Avoid costly materials mistakes

Common oversights keep plants from getting the most reliable and suitable vessels.

By Chip Eskridge, Jacobs, Steve Zoller, Enerfab, and Mike James, DuPont

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9. Check into non-metallics. Many applications don’t really require a metal tank. High density polyethylene (HDPE) tanks come in a wide range of sizes and configurations. From 200 to 12,000 gallons, these tanks cost a fraction of metal ones. The major disadvantages of HDPE tanks are pressure/temperature and anchoring limitations. They can’t be rated for pressure/vacuum nor can they be designed with load-bearing attachments or platforms. Nozzles can be added to customize the tank but also with limitations. Tanks made of reinforced thermoplastic resin (RTR), also referred to as fiberglass-reinforced plastic (FRP), offer a more robust alternative. They overcome the limitations of HDPE tanks and can be designed/fabricated to either manufacturer’s standards or to ASME RTP-1 — the later requiring a bit more testing, inspections and documentation, which all come at a price. Above 15-psig design pressure (i.e., for pressure vessels), ASME Section X can be used. However, only a handful of manufacturers in the U.S. can provide a Section X Stamp.

10. Use the extra metal to your benefit. After design parameters are set, the fabricator will determine the required wall thickness. For instance, a 150-psig, 300°F, 4-ft.-diameter carbon-steel vessel, spot X-rayed with 1/16-in. corrosion allowance, will have a required shell thickness of 0.326 in. The fabricator will purchase the next thicker commercially available plate, which would be 3/8 in. (i.e., 0.049-inches thicker than required). This additional wall thickness can be used in one of three ways, and you have a control over its use.

Option 1 is to use the extra metal to rate the vessel with a higher Maximum Allowable Working Pressure (MAWP) than the required design pressure, 178 psig instead of 150 psig here. This choice favors continuous processes, and gives production the option to operate the vessel harder (i.e., at higher pressure).

Option 2 is to set the MAWP equal to design (150 psig) and use the extra metal as additional corrosion allowance (1/16 in. plus 0.049 in.). This will give you a longer service life, which favors batch processes.

Option 3 is to set MAWP equal to design (150 psig) and use the extra metal to obtain a higher maximum design temperature. This option favors processes that have automatic temperature trips, such as exothermic reactors and fired heaters, and avoids possible fitness-for-service determinations if an excursion should occur.

The option selected can be changed later by performing a re-rate, although choosing Option 2 or 3 would require a new hydrostatic test. Also, when opting for Option 1 or 3, watch crossing into the next higher flange class.

11. Understand the difference in surface treatments. Pickling and passivating are surface treatments of carbon steel and corrosion resistant alloys that use acid or other solutions to remove surface oxides or improve corrosion resistance of the metal to a given process. Pickling is performed using a strong oxidizing acid, such as nitric or hydrofluoric acid, to remove the outer oxide layer. All stainless steels are pickled to various degrees after they’re made. Approximately 25 mils to 50 mils (one mil equals 0.001 in.) of oxide is removed during pickling. Carbon steels typically aren’t subject to pickling but are supplied in as-formed condition. On the other hand, passivation of stainless steel is performed using a weaker acid, such as weak nitric or citric acid, that preferentially removes the more easily extracted iron and nickel atoms from the oxide layer, leaving behind a chromium-oxide-rich surface. Usually less than 2 mils of metal are removed. Thus, after passivation, stainless steel has a bright finish.

Pickling typically is performed at the mill or by a material supplier while passivation usually is done by the fabricator after the vessel is complete.

Fabricators that handle both carbon steel and stainless steel carry an inherent risk of contaminating their stainless steel by picking up free iron from tooling. Iron from carbon steel can be embedded in the stainless steel surface from the forming process (e.g., contaminated plate rolls), grinding wheels, machining operations and via airborne particles if not carefully controlled. It’s good practice to passivate stainless steel vessels if a fabricator handles both carbon steel and stainless steel or if surface iron contamination and its deleterious effects on corrosion resistance can’t be tolerated. Another way to avoid this problem is to only allow fabricators who specialize in stainless steel or higher-end alloys to perform the work. Often, a shop that specializes in alloy materials will turn down bid requests for carbon steel work because of its low-end nature and the risk of contamination of alloy work.

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