- 1 Introduction
- 2 Precautions and Preventions before Tank Cleaning
- 3 Precleaning Measurement
- 3.1 Physical Properties
- 3.2 Precleaning Methods for various Materials
- 3.2.1 Products that are Water Soluble or Miscible
- 3.2.2 Products With a High Melting Point
- 3.2.3 Products with High Viscosity
- 3.2.4 Vapour Pressure/ Boiling Point
- 3.2.5 Flash Point / Flammable Range
- 3.2.6 Density / Specific Gravity
- 3.2.7 Product Characteristics
- 3.2.8 Evaporation of Light Substance
- 3.2.9 Reaction with Water
- 3.2.10 Reactions with Oxygen, Deying Products
- 3.2.11 Reactions with Water Hardness Compounds
- 3.3 Selection of Pre Cleaning Methods
- 4 Final Cleaning
The tank cleaning suggestions in this manual are guidelines and will be updated based on future experience. Sufficient tank cleaning should be performed after every discharge, irrespective of the next cargo to be loaded. Due to quality reasons many chemical products require ever-higher cleaning standards and improved analytical methods are able to detect any impurities or residues of prior cargoes. Survey companies will therefore perform more and more wall wash tests. Not only chlorides and water miscibility (Hydrocarbons) will be tested. Additionally the non volatile matter (NVM), the colour, the permanganate fading time (PTT), as well as ultra violet transmission (UV) will be tested. A.m. tests will be especially performed after vegetable or mineral oils, but also after various other restricted prior cargoes like paraffin’s, phthalates, olefins and other hydrocarbon products and chemicals. A fundamental understanding of the cargo to be cleaned as well as the cargo to be loaded is therefore of utmost importance for an efficient tank cleaning. Before loading of sensitive cargoes it is advisable to clean tanks and lines finally with a suitable cleaning solution in order to achieve the required cleaning standard. The tank cleaning manual is designed to assist tank cleaning and does not constitute an instruction. RX MARINE INTERNATIONAL cannot be held responsible for the content of the manual and will not accept any liability. The resistance guidelines of the coating manufacturer have to be carefully observed.
Precautions and Preventions before Tank Cleaning
Prevent Reaction with Water Hardness Compounds
Water hardness is formed by the calcium and magnesium content of the water. Sea water has a very high water hardness. Some products like fatty acids and vegetable oils with a high free fatty acid content will form white sticky residues, if they are cleaned with a water of a high water hardness (e.g. sea water). Isocyanates must never come into contact with water, not even the residues, because the reaction product and insoluble urethane (plus CO2) are very difficult to remove. Such products must be washed with a suitable solvent, that does not contain any water.
Drying and semi-drying vegetable and animal oils react with oxygen to form a varnish-like polymeric film. This is very difficult to remove from the bulkheads etc. Since heat increases the reaction speed the initial washing of these products must be done with water at ambient temperature without any delay after unloading the cargo.
Evaporation of volatile substances Cargoes consisting of mixtures with different vapor pressures should neither be cleaned by evaporation, nor prewashed hot. The evaporation of the light substances from a mixture could result in non-volatile residues, which are very difficult to remove.
The initial wash of products that tend to polymerise should be carried out with cold (ambient) water. Washing with hot water may result in polimeric residues being left in tanks and lines, which are very difficult to remove.
Pre-cleaning is the first cleaning step usually carried out without cleaning additives in order to remove the major amount of product residue.The first cleaning step is of utmost importance and should be carefully selected. It can be very difficult to obtain a satisfactory cleaning result after an initial failure.The selection of the proper pre-cleaning procedure should be based on the physical properties and characteristics of the product to be cleaned.
|Water miscibility||Percentage of product soluble/miscible in water completely / partly / not water soluble or miscible.|
|Melting point||Temperature at which a product change from solid to liquid. No solidification; solidification below 00 C; solidification above 00 C up to high melting points of over 600C.|
|Viscosity||Physical characteristic of the liquid: low viscosity = watery high viscosity: syrupy|
|Vapour pressure||Is the equilibrium pressure of the saturated vapour above a liquid expressed in bars at a specific temperature. The vapour pressure of a product is closely connected to the boiling point|
|Boiling point||Temperature at which a product exhibits a vapour pressure equal to the atmospheric pressure.|
|Flash point flammable products only||Temperature at which a product will give off enough flammable vapour to be ignited.|
|Explosive range/ Flammable range (flammable products only)||Is the range of gas concentration in air (%) between the lower and upper flammable (explosive) limits (LEL/UEL). Gas mixtures within this range are capable of being ignited and of burning.|
|Density||Is the ratio of the mass of a volume of a product to the mass of an equal volume of fresh water (1, 0). For a product of no or limited solubility, the density indicates whether it floats on water or sinks.|
Precleaning Methods for various Materials
Products that are Water Soluble or Miscible
Water soluble or miscible products are easy to clean with water. The solubility of the product might increase at higher temperatures. The use of a cleaning solution is only advisable for reduction of the cleaning time or for smell removing (e.g. Phenol).
Products With a High Melting Point
These products should be washed with a temperature above the melting point. During washing there should be no ballast water or cold cargoes adjacent to the tank to be cleaned. During cleaning special attention has to be taken on liquid and vapour line systems to avoid freezing/ solidification at cold line segments. NOTE: Washing as soon as possible after discharge is recommended.
Products with High Viscosity
These products should be washed with a moderate or hot temperature. In general the viscosity is closely connected to the temperature and will decrease at higher temperatures. During washing there should be no ballast water or cold cargoes adjacent to the tank to be cleaned. Washing as soon as possible after discharge is recommended.
The respective product characteristic of the product in question has to be observed. Some high viscose cargoes e.g. some lub additives have to be pre-cleaned with a warm or moderate temperature.
Vapour Pressure/ Boiling Point
Products with a high vapour pressure (higher than 50 mbar at 200 C) can be removed from the tank by evaporation (ventilation). As always when ventilating special care must be taken to prevent the risk of explosion (flammable products) or towards toxic vapours. All safety precautions have to be taken.
Products with low vapour pressure, or products which leave residues (NVM) or contain inhibitors should never be cleaned by evaporation (ventilation). The non-volatile matter (NVM) content in the specification of the product is a good indication to establish if a product might leave some residues.
Substances containing inhibitors, even if the quantities are low must be washed thoroughly. Any inhibitor left in the tank is difficult to remove and has nearly always a negative influence on the wall wash and on the next product to be loaded. Products mixtures with different vapour pressure of the individual compounds or crude products should never be pre-washed hot and should not be cleaned by evaporation (ventilation).
Flash Point / Flammable Range
The flashpoint is the lowest temperature at which a product give off sufficient gas to form a flammable gas mixture to be ignited. The flashpoint is measured in standard apparatus using a prescribed procedure (e.g. closed cup). The flammable range/explosion range is the gas concentration in air of a respective product between the lower (LEL) and upper (UEL) explosion limits. Gas/air mixtures within this range are capable of being ignited and of burning. In an undefined atmosphere, the vapours in the tank may be in the flammable range. The only way to guarantee that an explosion cannot occur during washing is to make certain that there can be no source of ignition. (Reference is made to ISGOTT Chapter 9)
Density / Specific Gravity
This is the ratio of the mass of a volume of a product to the mass of an equal volume of fresh water (1, 0). For a product with limited or no solubility in water, a density below 1.0 indicates that the product float on the water, a density above 1.0 indicates that the product sink in water.
Polymerisation Polymerization is a poly-reaction of a monomer compound which molecule contains double bonding or rings.The poly-reaction can occur under following influences:
- By an initiator (reaction partner)
- By heat (thermic polymerization);
- By light (photo polymerization)
- By radiation (radiation polymerization)
Some unsaturated compounds with one or more double bondings are able to polymerize:
- Vinyl – compounds
- Vinyliden – compounds
- Acryl – compounds
- Carbonyl – compounds (e.g. Aldehyde)
- Ethylene Oxide, Propylene Oxide
- Ethylene, Propylene, Isobutylene, Butadiene and Isoprene
Homo-polymerization = only one monomer is involved in the poly-reaction.
Co-polymerization = two different monomers are involved in the poly-reaction.
Polymeric = the product produced by the poli-reaction. To prevent the polymerization some products contain inhibitors
The initial wash of these products should be carried out with cold (ambient) water. Washing with hot water may result in polymeric residues being left in tanks and lines, which are very difficult to remove. To avoid any polymerization by loss of inhibitor-protection it is also recommended to wash the tanks immediately after discharge.
Evaporation of Light Substance
Cargoes which are mixtures of products with different vapour pressure and boiling point or cargoes which contains crude compounds should never be cleaned by evaporation (ventilation) and should never be pre-washed hot. The evaporation of the light substances from the mixture may result in residues being left in the tank, which are very difficult to remove.
Reaction with Water
An example for the reaction with water are the isocyanates (e.g. MDI or TDI). The reaction between water and isocyanates will produce urethane, which is insoluble and very difficult to remove. Therefore water can not be used to clean tanks from this kind of products.
These kinds of products must be washed with a suitable solvent like Methylene Chloride or Monochlorbenzene. It is of utmost importance that the solvents used for cleaning as well as all used equipment is free of any moisture. A second example is Sulphuric Acid which reacts vigorously with water in a very strong exothermic reaction (setting free large amount of heat). Furthermore diluted Sulphuric Acid is corrosive to stainless steel (in general between 10-90%). To absorb the heat and to minimize the corrosion of the stainless steel Sulphuric Acid must be initially washed without any interruption by large amounts of cold fresh water. The waste water has to be pumped continuously out without any delay.
Reactions with Oxygen, Deying Products
Drying and semi-drying vegetable and animal oils react with oxygen to form a varnish like polymeric film which is very difficult to remove. Heat increases the reaction speed. Therefore the initial washing of these products have to be done with cold (ambient) water or a temperature of max 100 C above the melting point of the product. Drying also takes place when a tank is left uncleaned therefore washing as soon as possible after discharge is recommended.Heated cargoes in adjacent tanks should be avoided.In case of doubt if a vegetable or animal oil is non-drying or drying always uses ambient water first.
Reactions with Water Hardness Compounds
The water hardness is formed by the calcium and magnesium content of the water. Sea water contains a lot of calcium and magnesium and has therefore a very high water hardness. Some products like fatty acids and vegetable oils with a high free fatty acid content will for mate white sticky residues (calcium and magnesium soaps) if they were cleaned with a water of a high water hardness (e.g. sea water). Heat increase these reaction therefore if sea water has to be used the initial wash has to be done with a temperature of max 10 to 150C above the melting point of the product.
Note: 100C increase in temperature double the chemical reaction rate.The best method to pre-clean these kinds of products is to prepare the necessary amount of fresh water for the pre-cleaning in a separate and clean tank and add a suitable water hardness remover.
If sea water or untreated freshwater has to be used for cleaning the formation of white residues or a white powdery film cannot be avoided. For the removal of white residues see cleaning suggestion for Fatty Acids.
A second example for a reaction with water hardness compounds is the Sodium Silicate Solution. Sodium Silicate Solution will form calcium and magnesium silicates which are very hard to remove. Sodium Silicate Solution should be therefore washed with fresh water only and never with sea water.
Selection of Pre Cleaning Methods
After evaluation of product data and other product specific characteristics, proper pre-cleaning method must be selected. The following are of utmost importance:
- Ambient : cold up to max 300 C
- Warm : 30 – 400 C
- Moderate : 550 C
- Hot : > 550 C
Select the pre-cleaning medium:
- Sea water
- Treated fresh water
- Demi water
Select the pre-cleaning time:
- Normal pre-cleaning time (1-2 circle of the tank cleaning machine)
- Extended pre-cleaning time (3-4 circles of the tank cleaning machines)
Select the pre-cleaning intensity:
- Normal amount of wash water initially to be introduced into the tank
- Large quantity of wash water initially to be introduced into the tank (additional cleaning machines)
Take following items into consideration:
- Outside temperature
- Sea water temperature
- Ballast conditions
Take following items into consideration:
- Sufficient number of tank cleaning machines (shadow areas)
- Sufficient position of tank cleaning machines (shadow areas)
- Sufficient pressure
- Sufficient heating capacity
- Availability of additional equipment and connections to carry out specific cleaning procedures like injection or recirculation.
Final-cleaning is the last cleaning step usually carried out in order to remove all product residues to reach the required cleaning standard. On certain products/product residues it is necessary to introduce a cleaner into the washing process to accomplish a more effective cleaning.For selection of the final cleaning procedure following items are of utmost importance:
Knowledge of the Product to be Cleaned
Physical properties and characteristics of the product as explained before
Nature of the Cargo to be Loaded
Cleanliness requirements of the product to be loaded
- Chemical standard: Visual clean, dry and free of smell or
- High purity standard: Visual clean, dry and free of smell
- Additional wall wash requirements like:
- Hydrocarbon-test (water miscibility)
- Chloride-test * NVM-test (none volatile matters)
- Permanganate time-test (PTT-test)
- UV-test (ultra violet transmission)
Selection of Proper Cleaning Agent
The selection of the proper cleaning agent depends on the following possibilities to remove product residues that are not water soluble:
- Combination of emulsification and saponification
- Soluble/reactive with acid
- Solving by a chemical/product
- Smell removal
Recommended Cleaning Agents
- RXSOL CTC-VOLC
- RXSOL AL 2005
- RXSOL ALK 2004
Emulsification in combination with Saponification
- RXSOL CTC-VOLC in combination with RXSOL AL 2005 or RXSOL ALK 2004.
Acid soluble or reactive
- RXSOL CTC-VOLC in combination with RXSOL MB 1011.
- Methylene Chloride for MDI/TDI
- Base oil or Gas oil for Lub Additives
- RXSOL – X
Smell removal in combination with emulsification
- RXSOL CTC-VOLC in combination with RXSOL-X
- To neutralise the smell of some chemicals (e.g. Acrylate, Nitrobenzene or Pygas)the use of a smell killer may be recommended. Special attention should be given to Gaskets in the tank- and cleaning hatches. If the next cargo is a sensitive cargo the gaskets should be cleaned or renewed.
- Our recommended smell remover is a solution of 0, 4% RXSOL CTC-VOLC and 2% RXSOL – X.
- A smell killer/remover should preferably used in combination with an emulsifier.
- The tanks should be cleaned by recirculation to guarantee that each drop of the smell producing cargo inside of cargo lines, stripping line, pumps and valve seats will be removed.
- The recommended cleaning temperature is 600 C
- Minor residues of a smell producing cargo left in lines, valves and pumps (including pump cofferdams) can contaminate a sensitive cargo.