Failure Evaluation of the Compressor Vanes of Combined Cycle Unit

Abstract

A compressor blade failure was experienced on a 69 MW gas turbine of a combined cycle (C.C.) unit after four years operation since its last overhaul (January 2005). The unit accumulated 27,000 service hours and 97 start-ups since the last overhaul. This unit consists of four gas turbine stages and 19 compressor stages and operates at 3600 rpm. In 2006, the unit was equipped with a fogging system at the compressor air inlet duct to increase unit power output during high ambient temperature days (hot days). These fog water nozzles were installed upstream of the compressor inlet air filter without any water filter/catcher before the water spray nozzles. Three unit failure events occurred within a small time period, which caused a forced outage. The first failure occurred in December 2008, a second event in March 2009 and the third event in May 2009. Visual examination carried out after the first failure indicated that the compressor vanes (diaphragms) had cracks in their airfoil initiating at the blade tenons welded to the diaphragm outer shroud at stages 3, 8, 9, 10 and 11. Also, a number of stationary vanes and blades at each stage of the compressor showed foreign object damage (FOD) and fractures at the airfoil. Visual examination performed for the second failure event after 60 unit operating hours indicated that many compressor vanes (diaphragms) and blades had FOD at the airfoil. This was attributed to fractures caused by the fogging system. The water spray carried over in the compressor flow path at high velocity causing the FOD damage. Visual examination completed upon the third failure event after two unit startup attempts indicated damage of compressor stationary vanes and blades, principally at stages 12 to 16, and also stages 17 to 19. The damage consisted of airfoil fracture in the stationary vanes and blades, FOD, blade tip rubbing, and bending of the stationary vanes, blades and diaphragm shrouds. A laboratory evaluation of stationary vane tenon fracture indicated a high cycle fatigue (HCF) failure mechanism, and crack initiation was accelerated by corrosion pitting on blade surfaces due to high humidity air generated by the fogging system. Stationary vane damage was caused by a rotating stall phenomenon, which generates vibratory stress in stationary vanes and blades during unit start-ups. During the third failure event, the stationary vane HCF damage was highly accelerated due to pre-existing partial fractures in the tenons generated during previous failure events which had not been detected by non-destructive tests. Stationary vane and moving blade failure was also influenced by high tenon brittleness in stationary vanes and blades generated during manufacture by welding the diaphragms, and repair welding the blades without adequate post-weld heat treatment (stress relieving). A compressor stationary vane and blade failure evaluation was completed. This investigation included cracked blade metallographic analysis, unit operation parameter analysis, history-of-events analysis, and crack initiation and propagation analysis. This paper provides an overview of the compressor failure investigation, which led to the identification of the HCF failure mechanism generated by rotating stall during unit start-ups, highly accelerated corrosion generated by the fogging system, and high brittleness in the stationary vanes and blades as the primary contribution to the observed failure.