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Special tank cleaning & tank cleaning best practice for chemical tankers

Water washing may be inadequate or inappropriate after the carriage of certain products, because tanks can only be adequately cleaned by special cleaning methods or cleaning agents. Where it is decided to use a special cleaning method, and well documented experience indicates that it is safe to do so, thorough company guidance should be provided that describes the procedures for the ship to follow.

Where a special cleaning method is to be used in port, local authorities may impose additional safety or environmental requirements.

Some cargoes may react with certain cleaning agents and produce large amounts of toxic or flammable vapours, or render equipment such as pumps inoperable. The choice of a tank cleaning agent should be made with full knowledge of the cargo characteristics.

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.

If, however, the only practical means of cleaning involves personnel entering the tank then the precautions should be strictly followed. No one should enter any cargo tank unless express permission to do so has been received from the responsible officer, and all appropriate precautions taken. The tank atmosphere should be safe for entry and an entry permit issued. Chemical absorption detectors should be used for detecting the presence of specific gases and vapours at TLV levels.

In exceptional circumstances the requirement might arise for wiping down product residues from the tank walls by using a chemical solvent in a localised area. The amount used should be small, and the personnel involved should be aware that its use may modify the atmosphere. The introduction of the solvent into the tank might also generate additional risks such as toxicity or flammability. Such risks should be carefully evaluated before starting the operation, which should not be undertaken unless the personnel involved can be effectively protected from those risks. Data sheets for the chemical solvent used should be available on board.

In addition, manufacturer's instructions or recommendations for the use of commercial products should be observed, and the resulting slops disposed of in accordance with the ship's P&A Manual.

Precautions during loading, discharging, sweeping, tank cleaning of various oils

Oxidise Means, absorption of oxygen from the air, and so the air remaining in the tank containing vegetable or animal oil, or coated with residual quantities of these oils, may not have enough oxygen in to support life. This is most likely to happen when a tank has been closed for a long time after the oils or fats have been discharged. The residual oil or fat on the structures in the tank starts to decompose (rot). When it does this, it not only absorbs oxygen, but also produces various toxic and asphyxiating gases such as methane, carbon dioxide and hydrogen sulphide. These gases may also be present in sumps or drains, where oils and fats of this nature remain.

Sometimes it is essential to sweep the tank during / after discharging of the vegetable, animal oils and fats, as many residues can remain on the bottom of a tank. In this case the physical tank entry is required. Therefore, it is dangerous to enter a tank, which has been used for the vegetable or animal oils or fats unless it is being continuously ventilated.

Tank cleaning best practices (Vegetable Oils - drying or semi-drying)

Animal and Vegetable Oils easily saponify when combined with Caustic Soda. To saponify means converting the oil into soap, or, in practice bringing into suspension. However, alkaline cleaners can only be used if the tank is made from stainless steel or coated with epoxy based coating.

Drying and semi-drying vegetable and animal oils react with oxygen to form a varnish-like polymeric film. This is very difficult to remove from the bulkheads. Heat increases the reaction speed. Therefore the initial washing of these products must be done with water at ambient temperature without any delay after unloading of cargo.

If full tank cleaning in port is not possible after discharge, a quick wash should be given to the tanks to ensure the atmosphere in the tank remains moist. As an alternative, a low pressure steam or limited hand-hosing may be applied.

Heated cargoes in adjacent tanks should be avoided. In case of doubt as to whether oil is non-drying or drying, always use ambient water first. Initial washing must be done using ambient fresh or salt water. Basic cleaning should be done for 3 hours followed by 3 hours hot washing (80C).

Related info:

Practical example of sampling chemical cargo

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

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