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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. Onboard a chemical tanker tank cleaning may be required for one or more of the following reasons:
  1. To carry clean ballast.
  2. To gas free tanks for internal inspections, repairs or prior to entering dry dock.

  3. To remove sediment from tank top plating. This may be required if the vessel is engaged in the repetitive carriage of chemicals or similar sediment settling cargoes. Although washing may not be necessary between the consecutive voyages, assuming the cargoes are compatible, many Ship Owners have found it prudent to water wash a small group of tanks on a rotation basis between voyages, thus preventing any large accumulation of sediments.

  4. To load a different and not compatible grade of cargo. Washing between different grades of cargo is the most common reason for tank cleaning. In most cargo sequences on chemical tankers, this cleaning may consist of no more than a simple hot or cold seawater wash. A simple water wash will disperse many types of chemicals and has been found effective between clean petroleum products such as gas oil and kerosene. However, it should be noted that there is a number of grade sequences, particularly in the chemical products trade, where no washing at all needs to be carried out. Thus, the decision for necessary tank cleaning required in such trades is often made only when knowledge of the next grade to be loaded is obtained.

Washing machines, their water supply and even the washing method are usually described by the term “Butterworth”. The machines, either fixed or portable, consist of revolving nozzles, which are moved by water driven gearing to create a spherical wash pattern or “cycle”.

With portable machines, both the machine and its flexible water supply hose are placed into the top of the tank to be cleaned through an opening called the “Butterworth Port”. The machines are progressively lowered down the height of a tank in stages or “drops” each usually of 10-15 feet. Graduation marks every 5 feet on the water supply hose are a useful check on the depth of the machine inside the tank. The lowest “drop” is normally about 5 feet above the “bottom” of the tank where the machine is positioned for a “bottom wash”. The wash duration at each drop is usually for one cycle of the machine, the cycle time varying between 30-60 minutes according to the size of the machine and its pump pressure.

Throughout the washing operation, cargo residues mixed with washing water are continuously stripped from the cargo tanks by the vessel normal cargo pumps. These washings are directed through the cargo line system into reception tanks, a slop tank or in some cases to shore facilities.
( Further reading Wärtsilä Encyclopedia of Ship Technology )

Pre-cleaning of cargo tanks :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. )

Related Info:

Fixed and portable tank cleaning equipments

Tank cleaning and risk with cargo contact

Practical example of solving tank cleaning problems

Tank cleaning fatality- case study & lessons learned

Pre-cleaning /washing of cargo tanks

Final cleaning of cargo tanks prior loading

Tank cleaning and posoning hazards

Testing of tanks and cargoes

Practical tank cleaning methods for various noxious liquid cargo

Special tank cleaning method

Determining proper tank cleaning by acid wash method

Supervision of all tank cleaning and gas freeing operations

Disposal of tank washings, slops and dirty ballast - safe method

Special tank cleaning method

Cargo tank damage during pigging operations

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