In our previous article we had taken a look at the deballasting procedure before the actual loading of cargo in the tanks and the role of the IG plant in the same. Now we will learn about the conditions when the cargo is being pumped into the tanks and after cargo is fully loaded.
Loading of Cargo
It is logical sense that as cargo is being pumped into the tanks it will generate lot of hydrocarbon vapours and these should not go back into the safe zone of the ship (see image in this article for layout plant of IG system on a ship). Hence the deck isolating valve should be kept in closed condition and the IG plant should be shut. But in case some of the tanks have not been deballasted and this operation is going along with the loading of other tanks then the IG plant cannot be shut down and the valves should be kept in appropriate positions depending of the tanks which are being deballasted and the ones that are being loaded with cargo.
Apart from this the risers and vents on the tanks being loaded should be closed to prevent unnecessary accumulation of the hydrocarbon vapours on the deck.
After Loading is Complete
Once the ship is fully laden with the requisite amount of cargo it is all ready to set sail in the deep waters. It must be ensured to maintain a positive pressure on all the loaded tanks which is of the order of 0.01bars above the atmospheric pressure. This prevents any outside air to come inside the tank from any possible leaks in the valves or any other place. If the voyage is substantially long this pressure might get lost over a period of time hence the tanks should be topped up routinely to maintain this positive pressure.
Proper care should be taken to ensure while topping up the cargo filled tanks that the oxygen concentration in the inert gas is not only below 8% but ideally below or near 5% concentration by volume. It might be necessary to vary the boiler load to ensure this low concentration of oxygen in the flue gases.
In the next article we will talk about the operations of crude oil washing