Avoiding brittle fracture of process vessels and piping
High-fidelity analysis avoids risk and unnecessary CAPEX
Brittle fracture is defined as the sudden rapid fracture under stress where the material exhibits little or no evidence of ductility or plastic deformation.
Whereas ductile materials are characterised by high toughness, brittle materials have low fracture toughness. Most steels undergo a ductile-to-brittle transition in fracture toughness as their temperature is reduced (figure, right).
In a hydrocarbon processing facility a brittle fracture of a pressure vessel or a pipe segment could lead to the release of flammable hydrocarbons with a significant risk of harm to people, as occurred in the Longford Gas Plant Explosion of 1998.
If a pipe or pressure vessel is subjected to a temperature below its lower design temperature it is considered embrittled and therefore at risk of fracture. Depending on the mechanical stresses the process equipment is subjected to, this can result in brittle failure.
Examples of significant mechanical stresses of pipes and vessels are:
- High internal pressure
- Shock chilling, resulting in sudden changes in temperature
- Vibrations such as those caused by choking in tailpipes
- Thermal expansion/contraction stresses in long pipes
- External impact.
To assess integrity risks and avoid brittle fracture the minimum temperature that process equipment can obtain is assessed against its specified mechanical design limits, in particular the Lower Design Temperature (LDT) or alternatively the Minimum Allowable Temperature (MAT).
Abnormal (usually transient) occurrences on the facility most commonly lead to low temperatures for operational events such as start-up, shutdown and emergency situations. Of specific concern are:
- System depressurisation
- (Re-)pressurisation of the process, for example during well start-up
- Flare piping thermal response during pressure relief and plant blowdown events
- Transient response following loss of heating
PSE Oil & Gas assesses such scenarios using best practice dynamic model-based approaches.
Brittle fracture during process blowdown and pressure relief events – Industrial Guidelines
API 521 – 6ed (2013) – Pressure Relieving and Depressuring Systems – is widely used as the basis of many operating companies standards and practices.
The 6th edition of the API standard highlights the importance of accurate assessment of brittle fracture risks both upstream of the blowdown valve (in the process) and downstream in the flare piping.
In both cases rigorous modelling techniques are recommended, with a particular focus on cases where metal temperatures are expected to impact metal selection criteria.
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Toughness vs. temperature plot
Lower Design Temperature
The lowest temperature at which equipment may be subjected to at its design pressure
Minimum Allowable Temperature
The permissible lower metal temperature limit: this can be an envelope of allowable temperatures as a function of pressure