Material Selection for Sour Service
A new pipeline is being designed to transport sour crude oil with 5% H2S and 3% CO2 at a partial pressure of 10 psi H2S. The operating pressure is 1000 psi, and temperature is 60°C. The pipeline will be buried and subject to external corrosion. The design life is 30 years. The project team is considering carbon steel with corrosion inhibitor, 316L stainless steel, or duplex stainless steel 2205. The budget is tight, but safety and long-term integrity are paramount.
1. Evaluate the suitability of each material for this application.
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Carbon steel with inhibitor: Cost-effective but requires reliable inhibitor injection and monitoring. However, at 10 psi H2S partial pressure, the risk of sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC) is significant. NACE MR0175/ISO 15156 limits carbon steel to a maximum hardness of 22 HRC and requires proper heat treatment. With 5% H2S, carbon steel may be susceptible to SSC if not properly controlled. Inhibitor effectiveness in sour service is challenging. 316L stainless steel: Not suitable for sour service with 5% H2S because it is susceptible to chloride stress corrosion cracking (Cl-SCC) and pitting in the presence of chlorides and H2S. Duplex 2205: Has good resistance to SSC and HIC, but at 10 psi H2S and 60°C, it may be acceptable if the pH is controlled. However, duplex is susceptible to chloride SCC at higher temperatures, but 60°C is generally safe. Duplex is more expensive but offers lower maintenance. Given the sour conditions, duplex 2205 is likely the best choice, but need to verify with NACE MR0175 boundaries.
2. If carbon steel with inhibitor is chosen, what additional measures are needed to ensure integrity?
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If carbon steel is chosen, the following measures are necessary: (1) Ensure the steel meets NACE MR0175 requirements for sour service (hardness ≤22 HRC, proper chemistry, heat treatment). (2) Implement a robust corrosion inhibitor program with continuous injection and monitoring. (3) Use internal coating to provide a barrier, but ensure coating is compatible with sour service and can withstand the temperature. (4) Install corrosion monitoring (coupons, probes) at critical locations. (5) Perform regular intelligent pigging to detect corrosion. (6) Control pH to minimize corrosion. (7) Consider HIC-resistant steel if the environment is severe. (8) Implement external CP and coating for external corrosion. Given the high H2S, carbon steel may not be the most reliable option for 30 years; therefore, a lifecycle cost analysis should compare with duplex.