On
most of the production units, oil and gas are directly piped to a refinery or
tank terminal. It is difficult to store gas locally, but in certain cases underground
mines, caverns or salt deposits can be used to store gas. On production units
without a pipeline, oil is stored in onboard storage tanks to be transported by
shuttle tanker. The oil is stored in storage cells above the shafts on concrete
platforms, and in tanks on floating units. On some floaters, a separate storage
tanker is used. To balance the buoyancy in both cases when the oil volume
varies, the usage of ballast handling is very important. For onshore, fixed
roof tanks are used for crude, floating roof for condensate.
Special
tank gauging systems such as pressure, level radars or float are used to
measure the level in storage tanks and cells. The conversion of level
measurement to volume is done via strapping table and a satisfied temperature
to provide standard volume. Float gauges can also calculate density, and so
mass can be established.
A
tank farm consists of 10-100 tanks of different volume for the maximum total
capacity in the area of 1-50 million barrels. Storage or shuttle tankers
normally store up to two weeks of production, one week for normal cycle and one
extra week for delays, e.g., weather and climate conditions.
TYPES OF STORAGE
TANKS USED IN REFINERIES
Tanks for any specific field fluid
are chosen according to the flash-point of
that substance. Generally in refineries and especially for liquid fuels, there
are floating roof tanks, and fixed roof tanks.
Fixed roof tanks are used for liquids with very high flash points,
(e.g. fuel oil, water, etc.,). They are insulated to prevent the coating or
thickening of certain materials, wherein the heat is provided by steam coils
within the tanks. When the storage pressure is slightly higher Dome roof tanks
are used.
Floating
roof tanks are broadly divided into external floating
roof tanks (usually called floating roof
tanks: FR Tanks) and internal floating roof types (IFR Tanks).
IFR tanks are used for liquids with low
flash-points (e.g., ethanol, ATF, gasoline). These tanks are nothing but cone
roof tanks with a floating roof inside which moves up and down along with the
liquid level. This floating roof traps the vapor from low flash-point fuels. Legs
are provided at the bottom to support floating roof . Floating Roof tanks do not
have a fixed roof (it is open in the top) and has a floating roof only. Medium
flash point liquids such as naphtha, kerosene, diesel, and crude oil are stored
in these tanks. One of the common types used in mining areas is the open roof
type tank, usually to store ore slurries. The manufacturing of this type tank
is quite easier.
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