Brief description of the usage characteristics of compensators in acid production projects

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In recent years, as demand in society has increased, acid production projects in China’s chemical industry have emerged one after another, with the scale of these projects growing larger and larger – from 150,000 tons per year to 300,000 tons per year, and now up to 800,000 tons per year. Compensators used in conjunction with such projects must be designed and manufactured taking into account the various characteristics of sulfuric acid production processes, so that specialized compensators can be created for use in these applications. I. Characteristics of the acid production project: The piping layout in an acid production project is very compact; many areas are not suitable for installing fixed brackets or hangers. As a result, compensators are needed to bear the weight of the piping itself, which places high demands on the stiffness of the components of these compensators as well as the strength of their structural elements. Additionally, to reduce heat loss, the compensator components are protected from corrosion by sulfur dioxide and sulfur trioxide in the air. Manufacturers generally provide external insulation. Since ordinary insulation materials contain chloride ions, additional protection for the corrugated components must be considered ； Furthermore, the high temperature of the entire piping system, along with the presence of corrosive gases such as sulfur dioxide and sulfur trioxide in the fluid, imposes higher requirements on the material selection for the corrugated components to prevent stress corrosion (it is recommended to use SUS321 and SUS316L). Because the temperature difference between each section is very large. Therefore, when applying external insulation, the thickness of the external insulation layer should also vary ； It is also well known that the actual service life of compensators is greatly affected by stress corrosion, and corrosion is the main cause of compensator failure. Therefore, in the manufacturing of compensators, the degree to which residual stresses are eliminated directly determines the service life of the compensator. II. It is recommended to use a protector cover for the compensator. Reason for this recommendation: As mentioned earlier, chloride ions in the external insulation layer can cause significant damage to certain components; therefore, protective measures are necessary. It is recommended to use an external protective cover, which serves the following purposes: it enhances the strength of the compensator, reduces or even prevents tearing due to excessive tensile forces on the ring welds between the components and the end tubes during handling, and also reduces deformation of the components caused by impacts ； It prevents damage to the component surfaces caused by impacts during installation, as well as by splashes of slag and welding slag during system installation. It can certainly also effectively prevent chloride ions in the insulation layer from adhering to the surface of the components, thereby reducing stress corrosion resulting therefrom. III. The selection of the insulation layer thickness is based on the accumulated practical experience of manufacturers of compensators used in acid production projects within the chemical industry. The general principle for determining the insulation layer thickness (δ) is as follows: when the temperature is ≤ 350°C, the thickness should be 200 millimeters ； When the temperature is <480°C and >350°C, the thickness should be 250 millimeters ； When the temperature is ≥480°C, a thickness of 280 millimeters should be used. IV. Based on the characteristics of acid production projects, the general principles that should be followed in design and selection are summarized: the compensation amount of the compensator should be set at a higher level, with more margin reserved, especially in areas near branch pipes and valves ； The corrugated elements should preferably be single-layer and slightly thicker ； For high locations with large lateral displacements and sections with small diameters, hinge-type connections should be used instead of composite tie rods whenever possible. Several questions about corrosion: 1. What is corrosion? The phenomenon or process by which a material deteriorates and is damaged due to chemical or electrochemical actions of external media is called corrosion. 2. What is intergranular corrosion? What are the causes of intergranular corrosion? Corrosion penetrates from the surface along the grain boundaries into the interior, with no signs of corrosion visible on the outside. Intergranular corrosion is caused by the precipitation of impurities at grain boundaries, or by an increase or decrease in the amount of a certain element. Taking austenitic stainless steel as an example, during welding, the area 2 to 3 millimeters on either side of the weld can be heated to 400°C to 910°C. At this point, chromium and carbon at the grain boundaries combine to form CR23C6, which precipitates from the solid solution. Since chromium moves very slowly and finds it difficult to diffuse from within the grains to the grain boundaries, low chromium regions are formed. In a suitable corrosive solution, a \"chromium carbide cathode\" and a \"chromium-deficient region anode\" cell is created, resulting in corrosion of the chromium-deficient regions at the grain boundaries. 3. What are the methods to prevent intergranular corrosion in austenitic stainless steels? 1) Heat treatment method (also known as solution treatment): The material is heated to 1100°C and then quenched in water; since chromium carbide dissolves at 1100°C, a homogeneous alloy steel can be obtained. 2) Add elements with a stronger affinity for carbon than chromium, such as titanium and nickel. 3) Reducing the carbon content to below 0.03% minimizes the amount of chromium carbide produced, thereby preventing intergranular corrosion. What is stress corrosion? Under what conditions does stress corrosion occur? What methods are used to prevent stress corrosion? Corrosion that occurs in metals and alloys under the combined effect of corrosion and stress is known as stress corrosion. Stress corrosion occurs only in certain specific “material-environment” systems. For example, in systems such as “austenitic stainless steel—chloride ions” and “carbon steel—nitrogen trioxide”. Of course, stress must also be present (external forces or residual stresses resulting from welding, cold working, etc.). There are generally two types of cracks in stress corrosion: one develops along the grain boundaries and is known as intergranular fracture ； Another type that passes through the grains is called transgranular fracture. There are also mixed types, such as intergranular cracks as the main cracks and transgranular cracks as the secondary cracks. The main methods of prevention include: generally, stress is eliminated or reduced through heat treatment ； A value lower than the critical stress corrosion cracking strength is adopted in the design ； Improve the design structure to avoid stress concentration ； Apply compressive stress to the surface ； Electrochemical protection, coatings, or corrosion inhibitors are used. What is stress corrosion cracking? What are those \"environment-material\" systems that induce stress corrosion cracking? Under certain temperature conditions, the fracture of metal caused by the combined effect of stress and corrosion is known as stress corrosion cracking. Since fracture is brittle and occurs without obvious precursors, it can easily lead to catastrophic accidents. The possible “material–environment” combinations are: 1) the material is carbon steel or low-alloy steel, and the environment is an alkaline solution. Such as nitrate solutions, anhydrous liquid ammonia, hydrogen sulfide, acetic acid, etc. 2) The material is austenitic stainless steel, and the environment includes chloride ions, chlorides, water vapor, wet hydrogen sulfide, alkaline solutions, etc. 3) The material is molybdenum-containing austenitic stainless steel, and the environments include alkaline solutions, chloride aqueous solutions, and solutions of sulfuric acid with copper sulfate, etc ； 4) The material is brass, and the environment includes ammonia gas and solutions, as well as ferric chloride and wet sulfur dioxide. 5) The material is titanium, and the environment includes methanol and ethanol containing hydrochloric acid, as well as molten sodium chloride. 6) The material is aluminum, and the environment includes hydrogen sulfide, seawater, etc. Introduction to common materials for several types of compensators. Common austenitic 300 series: SUS316L; its main components are 022Cr17Ni12Mo2, it can withstand temperatures up to 550°C and has good resistance to acids and alkalis. A high content of MO2 is beneficial for welding and improves the quality of welds. High resistance to stress corrosion ； SUS316 has a main composition of 0Cr17NI12MO2; it can withstand temperatures of around 500°C, has good resistance to acids and alkalis, and excellent weldability ； The main component of SUS304L is 00Cr19Ni9; it can withstand temperatures up to 500°C. The main component of 304 is 0Cr18Ni9, and it can withstand temperatures up to 450°C℃ ； The main component of SUS310L is 0Cr25Ni20, with a temperature resistance of around 900°C.