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 on the safe operation of internal floating roof storage tanks. 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 vent holes), 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\", and clear specifications are given for the opening area and installation location of these vents. (This is the case in countries around the world, with similar requirements; China’s design standards are based on API650.) See the table below: Location and total area of peripheral vent holes, as well as hole spacing. Sequence number, name and code of the standard, location of peripheral vent holes, total area of these holes, hole spacing and quantity. 1 GB50341-2014 Design Code for Vertical Cylindrical Welded Steel Storage Tanks: These vent holes should be installed on the tank wall or fixed roof above the designed liquid level. The total area of the vent holes must be A≥0.06D, where D is the inner diameter of the tank in meters. The spacing between vent holes should not exceed 10 meters; at least 4 holes 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 must still retain their function of preventing the accumulation of combustible gases and allowing pressure relief. However, in domestic applications of this regulation, the function of \"peripheral ventilation holes\" is understood merely as a normal ventilation function during tank operation; they are installed when sealing is not required, and there are no regulations 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 the even distribution of their installation location around the tank 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 thereby reducing the severity of the accident. Filling with an inert gas for protection prevents the formation of a space within 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 storage tank; the formation of a space within 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), Amount of air drawn in (negative pressure, m3/h), Amount of air expelled (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 such temperature drops are caused by climatic factors, they affect tanks in a wide area; dozens or even hundreds of tanks in the same region may need to have inert gas added to them at the same time, which is impossible to accommodate in practical terms. 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 warrantable discussion to stipulate unconditionally that floating roof tanks with sealing requirements should not be equipped with \"peripheral vent holes\". 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 activated, but they must still be installed. Therefore, sealing the \"peripheral vent holes\" of internal floating roof storage tanks has an impact on the safe operation of these tanks; it represents a serious safety hazard that should be rectified. If there are any mistakes in the above opinions, I welcome everyone’s criticism and suggestions.