combustion monitoring - ms9001

7/17/2019 Combustion Monitoring - Ms9001

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COMBUSTION MONITORING – MS9001

Bharat Heavy Electricals Limited, Hyderabad of 18

Control and Protections

Combustion Monitor Function

INTRODUCTION

Monitoring of the exhaust thermocouples to detect combustion problems is performed by the

SPEEDTRONIC micro processor based controllers 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 control sequence program for details of algorithm

implementation and identification of selectable display lists and PROM constants.

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 dischargetemperature 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.






COMBUSTION MONITORING FUNCTION SOFTWARE

The controllers contain a series of programs written to perform the monitoring task

(see Combustion Monitoring Schematic Figure CMFPR-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.

The exhaust thermocouples are scanned once every second and the data is stored in the

controllers’ memory. The stored temperature data is read by the combustion monitor as well as

the temperature control programs. From each set of readings, the median exhaust temperature

TTXM, the allowable spread TTXSPL, and three sets of actual spread values are calculated and

compared with specific exhaust temperature limits to determine whether alarm and trip

conditions have been exceeded.

ALLOWABLE SPREAD

The allowable spread is the sum of two values: a nominal 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 degree F.

During steady state operation, the bias value is 0 degree F. when a transient operational condition

occurs, such as rapid change in load, the bias value steps to 200 degree F. It remains at that value

until 2 minutes after the transient condition ends. Its value then decays exponentially to

0 degrees F on a 2 minute time constant. Refer Fig.






The transient conditions which cause the bias to step to 200 degrees F are:- Fuel transfer

- Turbine startup and shutdown

- Load changes produced by a governor RAISE or LOWER signal (Either Manual or Auto)

- Load changes produced by a rapid change in FSR.

SPREAD TEST

The most advanced algorithm, which is standard for gas turbines with redundant sensors, makes

use of the temperature spread and adjacency tests to differentiate between actual combustion

problems and thermocouple failures. The behavior is summarized by the Venn diagram

(Figure CMFPR-2) where:

Sallow (TTXSPL) is the "Allowable Spread", based on average exhaust temperature and

compressor discharge temperature.

TTXSPL = TTKSPL4 * TTXM – TTKSPL3 * CTDA + TTKSPL5

And:

SPREAD #1(S1): TTXSP1

The difference between the highest and the lowest thermocouple reading

SPREAD #2 (S2): TTXSP2

The difference between the highest and the 2nd lowest thermocouple reading

SPREAD #3 (S3): TTXSP3

The difference between the highest and the 3rd lowest thermocouple readings






The allowable spread must be between the limits TTKSPL7 and TTKSPL6 in degrees F, usuallyabout 30° and 125° F. The values of the combustion monitor program constants are listed in the

Control Specifications.

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.

Exhaust Thermocouple Trouble Alarm (L30SPTA)

If any thermocouple value causes the largest spread to exceed by a constant, TTKSP2 (usually

5 times the allowable spread) and persists for 4 seconds L30SPTA alarm "EXHAUST

THERMOCOUPLE TROUBLE" is displayed, the alarm will latch and remain on until

acknowledged and reset.

Combustion Trouble Alarm (L30SPA)

A combustion alarm can occur if a thermocouple value causes the largest spread to exceed by a

constant, TTKSP1 and persists for 3 seconds, the alarm will latch and the "COMBUSTION

TROUBLE" message will be displayed and remain on until acknowledged and reset.

High Exhaust Temp Spread Trip (L30SPT)

A high exhaust temperature spread trip can occur if combustion exhaust monitor (L83SPM) is

enabled

And1) Exhaust temperature spread # 1 “high” exists, and the second largest spread exceeds 0.8

times the allowable spread and the lowest thermocouples are adjacent

i.e TTXSP1 > TTXSPL & TTXSP2 > 0.8 * TTXSPL and the first and second lowest

T/C’s are adjacent

Or






2)

If Exhaust temperature spread # 1 exceeds 5 times the allowable spread and the secondlargest spread exceeds 0.8 times the allowable spread and the second and third lowest

thermocouples are adjacent

i.e TTXSP1 > 5 * TTXSPL & TTXSP2 > 0.8 * TTXSPL and the second and third

lowest T/C’s are adjacent

Or

3) If spread #3 exceeds the allowable spread (which means that spread 1 and 2 also exceeds

the allowable spread limit)

i.e All spreads exceeds the allowable spread limit

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.

Exhaust Temperature 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 exhaust temperature monitor is not enabled, no protective actions

are taken.

TROUBLESHOOTING WITH THE COMBUSTION MONITOR

FUNCTIONThe 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 thermocouplemaintenance. 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 TROUBLE 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 TROUBLE 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 ambient temperature. In

addition, the fuel oil nozzle pressures 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 CMFPR-3 through 5 are Troubleshooting flowcharts to help in that determination.

Condition Figure

Only the TC Failure Alarm occurs CMFPR-3

Combustion Alarms occurs

(With or without TC failure alarm) CMFPR-4

Combustion Alarm and Trip occur

(With or without TC failure alarm)

And gas turbine has tripped CMFPR-5






All possible problems though cannot be foreseen and initiative and judgment may be required totroubleshoot.

Thermocouple Checking

The <HMI> station 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 <HMI> station display. An open

thermocouple will indicate a value of -40F. 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.

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 and determine the average. Also compute 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.

Interpreting Failed Thermocouple Patterns

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 CMFPR-5






CAUTIONFollow 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 Bharat Heavy Electricals Limited Field Service

Representative be consulted.

COMBUSTION TROUBLE MODES TO BE CONSIDERED

Combustor

1. Failed Liner (Cracked or Burned)

2. Failed Transition Piece (Cracked or Burned)

3. Collapsed Liner

4. Hot Crossfire Tubes

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

Fuel Nozzle

1. Plugged Fuel Nozzle (Liquid or Gas)

2. Unscrewed Fuel Nozzle

3. Fuel Nozzle Erosion

4. Red Hot Fuel Nozzle






Atomizing Air System1. Break in Atomizing Air Line

2. Faulty Purge System

3. Atomizing Air Compressor Failure

4. Plugged Atomizing Air Passage at Manifold or Nozzle

Pressure Vessel Integrity

1. Cracked Combustor Casing

2. Blown Gasket

3. Damaged Crossfire Tube Piping

4. Cracked or Blown Sight Port.

5. Leakage at Flame Detector or Spark Plug

First Stage Nozzle

1. Burned Out First Stage Nozzle

2.

Plugged First Stage Nozzle

CORRECTIVE ACTION REQUIREMENTS

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 CMFPR-4 and the following:

** WARNING **

Keep personnel away from the vicinity of the turbine and accessory compartments until the

problem has been identified.






A. Look for abnormal smoke from the exhaust. Trip the turbine if abnormal smoke isobserved.

CAUTION

During troubleshooting watch the displays of temperature spread closely and trip the turbine

in the event of a continuous or sudden increase.

B. Read and record the exhaust thermocouple temperatures using the display, and the

printer.

C. Record the "spread" displayed.

D. Proceed with a normal turbine shutdown.

E. If faulty exhaust thermocouples have been discovered, replace the thermocouples.

F. Inspect the turbine for signs of leaks or damage.

G. If no evidence is discovered in (F), start the turbine in the crank position (unfired).

Inspect the turbine compartment for leaks or damage.

H. If no evidence is discovered in (G), 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 onliquid fuel fired gas turbines.

I. If an abnormal condition is observed, trip the turbine and take corrective action.

J. If no abnormal condition can be discovered at this point, the turbine should be shutdown

and a combustion inspection performed.






******** FURTHER ATTEMPTS AT DIAGNOSIS BY OPERATINGTHE MACHINE IS NOT RECOMMENDED ****************

K. 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.

L. The problem area discovered in the course of the combustion inspection should be

corrected and the turbine should be returned to normal operation.

Turbine Trip - Corrective Action (See Figure CMFPR-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:

A. It is likely that an abnormal condition exists within the turbine and corrective action

should be initiated.

B. Inspect the turbine compartment hardware for signs of leaks or damage.

C. 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.






Automatic Trip - Corrective ActionIn 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.

A. Replace the failed thermocouples.

B. Open the turbine compartment doors and keep all personnel away from line-of-sight of

the turbine compartment.

C. 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.

D. 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.

E. Any further trips due to the combustion monitor indicate an unsatisfactory condition, and

a complete combustion system inspection should be performed.

F. Otherwise resume normal operation.

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 re-fire 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 trouble shooting procedures in accordance with

Figure CMFPR-4. If a trip occurs during water injection operation, proceed with troubleshooting

procedures in accordance with Figure CMFPR-5.

combustion monitoring - ms9001

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