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Pre- cleaning ( washing) of tanks prior loading noxious liquid cargo onboard chemical tankers

Tank cleaning prior loading noxious liquid substances onboard chemical tankers involved numerous hazards.

Practically all cases of tank cleaning start by washing with water. This is mechanical removal of cargo residues. This method has a slight emulgating effect (forming minute droplets of cargo suspended in the washing water). The water pressure should preferably be 12 - 14 kp cm 2 with a capacity corresponding to 4 washing machines (80- 10o m 3 /h). The washing water heater should have a capacity of yielding 80 degr. C washing water with 2 - 3 washing machines working.

While washing one should simultaneously drain the tank at the same rate in order to assist the cargo residues in their flow towards the tank suctions. If not, the residues will have a-tendency to come to rest anywhere on the tank bottom.

a) Products with good solubility in water : Pre-cleaning can normally be carried out with cold water. Examples are: mineral acids (sulphuric acid, phosphoric acid), alkalies (caustic soda, ammonia/potash solutions), alcohols (ethanol, methanol, butanol), acetone. The more viscous water-soluble products may have to be pre-cleaned with hot water, examples: glycols, glycerine, molasses. Note: sulphuric acid must be washed with copious amounts of water to guarantee rapid dilution and reduce risk of heavy corrosion.

b) Products which are volatile and vaporise without any traces frequently need no tank washing , only ventilation and possibly steaming of the tank. Examples: acetone, methyl ethyl ketone, hexane, methanol, butanol, propanol, toluol, trichlor ethylene. If water flushing is not carried out: remember to drain all cargo lines, pumps etc. Draining out cargo may be a difficult process to carry out safely and therefore water flushing and subsequent draining of the piping may be an advantage. Thereafter draining of water from the piping system can be carried out.

c) Some vegetable oils and animal oils (fatty acids) oxidize and "dry" upon application- of air and heat. Examples are: castorseed oil, cottonseed oil, groundnut oil, linseed oil, spermoil, talloil. Pre-cleaning must then be carried out with cold water. Otherwise the residues will dry up and harden and may be very difficult to remove. Final washing, however, can be carried out hot,

d) Vegetable oils and animal oils of non-drying type should preferably be pre-washed directly with hot water (800C). Examples: coconut oil, palmkernel oil, palm oil, tallow, whale oil.

e) Polymerizing products should be. pre-washed with cold water or the tanks should be flooded with water. Hot water may cause deposits of polymerized material, sometimes very difficult to remove. Examples on such products: styrene monomer, vinyl acetate, acrylonitrile, vinyl chloride.

f) Heavy oils, lubrication oils, lubrication oil additives, gas oil are normally pre-washed with hot seawater (800C) although cold water can also be used.

g) (Crude oil is mentioned as a reference. Crude oil with a relatively high percentage of light fractions such as Arabian crudes are often pre-washed cold and then hot. If hot water is used the light fractions are liberated first and then the residue tends to be tougher and more difficult to remove. Heavy crudes, however, e.g Boscan crude with little or light fractions can be washed directly with hot water. )

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