Chemical Processing - May 2008 - (Page 30)

are associated with socket joints, are more prone to fatigue failures and aren’t easily radiographed. Return bends (180 deg.) and coils bent from pipe improve reliability by minimizing internal welds and fittings, which are the primary cause of coil failure. A heavy corrosion allowance is suggested. A coil will be buoyant if steam is used as a heating medium (because its specific gravity is less than that of the product); so, design for both hold-down and thermal growth. 5. Understand the role of code inspectors. Vessel inspections are performed in the shop by Code Authorized Inspectors, commonly referred to as AIs. They aren’t employed by the fabricator or vessel owner but by the fabricator’s insurance company or, sometimes, by the local jurisdiction (i.e., state or city government). Their purpose is to con rm vessel safety — not absolute quality — by ensuring the fabricator has followed the rules and procedures of the ASME code. They check to ensure the materials, welding and testing meet the rules of the code for which the vessel was designed and major dimensions, such as vessel diameter and overall length. It’s up to the owner or designer to perform quality checks. AIs don’t check for all nozzle locations or measure support lug location or many of the minor dimensional requirements (e.g., nozzle projection) needed for your project. They don’t check for special surface finishes, internal or external coatings or contractual requirements written in requisitions and purchase orders. 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 and is available online at www.ChemicalProcessing.com/articles/2008/003.html, covered common materials mistakes. The ﬁnal part of the series will address design mistakes. >> Full radiography Full RT NPS 20 RT-1 Full RT NPS 20 RT-2 Spot RT Figure 2. RT-2, which uses both full and spot X-raying, can enable savings in materials and fabrication. 30 • May 2008 They will check for special testing and examination requirements if specified on the fabrication drawings. If dimensional accuracy or special coatings and finishes are essential for your project, it behooves you to schedule shop inspections. 6. 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.) www.chemicalprocessing.com http://www.ChemicalProcessing.com/articles/2008/003.html http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - May 2008

From the Editor ChemicalProcessing.com Field Notes In Process Energy Saver Compliance Advisor WirelessHART signals a change at plants Avoid costly fabrication mistakes Watch out with variable speed pumping Hot cutover boosts control system migration Plant InSites Process Puzzler Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - May 2008

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