Basic knowledge of storage tanks

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Purpose of storage tanks: A steel-sealed container used for storing liquids or gases is known as a steel storage tank. Steel storage tank systems are 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 equipment 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 our country, oil storage facilities are mainly surface tanks, with metal structures being the most common type. Classification of storage tanks: Due to the varying storage media, storage tanks come in a wide range of forms. 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 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 mostly vertical tanks ； Storage tanks with a capacity of 100 m³ or less are considered small storage tanks, and they are usually horizontal in design. 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. 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 cubic meters ； The auxiliary facilities (including wind-resistant ring beams, locks, 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. Structure of storage tanks: At present, the types of storage tanks that are most widely used in China, and for which the manufacturing and installation techniques are the most mature, 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 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 arch-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 overlapped, while the longitudinal welds are butted. This form is commonly used for vault 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 butted. 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 spherical in shape, formed by welding multiple fan-shaped plates together. On the inner side of the roof, flat steel bars are used as reinforcing ribs. Overlapping welds are employed between the individual fan-shaped plates. The entire tank roof is welded to the angle iron ring at the upper part of the tank wall (also known as the locking ring). 2. Structure of floating roof tanks Floating roof tanks consist 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 are classified into single-disc floating roofs, double-disc floating roofs, and float-type floating roofs, among others. 2.3.1 Single-disc floating roof: A ring-shaped floating vessel composed of several separate compartments, with a single-disc deck on the inner side of the ring. The bottom of the single-disc roof is reinforced with multiple annular 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 vaporization of the liquid contained in the tank, while the external arch-shaped roof prevents rainwater, snow, dust, and other contaminants from entering the tank, thus ensuring that the liquid remains clean. Such storage tanks are mainly used to store light oils, such as gasoline and aviation kerosene. Internal floating roof storage tanks feature straight tank walls, with the wall plates butt-welded together; the dome is fabricated in accordance with the requirements for domed storage tanks. Currently, there are two types of internal floating roofs used in China: one is a steel floating roof, which is the same as that used in floating roof storage tanks ； Another type 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 equal. 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 reinforcing 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 external ring wall foundation ； 4. When space is limited, a ring wall foundation can be used. Construction of storage tank foundation 1. Earthwork excavation: Compaction of the foundation pit ； 2. Reinforced concrete and masonry works: (omitted) ； 3. Earthback filling: Compacted mechanically; the fill layer thickness is greater than 500 mm ； 4. Sand cushion: Use medium to coarse sand, with a laying thickness of 200–250 mm; compact it by sprinkling water and using a flat vibrator ； 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\". Tank fabrication and installation 1. The tank construction process is divided into two parts: 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 omitted). 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. 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. Anti-slip steps are provided on the top of the tank. 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 650mm. 4. Manhole: Three common specifications are available: DN500, DN600, 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 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 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 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, whose diameter is 150 mm and which is usually located at a distance of 1 m from the tank wall. Click to view--Summary of Chemical Engineering Skill Training Courses for 2023 8. Drain pipes and automatic water cutters: Drain pipes, also known as discharge pipes, are designed specifically to remove water and impurities from within tanks as well as to eliminate residual oil at the bottom of the tanks. The drain pipe is equipped with a valve on the side outside the tank; to prevent problems resulting from a faulty or damaged 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 fixed to 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 specifically for light oils Light oils (including gasoline, kerosene, diesel, etc.) are oils with low viscosity, low density, and high volatility. Oil tanks used to store such oils are equipped with various accessories designed specifically for oil tanks, which are suited to their characteristics and meet both production and safety requirements. 12.1 Tank breather valves: Tank breather valves are important devices that ensure the safe use of oil tanks and minimize 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 loading/unloading operations, it serves to ensure the safe sealing of the tank and prevent damage to it. 12.3 Flame arresters: Also known as tank flame arresters, flame arresters are fire safety devices for oil tanks. They are installed beneath mechanical breather valves or hydraulic safety valves, and contain numerous wire meshes or corrugated plates made of copper, aluminum, or other metals with high heat capacity. Should foreign flames or sparks somehow enter the flame arrester through the breather valve, the metal mesh or corrugated sheet can quickly absorb the heat from the burning material, extinguishing the flames or sparks 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 with ordinary arch-roof 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 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 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 the process of oil loading and unloading, a large amount of static charge accumulates on the floating roof of internal floating roof tanks. Since insulating materials are commonly used as sealing materials between the floating roof and the tank walls, the static charge accumulated on the floating roof cannot be discharged through the tank walls. 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 platform. 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 is designed 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 roof 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 pillars located on the floating roof. 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, 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 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 discharge 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 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 force, 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 is no oil transfer activity in the tank, the temperature of the gas space inside the tank, the evaporation rate of the oil, the concentration of oil vapor, and the vapor pressure change in response to the daily fluctuations in external temperature and pressure. The loss of oil and gas caused by this process of releasing oil vapors and inhaling 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 temperature between day and night, the greater the minor respiration losses. 2.2 Sunlight intensity in the area where the oil tank is located. The greater the sunlight 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 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, maintaining 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 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, so the amount of loss is minimal. Loading is done by immersion loading. Measures taken: Use of dip pipes for loading, and submerged loading method.