Introduction to the basic structure of pressure vessel
Pressure vessel is usually a welded structure composed of plates and shells. Among the pressure components, the shells corresponding to the cylindrical cylinder, spherical tank (or spherical head), elliptical head, dished head, spherical crown head, conical head and expansion joint are respectively cylindrical shell, spherical shell, ellipsoidal shell, spherical crown + ring shell, spherical crown, conical shell and annular plate + ring shell.
The flat cover (or flat head), annular plate, flange, tube plate and other pressure components respectively correspond to the circular plate, annular plate (the difference between the outer radius and the inner radius is greater than 10 times of the plate thickness), ring (the difference between the outer radius and the inner radius is less than 10 times of the plate thickness) and elastic foundation circular plate.
The above-mentioned 7 kinds of shells and 4 kinds of plates can be combined into a variety of pressure vessel structures. In addition, sealing elements, supports, safety accessories, etc. constitute a complete pressure vessel. Figure 1-1 shows the overall structure of a horizontal pressure vessel. The basic composition of the pressure vessel is briefly introduced in combination with the figure below.
- Sealing device
- Opening and connecting pipe
- Safety accessories
- Welding between parts of pressure vessel
The function of the cylinder is to provide the pressure space required by the process, and it is one of the most important pressure components of the pressure vessel. Its internal diameter and volume are often determined by the process calculation. Cylindrical cylinder and spherical cylinder are the most commonly used cylinder structures in engineering.
When the diameter of the cylinder is small (generally less than 1000mm), the cylinder can be made of seamless steel pipe, and there is no longitudinal weld on the cylinder; when the diameter is large, the steel plate can be rolled into a cylinder on the plate rolling machine or pressed into two half cylinders on the hydraulic press, and then the two cylinders can be welded together by the weld to form the whole cylinder. Because the direction of the weld is parallel to the longitudinal (i.e. axial) direction of the cylinder, it is called longitudinal weld, or longitudinal weld for short.
If the diameter of the vessel is not large, there is generally only one longitudinal weld; with the increase of the diameter of the vessel, there may be two or more longitudinal welds due to the limitation of the size of the steel plate. In addition, the vessel with short length can be directly connected with the head at both ends of a cylinder to form a closed pressure space, which also makes a pressure vessel shell.
However, when the container is long, due to the limitation of the size of the steel plate, it is necessary to roll and weld the steel plate into several segments of the cylinder (a segment of the cylinder is called a cylinder section), and then assemble and weld two or more segments into the required length of the cylinder. The connection weld between the barrel section and the barrel section and between the barrel and the end head is called circumferential weld because its direction is perpendicular to the axial direction of the barrel.
According to its structure, cylinder can be divided into two types: single layer type and combined type.
①. The wall of the single-layer cylinder is composed of a whole material in the thickness direction, that is, there is only one layer of the wall (in order to prevent internal medium corrosion, the anti-corrosion coating on the lining is not included). According to the manufacturing method, single-layer cylinder can be divided into single-layer roll welding type, integral forging type, forging welding type, non welding bottle type, etc.
The single-layer roll welding structure is one of the most widely manufactured and used cylinder forms at present. It uses steel plates to roll up the cylinder on a large-scale rolling machine, forms a cylinder section through welding longitudinal welds, and then assembles and welds with the head or end flange to form a container. The cylinder shown in Figure 1-1 is a single-layer roll welding structure.
The integral forging structure is the earliest form of cylinder. During manufacturing, the cylinder and flange can be forged as a whole or connected by thread, and the whole cylinder has no weld. After the development of welding technology, forging in sections appears, and then welding and assembling the whole forging welding cylinder.
There are two main manufacturing methods of non welded cylinder: one is made of high-quality seamless steel pipe by hot spinning at both ends; the other is hot spinning after ingot stamping. Generally, integrally forged and forged welded cylinders are mainly used in high-pressure and ultra-high-pressure vessels, while non welded cylinders are often used to manufacture non welded large volume pressure vessels.
1-flange; 2-support; 3-Head splicing weld; 4-head; 5-circumferential weld; 6-stiffening ring; 7-manhole; 8-longitudinal weld; 9-cylinder; 10-pressure gauge; 11-safety valve; 12-level gauge
The material of the integral forging cylinder has dense metallographic structure, high strength and good quality, which is especially suitable for the ultra-high pressure vessel made of high strength steel with poor welding performance. However, very large smelting, forging and machining equipment are needed in manufacturing, with large consumption of materials, low utilization rate of steel (only 26% ~ 29%), and large machining volume. Therefore, it is generally only used for small ultra-high pressure vessels with an inner diameter of ϕ 300 ~ 800mm and a length of no more than 12M, such as polyethylene reaction kettle, artificial crystal kettle, etc.
②. The wall of the combined cylinder is composed of two or more layers of mutually discontinuous materials in the thickness direction. According to the structure and manufacturing mode, the combined cylinder can be divided into two categories: multilayer type and winding type. The specific structure will be described in Chapter 4 of this book.
According to the different geometry, the head can be divided into spherical, elliptical, dished, spherical crown, conical shell and flat cover, among which the spherical, elliptical, dished and spherical crown heads are collectively referred to as convex heads.
When the container does not need to be opened after assembly (generally, there is no internal part in the container or there is internal part but no need to replace or repair), the head can be directly welded with the cylinder, so as to effectively ensure sealing, save materials and reduce the workload of processing and manufacturing. For containers that need to be opened for many times due to maintenance or replacement of internal parts, the connection between the head and the cylinder shall be detachable. At this time, there must be a sealing device between the head and the cylinder.
There are many sealing devices on the pressure vessel, such as the detachable connection between the head and the cylinder, the detachable connection between the vessel connecting pipe and the external pipeline, and the connection between the manhole and the handhole cover. Whether the pressure vessel can operate normally and safely depends on the reliability of the sealing device to a great extent.
Bolt flange connection (abbreviated as flange connection) is the most widely used sealing device. Its function is to ensure the sealing by connecting with bolts and tightening the bolts to compress the sealing elements. The flange is divided into vessel flange and pipeline flange according to the components connected.
The flange used for the connection between the vessel head (or top cover) and the cylinder and between the two cylinders is called the vessel flange; the flange used for the connection of the pipeline is called the pipeline flange. In the high pressure vessel, the vessel flange, which is used to connect the top cover and the cylinder and welded with the cylinder, is also called the end of the cylinder.
Due to process requirements and maintenance needs, openings and connecting pipes of various sizes are often set on the cylinder or head of pressure vessel, such as manhole, handhole, sight glass hole, material inlet and outlet connecting pipes, and connecting pipe openings for installing pressure gauge, liquid level gauge, safety valve, thermometer, etc.
Handholes and manholes are devices used to inspect, assemble, disassemble and wash the inside of containers. The inner diameter of the hand hole shall allow the operator’s hand to pass freely. Therefore, the diameter of hand hole should not be less than 150 mm in general. Considering that the length of human arm is about 650-700mm, it is not suitable to set handhole for vessels with diameter greater than 1000mm, but to set manhole instead.
There are two kinds of common manhole shapes: circular and elliptical. In order to make the operators have free access, the diameter of circular manhole shall be at least 400mm, and the size of elliptical manhole shall be 350mm × 450mm generally.
After the opening on the cylinder or head, the strength of the opening part is weakened and the stress increases. This weakening degree increases with the increase of the opening diameter, so the number of openings should be reduced as much as possible on the container, especially to avoid opening large holes. For the holes already opened on the container, the opening reinforcement design shall also be carried out to ensure the required strength.
The pressure vessel is supported by support and fixed on the foundation. The supports of cylindrical and spherical vessels are different. With different installation positions, the cylindrical vessel supports are divided into two types: the vertical vessel supports and the horizontal vessel supports. The vertical vessel supports include leg supports, supporting supports, ear supports and skirt supports. The spherical vessel supports are mostly column or skirt supports.
Due to the use characteristics of pressure vessels and the chemical process characteristics of the internal medium, it is often necessary to set up some safety devices and measuring and control instruments on the vessels to monitor the parameters of the working medium, so as to ensure the use safety of pressure vessels and the normal process.
Safety accessories of pressure vessel mainly include safety valve, bursting disc device, emergency shut-off valve, safety interlock device, pressure gauge, liquid level gauge, temperature measuring instrument, etc.
The above six parts (cylinder, head, sealing device, opening nozzle, support and safety accessories) constitute the shell of a pressure vessel. For the container for storage, the shell is the container itself; for the container for chemical reaction, heat transfer, separation and other process, the inner parts required by the process must be installed in the shell to form a complete product.
The six components of the pressure vessel shell are introduced above, and most of the connections between the components need to be welded, so the quality control of welding is a very important part of the whole vessel quality system. Although the welding quality control also involves many welding process problems, the main task of the design link is to design the welding structure and determine the NDT method, proportion and requirements.
The design of welding structure involves the form of joint (such as butt joint, lap joint, fillet joint), the groove form of joint, geometric dimension, etc. Because of the particularity of pressure vessel, it can be said that its requirement for welding quality is the highest one of all welding equipment. Therefore, the pressure vessel design engineer must understand the characteristics of the welding structure design and the basic requirements of the welding quality inspection. The specific problems of welding structure design will be discussed in the following article.
Source: China Pressure Vessels Manufacturer – www.secmachinery.com
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