Читать книгу Fabrication of Metallic Pressure Vessels - Maan H. Jawad - Страница 26

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.