General knowledge about various boiler and pressure vessels

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Manholes and handholes: Manholes and handholes are openings created in a container in order to inspect its internal space, clean it, and install or remove accessories inside the container. For vessels with a nominal diameter of 1000 mm or more, if it is not possible to access the interior using removable devices such as end caps or nozzles, manholes must be provided. This allows workers to enter the vessel during maintenance tasks in order to conduct a visual inspection or surface testing for defects such as corrosion, wear, or cracks on the inner walls of the vessel, or to carry out appropriate repairs for any such defects. For containers with a small diameter, if inspections cannot be carried out using other detachable structures, inspection manholes should also be provided. The common types of manholes or access holes are circular or oval. Circular holes are easy to manufacture and widely used. Elliptical holes are more difficult to manufacture, but their advantage is that the area of the opening in the wall of the vessel can be smaller. The oval manhole in vertical containers is usually located on the cylindrical body, as this allows the shorter diameter of the oval opening to be aligned with the container’s axis. This not only reduces the impact of the opening on the strength of the cylinder but also facilitates entry and exit for people, since the manhole is wider in the horizontal direction and narrower in the vertical direction. For horizontal vessels, the manholes provided on the cylinder should be circular; oval manholes are only used on disc-shaped heads. The manhole or access hole on the container is created by welding a short fitting to the opening in the shell, or by folding the edge of the hole into a short tube, which is then covered with a lid. The closure of manholes or access holes comes in two types: internal closure and external closure. The cover for an internally closing manhole or access hole is placed inside the opening, and secured with bolts to a crossbar that is positioned outside the hole and supported against its edge. This type often uses elliptical and irregularly grooved covers, as this facilitates the placement of gaskets and hole covers. Although installing inner-lidded hole covers is somewhat difficult, they offer good sealing performance. The pressure inside the container can further compress the hole cover, providing a self-sealing effect. In particular, it can prevent a large amount of medium inside the device from spraying out due to the loss of effectiveness of gaskets and similar components. Suitable for boilers as well as vessels with high-temperature or toxic gases as the medium. 24. Multiple-layer cylindrical vessel: A multiple-layer vessel usually refers to a thick-walled cylindrical container formed by welding multiple layers of plate cylinders together with forged end caps. The shell wall of a thick-walled cylinder made of multi-layer plates is composed of several or dozens of metal sheets that are closely fitted together. The methods for combining laminates include wrapping and welding, winding, and heat fitting. Multi-layer containers have the following advantages: (1) Through the layer combination process, contact pressure can be generated between the layers; as a result, compressive prestress exists in the inner layer of the cylinder, while tensile prestress exists in the outer layer. When the cylinder is subjected to internal pressure, the stress on the shell wall can be distributed more evenly, allowing for more efficient use of the shell material ； (2) When the medium is corrosive to carbon steel, a corrosion-resistant alloy can be used for the inner cylinder, while carbon steel or other high-strength low-alloy steel plates can be used as the lining plates; this allows for the full utilization of the advantages of both materials and helps to save precious metals ； (3) When the shell wall material has serious defects such as cracks (in the raw material or resulting from welding), these defects are not likely to spread across layers ； (4) The thin plates used possess better resistance to brittle fracture compared to thick steel plates of the same grade, resulting in a lower likelihood of brittle failure ； (5) Manufacturing does not require large-scale forging equipment. The disadvantage of multi-layer thick-walled cylinders is that their deep and narrow ring welds are difficult to heat-treat; in addition, the limitations associated with the connection between the various cylinder sections and the forged end flanges or heads can sometimes lead to brittle fracture. Due to the numerous advantages of the multi-layer container structure in terms of both design and manufacturing, this structure is widely used in high-pressure containers manufactured in recent years, especially large-scale high-pressure containers for the petrochemical industry. 25. Plate-wrapped thick cylinder: A plate-wrapped thick cylinder is formed by welding together several short cylinder sections and end flanges. The tube section consists of an inner tube formed by rolling and welding medium-thickness steel plates (usually 15–25 mm thick), with multiple layers (typically over ten layers) of thin steel plates (with a thickness of 6–12 mm) wrapped around and welded to its exterior. Each layer of laminate is first rolled into two circular shapes; during wrapping, it is pressed tightly against the outside of the inner tube, and after being tightened with special clamps, two seams are welded. The weld surface is smoothed with a grinding wheel, and then wrapped and welded layer by layer in the same manner until the desired wall thickness is achieved. The longitudinal welds between the various plate layers are offset from each other to reduce the weakening of the cylinder’s strength caused by these longitudinal welds. During wrap welding, the cooling and contraction of the welds in each layer enable the sheets to bond tightly together or generate interlayer contact pressure. A small hole that penetrates through all the layers of plates (excluding the inner cylinder) is usually provided on the tube section, serving as a signal hole (leakage hole). This allows any leakage that occurs due to a rupture in the container’s inner cylinder during operation to be detected promptly, thereby preventing the defect from worsening. Laminated wrapped welded high-pressure vessels were adopted by the American company A•O•Smich in the 1930s, and soon spread to countries around the world. Our country successfully developed a prototype by the mid-1950s. Many high-pressure vessels used in current fertilizer equipment still adopt this structure. General knowledge about various boiler and pressure vessels