The operating environment of chemical equipment pressure vessels is relatively complex. The quality problems caused by corrosion, crack and deformation are easy to cause fatal injuries. Therefore, compared with the post-processing of destruction, more attention should be paid to prevention and control in advance to ensure the safe and stable operation of pressure vessels of chemical equipment and effectively prolong the service life of vessels. Due to the diversified forms and causes of the destruction of pressure vessels in chemical equipment, it is necessary to strengthen innovation in preventive measures and management means, so as to give full play to the value of the management of pressure vessels in chemical equipment.
1. Accident characteristics of chemical equipment and pressure vessels
Pressure vessel belongs to pressure-bearing equipment, which refers to pressure-bearing special equipment and safety accessories that are dangerous and involve life safety, including pressure pipes and boilers. There are various types of pressure vessels, which can be classified into internal pressure vessels and external pressure vessels according to the pressure bearing methods. If the vessel is selected by the classification of pressure, it can be divided into a high pressure, low pressure and medium pressure vessels and ultra-high pressure vessels. If the vessel is selected by the classification of working temperature, it includes low temperature, high temperature and normal temperature containers. If it is classified according to the manufacturing method, it includes forging, welding and casting vessels. Although the classified contents are different, the control of safety and reliability of any chemical equipment pressure vessels can not be ignored during its use. The accident characteristics of chemical equipment and pressure vessels are mainly reflected in the following aspects:
(1) High risks
Chemical products are characterized by high temperature, high pressure, toxicity, flammability and explosion. The possibility of fire, leakage and explosion of pressure vessels is high, which is unmatched by other industries in terms of risks. Due to the strong reactivity, toxicity and flammability of some chemical raw materials, it is the main reason for frequent accidents in chemical enterprises. Fire, explosion, leakage and other serious accidents in chemical enterprises will not only cause staff injury or death but also cause great economic losses.
(2) Environmental pollution
The raw materials, products, intermediates and by-products used in chemical production have a certain degree of industrial toxicity and corrosion. Leakage into water or air, especially untreated industrial substances, will not only cause different degrees of environmental pollution but also cause economic losses and casualties. The serious accidents in chemical enterprises generally feature difficult governance and long influence time.
(3) Affecting the normal operation of equipment
Chemical enterprises have higher requirements for the operation of industrial equipment. However, most of the pressure vessels of chemical equipment operate in a harsh environment, which is influenced by factors such as vibration and internal corrosive medium. This will cause quality problems such as fatigue of equipment metals, which will directly affect the normal operation of chemical equipment. This is also a problem that can not be ignored by chemical enterprises in the safe management of equipment use.
2. Forms, causes and characteristics of chemical pressure vessel damage
(1) Excessive plastic deformation
Once the pressure load of the chemical pressure vessel itself exceeds the established value, the pressure vessel wall of chemical equipment will gradually become thinner, even in an unstable state. The container will also be deformed due to its over-shaping, and even lead to the direct rupture of the container. If the rupture is caused by over-shaping, the fractured state is a tearing effect, and the container will be accompanied by a small number of fragments, or no fragments will be produced. At the same time, the energy of pressure vessel explosion of chemical equipment directly determines the size of the vessel explosion opening.
(2) Excessive elastic deformation
The so-called elastic deformation means that when the chemical pressure vessel is subjected to external force, it will be deformed to some extent, which is a direct expression. When the external force is withdrawn, the object will gradually return to its original shape. Therefore, it can be called elastic property. This is a kind of reversible deformation, which is usually called elastic deformation. However, if excessive elastic deformation occurs in the use of chemical pressure vessels, it is easy to make the chemical equipment pressure vessels in an unstable state. With the enhancement of external force, the chemical equipment pressure vessels will gradually transition to the degree of instability.
(3) Large strain fatigue
Under the influence of alternating stress, the local area of chemical equipment pressure vessels, such as the discontinuity of local structure or the surrounding of perforated nozzles, etc., in which the metal grains are subjected to the greatest force, will slip, and then gradually show excessive micro cracks. With the gradual increase and expansion of both ends of the crack, the chemical pressure equipment eventually formed is prone to fatigue and obvious damage. The high-pressure part itself has extremely high stress. In the local area with extremely high stress, fatigue first appears, and high stress will cause the damage of large strain parts. Therefore, it can also be called large strain fatigue. At present, it is found that the fatigue failures of chemical pressure containers are mainly reflected in the following points: the deformation of containers is not obvious; Fracture mainly exists in two areas where fatigue cracks are generated to the propagation area and the last fracture area; failures due to cracking and leakage; Chemical pressure vessels are more prone to fatigue damage after repeated loading and unloading.
(4) Corrosion fatigue
Under the joint action, a brand-new failure form takes shape. If analyzed from the perspective of corrosion fatigue of materials, it can be found that it will lead to extremely obvious local damage on the metal surface and lead to fatigue cracks in the metal. Second, the protective film on the metal surface is damaged under alternating tensile stress, resulting in surface corrosion. After the alternating stress destroys the protective film, the protective mold can’t be formed again, which will also lead to the deposition in the corrosion pit, directly affecting the diffusion of oxygen. It will also lead to the failure of the protective film to recover in a short time, which will eventually form a vicious circle. However, the bottom of the corrosion pit is always in a relatively active state, which is easy to form the anode of the corrosion battery. Finally, under the dual action of alternating stress and corrosion, the crack will continue to develop until the metal finally breaks.
(5) Stress corrosion
When analyzing stress corrosion, it is considered that stress corrosion mainly refers to the combined action of tensile stress and metal corrosion medium, and then forms a unique failure form. When the phenomenon of stress corrosion of metals occurs in chemical pressure vessels, it is in a mutually reinforcing state due to two different factors of stress corrosion, in which corrosion will directly reduce the effective cross-sectional area of the metal itself, and gradually form a gap on the surface of the metal, which will make its stress more concentrated. At the same time, it is also necessary to consider that under the action of prestressing, the progress of corrosion will be gradually accelerated, and the gap on the metal surface will become larger and larger as a result of the foundation until the final fracture.
(6) Brittle fracture
A brittle fracture refers to a fracture without significant plastic deformation. Fragments often fly out from the container parts of brittle fracture and show a broken shape. Rupture accidents usually occur at low temperatures. Brittle fracture is more likely to occur on thick-walled vessels made of medium and low strength and pressure vessels made of high strength steel.
(7) Hydrogen corrosion cracking
Because pressure vessels of most industrial equipment are under high temperature and high pressure in the process of manufacturing and using, a large number of hydrogen molecules will be adsorbed on the surface of steel. However, hydrogen molecules will gradually decompose into hydrogen ions or atoms, which will lead to the phenomenon of solid solution in the steel surface layer, even diffuse into the steel, and constantly destroy the performance of steel through hydrogen corrosion and hydrogen embrittlement. From the analysis of hydrogen corrosion, hydrogen ions or atoms diffused into steel will combine into hydrogen molecules, and some of them will chemically react with nonmetallic inclusions or carbon and carbide to generate gas products that are not easy to dissolve. However, the products will gradually accumulate in the original tiny cracks in the grain boundary, and then gradually form local high pressure and force concentration, which will make the grain boundary gradually widen and produce certain microcracks, and the mechanical properties of steel will be gradually weakened at this stage.
Under the influence of the continuous change of external temperature and stress, the metal container wall is easily damaged after a long period of accumulation. Different from plastic deformation, the temperature is an important influencing factor of creep, and high temperature will weaken the rigidity of metal materials. In addition, the tensile stress will gradually weaken the performance and quality of the pressure vessel, thus leading to its damage.
3. Analyses of pressure vessel damage
The accident probability of pressure vessels is high. Once a vicious incident occurs, it will cause a shutdown, corrosion and so on, which are the direct causes of industrial equipment pressure vessel accidents. However, the factors such as chaotic management of pressure vessels and equipment, incomplete technical archives, inadequate implementation of safety production responsibilities, inadequate monitoring and inspection of safety production rectification and so on are the indirect causes of vessel accidents. In addition to simple investigation and analysis, it is also necessary to provide the inspection of whether the container is broken, figuring out the state of the fracture at the broken position and the physical and chemical properties of the container. If you want to really find out the cause of the accident, you can carry out the destructive simulation test of the container. At the same time as technical inspection and accident investigation, formulate targeted treatment and preventive measures.