A guide to materials and fabrication
This is the second article in a three-part series intended to provide guidance to individuals not versed with specifying welded equipment (e.g., pressure vessels, heat exchangers and tanks). The first part, which appeared in the January 2008 issue. The final part of the series will address design mistakes.
"Twenty-two weeks after receipt of approved drawings.” How often do you see this in a quote and don’t understand why it takes so long to fabricate equipment. With the loss of in-house fabrication-savvy personnel over the last two decades, many owner/operators and engineering companies often write purchase specifications that can add unnecessary time and cost to a project.
So, in this second article in our series, we’ll provide pointers on how to avoid delays and achieve savings in the fabrication process. Moreover, we’ll cover techniques that, if schedules permit, can improve vessel reliability and thus forestall costly repairs down the road.
- Know your fabricator’s limitations. Nearly all ASME-stamp-holder fabricators are qualified to weld carbon steel and stainless steel; some are qualified for high nickel. But when ordering vessels in an exotic alloy (e.g., titanium, tantalum or zirconium), another niche material (e.g., copper, aluminum or chrome-molys), or made via a specialized process (such as clad overlays), do your homework. Welding is a skill and all metals don’t weld in the same manner or require the same skill level to produce a quality weld. If a welder hasn’t used a specific welding process in the last six months, the ASME Code requires the welder to requalify. Therefore, strive to find fabricators that regularly weld the material you need. Call around before making your bidders’ list and request references or a rundown on recently fabricated equipment. This will eliminate “no bids” and less qualified fabricators.
Additionally, it’s a mistake to view a heat exchanger as just another vessel. Heat exchangers are “performance” vessels and “mechanical only” fabricators don’t have the necessary thermal performance software to appropriately analyze your process data to provide an optimum design. It may seem safe to award mechanical only fabricators “replace in kind” orders without performing a new thermal analysis but you may have missed an opportunity to improve your plant’s performance. Also, heat exchanger fabrication requires special processes such as tube-to-tubesheet welding and tube rolling, which depend upon acquired skills and knowledge. So, fabrication should be left to those companies well versed in manufacturing and analyzing heat exchangers.
- Be aware of wide loads. A wide load is a generic term for over-the-road shipments whose width exceeds 8 ft. or height exceeds 13.5 ft. (Length and weight restrictions also apply.) Western U.S. states have a legal height limit of 14 ft. When exceeding these limits, state (and sometimes city) “wide load” permits must be obtained — requirements vary by state — and driving restrictions such as dawn-to-dusk curfews are imposed. It’s always easier to obtain permits for excess width; states usually require at least one escort vehicle. When height limits are exceeded, utility company involvement and police escort become the rule rather than the exception.
Shipping arrangements are handled by the fabricator but it’s useful to know where the break points are for permits when developing your design. In general, a vessel 12-ft. diameter (or less) can be shipped with minimal permitting and attendant costs. This accounts for the deck height of a trailer and can mean the trailer must occasionally bypass an overpass. For larger diameters, the permit process gets more involved. A “stick trip” (travel of the entire route by a vehicle with a mounted stick to verify clearance prior to actual shipment) often is involved and the design of the equipment (i.e., location of nozzles and other external attachments) will impact the permit. If possible, orient attachments to minimize the load height. Purchase vessels of this size and magnitude from fabricators knowledgeable about handling wide loads. Experienced personnel will recognize if a vessel can be shipped in large sections (Figure 1) and then welded together and tested in the field.