Issues with ventilation holes in the inner ceiling of LNG atmospheric storage tanks
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For large LNG atmospheric-pressure suspended single-volume storage tanks, vents are usually installed on the ceiling to balance the pressure between the inner and outer tanks. What is the exact mechanism of these vents? If they are in direct connection, does that mean the temperature of the outer tank is the same as that of the inner tank? However, the minimum design temperature for the material of the outer tank in such tanks is around -40°C. How can it be ensured that the temperature of the outer tank remains above -40°C?
It’s not a breathing valve; it’s the same type of ventilation hole found between the ceiling and the vault of the inner tank. Its purpose is to ensure equality between the inner and outer tanks, so that the inner tank is not under pressure
Foreign-made products don’t have these ventilation holes; they are something that some domestic manufacturers added on their own. There’s no guarantee that such additions have any benefits, and they can even pose safety risks.
The pressure inside the atmospheric-pressure LNG tank is generally around 15 KPA; when pressure exceeds this level, safety valves come into action to release excess pressure. The interlayer is usually filled with perlite and nitrogen to maintain a slight positive pressure. As for the ventilation holes you mentioned, used to keep the internal tank and the interlayer at the same pressure level? ? ? ?
If the inner tank has an arched ceiling, it is the design approach you mentioned; but in the case of a suspended ceiling, there are differences
Insulation material is filled between the inner and outer tanks. During normal operation, a certain temperature gradient is established between the inner tank and the outer tank. Under normal conditions, the gas in the insulation layer is \"dead air\"; when the amount of gas vaporized from the inner tank is large and cannot be discharged promptly, it diffuses into the outer tank through the pores in the insulation material. At this point, the outer tank is subjected to gas pressure, while the inner tank is not. If the gas from the outer tank is continuously released due to an operational error, the temperature of the outer tank will drop. Under normal conditions, BOG is discharged from the inner tank, so the temperature between the inner and outer tanks does not decrease.
Indeed, temperature field analysis must be conducted for double-volume tanks and full-volume tanks to ensure the safety of the storage tanks.
Why are some tanks equipped with gas-phase connection between the inner and outer tanks, and even a pressure difference gauge?