Consult regarding the issue of avoiding excessive vapor release during LNG unloading.
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Dear experts, I’m sorry – I’ve only been in this industry for a short time and there are some issues I can’t figure out, so I’ve come to seek your advice. Please help me out. Parameter: On the ship’s side, the temperature is about 4°C higher than that on the shore side, and the density is lower than that of the tanks on the shore side. About 4KG. The BOG treatment unit can handle about 6 tons per hour. Under normal circumstances, when the temperature difference is not so large, the fluid with higher density should flow upward into the inlet, while the fluid with lower density should flow downward into the inlet. But now, with a temperature difference of 4°C, since the density on the ship’s side is lower than that on the shore side, liquid flows in and mixes thoroughly with the colder LNG inside the storage tank, which results in a large amount of BOG. The recovery system is not able to capture all of it. Are there any good suggestions? I would appreciate some guidance from industry experts.
The pressure on the ship is different from the pressure in the storage tank. When the liquid on board enters the storage tank, it undergoes throttling flash evaporation. The temperature of the liquid after flashing will not differ significantly from the temperature of the liquid in the tank. It is not possible to reduce the BOG volume. The only way to handle all the evaporated BOG and prevent overpressure in the storage tanks is by reducing the unloading rate.
This may require more detailed information such as the size of the storage tank, its capacity during unloading, and the estimated amount to be unloaded. Based on the available information, I have a few suggestions: 1. Reduce the pressure in the storage tank as much as possible before the ship arrives, and increase that pressure during unloading depending on the operating conditions. 2. If reducing the unloading rate is to be adopted, it is best to discuss this in advance with the shipowner. 3. The ship’s crew should have communicated with the ship’s owners prior to the ship’s arrival; why wasn’t this issue addressed in a timely manner given such a large temperature difference?
The temperature difference is just a superficial phenomenon; the root cause lies in the significant differences in saturation states resulting from compositional variations. Therefore, it is essential first to ensure uniform mixing, in order to prevent accidents such as Roll-Over that can occur due to stratification. In practice, apart from reducing the unloading speed, there is no good alternative; when the BOG treatment unit is unable to handle it, it is necessary to vent the gas promptly. Once again, avoiding sudden accidental discharge due to layer formation is the top priority.