A For ships built before 1 July 1994
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Statutory Documents - IMO Publications and Documents - International Conventions - MARPOL - International Convention for the Prevention of Pollution from Ships - Annex II of MARPOL 73/78Regulations for the Control of Pollution by Noxious Liquid Substances in Bulk - Appendix 6 - Prewash Procedures - A For ships built before 1 July 1994

A For ships built before 1 July 1994

 A prewash procedure is required in order to meet certain Annex II requirements. This appendix explains how these prewash procedures shall be performed.

Number of cleaning machine cycles to be used in each location

Category of substance Number of cleaning machine cycles
Non-Solidifying Substances Solidifying Substances
Category X 1 2
Category Y 1/2 1
Parent topic: Appendix 6 - Prewash Procedures

Prewash procedures for non-Solidifying Substances

  1 Tanks shall be washed by means of a rotary water jet, operated at sufficiently high water pressure. In the case of Category X substances cleaning machines shall be operated in such locations that all tank surfaces are washed. In the case of Category Y substances only one location need be used.

  2 During washing the amount of water in the tank shall be minimized by continuously pumping out slops and promoting flow to the suction point (positive list and trim). If this condition cannot be met the washing procedure shall be repeated three times, with thorough stripping of the tank between washings.

  3 Those substances which have a viscosity equal to or greater than 50 mPa.s at 20°C shall be washed with hot water (temperature at least 60°C), unless the properties of such substances make the washing less effective.

  4 The number of cycles of the cleaning machine used shall not be less than that specified in table 6-1. A cleaning machine cycle is defined as the period between two consecutive identical orientations of the tank cleaning machine (rotation through 360°).

  5 After washing, the tank cleaning machine(s) shall be kept operating long enough to flush the pipeline, pump and filter, and discharge to shore reception facilities shall be continued until the tank is empty.

Prewash procedures for Solidifying Substances

  1 Tanks shall be washed as soon as possible after unloading. If possible tanks shall be heated prior to washing.

  2 Residues in hatches and manholes shall preferably be removed prior to the prewash.

  3 Tanks shall be washed by means of a rotary water jet operated at sufficiently high water pressure and in locations to ensure that all tank surfaces are washed.

  4 During washing the amount of water in the tank shall be minimized by pumping out slops continuously and promoting flow to the suction point (positive list and trim). If this condition cannot be met, the washing procedure shall be repeated three times with thorough stripping of the tank between washings.

  5 Tanks shall be washed with hot water (temperature at least 60°C) unless the properties of such substances make the washing less effective.

  6 The number of cycles of the cleaning machine used shall not be less than that specified in table 6-1. A cleaning machine cycle is defined as the period between two consecutive identical orientations of the machine (rotation through 360°).

  7 After washing, the cleaning machine(s) shall be kept operating long enough to flush the pipeline, pump and filter, and discharge to shore reception facilities shall be continued until the tank is empty.

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