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A storage tank is a steel-sealed container used for storing liquids or gases; it is thus known as a steel storage tank. Steel storage tank systems represent essential and important infrastructure in industries such as petroleum, chemicals, grain and oil, food processing, firefighting, transportation, metallurgy, and defense. Our economic life relies heavily on these small steel storage tanks, which play an irreplaceable role in the development of the national economy. Steel storage tanks are specialized devices used for storing various liquid (or gas) raw materials and finished products. For many enterprises, production cannot proceed without such tanks; in particular, **the storage of strategic materials relies on storage tanks of different capacities and types. In China, most oil storage facilities consist of above-ground tanks, which are predominantly of metal construction. Due to the differences in the media stored, there are a variety of forms of storage tanks. 1. Classified by location: they can be divided into above-ground storage tanks, underground storage tanks, semi-underground storage tanks, offshore storage tanks, and subsea storage tanks, etc. 2. Classified by oil type: They can be divided into crude oil storage tanks, fuel oil storage tanks, lubricating oil tanks, edible oil tanks, fire water tanks, etc. 3. Classified by purpose: they can be divided into production tanks, storage tanks, etc. 4. Classified by shape: they can be divided into vertical storage tanks, horizontal storage tanks, etc. 5. Classified by structure: they can be divided into fixed-roof storage tanks, floating-roof storage tanks, spherical storage tanks, etc. 6. Classification by size: Tanks with a capacity of over 100 m3 are considered large tanks, and they are usually vertical tanks ； Storage tanks with a capacity of 100 m³ or less are considered small storage tanks, and they are usually horizontal in design. Common standards for storage tanks: 1. American Petroleum Institute standard API650 ； 2. British Standard BS2654 ； 3. Japanese Standard JISB8501 ； 4. German standard DIN4119 ； 5. Petroleum industry standard SYJ1016 ； 6. Petrochemical industry standard SH3046. Materials for storage tanks: The materials required for storage tank projects are divided into tank body materials and materials for auxiliary facilities. Tank materials can be classified as low-strength steel or high-strength steel based on their tensile yield strength or tensile ultimate strength; high-strength steel is commonly used in storage tanks with a capacity of over 5000 m3 ； The auxiliary facilities (including wind-resistant ring beams, joints, spiral staircases, guardrails, etc.) are made of ordinary carbon structural steel with lower strength, while other components and accessories are made of different materials depending on their intended use. Commonly used domestic steel grades for manufacturing tanks include 20, 20R, 16Mn, 16MnR, and the Q235 series. In terms of tank structures, currently in China, the most widely used and those with the most mature manufacturing and installation techniques are domed tanks, floating roof tanks, and horizontal tanks. 1. Structure of dome-top storage tanks A dome-top storage tank is a steel container with a spherical roof and a cylindrical body. Dome storage tanks are simple to manufacture and inexpensive, which is why they are widely used in many industries both domestically and internationally. The most common capacity range is 1000–10,000 m3; currently, the maximum capacity of dome storage tanks used in China has reached 150,000 m3. 1.1 Bottom of the tank: The bottom of the tank is assembled from steel plates; the steel plate in the middle of the bottom is a medium-width plate, while the steel plates around it are edge plates. The edge plate can be a strip-shaped plate or an arc-shaped plate. Generally, when the inner diameter of the storage tank is < 16.5 m, strip-shaped edge plates are preferred; when the inner diameter is ≥ 16.5 m, arch-shaped edge plates are preferred. 1.2 Tank wall: The tank wall is formed by welding multiple layers of steel plates together, and it comes in sleeve-type and straight-type configurations. The circumferential welds of the sleeve-type tank wall panels are lap welded, while the longitudinal welds are butt welded. This form is commonly used for vaulted storage tanks; its advantage is that it facilitates the alignment of the various wall panels, and construction using the inverted method is relatively safe. The circumferential welds of the straight-type tank wall panels are butt-welded. The advantage is that the diameter of the tank wall remains constant from top to bottom, making it particularly suitable for internal floating roof tanks; however, it requires high precision during assembly and installation, and the process is rather complex. 1.3 Tank roof: The tank roof is formed by welding multiple sectoral plates together to create a spherical shape. Reinforcing ribs made of flat steel are used on the inner side of the roof, and lap welds are employed between the various sectoral plates. The entire tank roof is welded together with the angle iron rings (also known as lock joints) located at the top of the tank wall panels. 2. Structure of floating roof storage tanks A floating roof storage tank consists of a floating roof that floats on the surface of the medium and vertical cylindrical tank walls. The floating roof rises and falls as the volume of the medium inside the tank increases or decreases. An annular sealing device is present between the outer edge of the floating roof and the tank wall, ensuring that the medium inside the tank is always directly covered by the inner floating roof, thereby reducing vaporization of the medium. 2.1 Bottom of the tank: Floating roof tanks generally have a large volume, and their bottom plates are equipped with arc-shaped edge plates. 2.2 Tank wall: A straight-type tank wall is used; the butt welds should be polished smoothly to ensure a smooth inner surface. The upper part of floating roof storage tanks is open. To increase the stiffness of the wall panels, wind girders and stiffening rings must be installed at the top of the tank walls, depending on the magnitude of wind loads in the respective region. 2.3 Floating roof: Floating roofs come in various types, such as single-deck floating roofs, double-deck floating roofs, and float-type floating roofs. 2.3.1 Single-deck floating roof: An annular floating vessel composed of several independent compartments, with a single-deck top plate on its inner annular side. The bottom of the single-disc roof is reinforced with multiple circular steel rings. Its advantages are low cost and easy maintenance. 2.3.2 Double-disc floating roof: It consists of an upper disc plate, a lower disc plate, and deck edge plates, and is divided into several separate annular compartments by radial and circumferential partitions. Its advantages are high buoyancy and excellent drainage performance. 3. Structure of internally floating roof storage tanks An internally floating roof storage tank is created by adding a floating roof inside a domed roof storage tank. The addition of this floating roof helps to minimize the volatilization losses of the stored medium. Meanwhile, the outer domed roof prevents rainwater, snow, dust, and other contaminants from entering the tank, thus ensuring the cleanliness of the medium inside. Such storage tanks are mainly used to store light oils, such as gasoline and aviation kerosene. Internal floating roof tanks feature straight tank walls, with the wall panels welded together at their joints; the vault is constructed in accordance with the requirements for vault-type tanks. Currently, there are two types of internal floating roofs used in China: one is a steel floating roof identical to that used in floating-roof storage tanks ； Another option is an aluminum alloy floating roof that is assembled in place. 4. Structure of horizontal storage tanks: The volume of horizontal storage tanks is generally less than 100 m3, and they are typically used in production facilities or gas stations. The circumferential welds of horizontal storage tanks use lap welding, while the longitudinal welds use butt welding. The ring plates are arranged interactively in an odd number so that the diameters of the end caps are identical. The end caps of horizontal storage tanks are divided into flat end caps and disc-shaped end caps; horizontal storage tanks with flat end caps can withstand an internal pressure of up to 40 kPa, while those with disc-shaped end caps can withstand an internal pressure of up to 0.2 MPa. Underground horizontal storage tanks must be equipped with reinforcement rings, which are fabricated from angle steel. Selection of storage tank foundations: 1. When the underlying soil layer can meet the requirements regarding bearing capacity and settlement limits, and there are no constraints regarding the site location, it is advisable to use a retaining-wall type or outer-ring wall type foundation ； 2. When the bearing capacity of the foundation soil layer does not meet the required design values, but the settlement amount is within the allowable limits, a ring wall foundation or an outer ring wall foundation can be used ； 3. When the foundation soil layer is soft soil, it is advisable to treat the foundation before adopting an outer ring wall type foundation ； 4. When space is limited, a ring wall foundation can be used. Construction of storage tank foundation 1. Earthwork excavation: Compacting the foundation pit ； 2. Reinforced concrete and masonry works: (omitted) ； 3. Earthback filling: Compacted mechanically, with the fill layer thickness exceeding 500 mm ； 4. Sand cushion: Medium to coarse sand is used, with a laying thickness of 200–250 mm; it is compacted by using a flat vibrator while sprinkling water ； 5. Asphalt sand cushion: Medium and coarse sand, along with No. 60 road petroleum asphalt, are used to create asphalt sand; this material is laid in layers and sections in a even manner, with a thickness of 80–100 mm. The surface of the tank foundation has a slope of 15–35‰ from the center outward ； 6. Slope protection construction: Slope protection work is carried out after the storage tank has undergone a hydrostatic test, with a slope width of 800~1000 mm. The gap between the slope protection and the tank bottom plate is filled with asphalt mastic. 7. During the design and construction of tank foundations, please refer carefully to SH/T 3083 \"Technical Specifications for Foundation Treatment of Steel Storage Tanks in Petrochemical Industries\" and SH 3086 \"Design Specifications for Foundations of Steel Storage Tanks in Petrochemical Industries\". Storage tank fabrication and installation 1. The construction process of storage tanks consists of two phases: prefabrication of semi-finished products and on-site assembly and installation. 2. Prefabrication of semi-finished products: Components such as the tank bottom, tank walls, and tank top all need to be prefabricated (the specific methods for prefabrication are not detailed here). 3. On-site alignment and installation methods: These are generally divided into the inverted construction method, the normal construction method, and special construction methods. Tank accessories 1. Oil tank accessories are an important part of the oil tank itself. Its settings can be divided into 4 types based on their functions: 1.1 To ensure the proper handling, receipt, and storage of oil, thereby facilitating production and operational management. 1.2 Ensure the safe use of oil tanks, and prevent and eliminate various types of oil tank accidents. 1.3 Facilitates tank cleaning and maintenance. 1.4 can reduce oil evaporation losses. 2. Spiral staircase (or straight staircase): with a landing, 650 mm wide, rotating counterclockwise. The tank top is equipped with anti-slip steps. The escalator is provided specifically for operators to climb onto the tank for gauging, temperature measurement, sampling, and inspections. It comes in two types: straight stairs and spiral stairs. Generally, straight ladders are used for small oil tanks, while spiral ladders are used for large oil tanks. 2. Fencing: height of 800~1000mm. 3. Overpass: Used for connecting tanks, with a width of 650 mm. 4. Manhole: Three common specifications are available: DN500, DN600, and DN750. Manholes are provided for operators to enter and exit the oil tank during cleaning and maintenance. In standard vertical oil tanks, the manhole is located on the lowest ring plate of the tank wall, opposite to the lighting opening at the top of the tank. The diameter of the manhole is usually 600 mm, with the center of the hole being 750 mm above the bottom of the tank. Generally, oil tanks with a volume of 3,000 cubic meters or less are equipped with 1 manhole, those with a volume of 3,000 to 5,000 cubic meters have 1 to 2 manholes, while oil tanks with a volume of more than 5,000 cubic meters must have 2 manholes. 5. Light-transmitting hole: Common specification: DN500. A light opening, also known as a ventilation opening, is provided for lighting and ventilation during the cleaning or maintenance of oil tanks. It is usually installed on the tank top above the inlet and outlet pipes, with a diameter of generally 500 mm; its outer edge is located 800–1000 mm away from the tank wall, and the number of such units is the same as that of the manholes. 6. Cleaning hole or drain hole: A drain hole is used for light oils, while a cleaning hole is used for heavy oils. There are three models: DN50, DN80, and DN100. 7. Oil measurement port: Common specification: DN150. The oil measurement hole is provided for gauge reading, temperature measurement, and sampling, and is installed near the tank roof platform. Each oil tank is equipped with only one measuring hole, which has a diameter of 150 mm and is usually located at a distance of 1 m from the tank wall. 8. Drain pipe and automatic water cut-off device: The drain pipe, also known as a discharge pipe, is designed specifically to remove water and impurities from within the tank as well as to eliminate residual oil residues at the bottom of the tank. The drain pipe is equipped with a valve on the side outside the tank; to prevent leaks or damage to the dehydration valve, two valves are usually installed. In winter, it is also necessary to insulate the drain valves to prevent freezing or cracking of the valves. 9. Import and export connection pipes. 10. Fire foam chamber: Also known as a foam generator, the fire foam chamber is a fire extinguishing device mounted on oil tanks. One end of the foam generator is connected to the foam pipeline, while the other end is fitted with a flange that is welded to the topmost ring plate of the tank wall. As the fire-fighting foam passes through the air intake of the fire foam chamber, it draws in a large amount of air to form foam, which then breaks through the partition glass (with a thickness of no more than 2 mm) and enters the tank, thereby reaching the fire source. 11. Grounding wire: The grounding wire is a device used to eliminate static electricity in oil tanks. 12. Special accessories for light oils. Light oils (including gasoline, kerosene, diesel, etc.) are oils with low viscosity, low density, and high volatility. Tanks used to store such oils are equipped with various special accessories designed to take into account their properties and to meet the requirements of production and safety. 12.1 Oil tank vent valves: Oil tank vent valves are important devices that ensure the safe use of oil tanks and help reduce oil losses. 12.2 Hydraulic safety valve: The hydraulic safety valve is another important device designed to enhance the safety of oil tanks during use; its operating pressure is 5~10% higher than that of mechanical relief valves. Under normal conditions, it remains stationary; however, when the mechanical breathing valve fails due to rust or jamming of its disc, or when there is excessive pressure or vacuum inside the tank as a result of abnormal operations related to filling and emptying the tank, it serves to provide a secure seal for the tank and prevent damage to it. 12.3 Flame arrestors: Also known as tank flame arrestors, these are fire safety devices for oil tanks. They are installed below mechanical breathing valves or hydraulic safety valves, and contain wire meshes or corrugated plates made of copper, aluminum, or other metals with high heat capacity. In the event that an external flame or spark manages to enter the fire extinguisher through the breathing valve, the metal mesh or corrugated plate can quickly absorb the heat from the burning material, extinguishing the flame or spark and thus preventing the oil tank from catching fire. 12.4 Sprinkler cooling system: The sprinkler cooling system is an energy-saving device installed to reduce the oil temperature inside the tank and minimize the breathing losses associated with the tank’s size. 13. Special accessories for internal floating roof tanks. Compared to ordinary dome-shaped tank tanks, internal floating roof tanks have various unique special accessories due to their different structure and the performance requirements associated with their use. 13.1 Vent holes: In internal floating roof tanks, the internal floating disk covers the oil surface, which virtually eliminates the gas space; as a result, evaporation losses are minimal. Therefore, no mechanical breathing valves or safety valves are installed on the tank top. However, in practice, leaks of oil and gas still occur at the floating roof ring gaps or other connections. To prevent the accumulation of oil and gas to dangerous levels, ventilation holes are provided on both the top and sides of the oil tank. 13.2 Static electricity discharge device: During oil loading and unloading operations in internal floating roof tanks, a large amount of static charge accumulates on the floating roof. Since insulating materials are commonly used as sealing materials between the floating roof and the tank wall, the static charge accumulated on the floating roof cannot be dissipated through the tank wall. To discharge this amount of static charge, an electrostatic discharge wire was installed between the floating tray and the tank top. It generally consists of 2 soft copper bare twisted wires, with their upper ends connected to the lighting opening and their lower ends pressed against the cover strip of the floating deck. 13.3 Anti-rotation steel cables: To prevent deformation of the oil tank wall and to ensure smooth lifting and lowering due to the rotation of the floating roof, two stainless steel cables are vertically tensioned between the top and bottom of an internal floating roof tank; these two cables are arranged symmetrically at both ends of the floating roof’s diameter. Under the restraint of steel cables, the floating top can only move vertically, thereby preventing the floating disc from rotating. 13.4 Automatic vent valve: The automatic vent valve is located in the middle of the floating disk. It serves to ensure that the oil tank can breathe properly when oil is being added to or removed from it, while the floating disk remains in its supported position; this prevents vacuum formation or pressure buildup below the floating disk. 13.5 Floating deck supports: After being in use for a period of time, internal floating roof tanks require maintenance and cleaning; at such times, the floating roof needs to be lowered to a certain height above the bottom of the tank, and it is supported by several supports located on the floating deck. 13.6 Diffusion tube: The diffusion tube is connected to the inlet pipe inside the oil tank; its diameter is twice that of the inlet pipe, and it is equipped with numerous small holes with a diameter of 2 mm, evenly distributed on both sides. It serves to reduce the flow rate when oil is being taken in by the tank, thereby protecting the floating pad supports. Emissions during crude oil storage and loading mainly include: large and small breathing losses from storage tanks, leaks and spills of the oil, and loading losses. 1. Large breathing loss of storage tanks: Large breathing refers to the breathing that occurs when oil is taken out of the tank. When oil is being poured into the tank, as the oil level rises gradually, the gas space decreases, causing the pressure inside the tank to increase. When this pressure exceeds the control pressure of the breather valve, oil vapors at a certain concentration begin to be released through the breather valve. This process continues until oil pouring into the tank stops, and the oil vapors released result in losses due to oil evaporation. When the oil tank dispenses oil, as the oil level continues to drop, the gas space within the tank gradually shrinks, resulting in a decrease in pressure. When this pressure falls below the vacuum level controlled by the breather valve, the tank begins to draw in fresh air. Since the air and oil mixture in the space above the oil surface is not saturated, this accelerates the evaporation of the oil, allowing it to reach saturation once again; as a result, the pressure inside the tank rises, causing some oil vapor to be expelled through the breather valve. The main factors affecting deep breathing are: 1.1 Oil properties. The lower the density of the oil, the more light fractions it contains, and the greater the losses ； 1.2 Oil intake and output speed. The faster the oil inflow and outflow rates, the greater the losses ； 1.3 Pressure rating of oil tanks. The better the pressure resistance of the oil tank, the lower the breathing loss. When the pressure resistance of the oil tank reaches 5 kPa, the loss reduction rate is 25.1%; if the pressure resistance is increased to 26 kPa, the minor breathing losses can be essentially eliminated, and the major breathing losses can be reduced to a certain extent. 1.4 is related to the geographical location of the oil tank, atmospheric temperature, wind direction, wind strength, and management level. Measures taken: The oil stored in this project is crude oil, which has lower volatility compared to gasoline and diesel. The various storage tanks are interconnected through pipelines, and internal floating roof tanks are used, as a result of which the amount of vapor loss from the crude oil is relatively low. 2. Minor breathing losses of storage tanks: When there are no operations of loading or unloading oil in the tank, as external temperatures and pressures change throughout the day, the temperature of the gas space inside the tank, the evaporation rate of the oil, the concentration of oil vapor, and the steam pressure also change accordingly. The loss of oil and gas caused by this process of releasing oil vapors and absorbing air is known as minor breathing loss. The main factors affecting minor breathing loss are as follows: 2.1 Fluctuations in day-night temperature differences. The greater the variation in temperature between day and night, the greater the minor respiration loss. 2.2 Sunlight intensity in the area where the oil tank is located. The greater the light intensity, the greater the minor respiration loss. 2.3 The larger the storage tank, the greater its cross-sectional area, and the greater the minor breathing loss. 2.4 atmospheres. The lower the atmospheric pressure, the greater the small breathing losses. 2.5 Degree of filling of the oil tank. When the oil tank is full, the volume of the gas space is small, resulting in low minor breathing losses. Measures taken: The oil stored in this project is crude oil, and the pipelines of various storage tanks are interconnected. Internal floating roof tanks are used, and cooling water is sprayed regularly during summer to prevent minor breathing losses; as a result, the amount of loss due to minor breathing of crude oil is low. 2.6 Losses caused by inadequate sealing of tank accessories. Measures taken: Strengthening the maintenance of equipment attached to oil tanks, ensuring their airtightness, and improving the operational management of these tanks – all as the most cost-effective and effective ways to reduce losses and prevent pollution. The flame arrestors, liquid seal oil, mechanical breathing valve discs, fire-resistant foam glass chambers, and oil measurement ports are thoroughly inspected twice a year to ensure that their airtightness meets the required standards. Fill the oil tank to the allowable level as much as possible; the lower the fill level, the greater the losses. 2.7 Losses during crude oil loading Losses during crude oil loading refer mainly to the losses that occur during the process of filling crude oil into tank trucks. 50% of the crude oil is transported by pipeline and then loaded onto ships, while 50% is transported by truck. The loading is done in a balanced manner, with a backpressure system in place, resulting in minimal losses. Loading is done by immersion loading. Measures taken: Use of dip pipes for loading, as well as submerged loading.