Tubes operating in Crude Heating service in one of the refineries were removed on sampling basis after 30 years of service. The sampling was done for the purpose of remaining life assessment, based on Part 10B of API579 / ASME FFS-1. The procedures in Part 10B require creep life calculations based on two special parameters relating to in-service ageing of material – referred to as Äcd and Äsr.
The material of constructions of these sample were: SA106 Grade B, A 200 Grade T22, SA 335 Grade P9.
The RLA study was carried out with the following step-by-step assessment:
1. Collection and study of available data and past inspection history,
2. Identification of different damage mechanisms that could attribute to life limitation,
3. Visual examination, Thickness & Dimensional measurements, Wet Fluorescent Magnetic Particle Inspection (WFMPI), Chemical analysis for composition checks
4. Macrostructural examinations for internal structural characterization, detection of carburization
5. Microstructural examinations for assessment of metallurgical degradation using Energy Dispersive Spectroscopic (EDS) analysis and Scanning Electron Microscopy (SEM),
6. Evaluation of physical properties by room temperature tensile test, design temperature tensile test, Accelerated Creep Rupture Test (ACRT) and hardness measurements.
The results of ACRT provides detail about materials’ creep characteristics, defining primary, secondary and tertiary creep modes within a reasonably short duration. The graphs for creep strain rate v/s time and the natural log of strain rate v/s strain are used to determine the critical parameters (Äcd and Äsr) that correlate to in-service ageing of the material. The calculations of remaining life are further, extended to future corrosion rates and extrapolation of tube wall thickness to precisely conclude the remaining life of tubes. The determination of Äcd and Äsr (omega parameters) becomes a useful exercise to correlate the remaining life of other similar tubes in service.
Based on the test results calculations are done as per detailed guidelines by deriving the Omega parameters and judgement on remaining creep life.
The analysis helped to be conceivable and beneficial to take management decisions.