Describe gas freeing procedure on board of tanker or gas carrier.

asked 22 May '17, 21:27

May 22, 2017, 9:27 p.m.
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1) Before gas freeing it is required to liquid free the cargo tanks. To liquid free the tanks it is possible either blow hot gas down in the pump sump or pressurize the tank and empty the tank through the empty blow line. It depends on the type of gas carrier we are on and the pipes in the cargo tanks. Blowing of hot gas done either through the condensate line, empty blow line or the pump. If blowing done through the pump check that the pump does not start to rotate. When the hot gas is blown down in the pump sump the liquid will be boiled off. As long as there is a liquid the cargo tank pressure will increase as hot gas blown. When there is no more liquid left in the cargo tank, pressure will be stabilized and only the tank shell temperature will increase. In the tank should be as high pressure as possible when liquid free the tank. On fully refrigerated gas carriers the cargo tank pressure to be observed at all times to avoid an uncontrolled venting. Continue to blow hot gas down in the cargo tank until temperature about 5 C above the seawater temperature. Read the temperature on both sides of the cargo tank shell. Keep in mind that the cargo tank shell can have a thickness of 20 mm or more.

2) Stop the cargo compressors when reached the planned cargo tank temperature. Then vent off the cargo tanks pressure, either through the vapor manifold into the water or to the vessels vent mast. If the vessel is equipped with purge tanks or gas recovery plant available use the cargo compressors and condensate the overpressure to the purge tank. The purge tank is a small tank "pressure vessel" located either on deck or in a hold space. When pressure in the cargo tank reached atmospheric commence to inert or purge the cargo tanks with nitrogen.

3) When using inert gas the oxygen content by volume has to be less than 5%, that is an IMO requirement. The inert gases have to be as dry and warm as possible before entering the cargo tank. If the vessel is equipped with a heater on the inert gas line it must be used to take advantage of the density difference between the inert gas and the cargo vapor. Difference in the density helps to achieve a good displacement purging. The differences in density determines if inert gas must be blown through the cargo tank vapor or liquid line.

4) Hold as low cargo tank pressure as possible while inerting to avoid turbulence in the cargo tank. Start the inerting with as low rate as possible, to get the most effective purging. Displacements purging means the cargo atmosphere is pressed out by the inert gas. Just after commenced the inerting measure the cargo tank atmosphere for HC (Hydro Carbon) vapour in the part of the cargo tank were blown in the inert gas. During the whole inert operation measure the cargo tank atmosphere for HC vapour content until planned content reached less or equal to LEL or the limit stated in the company QA manual. While inerting the oxygen content will not be higher than the oxygen content set on the inert gas generator e.g. 5% by volume. The HC content will be reduced as long inerting is in progress. No not stop the inerting before reached the LEL for the actual cargo. As an example the LEL on propane is 2% by volume, so inert until reach 1,5% by volume. When HC content get less than LEL on the actual cargo we commence ventilating the cargo tank with air.

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5) In the IMO regulations stated to use a safety factor of 2 as the margin for error on measurement and instruments. That means that when drawing a flammability diagram, the line for critical mixture with 2% oxygen by volume gives 4% HC content by volume. Therefore inert until read 2% HC by volume before we commence ventilating with air. It is important also to inert all liquid lines, condensers and cargo compressors before we stop the inerting, to have a neutral atmosphere in them.

6) Cargo compressors, cargo fan, inert gas blower, booster compressor or portable cargo fans can be used when venting cargo tanks with air. Actual equipment used depends on the vessel's cargo system design. If the vessel is equipped with a vent heater it should be used to get as warm and dry air as possible. While venting with air measure the oxygen content and also check that the HC content is reduced to 0% by volume. Before stopping the air ventilation measure the cargo tank atmosphere for 0% by volume of CO - CO2 (Carbon monoxide - Carbon dioxide) and the oxygen content must be 21% by volume.

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answered 22 May '17, 21:52

May 22, 2017, 9:52 p.m.
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