Cooler tubesheet assembly FEA hydrotest stress intensity plot
How long will they last? What stresses will they endure in service? How will they fail and what should be done to achieve a 25-year service life?
A leading manufacturer of heat exchangers presented a handful of two different types of cooling units for life assessment studies. The work called for an initial review of available information about their performance. Several stress analyses were conducted using FEA and hand calculations carried out to determine the operational stresses for the coolers.
A multidisciplinary team of corrosion engineers, metallurgists and structural integrity engineers examined all damage mechanisms and highlighted the potential active modes.
All possible failure scenarios for the tubesheets were identified and a life assessment methodology produced. Finally, the life assessment was conducted on the possible failure modes using the level 2 procedures in BS 7910.
The life assessment was significantly conservative with all assumptions being onerous, especially the operational stress values used, and the Paris law coefficients adopted for the fatigue assessment. These were for steel freely corroding in a marine environment.
The results of the life assessment showed for both cooler types that:
- The tubesheets would leak before they fractured.
- The lives of the tubesheets were greater than 25 years.
- An 11mm deep crack between the first two holes in the centre of the tubesheet on the shellside 11mm deep would have to be present to reach the limiting flaw size within 25 years.
In summary, the advice given was that the likelihood of any of the cooler tubesheets failing due to gross yielding or fatigue in 25 years is very low. Given that the other areas of the coolers have met the design and construction requirements of recognised pressure equipment standards, the investigating team concluded that the cooler tubesheets were safe to enter their intended service for 25 years. The assessment also highlighted that there is no need to conduct any extra inspection or maintenance, above that which is already planned, during the tubesheet life.
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