The minimum retirement thickness values were calculated for 6′-6″ dia. x 25′-0″ Sm. / Sm. horizontal vessel. The minimum thickness values were listed for the vessel re-rated to 150 psig @ 150 °F.
The original vessel built in 1969 had MAWP of 258 psig @ 125 °F. In 1996, this was re-rated to 100 psig @ 250 °F. In 2014, the same vessel was again re-rated to 150 psig @ 150 °F.
The vessel had general corrosion and severe pitting problem. Also, at the entire liquid line spanning from weld to weld, a groove (1″ wide x 0.250″ deep) was created in the west head.
To maintain the new re-rating, several repairs were made to the vessel. These are as follows:
- Even though the minimum thickness required for the ellipsoidal head was 0.410″ for the new design conditions, it was too close to the remaining wall thickness of 0.430″ for the West head. In order to make sure that we were not below the recommended minimum thickness values at any place in the groove, it was recommended to weld build up the groove in the West head to a thickness of 0.500″ to make sure that there was sufficient corrosion allowance for future operation.We also had to make sure that there were no sharp corners in this groove that will start fatigue cracks in the corroded groove. Hence, it is recommended to grind the weld smooth after the weld build-up.
- It was also recommended that the pits present in the vessel shell and heads be repaired by weld build-up. This ensured that we were not below the minimum recommended thickness values at any place in the shell or heads.
- The gasket surfaces for the three (3) 1-1/2″ nozzles were corroded. These were repaired by weld build up and subsequent grinding to obtain the desired surface finish.
- The bottom surface of the Manway was corroded. This was repaired by weld build up and subsequent grinding to obtain smooth finish.
- The bottom shell area (Size: 3″ x 1/2″) between the vortex breaker and circumferential shell-to-head weld was corroded. This needed to be repaired by weld build-up followed by grinding smooth.
With the specified retirement thickness values and subsequent repairs, the life of the vessel was significantly increased.