Crude oil normally contains fractions of light and heavier gases which cannot be transported together with the liquid fraction of the crude oil due to their volatility. Transport-technically it would be favourable if the crude oil were separated into a gas fraction, a wet gas fraction and a liquid fraction. However, separation and transport of the petroleum products in three fractions from a production field involve considerable extra expenses, and it is therefore more common for the crude oil to be separated into a gas fraction and a liquid fraction. The wet gas phase is therefore divided into a lighter fraction which is transported together with the gas fraction, and a heavier fraction which is transported with the liquid fraction.
When a liquid petroleum fraction containing heavier gases, such as propane and butane, is being loaded at a moderate positive pressure, gages continuously evaporate from the liquid fraction. To prevent a pressure increase in the transport or storage tanks, the evaporating gases, the volatile organic compounds, must be drawn from the tanks and be carried away for combustion or they must be carried to a re-injection plant.
It is known that there is relatively extensive evaporation of volatile organic compounds as oil is being pumped into larger tanks. Normally a pressure in the order of 1.05 to 1.07 bar is maintained in both storage and transport tanks. During the loading of a tanker for example, it is common for the oil to be pumped from a storage tank through a supply pipe to a position above the cargo tank, from where the oil is directed into the tank through a drop line down to the bottom portion of the tank. A drop line of this kind may have a length in the order of several tens of metres.
When the oil enters the upper end portion of the drop line, gravity will accelerate the liquid flowing down the drop line, whereby, a lower overall pressure is created in the supply pipe and the upper portion of the drop line. In these pipes, in which there is a lower overall pressure, there is considerable evaporation of volatile organic compounds, which will be re-liquefied only to a minor extent when the pressure increases again to the normal pressure of the tank.
The invention has as its object to remedy the drawbacks of known technique.