Root Cause Analysis (RCA) – Axial Compressor Example

In general, MSI’s role in helping to ensure equipment reliability takes place at three levels: (a) preconstruction audits and analysis during the design phase to help prevent problems, (b) using specialized testing and analysis techniques to solve performance, acoustic, vibration, stress, or thermal problems before failure takes place, and (c) performing root cause analysis (RCA) after a failure. MSI recently performed RCA to determine the root cause of cracks found in multiple blades on several rows in the low pressure (LP) section of a large axial compressor. The key deliverable was recommendations, supported by analysis results and experience, for extending blade life.

To accomplish the RCA, following inspection of the failed components and detailed interviews with End User personnel, MSI performed a detailed 3D Finite Element Analysis (FEA) of each failed blade and the assembly disk. The analysis was focused on determining the mean and worst-case alternating stresses during normal operation and surge events. Natural frequencies of each single blade and the assembly disk were also determined by analysis calibrated by non-operating modal testing in order to verify possible resonances in the system.

The cracks were found at the base of the blades (above the fillets) where they meet the platform. Based on MSI's RCA, the root cause of the cracks in the LP section blades was determined to be a consequence of several surge events that occurred during uncontrolled shut-downs of the units after power outages of the self generated power of the plant. These events generated non-linear loading that was 10 to 20x the gas bending load at low frequency, which increased the alternating load producing low cycle fatigue cycles in the material.

Mechanical Solutions, Inc. was contracted by a manufacturer to perform a seismic structural analysis on three identical large pump systems in a low-activity and high-activity nuclear waste treatment process. The scope of the analysis included the pressure containment casing, nozzle attachments, support system and driver. The pump systems were required to meet the specifications of Seismic Category 1 (SC-1).

Finite Element Analysis (FEA) techniques were employed to perform an equivalent static analysis to evaluate stress levels as a result of the seismic input in combination with nozzle loading, impeller thrust, and torque. The effects of the motor, motor attachment means and rotating components were incorporated into the model. A conservative seismic approach was taken due to the uncertainty in the design of the attached piping and pipe support configuration. Natural frequencies and mode shapes were also investigated and compared to running speed and other excitation sources.

The final analysis indicated that the seismic stresses in the pump components as well as all bolting were fund to be within acceptable levels. Peak stress levels were below the yield strength of the materials used. The major structural natural frequencies were identified and determined to be at a safe margin excitation sources, and therefore resulted in minimal increase in the quasi-statically calculated seismic stresses. The normal operating peak stresses due to internal pressure, nozzle load, and seismic and fatigue analysis criteria.

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