There are basically three wash oil supplying methods so called ―
- Bleed-off System.
- Closed Cycle
- Open Cycle
- Semi-open Cycle System:-
- Wash oil is introduced from a tank being discharged and drained oil is discharged into the aftermost centre tank or slop tank.
- Precaution:- The tank be used as a source of crude oil washing (Driving Oil) should be first discharged at least one metre in depth before supplying as a washing medium to suppress electrostatic charges.
- Closed Cycle System (re-circulation method):-
Wash oil is supplied from the aftermost centre tank or slop tank and drained oil is collected in the same tank, i.e., the same tank is used for both purposes
- Slop tanks loaded by the ―Load-on-Top‖ procedure should be discharged entirely and then filled with fresh oil (crude oil free from water and residue) prior to COW.
- It is considered better to discharge Driving Oil in Slop tank after washing a few tanks, not more than six tanks, and replace to fresh oil again as it may avoid the Driving oil to be sludge-full.
- Bleed-off System (Open Cycle System):- A method in which a part of the oil being discharged is branched off to the tank cleaning line. Some terminals call this system in which cargo is discharged simultaneously with COW.