CARGO STRIPPING SYSTEM :

The stripping arrangement for specialised ships such as chemical tankers and gas carriers must meet the requirements of Annexure II of MARPOL 73/78 convention and protocol.

When the main cargo pump is of centrifugal type, there is every likelyhood that the pump loses suction to discharge the bottom portion of cargo due to high suction lift. In order to strip-the tanks dry of cargo, there are three systems approved under MARPOL.

1) Pressurising the tank and stripping by means of a small diameter pipe led from the bottom of the tank well. The process is slow and requires a large quantity of nitrogen or inert gas, also careful setting of tank relief valve is required.

2) Using a stripping box in the tank, the box being connected to the stripping line. The box is alternately put under vacuum and pressure so that it fills with cargo and then when pressurised, the cargo is displaced up the stripping line. Back flow is prevented by non - return valve. This system is simple and does not rely on rotating machines. However, the non-return valves can give trouble if the cargo gets crystallised.

3) when pressurised, the cargo is displaced up the stripping line. Back flow is prevented by non - return valve. This system is simple and does not rely on rotating machines. However, the non-return valves can give trouble if the cargo gets crystallised.

4) Using the deepwell pump, running against a closed discharge valve, upper part of the riser column being pressurised with nitrogen. The stripping line is led from the top of the pump chamber to a point just after the main riser discharge valve. The arrangement requires the pump to operate with liquid cargo lubricating the shaft bearings supplied by a reservoir of pressurised cargo, provided for this duty.

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