Materials Performance

AUG 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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Page 47 of 92

45 MATERIALS PERFORMANCE: VOL. 57, NO. 8 AUGUST 2018 Using Coating Systems as Mechanical Barriers in Gas Processing Facilities Case History 1 At this chemical plant, there are over 40 GDUs. The composition of the natural gas processed in the facility is shown in Table 1. The absorber column on each GDU con- sists of several trays with bubble caps (Fig- ure 1). The gas to be dehydrated is sent into the column between the seventh and eighth trays. Glycol comes in from the top (above the first tray). The chimney tray is located below the eighth tray. The outlet at the chimney tray consists of water with con- densate and traces of glycol. Volum etric ultras onic t estin g (U T) phased array on the absorber column of a GDU at the plant discovered several indi- cations between the eighth tray and the chimney tray. This unit had been in opera- tion for 40 years. The absorber column typically operates at 105 °F (41 °C) and 1,200 psi (8.3 MPa). The materials of con- struction are ASME SA516 Grade 70 car- bon steel (C S) for the shell , heads, and nozzles; and the trays and bubble caps are made of Type 321 (UNS S32100) stainless steel (SS). The indications were identified as laminations and inclusion clusters. The origin of these defects is uncertain; how- ever, the facts suggest that they were prob- ably manufacturin g (mi l l) d efe cts. An internal visual inspection of the eighth tray and chimney tray was conducted during the turnaround. The surface had uniform corrosion and no indications of bulges or blisters. However, laminations and inclu- sion clusters of this kind had been reported on equipment at other GDUs built around the 1970s by the same manufacturer. A fitness-for-service (FFS) analysis was conducted to evaluate the magnitude and probable implications these defects could have on the safe operation of this unit. The FFS analysis was based on API 579, 6 Part 13, which applies to assessing these types of defects. The outcome of the assessment showed that the size of the indications was considerably large and the component could not continue in operation unless actions were taken. Possible rem edial options included repairing, rerating, eliminating, or isolating sources of atomic hydrogen that could lead to HIC. The indications were too large for a hydrogen charging service and, hence, did not pass the FFS assessment. One of the options considered by the asset integrity t e a m to li mit fu r th er d a m a ge to th e absorber column was to apply a mechani- cal barrier that would isolate the base TABLE 1. CHEMICAL COMPOSITION OF NATURAL GAS PROCESSED BY THE GDU IN CASE STUDY 1 Gas Chemical Composition (mole%) H 2 S 0.038 CO 2 5.759 Nitrogen 10.443 Methane 72.572 Ethane 2.797 Propane 1.532 i-Butane 0.300 n-Butane 0.893 i-Pentane 0.325 n-Pentane 0.467 n-Hexane 0.480 n-Heptane 0.353 n-Octane 0.210 n-Nonane 0.066 n-Decane 0.022 n-C11 0.004 Water 3.737 FIGURE 1 The sketch shows the general arrangement of the absorber column, its internal and main streams, and the area where the coating system was applied (marked in red).

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