Control.com - Nerds in Control

Hello sir!

I am Amir Mairaj, working as an Automation Technician in QAPCO Qatar (UAE).

I am facing a great trouble for two months. Our gas turbine With Anasaldo Generator before general shut down of turbine was working very well, but after startup Delta T of Exhaust exceeds 30 to 40 degree celcius. We have already changed new Nozzles and also new axial compressor rotor and all 12 exhaust thermocouples. The distance of all thermocouples is also same, about 6 mm from piping and randomly thermocouples changed but the problem yet same as starting from zero. We also loop calibrated but loop is right. Please accomodate me to find right troubleshooting. I will be very thankful to you for this act of kindness.

AMIR MAIRAJ
AUTOMATION TECHNICIAN
QAPCO QATAR
CELL # +9745108495

WISHING YOU A HAPPY AND PEACEFUL LIFE EVERYWHERE.

What occurred during the "general shutdown"? A Combustion Inspection? A Hot Gas Path Inspection? A Major Inspection? You say the axial compressor rotor was changed, so the hot gas path hardware was likely at least "disturbed" at some point (removed and reassembled).

You haven't said which fuel the unit is running on when this occurs? Or, does this happen on gas fuel *and* liquid fuel? If it's happening on liquid fuel, can you tell us the fuel pressures for each of the combustion cans from the selector valve/gauge near the Liq. Fuel Flow Divider? If it's happening on liquid fuel on a dual fuel (gas/liquid) machine, have you checked the Tell-tale Leakoff to see if any of the Atomizing Air Purge Check Valves are leaking?

I've seen people change fuel nozzles but not liquid fuel or atomizing air purge check valves and still have the same problems. There's more to the fuel system, especially with liquid fuel systems, than just the fuel nozzle. I've also seen people randomly replace fuel nozzles without regard for trying to match flow characteristics and actually make the exhaust spread problem worse--on both gas and liquid fuels. Many Customers "flow match" fuel nozzles as a set, and they do this to ensure minimal exhaust spread problems due to fuel nozzle characteristics.

Does the cold spot always stay in the same area? In other words, are the highest- and lowest T/C readings always in the same location? Can you tell us which T/Cs are hottest and which are coldest?

If you've changed all the fuel nozzles (you didn't say how many or which ones you've changed) and you've tested the T/C inputs from the exhaust JBs to the turbine control panel with a mv or T/C simulator, and you've changed random T/Cs, and you've inspected the T/C insertion in the radiation shields in the exhaust plenum, then the problem is likely something wrong with the mechanical reassembly of the hot gas path components (combustion can "hula skirt" seals; nozzle side seals; transition piece seals; etc.) or there's something affecting the exhaust flow in the exhaust duct. I've seen very slight "modifications" to exhaust duct transitions from the gas turbine to a heat recovery steam generator cause air flow disruptions which caused lots of problems with exhaust temperature readings.

But, it sure doesn't seem like this problem is controls-related from what you've told us. Most exhaust temperature spreads are not controls-related, but disassembling the unit requires more effort than the things you've already done. And one wouldn't want to take the unit apart again only to find the problem was controls-related and didn't need to be disassembled. So, you've done about everything you can, and the problem seems to be pointing at a mechanical issue.

We would appreciate it if you would let us know what is found to be the cause of the high exhaust temperature spreads.