The pressure vessel is usually a welded structure composed of a plate and a shell. Among the pressure components, the corresponding shells of cylindrical cylinder, spherical tank (or spherical head), 
elliptical head, dish head, spherical crown head, conical head and expansion joint are cylindrical shells. , Spherical shell, ellipsoidal shell, spherical crown + ring shell, spherical crown, cone shell and annular plate + ring shell.
The flat cover (or flat head), annular plate, flange, tube sheet and other pressure components correspond to round flat plates, annular plates (the difference between the outer radius and the inner radius is greater than 10 times the thickness of the plate), and the ring (outer radius) The difference between the inner radius and the inner radius is less than 10 times the plate thickness) and the elastic base round plate.
The above-mentioned 7 kinds of shells and 4 kinds of plates can be combined into various pressure vessel structure forms, together with sealing elements, supports, safety accessories, etc., constitute a complete pressure vessel. Figure 1-1 is the overall structure diagram of a horizontal pressure vessel. The basic composition of the pressure vessel will be briefly introduced below in conjunction with this figure.
(1) Cylinder
The function of the cylinder is to provide the pressure-bearing space required by the process. It is one of the most important pressure-bearing components of the pressure vessel. Its inner diameter and volume are often determined by process calculations. Cylindrical cylinders (i.e. cylinders) and spherical cylinders 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 the steel plate can be used in the hydraulic press The upper press is made into two semi-cylinders, and then the two are welded together with welding seams to form a complete cylinder. Since the direction of the weld is parallel to the longitudinal direction (ie, the axial direction) of the cylinder, it is called a longitudinal weld, or longitudinal weld for short.
If the diameter of the container is not very large, generally there is only one longitudinal weld; as the diameter of the container increases, due to the limitation of the size of the steel plate, there may be two or more longitudinal welds. In addition, a container with a shorter length can be directly connected to the two ends of a cylinder to form a closed pressure space, and thus a pressure vessel shell can be made.
However, when the container is long, due to the limitation of the size of the steel plate, it is necessary to first use the steel plate to coil and weld several sections of the cylinder (a section of the cylinder is called a cylinder section), and then be welded by two or more cylinder sections. Into a cylinder of the required length. The connection welds between the cylinder section and the cylinder section, and between the cylinder body and the end head, are called circumferential welds, or ring welds for short, because their direction is perpendicular to the axial direction of the cylinder.
According to its structure, the cylinder can be divided into two types: single-layer type and combined type.
① Single-layer cylinder The wall of the cylinder is composed of a whole material in the thickness direction, that is, the wall has only one layer (to prevent corrosion of the internal medium, the anti-corrosion layer on the lining is not included). According to the manufacturing method, single-layer cylinders can be divided into single-layer coil welding, integral forging, forging welding, and non-welding bottle type.
Among them, the single-layer coil welding structure is the most manufactured and used cylinder form at present. The flange is assembled and welded into a container, and the cylinder shown in Figure 1-1 is a single-layer coil-welded structure.
The integral forged structure is the earliest barrel form used. The barrel and flange can be forged as a whole or connected by threads during manufacture, and the entire barrel has no welds. After the development of welding technology, segmental forging appeared, and then welded and assembled into a whole forged-welded cylinder.
There are two main manufacturing methods for non-welded bottle-type cylinders: one is made of high-quality seamless steel pipes through hot spinning at both ends; the other is a steel ingot after stamping and then hot spinning. Generally, integral forged and forged-welded cylinders are mainly used in high-pressure and ultra-high pressure vessels, while non-welded cylinder cylinders are often used to manufacture non-welded large-volume cylinder pressure vessels.
The material of the integral forged cylinder has dense metallographic structure, high strength, and good quality. It is especially suitable for ultra-high pressure vessels made of high-strength steel with poor welding performance. However, very large smelting, forging and machining equipment is required during manufacturing, material consumption is large, steel utilization rate is low (only 26%~29%), and the amount of machining is large, so it is generally only used for inner diameter ϕ300~800mm, length Small ultra-high pressure vessels of no more than 12m, such as polyethylene reactors, artificial crystal kettles, etc.
② Combined cylinder The wall of the cylinder is composed of two or more layers of discontinuous materials in the thickness direction. Combined cylinders can be divided into two categories: multi-layer and winding according to structure and manufacturing method. The specific structure will be introduced in Chapter 4 of this book.
(2) Head
According to the different geometric shapes, the heads can be divided into spherical, elliptical, dish-shaped, spherical crown, cone shell and flat cover, among which spherical, elliptical, dish-shaped and spherical crown-shaped heads are collectively called convex Head.
When the container does not need to be opened after assembly (generally there is no internal parts in the container or there is no internal parts but there is no need to replace or overhaul the situation), the head can be directly welded to the cylinder, thus effectively ensuring the seal, saving materials and Reduce the workload of processing and manufacturing. For containers that need to be opened multiple times due to maintenance or replacement of internal parts, the connection between the head and the barrel should be detachable. At this time, a sealing device must be provided between the head and the barrel.
(3) Sealing device Pressure vessels need many sealing devices, such as the detachable connection between the head and the cylinder, the detachable connection between the container nozzle and the outer pipe, the connection of the manhole and the hand hole cover, etc., the pressure vessel Whether it can operate normally and safely depends to a large extent on the reliability of the sealing device.
Bolt flange connection (referred to as flange connection) is one of the most widely used sealing devices. Its function is to connect by bolts and tighten the bolts to ensure sealing. Flanges are divided into container flanges and pipeline flanges according to the connected parts.
The flange used for the connection between the container head (or top cover) and the cylinder and between the two cylinders is called the container flange; the flange used for the pipe connection is called the pipeline flange. In the high-pressure vessel, the vessel flange used to connect the top cover and the cylinder body and welded to the cylinder body is also called the end of the cylinder body.
(4) Opening and connecting pipes Due to process requirements and maintenance needs, various sizes of holes or installation pipes are often opened on the cylinder or head of the pressure vessel, such as manholes, hand holes, sight glass holes, and material import and export. Take over and install pressure gauges, liquid level gauges, safety valves, temperature measuring instruments, etc. to take over openings.
Hand holes and manholes are devices for inspecting, assembling, disassembling, and washing the inside of the container. The inner diameter of the hand hole should allow the operator's hand to pass freely. Therefore, the diameter of the hand hole should generally not be less than 150mm. Considering that the length of a human arm is about 650~700mm, the container with a diameter greater than 1000mm should not be provided with a hand hole, but should be replaced with a manhole.
Common manhole shapes are round and oval. In order to allow operators to freely enter and exit, the diameter of the round manhole should be at least 400mm, and the size of the oval manhole is generally 350mm×450mm.
After a hole is opened on the cylinder or the head, the strength of the hole is weakened and the stress there is increased. The degree of weakening increases as the diameter of the opening increases. Therefore, the number of openings on the container should be minimized, especially to avoid opening large holes. For the holes that have been opened on the container, the opening reinforcement design should also be carried out to ensure the required strength.
(5) Bearing
The pressure vessel is supported by the support and fixed on the foundation. Cylindrical container and spherical container have different supports. Depending on the installation position, cylindrical container supports are divided into vertical container supports and horizontal container supports. Among them, vertical container supports have leg supports, support supports, ear supports and skirts. There are four types of support; while the spherical container mostly adopts column or skirt support.
(6) Safety accessories
Due to the characteristics of the use 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 vessel to monitor the parameters of the working medium to ensure the safety of the pressure vessel and the normal progress of the process.
The safety accessories of pressure vessels mainly include safety valves, bursting disc devices, emergency shut-off valves, safety interlocking devices, pressure gauges, liquid level gauges, temperature measuring instruments, etc.
The above six major components (cylinder body, head, sealing device, opening nozzle, support and safety accessories) constitute the shell of a pressure vessel. For storage containers, this outer shell is the container itself; for containers used for chemical reactions, heat transfer, separation and other technological processes, the inner parts required by the craft must be installed in the outer shell to form a complete product.
Welding between pressure vessel parts
The six components of the pressure vessel shell are described above, and most of the connections between the components need to be welded. Therefore, quality control of welding is an extremely important part of the entire vessel quality system. Although welding quality control also involves many welding process issues, the main task of the design link is to design the welding structure and determine the non-destructive testing methods, proportions and requirements.
Welding structure design involves the form of joints (such as butt joints, lap joints, corner joints), joint groove forms, geometric dimensions, etc. Due to the particularity of pressure vessels, it can be said that its requirements for welding quality are the most demanding of all welding equipment. Therefore, pressure vessel design engineers must understand the characteristics of the welded structure design in the vessel and the basic requirements for welding quality inspection. Specific welding structure design issues will be discussed in a later article.
