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Practical tank cleaning method onboard modern chemical tankers

The tank cleaning process and the freshness involved have different standards depending upon the previous cargo and the state of purity required for the merchandise to be loaded. The relevant tank cleaning guides should always be discussed. Generally, an essential part of the tank cleaning process is Butterworthing with hot or cold seawater at enough pressure and the appropriate tank levels. It should be followed by fresh water washing to eliminate seawater residues.

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.

chemical tanker navigation at sea
It is summarized below practical tank cleaning methods for various chemical cargo.

Cleaning agent added directly to the washing water

The cleaning chemical is injected into the washing water utilizing a dosage pump, usually air-driven, through a fitting attached to the deck's washing pipe. Sometimes, the cleaning chemical is added to the washing water pump's suction side, thus eliminating the need for a dosage pump. But the dosage becomes less accurate, and the method is more cumbersome in the latter case.

The direct addition of cleaning chemicals to the washing water is quite common, but chemical consumption tends to be high, and its full cleaning effect is not always utilized. 1-2 hours of washing is usually to be recommended. The final rinsing being sea or freshwater, depending on the product to be loaded.

Recirculation of the washing water

Gives a better utilization of the cleaning chemical a mixture of 5 - 50 tons of hot water with the recommended amount of cleaning chemical is made in a cargo tank, a slop tank, a cofferdam, or a special tank for the purpose.

The water may be further heated using heating coils. A cargo pump may serve as a washing pump with delivery to the washing line on deck, and then to the washing machines. Another cargo pump or stripping pump drains the water back to the containment tank where the cargo residues can be removed by carefully drawing out the washing water from the bottom.

Washing should only be carried out because the tank atmosphere is well below LEL (lower explosive limit). Check during washing! Washing should be carried out under the inert gas atmosphere if cargo vapor concentration is above LEL. There are indications that contaminated washing water may cause incendive static electricity sparks. It refers in particular to recirculation washing.

The recirculation method can be used with all types of cleaning chemicals. One advantage is that one can easily incorporate major parts of the piping system in the cleaning circuit. Remember cross-overs and risers to deck! Finally rinsing with sea or freshwater.

Chemical tanker tank cleaning method using chemical agent
Fig: Chemical tanker tank cleaning method using chemical agent

The cleaning chemical can be applied undiluted directly onto the tank walls

This method is becoming quite common. The cleaning chemical is sprayed directly onto the tank bulkheads using a high-pressure (portable) pump and a long lance, which the operator can direct all round the tank. The operator must use protective oilskins, goggles, and preferably a breathing mask too. The cleaning chemical is left on the tank walls for 20 - 40 minutes. The tank is then washed by hot water. The process is repeated where necessary until the tank is clean. This method is very effective.

The cleaning chemical is added to an ejector in a similar operation, which draws it into the steam when steaming the tank. The tank should first have been steamed thoroughly. After steaming with a cleaning chemical, steaming is continued for a while. Finally rinsing with sea or freshwater. Remember to keep the hatch lid slightly open to eliminate excess pressure during steaming or sub-pressure after steaming is finished!

A floatation method is sometimes used

For the final cleaning to achieve a high degree of cleanliness after heavier hydrocarbons such as lubrication oils, lubrication oil additives, and before loading hydrocarbon-sensitive products such as methanol. (Normally a tank is "upgraded" to methanol standard via two-three intermediate cargoes of aromatics ). The floatation method should only be used after first washing with cleaning chemicals. The method is then very effective, but it is less effective if cargo residues on the tank walls are of any considerable thickness.

The tank is first filled to a level of 0, 4 - 0, 8 m from the bottom with a strong non-water-soluble solvent, which is also reasonably cheap. Toluene is usually used. Then the tank is slowly filled with water, thereby lifting the toluene on top of it. After filling, the water is slowly drained again until the tank is empty. The top layer of toluene is then transferred to the next tank, and the process repeated (with the same water used again to minimize solvent losses). The filling and draining can be done with a level change of 0, 5 - 2, 0 m/h depending on the degree of contaminants on the tank walls.

There have been advanced warnings that this floatation method, although frequently used, might involve hazards as regards static electricity charges in the interface between the two liquids. It is wise to pump slowly to not exceed I m/s in the filling pipe. The process should preferably be carried out with the tank inerted.

Steaming with a solvent

It can also be done as a final cleaning process, after washing with a cleaning chemical. The method will only be effective for removal of the very last traces of a previous cargo.

Fill the tank with water to just above the heating coils and add a non-water-soluble solvent, usually toluene or xylene. The water is then heated using the heating coils, and the solvent vaporizes. The solvent condenses to a the certain extent on the tank walls and dissolves minute amounts of cargo residues, which after draining the tank can be washed off using water with/without cleaning agents added.

The amount of solvent should be of such small quantities that forming explosive mixtures in the tank is avoided. Choosing 2/3 of LEL as a guide arrives at the following maximum amounts of toluene:

Tank size 200 m 3 abt 10 l of toluen
500 m 3 -//- 25 -//-
1000 m 3 -//- 50 -//-

For other solvents, the corresponding amounts can be calculated based on the fact that one gram molecule (=as many grams as the molecular weight) occupies 22,2 liters of volume at normal atmospheric pressure and temperature.

Rinsing with water is normally the final cleaning process

Unless the cargo remains can be vaporized entirely away. Remember that certain cargoes to be loaded may require freshwater rinsing (risk for chloride contamination), e.g., methanol, ethanol, and glycols. Stainless steel tanks shall always be finally rinsed with fresh water. Otherwise, there is a risk for tank pitting corrosion. Eventually, the tanks should be ventilated dry, and any water left is dried up with rags before loading.

Cleaning of stainless steel

Stainless steel can be cleaned with all the methods mentioned above.

Rust “descaling”

In the trade of chemicals, loose rust will never be accepted upon loading. The rust itself may be harmless for many products, e.g., gasoline, but the rust may contain residues from previous cargoes, contaminating the next shipment. Gas freeing may also become more complicated if large amounts of rust collect on the bottom of the tank. It is sometimes possible to scrape off loose layers of rust. Another method, sometimes used but relatively expensive, is "electrochemical descaling."

A provisional, but robust, anodic protection system consisting of Al/Mg strips is tack welded to the tank interior. The tank is then filled with (salt) seawater for a couple of days. The anodic alkaline process breaks off the bulkheads' rust, and the rust can be washed down. "Only" rust-removal remains. This method is intended for uncoated tanks only.

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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