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Corrosion surprise threatens costly shutdown extension

Connect, no. 149, July/August 2007, p.8

When one of the world's biggest energy companies discovered corrosion in a safety critical area of one of its petrochemical plants during a planned shutdown, it turned to TWI for a fast assessment of the problem.

Deep localised external metal loss had been found near the top of a regenerator column due to corrosion under insulation. The 3.5m diameter pressure vessel stood 30m high and was fitted with 10 stiffener rings welded to the shell. It had been in service for 25 years, with an operating temperature of 101°C at the top and 120°C at the bottom and with a design temperature of 155°C.

Revealed in the last few days of the planned shutdown, the significant localised metal loss was found within an area beneath the insulation, immediately above one of the stiffener rings, near the top of the column.

'Could the column be safely operated with the remaining wall thickness in the corroded area less than the construction code minimum allowable thickness?' asked the Member of TWI. A quick run/repair decision was required to minimise costly slippage in the shutdown schedule.

'Depending on the equipment design local areas of metal loss could be tolerated' senior project leader Sean Fewell from TWI's Structural Integrity Technology Group told Connect. 'An engineering fitness-for-service assessment based on published international assessment procedures could be used to establish whether the discovered corrosion would compromise the safe operation of the column.'

Due to the proximity of the stiffening ring and the profile of the corrosion, a finite element stress analysis was required to determine whether the remaining thickness was sufficient to avoid failure.

TWI created a FE model of the column with a corrosion profile measured onsite by the member. Two cases were run for each of the design internal pressure and full vacuum, with self weight considered in both cases.

The stresses predicted by the FEA were compared with the design by analysis rules specified in API RP 579, with the material's tensile properties corrected for temperature.

Within two days of accepting the brief TWI was able to conclude in its report that; 'using a finite element model of the vessel in the vicinity of the locally thinned area showed the vessel to be safe under both internal pressure and operation at full vacuum. The maximum von Mises stresses were well below the specified minimum yield strength corrected for the design temperature and were within the API RP 579 Appendix B allowable stress limit'.

TWI recommended deferment of the repair until the next shutdown, arrest of the corrosion by an appropriate method, such as the application of a coating, and regular inspection of the troubled area to ensure the situation deteriorated no further.

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