Shell Ring Courses Fabrication
Cylindrical ring courses for pressure vessels are most frequently fabricated by a process referred to as “rolling and welding.” Plate with the required thickness is first cut to the size required for the diameter of the vessel and the length of the ring course. Relatively thin plates can be mechanically cut by shearing, but thicker plates are generally cut by oxyfuel gas or plasma-arc cutting. Whenever possible, each ring course is made from one piece of plate cut to a length that is equal to the circumference of the vessel, thus requiring only one longitudinal weld. Large diameter, or thick wall vessels may necessitate two or more plates to obtain the circumference, and consequently two or more longitudinal welds.
Weld bevels can be machined on the plate edges after the plate is cut to size. This machining must remove all shear damaged material from the edges of plate cut by shearing. Oxyfuel gas or plasma-arc cutting is also used to obtain the correct angles for the weld bevels on the edges at the same time that the plate is cut to size. Oxyfuel gas or plasma-arc cutting can therefore significantly reduce the time and cost of machining, but some finish machining or grinding is sometimes required to obtain an acceptable surface for welding and to remove thermally damaged material.
After the weld bevels are prepared, the plate is then rolled into a cylindrical shape with the diameter of the vessel, as illustrated in Figure 600-2. The correct diameter is obtained when the edges of the rolled plate come together (with the proper gap) to form the joint preparation for the longitudinal weld. “Peaking” at the longitudinal weld is minimized by “crimping” of the plate edges prior to rolling. Some reduction in plate thickness can occur that must be taken into consideration by the fabricator when the plate is procured, but this reduction is usually not significant. ASME Code, Section VIII, Divisions 1 and 2, requires the roundness of a rolled cylinder be within 1% of the nominal diameter for which the cylindrical component was designed (Paragraphs UG-80 and AF 130).
Plates with thicknesses up to 2- or 3-inches thick are generally rolled cold, but thicker plates may have to be rolled hot to reduce the force required to form the plate into a cylinder. A stress-relief heat treatment is recommended after cold forming if the fiber elongation exceeds 5% for carbon steel or 3% for low-alloy Cr-Mo steel, according to the following equation:
This equation is from ASME Code, Section VIII, Division 1, Paragraph UCS-79. Rolled cylindrical shell components will rarely exceed these limits.
Hot rolling at high temperatures can cause grain growth in plate materials that were given a normalizing heat treatment by the supplier. This can reduce the CV-impact toughness. Therefore, plates that are hot rolled at temperatures above approximately 1750°F may have to be heat normalized after rolling to restore the minimum CV-impact toughness. The fabricator should foresee this need, and prepare his materials purchase specification to avoid the unnecessary cost of duplicating normalizing heat treatments by the supplier and again after hot rolling.
Welding of the longitudinal seam is discussed in Section 651. Cylindrical shell ring courses can also be manufactured by a process known as “ring forging.” This process produces a ring course of the required diameter and thickness without the need for longitudinal welds. Generally, only ring courses with thicknesses of 4 inches and greater are manufactured in this manner.
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