Introduction to information on various types of storage tanks

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I. Purpose of storage tanks: Steel sealed containers used for storing liquids or gases are known as steel storage tanks. Steel storage tank systems constitute essential and important infrastructure in industries such as petroleum, chemicals, grain and oil, food processing, firefighting, transportation, metallurgy, and national 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 equipment used for storing various liquid (or gaseous) raw materials and finished products. For many enterprises, normal production is impossible without such tanks; in particular, the storage of **strategic materials relies on tanks of various capacities and types.** In our country, oil storage facilities are mainly surface tanks, with metal structures being the most common type. II. Classification of storage tanks: Due to the different storage media, there are various types of storage tanks. 1. Classification by location: They can be categorized into above-ground tanks, underground tanks, semi-underground tanks, offshore tanks, subsea tanks, etc. 2. Classified by oil type: they can be divided into crude oil storage tanks, fuel storage tanks, lubricating oil tanks, edible oil tanks, fire water tanks, etc. 3. Classified by purpose: they can be divided into production oil tanks, storage oil 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. III. Standards for storage tanks: 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. IV. 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 5,000 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. V. Structure of storage tanks: At present, the most widely used storage tanks in China, with the most mature manufacturing and installation technologies, are dome-shaped storage tanks, floating roof storage tanks, and horizontal storage tanks. 1. Structure of dome-top storage tanks A dome-top storage tank is a steel container with a spherical roof and a cylindrical tank body. Roof-mounted 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 roof-mounted storage tanks 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 plates can be strip-shaped plates or arc-shaped plates. Generally, when the inner diameter of the storage tank is <16.5 m, strip-shaped edge plates are recommended; when the inner diameter is ≥16.5 m, segment-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 welds, while the longitudinal welds are butt welds. 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 the same from top to bottom, making it particularly suitable for internally floating roof storage tanks; however, it requires high assembly and installation standards, and presents greater difficulty. 1.3 Tank roof: The tank roof is spherical in shape, formed by welding multiple fan-shaped plates together. Inside the roof, flat steel bars are used as stiffeners. Lap welds are employed between the individual fan-shaped plates. The entire tank roof is welded to the angle steel ring (also known as the locking ring) located at the upper part of the tank wall. 2. Structure of floating roof tanks Floating roof tanks are composed of a floating roof that floats on the surface of the medium, along with 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 the floating roof tank is open; to increase the stiffness of the wall panels, wind-resistant ring beams and reinforcing rings must be installed at the top of the tank walls, depending on the wind load in the area. 2.3 Floating roof: Floating roofs come in various types, such as single-deck floating roofs, double-deck floating roofs, and pontoon-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 the inner side of the annulus. 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 partitions and circumferential partitions. Its advantages are high buoyancy and excellent drainage performance. 3. Structure of internal floating roof tanks Internal floating roof tanks are created by adding a floating roof inside an arch-shaped roof tank. The floating roof installed inside helps to reduce the loss of vaporized material, while the external arch-shaped roof prevents rainwater, snow, dust, and other contaminants from entering the tank, thus ensuring that the material stored inside remains clean. Such storage tanks are primarily 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 ； The other type is a prefabricated aluminum alloy floating roof. 4. Construction of horizontal storage tanks. The volume of horizontal storage tanks is generally less than 100 m³; they are typically used in production processes or at gas stations. For horizontal storage tanks, lap joints are used for circumferential welds, while butt joints are used for longitudinal welds. The ring plates are arranged interactively in an odd number so that the diameters of the end caps are identical. The end covers of horizontal storage tanks are divided into flat end covers and dished end covers. Horizontal storage tanks with flat end covers can withstand an internal pressure of up to 40 kPa, while those with dished end covers can withstand an internal pressure of up to 0.2 MPa. Underground horizontal storage tanks must be equipped with reinforcing rings, which are fabricated from angle steel. VI. Selection of storage tank foundations: 1. When the underlying soil layer can meet the requirements regarding bearing capacity and settlement, and there are no constraints on the site, a slope-type or outer ring wall-type foundation is advisable ； 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 external ring wall foundation ； 4. When space is limited, a ring wall foundation can be used. VI. 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 being greater than 500 mm thick ； 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 storage 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\". VII. Manufacturing and Installation of Storage Tanks 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 upside-down construction method, the normal construction method, and special construction methods. VIII. Tank Accessories 1. Tank accessories are an important part of the 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 ladders and spiral ladders. 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 650mm. 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 typical vertical oil tanks, the manhole is located on the lowest ring plate of the tank wall, opposite to the lighting opening above the tank roof. 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 capacity of less than 3,000 cubic meters are equipped with 1 manhole, those with a capacity of 3,000 to 5,000 cubic meters have 1 to 2 manholes, while oil tanks with a capacity of over 5,000 cubic meters must have 2 manholes. 5. Light-transmitting hole: Common specification: DN500. A light hole, also known as a ventilation hole, 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 pipes and automatic water cut-offs: Dewatering pipes, also known as drain pipes, are specifically designed to remove water impurities from the tank and clear away contaminated 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 top 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 site. 11. Grounding wire: The grounding wire is a device used to eliminate static electricity in oil tanks. 12. Accessories specific to 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 accessories designed specifically for them, in order 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 valve 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 arrestor: Also known as a tank flame arrestor, it is a fire safety device for oil tanks. It is installed below the mechanical breathing valve or hydraulic safety valve, and its interior contains 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 Spray cooling device: The spray cooling device is an energy-saving facility installed to reduce the oil temperature inside the tank and minimize the breathing losses associated with the size of the tank. 13. Special accessories for internal floating roof tanks. Compared with ordinary dome-shaped tanks, internal floating roof tanks have unique various special accessories due to their different structure and the requirements of their performance. 13.1 Vent holes: In internal floating roof tanks, the internal floating disk covers the oil surface, which essentially eliminates the gas space; as a result, evaporation losses are minimal. Therefore, no mechanical breather 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 joints where attachments are attached. 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 filling and draining 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 disc. 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 roof can only move vertically, thereby preventing the floating disk from rotating. 13.4 Automatic vent valve: The automatic vent valve is located in the middle of the floating roof. It is designed to ensure that the floating roof can “breathe” normally when it is in a supported position during the loading and unloading of oil from the tank, thereby preventing vacuum formation or pressure buildup beneath the floating roof. 13.5 Floating pad 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 pad. 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. IX. Emissions during crude oil storage and loading: These mainly include: large and small breathing losses from storage tanks, leaks and spills of oil products, and losses during loading. 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, and the pressure inside the tank increases. 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, and this continues until oil pouring into the tank stops. The oil vapors that are released cause losses due to the evaporation of oil. When the oil tank dispenses oil, as the oil level continues to drop, the gas space inside 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 inlet and outlet speeds, 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 virtually 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 product stored in this project is crude oil, which has lower volatility compared to gasoline and diesel. All storage tanks are interconnected via pipelines, and internal floating roof tanks are utilized; therefore, the amount of vapor loss from the crude oil is relatively small. 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 rate of oil evaporation, the concentration of oil vapor, and the vapor 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 respiratory loss are as follows: 2.1 Fluctuations in day-night temperature differences. The greater the variation in day-night temperature differences, the greater the minor respiration losses. 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 Level 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 the crude oil is low. 2.6 Losses caused by poor 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 cheapest and most effective ways to reduce losses and prevent pollution. The flame arresters, liquid seal oil, mechanical breather valves, 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 Crude oil loading losses Crude oil loading losses 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 back-pressure 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.