The Ultimate Guide to Cargo Operations for Tankers

• Fresh air is to be supplied to the deck by pressing the air venting push button
• Open the inert gas inlet to 1 Port
• The pressure of the air can be controlled by the pressure regulator button to deliver the desired pressure Gas Freeing Procedure Let us assume that gas freeing is to be carried out for one tank, i.e. 1 Port. The said tank has HC content less than 2 % and it is decided to blow air into the tank for gas freeing.
• For this, ensure all those tanks which are not to be gas freed are isolated except 1 Port
• All the cargo tanks, which are not to be gas freed, their pressure should be lowered up to 50 mm
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• The IG system is setup in the engine room, and the control is handed over to the cargo control room (CCR)
• Now, Open the IG deck main isolation valve
129. l Marine Insight Cargo Operations for Tanker I 130. Gas Freeing Using Fresh Air Blank Fresh Air Inlet to Open

changed over to another tank by closing the air inlet and vent openings for the previous tank and opening the inert gas inlet valve for the next tank
Gas-freeing can also be carried out using portable hydro blowers onboard. They are of exhaust type and can befitted over the top of the tank cleaning hatches by opening the cover, as shown in the figure. Once fitted, hydro blowers can be run continuously by water hoses and areused for gas freeing in addition to the Inert gas blowers
• As the air enters the tank via the inert gas line, the pressure or velocity of the air should be sufficient to force its way through the bottom of the tank as it has to dilute the whole tanks atmosphere
• Initially, for sometime the tank atmosphere can be vented by the gas freeing port. However, after sometime the tank hatches also can be opened for venting
• Keep monitoring the oxygen inside the tank at all levels until it rises to 21% O
• A thorough check should be carried out for lower explosion limit (LEL), which should be 0, and HS concentration if present should be within permissible limits
• Once a tank is gas freed, the process can be
131. l Marine Insight To ensure good velocity of air entering the tanks, it is advisable that lesser tanks are gas freed simultaneously, maybe one or two at a time. This results in air entering with good velocity, hence faster dilution and gas freeing becomes a speedy process Cargo Operations for Tanker I 132.

In gas freeing method, the flammable gases are replaced by inert gases by pushing out the former to the atmosphere. The release of gases at deck level or through tank hatch openings depends on the ventilation and number of openings for gas freeing operation. This should be controlled for producing an exit velocity sufficient to carry the gas clear of the deck. The inert gas pressure in adjacent tanks to be reduced to minimum (200mmAq) to prevent the ingress of hydrocarbon gas / IG through any leaking valves, pipelines or bulkheads. All other valves in the tank including machine valves) should be closed. If gas freeing is taking place while the vessel is at anchor, it maybe necessary to suspend operation during calm or no wind condition which means the harmful gases released in the atmosphere will not be displaced. During the gas freeing operation, atmosphere tests to be made at several levels and in each compartment of the tank. If satisfactory gas readings are not obtained, ventilation must be resumed.
133. l Marine Insight A thorough check should be carried out for LEL which should be 0, and HS concentration if present should be within permissible limits Cargo Operations for Tanker I 134. Extraction Fan Arrangement

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