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Final cleaning of tanks prior loading noxious liquid cargo onboard chemical tankers

Method of final cleaning to be used depends on both previous cargo and cargo to be loaded. As a general rule the tanks and piping shall be completely drained of water or residues before loading. The bottom of the tanks may have to be dried up with rags.

Some cargoes are very sensitive to chloride contamination. The tanks to be loaded with these products have to be washed out thoroughly with fresh water. Examples on such products are first of all glycols and alcohols (methanol) but also aromatics and acetates. Vegetable and animal oils, gasoline, heavier oils, caustic soda do not normally require a fresh water rinse of the tanks to be loaded.

Tank washing process

a) Products with good solubility in water: The final cleaning is a direct continuation of the pre-cleaning with water, possibly with hot water for viscous products. If sensitive cargoes are to be loaded any odours that remain must be ventilated and/or steamed away.

b) Volatile products , as mentioned above, do not often require cleaning after discharge.

c) and d) Vegetable oils (drying and non drying) and animal oils are best cleaned by using an alkaline chemical, usually caustic soda, added to the hot washing water. The oil residues will then be converted into soap, which is a water soluble product and which will be rinsed off the bulkheads.

The cleaning chemical is consumed quite rapidly if there are un necessary amounts of cargo left. About 100 kgs of caustic soda are required, counted as a 50 % solution, to saponify 50 kgs of oil. Usually the cleaning chemical is added to the washing water in a 2-3 % concentration of caustic soda (max 5 %. Warning: alkaline washing solutions are dangerous to eyes, use goggles!

IMPORTANT: Zinc silicate tank coatings are adversively affected by alkaline cleaning chemicals. Never use caustic soda or alkaline cleaning agents in zinc silicate coated tanks !

Mineral oils and waxes cannot be saponified. It is possible to remove vegetable and animal oil residues by the use of detergent cleaning chemicals. Detergents reduce the surface tension of the washing water, thus "lifting off" the residues. Cleaning with detergents usually works out more expensive than cleaning with alkaline chemicals but it might be the only solution with zinc silicate coated tanks.

e) Polymerizing products can, after a cold prewash, usually be cleaned off using cold or moderately warm water with a cleaning chemical of a detergent type, emulsifier or with synthetic soap added

f) Heavy oils, lubrication oils, lubrication oil additives, gas oils and similar often account for the most difficult cleaning problems, particularly if demanding products like methanol and aromatics are to be loaded. After the pre-wash: hot washing with cleaning chemicals of a detergent type, solvent cleaner, emulsifier or synthetic soap. Emulsifiers assist in forming minute droplets of the product suspended in the water.

The emulsifiers are usually of a type where one end of its molecules attract water and the other end oil. Soap from an alkaline treatment acts also as an emulsifier. A dosage of 1-2% to the washing water is usually recommended. Certain emulsifiers are so effective that the emulsion will not break up (water and product separate) after a long time, even if heated. If such a washing solution is to be discharged to a slop-receiving facility ashore difficulties may arise. Check with the supplier of the cleaning chemical and the slop-receiver before making your purchase of cleaning chemicals!

"Solvent cleaners" contain water-soluble and hydrocarbon dissolving agents (ketones, aromatics, alcohols), usually in addition to other cleaning effects (alkaline etc). Solvent cleaners often have relatively low flash points: treat them with care, as flammable products.

Detergents contain wetting agents (tensides) which reduce the surface tension of the water, thus "lifting off" impurities from the tank wall.

When several cleaning chemicals are used their sequence of application should be: (water), detergent/alkaline (caustic), synthetic soap, emulsifier, solvent cleaner, solvent.

Leaded gasoline leaves posionous lead compounds on the tank walls which is unacceptable if edible products are to be loaded next.

For edible products the tank is washed with 10 % acetic acid to remove lead remains. This method does not, however, guarantee that the tank will pass a chemist's test and be accepted for edible products. Wine has a particular tendency to absorb lead compounds from the tank walls. Edible products should not be carried until after several other intermediate cargoes.

It is becoming more and more common to apply the cleaning chemical in undiluted form

Related Info:

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

Cargo tank damage during pigging operations

Other info pages

Draegar Chemical detector tubes use and reading correction guideline
These instruments, often referred to as Draeger tubes, normally function by drawing a sample of the atmosphere to be tested through a proprietary chemical reagent in a glass tube. The detecting reagent becomes progressively discoloured if a contaminant vapour is present in the sample. The length of the discoloration stain gives a measure of the concentration of the chemical vapour which can be read from the graduated scale printed on the tube. Detector tubes give an accurate indication of chemical vapour concentration, whatever the oxygen content of the mixture

Requirements of various grade chemical cargo heating
: The voyage orders will contain heating information, if heating is required. As a rule the final heating instructions are given by the Shipper in writing to the Master / Chief Officer in the port of loading. If those written instructions are not given, the master should request them and issue a Letter of Protest if they are not received at departure. In the latter case the management office should be immediately informed.

Recommended temperature monitoring equipments onboard
:Temperature sensors are fitted so that the temperature of the cargo can be monitored, especially where required by the IBC Code. It is important to know the cargo temperature in order to be able to calculate the weight of cargo on board, and because tanks or their coatings often have a maximum temperature limit. Many cargoes are temperature sensitive, and can be damaged by overheating or if permitted to solidify. Sensors may also be fitted to monitor the temperatures of the structure around the cargo system.

Cargo instruments
:In order to maintain a proper control of the tank atmosphere and to check the effectiveness of gas freeing, especially prior to tank entry, several different gas measuring instruments need to be available for use. Which one to use will depend upon the type of atmosphere being measured.

Liquid level gauges
:The accuracy required of chemical carrier level gauges is high because of the nature and value of the cargo. To limit personnel exposure to chemicals or their vapours while cargo is being handled, or during carriage at sea, the IBC Code specifies three methods of gauging the level of a liquid in a tank - open, restricted or closed

Overflow control
:Certain cargoes require the designated tank to be fitted with a separate high level alarm to give warning before the tank becomes full. The alarm may be activated by either a float operating a switch device, a capacitive pressure transmitter, or an ultrasonic or radioactive source. The activation point is usually pre-set at 95% of tank capacity.

Oxygen analysers
:Oxygen analysers are normally used to determine the oxygen level in the atmosphere of an enclosed space: for instance, to check that a cargo tank can be considered fully inerted, or whether a compartment is safe for entry.

Vapour detection
:Ships carrying toxic or flammable products (or both) should be equipped with at least two instruments that are designed and calibrated for testing the gases of the products carried. If the instruments are not capable of testing for both toxic concentrations and flammable concentrations, then separate sets of instruments should be provided.

Alarm circuit
:An important feature of many modern measurement and control instruments is the ability to signal a particular situation. This can be a main operational alarm that gives an indication of a pre-set situation such as liquid level in a tank, or a malfunction alarm indicating a failure within a sensor's own operating mechanism. The designs and purposes of alarm and shutdown circuits vary widely, and their operating system may be pneumatic, hydraulic, electrical or electronic. Safe operation of plant and systems depends on the correct operation of these circuits and a knowledgeable reaction to them.

Gas freeing
:Gas freeing onboard chemical tankers is required for entry into cargo tanks, for hot works or washing for clean ballast tanks. Gas Freeing is one of the most hazardous operations routinely undertaken onboard a Chemical Tanker and the additional risk created by cargo gases expelled from the tanks, which may be toxic, flammable and corrosive, cannot be over-emphasised.

Cargo tank damage during pigging operations
:Blowing and pigging of pipelines at terminals poses inherent risks for the terminal and a chemical tanker. Frequent damages to tanks have occurred. If there are doubts about the shore operation or signs of problems ashore the OOW must immediately request clarification.

Reference publications

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