Gas Turbine Nozzle and HRSG Damage Due to Liquids in Natural Gas Fuel

摘要

Unusual damage was experienced by the first stage nozzles in a gas turbine and a heat recovery steam generator. (Nozzles are stationary vanes that channel hot gases to the rotating blades in the turbine). All pressure (concave) walls of the vanes experienced some degree of damage, least of which were cracking of the coating, erosion and wall thinning. About 30% of the vanes were damaged more severely. They experienced pitting, erosion, cracking and total disintegration of up to half of the pressure walls. The suction walls (convex) of the vanes showed very little damage and all leading edges were surprisingly fine. The only trailing edges that were severely damaged or partially missing were downstream of the severely damaged pressure walls. Metallurgical analysis revealed that the overheating caused oxidation and depletion of the coating and dense spiking of oxides into the coating. The oxide spikes facilitated dense cracking of the base metal that led to disintegration of the affected wall sections into tiny fragments. Even more unusual was the lack of evidence of melting at the rugged perimeters of remaining parts of the pressure walls. Also unusual was the relatively good condition of the first stage turbine blades despite the missing sizeable sections of the first stage nozzle pressure walls and trailing edges. The damage was attributed to localized overheating by liquid contaminants (most probably emulsions) in the natural gas fuel (97.8% methane).