8.1.17 Cleaning Categories
8.1.17.2 Mineral Oils
8.1.16 Cleaning
We may sub divide the cleaning procedure after the nature of the general category of cargoes. There are three categories:
• These can be carried out with water or water and a mixture of detergents, emulsifiers or synthetic soap. • Sometimes solvent naphtha is used for cleaning purposes. It is obvious that solvent naphtha creates a positive hydrocarbon test. Therefore, after having used such a cleaning product, a final cleaning with water and detergents may be required.
Cleaning is carried out with the aid of water or water and detergents.
Detergents can be applied either as in line injection or they can be sprayed onto the tank and allowed to soak. When applying a solvent cleaner by spraying it will be obvious that after the reaction time (minimum 30 minutes) the residues have to be washed away. Butterworth machines with cold or warm water should be used for this, but not too hot, otherwise the applied solvents will evaporate. The machines must be operated from the deck and must be lowered down into the tank at frequent intervals in order to obtain an effective cleaning throughout the tank. Throughout the cleaning operation excess wash water must not be allowed to collect on the tank bottom. Sometimes cleaning solutions may be used two or even three times. When plain water is used as the cleaning agent, the cleaning operation is based on solubility of the product in water. In this case it is not recommended to use recirculation; the water may be saturated with product. Cleaning procedures may continue for several hours depending on the product. After the tank has cooled down, inspection can take place. If traces of oil or chemical residues are found, further cleaning may be needed.
• Animal and Vegetable Fats and Oils
• Mineral Oils (Crude Oil, Petroleum Products, etc.)
• Petrochemicals and Solvents
8.1.17.1 Animal and Vegetable Oils
These easily saponify when combined with Caustic Soda. Saponification is the process of converting oil or fat into soap; or, in practice, bringing it into suspension. However, after saponification hard, brittle soap particles may be present which have low solubility in water. Potassium Hydroxide (KOH) is therefore recommended instead of Caustic Soda (NaOH) as this creates a soft soap which is completely water soluble, though, these alkalis can only be used if the tank is made of stainless or mild steel or has been coated with an epoxy paint. If the tank has been coated with a zinc silicate coating, saponification should not be used. After cleaning with an alkali, residues have to be removed with warm fresh water. To overcome white coloured deposits, often encountered after cleaning with Caustic Soda or other alkalis due to the formation of insoluble magnesium and calcium salts, the use of a simple detergent instead of the alkaline cleaning agents may be preferred.
8.1.17.3 Petrochemicals and Solvents
These materials have a low boiling point and a high Vapour pressure at ambient temperature that can be removed by evaporation or a light cleaning, but inhibitors do not evaporate. Products with a lower Vapour pressure are removed by water washing and steaming. In the first stage cold water is recommended because many volatile products show a higher solubility in cold water. Directly after the cleaning operations the tanks should be rinsed with hot or cold water. Butterworth machines should be used. It is most important that the cleaning solution should not be allowed to dry as this will hinder rinsing. Rinsing should normally continue for about two hours or until no residue can be found in the tank.
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