Abstract

The ammonia plant experienced increased process inlet gas temperatures, triggering a review of asset integrity. The initial fitness-for-service (FFS) work suggested that the critical location identified was the inlet “tee” and conservatively set their operating limits. The conservatism in the initial assessment, such as creep relaxed stress at 40,000 hours instead of fully relaxed stress and surface point failure criterion, was relaxed to determine realistic remnant creep life consequently providing more reliable operating limits. The creep damage indicated by the metallurgical replications allowed the material properties to effectively be calibrated. The material calibration process involved estimating creep life fraction based on the replication results and matching the FE predicted life at the replicated locations by adjusting creep material properties. Full creep stress relaxation behaviour was modelled by simulating the creep stress evolution behaviour from commissioning up until 2023, and a creep failure criterion based on net section stress was considered to reduce the conservatism associated with the surface stress failure criterion. In order to acquire a through-thickness stress distribution for the creep life assessment, reference stresses were calculated with guidance from API-579. Linearised membrane and bending stress components were calculated based on von-Mises stresses extracted from the FE model through the section of the tee. Three critical locations were selected and analysed. Given the operating temperature data, and the reference stresses determined from the FE analysis, the creep life consumption at the start of 2020 was calculated to be up to 52%. Given the 2020 creep life consumption of 52% calculated, future creep life consumption until 2023 was calculated based on various operating combinations of temperature and pressure scenarios. A correlation of operating conditions with remnant life was established, which enable to define integrity operating window. In addition, the safe operating envelope for the inlet tee achieving 100,000 hours of further operation was determined.