The impact of inert gas sealing on the safe operation of internal floating roof storage tanks

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The impact of inert gas sealing in internal floating roof storage tanks on their safe operation. Currently, in China, there is a trend to use inert gas sealing for the \"peripheral ventilation holes\" of closed internal floating roof storage tanks, as well as to implement systems for the recovery and treatment of exhaust gases. Whether this approach is technically necessary is a matter that requires further discussion; it will not be addressed here for now. However, this method requires the sealing of the \"peripheral vent holes\" in internal floating roof tanks (or the absence of such vents), which poses a serious safety risk to the proper operation of these tanks. Thorough evaluation is necessary before making a decision in this regard. In the design of internal floating roof tanks, it is required to install \"peripheral vent holes\", with clear specifications regarding their opening area and installation location. (This is the case in countries around the world, with similar specifications; China’s design standards are based on API650.) See the table below: Location and total area of peripheral vent holes, as well as the spacing between them. Sequence number, name and code of the standard, location of peripheral vent holes, total area of these holes, spacing and number of holes. 1 GB50341-2014 Design Code for Vertical Cylindrical Welded Steel Storage Tanks: These vents should be installed on the tank wall or on a fixed roof above the designed liquid level. The total area of the vent holes should be A≥0.06D m2, where D is the inner diameter of the storage tank in meters. The spacing between vent holes should not exceed 10 meters; at least 4 vents are required, and they should be evenly distributed along the circumference. 2 According to API650-2007, for steel welded oil tanks, the peripheral vent holes should be located on the fixed roof (unless otherwise specified by the buyer). The total net opening area in the diameter direction must be greater than or equal to 0.2 ft2 per foot. The maximum spacing between vent holes (in terms of arc length) is 32 feet; however, there must be at least 4 vent holes evenly distributed. Notes: 1. GB50341 stipulates that peripheral vent holes should be provided on internal floating roof storage tanks when no sealing is required. 2. API650 requires that if peripheral vent holes are not installed as specified, inert gas protection or other measures should be employed to prevent the accumulation of flammable gases. In addition, pressure relief protection measures for emergency situations should be implemented as required. Editor’s note: API650 specifies that when no peripheral vent holes are installed, they shall still retain their function of preventing the accumulation of combustible gases and allowing pressure release. However, in domestic applications of this regulation, the function of \"peripheral vent holes\" is understood merely as a normal ventilation function during tank operation; they are installed when sealing is not required, and there are no provisions regarding their use when sealing is required, without any additional conditions (unlike API650-2007). This is a misunderstanding; if it were merely a matter of ventilation, it would not be possible to explain the following issues: a) why such a large opening area is required? b. Why should it be evenly distributed around the perimeter of the tank to seal the upper part rather than concentrated on the tank top? The ventilation effect at the top of the tank is the best. In fact, the \"peripheral ventilation holes\" are an important safety component for ensuring the safe operation of internal floating roof storage tanks. Their main role and function is to prevent and protect against emergency situations that may arise during the operation of the tanks. (Verifiable from API650) Prevention: During normal operation of internal floating roof tanks, thanks to their uniform installation around the tank’s perimeter, circulating ventilation is used to dilute the small amounts of fluid that leak from the tank’s sealing gaskets, thereby preventing the accumulation of explosive gases. Protection: In the event of an emergency, the \"peripheral ventilation holes\" can release the energy of the gases generated by the accident, preventing the tank from being torn apart and reducing the severity of the accident. Filling with an inert gas for protection prevents the formation of a space inside the tank containing explosive gases, which means that \"peripheral ventilation holes\" are not necessary. However, this conclusion must be drawn carefully, with specific considerations and additional prerequisites: under any operating conditions and in any climate conditions, air must not be allowed to enter the tank; the formation of a space inside the tank containing explosive gases is the necessary condition for not installing (or sealing) \"peripheral ventilation holes\". In addition, API650 also requires compliance with discharge requirements in emergency situations. For large internal floating roof storage tanks, inert gas protection is subject to limitations; due to factors such as weather conditions, it is not possible to provide protection 24/7 across the entire surface. When the flash point of the storage medium is below 37.8°C, the amount of air drawn into the tank due to the contraction of the gas inside the tank as a result of the maximum temperature drop in the atmosphere, and the amount of gas expelled from the tank due to the expansion of the gas inside the tank as a result of the maximum temperature rise in the atmosphere are shown in the table below: (SH/T3007-2014) Tank volume (m3), Air intake (negative pressure) (m3/h), Air exhaust (positive pressure) (m3/h): 500 – 84.5, 84.5; 1000 – 169.0, 169.0; 2000 – 338.0, 338.0; 3000 – 507.0, 507.0; 5000 – 787.0, 787.0; 10000 – 1210.0, 1210.0; 20000 – 1877.0, 1877.0; 30000 – 2495.0, 2495.0. As can be seen from the table, the amount of air drawn into the tank during a maximum temperature drop is quite large. Since temperature drops are caused by climatic factors and affect a wide area, tanks in the same region are all affected. Dozens or even hundreds of tanks may need to have inert gas added simultaneously, which is impossible to accommodate in practical engineering applications. Therefore, the conditions for a gas space with explosive potential to form inside a storage tank are absolute, while the failure to form such a space is relative. It is inappropriate, problematic, and subject to discussion to require that floating roof tanks with sealed designs not be equipped with \"peripheral ventilation holes\" under unconditional regulations. The \"peripheral vent holes\" of internal floating roof storage tanks, as safety components, are normally not in use or activated for the most part of the time; moreover, they serve to prevent the accumulation of explosive gases. For example, the safety valves on most devices have never been activated, but they must still be installed. Therefore, sealing the \"peripheral vent holes\" of internal floating roof storage tanks has an impact on their safe operation; it represents a serious safety hazard that should be rectified. If there are any mistakes in the above opinions, please feel free to offer your criticism and suggestions.