Liquefied Natural Gas stored in the storage tanks evaporates due to heat transfer from insulation and kinetic energy provided by the motion of lng carrier.
The vaporized gas or Boil off Gas (BOG) increases the cargo tank pressure, due to increasing pressure boil off gas volume decreases but cargo temperature increase. This increase in cargo temperature further stimulate the boil of process.To prevent excessive pressure build up of boil off gas and in accordance with IGC Code 2 independent safety valves shall be provided in each tank. These safety valves helps to control maximum and minimum design pressure of tanks.
During the cargo handling operation more boil off gas is produced which will increase pressure of the tank. IAS will regulate the tank pressure through vent valve. When pressure of tank reaches at threshold value lets say design pressure of 240 m bar, vent valve will open automatically and it will remain open until it reaches to normal pressure of 200 mbar. High pressure alarm can be set below this pressure.
Low boil off gas pressure is also dangerous for tanks as pressure reaches negative due to over consumption. Low pressure might disrupt tanks insulation. Boil off is also used as fuel, so at low pressure fuel supply will also be interrupted. To manage this, as soon as pressure reaches to minimum threshold like 30 mbar, compressors will start and will increase the pressure of boil off gas until it reaches to minimum design pressure of 80 mbar.
When the system moves into above condition the boiler operates in dual fuel mode with the low duty compressor running, the fuel oil back up command is then reset at 30mbar. When vapour header pressure is decreased to 20 mbar, tank protection system will be activated so that
fuel gas master valve and low duty compressor will be tripped immediately.
Close to zero pressure, emergency shut down shall be activated. Two methods can be used to regulate excess pressure in tanks. First to release excess boil off gas in atmosphere or force this excess gas to boiler.
When boiler load is already at maximum and it cannot consume more boil off gas, it should be released on atmosphere.
During the laden mode, the operator should use absolute pressure or gauge pressure transmitter control to maintain the tank pressure efficiently. Gauge pressure mode can only be selected during the ‘Ballast’. Cargo tank pressure is also related with temperature of the tank interior. Thus in a while achieves spray to drop temperature of the tank interior in the ballast voyage, it is necessary to pay attention because the sub-cooling can decrease the cargo tank pressure. After loading LNG from the terminal, LNG will rapidly evaporate because of the membrane wall temperature. At this time, an operator should pay attention to control the cargo tank pressure.
Because the differential pressure between Insulation Space and Cargo Tank is important, it is necessary to consider the nitrogen pressure in the insulation space.
Excess BOG Dump
Normally insulation spaces are still warm compared to storage tanks and it takes approx 72 hours for insulation space to rech steady state temperature.
Vapour Header Pressure Control
If the vapour header pressure is higher than set point, the output of controller increases to decrease the vapour header pressure toward set point and the output is sent to boiler ACC for excess BOG dump control. If the vapour header pressure is lower than set point, the output of controller decreases to increase the vapour header pressure toward set point.
During normal operations, the pressure in the cargo tanks is controlled by the use of boil-off gas in the boiler as fuel or controlled via the forward vent mast. Vent mast is protected by nitrogen purge fire smothering system.
Vent control valve is controlled from the IAS and has four control levels as follows:
1) Cargo tank protection
2) Manual vent inhibit
3) Vent control at ‘Vent Mode’
4) Manual vent
In the cargo tank protection mode, the vent control valve will open to full flow (100% capacity) when a pressure on the vapour header exceeds the set value . The valve will stay in this mode until the pressure drops below the set value on the vapour header, at which point the valve will be closed.
In the manual vent inhibit mode, the vent valve will stay closed when the vent inhibit order is set from the wheelhouse. In this mode the manual operation of the vent mast valve is not available. The cargo tank protection mode will override the manual vent inhibit.
In the vent control at vent mode, the IAS controls the opening of the vent control valve according to the vapour header pressure regardless of whether BOG is being fed to the engine room for burning in the boilers. In this mode the manual operation of the vent mast valve is not available.