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 people using the equipments. The outside exports who are familiar with the operational details. To collect opinions and ideas, which are evaluated vis-à-vis the testing and studies conducted at a latter stage.
CALCULATION BASED APPROACH
Calculation procedures are often employed to determine the expanded lives of components 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 in time can be estimated.
Unfortunately, histories of plant are usually not available in sufficient detail. Errors of as little as 14°C in assumed temperature and small errors in assumed stress can lead to errors.
DESIGN APPROACH
Components which are operated bellow creep regime are generally designed on the basis of yield strength, tensile strength and fatigue strength with suitable safety factors.
Under normal condition deformation and fracture are not time dependant.
As long as the applied stresses do not exceed the design stresses these components should last indefinitely; but in practice various factors causes reduction in life.
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 OF THE TECHNOLOGICAL FEASIBILITY OF EXTENDING COMPONENT LIFE
THE SALIENT FEATURES INVOLVED ARE:
- Understanding the actual degradation mechanism
- HIGH CYCLE FATIGUE
- LOW CYCLE FATIGUE
- THERMAL FATIGUE
- THERMO MECHANICAL FATIGUE
- WEAR
- THERMAL AGING
- CREEP
- EMBITTERMENT
- CORROSION
- Visual examination
- NDT involving In-situ Metallo-graphy, 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 an good in-site 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 of the fitness of the equipment is done.
- SUGGESTIONS ON REPAIRING IF ANY
If required, repairing of the equipment is suggested, for life extension.
- JUDGMENT OF REMAINING LIFE BASED ON ANALYSIS
Finally, the estimates for remaining life is carried out.
Often the physical properties are verified and not so common tests like Stress ruptures are conducted. Having ascertained the extent of degradation, judgment on the remaining life is made. In addition to this periodic inspection procedures are spelled out to monitor the health of the equipment during the course of operation. The results point out if any operational mistake, restriction in free movement by thermal expansion or any other damage mechanism prevailing is reveled and preventive maintenance approach can be formulated.
DEFINITIONS OF COMPONENT LIFE
HISTORY BASED CRITERIA: 30 to 40 years have elapsed, Statistics of prior failures indicate impending failure, Frequency of repair renders continued operation uneconomical, Calculations indicate life exhaustion
PERFORMANCE BASED CRITERIA: Severe loss of efficiency indicating component degradation, Large crack manifested by leakage, severe vibration or other malfunction, Catastrophic burst
INSPECTION BASED CRITERIA: Dimensional changes have occurred, leading to distortions and changes in clearances, Inspection shows microscopic damage, Inspection shows crack initiation, Inspection shows large crack approaching critical size
CRITERIA BASED ON DESTRUCTIVE EVALUATION: Metallography or mechanical testing indicates life exhaustion
DATA REQUIREMENT FOR LIFE ASSESSMENT
| Item | Level – I | Level – II | Level – III |
| Feature | Least detail | More detail | Most detail |
| Failure history | Plant records | Plant records | Plant records |
| Dimensions | Design or nominal | Measured or nominal | Measured |
| Condition | Records or nominal | Inspection | Detailed inspection |
| Temperature and pressure | Design operational | Operational or measured | Measured |
| Stresses | Design or operational | Simple calculation | Refined analysis |
| Material properties | Minimum | Minimum | Actual material |
| Material samples required? | No | No | Yes |
