High temperature hydrogen attack (HTHA) is a significant risk to the operation of critical pieces of equipment in the refining industry. FEC were approached by a client after identifying HTHA in the head of a Feed Converter Exchanger of their Ammonia Plant. The exchanger was subject to the following design conditions:

  • 510°C design temperature
  • g internal design pressure (shell side)
  • High thermal shock loading on start-up
  • 50mm thick hemispherical head with significant damage
  • Site PWHT requirements

With the damage identified, the integrity of the lower head was determined non-compliant and a suitable repair strategy was required.

FEC completed high urgency work to help the client meet the Turnaround schedule and complete repairs. The considered repairs included a 75mm thick hemispherical head leak box with optimised welded connection to balance mechanical strength and thermal gradients. The analysis also included an assessment of the transient thermal response during start-up. FEC was responsible for the following scope of works:

  • Mechanical design of a leak box repair to API579
  • Assessment of transient thermal stresses
  • Fatigue assessment
  • Pipe stress design for connecting piping
  • Constructability reviews
  • Iteration of potential solutions considering material availability and minimisation of site welding