Sources of Evaporation Loss

Evaporation Loss Control

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 it is.

Unfortunately, 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.

An evaporation 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.

Evaporation 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.

The marketing of a weathered product can have a disastrous effect on sales volume through customer dissatisfaction.

Selection of 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.

Evaporation 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

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.

Heat from the sun expands the air-vapour mixture within a cone tank, resulting in the venting of vapour.

Normally, a 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 the vent.

As a cone roof cools, air drawn in soaks up vapour that is later lost through venting.

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 filling loss.

Product 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 maintenance.

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.

A roof floating on the liquid almost entirely eliminates the vapour space and minimises evaporation losses.