Fabrication of Metallic Pressure Vessels. Maan H. Jawad

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СКАЧАТЬ Fixturing

      As in the case of tooling for rolling a shell course, the value of fixturing for fit‐up and assembly is often limited for production of a single or low volume product, but as production rises the cost of fixturing may remain constant while the benefits increase.

      For a single shell, tack‐welded lugs, wedges, and clamps are often all that are needed and used for alignment, though in some cases hydraulic rams may be used. Lugs will be flame cut out of stock plate and welded in place – number of lugs, thickness of lugs, and amount of weld vary depending on how much “persuasion” the fit up is anticipated to require. After pushing shell edges (for example) into alignment so that they can be tacked together for welding, the wedges are removed, the lugs are ground free or knocked off, and any damage to the plate surface is repaired and ground flush.

      Compare this scenario to the shop that has many shells of either the same size or a small number of predictable sizes to be produced. In this case, design and construction of fixtures to accomplish the same thing can cut individual shell fit‐up time significantly. Once shell fit‐up fixtures are constructed, the following might take place: The rolled plate section is placed on the fixture. Portions of the fixture will be swung into place and pinned. Hydraulic rams will push the sides and ends of the shell to bring the longitudinal joint into rough alignment. Other rams are used to bring the edges into the same plane. The side rams may be further adjusted to provide the proper root opening. The longitudinal seam is tacked, the rams are released, the fixture arms are moved back to provide space to remove the shell course, and the next shell is brought in.

      There is often a sizable investment in a fixture such as this, and any such equipment that is developed will occupy shop space, so it shouldn’t be done without consideration of the returns. However, this investment can cut what may be an 8–16 (or even more) hour job to a matter of an hour or so.

      1.3.2.10 Welding

      A similar situation arises in the case of manual versus automated welding. A number of different processes may be used to produce welds. Each has its benefits and drawbacks. Chapter 7 discusses welding in detail and provides a comparison of various welding processes, including deposition rates. Items to consider include equipment and setup costs versus the benefits of more efficient placement of welds, design for production runs rather than individual fabrications, and weld configurations, such as narrow welds to minimize weld metal required and residual stresses.

      1.3.2.11 Hydrotesting

      Pressure testing is most often performed using water or other comparably incompressible fluids. Hydrotest of a single vessel is usually accomplished by filling it, pumping to pressure, holding, and draining the vessel. For single vessels, the water is usually dumped after use, and pumping is accomplished using a small positive displacement pump.

      If the quantity of vessels produced in accordance with a particular vessel design is such that multiple vessels are tested daily, then it is common to set up test fixtures and to salvage and recycle the water. As with other means employed to reduce per unit cost, the savings must be weighed against the up‐front cost of fixturing, constructing a reservoir, etc.

      1.3.3 Shipping

      For most pressure vessels, the cost of shipping is not more than a few percent of the total cost, yet even that is enough that it should be considered in the price of the product. For products that are extremely large or extremely heavy, however, that percentage may increase.

The size and load capacity of standard rail cars facilitate shipment of many vessels that might require permits as wide, long, or heavy loads if shipped by truck. Rail rates (per pound) are often much less than truck rates. Barges even more so, if the size of the product justifies them. This is especially the case if permits or special routing are needed for trucking. Rail shipments often take longer than trucks, however, due to the way that rail traffic is routed.

      In any case, unless the estimator is confident of knowing shipping costs with a good degree of accuracy, it would be good to verify costs with shippers prior to bidding a job. See Chapter 11 for more information regarding shipping.

      1.3.4 General approach to cost control

      Effective management of cost involves making trades based on actual costs of the delivered product. It therefore requires assessment not only of material and labor costs but also the cost of shipping. This will be especially important for shipment of large and/or heavy fabrications. These are likely to require permits and may require special equipment and routing, raising costs far above the usual cost per pound for shipping.

      A general rule, with some exceptions, is that labor costs outweigh material costs and that labor is therefore the area most ripe for cost reductions.

      If, for example, material represents 10% of the cost of a product, then any reduction in material costs must clearly be less than 10% of the cost of the overall product. This could be the case for a carbon steel vessel with complex fit‐up. For this, vessel reductions in labor likely do not increase material costs significantly and should be considered as ways of reducing overall costs.

      A vessel fabricated of certain nickel alloys, titanium, or zirconium, on the other hand, will have very high material costs. In this situation, reducing material costs may be effective in reducing overall costs.

      Seeking only the lowest hourly rates risks, at times, finding the lowest productivity, but where skilled labor is acquired cheaply, overall product costs may be low.

      Thus, it is important to assess the overall cost of a delivered product. When a design change is made, whether or not with the intent of reducing costs, overall costs must be reassessed. It will sometimes be found that the change results in even greater savings than anticipated, but it will also sometimes be found that the savings are eaten up by increases in other areas.

1.4 Fabrication of Nonnuclear Versus Nuclear Pressure Vessels

      The fabrication of nuclear components such as vessels, pumps, valves, piping, and storage tanks in the United States must meet the requirements of Section III Division 1 of the ASME Boiler and Pressure Vessel Code as well as the rules of the U.S. Nuclear Regulatory Commission (NRC). This book is written for nonnuclear applications. While the general fabrication processes such as forming, machining, and welding are the same for both nonnuclear and nuclear components, the quality control process is different regarding the details of these operations.

      Nuclear components constructed in accordance with the ASME code are considered in “classes” that are used to construct pressure equipment in accordance to its relative importance to safety. The three most common classes are Class 1, 2, and 3.

Class 1 components, including vessels such as reactor vessels, pressurizers and the primary side (tubes) of steam generators are exposed to radioactive coolant fluid, and they consequently are considered to bear the highest importance to safety. Class 1 vessels therefore require the most stringent levels of quality control for the various fabrication operations, compared to Class 2 and 3 vessels. Class 2 vessels generally resemble ASME VIII‐2 (editions prior to 2007) requirements, while Class 3 components are generally similar to VIII‐1 requirements.

      All three classes of nuclear components are subject to strict quality control during construction. Quality assurance requirements for nuclear applications are provided in Article 4000 of the СКАЧАТЬ