Design of pressure-relief systems for CCS plants (22.RP1.0116) – Completed

The challenge

One safety risk of CCS is failure of CCS equipment at high pressure, causing a large release of concentrated CO2 into the surrounding area, where people can be harmed by CO2 exposure. The pressure-relief system in a CCS facility is designed to prevent failure of CCS equipment at high pressure by keeping pressure within design limits.

Pressure relief for CCS involves a CO2-rich mixture—potentially a dense or supercritical phase fluid—flowing to a low-pressure disposal system such as an atmospheric vent. The wide range of possible fluid conditions within the CCS pressure-relief system presents a challenge to designers.

Without first recognising the range of fluid conditions and then selecting the best equations of state (EoS) and pressure safety valve (PSV) models for design, the pressure-relief system installed at a CCS facility may be insufficient for protecting against equipment failure.

The solution

Woodside Energy provided this project with realistic CCS process conditions. These process conditions informed the modelling of fluid flow through a CCS pressure-relief system to an atmospheric vent.

The project investigated five questions relevant to CCS pressure-relief system design:

1. Are CO2 mixture properties and phase behaviour predicted more reliably by cubic or multiparameter EoS.
2. Are significant differences in fluid transport and pressure-relief system design found when different EoS (SRK, GERG-2008, ‘Helmholtz-EoS’) are used.
3. Are significant differences in fluid transport and pressure-relief system design found when different PSV models (HEM, Delayed-HEM) are used.
4. Do plausible scenarios exist where a change in relief temperature causes a switch from condensing to flashing flow, and does this affect pressure-relief system design.
5. Can pressure-relief systems be undersized due to improper selection of EoS or PSV models.

Results from the above investigation can equip engineers with a better understanding of CCS pressure-relief system design.

The outcome

For the specific CCS process conditions provided by Woodside Energy, the project found:

1. The CO2 mixture could flow through the pressure-relief system as flashing liquid or as condensing gas depending on relief temperature. The required size of pressure-relief system for flashing liquid was found to be substantially different than for condensing gas. This complex situation shows the importance of identifying the full range of relief temperatures for the CCS process to ensure the pressure-relief system is adequately designed.

1. The part of the pressure-relief system most sensitive to EoS choice was choking pressure in the PSV—particularly for relief conditions far above the mixture’s critical pressure. Although PSV mass fluxes were broadly consistent across EoS, differences in the predicted choking pressure cascaded to design differences (diameters, lengths) for the PSV discharge piping.

The impact

Equipping engineers with a better understanding of the complexities of pressure relief for CCS will lead to:

• Fewer major process safety events, reducing costs associated with damage, fines, and litigation.
• Reduced reputational damage to CCS technology as a whole.
• Less rework required to rectify design issues.
• Reduced scrutiny of operations by safety regulators.
• Enhanced facility reliability and operability.

Next steps

Three key areas requiring further investigation were identified:

1. For complex CO2 mixtures, some EoS could not reliably produce complete dew curves. A review of existing experimental data is required to identify the most reliable EoS for predicting condensation of complex CO2 mixtures.
2. Pressures and temperatures through a CCS pressure-relief system span wide ranges. A review of experimental data underpinning the EoS used in this work is required to determine whether the data spans these wide pressure and temperature ranges. If not, further experiments are required to extend EoS range of validity.
3. This project focussed on changes to pressure-relief system sizing when different EoS and PSV models were used; for example, changes in PSV orifice size with different EoS. An alternative approach is recommended that keeps the pressure-relief system design fixed. Different EoS and PSV models would then predict different relief capacities for a fixed pressure-relief system. This approach would be useful for checking existing CCS installations.