Tank cleaning on Chemical tankers is preliminary done by washing with sea water followed by fresh water rinsing. Subsequent tank cleaning and treatment of tanks is done upon cargo to be loaded and previous cargo carried. This initial washing with seawater or fresh water is carried out either by fixed or portable machines.
Tank cleaning involves various standards of which Wall Wash standards are most stringent
Steps of Tank Cleaning on Chemical Tanker
- Pre-Cleaning with sea water (Pre-Wash)
- Main wash with sea water
- Chemical Washing
- Rinse with fresh water
- Draining tank
- Drying the tank with ventilation
- Test to verify the required standards
- Mop and Dry
Step 1 : Pre-Cleaning with sea water
No cleaning can be carried out unless mandatory Pre-Wash is carried out .Pre – cleaning is the first step for cleaning, with cleaning agents for removing majority of the cargo residue. The cleaning temperature and temperature of the adjacent tanks are very important parameters for successful cleaning.
Step 2 : Main Wash with Sea Water
Sea water is used to cleaning the tanks via tank cleaning the tanks at a pressure in the line of about 8-10bars is recommended for achieving good cleaning. Once cleaning is commenced rinse the wash water from the tank. The temperature of water used for cleaning is also important, it has to monitored as well for achieving the efficiency of cleaning. Washing is carried out using tank cleaning machines which can be either fixed or portable machines or combination of both. All the vent lines need to be washed thoroughly such as PV valve, manifold .
Step 3 : Chemical Washing
There are various chemicals being used for cleaning the tank to achieve the standards. Chemicals help in cleaning the tank more efficiently than using only water, without damaging the tank coating. Chemicals used for cleaning need to be approved. This washing is carried out in recirculation with the help of octopus and flexible hoses which are approved for tank cleaning. Inject at the rate of 2 litres chemical per cubic meter of wash water. All the lines need to be washed thoroughly such as PV valve, manifold . Discharging to be done only by complying MARPOL Annex 2.
Check MEPC circular 2 for approved cleaning chemicals. Rinse the chemical washing. They are used with fresh water. After washing is carried for the required time it has to rinsed.
Types of Cleaning chemicals
Emulsifier : One end of the molecule has an infinity for water, the other for oils or certain chemicals. Hence the oil etc.. can be pumped away from the surface of the tank.
Saponifiers : Chemically convert an animal or vegetable oil into a soap, which is soluble.
Combinations : Tank Cleaning chemicals may also include surfactants which reduces surface tension and give better wetting properties.
Step 4 : Rinse with Fresh Water
Once step 3 is completed, fresh water is introduced for rinsing the remaining chemicals and deposits from the tank. Washing is carried out using tank cleaning machines which can be either fixed or portable machines or combination of both. All the lines need to be washed thoroughly such as PV valve, manifold .
Step 5 : Drain the Tank
In this step the tank need to stripped thoroughly.
Step 6 : Drying the Tank through ventilation
Once step 5 is completed start the ventilation of the tanks for considerable amount of time so as to achieve standards for per entry. Check the gas contents of the tanks by stopping the ventilation of the particular tank. Do not carry out gas check while ventilation is running, it will show wrong readings.
Step 7 : Test to verify the required standards
- A different blog will be published for Test
Step 8 : Mop and Dry
Once step 7 is completed mop and dry is carried out for completely drying the tank, by complying all the procedure for enclosed space entry.
Water washing can be done even if the solubility of the chemical in water is low to as much as 0.3%. Such solubility always increases if the water temperature is higher. Hence always try to use wash water at high temperature unless the cargo being cleaned does not allow it. Solidified matter when melted flows away with the stripped out water, it need not be soluble.
Cargoes with high paraffin content which require heating during transportation should always be cleaned at ambient temperature to start with cleaning, and to reduce the waxy residue in the tank.
Ambient temperature means upto 35 deg C, Moderate temperature means upto 60 deg C and Hot temperature means greater than 60 deg C.The duration of cleaning at each step varies upon grade of cargo previously loaded and next going to loaded
by Hrishikesh Kanitkar