Fabrication of Metallic Pressure Vessels. Maan H. Jawad

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СКАЧАТЬ line, either available off the shelf or made to order. Those that do not have a product line and that bid for individual jobs within their field(s) of expertise are referred to as job shops or custom fabricators. Whether fabricating a piece of equipment on a job shop basis or producing a standard product, the organization must develop a design, procure or produce the component parts, and assemble them, all the while ensuring quality and maintaining quality assurance documentation.

1.2 Fabrication Sequence

      To provide a background for the remaining chapters, which delve into the details of each aspect of pressure vessel fabrication, consider a large pressure vessel for a process application. The fabrication process flow proceeds as follows:

      The pressure vessel manufacturer receives a request for quotation from the procurement organization for a petro‐chemical plant. A job file will be created and a project engineer or estimator will be assigned.

      If the design of the pressure vessel is fully defined by the purchaser, including all dimensions, materials, interfaces, etc., then the bidding process will be straightforward. However, if just interfaces and process requirements are provided, then this will allow the fabricator leeway to use its particular experience, efficiency, or capability. Either way a job file will be created to document what is required and what has been accomplished. This allows keeping track of preliminary analyses, decisions, and details, and it ensures that work and research such as sourcing of unusual components done at the bidding stage does not have to be repeated if the company is successful in getting the job.

More sophisticated customers, such as oil refineries and larger chemical companies, may provide a fairly refined design and will often have their own design specification. Such company specifications usually include requirements that may increase the cost of fabrication over that of a minimal design. The further details are usually based on company experience indicating that long term overall costs are reduced by the additional requirements. Others will leave much of the design to the fabricator, just defining interfaces and process requirements such as temperature, pressure, volume and envelope dimensions, and chemical compatibilities. Or they may provide the design of a vessel that is being replaced but still allow some design and fabrication flexibility for the new vessel.

      If only limited design information is received, then a preliminary design must be roughed out to produce a cost estimate. Even if the design is fully defined, the fabricator will still need to resolve items including many of the weld details, weld processes, and things such as whether a nozzle is fabricated using a pipe and a flange or a long welding neck (LWN) flange. Not every detail needs to be worked out at this stage, but there needs to be sufficient resolution of the design that a reasonable cost estimate can be produced. Accuracy should be precise enough that the company can be confident of making a profit on the job and at the same time be competitive on price and delivery. Extra time invested at this point can often find ways to keep overall fabrication costs down, resulting in a higher bid success rate and helping ensure that no unpleasant surprises occur after receipt of a contract.

      This book will not address the details of developing a bid on process equipment except to note that accurate bidding involves a thorough understanding of what it takes to produce the required equipment, and enough clarity in the estimate to ensure that all aspects of the effort are covered. Fabricators with standard products may use sophisticated internal estimating programs to develop pricing information. Other fabricators rely on the background and experience of their estimators to put together material and labor costs for each and every job, and some use standard industry programs to assist.

      Once the order is actually received, the design and process flow will be finalized and a quality assurance package begun.

      If not already accomplished at the bid stage, trade‐offs will be assessed, such as stronger material or additional inspection such as radiography or ultrasonic testing to allow increased joint efficiency to reduce vessel wall thickness. This can reduce total material weight and the amount of welding required. Some parts of the design may be decided based on shorter lead times for one option than for another. Some are based on the particular equipment and capabilities available within the company. Others are based simply on cost. After all aspects of the design have been defined, a detailed material list will be produced. This may be done using in‐house or specialized industry software, or it may be done by hand. Any material not available from stock must be procured, and process flow may be adjusted accordingly.

      It is usual to identify long lead time items and contract for them immediately. Typically, these include heads (if not made in‐house), special valves, filters, and forgings, any mill orders, anything made of exotic materials, and anything else that was identified during the bid stage as requiring extra time. Some custom manufacturers may stockpile such items as exotic materials and exotic weld supplies in anticipation of future orders to minimize lead time and get an edge over their competitors.

      Weld procedures may be developed at this time if they are not available, as coupons can then be produced and tested in parallel with the wait for materials and components without extending the overall schedule.

Also, at this time quality assurance personnel develop plans for the required inspections, tests, and hold points that will take place throughout fabrication. This will include a number of dimensional inspections, verification that reported test results are compliant with applicable requirements, verification of process control of welding and other processes, and review of radiographs and other nondestructive examination (NDE) results. Review by the Authorized Inspector will be included if the work involves an ASME code stamp, which for a pressure vessel it almost certainly does.

      Additionally, this is when the layout department is likely to become involved. The layout department personnel have a thorough understanding of geometry, trigonometry, fabrication, and some of the behavioral characteristics of materials while they are being fabricated. They are trained in how to lay out intersections of such items as pipe or cone sections with heads or shells. The layout department will plan for efficient use of materials, produce detailed layouts for the heads and shell sections, mark locations and contour cutouts for nozzle installations, etc. The first part of this effort takes place in the flat, when shell sizes and weld bevels are prepared. Other parts occur throughout fabrication. Shell layout will include allowance for weld shrinkage.

      As the material arrives, it will go through a receiving inspection and be checked for compliance with specifications, with material mill test reports and other documentation placed in the quality assurance file. Early arrivals are often stockpiled but segregated from non‐code materials that have not gone through quality assurance acceptance until enough components are available to begin work and continue through the flow without unnecessary starts and stops. Even if shell plates are available from stock, it is usual to postpone cutting them until the heads arrive so that actual head dimensions can be measured, or to request a “taping” (a measurement of the circumference) from the head manufacturer prior to shipping. This allows the shell diameter to be adjusted if needed, from its nominal dimension to permit an optimal fit to the heads. This slight adjustment to the shell circumference is often necessary since it is difficult to bring the head circumference to a precise dimension during forming due to the three‐dimensional nature of the head. The difference is usually fairly insignificant, but even an eighth of an inch (3.2 mm) of diameter can make fit up and welding more, or less, efficient.

      The shell sections are rolled subsequent to cutting to size and beveling for welds. Their longitudinal joints (straight seams) will be tacked into alignment, and then welded. The heat and stresses of welding will cause a certain amount of shrinkage and distortion. This may be, to some extent, controlled by alternating weld passes on the inside and outside of the weld. However, if distortion is excessive after welding, the shells will be reworked with hydraulic rams or will be re‐rolled to bring them back within tolerance. Working from the zero point on each shell course, nozzles and other appurtenances will be laid out full scale on the plates, with indications СКАЧАТЬ