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Failures in Pressure Vessels

 By Farzad Tabatabaie, Senior Equipment Engineer.

A) Categories of Failures in Pressure Vessels

  1. Material-Improper selection of material; defects in material.
  2. Design-Incorrect design data; inaccurate or incorrect design methods; inadequate shop testing.
  3. Fabrication-Poor quality control; improper or insufficient fabrication procedures including welding; heat treatment or forming methods.
  4. Service-Change of service condition by the user; inexperienced operations or maintenance personnel; upset conditions. Some types of service which require special attention both for selection of material, design details, and fabrication methods are as follows:

·         Lethal

·         Fatigue (cyclic)

·         Brittle (low temperature)

·         High temperature

·         High shock or vibration

·         Vessel contents: Hydrogen; Ammonia; Compressed air; Caustic; Chlorides; Hydrocarbons.

 B) Types of Failures

  1. Elastic deformation-Elastic instability or elastic buckling, vessel geometry, and stiffness, as well as properties of materials, are protection against buckling.
  2. Brittle fracture- Can occur at low or intermediate temperatures. Brittle fractures have occurred in vessels made of low carbon steel in the 40-50 °F range during the hydrostatic tests where minor flaws exist.
  3. Excessive plastic deformation-The primary and secondary stress limits as outlined in ASME Section VIII, Division 2, are intended to prevent excessive plastic deformation and incremental collapse.
  4. Stress rupture-Creep deformation as a result of fatigue or cyclic loading, i.e., progressive fracture. Creep is a time-dependent phenomenon, whereas fatigue is a cycle-dependent phenomenon.
  5. Plastic instability-Incremental collapse; incremental collapse is cyclic strain accumulation or cumulative cyclic deformation. Cumulative damage leads to instability of vessels by plastic deformation.
  6. High strain-Low cycle fatigue is strain-governed and occurs mainly in lower-strength high-ductile materials.
  7. Stress corrosion-It is well known that chlorides cause stress corrosion cracking in stainless steel; likewise, caustic service can cause stress corrosion cracking in carbon steel. Material selection is critical in these services.
  8. Corrosion fatigue-Occurs when corrosive and fatigue effects occur simultaneously. Corrosion can reduce fatigue life by pitting the surface and propagating cracks. Material selection and fatigue properties are the major considerations.



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