High temperature headers are among the most difficult and critical components for risk assessment. They bring together questions of weld integrity, defect assessment and creep-fatigue damage in thick sections. Recommendations which reflect the real risks and options are key.
A systematic approach to header reliability requires Level I scoping techniques to identify at-risk headers, followed by more detailed approaches to define specific recommendations. Superheater and reheater outlet headers are usually thought of as the highest risk headers. This is not always true. Seam-welded superheater platen outlet headers could occupy the top slot, which is why an initial survey to rank headers in terms of likely risk is recommended.
The new API-1/ASME FFS-1 post-construction standard provides data and a framework for pressure part risk assessment. Risk-based assessments can provide a rational basis for dealing with the meaning of “exceeding minimum rupture life”.
The level of information available defines the amount that can be done. Typical examples are:
Due to the statistical spread of properties, calculation of header remaining life can only be a probabilistic exercise unless physical evidence is available. Metallurgical evidence can provide an indication, as does swelling. Welds (seam or girth) coinciding with tube penetrations are often the first areas of creep damage. On thicker headers, thermal creep-fatigue can be a factor. Top-to-bottom differentials can give rise to distinctively oriented cracking. Where creep damage is identified or suspected, removing samples and subjecting them to accelerated creep and rupture testing provides the strongest basis for determining remaining life. Testing overcomes the problem of the unknown initial creep strength, since the test directly measures the property of interest. Extracting a core sample, as though a tube penetration was required, is the easiest sampling technique to repair. Samples from the core are then fabricated into a test sample for creep and rupture testing.
Header Assessment May Be Required When:
Estimation of remaining life of high temperature headers follows a number of possible paths depending on the available information. The following possibilities are based on the use of API-579/ASME-FFS mechanical properties data.
Typical information requirements necessary to make an assessment include:
We can provide a defensible basis for decisions such as:
Due to the complexity of header remaining life predictions, leverage of plant experience and expertise is essential. We will work closely with plant personnel to develop a comprehensive plan for header risk management.