Structural Integrity Analysis & Evaluation of Fitness for Service (FFS) and RLA of a 500 MW Power Plant
Critical piping systems of a 500 MW super thermal power plant were analysed by ProSIM (www.pro-sim.com) for structural integrity. Study included MS, HRH, CRH, HPBP, LPBP pipelines. Plant was in service for more than 25 years. Study conducted by ProSIM to a primary EPC contractor included site inspection, data collection on site, comparison of the design drawings with as as-is-where-is condition of the plant piping, conducting finite element analysis (FEA), analysis of effect of defects including microscopic defects, corrosion effects, and weld defects / discontinuity.
As these plants were old, we analyzed the plant design for as-designed condition. Based on the NDT evaluations current situation (as-is-where-is condition) of the pipelines were obtained including deteriorated / altered dimensions, deflection of hangars etc. Using these data, an analysis of as-is-where-is condition was conducted. Using pipe stress analysis forces, moments were obtained. Finite element analysis (FEA) was conducted to determine the stresses in the entire piping system including, hangars, supports, valves, etc. Non-linear FEA was conducted to account for the material damage including fatigue, creep, plastic deformation. Larsen Miller Parameter (LMP) was used to estimate the time for stress rupture. Creep strength reduction factor were determined as defined in the API codes for Fitness for service (FFS). FFS level-3 protocols as per API 579 / ASME-FFS-1 were imposed for remaining life assessment (RLA) of the components in the critical pipeline of super thermal power plant.
Several hangars/ supports were found to be damaged during physical inspection and these damaged hangers/ supports were considered in as-is-where-is condition stress & RLA. ProSIM team suggested several recommendations for life extension (RLE).
Analysis for RLA and RLE included combination of gravity loads, operational loads (temperature and pressure), wind load, and seismic load.
Suggestions for replacement / repair were provided by ProSIM based on FFS, RLA and RLE studies using numerical method (FEA). Three of the support had to be changed with new ones. One of the supports could be repaired and reused. Reminder of the supports were found to be qualifying in all conditions (as-is-where-is, as recommended).
Some of the corroded sections in bends, and reducers were found to be failing (due to corroded sections, reducing the wall thickness). After refurbishment, they were qualifying as per the code requirements.
Dr S Shamasundar
Managing Director, ProSIM R&D Pvt Ltd