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

A Case Study: LP Shrunk-On Disc and Blade Attachment

Inspection Findings

During a recent outage, phased array ultrasonic testing (PAUT) of double-flow LP shrunk-on discs revealed no recordable indications in any disc bores or keyways. 

Original Equipment Manufacturer (OEM) guidelines recommended multiple reinspections between major overhauls, based on an hourly schedule. However, since these intervals were established, the unit’s operational profile has changed significantly.

Analysis Overview

Two crack propagation mechanisms were evaluated:

1. Stress Corrosion Cracking (SCC): SCC is a well-documented phenomenon in wet steam environments, with increasing probabilities as service hours exceed 150,000. SCC crack growth is influenced by factors such as material yield strength, stage operating temperature, and steam chemistry. Unlike other mechanisms, SCC depends on operating hours rather than the number of cycles.
2. Low-Cycle Fatigue (LCF): LCF arises from repeated stresses during normal start-up and shut-down cycles and overspeed events.

Using finite element analysis (FEA), ENTRUST Solutions Group quantified startup and overspeed stresses in the shrunk-on disc keyway and the blade attachment. Maximum stress levels observed in the rotor steeples, disc bore, and conservative material properties were used to determine the minimum critical crack size. A safety factor was applied to this value to establish a reinspection crack size.

Crack Growth Evaluation

Based on fracture mechanics principles, the crack growth analysis assumes an initial flaw of the smallest size detectable by the clean PAUT inspection. Crack propagation from SCC and LCF was calculated based on operating hours and cycles. The cumulative crack growth was compared to the reinspection crack size to determine appropriate reinspection intervals.

Dynamic Reinspection Plots

Dynamic reinspection intervals were visualized using plots. The solid red line represents the reinspection crack size, and the dashed red line depicts the calculated crack growth based on the unit’s duty cycle inputs. A re-inspection is required when the two lines intersect.

The plot separates the contributions to crack propagation from SCC (green line) and LCF (blue line). Circles on the plot indicate current crack growth contributions from SCC (green) and LCF (blue) based on inputted operational data.

Scenario 1:

Figure 1 illustrates a base-loaded operational profile. Most crack propagation comes from SCC, shown by the dominant green line, with minimal contribution from LCF. The solid black line indicates that crack propagation is 73.8% of the reinspection crack size.

Scenario 2:

Figure 2 presents a cycling-operational profile with three times as many cycles and significantly fewer operational hours than Scenario 1. Despite the change in cycles versus hours, total crack propagation remains in a similar range across SCC and LCF sources.

Improvements

After evaluating the unit’s actual operational profile, it was determined that the OEM-recommended reinspection intervals for the shrunk-on disc and blade attachments were overly conservative. 

Using the dynamic reinspection interval methodology, ENTRUST identified the opportunity to reduce the frequency of inspections. This adjustment decreased outages driven by reinspection requirements for the shrunk-on disc and blade attachments, improving operational efficiency and reducing maintenance downtime.

Final Notes

By aligning reinspection intervals with actual operational data, operators can significantly reduce unnecessary inspections while maintaining safe and reliable turbine operation. In this case study, the dynamic evaluation framework provided a critical tool for optimizing maintenance cycles.

***

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.

Ali_ENTRUST

See Full Bio