Physical Properties of the cargoes
The physical properties of the cargoes listed below are indicated for each cargo in the Cargo Details section of this publication.
Colour
Maybe the most important physical property that must be considered before planning a tank cleaning is the colour of the cargo. Black cargoes are more difficult to clean than colourless cargoes. The most common colour scales are ASTM colour and APHA colour. More details about Colour
Specific Gravity
For a product with limited or no solubility in water, a specific gravity below 1.0 indicates that the product will float on water. A specific gravity above 1.0 indicates that the product will sink in water. More details
Solubility in water
Where the data is available, water solubility values are given in percent levels.
For many cargoes, the solubility in water is described as a range, for example complete (means 100%), partly soluble, slightly or insoluble (means less than 1%). Generally, cargoes that are soluble in water are easier to clean than cargoes that are insoluble in water. Moreover, the solubility in water tends to increase with temperature, meaning hot water washing is preferred.
Boiling point
Boiling point is the temperature at which a liquid change to a gas (vapour) at normal atmospheric pressure. Values are in °C. More details about Boiling Point
Melting point
Melting point is the temperature at which a product changes phase from a solid to a liquid. Values are in °C. More details about Melting Point
Products with a high melting point should be washed at a temperature of 20-25ºC above the melting point. Ideally and if it is safe to do so, during washing there should be no ballast water or cold cargoes adjacent to the tank being cleaned. During cleaning, special attention should also be given to the liquid and vapour line systems to avoid freezing/solidification of previous cargo residues at cold line segments. Washing as soon as possible after discharge is recommended.
Products with a high viscosity
Viscosity is the resistance of a fluid to flow. Values are in mPa*s at 20°C
In general the viscosity of a product is closely related to temperature and will decrease at higher temperatures, meaning products with high viscosity should ideally be washed at higher temperatures. Ideally and if it is safe to do so, during washing there should be no ballast water or cold cargoes adjacent to the tank to be cleaned. Washing as soon as possible after discharge is recommended.
Flash Point
The lowest temperature at which a liquid gives off sufficient gas to form a flammable gas mixture near the surface of the liquid. It is measured in a laboratory in standard apparatus using a prescribed procedure. Values are in °C. More details about Flash Point
The pre-cleaning temperature must be well below (minimum 10°C) the flashpoint.
If this is not possible, because the flash point of the product is lower than the ambient temperature, there is a risk of a flammable mixture forming inside the tank that could under certain circumstances ignite during the cleaning operation.
In this case the tank should be in inert condition before cleaning operations commences.
Non inert tank washing in this case requires careful removal of the cargo residues from the tank and piping system by either bottom flash or effective stripping. The tank atmosphere must be controlled before and during the tank washing. Only if the LFL is below 10% the tank washing may take place. For more details see ISGOTT and CTSG.
