After crude oil has been discharged from a tank, there remains a certain amount of clingage - oil that sticks to the tank sides - together with sands or waxy deposits on the bottom of the tank. These residues result in some cargo loss, so cleaning guns are fitted within the tanks. During discharge, about 10% of the cargo is recirculated under pressure through these rotating guns to wash down the clingage (residue of crude clinging to tank walls). By the time pumping is complete very little residue is left and what remains can be removed if the tanks are water washed and gas freed if this is required. Crude oil washing and double-hull ballast tanks have removed the main reason for water washing cargo tanks for the carriage of ballast but occasionally water washing may be needed if the tanks need to be physically entered for maintenance or drydock and occasionally when a crude cargo with some specifìc characteristic, such as high wax content, H2S or mercaptan sulphur, means that residues could prove problematic to the next crude cargo, particularly at the next load port.¹⁾

The use of eductors is another method of cargo tank stripping. Eductors for this purpose are large and are located in the pump room. The large motive fluid requirements are generally supplied by the main cargo pumps. Eductors are widely used with crude oil washing systems.²⁾