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Monday, March 15, 2010
TankcLEANIng In GenEral
By Dr. A Verwey
Chemical Laboratories & Superintendence Company
II TANKCLEANING IN GENERAL
The procedure for tank cleaning may include the following stages :
a) Precleaning (butterwothing with water)
b) Cleaning ( butterworthing with water or water and detergents)
c) Rinsing (butterworthing with water)
d) Flushing (with fresh water)
e) Steaming
f) Draining
g) Drying
a)Precleaning
The purpose of precleaning tanks with butterworth machines using either salt or fresh water, is to remove oil and its residues from bulkheads, tankbottom and underdeck.
The sooner the precleaning is carried out after the discharge, the easier oils and residues will be removed. It is therefore recommend to order the issue of the Empty-Tank-Certificate immediately after the tank is empty and then commence precleaning without delay.
Precleaning is highly important since large deposits of oil or other residues in the tank must be removed to ensure an effective cleaning operation (second stage).
Under all circumstances the water used for precleaning should have a minimum temperature of 20 celsius. Such tanks should be butterworthed with cold water. Next, warm or hot water may be applied if necessary.
No exact duration of precleaning operations can be advised. It fully depends on the grade of material to be removed. The size of the tanks does not affect duration of cleaning procedures in most cases. Smaller tanks usually contain more obstacles, such as stringers and girders, which encumber the cleaning operation.
During precleaning the butterworth machines should be moved frequently, vertically as well as horizontally, around the tanks should be inspected and, if oil and/or other residues are found, the butterworth machines should be centred over the areas for further cleaning.
b)Cleaning
Cleaning is carried out with the aid of water or water and detergents. In most cases we recommend 0, 0.5% of a detergent like Teepol.
The cleaning solution can be applied cold or hot, using butterworth machines. When applying a solvent cleaner by spraying it will be obvious that after the reaction time (minimum 30 minutes) the residues have to be washed away.
Butterworth machines with cold or warm water should be used for this, not too hot, otherwise the applied solvent will evaporate.
The machines must be operated from the deck and must be lowered down into the tank at recurring intervals in order to obtain an effective cleaning throughtout the tank.
The cleaning solution (approximately 10 tons for tanks up to 500 cubic meters) is stored in a small tank.
A butterworth pump will carry it via heaters and then through the butterworth machine into the tank to be cleaned.
From this tank the solution has to be pumped back into storagetank or in the sloptank by means of the cargopump. Throughout the cleaning operation the tankbottom has to be kept as dry as possible by constantly pumping.
The cleaning solution can be used two or three times if light petrochemical products are to be removed. However, precautions have to be taken in order to prevent build up of stactic electric charges.
When plain water is used as the cleaning agent, the cleaning operation is based on solubility of the product in water. In this case it is not recommended to use recirculation; the water may be saturated with product!
Cleaning procedures should be continued for several hours depending on the product. After the tank has cooled down, inspection can take place.
If traces of oil or chemical residues are found, cleaning has to be prolonged, mainly where needed.
We may sub-divide the cleaning procedure after the nature of the general category of cargoes. There are three categories:
a) Animal and Vegetable Fats and Oils
b) Minerals Oils
c) Petrochemicals and Solvents.
General cleaning outlines for these group are:
Sub (a)
Animal and vegetable oils easily saponily when combined with Caustic Soda. To saponity means converting the oil into soap, or, in practice, bringing into suspension.
However, after saponification hard, brittle soap-particles are present which are almost insoluble in water.
We therefore recommend instead of Caustic Soda (NaOH) using Potassium Hydroxyde (KOH) which creates a soft soap which is completely water-soluble.
However, these alkalies can only be used if the tank is made of stainless or mild steel, or alternatively, has been coated with an epoxy-based paint. If the tank has been coated with a zinc silicate-based protective, saponification should not be used.
After cleaning with an alkali residues have to be removed with warm fresh water. To overcome white coloured deposits, often encountered after cleaning with Caustic Soda or other alkalies due to the formation of insoluble magnesium and calcium salts, we recommend the use of a simple detergent instead of the alkaline cleaning agents.
Sub (b)
Cleaning of Mineral Oils can be carried out with water and a mixture of detergents, emulsifiers or synthetic soap.
Sometimes the manufacturer of the cleaning product adds to the mixture a solvent such as Solvent Naphtha.
It is obvious that such a solvent also creates a positive hydrocarbon test.
Therefore, after having used such a cleaning product, a last cleaning with water and some detergent always has to be performed.
Sub (c)
Petrochemicals has Solvents are removed by one of the following(general) procedures:
1.products with a low boiling point and a high vapour-pressure (50 mbar or mode) at ambient temperature can be removed by evaporation or a light cleaning. The ventilation procedure cannot be used for volatile monomers like vinylacetate; the inhibitor does not evaporate!
2.products with a lower vapour-pressure are removed by water-washing and steaming. In the first stage cold water is recommended; most volatile products show a higher solubility in cold water.
c)Rinsing
Directly after the cleaning operations the tanks should be rinsed with hot or cold water.
Butterworth machines should be used.
It is most important than cleaning solution should not allow to dry as this will encumber rinsing. Rinsing should normally continue for about two hours or until no residue can be found in the tank.
d)Flushing
The tank is thoroughly flushed with fresh water. The water is pumped from the storagetank through a 2”flexible hose with nozzle.
The water is sprayed on tanktop, bulkheads and tankbottom.
Steaming
The tank-hatch is closed but not secured. Steam is blow into the tank.
The duration of the steaming depends on the tempeture of the steam and pressure in the tank.
The pressure in the tank should not be too high; therefore the tank-hatch must not be secured.
Condensation water resulting from the steaming, is pumped away by the cargopump.
Upon finishing steaming the tank-hatch and all holes should be opened.
An air-fan is placed in butterworth hole to remove the steam and the nitrogen gas.
For very sensitive cargoes such as alcohols etc., it is advisable to flush the tank with chloride-free water.
Then tests for hydrocarbons, chloride and a permanganate-time-test(P.T.T) should be conducted to check whether the tank is clean enough for sensitive products.
e)draining
Tank, line and pump are drained carefully. All plugs etc. should be removed. Blowing with compressed (dry) air may be very helpful.
An ejector may be used to remove and quantity of water remaining on the bottom of the tank.
Referents
** Dr. A Verwey,Tank Cleaning Guide,1992 **
Chemical Laboratories & Superintendence Company
II TANKCLEANING IN GENERAL
The procedure for tank cleaning may include the following stages :
a) Precleaning (butterwothing with water)
b) Cleaning ( butterworthing with water or water and detergents)
c) Rinsing (butterworthing with water)
d) Flushing (with fresh water)
e) Steaming
f) Draining
g) Drying
a)Precleaning
The purpose of precleaning tanks with butterworth machines using either salt or fresh water, is to remove oil and its residues from bulkheads, tankbottom and underdeck.
The sooner the precleaning is carried out after the discharge, the easier oils and residues will be removed. It is therefore recommend to order the issue of the Empty-Tank-Certificate immediately after the tank is empty and then commence precleaning without delay.
Precleaning is highly important since large deposits of oil or other residues in the tank must be removed to ensure an effective cleaning operation (second stage).
Under all circumstances the water used for precleaning should have a minimum temperature of 20 celsius. Such tanks should be butterworthed with cold water. Next, warm or hot water may be applied if necessary.
No exact duration of precleaning operations can be advised. It fully depends on the grade of material to be removed. The size of the tanks does not affect duration of cleaning procedures in most cases. Smaller tanks usually contain more obstacles, such as stringers and girders, which encumber the cleaning operation.
During precleaning the butterworth machines should be moved frequently, vertically as well as horizontally, around the tanks should be inspected and, if oil and/or other residues are found, the butterworth machines should be centred over the areas for further cleaning.
b)Cleaning
Cleaning is carried out with the aid of water or water and detergents. In most cases we recommend 0, 0.5% of a detergent like Teepol.
The cleaning solution can be applied cold or hot, using butterworth machines. When applying a solvent cleaner by spraying it will be obvious that after the reaction time (minimum 30 minutes) the residues have to be washed away.
Butterworth machines with cold or warm water should be used for this, not too hot, otherwise the applied solvent will evaporate.
The machines must be operated from the deck and must be lowered down into the tank at recurring intervals in order to obtain an effective cleaning throughtout the tank.
The cleaning solution (approximately 10 tons for tanks up to 500 cubic meters) is stored in a small tank.
A butterworth pump will carry it via heaters and then through the butterworth machine into the tank to be cleaned.
From this tank the solution has to be pumped back into storagetank or in the sloptank by means of the cargopump. Throughout the cleaning operation the tankbottom has to be kept as dry as possible by constantly pumping.
The cleaning solution can be used two or three times if light petrochemical products are to be removed. However, precautions have to be taken in order to prevent build up of stactic electric charges.
When plain water is used as the cleaning agent, the cleaning operation is based on solubility of the product in water. In this case it is not recommended to use recirculation; the water may be saturated with product!
Cleaning procedures should be continued for several hours depending on the product. After the tank has cooled down, inspection can take place.
If traces of oil or chemical residues are found, cleaning has to be prolonged, mainly where needed.
We may sub-divide the cleaning procedure after the nature of the general category of cargoes. There are three categories:
a) Animal and Vegetable Fats and Oils
b) Minerals Oils
c) Petrochemicals and Solvents.
General cleaning outlines for these group are:
Sub (a)
Animal and vegetable oils easily saponily when combined with Caustic Soda. To saponity means converting the oil into soap, or, in practice, bringing into suspension.
However, after saponification hard, brittle soap-particles are present which are almost insoluble in water.
We therefore recommend instead of Caustic Soda (NaOH) using Potassium Hydroxyde (KOH) which creates a soft soap which is completely water-soluble.
However, these alkalies can only be used if the tank is made of stainless or mild steel, or alternatively, has been coated with an epoxy-based paint. If the tank has been coated with a zinc silicate-based protective, saponification should not be used.
After cleaning with an alkali residues have to be removed with warm fresh water. To overcome white coloured deposits, often encountered after cleaning with Caustic Soda or other alkalies due to the formation of insoluble magnesium and calcium salts, we recommend the use of a simple detergent instead of the alkaline cleaning agents.
Sub (b)
Cleaning of Mineral Oils can be carried out with water and a mixture of detergents, emulsifiers or synthetic soap.
Sometimes the manufacturer of the cleaning product adds to the mixture a solvent such as Solvent Naphtha.
It is obvious that such a solvent also creates a positive hydrocarbon test.
Therefore, after having used such a cleaning product, a last cleaning with water and some detergent always has to be performed.
Sub (c)
Petrochemicals has Solvents are removed by one of the following(general) procedures:
1.products with a low boiling point and a high vapour-pressure (50 mbar or mode) at ambient temperature can be removed by evaporation or a light cleaning. The ventilation procedure cannot be used for volatile monomers like vinylacetate; the inhibitor does not evaporate!
2.products with a lower vapour-pressure are removed by water-washing and steaming. In the first stage cold water is recommended; most volatile products show a higher solubility in cold water.
c)Rinsing
Directly after the cleaning operations the tanks should be rinsed with hot or cold water.
Butterworth machines should be used.
It is most important than cleaning solution should not allow to dry as this will encumber rinsing. Rinsing should normally continue for about two hours or until no residue can be found in the tank.
d)Flushing
The tank is thoroughly flushed with fresh water. The water is pumped from the storagetank through a 2”flexible hose with nozzle.
The water is sprayed on tanktop, bulkheads and tankbottom.
Steaming
The tank-hatch is closed but not secured. Steam is blow into the tank.
The duration of the steaming depends on the tempeture of the steam and pressure in the tank.
The pressure in the tank should not be too high; therefore the tank-hatch must not be secured.
Condensation water resulting from the steaming, is pumped away by the cargopump.
Upon finishing steaming the tank-hatch and all holes should be opened.
An air-fan is placed in butterworth hole to remove the steam and the nitrogen gas.
For very sensitive cargoes such as alcohols etc., it is advisable to flush the tank with chloride-free water.
Then tests for hydrocarbons, chloride and a permanganate-time-test(P.T.T) should be conducted to check whether the tank is clean enough for sensitive products.
e)draining
Tank, line and pump are drained carefully. All plugs etc. should be removed. Blowing with compressed (dry) air may be very helpful.
An ejector may be used to remove and quantity of water remaining on the bottom of the tank.
Referents
** Dr. A Verwey,Tank Cleaning Guide,1992 **
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