Fabrication of Metallic Pressure Vessels. Maan H. Jawad

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      The fitter assembles the shell courses, referred to as “courses” or “cans,” to each other and to the heads. Circumferential shell welds are usually welded on positioning rolls to allow welding to be performed in the flat position, which is the preferred position because it is the easiest orientation for producing high volumes of high‐quality weld. Next, nozzles will be fabricated, and reinforcing pads laid out, cut, and formed, then fit in place and welded, either preceded or followed by fitting and welding of supports.

      Note that while this description looks simple, the work involves a high level of training and skill on the part of the layer out, fitter, and welder. The fitter has the job of fitting and tacking together the assembly within fairly tight tolerances and the welder must be able to produce hundreds of feet of weld with the least amount of rejectable indications.

Nozzles, for example, must be laid out and then fit accurately in the holes cut in the shell sections, correctly oriented, with the proper projection, and with bolt patterns on the flanges in the correct orientation. This is essential in order they fit correctly with piping that may already exist in the field or that may be assembled elsewhere. The fitter will typically install “spiders” and other braces to minimize distortion such as shells going out of round or nozzles sinking excessively during welding. The welder also has a part, controlling his welding within the parameters of the Welding Procedure Specification, in accordance with which he has already demonstrated the ability to produce top quality welds. The welder also maintains preheat and interpass temperatures and speed of progression, and makes in process adjustments as his experience dictates to maximize productivity, avoid weld defects, and control distortion.

      After welding is completed, the welds will be inspected. Common inspection methods include the following:

      1 Visual.

      2 Magnetic particle for ferromagnetic materials such as steel.

      3 Dye penetrant for either magnetic or nonmagnetic materials.

      4 Dimensional inspections.

      5 Radiography.

      6 Ultrasonic examination.

      It is usual to do these inspections before any required post weld heat treatment (PWHT), even if they are required after PWHT as well, so that any needed repairs can be completed prior to final heat treatment. This is because repair of a defect found after PWHT will normally require repair and an additional heat treatment. Such additional heat treatment can be costly as well as have the potential to reduce material mechanical properties.

      After PWHT and required final NDE, any final machining that is needed takes place. Intermediate machining processes may already have taken place if thick welds require J‐grooves or if unique machining is required because of special configurations. Also, for vessels such as heat exchangers requiring tubesheets or other special components, machining of these tubesheets and components is accomplished in parallel with other work on the vessel.

      Next, the vessel will be pressure tested when inspections and NDE demonstrate compliance with all requirements. Pressure testing is done either by a hydrostatic test, which is preferred for safety reasons, or by a pneumatic test. Although failures are not anticipated, access is usually restricted during such tests due to the potentially high levels of stored energy. This is especially true during pneumatic tests, but even though water is considered an incompressible fluid, the energy stored by compression of water or other liquid and any trapped air and the stretch of the metallic shell can result in a significant hazard during such tests.

      Once the pressure test is completed and all other quality requirements are verified, the vessel is ready for final cleaning and application of any required paint, conversion finishes, anodizing, or other surface treatments. A name plate describing various vessel parameters is then attached to the vessel, indicating compliance with the applicable code and other requirements.

      Finally, with fabrication, inspection, NDE, and testing completion, and coatings applied, the pressure vessel is readied for shipment. Shipment may include low level pressurization with a clean, dry, inert gas, sometimes referred to as “pad pressure.” It is used to ensure that nothing is sucked into the vessel on cold days and to prevent condensation. Shipment also includes blocking or cribbing, special supports, possible packaging, and tie‐down on the truck, railcar or barge for shipping.

      Once the product arrives at the customer’s facility, it will often undergo further inspection to ensure that all of the requirements have been met and that there has been no damage during shipping. The Quality Assurance package, when supplied, will be reviewed in detail and placed on file. Only then can the vessel be installed and put into service.

1.3 Cost Considerations

      The cost of a pressure vessel is a function of many parameters. In areas where labor is costly, it is often the biggest single factor, but many decisions by both the designer and fabricator influence overall cost. The most effective design from a cost standpoint will be one in which schedule, cost and availability of materials, cost and capability of labor, inspection options, and available equipment and tooling are all considered. In addition, short versus long term product cost considerations may need to be discussed with the customer.

      It follows that the designer will either have some experience in all of these areas or will work closely with people who do. Similarly, the shop management will be familiar with a wide range of production techniques, including means of cutting and machining, forming, fixturing and fit up, welding, heat treatment, inspection and testing, cleaning, painting and other surface treatments, and packaging and shipping options and requirements.

      If large numbers of vessels of the same or similar designs are fabricated, design and fabrication choices will be different from those involving fabrication of a single unit.

      The particular capabilities of a vessel fabricator often make one variation of a design more cost effective than another, and if the designer is not directly associated with the fabricator, it makes sense for these two parties to discuss design options with an eye on cost reduction.

      This book is not about fabrication cost estimating, and this chapter does not address actual product cost. It, however, addresses a number of considerations affecting the cost of an overall pressure vessel fabrication to help the user, designer, and fabricator make judicious choices regarding design and fabrication approaches.

      1.3.1 Types of costs

      For a business, one way of dividing costs is to separate them into either capital or operating costs. Capital costs are the one‐time expenses such as purchase of land, construction of a plant, and major equipment purchases that are expected to last a long time. A small hand grinder, for example, would not be considered a capital cost, while the costs of constructing a building or purchasing a large forge would be. Operating costs are the other costs of being in business, including wages and salaries, real estate expenses (rent, taxes, etc.), materials, furniture, consumables, maintenance, etc.

      This way of looking at expenses is useful in understanding what things cost overall, and it might be enough for a company with a single product line. For calculating and controlling costs of production of individual products in a job shop, it is usually easiest to work with burdened labor rates that represent the hourly cost of performing an operation, plus material and other direct costs of a particular job, plus capital costs. The burdened labor cost includes such items as direct wages, cost of vacations and holidays, social security and other tax cost, sick leave, and pension or 401k plans.

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