a welding neck flange, it has the advantage of eliminating the following costs: flange to nozzle weld, reinforcing (or insert) plate, reinforcing or insert plate layout, forming of reinforcing plate, drilling and tapping of reinforcing plate vent hole, fit up of reinforcing plate, and welding of reinforcing plate both to the shell and to the nozzle.
1.3.2.6 Enhanced inspection for higher joint efficiency
Enhanced inspection to increase joint efficiency can result in a significant reduction in wall thickness on a heavy wall vessel. This, in some cases, sufficiently reduces the wall thickness to allow the use of the next smaller stock thickness, thereby reducing material and other fabrication costs. When inspection has not been performed to allow 100% joint efficiency of shell longitudinal or head welds, however, then locating shell longitudinal or head welds so that welds aren’t included in zones used for reinforcement may, in some cases, be enough to eliminate the need for extra reinforcement, since the excess material in the shell can be counted based on the 100% joint efficiency of the parent material.
Major considerations in deciding whether to perform inspections to reduce other costs include the cost of the inspection, the anticipated labor and material cost savings, and the level of confidence that the welds will pass inspection the first time. If inspection shows that weld repairs are required, all savings in labor and material may be wiped out by the cost of repairs and reinspection, resulting in no benefit to the fabricator and a loss in terms of schedule, and tolerances may be affected as well.
Example 1.1
This example illustrates an actual vessel for which the design approach eliminated a large number of operations as well as fabrication risk by using a much heavier wall than originally specified.
The heavy wall vessel shown in Figure 1.2 is 16 ft long, 30 in. diameter, 4 in. nominal wall, with flat bolted heads, 88 total penetrations, and the full shell length machined inside. Figure 1.3 shows the side views of the same vessel. The vessel might have been fabricated of much thinner material, but was fabricated this way to reduce cost.
The vessel was originally designed using a 1‐3/4 in. thick shell with a number of heavier shell plate inserts with blind drilled and tapped holes for attaching instrumentation. The original design also had an added heavy section at each end with drilled and tapped holes for installation of cover flanges. The fabricator evaluated four approaches before making a proposal. Each approach included the large nozzles welded into fabricated shell sections. The four approaches were (1) as designed originally; (2) a centrifugal casting with flats machined and drilled and tapped for small penetrations, with the internal surface machined after insertion of the large nozzles so as to meet internal tolerance requirements; (3) a single piece, trepanned, heavy forged cylinder with the same approach as (2) for nozzle penetrations, and (4) a rolled and welded heavy plate wall shell with the same approach as (2) for nozzle penetrations.
Figure 1.2 A vessel fabricated with a heavy wall to minimize cost
(Source: Los Alamos National Laboratory)
Figure 1.3 Side views of the vessel in Figure 1.2
The rolled and welded design proved to be the least costly. All of the heavy walled designs eliminated two circumferential welds at the ends, as well as the cost of layout and cutting of holes, and welding in the plates for the small openings. They also reduced the risk of distortion by minimizing the amount of welding required. The centrifugal cast and forged designs had higher costs for the basic cylinder than did the rolled and welded design.
When the user recognized the costs and the tolerance risks associated with welding a large number of nozzle plates, the rolled and welded design was accepted. The rolled and welded shell was produced by a pipe fabricating shop, helping to manage costs.
1.3.2.7 Process choices
Often decisions about cost of fabrication depend on the quantity of product being produced. Vessels will be more economical to produce if processes are optimized, but sometimes the cost of optimization is not warranted. For a single vessel, or even a small number of vessels, the cost of procuring forming equipment and optimal welding equipment and costs of developing tooling will likely exceed any profit on the job. Sometimes even setting up existing tooling for a vessel will not pay for itself, and it will be less expensive to fabricate the product using less efficient means but with essentially no initial setup cost.
1.3.2.8 Forming
Vessel fabricators will usually use one of four ways of making vessel shells.
First, as noted in Section 1.3.2.3.3, the least expensive way of producing a vessel shell is almost always to purchase a standard size of pipe, if it is available. This is usually true even for rolled and welded pipe.
Second, if large quantities are to be produced, is to develop dies and form shell sections using a large press. The cost of this tooling, even ignoring the cost of a press large enough to perform this type of work, is high, and it will only be justified by large quantities of product. For large quantities, however, this approach allows the production of shell sections (usually halves or thirds) with a single stroke of a press. Even the cost of installing the dies may be fairly high, and may not be cost effective for single vessels.
Third, rolled shell sections may be produced using forming rolls as described in Section 4.3, followed by placement of longitudinal welds. This technique is especially useful for diameters and shell lengths that can be rolled out of a single piece, since it efficiently produces cylinders requiring only a single longitudinal weld.
Finally, shells can be produced on a press brake. This is usually more labor intensive than either forming rolls or forming dies, but for small quantities of shell courses or if control of all aspects of the production is needed, it can make sense. For a company possessing a press brake but not forming rolls, rolling of pressure vessel shells can be accomplished in either of two ways: first, the shell can be “bump rolled” on the press, usually in sections, and second, the company can either buy the finished product or send shell material to a fabricator possessing a set of rolls for rolling. If the first approach is taken, the labor cost for bump‐rolling itself is probably greater than that for a product produced using forming rolls, but the cost of extra layout, pre‐crimping or cutting off extra material allowed in place of crimping (thicker sections), and shipping the product both directions are eliminated. Thus, for a single product, bump‐rolling may be adopted, while the second approach is likely if a number of shells are required and the roll setup costs can be better distributed over the number of shells produced.
If a very large quantity of the product is to be produced, particularly if it is to be produced on an ongoing basis, then a company may invest in a set of forming rolls. The cost of the rolls is then amortized over the life of the product line, costs go down, and the company has a new capability.
1.3.2.9
