The design buoyancy of a prefabricated internal floating roof should not be less than twice its own weight plus the frictional force between the sealing device and the tank wall. The design buoyancy of a fully liquid internal floating roof should not be less than 2.5 times its own weight plus the frictional force generated when the floating roof rises. When using an efficient fully compensated elastic sealing device, a gas phase pressure balance facility should be installed between the edge sealed gas phase space and the gas phase space above the internal floating roof to control the pressure difference between the two below the set value. If a closed internal floating roof storage tank (i.e. with nitrogen sealing without ventilation holes) is used, the compression effect of the gas phase space pressure on the thickness direction of the internal floating roof should be considered when designing the internal floating roof structure. The sentence is:
Explanation: If the airtightness of the high-efficiency fully compensated sealing device is very good, there is always a large pressure difference between the gas phase space above the liquid level on the lower side of the fully compensated sealing sliding elastic plate and the gas phase space above the floating roof. This pressure difference is close to the gas phase space pressure of the inner floating roof storage tank (there is a certain slight positive pressure in the gas phase space above the liquid level on the lower side of the elastic plate, which is established when the liquid level rises to the lower surface of the floating roof). If considering the gas phase space pressure of the storage tank at 2000Pa, in order to balance the pressure difference, the immersion depth of the inner floating roof needs to be increased by 200mm (calculated based on the density of water, if calculated based on the density of gasoline, the immersion depth needs to be increased by 285mm). The immersion depth has greatly exceeded the height of the inner floating roof, that is, the liquid level has exceeded the upper surface of the inner floating roof, resulting in a sinking accident of the inner floating roof. Therefore, this pressure difference must be eliminated to ensure the safe operation of the inner floating roof 1.2 Analysis of immersion depth for nitrogen sealed internal floating roof storage tanks using "fully connected liquid efficient internal floating disc+large compensation elastic seal"
1.2.1 Analysis of immersion depth under feeding conditions
(1) The floating disc transitions from a bottom falling state to a just floating state
When feeding in the state of the floating disc falling to the bottom, the liquid level touches the bottom surface of the floating disc, and the gas phase in the sealed annular space at the edge is sealed. At this time, the pressure in the annular space and the tank is equal, represented by p0. After the gas phase in the edge sealed annular space is sealed, the liquid level in the annular space slightly rises, and the gas phase in the annular space is compressed and the pressure increases. The gas phase pressure in the annular space has an upward effect on the pressure exerted on the lower surface of the primary seal.
The gas phase pressure above the floating disc is equal to the pressure on the liquid surface inside the opening of the floating disc body. The liquid surface in the opening of the floating disc body rises rapidly, and the liquid surface in the opening of the disc body (such as the oil measuring hole and the vent hole) is higher than the liquid surface in the primary sealed annular space. The pressure difference generated by the liquid surface height difference is equal to the pressure difference between the gas phase in the primary sealed annular space and the gas phase on the liquid surface inside the opening of the floating disc body. When the liquid level in the opening of the floating disc reaches the required immersion depth h for floating, the sum of the buoyancy force of the liquid and the lifting force of the annular sealing gas on the floating disc is equal to the sum of the weight and friction force of the floating disc. The floating disc transitions from the bottom state to the initial floating state. At this time, the immersion depth in the sealed annular space is h ^ -1, and the gas phase pressure in the annular space is p ^ -1. The process from the liquid level touching the bottom of the floating disc to just starting to float can be regarded as an instantaneous jumping process. During this process, the volume of gas phase in the tank can be regarded as unchanged, and the gas phase pressure in tank 1 remains at p0 without any increase.
(2) The floating state of the floating disk rises, and the gas phase pressure of the storage tank increases
As the float rises and/or the gas phase temperature above the inner float rises, the gas phase space above the float is compressed and the pressure increases. When the pressure in the gas phase space above the float reaches the set exhaust pressure value of the storage tank, it can be discharged, so the storage tank pressure will not continue to rise and will stabilize at the exhaust pressure value.
The immersion depth in the edge sealed annular space has increased, and the gas phase is compressed, resulting in an increase in pressure. However, the increase in gas phase pressure in the annular space is smaller than that in the upper part of the floating disc. The pressure difference between the gas phase in the edge sealed annular space and the gas phase in the upper part of the floating disc decreases, and the height difference between the liquid level in the opening of the floating disc body and the liquid level in the first sealed annular space decreases. The gas phase pressure in the edge sealed annular space is always higher than that in the upper part of the floating disc, and the liquid level is always lower than that in the opening of the floating disc body; The lifting force generated by the pressure difference between the upper and lower surfaces of the edge seal is slightly reduced, and the liquid buoyancy required for floating the floating disc is slightly increased. The immersion depth h required for floating the floating disc is extremely small, and it is almost imperceptible.
