For a pressure vessel with a conical head, due to the special structure of the head, the pressure vessel is often scoured by the fluid medium during operation, and it is inevitable that the wall thickness at the conical head will be reduced due to corrosion. . This article will analyze the failure of the conical head of the pressure vessel due to local corrosion, and discuss the preventive measures.

　　Corrosion cause analysis

　　The damage caused by the chemical or electrochemical interaction between the metal and the surrounding medium is called metal corrosion. According to the corrosion mechanism, metal corrosion can be divided into two categories: chemical corrosion and electrochemical corrosion. Chemical corrosion refers to the damage caused by the direct chemical interaction between metal and non-electrolyte. The corrosion process is a pure chemical reaction of pure oxidation and reduction, that is, the corrosion medium directly interacts with atoms on the metal surface to form corrosion products. There is no current generated during the reaction, and the process conforms to the laws of chemical kinetics. For example, the corrosion of lead in carbon tetrachloride or ethanol, and the stage when metal is just forming a film in high temperature gas are all chemical corrosion. Electrochemical corrosion is the damage caused by the electrochemical interaction between metal and electrolyte solution. For example, the corrosion of metals in seawater, atmosphere, and various acid-base salt solutions is electrochemical corrosion. According to the characteristics of metal damage, metal corrosion can be divided into general corrosion and local corrosion. General corrosion means that the corrosion damage occurs on the entire metal surface, making the entire wall thickness of the pressure vessel gradually thinner. Localized corrosion means that the corrosion occurs locally in the metal, causing the metal to be corroded in a small area, while the other parts have almost no corrosion or corrosion is very slight, which is a corrosion form that is more dangerous than general corrosion. This is because the characteristics of general corrosion are obvious, and because the corrosion rate is not much different, the corrosion rate can be easily calculated, and then the loss caused by the damage and the time to replace the equipment can be estimated to maximize the utilization of the equipment. On the contrary, local corrosion is the local damage of the container, which is often hidden and difficult to find. In addition, the reaction mechanism and reaction conditions of local corrosion are more complicated, so the reaction rate cannot be determined. This will easily cause sudden damage to the container and accidents, causing serious casualties. There are many types of local corrosion, common ones are impact corrosion (also called wear corrosion), pinhole corrosion, crevice corrosion, stress corrosion, intergranular corrosion, etc.

　　Since the thickness values ​​of other parts of the container are normal, only the wall thickness of the upper head pipe is directly flushed (that is, on the tapered head). Combined with the classification of metal corrosion and the medium of the container, analyze and judge the The thinning is caused by the combined action of impact corrosion and electrochemical corrosion. Impact corrosion refers to the metal damage phenomenon caused by high-speed relative movement between the metal surface and the fluid medium (whether corrosive or not), especially when there are more hard solid particles suspended in the fluid, this damage will be aggravated. Generally speaking, the higher the relative velocity, the more and harder the solid particles suspended in the fluid, the faster the impact corrosion speed, which is common in the pressure vessel where the medium inlet is straight or where the medium direction changes suddenly. In addition, the medium of the container is mainly sinter dust ash and air. The main components of dust ash include TFe, SiO2, CaO, MnO, MgO, Al2O3, P2O5, TiO2, sulfide, etc. Due to the fine particle size and large surface area of ​​dust removal ash, it has strong water absorption and high moisture content. The air contains a small amount of SO2. In a dry environment, the corrosion effect of SO2 is very weak, but if it is combined with water to generate sulfurous acid, the PH value (an important factor affecting corrosion) can reach 3, which will cause metal Strong corrosive effect. In the normal working state of the container, the passivation film and metal ions on the metal surface at the point where the medium inlet of the upper head pipe is directly flushed are impacted by the high-speed solid dust particle fluid, and the wall shear stress is first generated on the metal surface, causing it to passivate. The film or covering layer is mechanically damaged and leaves the metal surface, causing impact corrosion, and the fresh metal surface is constantly being scoured and worn. At this time, the fresh metal surface formed by impact corrosion and sulfurous acid electrochemically corrode and expand into the material continuously. Repeatedly, the formation of material passivation-fluid erosion-electrochemical corrosion damage-and then passivation alternate destruction process, and ultimately lead to the thinning of the wall thickness at the direct impact.

　　Next, let’s discuss the issue of impact corrosion and electrochemical corrosion in this accident. The material of the container in this article is Q235-B. Checking the corrosion resistance table shows that the corrosion rate of Q235-B in a sulfurous acid environment is> 1.5mm/a. It is not suitable for use in a sulfurous acid environment due to severe corrosion. If electrochemical corrosion plays a leading role in this accident, the container should undergo general corrosion rather than local corrosion at the lower conical head. But this is not the case, why? This shows that electrochemical corrosion is not the main cause of this accident. Then there is only impact corrosion. Under the impact of a high-speed fluid containing more hard TFe, SiO2, CaO, MnO and other solid particles at the inlet of the pipeline, the passivation film or covering layer on the surface is destroyed, and the fresh metal surface is mechanically damaged. Constantly being scoured and worn. This gives the sulfurous acid an opportunity to cause electrochemical corrosion on the fresh metal surface, further accelerating the corrosion damage. It can be seen that impact corrosion is the main cause, and electrochemical corrosion is secondary. If measures can be taken to avoid impact corrosion at the inlet of the pipeline medium, the chance of local corrosion can be effectively reduced.

　　Incident handling

　　In accordance with the relevant provisions of the "Regulations for the Safety Technical Supervision of Stationary Pressure Vessels" and the "Regulations for Periodic Inspection of Pressure Vessels", a notice of special equipment inspection opinions was immediately issued to the users, requesting them to stop using them immediately, and it is recommended that the users follow the relevant procedures Find a qualified unit for maintenance. After receiving the notice, the user immediately got in touch with us and actively looked for the maintenance unit. After repeated communication with the three parties, it was finally determined to repair the local corrosion parts, and to install a buffer plate in the direct impact area of ​​the medium inlet to prevent the reoccurrence of impact corrosion. After the maintenance is completed, it will be put into use after passing the supervisory inspection. After several months of trial operation, the local corrosion and the surrounding wall thickness have not been thinned, and the accident treatment has achieved the expected results.

　　The medium inlet of the upper head pipe of the pressure vessel is flushed by the high-speed fluid, causing the passive film or covering layer on the surface to produce mechanical damage and leave the metal surface, causing impact corrosion. The fresh metal surface is constantly being scoured and worn, and is affected by the erosion. The sulfurous acid generated by the combination of SO2 with air and water in the dust removal ash undergoes electrochemical corrosion, continues to expand into the material, cyclically, and finally causes the wall thickness of the direct impact to be thinned and the strength is weakened, which cannot meet the safe and stable operation of the container. Therefore, in the process of equipment management, daily monitoring should be strengthened, and the thickness of the parts prone to impact corrosion should be measured regularly, problems can be found in time, and the safety technology status should be grasped at any time. If problems are found, they should be dealt with promptly, and if necessary, they should contact special equipment inspection and testing institutions or manufacturers to ensure the safe and stable operation of pressure vessels.