Cargo Operations for Tanker I 124.
Explosimeter
With the check switch is closed, the zero adjustments of the meter are checked.
Precautions for gas freeing operation:
• Chief officer to ensure only personnel designated for the Gas freeing operations should be allowed on the main deck
• The proposed duration for the gas freeing operation to be decided well ahead of the operation in a pre-operation meeting
• If there are small crafts alongside the vicinity of the tanker, their personnel should also be notified about the gas freeing operation and hazards related to the same
• The atmosphere near the operational area should
125. l Marine Insight
hydrocarbon vapors
• All crew involved in the gas freeing activities must wear appropriate protective gears and clothing
Cargo Operations for Tanker I 126.
Tank Entry
freeing operation is being drawn into the accommodation, the air conditioning and
mechanical ventilating systems should be stopped immediately
• Many tanks have common venting arrangement for all cargo tanks. In such cases, each tank should be isolated to prevent the transfer of gas to or from other tanks
• When the ship is underway, it may become
necessary to slightly alter the course for preventing hydrocarbon vapors from accumulating in the accommodation areas
• During the gas freeing operation, the intakes of central air conditioning or mechanical ventilating systems should be monitored and adjusted, if required, to prevent the entry of harmful gases
• Ensure to maintain Positive pressure in accommodation
• If it is suspected that harmful gases from gas
127. l Marine Insight
1. Open 2. Shut
Cargo Operations for Tanker I 128.
Open and Shut Ventilation Ports
• 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 WG
• The IG system is set up 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 be fitted 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 tank‟s atmosphere
• Initially, for some time the tank atmosphere can be vented by the gas freeing port. However, after
some time the tank hatches also can be opened for venting
• Keep monitoring the oxygen inside the tank at all levels until it rises to 21% O2
• A thorough check should be carried out for lower explosion limit (LEL), which should be 0, and H2S 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, may be 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 may be 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 H2S concentration if present should be within permissible limits.”
Cargo Operations for Tanker I 134.
Extraction Fan Arrangement