The CSB Investigation identified that: A steel pipe elbow containing high concentrations of nickel and copper had become severely thin from HF corrosion and ruptured to initiate the incident. Carbon steel with high nickel and copper content is known within the industry to corrode faster from contact with HF than
carbon steel with lower nickel and copper content. While the pipe elbow had become severely thin from corrosion, adjacent piping components lower in nickel and copper content had not corroded as quickly and were not thin.
Well before 2019, refining industry corrosion/materials SMEs, readers/users of API 751, and HF Alky unit refinery inspectors/analysts, should have been aware of, had experience and/or information about the adverse effect of residual copper, nickel, and chromium on the corrosion resistance of carbon steel in HF acid alkylation service. Yet effective corrosion control/management, effective piping CML programs and Retro-PMI programs, and Process Safety still are challenging us....why? What are your thoughts?
With limited budgets and resources, risk-based inspection studies/programs, supported by high-quality Damage Mechanism Reviews (DMR) studies and corrosion control guidance (documents,) seem essential to focusing our efforts on high-risk concerns/issues.
A Corrosion 1993 paper published by NACE (now AMPP) titled, Effect of Residual Copper, Nickel, and Chromium on the Corrosion Resistance of Carbon Steel in Hydrofluoric Acid Alkylation Service, analyzed field failures of piping in HF service with elevated copper, nickel, and chromium content over a ten-year period, beginning with the first documented field failure in 1980. In summary, the paper, and subsequent data and technical literature within the industry communicated and documented that steel piping with high nickel and copper content will experience higher corrosion rates and more likely to fail (leak) due to HF corrosion.
Further, a NACE Corrosion 2003, Paper 03651 indicated that the combination of carbon (C) content and residual element content (Cr, Ni, Cu) could increase this non-uniform corrosion by up to 5 fold compared to moderate measured corrosion rates. This same study concluded that non-uniform corrosion would be minimized if: C > 0.18 wt% and the Cu + Ni + Cr < 0.15 wt% and that the non-uniform corrosion would be maximized if C < 0.15 wt.% and Cu + Ni + Cr > 0.30 wt%. This information can assist an operator in focusing their inspection programs for potential localized corrosion due to carbon steel chemistry.
Should we expect that with long-standing retro-PMI programs and the availability of portable optical emission spectrometers/analyzers, the industry has the technical capability to purchase and validate receipt of this type of low residual element material for a plant. ASTM standards for typical [carbon steel] specifications such as A516, A106, A333, A960, A961 now include supplementary requirements for HF service application [34, p. 38].
At the time of the Philadelphia Energy Solutions HF Acid Alky Unit incident on June 21, 2019, published industry standards and recommended practices did not require refineries to conduct 100% component inspection of carbon steel piping in HF service to identify any piping components corroding and thinning faster than others, which as shown by this incident, can lead to hazardous loss of containment events.
However, an effective retroactive positive material identification (RPMI) program should be able to readily identify carbon steel with elevated nickel and copper levels.
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William L. (Bill) Valerioti
Chair, Process Industries Technical Committee TC 07
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