Using Portable Material Property Devices for Pipe Grade Determination

Proposed U.S. gas pipeline industry safety regulation changes were announced in 2016, many of which were prompted by the U.S. Department of Transportation (DOT) incident findings over the last decade. Of special note is the proposed new requirement in 49 CFR Part 192.607 to determine and verify the physical characteristics of any installed line pipe, valve, flange and component where material records are not available. To satisfy this requirement, Reliable, Traceable, Verifiable and Complete (RTVC) records will be needed in High Consequence Areas (HCA), Class 3 or Class 4 locations.

The industry has been developing in-situ techniques to measure pipe properties in lieu of other destructive approaches (e.g., cut-outs and tensile testing) for the last 20 years. Part 192.607(c) has proposed that any non-destructive method used to determine strength be able to produce results accurate within 10% of the actual value with 95% confidence. Furthermore, the operator must use methods, tools, procedures and techniques that have been independently validated by subject matter experts in metallurgy and fracture mechanics.

The approaches being pursued to date for assessing properties include: hardness testing (e.g., Brinell, Leeb) combined with metallography and chemical characterization techniques; instrumented indentation (e.g., progressive indentation and unloading); as well as frictional sliding techniques. Most of the more advanced approaches has been commercially developed, so their theory and interpretation basis is proprietary. Very little testing data have been made available to support an independent analysis of each supplier’s performance claims. This places the performance specification burden of accuracy and confidence on the end user.

Hardness testing is essentially an empirical measurement of a material’s resistance to plastic deformation; the measured hardness is related to the size or depth of an impression made at a defined load. But fundamental material properties, such as yield strength, are not directly measured due to the complex stress-strain state involved in the deformation. Despite this complexity, the measured hardness is clearly related to the elastic-plastic properties of the material, and the hardness value usually correlates reasonably well with strength, particularly within a given material class. Somewhat counter-intuitively, the correlation is usually better for ultimate tensile strength than yield strength, and standard correlations between tensile strength and hardness for various alloy systems are given in ASTM and CEN specifications.

Joint Industry Development efforts have made progress toward understanding the correlation between hardness and mechanical properties. GRI, PRCI and private industry projects have all applied research dollars toward this pursuit. Promising new techniques include: Progressive indentation; Frictional sliding principles, and the recent repurposing of Inline Inspection (ILI) signal responses for classifying pipe joints with similar bulk magnetic properties.

A formal, written material testing procedure can be a beneficial tool to increasing the performance and reliability of these nondestructive tests. An effective procedure defines roles of responsible personnel, describes the process for determining the number of tests required to verify the materials, describes the sample collection process and field data collection process, standardizes documentation, and provides guidelines for determining the test locations. The use of record data (as-built drawings, bills of materials, etc.) and its interaction with test results must be considered carefully. Application of a rigorous procedure that provides a clear method and criteria can improve consistency and traceability in material testing.

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