[Common issues in pipeline design by Haichuan] Anti-corrosion and thermal insulation of low-temperature pipelines

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For anti-corrosion and thermal insulation of cryogenic pipelines, the anti-corrosion and insulation layers are generally arranged in a four-layer structure from the inside out: an anti-rust layer, a cold-insulation layer, a moisture barrier layer, and an outer protective layer. For stainless steel pipes equipped with a thermal insulation layer, applying an anti-corrosion coating is unnecessary. This is because at temperatures below -45°C, no moisture remains on the pipe surface. Based on the mechanisms of metal corrosion, there is no need for any additional anti-corrosion coating in such conditions. The surfaces of above-ground low-temperature pipelines, uninsulated carbon steel pipelines and equipment, uninsulated stainless steel pipelines and equipment, steel structures, cryogenic carbon steel, and low-temperature steel pipelines and equipment shall be painted. When the mass concentration of chloride ions in the insulating material exceeds 25×10-6 and there is no stress corrosion inhibitor, the insulated stainless steel pipes must also be painted. Thermal insulation: Given that the cost of obtaining cooling energy during the operation of chemical plants is relatively high, it is even more important to ensure proper thermal insulation for low-temperature pipelines. The calculation of the thickness of the thermal insulation layer can be done by referring to Equation (5.3.3-1) in GB 50264-2013 “Design Code for Thermal Insulation Engineering of Industrial Equipment and Pipelines” to determine the theoretical insulation thickness. Additionally, an appropriate thickness margin must be added based on the specific conditions of each project to ensure effective thermal insulation. Currently, the commonly used thermal insulation materials in design include: flexible foam rubber and plastic products, rigid closed-cell polyurethane foam, foam glass, polyisocyanurate (PIR), and high-density polyisocyanurate (HDPIR). Vapor barrier: The primary function of vapor barrier materials in thermal insulation structures is to resist steam penetration, prevent water ingress, and block moisture. These materials must possess stable chemical properties; they must not corrode or dissolve the insulation layer or protective layer. Currently, commonly used damp-proof layer materials include: asphalt mastic, medium-alkali coarse woven glass cloth, and flame-retardant asphalt mastic. When PIR is selected as the thermal insulation material for cryogenic pipelines, a secondary moisture barrier should be used for the outermost and second outermost insulating layers; a primary moisture barrier should be placed inside the metal protective layer that covers the insulation layer. The outer protective layer should be made of a material with high strength, resistance to softening and brittleness, as well as good aging resistance. It must also possess waterproof, moisture-proof, non-flammable, and corrosion-resistant properties against atmospheric influences. Additionally, its chemical properties should be stable, so as not to cause any corrosion to the insulation layer or moisture barrier. The material used for the outer protective layer shall be at least of Class C as specified in **Standard GB 8624-2012 “Classification for fire performance of building materials and products”. Currently, the commonly used outer protective layer materials include: aluminum alloy sheets, stainless steel sheets, and galvanized steel sheets.