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Pressure vessels used to store liquid ammonia are widely employed, and in most cases the liquid ammonia environment constitutes a stress-corrosion condition; therefore, special requirements should be specified in the design, manufacturing, inspection, and acceptance of such vessels. However, designers often overlook these regulatory requirements. Environmental conditions for stress corrosion in liquid ammonia: 1. The medium is liquid ammonia with a low water content (≤0.2%), and in situations where it may be contaminated by air (O2 or CO2) ； 2. Use at temperatures above -5°C. Material selection: 1. The minimum yield strength Rel specified by the material standards shall be ≤355 MPa ； Note: Q245R, Q345R, Q370R (>36mm), 20 forged, and 16Mn forged meet the requirements. 2. The actual measured tensile strength Rm of the material ≤ 630 MPa ； Note: The standards for Q245R, Q345R (>16mm), 20 forged, and 16Mn forged meet the requirements ； Q345R thin plates (3~16mm) require actual measurement. 3. The material should be in a normalized or normalized+tempered state, or in an annealed or quenched and tempered state ； Note: The standards for Q370R, 20 forged, and 16Mn forged meet the requirements ； Additional requirements apply to Q245R and Q345R. 4. Carbon equivalent limit (when the carbon equivalent limit is exceeded, the monitoring frequency of the hardness limit should be increased): Low-carbon steel and carbon-manganese steel: Ce≤0.43, Ce=C+Mn/6; Low-alloy steel (including low-temperature nickel steel): Ce≤0.45, Ce=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15. Note: Q245R and 20 forged grades meet the requirements ； Additional requirements apply to Q345R, Q370R, and 16Mn forgings. 5. For non-welded parts or materials that have been normalized or tempered after welding, the hardness limits are as follows: Low-carbon steel – Hv(10) ≤ 220 (single value); Low-alloy steel – Hv(10) ≤ 245 (single value). Welding requirements: 1. All welds must undergo welding procedure qualification, including butt welding, repair welding, welding of pipes to tube sheets, cladding welding, fillet welding, etc ； 2. Where strength requirements are met, low-strength welding materials should be used as much as possible ； 3. Hardness limit requirements for welded joints (including welds, heat-affected zones, and base metal): for low-carbon steel, Hv(10) ≤ 220 (single value); for low-alloy steel, Hv(10) ≤ 245 (single value) ； 4. Hardness testing shall be conducted on the welding procedure qualification specimens, welding test plates, and the welds of products produced using each welding procedure, to ensure compliance with the requirements specified in item 3 above. The hardness measurement of the product should be taken on the surface on the side in contact with the medium ； Process qualification and hardness testing on test plates shall be carried out at the cross-section (1.5 mm from the surface) ； 5. All arc initiation and striking points outside the welds (including those at temporary welds) must be ground to a depth of more than 0.3 mm prior to the final stress-relief heat treatment, and subjected to magnetic particle or penetrant testing ； Heat treatment requirements: 1. Post-weld heat treatment or welding using a welding process with a hardness not exceeding 185HB ； 2. Pressure vessels subject to stress corrosion and their pressure-bearing components shall undergo post-weld heat treatment ； 3. For pressure vessels subject to stress corrosion and their pressure-bearing components made of steel plates, if the deformation rate of these pressure-bearing components exceeds the range specified in Table 4 of GB/T150.4-2011, appropriate heat treatment should be carried out after shaping to restore the mechanical properties of the material ； 4. For pressure vessels subjected to stress corrosion, if any welding repairs are carried out after weld heat treatment, heat treatment should be applied again to the repaired areas ； Takeover flange: 1. The pipe flange shall comply with the provisions of the HG/T20592–HG/T20635 series of standards, and a necked butt-welded flange, a metal wound gasket with reinforcing rings, and a special-grade high-strength bolt combination shall be used (for moderately hazardous media with high permeability) ； 2. The nominal stress grade of the connection flange should be no less than 2.0 MPa (for moderately hazardous media with high permeability). Example of technical requirements from the drawings: 1. The Q345R steel plates used for the shell shall comply with the specifications in GB/T713-2014 \"Steel Plates for Boilers and Pressure Vessels\", and shall be supplied in the normalized state. It should also meet the following requirement: material carbon equivalent: Ce≤0.43, Ce=C+Mn/6 ； Material hardness: Hv(10)≤220 (single value) ； The actual measured tensile strength Rm of the material is ≤ 630 MPa ； 2. 16Mn forgings shall be manufactured, inspected, and accepted in accordance with the requirements of Grade II specified in NB/T 47008-2017 \"Forgings of carbon steel and alloy steel for pressure equipment\". It should also meet the following requirement: material carbon equivalent: Ce≤0.43, Ce=C+Mn/6 ； Material hardness: Hv(10)≤220 (single value) ； 3a. Steel pipes with diameters of 10 and 20 mm shall be manufactured, inspected, and accepted in accordance with the provisions of GB/T9948-2013 \"Seamless steel pipes for petroleum cracking\". For 10 and 20 steel pipes with an outer diameter of not less than 70 mm and a wall thickness of not less than 6.5 mm, impact tests at -20°C and 0°C shall be conducted respectively ； 3b. 20. Q345C/D steel pipes shall be manufactured, inspected, and accepted in accordance with the provisions of GB/T6479-2013 \"Seamless Steel Pipes for High-Pressure Fertilizer Equipment\". Q345C/D steel pipes shall also meet the following requirements: material carbon equivalent: Ce≤0.43, where Ce=C+Mn/6 ； Material hardness: Hv(10)≤220 (single value) ； The actual measured tensile strength Rm of the material is ≤ 630 MPa ； 4. All welds shall undergo welding procedure qualification, including butt welding, repair welding, welding of pipes to tube sheets, surfacing welding, fillet welding, etc.; low-strength welding materials shall be used as much as possible provided that the strength requirements are met ； 5. Hardness limit requirements for the welded joint (including the weld seam, heat-affected zone, and base metal): Hv(10) ≤ 220 (single value) ； 6. Hardness testing shall be conducted on the results of welding procedure qualification, welding test pieces, and the welds of products manufactured using each welding procedure. For products, hardness testing should be carried out on the surface facing the medium in contact with it; for procedure qualification and test pieces, hardness testing should be performed on the cross-section at a depth of 1.5 mm from the surface. The allowable hardness limit is Hv(10) ≤ 220 (for a single value) ； 7. Unless otherwise specified, all butt welds and fillet welds must have a fully penetrated structure, with a smooth transition from the weld to the base material. There shall be no defects such as surface cracks, incomplete penetration, lack of fusion, surface pores, cratering, unfilled areas, or visibly visible slag inclusions; moreover, the welds must not exhibit undercutting, and the shape of fillet welds should have a concave transition. Unless otherwise specified, the root thickness of fillet welds is the thickness of the thinner plate ； 8. The equipment shall undergo post-weld stress-relief heat treatment, and no further welding shall be performed on the equipment after this heat treatment ； 9. All arc initiation and striking points outside the welds (including those at temporary welds) must be ground to a depth of more than 0.3 mm prior to the final stress-relief heat treatment, and magnetic particle testing shall be conducted. 100% magnetic particle testing is required for the joints where the leg shims meet the lower head; all such tests shall meet the requirements of Grade I as specified in NB/T47013.4-2015 ；