Precautions and Insulation Technology for Low Temperature Storage Tanks

&Attention to Liquid Oxygen Storage Tanks Low temperature storage tanks are equipment used for transportation and storage. In order to confirm safety, we need to understand the corresponding knowledge to avoid the occurrence of risks. Liquid oxygen is non flammable, but it can strongly support combustion and has a fire risk of Class B. It generally cannot spontaneously ignite when in contact with fuel. If two liquids come into contact, liquid oxygen will cause the liquid fuel to cool and condense. The mixture of condensed fuel and liquid oxygen is sensitive to impact and often turns into an explosion under pressure. There are two types of incineration reactions, depending on the mixing ratio of oxygen and fuel and the incineration situation: one is that the fuel and liquid oxygen do not ignite when mixed, but this mixture can explode when burned or subjected to mechanical impact; Another type of liquid oxygen burns before or when it comes into contact with fuel, igniting or burning with repeated explosions. The intensity of incineration reaction depends on the function of the fuel. &II. LNG Low Temperature Storage Tank Insulation Technology and Vacuum Application Unveiled 1. Purpose and Method of Insulation The main purpose of low-temperature insulation is to reduce heat conduction between the environment and LNG, that is, to reduce cold loss. For low-temperature storage tanks, it can reduce gasification losses or create conditions for long-term storage and long-distance transportation. Especially for natural growth gas storage tanks and LNG storage containers, liquefaction and storage are difficult to achieve without good insulation. Therefore, low-temperature insulation not only has economic significance, but also technical significance. In addition, insulating equipment and pipelines can also prevent condensation and frosting on the outer surface, as well as avoid the feeling of coldness or even coldness when human skin comes into contact with them. This is necessary for changing working conditions and preventing accidents from happening. 2、 The classification of insulation methods for low-temperature insulation is divided into ordinary insulation and vacuum insulation. 1. Ordinary (stacked) insulation is an early traditional insulation method, which involves laying solid porous insulation materials on the outside of equipment, containers, and pipelines, and filling the gaps of the insulation materials with air (or other gases) under atmospheric pressure. This insulation method has poor insulation performance, but its structure is simple and cost-effective, so it is widely used in situations where insulation requirements are not high. Nowadays, LNG refueling stations often use this insulation method for their pipelines. 2. Vacuum insulation is the process of making an insulation structure into a sealed interlayer, with the internal space evacuated to a certain degree of vacuum to reduce heat transfer. There are three basic types of vacuum insulation: high vacuum insulation, vacuum porous insulation, and vacuum multi-layer insulation. 2.1 High vacuum insulation, also known as simple vacuum insulation, simply draws the interlayer space to a vacuum of 1.38 * 10-3Pa (1.0 * 10-5mmHg) 1. The common vacuum insulated cup adopts this insulation method. In a vacuum interlayer, there is only radiative heat transfer between two walls and thermal conduction of residual gases inside the interlayer. If the inner surface of the interlayer is polished or coated with a reflective material, a vacuum interlayer with good reflective performance can be formed, and the radiation heat transfer can be reduced. This insulation structure has poor insulation performance due to radiative heat exchange; But its structure is relatively simple, lightweight, and has a small heat capacity, so it has been widely used since the beginning of the last century. 2.2. Vacuum porous insulation is the process of filling a porous insulation material in the interlayer and then pumping it to a determined vacuum. For the convenience of the process, it is generally filled with powder or fiber materials, so this insulation method is also known as vacuum powder insulation or vacuum fiber insulation. The insulation material in the vacuum interlayer weakens the radiative heat transfer between the walls, so its insulation performance is better than that of high vacuum insulation, and the requirement for vacuum can be reduced, generally reaching around 1.33Pa (10-2mmHg). If a certain proportion of highly reflective metal powder, such as aluminum powder or copper powder, is added to powder or fiber materials to reduce the radiative heat transfer between particles or fibers in the material, it is called light blocking effect, which can greatly improve the insulation performance. Vacuum powder and vacuum fiber insulation are widely used in LNG liquid storage and transportation equipment. 2.3. Vacuum multi-layer insulation involves installing multiple radiation screens in a vacuum interlayer to reduce radiative heat transfer between walls. There are two basic forms of multi-layer insulation. One is to use metal foil as a radiation screen, with low thermal conductivity spacer materials filled between the screens; One method is to use a single-sided aluminum sprayed polyester film as a radiation screen, and press it into a corrugated shape to form a concave convex shape, in order to reduce the contact heat transfer between the screens. Vacuum multi-layer insulation requires a vacuum of about 1.3 * 10-2Pa (10-4mmHg). Vacuum multi-layer insulation is one of the current insulation methods with good insulation performance, often referred to as "large insulation". It is mostly used for LNG/liquid hydrogen and liquid helium storage and transportation containers, and is now also used for small liquid oxygen and liquid nitrogen containers. The disadvantages of vacuum multi-layer insulation are that the construction is relatively complicated, the cost is relatively high, and the insulation performance varies with the construction quality. 3、 The insulation performance of graded insulation structures can be evaluated by their thermal conductivity (or apparent thermal conductivity, including convective and radiative heat transfer), and the smaller its value, the better the insulation performance. The variation range of thermal conductivity for various insulation methods is shown in Figure 4-2. From the figure, it can be seen that multi-layer insulation has much higher insulation performance than other insulation methods, while non vacuum insulation methods have poorer insulation performance. LNG commonly uses low-temperature insulation technology in various insulation methods, except for high vacuum insulation, which requires the use of insulation materials. Insulation materials are used to enhance the insulation performance of insulation structures, in order to reduce the heat transfer through the insulation structure. Understanding the properties of insulation materials is the foundation for designing insulation structures. 1. There are many types and general characteristics of insulation materials, and their properties vary greatly from each other. Thermal insulation materials can be divided into two categories based on their materials: mineral materials and organic (based on actual reports) materials. Mineral materials are commonly used in low-temperature devices. Thermal insulation materials can be divided into three types according to their organizational structures: foam like materials, powder like materials and fibrous materials. Their organizational structures are different, the mechanism of heat transfer process carried out in them is different, and the types of thermal insulation structures formed by them are not all the same. 1.1 Foamed plastic is made from polymer or synthetic resin by heating and foaming with foaming agent and stabilizer. The advantages of foamed plastics used as thermal insulation materials are that their density and thermal conductivity are small, they can be applied to low temperature, their water absorption is small, they can resist acid and alkali erosion, their combustion is poor (they can self extinguish after leaving the ignition source), and they are easy to cut and construct. Therefore, their applications are increasingly widespread. 1.2 Mineral wool, also known as slag wool, is a mineral fiber made by blowing molten iron slag (or mudstone) into a molten state with high-pressure steam (the fiber often contains glass like small balls). It is fire-resistant, frost resistant, odorless, and has low density and thermal conductivity, making it a good insulation material at a reasonable price. Mineral wool is commonly used in air separation units and transport equipment. Mineral wool 1.3 pearl sand pearl sand, also known as expanded perlite. Perlite is an acidic glassy lava erupted by volcanoes, mainly composed of SiO2 and AlO2. When magma flows out of the surface, due to rapid cooling, the water cannot escape completely, so the rock contains a certain amount of crystalline water. After crushing the rock into fine particles, the speed is suitable for heating to 700-1000 ° C, and the speed of crystallization water is suitable for vaporization. The volume of the rock can increase by 4-20 times, and it is deeply immersed in light colored and white pearl sand, most of which have a size between 0.3-0.6mm. Pearlescent sand has a low density and thermal conductivity, making it a good insulation material. Pearlescent sand is non combustible, non moldy, odorless in toxin detection, and non corrosive; It has good fluidity and can be transported by wind pressure; In addition, it also has the properties of sound insulation and stable performance, and with a wide range of sources, it is widely used. Pearlescent sand is mainly used in air separation units and gas liquefaction units, but its disadvantages are high water absorption and sinking phenomenon. Pearlescent sand also has the ability to adsorb gases. When it adsorbs, it should be replaced before maintenance and construction to confirm the safety of the construction. 1.4 Magnesium carbonate powder has good insulation performance and low price, so it has been widely used for vacuum insulation of low-temperature devices and equipment in the past. But magnesium carbonate is prone to clumping (and difficult to produce after clumping), and often contains a certain amount of moisture, making it difficult to evacuate, so it is gradually replaced by other materials. 1.5 Aero gel and silica gel powder aero gel, also known as silicic acid aero gel, are deeply rooted materials by removing liquid from silicic acid gel without obviously compressing its skeleton, and are known as relatively thermal insulation materials at present. Aerogel has small thermal conductivity, good fluidity and a little elasticity. It will not sink due to vibration after one filling, and is easy to be evacuated in vacuum insulation. However, aerogel is very expensive, and it forms silicic acid gel after being immersed in water. The density will increase nearly ten times, so it can no longer be used as thermal insulation material. Silica gel powder is a powder composed of silica, and its density and thermal conductivity are larger than those of aerogel. 2. The commonly used LNG low-temperature insulation technology 2.1 is vacuum porous insulation, which mostly uses pearl sand as the filling medium. 2.2 Vacuum multi-layer insulation: Aluminum foil composite materials are often used to wrap the inner liner, and the interlayer is evacuated to high vacuum to achieve better insulation effect. Representative products include: car bottles, quick and easy to cool, low-temperature tank trucks, tank containers, etc.