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