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.
Gas Dehydration Units (GDU) are primarily used upstream to remove water from the raw natural gas. Natural gas, before treatment, may be saturated with water, which can cause downstream damage. Upstream natural gas equipment with high water content in the stream is more susceptible to aqueous damage mechanisms, freezing and damaging the piping when at low temperatures, and reacting with the natural gas to form hydrocarbons like methane hydrate. The composition of the natural gas processed in this facility is shown in Figure 1.
The absorber column, which is the subject of the case study 1 below, typically operates at 105°F (41°C) and 1200 psi (8.3 MPa) and the materials of construction are carbon steel SA-516 Gr 70 for shell, heads and nozzles, and the trays and bubble caps are made of 321 stainless steel (SS). Gas dehydration units are susceptible to damage mechanisms including stress corrosion cracking (SCC) and wet H2S damage (sulfide stress cracking SSC, blistering, hydrogen induced cracking HIC, and stress oriented hydrogen induced cracking SOHIC). Wet H2S damage occurs when there is increased hydrogen permeation due to increased H2S partial pressure. It is possible at operating conditions between ambient temperature and 300°F (149°C) and at an acidic or basic pH.1 Excess hydrogen permeates the metal, clusters at inclusions and laminations creating overstressed areas that eventually lead to hydrogen damage in the form of blisters and/or cracking.