Nondestructive testing of pipeline materials: analysis of chemical composition from metal filings

摘要

The Pipeline and Hazardous Materials Safety Administration(PHMSA)has proposed new requirements for verification of pipeline materials where traceable, verifiable, and complete(TVC)records do not exist.This verification requires knowledge of material properties, including strength and chemical composition.The Pacific Gas and Electric Company(PG&E), as a part of its Facilities and Integrity Management Program(FIMP), is evaluating the use of nondestructive examination(NDE)technologies to determine these properties.Oftentimes, the use of NDE not only minimizes interruptions to operations, but also, in the case of facilities components, is the only practical means of measuring mechanical properties since the diversity of features in a given station precludes destructive materials testing for verification.This paper focuses on the determination of the chemical composition of features by NDE.The chemical composition of a feature, along with processing history and manufacturing practices, influences its mechanical properties.It can also be a marker of historical manufacturing practices, and has therefore been used to estimate the intended grade or the mechanical properties of features that are lacking records(i.e., to determine the design maximum allowable operating pressure [MAOP] for features that are lacking records).A probabilistic pipeline feature grade prediction tool developed by PG&E and Kiefner and Associates, Inc.(Kiefner)utilizes chemical composition in this manner.1 When using such calculators, it is important to understand the reliability and accuracy of analytical methods for determining key chemical elements found in line pipe, as variability in the measured chemical compositions could influence the output.Furthermore, examination of some of the common analytical tools used for chemical composition determination is valuable for many aspects of pipeline integrity management, which require knowledge of material properties.For example, chemical composition is an input to some algorithms that estimate Charpy impact properties,2 and impact toughness is a critical consideration in determining failure pressure of planar flaws and leak vs.rupture behavior.3