Remaining Life Assessment (RLA)
Need for Remaining Life Assessment
- Increasing cost of new equipment and diminishing resources
- Extended lead time in plant construction
- Stringent environmental, safety and other regulations
- Increasing awareness for exploring the technological feasibility in extending component life
A pragmatic approach is adopted in handling the cases of remaining life assessment. Efforts are put in to collect as much data as possible on the component / equipment history. Often brain storming sessions are conducted with the concerned engineers and technicians using the equipment as well as the other experts who are familiar with the operational details.
Calculation Based Approach
Calculations are often done to determine the expended life of the component under creep, fatigue and creep-fatigue conditions. From plant records, information on temperature and cycling history is gathered. By use of standard material properties and damage rules, the fractional life expended up to a given point of time is then estimated.
Quite often, background history of plant & equipment are usually not available in sufficient details. Due care is therefore taken to avoid errors in assumption of operating temperature and stress that can otherwise lead to an erroneous error in estimation of remaining life of the concerned component.
Components which are operated below creep regime are generally designed on the basis of yield strength, tensile strength and fatigue strength with suitable safety factors. Under normal operating conditions, deformation and fracture are not time dependent. As long as the applied stresses do not exceed the design stresses these components should theoretically last for indefinite time; but in practice various factors cause reduction in life.
The Salient Features Involved are:
- UNDERSTANDING THE ACTUAL DEGRADATION MECHANISM
- High cycle fatigue
- Low cycle fatigue
- Thermal fatigue
- Thermo mechanical fatigue
- Thermal aging
- VISUAL EXAMINATION
- NDT involving In-situ Metallography, Ultrasonic Testing, Magnetic Particle Inspection, DP Test, Ferrite Measurement.
- STRESS ANALYSIS : Stress analysis is carried out to know the strength of the material.
NON DESTRUCTIVE TESTING : NDT inspection data provide good insite to the component integrity.
- LABORATORY TESTING : Laboratory testing of cut samples provide valuable information about the material soundness.
- JUDGMENT OF FITNESS OF THE EQUIPMENT : Based on available data, a judgement about the fitness of the equipment: If required, repairing of the equipment is suggested, for life extension.
- JUDGMENT OF REMAINING LIFE BASED ON ANALYSIS : Finally, the estimation for remaining life is carried out.
Often the physical properties are verified and tests like stress-rupture, that are not so common are conducted. Having ascertained the extent of degradation, judgment on the remaining life is made. In addition to this, periodic inspection procedures are formulated and followed to monitor the health of the equipment during the course of operation. RLA is done based on life limiting factors like corrosion, creep and microstructure degradation with respect to time. There are different approaches available to assess the remaining life based on thermal cycles, creep calculation and fatigues cycles. Based on these aspects, the remaining life is ascertained with integration of rate of degradation v/s available material properties.
Data Requirement for Life Assessment
||Design or nominal
||Measured or nominal
||Records or nominal
|Temperature and pressure
||Operational or measured
||Design or operational
|Material samples required?