The following are some generalized or typical procedures that may be included in a tank cleaning operation: • Precleaning (butterworthing with cold sea water) - one sequence. • Cleaning (butterworthing with warm or hot sea water or sea water and detergents) - three sequences. • Rinsing (butterworthing with cold sea water) - one sequence. • Steaming (usually to remove salt [chlorides] residues left behind from washing with sea water).
On some vessels the Butterworth machines are permanently mounted in the tanks, but portable units are still used. A typical tank cleaning operation will use 2 to 4 Butterworth machines per tank and will run each sequence once per machine at three levels in the tank [upper, middle, and lower]. It usually takes 6 to 9 hours to clean a tank. Each Butterworth machine uses approximately 2000 gallons of water per hour. The Butterworth system is normally connected to an in-line heater which gives the option of using cold, warm, or hot water. Tank cleaning is as much of an art as it is a science. It is part of the skill of the mariner and how it is done depends upon so many factors that it is not possible to document it in a few pages.
Although it is the vessel’s responsibility to ensure that it meets its obligations under the charter party in respect to cargo segregation, in scenarios where a vessel is loading multi-grade cargoes that have high and low flash-points, we need to consider the possibility of Vapour contamination and advise the vessel that any common Vapour lines such as inert gas and ventilation lines are to be isolated.
8.1.13 Tank Cleaning
While tank cleaning is not a service that we would normally be directly involved with, the way in which a vessel cleaned its tanks can be important to us when evaluating a tank’s cleanliness to load a subsequent cargo and therefore it is incumbent upon us to have an understanding of how it is performed.
8.1.15 Precleaning
The purpose of precleaning tanks with Butterworth machines using either salt or fresh water, is to remove the bulk of the oil and its residues from bulkheads, tank bottoms and under deck. The sooner the precleaning is carried out after the discharge, the easier oils and residues will be removed. Precleaning is highly important since large deposits of oil or other residues in the tank must be removed to ensure an effective cleaning operation. No exact duration of precleaning operations can be advised. It fully depends on the grade of material to be removed. The size of the tanks does not affect duration of cleaning procedures in most cases.
8.1.14 Cleaning Machines
Tank cleaning on marine vessels is usually performed by using a cleaning device that is called a ‘Butterworth machine’ This is a nozzle head suspended from a hose or pipe, that can rotate in two directions. The nozzle itself usually rotates in a vertical plain while the entire unit rotates in a horizontal plane. These rotations are controlled by turbine driven gearing which is driven by the water passing through the nozzle. This controlled rotation allows a stream of water to reach every part of a tank within the programmed sequence of the machine. These sequences usually take from 2 to 3 hours.
Butterworth Type K machine
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