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Practical tank cleaning method onboard modern chemical tankers

Tank cleaning method and the cleanliness involved have different standards depending upon the previous cargo and the cargo to be loaded. 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 by means of a dosage pump, usually air-driven, through a fitting attached to the washing pipe on deck. Sometimes the cleaning chemical is added to the suction side of the washing water pump, thus eliminating the need for a dosage pump. But the dosage becomes less accurate and the method is more cumbersome in the latter case.

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

Recirculation of the washing water

Gives a better utilisation 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 by means of 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 on condition that the tank atmosphere is well below LEL (lower explosive limit). Check during washing! Washing should be carried out under inert gas atmosphere if cargo vapour concentration is above LEL. There are indications that contaminated washing water may cause incendive static electricity sparks. This 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 fresh water.

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 by means of 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.

In a similar operation the cleaning chemical is added to an ejector, 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 fresh water. 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 appreciable 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, say 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 in order not to exceed I m/s in the filling pipe . The process should preferably be carried out with the tank inerted .

Steaming with a solvent

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 by means of the heating coils and the solvent vaporises. The solvent condenses to a certain extent on the tank walls and dissolves minute amounts of cargo residues, which after draining of the tank can be washed off using water with/without cleaning agents added.

The amount of solvent should be of such small quantities that the forming of explosive mixtures in the tank is avoided. Choosing 2/3 of LEL as a guide one 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 grammolecule (=as many grams as the molecular weight) occupies 22,2 litres of volume at normal atmospheric pressure and temperature.

Rinsing with water is normally the final cleaning process

Unless the cargo remains can be completely vaporised away. Remember that certain cargoes to be loaded may require fresh water 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 . Finally the tanks should be ventilated dry and any water left be dried up with rags prior to 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, which may contaminate the next cargo. Gas freeing may also become more difficult 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 very strong, 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 rust from the bulkheads and the rust can be washed down. "Only" rust-removal remains. This method is intended for uncoated tanks only.

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