Supercritical CO2 (sCO2) pipelines in CCUS plants operate above 31°C and 73.8 bar, where CO2 behaves as a dense fluid with low viscosity and high diffusivity. This makes weld integrity critical: any micro-leak can propagate rapidly due to the high pressure and the aggressive nature of dry CO2.
The physical mechanism behind sCO2 weld failures is often related to the absence of a protective oxide layer at weld toes. CO2 at high pressure can cause stress corrosion cracking (SCC) in carbon steel HAZ if residual stresses are not relieved. Additionally, the low viscosity of sCO2 means that even a 0.1 mm root gap can produce a detectable leak, making root pass quality paramount.
Engineering Failure Analysis papers on welded pressure equipment frequently trace root cause to a combination of weld toe geometry, misaligned fit-up, and delayed hydrogen cracking rather than a single code nonconformance. For sCO2 service, this is amplified because the high-pressure dense phase can penetrate flaws that would be harmless in natural gas.
Key decision rules for sCO2 pipeline welding:
- Use GTAW for root passes with 100% argon backing to ensure full penetration and smooth ID profile.
- Limit heat input to 1.0-1.5 kJ/mm to control HAZ grain growth and maintain toughness at -30°C (typical design temperature).
- Apply PWHT for wall thickness above 25 mm to reduce residual stresses and mitigate SCC risk.
- Perform 100% volumetric NDT (PAUT or RT) plus a dedicated hydrotest at 1.5× design pressure with dry CO2-compatible seals.
A common pitfall is assuming that a standard API 1104 girth weld for natural gas is directly transferable to sCO2. The combination of high pressure, low viscosity, and potential for SCC demands tighter acceptance criteria on root concavity and reinforcement height.
How does your project handle sCO2 weld acceptance criteria?