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

A Case Study

Inspection Findings

During a planned major outage, last-stage blades were discovered with cracking in the blade attachment. The blades were of a free-standing design, meaning they were not interconnected with adjacent blades through shrouds, snubbers, or tie wires. The observed cracking was clustered in specific groups and was not present consistently around the entire 360-degree row.

Analysis

To investigate the issue, ENTRUST Solutions Group used blue light scanning technology to create a solid blade model. This model was then meshed to conduct a Finite Element Analysis (FEA). The FEA enabled calculations of blade natural frequencies and root stresses.

The natural frequencies of large blades are a critical parameter, as they ensure the blade is well-tuned and not subject to vibration excitation from multiples of the turbine’s running speed.

Findings

The analysis revealed that the blade was effectively tuned against multiples of the running speed. However, a significant discovery was that stresses in the blade attachment were highest at the observed cracking locations when the blade vibrated at its first natural frequency.

Free-standing blades, such as these, are highly susceptible to aeroelastic vibration under certain conditions. They may be affected by unstalled flutter at high flow rates, stall flutter at low flows, and specific combinations of low-pressure exhaust conditions. Adjacent free-standing blades with nearly identical natural frequencies are particularly vulnerable to unstalled flutter.

An established industry best practice to mitigate aeroelastic vibration in free-standing blades is mix-tuning. This involves ensuring a variation of several Hertz in natural frequencies between adjacent blades. Mix-tuning can be achieved passively through manufacturing tolerances or actively by designing blades with slightly different geometries, often using two unique part numbers to produce the desired frequency variation.

Improvements and Solutions

Since the cracking was detected during a routine major outage, design change options were limited due to the urgency of returning the unit to service. ENTRUST recommended the following solutions to address the issue effectively while minimizing downtime:

• Blade Mix-Tuning: The investigation confirmed that the blades were not originally mix-tuned. Implementing mix-tuning created a frequency variation between adjacent blades, reducing the risk of flutter-induced vibration.

• Material Upgrade: Replacing the blades with an upgraded material offered improved strength and reliability.
• Shot Peening: Shot peening the blade attachments introduced a compressive stress layer on the surface, enhancing crack resistance.

These solutions were implemented successfully, and no issues have been reported with the upgraded blades.

Best Practices

For turbines equipped with large free-standing blades, ENTRUST recommends the following measures to ensure long-term reliability:

• Mix-Tune Blades: During blade assembly or replacement, verify that the blades are mix-tuned.

• Audit Flow and Backpressure Conditions: Periodically review flow rates and low-pressure exhaust conditions to confirm compliance with allowable parameters.
• Thorough Inspections: Inspect blade roots and rotor attachments during major outages whenever possible. Additionally, L-0 blade inspections can be performed through the condenser following peak operational seasons for many turbine designs.

Closing Remarks

By following industry best practices and implementing targeted improvements, turbine operators can significantly reduce the risk of blade cracking in free-standing designs. In this case, ENTRUST analysis and recommendations provided a reliable path to addressing the identified issues while minimizing operational disruptions. 

Contact us today to discover how we can support your steam turbine blade health. 

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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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