Avoid Costly Fabrication Mistakes

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

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

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Full radiography often is confused with 100% radiography, with the later requiring X-raying of all butt welds, including small diameter and thin wall nozzles. Lethal services require 100% radiography. A savvy designer knows the difference and may specify 100% in non-lethal services where process reliability is crucial (e.g., continuous processes vs. batch) or where accessibility to certain joints will be restricted and hamper future repairs (e.g., jacketed designs).

“Spot radiography” is a common choice in the chemical industry for normal service fluids. It involves a 6-in. shot for every 50-ft. of weld seam at locations specified in the code and incurs only a slight, 15%, wall thickness penalty. ASME designates this as “RT-3.” On small and low hazard vessels, often no radiography or “none” is specified; this will increase the wall thickness another 15% (i.e., 30% thicker than for full X-ray).

RT-2, which is a hybrid between RT-1 and RT-3, seldom is specified by owners but offers economic advantages by permitting thinner wall vessels (Table 1). All long seams are fully X-rayed (similar to RT-1) and the longer circumferential seams are only spot X-rayed (similar to RT-3) with one extra quality shot at the T-junctions (Figure 2). Why RT-2? The long seam is stressed two times higher than circumferential seams for most vessels where wind load doesn’t govern design (i.e., vessels under 50-ft. tall). So, have your fabricator quote an optional price for RT-2 on vessels constructed of alloy materials. A vessel engineer can work you through the rules.

In summary, full radiography was once considered only necessary for vessels containing highly hazardous processes or requiring maximum reliability. However, with the ever increasing cost of alloys, consider specifying RT-2 radiography for all welded equipment as a potential way to achieve material/fabrication cost savings.

  • Release the head for fabrication ASAP. Often, the first thing fabricators do when they get the go-ahead to begin fabrication is to order the heads. The design pressure, temperature, vessel diameter, material of construction, and amount of radiography are all that are needed to release the heads for forming. The fabricator usually will wait until the heads arrive (two-to-four weeks later) and check for dimensional accuracy before rolling the shell. So, to ensure the project remains on schedule, don’t delay releasing the heads while you’re finalizing nozzle sizes and locations. A few major fabricators have the equipment to form their own heads. Knowing this ahead of time can be helpful if the project is schedule driven.
  • Get anchor bolt templates. Templates supplied by the fabricator, cut to match the equipment’s actual anchor bolt pattern, can be very useful to your on-site construction contractor when project field schedules are tight. In these situations, concrete pads with anchor bolts often are poured and pre-set before the vessel arrives. If time allows, a contractor may wait for the vessel to accurately determine exact anchor bolt locations before pouring the pad. However, this has obvious time and cost consequences (e.g., crane rental). So, when warranted, have the fabricator provide (with the vessel or ahead of time) a ¼-in.-thick metal or wood template of the vessel’s actual anchor bolt circle pattern. As bolt circles get larger, base rings become thicker and the ability to correct out-of-tolerance bolt patterns becomes more difficult. Small diameter vessels with welded-on legs are just as susceptible to arriving with unacceptable tolerance deviations because they are mounted independent of each other.
  • Opt for dual stamping. The code allows vessels to be dual stamped (i.e., separate code nameplates for two sets of design pressure and temperature). Pilot plant vessels also are good candidates because they potentially may handle an endless number of products/processes. The drawback to dual stamping is if both stamped pressures are over 15 psig, then the vessel then would require over-pressure protection, i.e., two separate pressure-relieving devices — but this inconvenience can be minimized by installing a three-way full-port selector valve. It’s possible to add a second nameplate after the equipment has been placed in service by “R-Stamping,” but all the code requirements must be met for the alternative design conditions.
  • Insist on a mechanical guarantee. Fabricators aren’t responsible for classifying a vessel for lethal service, selecting materials of construction or determining corrosion allowances. Nor are they required to provide you with a vessel free from imperfections. They merely need to follow the ASME Code rules and perform the necessary calculations, inspections and tests. The vessel you get won’t be perfect. Therefore, add language in the purchase order requiring a limited mechanical guarantee. Fabricators won’t agree to a lifetime guarantee but will accept one for a set period to repair failures due to mechanical workmanship not caused by neglect or mis-operation. A common term is 18 months after receiving the vessel or 12 months after placing it in service, whichever comes first. Insist that any repair performed resets the 12-month clock.

    If the vessel maker completely overlooks a fabrication requirement listed in the specification that affects reliability, then push for a longer service guarantee, such as five years, for the missed item (e.g., rounded corners on pads that cause stress risers and can lead to premature fatigue cracking). You are in an excellent negotiating position once a vessel is fabricated with a design specification error and final payment is pending.

The last part of this serieswill delve into design issues, such as choosing the appropriate pressure and temperature, head and jacket choices, and how to design a rectangular tank.

Chip Eskridge is a principal engineer with Jacobs, Louisville, Ky. Mike James is a senior consultant, materials engineering, for DuPont, Houston. Steve Zoller is director of fabricated equipment for Enerfab, Cincinnati. Reach them via e-mail at chip.eskridge@jacobs.com, Michael.M.James-1@USA.dupont.com and Steve.Zoller@enerfab.com.

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