Tank cleaning methods onboard chemical tankers
Chemical tanker safety guideline
The tanks of Chemical Tankers may be constructed or coated with various different types of materials and 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, use of cleaning chemicals etc which may be applicable to the vessel.
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.
Examples: glycol intended for cosmetics or pharmaceutical purposes requires cleaner and completely odourless tanks than does glycol intended for antifreezes; caustic soda for making paper is more sensitive to iron contamination than caustic soda for the aluminium 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 tankers 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 consideration the nature of the previous cargo and the cargo to be loaded, time factor, available equipment and cleaning chemicals etc.It is stated the necessary degree of cleanliness for a number of products, 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 in order to deliver all cargo and 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 on measures on how to obtain the best possible cargo stripping:
i) Due regard to ship's trim and heel.
ii) Viscous cargoes 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 be 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).
Practical examples on solving problems
The following text is intended as a general guide and will give some practical examples on problems and methods. The information given should not substitute your own or others' good and proven methods! Also consult shippers and tank inspectors coming on board.
Analyze the properties of the previous cargo and take advantage of its "weak points", e.g as follows:
i) Water soluble? If the cargo is reasonably soluble in water then chemical cleaning agents are unnecessary in most cases.
ii) Will an increased cleaning temperature cause a beneficial reduction of cargo viscosity and lower surface tension or could it cause the opposite: that cargo residues polymerize or oxidize ("dry"). Polymerization and drying must be avoided, therefore the first cleaning operation must be carried out cold.
iii) Is it possible to emulgate the cargo in water or in water with emulgators added? Make a test on board.
iv) Will the product be affected by alkalies? Caustic soda is a relatively cheap and easily available alkaline chemical.
v) Will the product dissolve in other easily available products by which the tank walls can be treated? (Succesively "upgrading" or "floatation" methods).
vi) Will cargo residues vaporize without leaving any traces?
vii) Can cargo remains be safely mixed with the cargo to be loaded? In many cases it is not known what the next cargo will be but sometimes this method can be applied.
Some products are very sensitive to foreign odours, usually stemming from previous cargoes in the same tank. Examples of sensitive cargoes are: glycols, glycerine, vegetable and animal oils, molasses.
Odours remaining after a thorough tank cleaning are usually best removed by steaming and/or ventilation of the tank. Steaming "sweats out" cargo from pores etc. Cargo piping may also have to be steamed out. Epoxy coatings should not be heated above 60 - (80) degr.C, zinc silicates tolerate somewhat higher temperatures. So called deodorant fresh air sprays have an effect only on the atmosphere in the tank. Usually the odours stem from cargo residues on the actual tank walls and will therefore soon come back. The spray method is more of a symbolic value with regard to the care of the cargo.
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
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