We, constellation marine services as marine and ship surveyors provides an expert guidance and offer supervision on Tank cleaning of a chemical tanker which is more efficient and economical, including cargo contamination claim advisory & cargo expeditors for oil & chemical cargoes.
There are several operations that had to be carried out to maximize the efficiency of tank cleaning. That new innovative technologies in tank cleaning have raised the standards in marine industry. There are many ways to realize the optimum cleaning efficiency for different tanks. The evaluation of tank cleaning options starts with the audit of operations which are as follows,
1. Number of tanks require cleaning.
2. Are obstructions in the tanks present (e.g. agitators, mixers).
3. What residue needs to be removed.
4. Are cleaning agents required or is water sufficient?
5. What are the methods that can be used for tank cleaning?
After these steps, verify the results obtained and ensure that the best cleaning values can be achieved in terms of accuracy and reliability. Advancements in technology have made it easier to remove stubborn residues, shorten cleaning cycle times and achieve higher levels of automation. Presented in this paper are the methods used to perform tank cleaning to a standard which complies with the current legislation? If tank cleaning technologies are effective, then operating costs are minimal.
Tank cleaning methods onboard chemical tankers
On chemical Tankers, cargo tanks may be coated with various types of materials. Therefore, it is important to check with the P&A manual and the Paint Manufacturers Coating Resistance list prior to commencing Tank Cleaning Operations in order to ascertain the tank coating materials and any limitations with regards to temperature and use of cleaning chemicals which may be applicable to the vessel.
Tank cleaning on chemical a tanker is generally the responsibility of the ship staff. Tank cleaning must be planned according to the previous cargo 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 the final intended purpose of the cargo.
Examples: glycol is intended for cosmetics or pharmaceutical purposes, this requires completely cleaned and odorless tanks. However, glycol intended for antifreezes; caustic soda for making paper, is more sensitive to iron contamination than caustic soda for the aluminum industry.
Fig: Modern Chemical tanker tank cleaning process using steam spray
It must be mentioned first that the majority of cleaning operations on board chemical tanker are being carried out by means of water washing only. Further chemical cleaning is required for only a limited number of cargoes, but these cases may be very important.
One must take into considerations, the nature of the previous cargo, and the cargo to be loaded, time factor, available equipment and cleaning chemicals etc. It is stated that the necessary degree of cleanliness for several products is in line with what cargo surveyors normally require.
Generally speaking one should use mechanical tank cleaning methods, that is usually washing with water, before applying more expensive methods involving chemical cleaning agents. The most expensive, and least safe, method is manual cleaning, which should be kept to a minimum. Manual work should preferably be reduced to inspection and possibly to a final drying up of washing water only.
It is important to drain the tanks as much as possible to reduce pollution of the seas to an absolute minimum. This will now be even more important with the anti-pollution Convention of 1973.
Examples of measures on how to obtain the best possible cargo stripping:
i) Due regard to ship’s trim and heel.
ii) Viscous cargos may first be stripped from the various tanks to one tank near the pump room and from there be pumped ashore.
iii) Keep the cargo temperature sufficiently high so that the cargo drains also from remote corners of the tanks, especially in cold climates.
iv) Waxy deposits under the heating coils can sometimes be melted out by means of filling with water and then applying heat to the coils.
v) Sometimes steaming is allowed during discharge of molasses, which facilitates draining of molasses from the bulkheads.
vi) Vegetable oil tanks may in the last phase of discharge be recirculated and hosed down with vegetable oil taken from the cargo pump delivery side . Similarly, phosphoric acid can be recirculated to loosen sediments on the tank bottom.
vii) Drain cargo piping to shore. It is useful to have a small stripping pump with 50 mm delivery line to the hose connection for delivery of contents in the cargo piping to shore.
viii) Before loading sensitive cargoes: mudboxes, valve bodies and pump housings must be drained by opening the drain plugs (with due regard to personal safety).
Water washing may be inadequate or inappropriate after the carriage of certain products because tanks can only be cleaned by special methods or cleaning agents. Where it is decided to use these special cleaning methods, 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 are 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 vapors 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 localized 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.
Thus, we can conclude that tank cleaning on a chemical tanker is a challenging task and planning of carrying out same plays a very important role, as there is lot of commercial pressure and it has to be completed in a given time limit.
Capt. Manish Kumar
Principal Consultant (chemical and oil/products cargoes)
Constellation marine services.
(Prime area of work – UAE ports, Fujairah, Abu dhabi, Ras Al Khaima, Mina Sagar, Sharjah, Ajman, Dubai, Hamriyah, Jabel Ali, Bahrain, Kuwait, Qatar (Mina Hamad) (mesaieed), Egypt (Alexandria) or fuel, Sohar, Muscat, Salalah).