Developing a Fitness for Service Approach for Reduced Toughness Carbon Steel Piping, Flanges & Fittings

Introduction

It has been reported by multiple sources that recently manufactured piping, fittings, and flanges have reduced toughness and do NOT meet the ASME BPVC Section VIII Division 1 -20°F impact test exemption for listed components. All ASME B31.3 Figure 323.2.2A Curve B materials are considered to be potentially at risk, although the issue has not been found in piping components manufactured from plate materials. Recently, several owner/operators have undertaken toughness audits of new project components to quantify this issue. The results confirm a widespread and endemic problem with a substantial population of supplied components not meeting the toughness requirements for exemption. Industry response to date has been focused on collecting additional evidence to quantify the scale of the issue and to propose various changes to industry standards and specifications, to mitigate the potential for further issues with new production components. However, there currently is no focused industry effort in addressing what is suspected to be a substantial population of in-service components having reduced-toughness.

Despite the reduced-toughness of some recently manufactured fittings and flanges and the understanding that many are currently operating in systems that may present an increased probability of fracture, there have not been a significant number of reported failures. There appears to be some margin between design and operating conditions, resulting in a substantial reduction in the probability of brittle fracture. However, the size of this margin is not well understood therefore, making it difficult to identify which systems are fit for continued service and safe to operate. This demonstrates a need for a Fitness-for-Service (FFS) screening and analysis approach that can be utilized by industry to identify the most at-risk systems/components and focus the scope of mitigations necessary to avert a potential brittle fracture failure.

This article is an overview of the issue of reduced fitting toughness; the possible contributors to the problem; the measures that have been taken to-date to address the issue; and an introduction of a proposed fitness-for-service (FFS) approach to address the issue, specifically for components that are currently in-service.

Understanding why brittle fractures occur

Brittle fracture is the sudden and rapid fracture of a component under stress (residual or applied), where the material exhibits little or no ductility or plastic deformation prior to failure. Brittle fracture typically occurs when all of the following criteria are met, as presented in the brittle fracture triangle shown in Figure 1: