After consolidation of the data and understanding the data, the next step is to do a set of field tests. These field tests include.
Visual inspection for
- Erosion, corrosion
- Swelling, scaling
- Deposits, misalignments
- Supports, pipe hangers etc
- Outside diameter
- Non-destructive examination
- Penetrant testing including fluorescent type
- Magnetic particle inspection (Wet fluorescent & Dry)
- Ultrasonic tests
- In-situ hardness checks
- Eddy current testing
- Tube sampling – water walls for internal deposit analysis
- Superheater and reheater sample if needed
Special examinations for
- WW H2 embrittlement
- Superheater/Reheater for oxide scales,
- Metallographic examination of thick wall component
- Fibroscopic inspection of headers and other regions needed
- Spot chemical check when needed
Once when the field tests are completed, a few laboratory examinations and tests are carried out. The tube samples taken from the water walls, superheater, and reheater are subjected to microscopic examination (Light Microscopy & Scanning Electron Microscopy) to understand the microstructure of the tube material. The water wall tubes are subjected to internal deposit analysis. Both the quantity of the deposit and the chemical composition are carried out to evaluate the need to carry out post operational acid cleaning. Based on this, the solvent for post operational acid cleaning is decided. Other mechanical and metallurgical tests are also carried out for the tube samples collected. If any external deposits are collected from locations in the boiler, they are also analysed to understand the cause of the deposition. The presence of both external and internal corrosion is also evaluated based on the deposit analysis and the tube sample study. If the failure data gives a clue for any hydrogen embrittlement in the water wall tubes, a detailed field test and analysis is carried out to decide the area requiring replacement. The samples taken from the thick walled components are analysed for spheroidisation and cavitation level. Based on the level the repair and replacement strategy is arrived at.
Remaining life prediction techniques
There are many methods used by engineers to predict the remaining life of boiler pressure parts. The most commonly used technique is the steam side oxide scale thickness growth and life fraction rule using Larson-Miller parameter. Every tube in service has a Larson-Miller parameter that increases with time. The oxide scale thickness is correlated by many equations which is used to arrive at the remaining life along with the life fraction rule.
The strain measurement technique is another method used, however this requires data with a large time interval.
There are advanced techniques like crack growth propagation analysis which is being tried with limited success.
Many other tailormade techniques are used by different boiler makers to estimate the remaining life of pressure parts. However due to the large scatter in stress values of the materials used along with the safety margins and other special allowances used during design, it is very difficult to predict the remaining life with good accuracy levels.