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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As the vessel’s diameter, weight or length increase, shipping costs can become an overriding factor on which fabricators you invite to bid. Vessels more than 16-ft. diameter can only be trucked a short distance economically; hauling by either rail, barge, or shop fabricated in pieces and field assembled becomes the norm. Between 12-ft. and 16-ft diameters, it becomes very situational, so chose your bidders wisely. Moving a large vessel from the fabricator’s shop through a city to a major interstate or river can cost $10,000/mile and usually the most direct route can’t be taken. Factor in geographic location. It’s easier to ship wide loads in Western U.S. states than in the more congested Northeast.

  • Move carefully with used and relocated vessels. Several suppliers specialize in used equipment; check the classified ads in Chemical Processing and other magazines. Such vessels can be purchased at a discount and are available for your inspection if desired. Also, some operating companies often consolidate operations into a single location, which can lead to relocating vessels and other major equipment across state lines. When relocating a used ASME-code vessel to your state, first check with the office of the Chief Boiler Inspector in your state capital. Many state laws require pre-approval and inspection by that office before a vessel can enter the state. Failure to comply with state law carries monetary penalties.
  • Pay attention to internal coils. Internal coils can be included as part of an ASME vessel but aren’t required to be code stamped. It’s good practice to include internal coils in the pressure vessel scope so that strict quality control procedures are followed and third party inspections are performed. Internal coil failure can lead to forced shutdown, off-spec product quality and possibly safety concerns.

 

Table 1. The level of X-raying directly affects the necessary thickness of material, as illustrated here for an 8-ft. diameter, 16-ft long shell vessel with the same design pressure and temperature and with internal pressure governing the design. (Note: costs are approximate and can vary greatly depending on set-up costs, wall thickness (i.e. exposure source/time) and film type/length.)

For internal coils, specify butt-type joints with 100% radiography and avoid internal flanges, threads, couplings and socket-type joints. Fillet welds, which 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.

  • 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 confirm 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.

 

Figure 1. This wide load represents only one section of a large vessel that will require field assembly.

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. 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.

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