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By Tom Reid, Vice President of Power Generation Services, ENTRUST Solutions Group<\/i><\/p>\n
Combustion turbine blade maintenance and refurbishment intervals depend on several design and operational factors. Hot-section turbine blades and vanes are subjected to various damage and failure mechanisms, as discussed below.<\/span><\/p>\n Determining appropriate inspection and refurbishment intervals is complex, requiring a balance between premature scrapping of components and forced outages caused by underestimating wear and tear. Damage mechanisms identified through operating experience and inspection reveal patterns that can refine maintenance schedules. Some key examples of these mechanisms are highlighted in this article.<\/span><\/p>\n Oxidation damage on stationary vanes can be caused by prolonged high-temperature exposure. Advances in vane coating technologies could mitigate this damage and extend maintenance intervals.<\/span><\/p>\n Similarly, cracking reveals the impact of thermal-mechanical fatigue. Upgrading materials may prolong TMF life, especially if replacement vanes are made with improved high-temperature alloys.<\/span><\/p>\n Unexpected vibration, though rare, can also lead to catastrophic failure before a component reaches its scheduled refurbishment period. The cause could be an installed flutter condition at high mass flow rates compounded by detrimental blade-to-blade frequency relationships. This failure mode, linked to original equipment manufacturer (OEM) design issues, can be addressed by redesigning the blade to increase the separation between its first-mode natural frequency and other resonances.<\/span><\/p>\n Many industrial combustion turbine OEMs use equivalent operating hours (EOH) formulas to guide refurbishment intervals. These formulas generally include the following factors:<\/span><\/p>\n The outcomes of these EOH formulas can vary significantly depending on the unit\u2019s operating conditions. For instance, a unit running at baseload for three years might accumulate an EOH of ~22,000 hours. By comparison, the same formula applied to a cycling unit may calculate an EOH of ~38,000 hours. This variation highlights the differing damage mechanisms between baseload and cycling operations.<\/span><\/p>\n However, damage in cycling and baseload modes often appears in different component areas and is not necessarily additive, as implied by EOH calculations. This limitation is why certain OEMs recommend intervals based on total operating hours or cycles, whichever comes first.<\/span><\/p>\n To illustrate, consider two units with identical designs but different operating profiles. One consistently operates in cycling mode, while the other runs at baseload. Over time, blades in the cycling unit may retain considerable creep life. If transferred to the baseload unit, these blades could potentially extend their service life. Managing a pool of blades with varied operating exposure could provide cost-effective opportunities to extend blade life.<\/span><\/p>\n Rigid reliance on EOH calculations may overlook such opportunities.<\/span><\/p>\n Blade and vane replacement costs represent a substantial share of a plant\u2019s operating and maintenance budgets during major overhauls. A systematic evaluation of failure mechanisms and reviewing OEM-provided maintenance interval guidelines could offer opportunities to refurbish parts rather than replace them. For example, advanced coatings that resist oxidation and hot corrosion could help prolong component life.\u00a0<\/span><\/p>\n Similarly, thermal barrier coatings may reduce parent material creep damage, while blades with limited life in cycling conditions may operate reliably in baseload applications.<\/span><\/p>\n Taking a proactive approach to refine predictive refurbishment intervals can lower the costs associated with replacement parts. Destructive evaluations of components nearing the end of their life are especially valuable. Metallurgical sectioning, creep life assessments, tensile property evaluations, and coating effectiveness studies can yield critical insights to improve refurbishment strategies and extend operating life.<\/span><\/p>\nCommon Failure Mechanisms<\/span><\/h2>\n
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Maintenance and Refurbishment Interval Calculations<\/span><\/h2>\n
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Moving Forward<\/span><\/h2>\n