See Automatic Trip/Corrective Action
See Turbine Trip Corrective Action Do Not Reset Alarms Until Ready To Restart Turbine
Figure 5.
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GE Power Systems General Electric Company One River Road, Schenectady, NY 12345 518 • 385 • 2211 TX: 145354
