By Tom Reid, Vice President of Power Generation Services, ENTRUST Solutions Group
Combined-cycle power plants are expected to cycle on and off more frequently than traditional fossil-fired units. Plant owners and developers must thoroughly evaluate cycling requirements when specifying or procuring steam turbines to achieve reliable performance. Specifications should ensure that the chosen turbine can respond rapidly and operate reliably under rigorous cycling conditions.
Key design criteria for steam turbines must include optimal materials, detailed stress and low-cycle fatigue (LCF) analyses, verification testing, fracture mechanics evaluations, and advanced non-destructive examination (NDE) methods.
Each start, stop, and load change reduces the LCF life of a steam turbine. To combat fatigue-related issues, it’s essential to minimize crack initiation and limit the growth of cracks from inherent component flaws.
The first step involves reviewing operating procedures to identify opportunities for reducing LCF damage. Next, product design enhancements must address the unique demands of the turbine’s application and duty cycle.
Thermal stress plays a critical role in crack initiation, particularly in steam turbines’ high-pressure (HP) and intermediate-pressure (IP) sections.Â
Reducing the temperature difference between metal turbine components and the steam flow can mitigate thermal stress. Strategies to achieve this include:
Among these, particular attention should be given to the shutdown cycle for two main reasons. First, rapidly starting up and changing loads is a key requirement for meeting peaking power demands. Second, modifications to the shutdown cycle can reduce the overall stress range as effectively as the startup cycle changes.
One approach to consider is transitioning quickly from steady-state operation to no-load conditions. This minimizes the time the turbine is exposed to high mass flow rates and low steam temperatures.Â
However, this approach depends on the power grid dispatcher’s decisions. Thermal stress analysis typically shows that gradually decreasing load before shutdown (forced cooling) significantly shortens turbine life.Â
Conversely, rapid shutdown from full-load or steady-state conditions can double the turbine’s lifespan compared to forced cooling.
Both thermal and centrifugal stresses significantly contribute to LCF damage in HP and IP turbines. However, centrifugal stress alone is the primary cause of LCF damage in low-pressure (LP) turbines and generator rotors. Thus, designing components to minimize centrifugal stress is vital to utility specifications.
Finite-element analysis (FEA) is vital for identifying areas of peak centrifugal stress. For example, in one project, FEA revealed high-stress concentrations at the filleted contours of a conventional LP rotor disk under planned operating conditions. Stress concentrations were eliminated by specifying a smoother, more gradual rotor disk contour design.
Designing the last-stage blade attachment zone is also critical, as this area experiences significant centrifugal stress with minimal thermal influences. Given the high potential for LCF damage in this zone, turbine suppliers should be required to provide FEA data that identifies peak stresses in this critical area.
When comparing peak-stress analyses from turbine suppliers, it’s essential to examine a range of factors, including:
Discrepancies in finite-element analysis methods and assumptions among manufacturers affect the results and require careful evaluation. Additionally, corrections for plastic strain are necessary when peak stresses exceed the material’s yield strength. Ignoring these corrections can result in significant errors in predicting LCF damage and component life. For this purpose, methods like Neuber’s rule should be applied, though explaining them is beyond the scope of this blog.
Some OEMs offer verification testing of actual root-groove configurations, which is the most accurate way to evaluate component designs. This testing eliminates the need to approximate peak stresses and considers actual root-to-groove interface conditions.
Creep is another critical factor in the HP and IP turbine sections. Long-term creep contributes to crack initiation. Analyses of a material’s Larson-Miller stress-rupture curves can correlate allowable rotor stress with exposure time, temperature, and material properties.
By comparing steady-state maximum stresses with these curves, it’s possible to estimate the fractional life reduction of critical components. If the time-to-creep-rupture exceeds the plant’s design life, fractional damage can be approximated as the ratio of the design life to creep-rupture time. Many turbine manufacturers integrate creep data into their LCF design charts.
The presence of small cracks does not necessarily mark the end of a turbine’s service life. Crack propagation to critical size must also be evaluated.
Since every turbine component has microscopic flaws from the forging and fabrication processes, Linear Elastic Fracture Mechanics (LEFM) should be used to ensure flaws won’t grow to critical size within the operational duty cycle. Specifications must set conservative criteria for LEFM analyses to provide reliable crack growth predictions.
Stress intensity at a crack’s tip depends on flaw geometry. For conservatism, manufacturers should treat all flaws as elliptical with a 5-to-1 aspect ratio. Flaws near the rotor surface must be treated as surface flaws, as surface cracks grow faster and reach critical size sooner. The specification must also account for flaw-to-flaw interactions and other system effects using validated fracture-mechanics codes such as EPRI’s SAFER software.
Material selection is another crucial factor in ensuring LCF and creep resistance. High-ductility materials are preferable because they allow larger critical crack sizes and longer component life. Fracture toughness, directly related to material ductility, influences critical crack size and can be approximated using a material’s impact energy.
Another property to consider is the Fracture Appearance Transition Temperature (FATT), which indicates the temperature at which a material shifts from brittle to ductile behavior. Specifying materials with low FATT minimizes startup-related risks and reduces pre-warming requirements.
Power producers can ensure their combined-cycle steam turbines operate reliably under demanding cycling conditions with diligent design evaluations, detailed specifications, and thorough supplier review processes.Â
Properly designed turbines will deliver long-term, trouble-free cycling performance while meeting the stringent requirements of modern power operations. If you’re looking to safeguard your combined-cycle power plant’s steam turbine, ENTRUST is here to help.Â
Contact us today to learn more about how we can support your performance and help you avoid costly failures.
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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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