Most of the high temperature headers designed 30 to 40 years ago were expected to be operated continuously and cycling effects were not considered. However, due to the changing nature of power demand, the power industry has been forced to operate their plants in a more cyclic manner than originally intended. Such operation introduces cyclical stress and strain on the header. The frequent change in boiler load during steady state operation can introduce additional higher frequency fatigue damage. In particular, the thick walled superheater or reheater headers subject to cyclic operation at high temperatures are susceptible to ligament cracking. The cracking initiates at the inner surfaces of the header and eventually propagates through the header wall.
Ligament cracks initiate and radiate from tube bore holes. Linking of adjacent tube radial or circumferential cracks creates the ligament crack. They grow radially and circumferentially until completely breaking the tube from the header. Linking of multiple ligament cracks can eventually lead to explosive expulsion of tubes and severe shortening of the header life. The final superheater outlet header is one of the most critical components in power stations because its replacement cost is high. Assessments of the structural integrity of the superheater header and an estimation of remaining life have become essential for the safe and economic operation of power plants.
The assessment of high temperature creep fatigue degradation can be complex and involve finite element modeling, fracture mechanics, material property data and an understanding of the evolution of damage under creep/fatigue conditions.
By aligning these disciplines with approved codes (e.g. API 579-1/ASME FFS-1, BS 7910 and British Energy procedures, R5), Quest Reliability provides capabilities in high temperature assessment that include:
- Determination of the critical flaw depth that will cause failure by fracture, ductile overload or creep rupture
- Comprehensive thermo-mechanical finite element analysis to identify stress levels during starts and temperature fluctuations
- Calculation of creep crack growth and fatigue crack growth
- With respect to flaws found during inspection, determination of safe operating life to failure as a function of plant operation
- Recommendations for change in operation conditions and inspection schedule
- Provision of a customized life assessment online-monitoring program for determination of crack growth based on the actual operating data (temperatures and associated times). The program calculates the actual contribution from each thermal cycle and temperature fluctuation to provide operators with an instantaneous prediction of the current crack dimensions and associated growth rate.
Related services:
- Evaluation of current material condition from on-site/off-site metallurgical assessment
- Refinement of material properties by carrying out standard mechanical testing; e.g., tensile, toughness, hardness, etc
- Fracture toughness testing at low/high temperatures
- Creep rate, creep relaxation and creep crack growth tests
- Low cycle fatigue testing at high temperatures
- Failure analysis on the ligament cracked header
Results of the assessments of the structural integrity of the high temperature headers are used:
- To achieve safer operation due to better understanding of safety margins
- To curtail harsh operating conditions that may be causing excessive crack propagation and header damage.
- To reduce unplanned repairs and subsequent loss of generation
- To optimize maintenance and repair/replace scheduling
- To extend the life of headers for increased profitability
To develop recommendations about inspection intervals.
For further information about Superheater/Reheater Headers, please contact us.