Creep is defined as time-dependent deformation at elevated temperatures. Boiler tubes, superheater components, hot re-heat piping, and reformer tubes, for example, are subject to creep damage. It is important for the management of these assets to have confident estimates of their remaining life. API 579-1/ASME FFS-1 recognizes the importance of creep damage and presents a standard approach to its analysis. The API/ASME standard implements the Omega model, which captures the creep behavior of a wide range of alloys.
Crack growth can occur at elevated temperatures, particularly in components that contain pre-existing planar flaws. The API/ASME FFS standard includes a predictive model for creep crack that combines the Omega method with the principles of fracture mechanics.
Quest Reliability staff has long been involved in the development and application of assessment methods based on the Omega model. We have an extensive suite of analysis tools and laboratory facilities for assessing creep damage and creep crack growth at elevated temperature, including:
- Signal Fitness-For-ServiceTM software, which includes a module for performing Level 2 creep damage assessments and creep crack growth analyses
- Level 3 finite element analysis of creep, using the Abaqus commercial software and a user subroutine that implements the Omega material model
- LifeQuestTM HEP software, which performs stress analysis and life prediction of high-energy piping that operates at elevated temperatures
- State-of-the-practice creep laboratory, which has the capability to perform Omega testing
- Materials laboratory, which can be used to characterize microscopic creep damage in components that have failed or have been retired from service
Some specialty alloys do not fit the Omega creep model. For example, modern HP alloys, from which reformer tubes are manufactured, exhibit unusual creep behavior which is not adequately captured by the Omega model. Quest Reliability has performed extensive creep testing of HP alloys and has developed a proprietary model that incorporates and adjusts for the unique behavior of these materials. This model and the underlying data are incorporated into a life assessment tool for reformer tubes.
For further information about Creep Rupture/Creep Crack Growth, please contact us.