Vibration analysis for detecting failure of compressor blade

In a 210 MWe gas turbine, compressor blades in stage 3 failed and caused large scale collateral damage to the plant. The plant outage resulted in heavy loss to the generation company. The blade fractured at root of the airfoil due to high cycle fatigue as evidenced by beach marks and microscopic striations on the leading edge of the blade. Corrosion related pitting was also noticed on the leading edge fillet region on many blades of stage 3 and also on many blades of other stages. The Campbell diagram indicated possibility of high vibratory stresses at the leading edge due to excitation of the fundamental mode of the blade. Due to large number of startups caused due to technical reasons, it was estimated that the response of the blade could have initiated crack in the pitted region. It was further estimated that after the startup, crossing of the other fundamental modes at the synchronous speed could have propagated the crack to failure under normal operating stress.On the two other working units of similar make and capacity, it was necessary to assess the health of the compressors blades and take necessary action if warranted. The paper provides failure analysis of the failed blade and a new approach based on casing vibration analysis for assessing health of compressor blades in two operating plants. The innovative approach identified abnormalities in both the operating units. In the immediate plant outage, the compressor blades were examined by ultrasonic technique and cracks were detected in several blades at the root. The unit was thus saved from yet another major mishap.

► Compressor blades are one of the most critical components in gas turbines.
► Compressor blades vibrate during off design condition.
► Direct measurement of blade vibration not possible.
► Blade vibration can be detected by analyzing casing vibration.
► The method can also be used for detecting crack in operating blades.