Core Guidelines for Engineering Design of High-Temperature Molten Salt Systems in Chemical Plants
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I. Selection of Core Equipment and Design Specifications The core equipment of a molten salt system mainly includes molten salt tanks, molten salt pumps, molten salt heaters, and process heat exchangers (reactors). 1. Melting Tank/Storage Tank: Volume design – The melting tank is not only the device used to initially melt solid salts, but it also serves as a container for storing and buffering the molten salts throughout the entire system. Its effective volume must be capable of holding the total volume of the liquid molten salt in the entire system (including all pipelines, heat exchangers, and furnace tubes) at the highest operating temperature, with at least a 20% safety margin for expansion. Heating method: A steam coil (using high-pressure steam) or an electric heater is usually installed in the tank for bottom heating. The surface heat load of the heating element (watt density) must be strictly controlled to prevent local overheating that could lead to the decomposition of nitrites (it is usually kept below 2.0 W/cm2). 2. Molten salt pump (Circulation Pump): Structure type – a vertical cantilever submersible pump that is installed directly on top of the molten salt tank. This design eliminates the risk of seal leakage at the bottom of the pump body. Bearings and cooling: The temperature of the molten salt is usually between 350°C and 500°C. The bearings of the pump must be located above the level of the molten salt, and they must be equipped with a separate cooling water or cooling air system to prevent heat from spreading along the axis of the pump and causing damage to the motor or bearings. Material requirements: The impeller and pump casing are prone to erosion corrosion under high temperatures and high flow rates; therefore, heat-resistant stainless steels such as 304H, 316H, or 347H are typically used.
II. Pipeline System and the Principle of “Gravity Drainage” – The success of the pipeline design in a molten salt system directly determines the safety and operability of the installation. The molten salt solidifies at around 140°C (the freeze-bag phenomenon); once it forms clumps inside the pipes, this can cause the pipes to burst or the entire system to fail. 1. Absolute gravity draining: Slope design – All molten salt pipelines within the system (including horizontal sections) must not have any U-shaped dead legs at all. All pipelines must have a continuous slope (it is generally recommended that the slope be no less than 1:100 or 1:200) to ensure that, in the event of pump failure or an emergency, the molten salt can flow back to the molten salt tank automatically due to its own weight by 100%. Top exhaust and bottom drainage: An exhaust valve must be installed at the highest point of the system (leading to the atmosphere or a safe collection system), while the lowest point must be connected to the molten salt tank. 2. Thermal stress and flexibility analysis · The operating temperature range is wide (from room temperature to 500°C), resulting in extremely severe thermal expansion of the pipes. When performing pipeline stress analysis (such as using Caesar II), it is necessary to properly configure anchor supports, guide supports, and spring hangers, and make extensive use of natural compensators (such as Z-shaped and Π-shaped bends) to absorb large thermal displacements and prevent excessive stress on the equipment’s interface flanges.
III. System heating and insulation strategies: Due to the solidification properties of molten salts, heating the entire system is essential. 1. Selection of heating method: Electric heating (MI heating cables): Mineral-insulated (MI) heating cables are the most commonly used heating method for molten salt pipelines. It can provide a very high and uniform heating temperature. Casing heating (heat transfer oil): In certain specific areas, such as high-risk valves or dead corners where condensation is likely to occur, high-temperature heat transfer oil can also be used for casing heating, but this increases the complexity of the system. 2. Heat tracing control logic · The heat tracing system cannot be merely “on/off”. An excellent design divides the entire system into dozens or even hundreds of separate temperature control circuits (Zones). Before driving, the system needs to be preheated according to a specified heating curve (usually 10–15°C/h). The highest points and vertical sections of the system must be heated first, followed by the lowest points and valves, in order to ensure that there is a path for the expanding air to escape and to prevent local blockages that could lead to pipe rupture.
IV. Material Selection and Safety Measures 1. High-temperature corrosion and material upgrades: The limits of carbon steel: Below 400°C, dry molten salts free from impurities cause minimal corrosion to carbon steel, allowing the use of high-quality carbon steels such as 20G or A106 Gr.B. Applications of stainless steel: When the operating temperature exceeds 400°C, or due to process-related factors such as local overheating or the formation of alkaline substances from salt degradation, the oxidation and decarburization resistance of carbon steel drops significantly. At this point, it must be upgraded to austenitic stainless steel (such as 304/304H, 316/316H, 321). Note that austenitic stainless steels may be at risk of chloride stress corrosion cracking (SCC) in molten salt environments; therefore, the chloride impurity content in the nitrate purchased must be strictly controlled (usually required to be ≤ 0.05%). 2. Core safety precautions: Water and organic substances resistance. Explosion upon contact with water: When high-temperature liquid molten salts come into contact with liquid water, the water vaporizes and expands instantly (its volume increasing by more than a thousand times), resulting in a violent physical explosion. Therefore, water pipelines are strictly prohibited in the molten salt tank area, and water or foam must not be used in the fire extinguishing system; dry sand or specialized dry powder must be available. Oxidation of organic materials: Nitrates are strong oxidizers; they must not be mixed with flammable materials or organic substances such as engine oil, wood, and fabric, as this can lead to intense combustion or even chemical explosions at high temperatures.
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