Design and precautions of ammonia water storage tank
(1) In terms of material selection: Practice has proved that the higher the material strength, the greater the possibility of stress corrosion. However, the minimum strength limit where stress corrosion does not occur is related to factors such as impurity content and characteristics, stress magnitude, and operating speed. In order to prevent stress corrosion, under the comprehensive consideration of operating pressure, residual stress, and safety and economy, low-strength steel should be selected as much as possible.
(2) For new storage tanks before being put into use, the air in them should be completely removed; during filling, discharging and maintenance, certain measures should be taken to avoid bringing in any air. For large storage tanks, ammonia vapor should be condensed continuously, and most of the non-condensable gas is air, which should be discharged. Use smaller pumps or transpiration to remove air from the tank. In short, eliminating air pollution inside the storage tank can effectively prevent stress corrosion.
(3) Adopt reasonable structure and welding process. Stress concentration caused by excessive welds, excessive concentration, asymmetric welds, weld crossings and unreasonable welding sequences should be avoided on the structure. Strong assembly welding should be avoided during manufacturing to prevent defects such as undercuts and misalignments, and to ensure that the surface in contact with the medium is as smooth as possible. After manufacturing, annealing heat treatment should be performed to remove residual thermal stress after welding. The correct post-weld heat treatment can greatly reduce the residual stress in the manufacturing process, and can reduce the peak hardness of the welding heat affected zone.
(4) Regularly check the liquid ammonia concentration and water content, and if the water content is lower than the critical concentration, it should be replenished in time to keep the water content in the range of 012% 1%. In addition, other inhibitors, such as 100Lgög freezer oil or 5Lgög rapeseed oil or 1050Lgög silicone oil as stress corrosion inhibitors, can effectively inhibit stress corrosion caused by liquid ammonia.
(5) The newly put into use storage tanks shall be subject to internal and external inspections as well as periodic periodic inspections. The liquid, gas phase interface, lead-in and arc-out joints and T-joints should be inspected with emphasis on corrosion; 100% magnetic powder or ultrasonic flaw detection should be performed on all welds below the liquid surface. flaw detection. The cracks detected shall be evaluated. Because the stress corrosion limit fracture toughness JISCC is only about 1ö5 of the material's conventional fracture toughness D0105, the safety level of the crack should be evaluated according to the fracture mechanics criteria, and the treatment opinions and the next inspection time should be given. For shallow cracks not exceeding 1ö4 wall thickness and less than 4mm in depth, mechanical grinding can be used to eliminate them, but the grinding process must be strictly controlled; for deeper cracks, grinding must be performed before repair welding. Before repair welding, preheating and heating should be performed to prevent welding hardening. Low hydrogen electrodes should be used during welding. After welding, check for flaws and perform stress relief treatment.