Anran measures and acoustic emission detection technology in the use of LNG storage tanks

{1} The safety measures in the use of liquid ammonia storage tanks should be taken into account during the use of low-temperature storage tanks. From design, manufacturing to use, excellent measures should be taken in every aspect of the quality protection system from design, manufacturing to use. (1) Material selection. Practice has proven that the higher the strength of the material, the greater the likelihood of stress corrosion. The lower strength limit that does not undergo stress corrosion is related to factors such as impurity content and characteristics, stress magnitude, and operating speed. In order to prevent stress corrosion, steel with lower strength should be selected as much as possible, taking into account operating pressure, residual stress, safety, and economy. (2) Adopt a reasonable structure and welding process. Structurally, stress concentration caused by excessive or concentrated welding seams, asymmetric welding seams, intersecting welding seams, and unreasonable welding sequence should be avoided. During manufacturing, strong welding should be avoided to prevent defects such as undercutting and misalignment, and the surface in contact with the medium should be protected as smoothly as possible. After manufacturing, annealing heat treatment should be carried out to remove residual thermal stress after welding. Proper post weld heat treatment can significantly reduce residual stresses during the manufacturing process and lower the peak hardness of the welding heat affected zone. (3) Before putting new storage tanks into use, the air inside should be thoroughly removed; During the filling, discharging, and maintenance processes, take specific measures to avoid introducing any air. Large storage tanks should continuously condense ammonia vapor, while non condensable gases are mostly air, which should be discharged. Use suction or transpiration to remove air from the storage tank for smaller devices. In short, removing air pollution from storage tanks can prevent stress corrosion. (4) Newly put into use storage tanks should undergo internal and external inspections according to regulations and periodic inspections. Inspection should be conducted on the liquid and gas phase interfaces, arc initiation and termination points, and T-joints that are prone to corrosion; All welds below the liquid level should undergo 90% magnetic particle or ultrasonic testing, and if conditions permit, all welds should undergo 90% magnetic particle testing. (5) Regularly monitor the concentration and moisture content of liquid ammonia. If the moisture content is below the critical concentration, it should be replenished in a timely manner to maintain the moisture content within the range of 0.12% to 1%. {2} In the 1960s, Creen et al. began the application of acoustic emission technology in the non-destructive testing industry for LNG storage tanks. Dunegan applied acoustic emission technology to research in pressure vessels. The application of acoustic emission technology in the structural integrity detection and evaluation of LNG storage tanks can be divided into three aspects: acoustic emission detection and evaluation of newly made liquefied natural growth gas storage tanks, acoustic emission detection and evaluation of in use LNG storage tanks, and online monitoring and evaluation of acoustic emission of LNG storage tanks. LNG storage tanks are prone to crack formation under the influence of medium temperature and pressure. During the process of crack formation and propagation until cracking, acoustic emission energy signals of different sizes are emitted. Based on the magnitude of these energy signals, the occurrence and extent of crack propagation can be determined. An important feature of acoustic emission testing is the loading of LNG storage tanks during testing. The commonly used loading method is to conduct a pressure test on the LNG storage tank, and sometimes the working medium is directly loaded. If there is an acoustic emission localization source signal generated at the defect site during the entire loading process, the defect is judged to be active; otherwise, the defect is judged to be inactive. The advantage of acoustic emission detection is that it can detect active defects, such as material fracture and crack propagation, providing a basis for usability evaluation; Can be operated remotely, allowing for long-term monitoring of equipment's allowable status and defect expansion; The device is relatively lightweight; Its limitation is that the equipment is expensive; High quality requirements for operators; There are many interfering factors during the detection process; After the acoustic emission testing is completed, ultrasonic testing is generally required for retesting.