Exhaust Temperature Spread

BHEL-GE Gas Turbine Services EXHAUST TEMPERATURE SPREAD 1.0 Spread And Its Effects Exhaust temperature spread is the difference between maximum and minimum temperatures recorded by exhaust temperature thermocouples If spread is allowed with out correction results in reduction of residual life of down stream components 2.0 Spread Concerns • • • •

Any unit running below allowable (125° F on MS6B/ 9E; 175° F on F class machines) is acceptable to run. If a unit starts from the beginning at a "perceived" high spread (90 F- 100° F), and does not increase, there is no cause for concern. If more than one unit are at a site, the spreads on the units will be different. Also, if one unit runs at 60 F spread and the next at 85° F, does not mean there is something "wrong" with the unit. Time to get concerned: a) if there is a sudden jump in spread b) if spread continues to increase with time

3.0 Causes Of Spread Following is a typical cause and effect diagram for high exhaust spread.

Machine

People Internal Assembly

System

Trans.pc.seals Fuel transfer

Plugged/Damaged fuel tips

Piping

A/A comp. Moisture contamination Insufficient output Lube leaks

Incorrect hardware

Water wash

High Exhaust Spread Exh. Thermocouples

Fuel Contamination

Fuel nozzles Card Failure

Check Valves Water Injection

T/C wiring

Flow Divider Purge air valves

Hardware

Operation

Controls

BHEL-GE Gas Turbine Services Spread Related to Combustion components -

Fuel Nozzles Liners T/ P's (include floating & side seals)

Spread Related to Fuel system components -

Check valves (Liquid and purge) Flow divider Gaskets in pigtails Selector valve (internal leakage)

Spread related to human errors -

Pigtails connected wrong Gas flange taped and the tape not removed Use of Teflon tape in fuel lines Two check valves in one line One nozzle of a different design

4.0 Spread Related To Fuel Nozzles Brief description Optimum gas turbine operation, fuel nozzles must be removed from service at scheduled intervals for cleaning, inspection, reconditioning and calibration. Turbines operated with dirty, damaged or defective fuel nozzles run a high risk of extensive damage with the resulting expense of downtime and the repair / replacement of major components. Damage may extend from combustion liners and transition piece assemblies to include pre-mature failure of the first-stage turbine nozzle and buckets.

What can go wrong with fuel nozzles leading to spread? •

Worn internals not replaced

•

Internals and fuel passages not cleaned properly

•

Wrong part installed during replacement of internals

•

Fuel nozzle internals not properly assembled (torque and assembly checks)

•

Large variation in flow among set of fuel nozzles

•

Accumulation of dust during storage

BHEL-GE Gas Turbine Services Fuel Nozzle Refurbishment At BGGTS

BGGTS fuel nozzle testing facility developed critical processes of cleaning, inspection and calibration of the fuel nozzles in fuel nozzles refurbishment in line with GE process requirements. Fuel nozzles calibration facility equipped with Rose Mount mass flow meters and coupled with data logger software for capturing test data.

Following are ensured during refurbishment of fuel nozzles at BGGTS •

Pressure testing of fuel nozzles during assembly process ensuring proper assembly of internals

•

Dimensional and visual inspections of parts thoroughly carried out before recommending for parts replacement

•

Correct parts as per GE recommendations are replaced during refurbishment

•

Internal assembly carried out as per GE recommendations (torque, in process assembly checks etc.)

•

Flow testing performed on test benches duly calibrated as per GE specification requirements

•

Flow ranges adjusted to close tolerance among fuel nozzles (below 2%)to ensure equal fuel flow through fuel nozzles.

BHEL-GE Gas Turbine Services Fuel Nozzle Testing facility

Cleaning facility

Pressure testing facility

Liquid Flow system

Liquid Flow testing in progress

Air Flow system – gas /air

Gas flow testing in progress

Data Logger software

Rose mount flow meter

BHEL-GE Gas Turbine Services 5.0 Precautions To Be Taken At Site Following fuel line assembles checked to avoid spread related issues Flow Divider • Leakage, Abnormal Noise • Discharge Pressure Differential • Speed Pickups Secure/Gapped correctly Check valves • Leakage • Corrosion Note: Check valves to be replaced in sets and tested to tolerance of 0.5 kg. Crack pressure Fuel Nozzle Selector Valve • Improper Stem Operation • Leakage at Connections • Internal leakages False Start Drain Valve • Incorrect Sequencing During Startup and Shutdown • Improper Operation of Valve Actuator • Air and Oil Leaks • Worn Diaphragm in Valve Actuator Piping • Leakages • Corrosion and accumulation of debris/dirt Note: Do not use Teflon in assembly of fuel passage systems

6.0 Trouble Shooting Data required for troubleshooting of exhaust temperature spread • • • •

Exhaust temperature thermocouple readings (TTX's) at various loads TTX's, MW on more than one fuel if used TTX's, MW with and without water/ steam injection Fuel line pressure readings (Liquid only) before and after flow divider

Above data is analyzed, probable cause identified and corrective actions can be suggested by BGGTS Services and Engineering team.