Information on vertical storage tanks

I’m sorry; the internet connection was poor just now. I thought it hadn’t been sent successfully, so I sent it again

The medium mentioned by the poster doesn’t seem to fall under the category of pressure vessels. I’m not sure what the poster intends to do or what the issue is. It’s best to do it by yourself first

Large storage tanks have advantages such as saving steel, occupying less space, reducing investment costs, being easy to operate, and being convenient for management. With the rapid development of the national economy, oil storage tanks in our country are becoming increasingly large in scale. China’s first large-scale steel floating roof crude oil storage tank with a capacity of 100,000 cubic meters was introduced from Japan in 1985. Developed countries have been constructing and using large storage tanks for nearly 30 years, while China is still in the initial stages of this development. There are many factors that affect the safe operation of large storage tanks; once an accident occurs, it can lead to serious consequences with extremely heavy losses. Therefore, there is an urgent need to summarize experience in a timely manner and propose improvement measures. The author analyzes the main security issues involved and proposes countermeasures to provide a reference for engineering design. 1 Hazard analysis of large-scale crude oil storage tank projects 1.1 Hazard analysis of crude oil Crude oil is a Class A flammable liquid, meaning it is combustible; its explosive range is narrow although the values are low, which gives it a certain degree of explosion risk. Additionally, the tendency of crude oil to boil over requires special attention during firefighting operations. 1.2 Analysis of the causes of fire and explosion accidents: The properties of crude oil determine that the risk of fire and explosion is the main and most significant hazard factor for large crude oil storage tanks. The three necessary conditions for a fire to occur are: a heat source, combustible material, and air. The issue of ignition sources is mainly addressed by strengthening management, while the problem of flammable material leaks must be prevented and controlled during the design of storage tanks. Crude oil that has leaked and is exposed to air becomes a combustible material. Crude oil leaks occur fairly frequently during storage and transportation. The main causes include oil leakage due to overfilling of tanks, oil leakage resulting from dehydration, oil leakage caused by damage to equipment, pipelines, and valves, as well as oil and gas evaporation due to poor sealing. In addition, there is a possibility of major leakage incidents such as weld failures at the bottom of tanks or the sinking of floating decks. Corrosion is one of the key factors that lead to leaks. There have been numerous oil leakage accidents at home and abroad caused by corrosion at the bottom of oil tanks. The results of a preliminary investigation into the corrosion conditions in crude oil storage tanks indicate [1] that corrosion at the bottom of the tanks is severe, manifesting as ulcerative pitting corrosion, primarily occurring in the weld heat-affected zones, depressions, and areas with deformation. Corrosion at the top of the tanks is less severe; it takes the form of uneven general corrosion accompanied by pitting. Corrosion on the tank walls is mild, appearing as uniform pitting, mainly occurring at the oil-water interface and the oil-air interface. Relatively speaking, external corrosion at the bottom of the storage tank is more severe, mainly occurring on the side where the edge plate contacts the ring beam foundation. The floating disk sinking accident is one of the serious and severe equipment accidents that must be avoided at all costs during the operation of floating-roof oil tanks. The occurrence of such accidents reflects serious deficiencies in design, construction, management, etc., and at the same time leads to massive crude oil leaks, severely disrupting production, polluting the environment, and creating fire hazards. The cause analysis of the floating pad sinking incident is shown in Figure 1 (omitted). Figure 1 Analysis of the causes of floating platform sinking accidents 2 Major safety issues in the design of large crude oil storage tanks and their countermeasures 2.1 Tank foundations and footings The geological investigation for tank foundations and the design of these footings are the most fundamental guarantees for the safe operation of large storage tanks. According to the petrochemical industry standards [2], engineering geological surveys must be conducted during the site selection process. The conditions of ordinary foundations, soft soil foundations, mountainous area foundations, and special types of foundations need to be assessed separately, with corresponding foundation treatment methods proposed. Additionally, an evaluation of the seismic effects on the site and foundations is necessary, in order to avoid constructing on foundations with uneven hardness or within the influence range of active geological fault zones. Common forms of tank foundations include the ring wall (beam) type, outer ring wall (beam) type, and slope protection type. The selection should be based on geological conditions. The tank foundation must possess sufficient overall stability and uniformity, as well as adequate planar bending stiffness. The stiffness of the foundation structure directly beneath the tank walls should be enhanced, and the subgrade supporting the base plate should be flexible enough to absorb welding-induced deformations. A waterproof and oil-proof layer, along with oil leakage detection pipes, should be installed. The distance between the groundwater level and the top surface of the foundation must not be less than the height that capillary action can reach (usually 2 m) [3]. 2.2 Sealing devices for floating roof tanks: Fires involving the sealing rings of floating roof tanks occur fairly frequently, mainly due to poor sealing, which leads to high concentrations of oil and gas. Further reasons mainly include: a. During construction, deviations in the ellipticity, verticality, and local unevenness of large storage tanks are inevitable ； b. During the operation of the storage tank, factors such as the medium involved, climate conditions, temperature, and settlement of the tank’s foundation can cause changes in the geometry and dimensions of the tank and its floating roof ； c. Deformation of the existing sealing rubber caused by sunlight exposure, wind erosion, and high temperatures that may result from the wax-polishing mechanism ； d. Factors such as wind force and the inflow and outflow of medium cause the floating disk to \"drift\" inside the tank. Therefore, the reliability and tightness of the sealing device play an important role in reducing liquid evaporation and ensuring safe operation. To further address the shortcomings of the commonly used sealing devices at present, a new type of \"roller skeleton seal\" has been developed in China [4]. This seal utilizes several arc-shaped sealing skeletons that are connected by shafts, allowing the sealing skeletons to change shape along with the storage tank under the action of spring force, similar to a chain. Rollers are installed at the ends of the skeleton; as the floating roof moves up and down, these rollers move along the tank wall, maintaining a constant distance between the sealing skeleton and the tank wall. This device has multiple functions such as rain protection, wax removal, and dual sealing. 2.3 Signal alarms and interlock systems: Large crude oil storage tanks have a fast rate of oil intake and discharge. To prevent accidents such as the tank overflowing or the floating disk hitting the bottom, these tanks should be equipped with high and low liquid level alarm devices. The alarm settings should ensure that the liquid level does not exceed the limit within (10–15) minutes of the alarm being triggered. In addition, interlock devices should be installed to shut off the oil intake (or discharge) valves when the liquid level reaches its limit. A fixed combustible gas detection and alarm system should be installed within the fire dike of the crude oil storage tank. The distance between the tank’s drainage outlets, sampling ports, or the flanges and valves at the bottom (sides) of the tank and the detectors should not exceed 15 meters. It is recommended to install fixed combustible gas detection and alarm systems every 30 meters around the sealing ring at the top of the storage tank. In addition, fire alarm devices and industrial monitoring systems should also be installed around the sealing ring at the top of the storage tank, so as to detect fires early, extinguish them promptly, and minimize the damage caused by fires. 2.4 Design requirements for preventing the floating disk from sinking to the bottom: During normal operation, the floating disk on a floating roof tank can float freely as the liquid level of oil inside the tank changes. When the gravity on the floating disc increases or the disc becomes stuck due to external forces and cannot move freely, rapid oil withdrawal will cause the disc to be submerged, eventually leading to it sinking to the bottom. 2.4.1 Wax-scraping mechanism: The crude oil produced in oil fields such as Daqing, Shengli, and North China in China usually contains a high proportion of wax. When the oil temperature drops, this wax tends to precipitate first and accumulate on the walls of the tank. If no wax-scraping mechanism is in place or if it is not effective, as the floating deck descends, the wax accumulated on the upper part of the tank walls, under the influence of sunlight and its own weight, will fall onto the floating deck. This not only increases the weight of the floating deck but, more importantly, under the force of flowing water, the wax will be discharged through the central drainage pipe. Due to the high freezing point of wax, this can easily cause blockages in the central drainage pipe, preventing large amounts of rainwater from being drained promptly and leading to the sinking of the floating deck. Therefore, for the storage and transportation of heavy crude oil, attention must be paid to the design of the wax-scraping mechanism, while also striving to minimize the accelerated aging effect that the wax-scraping heating system may have on the sealing devices. 2.4.2 Central Drain Pipe The central drain pipe plays an important role in quickly draining the water accumulated on the top of the tank. It expands and contracts as the floating pad moves up and down, and is prone to plastic deformation when subjected to continuous tension or compression. Therefore, during design, its quality grade and technical standards should be appropriately increased to ensure flexibility and durability. 2.4.3 Other factors that affect the flexibility of the floating deck’s movement include: sealing devices, guiding devices, oil measurement pipes, floating deck walkway tracks, corrosion of the floating deck’s compartments, as well as deviations in the verticality of the tank walls caused by uneven settlement of the foundation. All of these aspects should be given due consideration in the design process. 2.5 Drainage design: Focuses on the issue of drainage ditches for fire dikes. The function of a fire dike is to prevent the liquid that leaks out of an oil storage tank in the event of an explosion or tank failure from spreading everywhere and causing a large-scale fire. Therefore, the fire dike should have good sealing properties. The current Code for Design of Oil Storage Tanks stipulates that at the points where the rainwater drainage pipes in the oil tank area pass through the fire dike, a closed device that can be operated from outside the dike must be installed. ”Research shows that current measures to prevent flammable liquids from leaking outside the dike mostly rely on movable gates. Open it when it rains, and close it after the water has drained. If an oil spill occurs due to the gate not closing in time, or if sewage accumulates because the gate does not open promptly when it rains, this will affect accident control or have a negative impact on production. Furthermore, when an oil tank catches fire and ruptures, the gate is in a closed position, keeping the oil contained within the fire dike. As firefighting operations proceed, large amounts of cooling water and the water released by foam will cause the oil level to rise, eventually causing it to overflow the fire dike. Therefore, it is recommended to install a fire and oil barrier drainage system outside the fire dike, consisting of a water seal well and an oil cutting and collection device [5]. This approach eliminates the need for manual intervention; it solves the drainage problem altogether, and at the same time allows for the recovery of some of the oil in case of an accident, thereby reducing the losses and damage caused by fires. This system requires an accident fluid storage tank of sufficient capacity to recover the oil. 2.6 Corrosion prevention measures: A certain thickness of brine always accumulates at the bottom of crude oil storage tanks. When storing heavy oils or those with high sulfur content and acid values, higher requirements are placed on corrosion prevention. Although there is currently no unified standard for the anti-corrosion design of storage tanks, for large crude oil storage tanks with huge storage capacities and high corrosivity, the importance of thoroughly designing and implementing anti-corrosion measures is self-evident. 2.6.1 Corrosion protection of the outer wall at the bottom of the tank: In addition to applying conventional external corrosion protection coatings to the outer wall at the bottom of the tank, it is advisable to follow the petroleum and natural gas industry standard SY/T0088-95 \"Technical Standards for Cathodic Protection of the Outer Wall at the Bottom of Steel Storage Tanks.\" Sacrificial anodes or forced-current cathodic protection methods can be used; such anodes can also serve as lightning and static discharge grounding electrodes for the tank. It is important to note that the traditional copper grounding electrode must be replaced – as copper acts as the cathode in this case, while the steel of the tank becomes the anode, thereby accelerating corrosion. It is advisable to use zinc or magnesium electrodes instead. 2.6.2 Corrosion protection of the inner wall at the bottom of the tank: Even with coating-based corrosion protection, it is still necessary to consider the use of sacrificial anodes, depending on the circumstances, in order to reduce corrosion in the event of defects in the coatings. Anti-static anti-corrosion coatings must never be used for coating, as using them in combination with a sacrificial anode will accelerate the dissolution of the anode, thereby eliminating its proper cathodic protection function [6]. Aluminum (Al)-based alloy anodes are recommended for sacrificial anodes on the inner wall. 2.6.3 Corrosion protection of tank walls: The focus of corrosion protection for tank walls is in the area corresponding to the height of the water layer at the bottom. An epoxy-based insulating coating suitable for oil and saline-resistant use should be applied to an area of 1 meter high on the inner wall at the bottom of the tank, while antistatic coatings specific for oil tanks can be used in other areas. 3 Conclusions Based on the analysis of the hazards associated with large-scale crude oil storage tank projects, as well as an examination of the main safety issues in their design, the following conclusions are drawn: 1) Storage tanks should be avoided being constructed on foundations with uneven hardness, or within the influence area of active geological fault zones ； 2) The sealing device should have multiple functions such as rain protection, wax removal, and double sealing ； 3) Fixed combustible gas detection and alarm devices, fire alarm systems, and industrial surveillance television systems should be installed on the tops of large oil tanks ； The storage tank should be equipped with high and low liquid level alarm devices as well as liquid level limit interlock shut-off devices ； 4) When storing crude oil with a high wax content, it is essential to pay attention to the design of the wax-scraping system, and to minimize any accelerated aging of the sealing devices that may result from the wax-scraping heating system ； 5) The central drainage pipe is crucial; its quality grade and technical standards should be appropriately improved ； 6) It is recommended to install a fire suppression, oil separation, and drainage system outside the fire dike, consisting of a water seal well and an oil cutting and collection device ； 7) It is recommended to use sacrificial anodes and forced-current cathodes for lightning and static electricity protection grounding of storage tanks ； Aluminum (Al)-based alloy anodes are suitable as sacrificial anodes for the inner wall ； The inner wall of the tank bottom can be coated with an epoxy-based insulating coating designed for oil tanks, which is resistant to oil and saline water; other areas can be coated with an antistatic coating specific for oil tanks. Author Introduction: Liu Minyan (Associate Researcher), Li Qingxiang (Engineer). Affiliation: Institute of Water Transport Science, Ministry of Transport. References: 1 Huang Zhi. Corrosion and Protection in Oil Tanks. Oil & Gas Storage and Transportation, 1996, 15(7): 15–17. 2 SH3068-95 Code for Design of Foundations of Steel Storage Tanks in Petrochemical Enterprises. 3 Sun Weili. Structural Characteristics of Large Oil Tanks and Their Requirements for Foundations. Oil & Gas Storage and Transportation, 1993, 12(6): 28–31. 4 Chen Jianping. A New Type of Sealing Device for Floating Roof Tanks. Oil & Gas Storage and Transportation, 1998, 17(9): 21–23. 5 Fu Zhimin et al. The Impact of Fire Dikes and Drainage Systems in Oil Tank Areas on Fire Safety. Oil & Gas Storage and Transportation, 1998, 17(4): 43–45. 6 Li Genzhao. Long-term Anti-corrosion Measures for Crude Oil Storage Tanks. Oil & Gas Storage and Transportation, 1996, 15(5): 16–19.

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