Tank washing machines either fixed or portable. They consist of revolving nozzles moved by water-driven gearing to create a spherical wash pattern or “cycle”.
There are two types of crude oil washers; single nozzle and twin-nozzle ones. In single nozzle units, it is possible to set the machine to wash the tank in two or three stages. This allows the machines to start washing the upper part of the tanks soon after the discharge begins. As the level is lowered, the second stage is washed and then the third when the tanks are almost empty. A twin-nozzle machine cleans the entire tank in a single programme.
With portable machines, both the machine and its flexible water supply hose are placed into the top of the tank to be cleaned through an opening called the “Butterworth Port”. The machines are progressively lowered down the height of a tank in stages or “drops” each usually of 10-15 feet. The graduation markings every 5 feet on the water supply hose are a useful check of the depth of the machine inside the tank. The lowest “drop” is normally about 5 feet above the “bottom” of the tank where the machine is positioned for a “bottom wash”. The wash duration at each drop is usually for one cycle of the machine, the cycle time varying between 30-60 minutes according to the size of the machine and its pump pressure.
Throughout the washing operation, cargo residues mixed with washing water are continuously stripped from the cargo tanks by the normal cargo pumps. These washings are directed through the cargo line system into the reception tanks, a slop tank or in some instances to shore facilities.