Descripción: Transmisión electrónica a través de un potencial silla en presencia de espín-órbira (primer borrador aun no terminado)
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Gassing Up (Purging) Cargo Tanks
After lay up or dry dock, the cargo tanks are filled with inert gas or nitrogen. If the inerting has been done done with inert inert gas, when the vessel arrives arrives at loading port port the cargo cargo tanks must must be gassed up (purged) prior to cool down. Inert gas contains a small quantity of water vapour and has a dew point of approximately -40 C. Gassing up lowers the dew point and avoids ice formation during loading. Additionally inert gas contains approximately 15% carbon dioxide. CO 2 sublimates from vapour to solid at -78 C, gassing up removes CO2. LNG is supplied by the terminal to the liquid manifold where it passes to the stripping/spray header via the appropriate ESD liquid valve. It is then fed to the LNG vaporizer and the LNG vapour produced is passed at 20 °C to the vapour header and then into each tank. As LNG vapour vapour at 20 °C is lighter than inert gas, the inert gas is therefore therefore exhausted up the tank loading line to the liquid header. The inert gas is then vented to atmosphere via the no.1 mast riser. When 5% methane is detected at the no.1 mast riser, the purged gas must be directed ashore, preferably preferably by free free flow to to avoid turbulence turbulence within the cargo cargo tanks. Gassing up is normally considered complete complete when the methane content, content, as measured measured at the top of the cargo line, exceeds exceeds 80%. The target value for Nitrogen and inert gas CO 2 is equal or less than 1%. This will usually entail a minimum of one and a half times the change in tank volume. Some operating procedures may recommend two times the change in tank volume. Cooling Down Cargo Tanks
Prior to loading a maiden cargo, or after circumstances have required the vessel to be gas free, the cargo tanks will be inert and at ambient temperature. After the cargo system has been gassed up, the headers and cargo tanks must be cooled down before loading can commence. The cool down follows immediately after the completion of gassing up, using LNG supplied by the terminal. The rate of cool down is limited for the following reasons: To avoid excessive pump tower stress • Vapour generation must remain within the capacity of the high duty compressors • To remain within the capacity of the nitrogen system to maintain the required pressures • within the interbarrier spaces. LNG is supplied from the terminal to the spray header which is open to the cargo tanks. When the cargo tank cool down is nearing completion, the liquid manifold crossovers, liquid header and loading lines are cooled down. Gene General rally ly the the cool cool down down of he cargo cargo tanks tanks is cons conside idere red d comp complet letee when when the the 4 lowe lowest st temperature temperature sensors in each tank reach -130 °C (Membrane type) or the equator equator temperature temperature is 1 below -130 °C (Moss (Moss type) . Cool down should take between 8 and 12 hours. Cool down of moss type vessel takes longer than membrane vessels. 1
On some vessels the temperature temperature requirement to to commence commence loading is -110 °C and not -130°C
Different methods are used to determine the amount of cool down required; in our report 4 methods are implemented: Method 1: The total LNG quantity required for cooling down is calculated by multiplying the difference of the equator temperature in each tank between before and after cool down by the cubic meters LNG needed to lower the tank equator temperature 1 °C (known value). This method is only used for a moss type design. Method 2: The ship has cool down tables that gives the amount of LNG (expressed in MMBTU) required to cool down the tanks from the temperature 2 before cool down to the temperature required to start loading (often -130°C). This method can be used for Moss type and membrane vessels Table 1 is an example of a cool down table for a Moss type, Table 2 is an example of cool down tables of a membrane vessel. Table 1: Cool down table Moss type
Remark: - tables are for all tanks (cargo volume= +/- 155000 m³) - LNG density= 442.72 kg/m³ - Calorific Value= 51857 BTU/kg Method 3: Same as method 2 however the cool down tables express the quantity of LNG needed for the cool down in cubic meter LNG and not in MMBTU Table 3 is an example of a cool down table for a Moss type, Table 4 is an example of cool down tables of a membrane vessel. Table 3: Cool down table Moss type
Remark: - tables are for all tanks (cargo volume= +/- 155000 m³) - LNG density= 442.72 kg/m³ - Calorific Value= 51857 BTU/kg Method 4: This method calculates the amount of LNG used during the cool down operation based on the number of nozzles, duration of the cool down, flow capacity. In this method a formula ( to be supplied by the Ch.Off ) must be used that calculates the amount of cubic meters LNG used for the cool down.
