The measurement of oil and petroleum products is an extensive subject and remains integral part of every sale&purchase contract. Here I will outline a brief introduction to the subject matter.
There are three methods of oil quantity measurement in a wider use:
1. Measurement of volume in tanks
Can take place on shore or in ship. An accurate adjustment of the volume in the tank is made against the height of liquid measured with a dip tape or automatic level gauge. Such adjustment is usually supervised by a contracted, third party surveyor (as for instance SGS) and an eye should be kept for possible inaccuracies derived from uneven position of the tank.
Furthermore, the temperature of oil in the tank needs to be precisely measured, since the volume is usually converted to a standart temperature of 60°F.
The knowledge of specific oil gravity will also be needed, firstly to convert a volume to weight, secondly it is highly relevant during a process of employing temperature conversion tables as specified by ASTM and IP. Therefore, the content of the tank must be the same and samples must be taken at several levels.
Meter measurement is typically realized through turbine meters placed in a way to give a measure of volumes moving through a pipe.
When blends are made directly into the ship this mode of measuring is always used. Accuracy also counts as an advantage of this method.
However in order to maintain it through the whole process of discharge, the adjustment of turbine meters need to be confirmed regularly through the whole length of discharge by sending the loops of previously confirmed volumes. Sampling and temperature measurement with this method tend to be also more accurate and can be applied automatically.
The error margin of volume measurement in loading the oil on a ship no matter which method applied stays usually around f0.3 per cent. Error margin on the overall operation of loading followed by discharge at the destination is usually estimated at the level of f0.4 per cent (excluding water measurement).
Weigh bridges are usually used in road transportation and for rail car’s oil measurement. This method is also applied for the products sold by weight and difficult to measure by meter (for instance bitumen).
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