Damage Control: Brittle Fracture 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

The previous installment of Damage Control summarized the fundamentals, damage characteristics, and non-destructive examination techniques associated with crack-like flaws and brittle fracture and offered a historical overview of notable brittle fracture failures across numerous industries. Additionally, critical factors influencing brittle fracture susceptibility, including weld and steel properties, operating metal temperature, process conditions, and post weld heat treatment (PWHT) were described. Part 2 of this three-part series on brittle fracture focuses on methods for evaluating the risk for brittle fracture in pressure equipment. Specifically, the evolution of industry brittle fracture screening and assessment methods (and associated limitations) will be explained and a summary of modern fracture mechanics-based fitness-for-service (FFS) methods that can be leveraged to qualify crack-like flaws will be outlined. Understanding how to determine critical flaw sizes (that is, crack-like flaws that reach an unstable size) in pressure equipment can be beneficial for not only managing the risk of a catastrophic fracture but extending the useful service life of critical process equipment. Furthermore, an overview of the standard methodologies for determining minimum design metal temperature (MDMT) in accordance with ASME pressure equipment design Codes will be discussed herein. It is worthwhile for equipment designers to comprehend these approaches and to identify any associated potential inadequacies in the existing methodologies. Moreover, it is imperative that plant engineering, reliability, and maintenance personnel understand the original construction code basis for component MDMT and recognize that FFS techniques based in fracture mechanics can qualify unanticipated damage and help guide safe pressurization procedures to alleviate the risk for brittle fracture. To this end, the concept of establishing technically based minimum pressurization temperature (MPT) envelopes will be outlined in this article. This notion is especially relevant for heavy-walled, low chrome hydroprocessing equipment subject to in-service temper embrittlement and high-pressure hydrogen process environments. Lastly, a concise overview of ductile tearing assessment methods is offered, and commentary on proposed, technically justified modifications to the current API 579, Part 3 brittle fracture screening procedures is provided.

Design Code MDMT Guidance and Impact Testing

The ASME Boiler and Pressure Vessel Code (e.g., ASME Section VIII Division 1 [1] and Section VIII Division 2 [2]) use the term MDMT to describe the lowest permissible metal temperature at the vessel Maximum Allowable Working Pressure (MAWP). Furthermore, the MDMT can be determined either through destructive testing methods (e.g., Charpy impact testing) or through an engineering evaluation [3,4]. It is important to note that prior to 1987, ASME Section VIII Division 1 permitted operation of carbon steel pressure vessels to temperatures as low as -20°F without any formal testing or engineering analysis. As such, most of the pressure equipment operating in plants today did not receive a formal assessment to quantify the risk for brittle fracture during original design. The 1987 addenda (to the 1986 Edition) of ASME Section VIII Division 1 introduced notable revisions to Part UCS-66 due to industry-wide brittle fracture concerns associated with carbon and low alloy steel pressure vessels.