Valves for CO2 Carbon Capture and Storage Service — CCS and CCUS

Carbon Capture and Storage (CCS) and Carbon Capture, Utilisation and Storage (CCUS) projects — including post-combustion capture, pre-combustion capture (IGCC), oxyfuel combustion, and direct air capture — involve capturing CO2 from industrial sources and either permanently storing it in geological formations or utilising it for enhanced oil recovery (CO2-EOR) or industrial use. The valve requirements for CO2 service differ significantly from standard oil, gas, and steam service because of CO2's unique corrosion behaviour and phase properties.

CO2 Corrosion Mechanisms

Dry CO2 (< 50 ppm water) is non-corrosive to carbon steel. Wet CO2 — even with as little as 50 ppm water — forms carbonic acid (H2CO3) in contact with carbon steel, causing electrochemical corrosion at rates of 1–10 mm/yr at pipeline temperatures. In CCS applications, CO2 streams from power plants and industrial sources often contain trace impurities (O2, H2S, SO2, NO2, H2O) that significantly accelerate corrosion: O2 content > 50 ppm dramatically increases corrosion rates; H2S > 50 ppm triggers sour service requirements (NACE MR0175); SO2 forms sulfurous acid in the presence of water, attacking carbon steel; combined CO2-H2S-H2O service represents the most aggressive CCS pipeline environment.

Supercritical CO2 Properties — Valve Design Implications

CO2 becomes supercritical above 31°C and 73.8 bar (the critical point). Supercritical CO2 (scCO2) has liquid-like density (~500–700 kg/m³) but gas-like viscosity (~0.03–0.06 mPa·s), compared to liquid water at ~0.9 mPa·s. This means: scCO2 can permeate seals, elastomers, and PTFE seats much more aggressively than conventional liquids; elastomers (Viton, EPDM, Buna-N) swell and lose strength in scCO2 — leading to rapid seat extrusion failure; PTFE creeps under sustained compressive loading in scCO2 — especially at elevated temperatures; metal-to-metal seats (Stellite or hardened SS 316) are preferred for scCO2 ball and gate valves above 100 bar.

Recommended Materials for CO2 Service Valves

CO2 Pipeline Standards

CCS pipeline design draws from multiple standards: DNV-RP-F104 (Design and Operation of Carbon Dioxide Pipelines) is the primary CCS-specific standard; ISO 27913 (Carbon Capture, Transportation and Geological Storage — Pipeline Transportation Systems) covers CO2 pipeline design internationally; ASME B31.4 (Liquid Hydrocarbon Pipelines) and ASME B31.8 (Gas Transmission) are applied by engineers based on CO2 phase state; API 6D covers pipeline ball and gate valves — specified with metal-to-metal seats, NACE MR0175 compliance if sour impurities present, and fugitive emissions per ISO 15848-1 Tightness Class A. Valve ESD (emergency shut-down) actuation is required at maximum intervals of 1 km in dense-phase CO2 pipelines per DNV-RP-F104.

Valve Sizing Considerations for Dense-Phase CO2

Dense-phase CO2 (above critical pressure, below critical temperature) behaves similarly to a low-viscosity liquid but has unusually high compressibility near the critical point. Key sizing considerations: Cv sizing for dense-phase CO2 uses liquid density (~800 kg/m³) not gas density — use liquid sizing equations; avoid flashing across control valves by maintaining outlet pressure above critical pressure wherever possible; fast-acting ESD ball valves can cause pressure waves ('water hammer') in dense-phase CO2 pipelines — use timed-close actuators (typically 30–60 second closure) with hydraulic snubbers; metal-seated ball valves in ASME Class 600–900 are standard for CO2 pipeline block valves.