Chemical Tankers / Day 3 / Day 3 Topic 5 Tank Cleaning Guidelines

5.3 Industry best practice in tank cleaning

Type and condition of tank linings

Examples of cargoes with high factor are Methanol, EDC, Acetone, Vinyl Acetate and Cyclohexanone. The swelling reduces the density of the coating and enables the cargo molecules to migrate into the coating. Typically such that the cargo is absorbed within a couple of days to the maximum level and stays constant for the reminder of voyage.

5.3 Industry best practice in tank cleaning

Type and condition of tank linings

The removal of cargo residues from the coating by ventilation

starts also at a rapid rate. After a couple of days there is no significant further loss of the retained cargo. Unfortunately the removal comes to a standstill before all cargo has been released by the coating.

5.3 Industry best practice in tank cleaning

Type and condition of tank linings

The remaining amount varies depending on coating type, film thickness and type of cargo.

If the coating is not soft, ventilation is not necessary and the cleaning should be performed as advised.

5.3 Industry best practice in tank cleaning

Type and condition of tank linings

If the coating is soft, tank should be ventilated for at least 24 hours after discharge or longer, if the coating system has not fully recovered. Cargoes with a vapor density higher than air have to be removed by forced circulation to avoid accumulation on the tank bottom.

5.3 Industry best practice in tank cleaning

Type and condition of tank linings

Ventilation must provide moisture-free dehumidified air to the lowest point of the tank. Water and steaming should not be used before full recovery of the coating system to avoid adverse effects on the coating.

5.3 Industry best practice in tank cleaning

Type and condition of tank linings

Zinc Silicates – it is anti corrosive paint system that is based on zinc dust with some additives and a binder. Zinc coatings have a tendency to hold or trap previous cargoes making them difficult to clean to a Wall wash standards. Zinc coatings have a good resistance against solvents, but are not resistant against strong acids and bases.

5.3 Industry best practice in tank cleaning

Type and condition of tank linings

Before cleaning of zinc coated tanks an instructions from coating supplier should be followed especially in regard to pH limitations. Quite frequently, when the previous cargo is not water soluble, soap or solvents will be needed as part of the cleaning process to remove all traces of the last cargo.

5.3 Industry best practice in tank cleaning

Type and condition of tank linings

It should be remembered that not all zinc coatings will become clean by the same method. This depends upon not only last cargo but other factors such as how old the coating is and whether is blistering or areas of open rust.

5.3 Industry best practice in tank cleaning

Type and condition of tank linings

Cargoes which are to be carried in zinc tanks and have a pH notations should be checked for the amount of pH prior to loading (for example, Fatty Acids). Some drying oils have a significant quantities of moisture presents which is settles out during the voyage, softening the zinc coating in the lower areas of the tank during the voyage and causing cleaning problems and damage the tank linings.

5.3 Industry best practice in tank cleaning

Type and condition of tank linings

Following the carriage of Acetate cargoes, proper ventilation must be carried out so that any free liquid remaining is removed, as such residue mixed with water will damage the coating.

Stowage of viscous-high melting point slops or dark colored liquids should be avoided due to porous nature and likelihood of coating damage.