Life assessment of a steam header after long term high temperature service

Abstract

Life assessments of essential industrial systems and their components pursue to manage the risks of unexpected disruptions and to improve the efficiency, reliability, predictability, and continuity of service. By successful characterization and holistic impression of present and future condition, the responsible operator can define and realize appropriate decisions and actions for inspection and maintenance on time. In this thesis, such a life assessment of a steam header (nominally 490 ℃/ 84 bar) of a pulp mill facility after 268 000 h of service was carried out, to support the maintenance and investment plans of a client company of Kiwa Inspecta that provided the inspection services for the assessment.

The thesis was initialized with supportive literature review thereafter the steam header was investigated by applying computational and measurement-based methods to assess the effects of life-limiting mechanisms of creep and fatigue and associated damage. Available data were utilized such as records of design, fabrication, material (10CrMo9-10), operation, and maintenance, with additional measurements of the current state.

The results mapped the condition and predicted safe life for nine studied welded locations of the steam header. As a result of relatively moderate operation conditions and stable type of baseload, computational analyses emphasized the dominance of creep damage and abundant remaining life exceeding 700 000 hours even by applying the conservative assumptions. Latest damage class indications of reported replica testing (2b - 3b), and earlier indications of macroscopic cracking in magnetic particle testing mostly emerged to the coarse-grained-heat-affected zone of weld, having generally stable or decreasing trend of damage class over the recorded history.

The overall condition of the header was considered as quite satisfactory for its time in operation. No evidence was found suggesting impending significant damage, necessity for repairs, replacement, or any other essential activity of predictive maintenance. For the assessed locations, recommendations were given on scheduling (20 000 h - 30 000 h) and criticality of future inspections in terms of defined measures and additional observations.