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CO2 pipelines risk assessment. Brittle fracture

The aim of this publication is to provide an overview on the problematic of assessing dense phase CO2 pipelines against brittle fracture. If you want to know more about it, contact us.

In a previous publication we made an introduction to the aspects to be accounted when assessing fracture control of dense-phase co2 carrying pipelines. As mentioned, one of the mechanism to be analysed is the named brittle fracture.

The brittle fracture mechanism arises as the result of the concurrence of an operating temperature below the ductile to brittle transition temperature of the material with a stress situation greater than the threshold stress for brittle fracture

To prevent brittle fracture, it is essential that the pipeline steel exhibits a ductile to brittle transition temperature lower than the specified Minimum Design Metal Temperature (MDMT) and that the pipeline is operating on the upper shelf of the ductile to brittle transition curve. 

In order to avoid the occurrence of this phenomena it is necessary to ensure a correct material selection giving special attention to mentioned MDMT.

Figure 1. Minimum Temperatures Without Impact Testing for Carbon Steel Materials. Source: ASME B31.3-2020

CO2 depressurization

The materials employed in the dense phase CO2 transportation and injection system may be exposed to low temperatures due to process events (e.g. depressurisation) and/or atmospheric conditions. Low temperatures can cause embrittlement in susceptible materials, resulting in brittle fracture. Materials shall be selected suitable for service at Minimum Design Metal Temperature (MDMT).

Potentially very low temperatures can result from unintended dense phase CO2 blowdown scenarios such as pinhole leaks or flange leaks.

Some certainly complex physical phenomena take place once this occurs:

  • CO2 liquid flashing
  • Joule-Thomson effect in the produced CO2 vapour downstream the leak hole
  • CO2 real gas behaviour
  • Supersonic CO2 vapour expansion downstream of the leak hole
  • Conversion of CO2 gas internal energy into kinetic energy when flowing with high velocity as per fundamental law of conservation of energy
  • Conjugate heat transfer between flowing/expanding gas and the metal upstream, inside and downstream of the leak hole, friction is accounted for
  • A dilution with ambient medium downstream of the leak depending on the location of a leaked flange

The expansion of CO2 from a high pressure cylinder to atmospheric pressure through a nozzle is ideally a constant enthalpy process, so the pressure decreases along a constant enthalpy line as the CO2 passes through an orifice.

The discharge and dispersion of high-pressure CO2 pipelines differs from the hydrocarbons pipelines in involving complex physics including cool temperature, phase transition, sonic multiphase flow, and heavy gas dispersion.

A schematic of the highly under-expanded jet is shown in figure below. An expansion barrel structure is generated at the nozzle lip as the flow expands into the atmosphere. 

The ratio Stagnation Pressure/Ambient Pressure is an important parameter to describe the expansion level. When this ratio is above 15, the shock waves diamond pattern will form.

Except for the intercepting shock in the interior of the jet, the Mach disc normal to the flow and it is unique for under-expanded jet. The flow front the Mach disc is supersonic, whereas the flow behind the Mach disc is obviously subsonic.

Figure 2. The schematic of the under-expanded jet. Source: B. J. L. Y. W. C. Teng Lin, “An Experiment on Flashing-Spray Jet Characteristics of Supercritical CO2 from Various Orifice Geometries,” Frontiers in Energy Research, vol. 9, 2021.

The flashing-spray jet structure has a significant impact on the cooling process that takes place on the metal in conditioning the extension of the low temperature fluid in contact with the pipe wall.

There are some other cases in which the shockwave system shows a fan-shaped structure instead of the mentioned barrel-shape. The Mach disc is unobserved in this cases the CO2 flows radially after shock wave.

All the above mention applies to a multispecies fluid with the addition of the proper complexity that arise from the different species involved. Moreover, in the event of having a strong interaction between then it would be necessary to account it.

Figure 3. Example of velocity field (left) and temperature (right) results from a previous CFD analysis performed by SDEA. Contour legend intentionally not included.

It is for the above mentioned that distributed analysis methods, e.g. CFD, comes to relevance to determine the MDMT. In SDEA_Engineering Solutions we have plenty expertise in performing this type of assessment and a lot more likewise challenging.

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