Patent Number: 050874098
Section: summary

BACKGROUND OF THE INVENTION This invention relates generally to pressure vessels and in particular to pressure vessels for holding high temperature material under high pressure that require periodic inspection of welds and internal structural members. Reference is made to inventor's prior patent No. 4,767,593, for a multiple shell pressure vessel with interspace metallic fillers of which the invention disclosed and claimed herein is an improvement. For pressure vessels containing toxic material at high pressures and temperatures, such as, nuclear reactors or containment vessels for toxic chemical reactions, safety requirements, such as those required by the American Association of Mechanical Engineers and other organizations both public and private, dictate periodic inspection and re-examination of all pressure loaded joints. The joints, such as, shell-to-flange girth welds and all other welds, must be exposed for non-destructive testing, such as, by ultra-sound, X-ray or other methods for determining soundness of a structure. For a single wall pressure vessel of the prior art, assembly and disassembly of the vessel was fairly straightforward. All welds were generally readily available for inspection. For a multiple-shell pressure vessel, the welded assembly of pressure vessel shells rendered it difficult, if not impossible, to gain ready access to the welded joints of the inner shells of the vessel without breaking or cutting the vessel apart. For a single wall pressure vessel of the prior art, a crack in the shell could easily propagate through the wall causing a catastrophic failure. For a multiple shell pressure vessel, such as, the present invention, a crack in one shell-layer cannot propagate to the other layers. For the present invention, overpressure may cause "gapping" of the modules as the tendons stretch, with subsequent "leak-before-break" failure mode. In this failure mode the vessel fluid will leak out into the external (water) coolant thus relieving the overpressure. In addition, the prior art pressure vessels could fail due to excessive creep, or creep buckling. For the present invention, the tendons may be tightened and filler introduced in the outermost filler space, while the pressure vessel is in service, so that the main vessel shell layers will not be able to creep. Furthermore, the single or multilayer pressure vessels of the prior art cannot be cooled through the vessel wall. Therefore, internal thermal insulation, or cooling of the internal vessel wall surface is necessary. For prior art pressure vessels, this posed an awkward problem. The insulation would be exposed to the vessel fluid and maintenance and repair was cumbersome. For the present invention, the thermal insulation is located inside the wall and its outer surface and the outer vessel shells can be kept cool, due to good thermal bonding from metallic fillers in the interspaces between the outer pressure vessel shells, so that they can carry a larger pressure. SUMMARY OF THE INVENTION The multiple shell pressure vessel of the present invention eliminates these problems by comprising three modular sections, namely, a top head modular section, a nozzle course modular section and a bottom shell course modular section. The three modular sections each comprise at least an inner pressure vessel shell and an outer pressure vessel shell telescopically enclosing the inner pressure vessel shell. The rim or rims of each shell are provided with separate flanges having upper and lower bearing surfaces. When telescopically assembled, the lower bearing surface of one shell flange is adapted to engage the upper bearing surface of the adjacent shell flange or flange of an adjacent vessel module either directly or through an intermediate spacer plate or ring. Top and bottom tendon skirts are adapted to respectively engage the outer flange of the top head modular section and the outer flange of the bottom shell course modular section and compress the bearing flanges together by means of tension members or "tendons" linking the top and bottom tendon skirts. The "tendons" are hydraulically or mechanically tensioned to create the necessary force on the vessel flanges to seal the vessel. All the tendons are located outside the vessel where they are kept cool and not subjected to heat or radiation. Being located outside the pressure vessel, the tendon stresses can be monitored during service. It is, therefore, an object of the present invention to provide a large scale multiple shell pressure vessel that can be disassembled for inspection. It is another object of the present invention to provide a large scale multiple shell pressure vessel that can be assembled in two or more prefabricated modules for ease of transport and assembly. It is a further object of the present invention to provide a multiple shell pressure vessel in which the individual shells can be disassembled for inspection, testing and repair. It is yet another object of the present invention to provide a multiple shell pressure vessel where all shells can be stressed simultaneously to their design limit during normal operation. It is also an object of the present invention to provide a multiple shell pressure vessel which can tolerate large differences in temperature across its walls so that the inner shell layers may be very hot while the outer shell layers are kept cool in order to carry the pressure load. It is a further object of the present invention to provide a multiple shell pressure vessel in which the hoop and axial stresses are continuously monitored. It is another object of the present invention to provide a multiple shell pressure vessel in which the torsional forces on the flanges are reduced or eliminated. It is yet a further object of the present invention to provide a multiple shell pressure vessel in which the hoop and axial stresses can be adjusted while the vessel is operating. These and other objects of the present invention will be manifest upon study of the following detailed description when taken together with the drawings.