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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6. Know the relative cost of materials. It’s common knowledge that stainless steel is more expensive than carbon steel. However, the cost difference between a 300-series stainless steel and a lean duplex, duplex or super duplex stainless steel — or between stainless steel and high nickel alloys or zirconium — is less obvious. It’s helpful to have a rough idea of the relative costs of materials so that meaningful discussions can take place during project development Table 2 lists the relative costs (excluding fabrication costs) of commonly used materials. Note, though, the cost and delivery for any given alloy can vary greatly from vendor to vendor based on current stock and availability. It’s always good practice to question the fabricator about how many material suppliers it got quotes from or to make independent inquiries into material costs, especially if you intend to sole source.

Table 2

7. Keep critical metal temperatures in mind. We learn at an early age that water boils at 212°F (100°C) and freezes at 32°F (0°C) at atmospheric pressure. Engineers know that as water crosses these points, its physical and thermodynamic properties change and a new set of conditions apply. Many engineers, however, don’t appreciate that solids also have temperature limits that, when crossed, create problems for the designer and thus can add additional steps (and cost) to the fabrication process. The most common material limit occurs at low temperature and is called the ductile-to-brittle transition temperature. It’s an issue with carbon steels and other metals with a body centered cubic structure and manifests as a loss of ductility — i.e., the metal becomes brittle. Stainless steels, nickel-base alloys, aluminum, and copper (e.g., FCC and HCP) also have limits but at temperatures below —325°F. Carbon steels’s ductility decreases with temperature and as carbon content increases. The ASME code protects against brittle failures by limiting carbon content to no more than 0.35% and by mandating the material either to be heat treated or impact tested when the ductile-to-brittle transition zone is approached. For thin-wall vessels, common carbon steel materials (SA-105, SA-106, SA-516-70) require heat treatment or impact testing at design temperatures below -20°F [6]. So, when possible, specify a warmer minimum design metal temperature to avoid these costs.

Below -55°F, all welders and weld procedures require a special qualification, which many fabricators may not have developed. For API-650 tanks, -40°F is the critical temperature where impact testing and welder re-qualification are required. For ASME B31.3 pipe, -50°F is the limit. Temperature limits are a function of weld thickness; the above values are for 3/8 in. and thinner. As weld (wall) thickness increases, these temperatures rise, that is, get warmer. The key here is when specifying carbon steel at low design temperatures, crossing over a critical metal temperature by 1°F will add cost to an ASME code vessel, heat exchanger, piping system or API-650 storage tank.

8. Consider coatings. The practice of coating isn’t new but remains under-utilized [7]. When quoting, fabricators often don’t suggest coatings, though, because it’s the owner’s responsibility (per code) to specify materials of construction. Plus it means adding an additional manufacturing step (and thus one to four weeks) to an often already tight schedule. Furthermore, most fabricators really aren’t expert in coating selection.

Proper selection of a coating that will resist the process is key. Coatings have limitations, primarily temperature. Many are restricted to 200°F to 300°F; they have a different coefficient of thermal expansion than the base metal they cover, which may make them more susceptible to separation over their service life. Like metallic vessels, coated equipment also requires periodic inspections. However, for moderate design temperatures, coating a carbon-steel vessel can be much more economical than purchasing a high alloy vessel or clad carbon-steel vessel. For instance, estimates for ethanol plants show savings of as much as 35% for coated carbon steel tanks compared to stainless ones.

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