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

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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. When cargo with a high melting point has been carried, tanks should be washed with hot water. If possible, steam should be used to ensure the residues are effectively melted and cleared. The cleaning process must also include the tank lines, tank lids and vent lines, including pressure vacuum valves and risers. Examples of cargoes with high melting points include phenol and waxes.

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:

1. Fixed and portable tank cleaning equipments
   The installation of fixed tank washing machines within a cargo tank allows an inert atmosphere to be maintained during the washing operation, and thus permits cleaning in a closed mode in compliance with port regulations prohibiting release of noxious vapours. Their installation and use also reduces crew exposure to cargo vapours and inert gas.
   


2. Tank cleaning and risk with cargo contact
   Different chemicals affect the human body in many different ways. A general information and some practical advice are available in Appendix,7, of "Medical first aid guide for use in accidents involving dangerous goods" published by IMO, WHO and ILO ref (36).


3. Practical example of solving tank cleaning problems
   Cleaning of tanks is usually the responsibilty of the ship. Tank cleaning and the cleanliness involved have different standards depending upon the previous cargo and the cargo to be loaded. But the matter can be still more complicated, as cleanliness for one and the same product may vary, depending on who the receiver is and for what purpose the cargo is finally intended.


4. Tank cleaning fatality- case study & lessons learned
   In the trade practice onboard chemical tankers, it is not uncommon that the use of methanol or other chemical/detergents is undertaken to achieve the required standard of cleanliness inside the cargo tank, pipelines, cargo hoses, manifold adaptors/reducers, gauging equipment, etc. Industry publications clearly highlight and caution regarding their use due to their flammability and toxicity hazards.


5. Pre-cleaning /washing of cargo tanks
   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.


6. Final cleaning of cargo tanks prior loading
   Method of final cleaning to be used depends on both previous cargo and cargo to be loaded. As a general rule the tanks and piping shall be completely drained of water or residues before loading. The bottom of the tanks may have to be dried up with rags.


7. Tank cleaning and posoning hazards
   Certain substances affect the tissues locally as an irritant (cashew nut shell oil) or cause grave damage to the eyes, skin or mucous membranes (e g strong acids and caustic). Other substances may be absorbed by contact to the skin without local effects (e g nitrobenzene, aniline).


8. Testing of tanks and cargoes
   Most common tests and checks for oil and chemical cargoes include testing tank walls for cleanliness. Testing is normally carried out by independent surveyors who, according to local practice or a written agreement in the charter party, are accepted by shipper, receiver and owner.


9. Practical tank cleaning methods for various noxious liquid cargo
   Tanks that may have contained monomer or drying oils should first be cleaned with sufficient cold water quantities to avoid polymerization of cargo residues. In some cases, it is necessary to employ tank cleaning chemicals, but their use is generally limited as it may be difficult to dispose of slops.


10. Special tank cleaning method
    If a special method involving cleaning agents is to be used, it may create an additional hazard for the crew. Shipboard procedures should ensure that personnel are familiar with, and protected from, the health hazards associated with such a method. The cleaning agents may be added to the wash water or used alone. The cleaning procedures adopted should not entail the need for personnel to enter the tank.


11. Determining proper tank cleaning by acid wash method
    The acid wash method is used if there is any suspicion that a cargo of aromatics may have been contaminated by a previous oil cargo. The method is also used as a check that a tank is sufficiently cleaned before loading aromatics.


12. Supervision of all tank cleaning and gas freeing operations
    Tank cleaning is essential on-a chemical tanker, but it must be recognised as a potentially hazardous operation, and rigorous precautions should be observed throughout the process. Together with gas freeing, it is probably the most hazardous operation routinely undertaken on a chemical tanker.


13. Disposal of tank washings, slops and dirty ballast - safe method
    During normal operations of a chemical carrier, the main need to dispose of chemical residues, slops or water contaminated with cargo will arise during or immediately after tank cleaning. Final disposal of slops or washwater should be in accordance with the ship's P&A Manual. Tank washings and slops may be retained on board in a slop tank, or discharged ashore or into barges.


14. Cargo tank damage during pigging operations
    Blowing and pigging of pipelines at terminals poses inherent risks for the terminal and a chemical tanker. Frequent damages to tanks have occurred. If there are doubts about the shore operation or signs of problems ashore the OOW must immediately request clarification.


15. Determining water contamination in chemical cargo
    Presence of free water in non water-soluble products can, very roughly be determined on board by warming a sample of the product in a test tube or in a bottle. Water will then collect at the bottom of the tube and can be seen after some time.


16. Determining Sulphur contamination (sulphides) in chemical cargo
    Certain products, in particular "virgin naphta feedstock'' (petroleum naphta) are severely contaminated by minor amounts of sulphides (and also lead compounds), which poison catalysts in further processing. Previous heavy oils or dirty harbour ballast water may have left traces of sulphides in the cargo tanks.


17. Chloride contamination in chemical cargo - how to resolve?
    Certain cargoes are very senstive to chloride contamination, in particular glycols, methanol, ethanol . The tanks should be finally washed with fresh water. However, chlorides (salt) may still be present and a check might be useful. The greatest risk for salt deposits is on horizontal surfaces.


18. APHA (Hazen) method for determining color of very light chemical products
    A method called APHA (Hazen) is often used for very light products, defined in ASTM D-1209, viz aromatics, ketones. This colour scale is defined with an origin in 100 cc distilled water (value 0) to succesively higher values (max 500) by adding APHA-solution (a platinum-cobolt salt solution).


19. Loading, discharging & care of Phenol - Safety guideline
    PHENOL is carried at sea in a generally pure state. As a result, it has a high freezing point of approximately 40~ 41deg C. PHENOL is also extremely dangerous when it comes into contact with the eyes or skin and can be fatal.


20. Hazards of Phenol - safe handling of Phenol on chemical tankers.
    Phenol must be carried at temperatures within charterers instructions, typically between +50 and + 60 degrees C. Heating instructions of the Shipper or Owners must be followed to avoid protests and delays in the port of discharge. Overheating can damage this cargo. A full cargo heating log must be maintained.


21. Handling benzene & methanol safety precautions
    Benzene is known as a strong carcinogen and known to cause leukaemia. When handling cargoes with more than Benzene concentration of 0.5%, the Master is to ensure that all personnel involved are aware of the long term hazards.


22. Personal protective equipments for carcinogens & cyanide-like cargoes onboard chemical tankers
    A carcinogen is a substance that may cause cancer by contact or by inhalation. It is therefore essential that the highest safety precautions are taken when handling these cargoes. Access to deck areas must be restricted to duty personnel only. All accommodation doors and ports must be closed and ventilation put on recirculation. Any member of crews involved in cargo operations must wear chemical protective suits and breathing apparatus


23. Handling ACRYLONITRILE safety precautions
    ACRYLONITRILE are high value and require sophisticated handling for safety, health and loss prevention reasons. They need careful consideration prior loading , tank coating compatibility, cross compatibility with other cargoes carried, environmental controls if required (inerting).


24. handling ISOCYANATES safety precautions
    Product safety data sheets may be available from various sources. For safety preparation, until the specific product safety data sheet can be obtained, Chemical Data Guide for Bulk Shipment by Water (U.S. DoT), should be used.


25. Loading, carrying & discharging of Sulphuric acid - regulatory requirements & special handling methods
    IBC code compatibility chart strictly prohibits water in adjacent compartment to Sulphuric acid as you are aware if both come in contact with each other will generate a violent reaction. It is therefore recommended that the during loading of sulphuric acid adjacent ballast tanks to be always stripped dry to the maximum efficiency of the deballasting equipment used.

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