Damage Control: High-Temperature Creep Assessment

Editor’s Note: This regular column offers practical insights into various damage mechanisms affecting equipment in the O&G, petrochemical, chemical, power generation, and related industries. Readers are encouraged to send us suggestions for future topics, comments on the current article, and raise issues of concern. All submissions will be reviewed and used to pick topics and guide the direction of this column. We will treat all submissions as strictly confidential. Only Inspectioneering and the author will know the names and identities of those who submit. Please send your inputs to the author at damagecontrol@inspectioneering.com.

Introduction

Part 1 of this Damage Control series on high-temperature creep discussed the fundamental damage mechanics associated with time-dependent creep in metals operating at elevated temperatures. Additionally, typical creep damage morphology was highlighted, including the formation of isolated creep voids that tend to coalesce over time to form micro-cracks, and eventually, macro-cracks that can lead to component failure/rupture. The limitations associated with traditional surface and volumetric inspection practices, attempting to identify creep damage, were also explained. This installment of Damage Control provides practical commentary on performing engineering evaluations to approximate creep damage in pressure components and how the Materials Properties Council (MPC) Omega Creep Method, as described in Part 10 of API 579-1/ASME FFS-1, Fitness-For-Service (API 579), can be leveraged to estimate equipment remaining life [1]. A brief overview of the creep rupture data captured in API 530, based on the Larson Miller Parameter (LMP), is also rendered [2,3]. Furthermore, an overview of the advantages of Omega creep testing is offered herein (including the development of material properties for next-generation high-temperature alloys), and relevant examples of how the API 579-based assessment methodology can be incorporated in advanced, inelastic computational simulation techniques such as finite element analysis (FEA) are provided.

Conventionally, two methods are used to estimate creep damage accumulation and predict pressure component remaining life. They are described below:

• API 530 (Larson Miller Parameter [3]): Creep rupture data is provided for minimum and average properties in terms of the LMP. This method is commonly used for fired heater tube thickness determination and remaining life estimates.
• API 579 MPC Omega Method: Creep data is based on strain rate and Omega parameters, and assessment options are available to account for minimum and average properties.

In general, for any creep life prediction approach, material properties, stress, and operating metal temperature history (ideally based on thermocouple readings) of a component must be known with reasonable certainty to ascertain meaningful remaining life estimates. Operating data also needs to be documented at a sufficient resolution/frequency to capture any fluctuations (e.g., process upset events) in pressure or temperature. High-temperature excursions can contribute significantly to creep damage accumulation over time, and as such, they should be considered in any fitness-for-service (FFS) assessment. Furthermore, acquiring and compiling tabular operating data that can be manipulated into loading histograms, which incorporate statistical analysis to bound the observed operating trends can be beneficial (the value of this for in-service equipment will be discussed further in Part 3 of this Damage Control series). Such data can also be exercised to perform sensitivity analysis on these input parameters. The reason for requiring complete process data sets in an engineering evaluation is that in general, creep damage accumulation is highly dependent on these parameters, and small changes in long-term metal temperature (or applied stress) can dramatically influence overall behavior in the creep regime and ultimately, remaining life. This phenomenon is discussed further herein, along with other pragmatic insights into state-of-the-art, high-temperature FFS assessment methods and remaining life predictions.

The Larson-Miller Parameter (LMP)

The LMP approach offers an extrapolation technique for correlating both time and temperature to other quantities such as applied stress. Given this, equivalent times corresponding to stress rupture (end of useful operating life) in a material can be determined as a function of operating metal temperature. In its most general form, the LMP is defined as follows: