One of Gas turbine installed at our facility has undergone Hot Gas Path inspection. After the Inspection when it was started it was observed that 1st Stage Aft thermocouple were reading temperature close to 410 C at 12 MW load(Alarm:427).
Activities carried out during the HGPI
1. 1st stage nozzle replacement
2. 2nd Stage nozzle replacement
3. Diaphragm and Packing replacement
Soon after the machine was started it was seen that the 1st Aft thermocouples (04 installed) all were reading temperature close to Alarm value (stated above). Difference between the thermocouple readings were not more than 10 C.
Thermocouple on 2nd stage nozzle is fixed type thermocouple that are fitted before nozzle is assembled in turbine casing. There are 04 thermocouple recording 1st Aft and 04 recording 2nd FWD.
Now we inspected compared the both 2nd stage nozzle, one that we took out and other that we installed. The difference was of 04 extra holes for cooling of wheelspace in the installed nozzle. This installed nozzle was working in one of the other machines and was refurbished to be used again in this machine.
We need advice from valued members of this prestigious forum to help us troubleshoot the problem.