Failure and Numerical Analysis

Expert Assessment Services by Generation Plant Specialists

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Assessing a key component

Vibratory stresses arising from resonance, surge, and flutter can lead to high cycle fatigue (HCF) damage accumulation in compressor blades, particularly those in the latter stages of a compressor in a gas turbine (GT).

In addition, variability in critical blade dimensions due to limitations in fabrication processes, lack of compliance with minimum requirements for material properties, and relaxation of mounting conditions following assembly can exacerbate the accumulation of damage during the stated flow-related conditions and lead to premature HCF failures.

An undetected problem

It is common for a blade failure to go undetected until the next scheduled inspection. Typically, the first step in assessment of a failed blade is accurate identification of the failure mechanism. Subsequent numerical simulations covering system gas path analysis and single blade vibration response can be employed to ascertain the probable cause of failure.

Accurate identification

For HCF failures, evidence of fatigue striations, coupled with cycle count estimations derived from fractography, secure proper identification of the failure mechanism.

Frequently, broken blades sustain significant post-fracture damage and sufficient fracture surface oxidation rendering identification of the fine striations attributable to HCF difficult.

Advanced technology and expertise

With the aid of a field-emission scanning electron microscope and extensive know-how in assessing fractures, Kinectrics can identify the failure mechanism. Using finite element analysis, a 3D model of an intact blade can be generated and evaluated for resonance against engine speed.

A survey of varying degrees of blade-mounting conditions can also be applied to determine if lack of constraint is a potential contributor to failure. Natural frequencies for an adequate number of vibration modes are assessed for each boundary condition, thereby yielding corresponding modal shapes.

The resultant data is then plotted on a Campbell Diagram.

By incorporating a gas-path analysis, vibratory stresses can be predicted for any blade geometry, and crack-initiation sites identified during failure analysis can be correlated with high-stress locations predicted numerically.

This information enables operators to focus on performance issues and mitigate the occurrence of future failures.

Keith Ellison, 416.207.6000 x6315, keith.ellison@kinectrics.com

Allan Jarvine, 416.207.6000 x5785, allan.jarvine@kinectrics.com