Gas turbine components and coatings are subject to a number of critical damage modes that impact the safe and cost effective operation of the turbine units. Typically, damage may be due to hot gas path corrosion and erosion , creep, low cycle fatigue, coating cracking and spallation, and microstructural degradation of the alloys used. The severity of each damage mode depends on the duty cycle and fuel(s) used during the lifetime of the parts.
Many turbine components are routinely inspected and replaced or refurbished when incipient damage is detected, or at arbitrary time intervals where no formal fitness-for-service analysis is performed. High value or difficult to service parts may appear to be ideal candidates for quantitative or semi-quantitative FFS-style analysis when inspection or materials testing detects flaws, but this is not always the case. While API 579-1 is intended as a comprehensive FFS standard for the petroleum and petrochemical industries, it does not apply to rotating equipment. Indeed, there are few formal standards (API, ASME or BSI) for fitness-related analysis of most rotating equipment – including gas turbines, and API-compliant FFS assessment is never indicated due to the unmanageable data requirements, computational complexity and improbability of a definitive result. More commonly, operators require a “condition assessment and remaining life prediction” that centers on damage evaluation and provides recommendations for operating strategies. In these evaluations, the life predictions are tied to specific damage mechanisms.
Gas turbines present unique challenges to fitness evaluations and remaining life assessment. Initial original equipment manufacturer service intervals only represent guidelines on gas turbine maintenance and the data upon which they are based are often proprietary. They generally do not address the specific operation of each individual unit and tend to be generic in nature. To manage maintenance intervals and procedures efficiently and more cost effectively, operators need to develop an understanding of attrition rates and the mechanisms by which their components degrade in service; these attrition rates and mechanisms are often unique to individual turbines owing to their distinct operating conditions.
Quest Reliability engineers are highly skilled and experienced in performing critical engineering evaluations of turbine components to determine their fitness-for-service. Quest Reliability can provide experts with advanced training in physical metallurgy, corrosion, structural integrity and tribology, and that are specialized in remaining life assessment and management of gas turbines. Quest Reliability applies a battery of skills and techniques to identify damage mechanisms in rotating equipment, assess and quantify conditions, and establish the rates at which damage is occurring. On this basis, Quest Reliability can develop a highly confident quantitative prediction of the remaining life of rotating equipment components. The remaining life estimate is a key factor in adjusting operating levels and performance expectations with a higher level of confidence, and in establishing inspection and maintenance programs that promote and enhance plant reliability.
The remaining life prediction studies can be improved significantly with the incorporation of physical inspection and testing data. For example, prediction of the remaining life of a turbine blade set can be achieved much more accurately by using metallographic examination and creep testing of samples removed from individual blades. Destructive testing of selected blades from a set can be cost-effective when assessing the viability of the set as a whole for refurbishment and further duty. Quest Reliability has developed a specialized protocol for miniature creep testing specifically for the purpose of predicting remaining life of turbine blades.
For further information about Gas Steam Turbines & Rotating Equipment, please contact us.