CaptainAR ´s Port Captain assist Master and Chief Officer during Tank Cleaning operations.
We check that the operations are done under standards and correct procedures, always maintaining safety first.
Tank cleaning method and the cleanliness involved have different standards depending upon the previous cargo and the cargo to be loaded. The most common is the following:
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 utilized. 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 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 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.
Rinsing with water is normally the final cleaning process
Unless the cargo remains can be completely vaporized away. Remember that certain cargoes to be loaded may require fresh water rinsing (risk for chloride contamination), eg 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.
In the trade of chemicals loose rust will never be accepted upon loading. The rust itself may be harmless for many products, eg 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.