By Tom Reid, Vice President of Power Generation Services, ENTRUST Solutions Group
New environmental regulations and the surge in renewable energy installations have led to the retirement of many older steam turbine generating units. These legacy units typically operated in a “base-loaded” fashion, accumulating extensive operating hours with minimal start-stop cycles. To ensure successful operation up to their planned retirement dates, addressing the long-term, temperature-dependent damage mechanisms affecting turbine rotors is essential. This article reviews two notable case studies and operational strategies to support life extension for such units.
High-pressure (HP) and intermediate-pressure (IP) rotors, often made from materials similar to ASTM A470, operate at steam inlet temperatures of around 1,000°F. Prolonged exposure to these conditions can initiate creep cracks in highly stressed areas and cause significant material embrittlement, reducing ductility over time. These mechanisms must be carefully monitored in rotor bores and blade attachment sections.
Creep is a time, temperature, and stress-dependent phenomenon that affects rotors subjected to high operating temperatures (above 800°F) and sustained stress. OEMs typically estimate design creep life at around 30 years for high-temperature rotor designs, using the Larson-Miller parameter (LMP) curve to relate time, temperature, and stress for crack initiation.
The LMP curve inherently includes safety margins and does not account for crack propagation or interactions between creep and fatigue from on/off cycling. While many units exceed their 30-year design targets, pre-existing forging flaws limit rotor life, especially in rotors manufactured before vacuum degassing technology.
Studies by Dr. Swami Swaminathan (EPRI Project 2481-5) on a retired rotor revealed material property changes over time. Testing indicated rotor material experienced significant embrittlement in less than 200,000 operating hours. The degree of embrittlement was represented by a shift in Fracture Appearance Transition Temperature (FATT), which is the point where rotor steel exhibits 50% brittle and 50% ductile behavior.
For long-term operation, the FATT’s shift by over 200°F decreases ductility and limits the rotor’s ability to withstand high stresses during startup. Interestingly, high-temperature zones above 1,000°F showed less embrittlement than intermediate zones operating near 800°F. These findings influence assessments of existing flaws and operational adjustments, such as reducing stops and starts or modifying startup procedures, to avoid premature rotor retirement.
A utility experienced a failure in the stage 2 dovetail of an IP rotor with over 425,000 operating hours. The rotor’s FATT increased by 300°F compared to non-embrittled steel with similar chemistry, reaching more than 600°F.
The utility sought to extend the unit’s operation until its planned retirement with minimal cold starts. ENTRUST Solutions Group adjusted the rotor’s startup procedure to address this challenge to account for the FATT shift. Modified simulations examined various speed hold times, ramp rates, and differential expansion to avoid high stresses in brittle material states.
The analysis demonstrated that extending soak periods at lower speeds allowed the rotor to achieve ductile properties before experiencing high stresses. This adjustment resulted in a 400% improvement in critical crack size (see Figure 1), enabling the utility to maintain its retirement plan. Validation tests showed the revised startup procedures closely aligned with simulation recommendations.
An HP turbine operating for over 300,000 hours experienced a failure in its inlet-stage control disc (see Figure 2). Testing on the rotor material identified high-temperature creep notch sensitivity in stress-concentrated rotor dovetail areas.
Comprehensive evaluations included boresonic and material tests, tensile strength assessments, and accelerated creep rupture tests. Based on the findings, a repair strategy was implemented to remove the damaged section, rebuild the rotor, relieve stress, and re-machine it to accommodate new blade installation.
The repair was successful, allowing the unit to operate reliably for an additional 10 years and meet its retirement target.
To extend the life of aging steam turbine rotors, effective operational strategies and assessments are vital:
Utilities can maximize rotor life and safely achieve retirement schedules without compromising reliability or safety by addressing high-temperature damage mechanisms and implementing tailored strategies.Â
Contact our experts at ENTRUST Solutions Group today to find out how.Â
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Tom has spent the entirety of his 15-year career in the power generation industry.Â
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.Â
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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