Which storage tanks require a nitrogen sealing system

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According to the \"Code for Fire Protection Design of Petrochemical Enterprises (2018 Edition)\” (GB50160-2008), 6.2.2 states that for storing liquids of Class A B and Class B A, floating roof tanks or internal floating roof tanks with metal floating decks should be used. For materials with special requirements or storage tanks with a volume of 200 m3 or less, other types of storage tanks can be used after implementing appropriate safety measures. The floating roof should be selected based on the properties of the flammable liquid and the strength of the material used, and it must comply with the following requirements: 1 When internal floating roof storage tanks with a single tank volume of 5000 m3 or less use floating roofs made of fusible materials, safety measures such as nitrogen protection should be implemented ； →A fusible material floating roof refers to a floating platform made of fusible metal or fusible alloy materials. For internal floating roof storage tanks with a single-tank capacity of more than 5000 m3, steel single-plate or double-plate floating roofs should be used ； 3 Floating roof storage tanks with a single tank volume of 50,000 m3 or more shall use steel double-disc floating roofs. Explanation of the provisions: Storing Class A-B and Class B-A liquids in floating roof tanks or internal floating roof tanks can reduce the likelihood of fires in these storage tanks and lessen the severity of such fires. There is virtually no gas space inside the tank; in the event of a fire, it burns only at the seal between the floating roof and the tank wall. The fire is not intense, making it easy to extinguish, and it also helps to reduce oil and gas losses as well as pollution of the atmosphere. Given that shallow-plate floating pads have been phased out, it is stipulated that metal buoyant tank-type floating pads must be used to avoid employing shallow-plate floating pads. Metal floating tank types include steel floating tanks and those made of fusible materials (such as aluminum floating tanks and stainless steel assembled floating tanks). The selection of floating discs is greatly influenced not only by the properties of the flammable liquid (including its physical and chemical properties as well as its corrosiveness) and the strength of the material itself (carbon steel, alloy steel, or austenitic stainless steel), but also by safety and environmental considerations aimed at controlling volatile gas emissions at the source and reducing environmental pollution. For liquid materials of Class A B and Class B A with special requirements, such as styrene, esters, hydrogenation feedstocks, acrylonitrile, and other liquids that are prone to polymerization, oxidation, or are toxic, it is also feasible to use fixed-roof storage tanks or horizontal storage tanks with nitrogen blanketing for storage ； For materials with high saturated vapor pressures such as degumming oil and light naphtha, they can be stored in fixed-roof tanks by cooling down, or in low-pressure fixed-roof tanks.     For storage tanks with a volume of 200 m3 or less, the range of floating roof or internal floating roof types is limited, making it difficult to choose an appropriate option. This is especially true for vertical storage tanks with a volume of 100 m3 or less – such tanks are difficult to manufacture and may not be cost-effective. To meet the requirements for storing small quantities of materials and the needs of fine chemical processing units, fixed-roof tanks with gas sealing or horizontal storage tanks can be used. In internal floating roof storage tanks with a volume of more than 5000 m2, if fusible material floaters are used, a fire can easily spread across the entire tank area in the event of a fire, making it difficult to extinguish. Furthermore, in view of comprehensive safety and environmental protection considerations, it is stipulated that floating roof or internal floating roof storage tanks with a volume greater than 5000 m3 shall use single-disc or double-disc steel floating roofs. 6.2.4A Fixed-roof tanks for heavy oil with a storage temperature exceeding 120°C shall be provided with nitrogen protection, and floating-roof tanks in areas prone to lightning strikes with a single-tank volume of 50,000 m3 or more shall have measures taken to reduce the space between the primary and secondary seals. Explanation of the provisions: Heavy oil stored at temperatures above 120°C has a relatively low risk of fire, but in the event of a fire, there are no reliable and effective measures or methods for extinguishing it. Therefore, it is stipulated that nitrogen protection should be provided; in the early stages of a fire, nitrogen is introduced into the storage tank as an auxiliary means for extinguishing the fire, thereby reducing the amount of air that can enter the tank and contribute to combustion, which would otherwise make it more difficult to control the fire. According to the \"Fire Protection Code for Engineering Design of Fine Chemical Enterprises\" (GB51283-2020), 6.2.2 states that for the storage of Class A B and Class B A liquids with a single tank volume of 100 m3 or more, internal floating roof tanks should be used. When using fusible materials to make floating pads, safety measures such as nitrogen protection should be implemented. When using fixed-roof tanks or low-pressure tanks, they should be sealed with nitrogen or an inert gas, and measures should be taken to reduce temperature rises caused by sunlight exposure. Explanation of the provisions: There is virtually no gas space inside a floating roof tank. In the event of a fire, combustion occurs only at the seal between the floating roof and the tank wall; thus, the fire is minor and easy to extinguish. This also helps to reduce the loss of flammable vapors and pollution of the atmosphere. For the storage of Class A B and Class B A liquid materials with a single tank volume of less than 100 m3, or those with special requirements such as being prone to oxidation or polymerization, fixed-roof tanks sealed with nitrogen or inert gases (or even accompanied by a water seal) can be used. For example, carbon disulfide is a Class A B flammable liquid; it has a low auto-ignition point, boiling point, and flash point, is highly volatile, flammable, and explosive. It is heavier than water and insoluble in water, so storing it in fixed-roof tanks with both a water seal and a nitrogen seal is safer and more reliable than using simply internal floating-roof tanks.     Implementing measures to reduce the temperature rise of storage tanks due to sunlight is aimed at preventing the evaporation of oil and gas and improving the safety of these tanks. Common measures include fixed cooling water spray (mist) systems, gas condensation recycling, and application of thermal insulation coatings. When the storage tank is equipped with an insulating or thermal-insulating layer, the impact of sunlight on the tank is minimal, so there is no need to take any additional measures to protect it from sunlight. GB50016-2014 \"Code for Fire Protection Design of Buildings (2018 Edition)\\" and AQ3053-2015 \"Safety Technical Regulations for Vertical Cylindrical Welded Steel Storage Tanks\" do not mention the requirement specified in SH/T3007-2014 \"Code for Design of Tank Areas in Petrochemical Storage and Transportation Systems\": For storing Class A/B liquids with a boiling point below 45°C or a saturated vapor pressure greater than 88 kPa at 37.8°C, pressure tanks, low-pressure tanks, or atmospheric-pressure tanks with cooling systems should be used, and the following requirements must be met: a) When using pressure tanks or low-pressure tanks, measures must be taken to prevent air from entering the tanks, and the gases discharged from the tanks must be collected and treated in a sealed manner ； b) When using atmospheric-pressure storage tanks with cooling for storage, one of the following measures should be taken: -- Use internal floating-roof tanks and install a nitrogen or other inert gas sealing system to control the storage temperature so that the vapor pressure of the liquid does not exceed 88 kPa ； --Fixed-roof storage tanks shall be used, along with a nitrogen or other inert gas sealing system, to keep the storage temperature 5°C or lower than the liquid’s flash point ； --Fixed-roof storage tanks are used, along with a nitrogen or other inert gas sealing system to maintain the storage temperature at such a level that the vapor pressure of the liquid does not exceed 88 kPa. The gases emitted from the tank are collected and treated in a sealed manner. 4.2.5 For storing Class A B and Class A C liquids with a storage boiling point of 45°C or higher, or with a saturated vapor pressure of not more than 88 kPa at 37.8°C, floating roof tanks or internal floating roof tanks should be used. For other liquid chemical products classified as Class A-B and Class B-A that require special storage conditions, fixed-roof storage tanks, low-pressure storage tanks, or horizontal storage tanks with a capacity of 100 m3 or less can be used; however, the following measures must be taken: -- Install a nitrogen or other inert gas sealing system to contain and treat the gases emitted from the tank ； --Install a nitrogen or other inert gas sealing protection system to keep the storage temperature 5°C or lower than the liquid’s flash point. 4.2.6 Floating roof tanks, internal floating roof tanks, fixed roof tanks, and horizontal tanks can be used for storing Class A and Class C liquids. 4.2.7 For storage tanks with a capacity of 100 m3 or less, horizontal storage tanks can be used. 4.2.10 Storage tanks for Class A B and Class A C liquids with toxicity levels I and II shall not exceed 10,000 m3, and shall be equipped with a nitrogen or other inert gas sealing protection system. GB 50074-2014 \"Code for Design of Oil Depots\" 6.1.2 For storing Class A-B liquids with a boiling point below 45°C or a saturated vapor pressure greater than 88 kPa, pressure tanks, low-pressure tanks, or low-temperature atmospheric-pressure tanks shall be used, and the following requirements shall be met: 1 When using pressure tanks or low-pressure tanks, measures shall be taken to prevent air from entering the tanks, and the gases discharged from the tanks shall be collected and treated in a sealed manner. 2 When using storage tanks at low temperature and normal pressure, one of the following measures should be taken: 1) For internal floating roof tanks, a nitrogen sealing system should be installed, and the storage temperature should be controlled so that the vapor pressure of the liquid does not exceed 88 kPa ； 2) When using fixed-roof storage tanks, a nitrogen sealing protection system should be installed, and the storage temperature should be kept 5°C or lower than the liquid’s flash point. 6.1.3 For storing Class A and B liquid chemical products as well as light naphtha with a storage boiling point of not less than 45°C, or a saturated vapor pressure at 37.8°C of not more than 88 kPa, external floating roof tanks or internal floating roof tanks should be used. When special storage requirements exist, fixed-roof storage tanks with a capacity of 10,000 m3 or less, low-pressure storage tanks, or horizontal storage tanks with a capacity of 100 m3 or less can be used. However, one of the following measures must be taken: 1) A nitrogen sealing system should be installed to ensure that the gases released from the tank are captured and treated; 2) A nitrogen sealing system should be installed, and the storage temperature should be kept 5°C or lower than the liquid’s flash point.     6.1.8 The capacity of a single storage tank for Class A B and Class A C liquids with toxicity levels I and II shall not exceed 5000 m3, and a nitrogen purge system shall be installed. Note: In some chemical enterprises built in the early days (or the hazardous chemicals sections of regular enterprises), due to imperfect design at that time, atmospheric pressure storage tanks that were not designed properly (or issues related to the standards used in the design), or modifications made to existing tanks, there are still cases where nitrogen sealing is not installed on these tanks, or where the installation does not comply with current regulations ； If a nitrogen sealing system is added, it is necessary to strictly adhere to the requirements for change management (including review by the design unit) to avoid repeated corrections ； In many new projects, nitrogen sealing systems are also installed even when the volume of a single tank is less than 100 m3.