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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  • Address documentation and archiving. How many times have you needed to replace a 20-plus-year-old vessel and worried about finding adequate documentation in the company files? Often only a drawing can be located; the code calculation, material certifications and testing records are long gone. The good news is that a vessel’s drawing is enough for a fabricator to provide you with an adequate bid — keep in mind, though, many of the fabrication practices and materials of the past are now obsolete. If documentation can’t be found, there are alternatives you can explore in lieu of starting from scratch.

Perhaps the original fabricator still has the print. For ASME-stamped equipment, fabricators are required to retain what’s called the “Manufacturer’s Data Package” for five years, though many will hold on to this information much longer. However, over the last 20-plus years, substantial consolidation and attrition have reshaped the vessel industry. So, while it’s worth trying, realistically you may find recovering information this way futile.

Your last chance for digging up old information is contacting the National Board in Columbus, Ohio. Besides training and accrediting code inspectors and auditing code stamp holders, it stores decades of manufacturers’ data reports (U-1 forms). A data report is much like the birth certificate of the vessel; it provides a wealth of information, such as major dimensions, materials of construction, shell and head thicknesses, nozzle construction, radiography and hydrotest pressure. To find out if the U-1 form is available, first check the vessel’s code nameplate in the field. If there’s a NB number stamped on it (or on the drawing), simply call the National Board, provide this number and the manufacturer’s name, and within days, for a nominal fee (i.e., $20 to $50), you’ll have the data form faxed to you. (Same day service is available for a small additional fee.)

 

Figure 2. RT-2, which uses both full and spot X-raying, can enable savings in materials and fabrication.

Such difficulties teach an important lesson. Be sure when purchasing vessels to include any special information (e.g., company purchase order, equipment number, special heat treatment or nondestructive evaluation) somewhere on the drawing because 20-plus years from now, a drawing may be all your successor is able to locate. Also, register and archive your vessel with the National Board — most states require registration [1], but it’s good practice even where not mandated. The cost is typically less than $50 and your fabricator will handle the registration.

  • More (radiography) is less (metal). The continuing climb in alloy prices necessitates a paradigm shift in thinking. It’s becoming more economical to specify more radiography (i.e., X-raying), not so much for joint quality but to reduce wall thicknesses.
    Vessel data sheets require the engineer to specify the amount of radiography required for the service. Typically the choices are “full,” “spot” or “none.” This tells the fabricator how much X-raying it should estimate for the job but, more importantly, it affects the required shell and head thicknesses based on ASME code rules. The ASME Code recognizes two types of “full” radiography, RT-1 and RT-2. RT-2 vessels provide the best balance in terms of risk and design economy [2].

“Full radiography” indicates to the fabricator that “all” pressure-retaining butt joints in the main vessel (excluding small diameter and thin wall nozzles) are to be X-rayed. This can be costly but actually can provide substantial savings when fabricating vessels from expensive materials by avoiding any penalty in vessel shell/head thickness. In ASME Code terms, this amount of X-raying is referred to as “RT-1” (Figure 2) and is stamped as such on the code nameplate.

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