Pressure vessels are widely used in all walks of life, is a special equipment with the risk of explosion, high production process requirements, the use of harsh conditions, and withstand a variety of loads, and high operational requirements, which determines the use of pressure vessels are different from other equipment. Now briefly described as follows.

First, the production process requires high

Many pressure vessels use units, especially petroleum, chemical and other industries, long production lines, many types of containers, a large number of interrelated, and mostly continuous production. With the development of production, scientific and technological progress, centralized control, automatic regulation is becoming more and more widespread, the reliability of equipment operation requirements are increasingly high.

Second, the use of harsh conditions

In order to meet the needs of the production process, pressure vessels to withstand a certain pressure, or even higher pressure; also due to intermittent operation or alternately add different media, the pressure fluctuations; in addition, pressure vessels also have to withstand different temperatures, some need to work in the low temperature state below -20 ℃, some need to work in the high temperature state above 450 ℃. Intermittent operation of the container and some heating, cooling alternately by the temperature of the container also subject to substantial changes in the impact; in addition, the pressure vessel is also subject to the medium of its corrosion, pressure vessels within the medium, some have a certain corrosive, corrosive media will produce different degrees of corrosion of the container; some pressure vessels within the medium have flammable and explosive or high degree of toxicity, once the leak or encounter the source of fire. It will cause poisoning or cause combustion, explosion, etc.

Third, subject to a variety of loads

(1) the pressure of the medium (internal pressure, external pressure or differential pressure) and pressure fluctuations, where the main load.

(2) the static pressure of the liquid.

(3) the self-weight of the container (including the inner parts and fillers, etc.) and the normal operating conditions or test state, the gravity load of the material contained inside.

(4) the gravity load of the appurtenant equipment and insulation materials, lining, pipes, escalators, platforms, etc. installed on the container.

(5) wind load, snow load and seismic load, especially the outdoor installation of tall tower-type equipment.

(6) the load of other factors.

1 support and support members of the reaction force.

2 connected pipes and other parts of the load caused by vibration.

3 temperature difference caused by the load, especially the intermittent operation of the container and alternating hot and cold containers, the temperature of each part is different, while the constraints of neighboring components, not free to expand and contract the resulting load.

4 impact reaction forces, such as those caused by fluid impact, etc.

5 The force during transportation or lifting.

Fourth, high operational requirements

The operation of the vessel mainly relies on instrumentation monitoring. Temperature and pressure changes in the vessel often occur in an instant, and more influencing factors, once the operation error, accidents can occur, which can seriously lead to equipment explosion accidents.

Therefore, this requires the pressure vessel operator to have a high degree of responsibility and certain technical quality, strict implementation of the equipment operating procedures, when encountering emergency situations can be eliminated in a timely manner to prevent accidents.

Five, the pressure vessel parameters

1、Pressure

(1) the highest working pressure: for containers subject to internal pressure refers to the highest possible pressure at the top during normal use; for containers subject to external pressure, refers to the highest possible pressure difference during normal use; for jacketed containers refers to the highest possible pressure difference at the top of the jacket. The highest working pressure in the container factory has been made, in the factory "pressure vessel product quality certificate" has been clear, the product nameplate and registration nameplate is also indicated. In use some containers found after inspection defects, inspection units and re-approved its allowable operating pressure (referred to as P allow), pressure vessel operators operating pressure vessels, shall not be higher than the above pressure. The maximum working pressure of the pressure vessel should not be greater than the design pressure of the pressure vessel or the pressure standard approved by the inspection unit.

(2) design pressure: refers to the highest pressure set on the top of the container, and the corresponding design temperature together as the design load conditions, the value is not less than the highest working pressure (i.e., marked on the nameplate design pressure of the vessel).

Pressure vessel "design pressure" shall not be lower than the container's "maximum working pressure". Pressure vessels equipped with safety relief devices, the "design pressure" shall not be lower than the safety relief device opening pressure or bursting pressure of the rupture disc. The design pressure of the fixed pressure vessel for liquefied gases, should follow the "capacity" and other relevant provisions.

(3) the maximum allowable working pressure: refers to the design temperature, the top of the container is allowed to withstand the maximum pressure. The pressure is calculated according to the effective thickness of the container pressure components, and take its minimum value. When using the maximum allowable working pressure, should be indicated in the drawings and nameplate.

(4) test pressure: refers to the pressure vessel in the pressure test, the pressure at the top of the vessel.

2、Temperature

(1) metal temperature: refers to the average temperature of the container pressurized components along the thickness of the cross-section. In any case, the surface temperature of the component metal shall not exceed the allowable use temperature of steel.

(2) design temperature: refers to the container in normal operating conditions and in the corresponding design pressure, set the metal temperature of the pressurized components. Container design temperature (i.e., marked on the container nameplate design temperature) shall not be lower than the component metal in the working condition may reach the highest temperature; for the metal temperature below 0 ℃, the design temperature shall not be higher than the component metal may reach the lowest temperature.

(3) test temperature: refers to the pressure test, the metal temperature of the container shell.

3, diameter

(1) diameter: generally refers to the inner diameter of the pressure vessel, is to determine the pressure vessel volume of one of the main dimensions.

(2) nominal diameter: refers to the size of the standardized, serialized, to meet the needs of the standardization of the vessel, serialization.

　　

Six, the source of stress in the pressure vessel

1, the basic concept of stress and deformation

Pressure vessel in operation, due to a variety of loads, will change the original shape of the deformation, and in its internal resistance to the role of external forces, the role of external forces and cross-sectional forces equal to the sum. In general, the internal force and deformation increase with the increase of external force. The distribution of internal forces in the cross-section and must be uniform, so it is necessary to find the internal force of each tiny unit area of the cross-section, the internal force acting on the unit area is called stress. The unit of stress is also called MPa.

2, the source of stress in the pressure vessel

The main sources of stress in a pressure vessel are

(1) by the pressure (internal or external pressure) and the stress, is the most important stress of the pressure vessel, and is the main factor to determine the wall thickness of the vessel. Vessels subject to internal pressure due to the shell under pressure to expand outward, and therefore always produce tensile stress in the wall.

In addition, the pressure vessel in operation, due to fluctuations in pressure and open and close, hydraulic test, etc. will produce stress.

(2) by the weight of the stress. Pressure vessel in addition to the vessel itself has a certain weight, in addition to the medium, process equipment accessories and other additional devices outside the vessel often have a large weight, all these weights acting on the vessel wall will also make the wall stress.

(3) other external loads caused by the stress. In addition to the above river pressure and gravity and other loads, there are some other loads will also make the shell wall stress. Such as wind load, snow load, earthquake load, etc.. In addition, there are additional moments such as takeover moments generated by the stresses.

(4) the stress caused by temperature. Due to the change of temperature, the container components are subject to thermal expansion and cold contraction, but it will be restrained by the adjacent parts or other components and cannot produce deformation according to the law of thermal expansion and cold contraction, so the internal components should produce stress.

(5) When the head is connected with the cylinder, the additional bending moment will also cause axial and circumferential stresses in the outer boundary area of the connection.