There is no single fail-proof method to get your tanks cleaned to methanol standard after Vegetable oil. Sorry!!!
I wish if I could tell you that if you did this particular thing your tanks will be ready for methanol standard.
But it just doesn’t happen that way.
The reality is that you can’t rely on one technique to clean your tanks after Vegetable oil.
You have to approach it using a combination of different strategies.
But positive thing is that there are few principles that we need to follow to make the tank cleaning much easier and less troublesome.
Let us discuss.
Type of Vegetable oil
The first and foremost, we must know the type of palm oil that we had in the tanks. This is important because the tank cleaning method would depend on that.
If we make a mistake in this, we can never be able to clean the tanks to methanol standard. Never!!!
So here is the type of palm oils that I am referring to.
The question is, how to find which category palm oil loaded on your vessel falls into. Tank cleaning software (provided on board by of the chemical tanker operators) helps in that.
Milbros is one of such software. Let us say we just discharged following Vegetable oils
- Sunflower oil
- Crude palm oil
- Palm Stearin
And we want to check the type of Veg oil for “Sunflower oil”. Open Milbros on the computer.
It will lead you to the initial screen.
In the search option, type Sunflower oil and choose sunflower oil from the options.
In the sunflower oil screen, go to notes and under Cleaning, look for any information of describing sunflower oil as drying or Semi-drying oil.
As you can see, Sunflower oil, in fact, a drying or Semi-drying oil.
This information can also be found in other resources like Dr. Verwey’s tank cleaning guide.
Precaution with drying and Semi-Drying palm oils
Now, why is it so important to know if the palm oil is drying or Semi-drying type?
The drying and semi-drying oils get to harden when it comes in contact with air.
If allowed to come in direct contact with air, these palm oils would dry quickly and become hardened on the tank coating. There are few conditions in which these oils can dry on the coating.
- If there is no moisture in the tank and/or
- If there is the high temperature in the tank.
Why high temperature? Because higher temperature absorbs the moisture from the atmosphere. This is particularly important while washing the tanks containing drying or semi-drying palm oils.
We must initially wash these tanks with ambient seawater. If we use hot sea water, the palm oil will become hardened on the coating and it will not be possible to remove this from the coating.
Another important point is to wash these tanks as soon as discharging is completed and empty tank certificate issued by the surveyor. If the terminal does not allow tank cleaning at berth, we can just introduce some water in the tank and recirculate it for few seconds in every few hours to keep the tank bulkheads moist.
This may look to be a small step but it really helps in a big way.
Take-away points for drying/Semi-drying palm oils:
- Initially wash the tanks containing drying or semi-drying palm oils with ambient seawater (or fresh water) for 1-1.5 hours.
- Keep the tank moist till the time tank cleaning is commenced.
General tank cleaning procedure
There are tons of resources to advise the general procedure to follow for cleaning the tank after a cargo. As discussed Milbros is one of these resources.
In the Milbros software, go to “From/to cleaning” section.
Enter the “from and to” cargoes and click on “View Results”.
This will show you the recommended cleaning, step by step.
Now if you follow these steps exactly as it is, there is no guarantee that your tanks will be ready to wall wash standard.
You may have to clean for an extended period (2-3 hrs mentioned in these steps may not be enough). Usually, 4 hours washing cycle is considered to be sufficient.
Main principles of tank cleaning
There are a number of tank cleaning chemicals that we may use during tank cleaning but the use of these chemicals helps only to a certain extent.
There are somethings other than the use of chemicals that brings wonder results for the tank cleaning.
An efficient cleaning of tanks require
- Physical Cleaning of the tanks
- Chemical cleaning of the tanks
Physical cleaning of the tank
Remember in old time (and in some places, even today) how the clothes are washed manually.
Yes, mainly by use of force.
More force, better cleaning. Though damage to the clothes needs to be kept in mind when using too excessive force.
Same applies to the tank cleaning on chemical tankers. The pressure of tank cleaning water is very important for efficient cleaning.
Washing of the tanks at 6 bars pressure will achieve nothing. More pressure the better.
Usually, 8 to 10 bars is considered to be good pressure, 8 Bars being the minimum. If our system allows, we must try to achieve at least 9 bars pressure in the tank cleaning line.
This is because of the fact that pressure in tank cleaning line and at the tip of the tank cleaning machine will not be same as the pressure that hits the bulkheads.
Take-away points: Higher the pressure of the tank cleaning medium, better will be the cleaning. Reduce the number of tanks being washed if pressure is less.
Chemical cleaning of tanks
When I say, cleaning of the tanks chemically, I do not mean the use of chemicals. There is something else more important than using chemicals for cleaning.
That is temperature. Yes, the temperature is the best chemical you will have during tank cleaning.
More the temperature of the cleaning medium better will be the cleaning.
Allow me to explain with the same analogy of washing of clothes. Dip a similar dirty shirt in cold water and in hot water buckets and leave it there for few hours.
After few hours when we take out the shirts, water in which of the bucket you would expect to be cleaner?
Of course, the cold water will be cleaner as hot water would have taken more dirt out of the shirt. But how?
This is the chemical reaction I am talking about here.
When I talked about the pressure, I talked about the pressure at which the water hits the bulkhead and not the pressure at the nozzle tip or in the tank cleaning line.
Same applies to the temperature.
If the bulkhead itself is cold or has the ballast on the other side of the bulkhead, the cleaning will not be effective.
I totally agree that removing the ballast from the adjacent ballast tanks is a real pain but believe me when I say it. Gains in terms of effectiveness of the cleaning outshines the pain of removing the ballast.
So we must lower the ballast water level from the adjacent tanks to a level where the water is not touching any of the cargo tank bulkheads.
But the use of higher temperatures is not true every time. There are some exceptions to this like the initial washing of drying and semi-drying palm oils (More exceptions later in this blog).
Take-away points: Barring few exceptions, higher the temperature, better will be the cleaning.
Use of chemicals
While high pressure and higher temperatures will do most of the job, cleaning to wall wash standard would still most likely require the use of chemical re-circulation in the tanks.
And the question is which chemical to use?
The first condition for choosing the tank cleaning chemical is that chemicals need to be IMO Approved. MEPC circular list all the chemicals with their makers that are approved for use by IMO.
Check if the chemical you have planned to use are in this list.
The second condition is that the chemicals need to be safe to use with respect to the tank coating. This information can be found from the information provided by the chemical maker.
Even when these conditions are satisfied, we still need to choose from the number of chemicals available in the market. And which chemicals work best will usually come by the experience of tank cleaning.
Generally, any of these chemicals are good for cleaning the palm oil tanks
- Grato 50 (for stainless steel tanks) and Grato 14 (for Zinc/Epoxy coated tanks) Make: CP Metal Chemicals
- Marclean SC or Marclean AC+ Make: Teca
- Caretank Eco Make: Marine Care
These are the chemical to do the main wash which is to make the tanks clean in all respects. But apart from these chemicals, you may need to have few other chemicals too just in case you.
You may need to have chemicals to remove odor from the tanks or color from the samples.
Well, some companies do not give many options to choose from and they have fixed chemicals to choose from and have the recommended list and quantities of the chemicals to maintain on board.
This may in a way remove some load from the chief officer.
We also need to have an estimate of what quantity of the chemicals would be required for tank cleaning. This can easily be calculated from the concentration of chemical required in the chemical solution and minimum volume of solution required for the pump to take suction during re-circulation.
For example, 0.5% of Caretank Eco is recommended for re-circulation. Let us say 3 m3 (3000 liters) of water is required for the pump to maintain good pressure during the re-circulation.
Then the quantity of Caretank Eco for one tank would be 3000 x 0.5/100 = 15 Liters.
For the chemical re-circulation to be effective, there is this one pre-condition.
The initial cleaning needs to be effective. This means that there should not be any traces of previous cargo in the tank. The tanks need to be absolutely clean visually. And higher temperature/Higher pressure during initial cleaning really helps in that.
The higher temperature-better cleaning is applicable to the chemical re-circulation also but there are few points that we need to keep in mind.
1. Caustic based chemicals can make your tanks white if heated to higher temperatures.
If caustic based chemicals are used for re-circulation, we need to be careful with heating the solution.
The temperature of the cleaning solution should not be increased to more than 40 C.
It is also recommended to not heat the solution at all but sometimes it is unavoidable especially when cleaning in a low-temperature environment.
2. Using fresh water for making the cleaning solution
If you need to heat the chemical solution used for recirculation, be aware of what water you will use to make the chemical solution.
If you use sea water and keep the steam in heating coils on, there are chances of hard solid salt deposits on the entire heating coils of the tanks.
This salt deposits could become difficult to remove.
We can just use the fresh water to make the chemical solution to avoid any of these solutions.
But sometimes we are short of fresh water and we need to use sea water for making the chemical solution.
In that case, there are few things you can do to avoid this.
First, stop the steam in the heating coils at least 30 minutes before stopping recirculation.
Second, rinse the tank with fresh water for 5-10 minutes immediately after chemical recirculation.
Keeping tank coating in mind
More temperature more pressure is good for removing the impurities and better cleaning of the tanks.
But it may not be so good for the tank coating.
The vessel must be aware of the maximum temperature allowed by the tank coating manufacturer.
Usually, coating manufacturer will have some temperature limitation but these limitations are for the temperature of the cargo.
Exposure to higher temperatures for short period is usually allowed and that does not affect the tank coating.
In any case, the tank coating manufacturer must be consulted to have the clarity on the use of higher temperatures than specified for tank cleaning.
Cleaning of the cargo tanks with palm oil as previous cargo is a difficult task.
And if you have to clean these tanks to load wall wash cargo, the task becomes even more difficult.
But this difficult task can become easy to a great extent if few simple principles of tank cleaning are followed.
More pressure and more temperatures (barring few exceptions) are one of such principle.
Choosing the correct chemical for cleaning and following the instructions for the chemicals to be effective also helps.
About Capt Rajeev Jassal
Capt. Rajeev Jassal has sailed for over 19 years mainly on crude oil, product and chemical tankers. He holds MBA in shipping & Logistics degree from London. He has done extensive research on quantitatively measuring Safety culture onboard and safety climate ashore which he believes is the most important element for safer shipping.
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