Evaporation loss in the storage of crude oil and its products can
be the unseen thief of countless US dollars each year. Replacement
means duplication of all costs from exploration work up to the stage
at which loss of usable liquid occurs. The later the loss occurs
in handling between well head and final consumer, the more costly
evaporation losses – which disappear as vapours – are not immediately
apparent. Loss of valuable vapours frequently shows up only on the
annual balance sheet, eating deeply into anticipated profit.
It is a needless
loss for the most part. Recognition of the fact the evaporation
loss of crude oil and products is very costly and application of
well proved conservation equipment can cut it to the bone. The result
– better quality products, lower storage cost and higher profit.
loss of 1% is accompanied by a drop in gravity of the crude oil
of approximately 0.4 degrees API. Since the value of crude decreases
with its gravity, the economic loss from this source alone is appreciable.
losses from motor gasoline have an adverse effect on volatility
and octane rating. A volumetric loss of 1% is accompanied by a rise
of 5 degrees Fahrenheit in the 10 per cent point – which is the
measure of starting ability – and a drop of one point in the octane
rating, which is the measure of operating performance.
of a weathered product can have a disastrous effect on sales volume
through customer dissatisfaction.
the proper storage tank has a tremendous effect on evaporation losses.
Experience shows that one 80,000 barrel cone roof tank can be expected
to lose almost 2,700 barrels of gasoline each year through evaporation.
Approximately 94% of this loss could be saved by equipping the tank
with a floating roof.
One 80,00 barrel
cone roof tank can suffer a yearly evaporation loss of 2,700 barrels
of gasoline. A floating roof would prevent 94% of this loss.
losses are costly and should be held to a minimum through the use
of appropriate conservation equipment, selected through a study
of the problem and its economics.
Cone roof tanks,
the minimum acceptable container for crude oil and its products
that do not boil at ambient temperature, are subject to evaporation
losses from two sources: breathing and filling.
from the sun expands the air-vapour mixture within a cone tank,
resulting in the venting of vapour.
cone roof tank will be partially full of liquid with the space above
it containing a mixture of the liquid’s vapour and air. As the sun
warms the tank, the volume of the air-vapour mixture in the vapour
space will expand. Since most cone roofs cannot withstand much pressure,
the excess air-vapour mixture must be released from the tank through
a cone roof cools, air drawn in soaks up vapour that is later lost
When the vapour
space cools with the outside temperature, air must enter through
the vent to prevent the tank shell from collapsing under vacuum.
Air breathed into the tank mixes with the vapour and, when the next
out-breathing cycle occurs, additional vapour will be lost through
the vent. Because the liquid in the tank is continually trying to
saturate the air-vapour mixture, the liquid volume is reduced each
time breathing occurs. This is known as breathing loss.
Air also enters
the vapour space when liquid is withdrawn to make deliveries. Additional
liquid evaporates tending to saturate the air and reduce the liquid
volume each time product is withdrawn from the tank. When the tank
is again refilled, each barrel of liquid entering forces an equal
volume of air-vapour mixture out through the vent. This is called
movements alternately pull in air and force out an air vapour mixture
causing a filling loss.
If a cone roof
tank and its accessories are permitted to fall into a state of disrepair,
additional losses will occur. Holes in the roof caused by corrosion,
leaky fittings or fittings stuck in an open position allow wind
to enter one opening, sweep through the vapour space and carry vapour
out through other openings. These losses can be prevented by good
Floating Roof Tanks
Since the vapour
space of a fixed roof tank is the only source of evaporation, losses
can be minimised by eliminating the vapour space. This is best accomplished
by the use of a roof which floats on the liquid surface. Breathing
and filling losses are eliminated for all practical purposes because
the vapour space is reduced to a narrow annular space which in turn
is covered by a Rim Seal.
roof floating on the liquid almost entirely eliminates the vapour
space and minimises evaporation losses.