ABSTRACT
In a refi nery, crude oil is received and stored in a fl oating roof tank. The roof is made of compartmented deck or pontoon fl oating over the oil to avoid loss of hydrocarbon vapours of dissolved hydrocarbon gases and low boiling fractions present in crude oil. The fl oating roof also fl oats up and down to compensate for the breathing operations during pumping in and out of the tank thereby safeguarding the loss of vapours and ingress of atmospheric air into the tank. A fl oating roof storage tank of crude oil is shown in Figure 3.1, where a fl oating roof (1), made in the form of a ship’s deck, fl oats over the crude oil stored. As the vapour is formed, fl oating roof does not allow the vapour to leak into the atmosphere, thereby avoiding loss. During low level in the tank the roof is supported by legs (2) while it rests on the tank bottom. A dip pipe (4) is provided to measure the level of crude stored, through a dip hatch (5) with a cover (6). The level of liquid in the tank or dip can be manually measured with a graduated dip tape by climbing onto the tank from the operating platform (7) via a ladder (8). A manual inspection using a swing ladder (3) onto the surface of the fl oating roof can be done for deck compartments, the rolling seals (16), and the roof drain (14) connected with a fl exible steel pipe, which is mobile with the roof. Crude oil received from tank cars, pipelines, or from tankers (ship) may contain much water either carried with the cargo or due to fl ushing of the receiving line. This water must settle in the receiving tank before oil accounting is started. Temperature, oil cut, water cut, and the density of the sample crude are the most important parameters in determining the quantity of oil received. After the quantity and valuation of oil are established, the water in the tank must be drained. Steam coils and side mixers are usually provided near the bottom of the receiving tank to homogenise the crude layers before processing. However, water and salt, which do not settle in the tank, can only be removed by desalting located within the battery limit of the crude distillation unit.
