Tank cleaning methods onboard chemical tankers
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.
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.
tank cleaning process
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).
Tank Coating
The suitability of a vessel's particular tank coating for loading a specific chemical cargo can be ascertained by reference to the coating manufacturer's "resistance list" which may also advise on special tank cleaning procedures.
The vessel's officers should have up-to-date knowledge of the condition and integrity of the cargo tank coatings, since degraded and/or perforated/missing coating may cause tanks to be rejected for loading a particular chemical grade.
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.
Odour
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.
Tank Cleaning reference Guide
"The Tank Cleaning Guide", 8th Edition, 2011 by Dr A. Verwey lists over 400 liquid substances carried in bulk. The cleaning charts recommends the tank cleaning procedure for each listed product to clean from and to in a matrix form. The guide has been developed from the rules and recommendations set out in the various IMO publications relating to dangerous chemicals in bulk and to ships carrying those chemicals.
Related info:
Main Info pages!
Home page ||| Chemical hazards ||| Cargo planning & Stowage ||| Cargo loading ||| Cargo documents ||| Safe stability ||| Cargo care ||| Preparation for unloading ||| Inert gas systems |||Gas freeing ||| Nitrogen handling ||| Chemical handling Safe practice |||Handling equipments ||| Cargo & Ballast pumps ||| Cargo tanks |||Tank cleaning |||Special cargoes |||Spills emergencies |||Fire protection
Copyright © 2011 Chemical Tanker Guide.com. All rights reserved.
Chemicaltankerguide.com is merely an informational site about various aspects of chemical tankers safe practice that should be adopted by any prudent mariner. If you are interested in finding out more about chemical tanker guideline please visit IMO official website www.imo.org.
tank cleaning process
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).
Tank Coating
The suitability of a vessel's particular tank coating for loading a specific chemical cargo can be ascertained by reference to the coating manufacturer's "resistance list" which may also advise on special tank cleaning procedures.
The vessel's officers should have up-to-date knowledge of the condition and integrity of the cargo tank coatings, since degraded and/or perforated/missing coating may cause tanks to be rejected for loading a particular chemical grade.
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.
Odour
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.
Tank Cleaning reference Guide
"The Tank Cleaning Guide", 8th Edition, 2011 by Dr A. Verwey lists over 400 liquid substances carried in bulk. The cleaning charts recommends the tank cleaning procedure for each listed product to clean from and to in a matrix form. The guide has been developed from the rules and recommendations set out in the various IMO publications relating to dangerous chemicals in bulk and to ships carrying those chemicals.
Related info:
-
Fixed and portable tank cleaning equipments
The installation of fixed tank washing machines within a cargo tank allows an inert atmosphere to be maintained during the washing operation, and thus permits cleaning in a closed mode in compliance with port regulations prohibiting release of noxious vapours. Their installation and use also reduces crew exposure to cargo vapours and inert gas.
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Tank cleaning and risk with cargo contact
Different chemicals affect the human body in many different ways. A general information and some practical advice are available in Appendix,7, of "Medical first aid guide for use in accidents involving dangerous goods" published by IMO, WHO and ILO ref (36). -
Practical example of solving tank cleaning problems
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. -
Tank cleaning fatality- case study & lessons learned
In the trade practice onboard chemical tankers, it is not uncommon that the use of methanol or other chemical/detergents is undertaken to achieve the required standard of cleanliness inside the cargo tank, pipelines, cargo hoses, manifold adaptors/reducers, gauging equipment, etc. Industry publications clearly highlight and caution regarding their use due to their flammability and toxicity hazards. -
Pre-cleaning /washing of cargo tanks
Washing between different grades of cargo is the most common reason for tank cleaning. In most cargo sequences on chemical tankers, this cleaning may consist of no more than a simple hot or cold seawater wash. A simple water wash will disperse many types of chemicals and has been found effective between clean petroleum products such as gas oil and kerosene. - Final cleaning of cargo tanks prior loading
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. - Tank cleaning and posoning hazards
Certain substances affect the tissues locally as an irritant (cashew nut shell oil) or cause grave damage to the eyes, skin or mucous membranes (e g strong acids and caustic). Other substances may be absorbed by contact to the skin without local effects (e g nitrobenzene, aniline). - Testing of tanks and cargoes
Most common tests and checks for oil and chemical cargoes include testing tank walls for cleanliness. Testing is normally carried out by independent surveyors who, according to local practice or a written agreement in the charter party, are accepted by shipper, receiver and owner. -
Practical tank cleaning methods for various noxious liquid cargo
Tanks that may have contained monomer or drying oils should first be cleaned with sufficient cold water quantities to avoid polymerization of cargo residues. In some cases, it is necessary to employ tank cleaning chemicals, but their use is generally limited as it may be difficult to dispose of slops. -
Special tank cleaning method
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. -
Determining proper tank cleaning by acid wash method
The acid wash method is used if there is any suspicion that a cargo of aromatics may have been contaminated by a previous oil cargo. The method is also used as a check that a tank is sufficiently cleaned before loading aromatics. -
Supervision of all tank cleaning and gas freeing operations
Tank cleaning is essential on-a chemical tanker, but it must be recognised as a potentially hazardous operation, and rigorous precautions should be observed throughout the process. Together with gas freeing, it is probably the most hazardous operation routinely undertaken on a chemical tanker. -
Disposal of tank washings, slops and dirty ballast - safe method
During normal operations of a chemical carrier, the main need to dispose of chemical residues, slops or water contaminated with cargo will arise during or immediately after tank cleaning. Final disposal of slops or washwater should be in accordance with the ship's P&A Manual. Tank washings and slops may be retained on board in a slop tank, or discharged ashore or into barges. -
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. -
Determining water contamination in chemical cargo
Presence of free water in non water-soluble products can, very roughly be determined on board by warming a sample of the product in a test tube or in a bottle. Water will then collect at the bottom of the tube and can be seen after some time. -
Determining Sulphur contamination (sulphides) in chemical cargo
Certain products, in particular "virgin naphta feedstock'' (petroleum naphta) are severely contaminated by minor amounts of sulphides (and also lead compounds), which poison catalysts in further processing. Previous heavy oils or dirty harbour ballast water may have left traces of sulphides in the cargo tanks. -
Chloride contamination in chemical cargo - how to resolve?
Certain cargoes are very senstive to chloride contamination, in particular glycols, methanol, ethanol . The tanks should be finally washed with fresh water. However, chlorides (salt) may still be present and a check might be useful. The greatest risk for salt deposits is on horizontal surfaces. -
APHA (Hazen) method for determining color of very light chemical products
A method called APHA (Hazen) is often used for very light products, defined in ASTM D-1209, viz aromatics, ketones. This colour scale is defined with an origin in 100 cc distilled water (value 0) to succesively higher values (max 500) by adding APHA-solution (a platinum-cobolt salt solution). -
Loading, discharging & care of Phenol - Safety guideline
PHENOL is carried at sea in a generally pure state. As a result, it has a high freezing point of approximately 40~ 41deg C. PHENOL is also extremely dangerous when it comes into contact with the eyes or skin and can be fatal. -
Hazards of Phenol - safe handling of Phenol on chemical tankers.
Phenol must be carried at temperatures within charterers instructions, typically between +50 and + 60 degrees C. Heating instructions of the Shipper or Owners must be followed to avoid protests and delays in the port of discharge. Overheating can damage this cargo. A full cargo heating log must be maintained. -
Handling benzene & methanol safety precautions
Benzene is known as a strong carcinogen and known to cause leukaemia. When handling cargoes with more than Benzene concentration of 0.5%, the Master is to ensure that all personnel involved are aware of the long term hazards. -
Personal protective equipments for carcinogens & cyanide-like cargoes onboard chemical tankers
A carcinogen is a substance that may cause cancer by contact or by inhalation. It is therefore essential that the highest safety precautions are taken when handling these cargoes. Access to deck areas must be restricted to duty personnel only. All accommodation doors and ports must be closed and ventilation put on recirculation. Any member of crews involved in cargo operations must wear chemical protective suits and breathing apparatus -
Handling ACRYLONITRILE safety precautions
ACRYLONITRILE are high value and require sophisticated handling for safety, health and loss prevention reasons. They need careful consideration prior loading , tank coating compatibility, cross compatibility with other cargoes carried, environmental controls if required (inerting). -
handling ISOCYANATES safety precautions
Product safety data sheets may be available from various sources. For safety preparation, until the specific product safety data sheet can be obtained, Chemical Data Guide for Bulk Shipment by Water (U.S. DoT), should be used. -
Loading, carrying & discharging of Sulphuric acid - regulatory requirements & special handling methods
IBC code compatibility chart strictly prohibits water in adjacent compartment to Sulphuric acid as you are aware if both come in contact with each other will generate a violent reaction. It is therefore recommended that the during loading of sulphuric acid adjacent ballast tanks to be always stripped dry to the maximum efficiency of the deballasting equipment used.
Main Info pages!
Home page ||| Chemical hazards ||| Cargo planning & Stowage ||| Cargo loading ||| Cargo documents ||| Safe stability ||| Cargo care ||| Preparation for unloading ||| Inert gas systems |||Gas freeing ||| Nitrogen handling ||| Chemical handling Safe practice |||Handling equipments ||| Cargo & Ballast pumps ||| Cargo tanks |||Tank cleaning |||Special cargoes |||Spills emergencies |||Fire protection
Copyright © 2011 Chemical Tanker Guide.com. All rights reserved.
Chemicaltankerguide.com is merely an informational site about various aspects of chemical tankers safe practice that should be adopted by any prudent mariner. If you are interested in finding out more about chemical tanker guideline please visit IMO official website www.imo.org.