In our previous article we had taken a look at the generic procedures related to the operation of the inert gas plants on board ships such as starting checks and procedures, stopping procedures and so on. In this article we will move further by studying the various types of cargo tank operations and their procedures as follows.
Tank Inerting Process
Tank inerting refers to the process of creating inert atmosphere within that tank which in turn means that the oxygen content in that tank should not exceed 8% by volume under any conditions. Not only is the tank inerted but it is also kept at a slightly positive pressure with respect to the atmosphere to prevent any ingress of outside air which would increase the oxygen levels inside the tank.
A tank is not only inerted when it is carrying cargo but even during ballast voyages as well. The purpose of doing so is to simultaneously check the operation and efficiency of the inert gas system before the actual cargo handling operations commence on the loading port. For inerting the empty tanks the vents as well as the purge pipes are made open to the atmosphere and inert gas is introduced inside the tank. When the requisite conditions of oxygen level are reached, these are shut down and the tank is made to attain a positive pressure with respect to atmosphere.
When the above mentioned process is actually in operation, it should be ensured not to insert any sort of equipment within the tank for the fear of generating a spark unless inert condition is reached. Once all the required tanks have been inerted, they can have a common pressure with the main line of the inert gas discharge which also is above the atmospheric pressure. The magnitude of this positive pressure is of the order of around 100 mm of water gauge, which is roughly equivalent to 0.01 bars. Just for your reference the standard atmospheric pressure is around 1.013 bars hence you can see that the magnitude by which the pressure inside the inert tank exceeds the atmospheric pressure is quite low of the order of 0.99% higher than the atmospheric pressure.
Before the ship reaches a port or the loading destination and starts loading the cargo, the inert tanks need to be emptied from the ballast water so that cargo operations could begin. This operation is known as Deballasting and it should be ensured that the IG system keeps pumping the gas inside the tanks whilst the water is being discharged. All openings from the tanks are closed during this operation except the valves connecting the tanks to the IG main line and the relevant deballasting valves.
In the next article we will study two more conditions namely when the cargo is being loaded and when it has been fully loaded.