Flashback thermocouple ring relocation
Central de Ciclo Combinado Saltillo
Mitsui & Co Ltd and Tokyo Gas Co Ltd
250-MW, gas-fired, 1 × 1 combined cycle located in Saltillo, Coahuila, México
Plant manager: René Villafuerte
Key project participants: Gerardo Rasgado, I&C coordinator; Roberto Hernández, maintenance manager
The two flashback thermocouples from each of the 16 combustors on the W501FD are channeled inside a ring that is originally located on top of the combustion shell (Fig 115). Through the history of the plant several problems attributedto overheating, melting, and wire insulation failure have caused many incidents, even an auto-unload.
During past outages, some refurbishment was made so to that thermocouple ring would be further from the heat (Fig 116), but the problems appeared again. On the “hot” side (combustor area) the temperature could reach up to 110F after the insulation blanket and the radiant heat soaked into the electric duct (the ring) causing a slow insulation degradation.
During the last major inspection outage in October 2010, the flashback thermocouple ring was completely rebuilt, but in this case, the new design was relocated around the “cold” side (compressor area) about 4 ft upstream and expanded in diameter (Figs 117 and 118). Temperatures on the compressor side are no more than 100F after the insulation blanket but with no radiant heat, and the instrumentation conduit remains quite cool.
Relocating the flashback thermocouple ring on the “cold” side (compressor side) eliminated the incipient failures that were common after unit startup and thus enhanced significantly the reliability and availability of the gas turbine.