12. Pay attention to surface finish. Polishing usually is specified for process reasons, although it can be very effective in improving equipment fatigue life in cycle service.
Unless directed otherwise, fabricators usually will provide a mechanical polish that will leave very fine scratches, small burrs, and a less-than-uniform microscopic surface appearance. Often this is sufficient and cost effective for a surface finish no smoother than 10 Ra (average roughness or the measure of the peaks and valleys with respect to the mean surface). On the other hand, electro-polishing generally is more cost effective below 10 Ra. Electro-polishing essentially is controlled uniform corrosion. The surface is first mechanically polished (to a rougher finish than the specified final finish) and then placed in an electrolytic solution. Sacrificial cathodes are strategically placed in the solution, thereby causing the vessel to be anodic. A voltage is applied, resulting in a small amount (1 mil to 5 mils) of surface metal (and debris) being uniformly electro-chemically removed. Iron and nickel are more anodic than chromium and are removed more, leaving the surface chromium-rich and very shiny. This operation simultaneously performs a surface passivation. Electro-polishing is a superior finish but comes with a heftier price tag; it typically costs $10,000 to $20,000 more than mechanically polishing, depending on the size of the vessel.
Be precise when specifying the quality of finish. A U.S. supplier will interpret 10 Ra as micro-inches while a non-U.S. supplier may be thinking micro-meters. Of course, 10 Ra micro-inches is vastly different than 10 Ra micro-meters.
The next article in the series will address fabrication issues such as head, jacket and tubing choices, how to reduce fabrication costs using radiography and related topics.
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.
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