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Engineering Assessment of Substandard In-Line Isolation Split Fitting


 In-line isolation fittings used in TC Energy systems typically include a split tee assembly, flange, completion plug and bolt-up hardware, and often form part of an in-line isolation train plugging system during maintenance projects. Due to potential concerns about a few large-size split tee flanges not meeting required strength and toughness properties, TC Energy undertook a fitness-for-service assessment. This assessment involved physical proof testing coupled with high-fidelity continuum finite element analysis (FEA) and engineering critical assessment (ECA), accounting for material deficiencies and hypothetical fracture indications.
 Proof testing in excess of 2 times (2 x) maximum allowable pressure (MOP) at a temperature of −46°C was performed, which involved introducing strategically sized, positioned and oriented electrical discharge machining (EDM) notches to approximate hypothetical crack-like flaws. Both the analysis and full-scale pressure proof testing demonstrated that the flanges did not experience any form of crack growth, though mechanical destructive testing showed that at certain locations the Charpy V-notch (CVN) toughness was as low as 3 J. Analysis predicted high plastic strain, which was confirmed in the full-scale testing without causing plastic collapse, burst or propagation of the preexisting flaw introduced by EDM. Test results showed reasonable consistency with the analytical results, supporting the appropriateness of applying the proposed analytical approach to location-specific assessment for current and potential similar future issues.
 Based on the assessment, confidence has been gained that for split tee flanges with low toughness and low tensile strength. The testing validated analytical approach adopting sound design parameters with installation specific design loading is reasonable to determine fitness for service. The engineering assessment methods, testing setup, results and findings are summarized in this paper.