By Tom Reid, Vice President of Power Generation Services, ENTRUST Solutions Group

A Case Study

Client Request

The client sought to evaluate the risk of postponing a boresonic inspection of their generator field beyond the Original Equipment Manufacturer (OEM) guidelines. According to the OEM, the field bore should be reinspected after a specific number of operating years. However, the plant had already exceeded this recommended time limit.

Analysis

ENTRUST Solutions Group conducted a thorough evaluation, beginning with a review of the unit’s operating history since the last boresonic test. This assessment included a tally of stop/start cycles and overspeed test events.

Crack growth in the field bore is primarily driven by Low Cycle Fatigue (LCF), which is influenced by stop/start cycling and overspeed events, including overspeed testing. Using historical and projected event data, ENTRUST calculated potential crack growth rates for any existing flaws in the bore. Conservative assumptions were made regarding cycles and overspeed events from the current date through the projected next inspection.

A Finite Element Analysis (FEA) model of the generator field was developed based on a shipping drawing provided by the plant. This model incorporated the field body section, including slot dimensions. The specific rotor material’s properties were also considered. Initial crack sizes from the last boresonic inspection results served as the starting point for the fracture mechanics calculations.

Key Analysis Considerations

1. Surface Flaws: Surface flaws on the bore are most concerning due to their exposure to free surfaces, where crack growth rates are highest. The hoop stresses created during rotor rotation are also greatest at the bore surface.
2. Stress Distribution: Larger-diameter sections experience higher stress levels due to increased mass. However, the stress distribution in field body sections is further complicated by the presence of winding slots.

The FEA model calculated stress levels at each flaw’s location. These stress values were used in a linear fracture mechanics analysis, which assumed initial flaw sizes and incorporated the actual mechanical properties of the metal. The analysis calculated the number of remaining cycles and overspeed events before existing cracks could reach critical size. A safety factor was applied to the critical crack size following standard OEM practices. The calculated critical crack sizes were cross verified with the rotor geometry to ensure physical feasibility.

Findings

The fracture mechanics analysis indicated a low risk of delaying the boresonic inspection for multiple years. 

Operating data revealed that the unit had undergone minimal cyclic behavior since the last inspection over 10 years earlier. If a boresonic inspection had been performed immediately, there would likely have been no measurable difference in flaw size compared to the previous inspection.

ENTRUST provided the plant with allowable limits for total overspeed events and normal stop/start cycles the unit could undergo before the next boresonic inspection.

Improvements

The analysis enabled the plant to defer the boresonic inspection until the next planned major outage, integrating it into the broader scope of maintenance activities. To date, there have been no issues with the generator field, confirming the analysis and decisions based on its recommendations were sound.

Best Practices

Deferring maintenance should always be guided by robust engineering analysis. Whenever possible, calculations should assess the risk of extending inspection intervals. 

In the case of boresonic inspections, the related failure modes are well-documented and understood. While OEM guidelines often provide generic recommendations, conducting a tailored evaluation of your specific rotor can yield more precise reinspection intervals. 

By applying actual flaw data from your rotor, you may find opportunities to safely extend inspection periods.

This case highlights the importance of integrating engineering expertise and historical unit data to optimize maintenance schedules, ensuring reliability and cost efficiency. 

Reach out to one of ENTRUST’s experts today to find out how we can support, and improve, your inspection planning. 

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Tom has spent the entirety of his 15-year career in the power generation industry. 

In his current role as Vice President of Power Generation for ENTRUST, Tom oversees a team of approximately 100 engineers, whose expertise covers power plant equipment, modeling, and testing. 

Prior to ENTRUST, Tom held turbine design and repair roles at General Electric. Tom is a graduate of GE’s Edison Engineering Development Program and holds 7 U.S. patents. He holds an BSME degree from Virginia Tech, an MSME degree from Georgia Tech, and is a registered professional engineer in the state of Delaware.

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