Volumetric UT (phased array) thickness measurements performed on the absorber column of a gas dehydration unit (GDU) found several laminations and clusters of inclusions around the chimney tray. This unit had been in operation for 40 years. A Fitness For Service (FFS Levels 1 and 2) analysis was conducted to evaluate the magnitude and implication of these defects on the safe operation of the column. The outcome of the FFS assessment indicated that the defects were of considerable size and hence the component could not continue in operation unless mitigating actions were taken.
Several possible remedial options were considered: Repair by replacing the defective plates, rerate or eliminate the hydrogen permeation that could lead to hydrogen induced cracking, or provide a mechanical barrier in the area between the eight tray and the chimney plate to isolate the base metal from the hydrogen charging environment. A coating system consisting of a metal particulate reinforced polymer primer with a ceramic filled epoxy top coat was applied on the ID of this column, which ensured that the cathodic reaction that releases atomic hydrogen as a corrosion by-product did not take place at the affected areas. The application of this coating saved the huge cost of replacing the plates, while ensuring the continued safe operation of the unit.
Keywords: Fitness for service, HIC, wet H2S damage, hydrogen permeation, coatings