Atenção: As questões de números 10 e 11 referem-se ao texto que segue.

Underground crude oil contains many lighter hydrocarbons in solution. When the oil is brought to the surface and processed, many of the dissolved lighter hydrocarbons (as well as water) are removed through a series of high-pressure and low-pressure separators. The crude oil is then injected into a fixed roof tank to await sale and transportation off site; the remaining hydrocarbons in the oil are emitted as vapors into the tank.

These vapors are either vented, flared or recovered by vapor recovery units (VRUs). Losses of the remaining lighter hydrocarbons are categorized in three ways:

Flash losses occur when the separator or heater treater, operating at approximately 35 pounds per square inch (psi), dumps oil into the storage tanks which are at atmospheric pressure.

Working losses refer to the vapors released from changing fluid levels and agitation of tank contents associated with the circulation of fresh oil through the storage tanks.

Standing losses occur with daily and seasonal temperature changes.

The volume of gas vapor coming off a storage tank depends on many factors. Lighter crude oils flash more hydrocarbon vapors than heavier crudes. In storage tanks where the oil is frequently cycled and the overall throughput is high, there will be more "working vapors" released than in tanks with low throughput and where the oil is held for longer periods and allowed to "weather." Finally, the operating temperature and pressure of oil in the vessel dumping into the tank will affect the volume of flashed gases coming out of the oil.

VRUs can recover over 95 percent of the hydrocarbon emissions that accumulate in storage tanks. Because recovered vapors contain natural gas liquids (even after condensates have been captured by the suction scrubber) they have a Btu content that is higher than that of pipeline quality natural gas (between 950 and 1,100 Btu per standard cubic foot (scf)). Depending on the volume of NGLs in the vapors, the Btu content can reach as high as 2,000 Btu/scf. Therefore, on a volumetric basics (i.e., Mcf), the recovered vapors can be more valuable than methane alone

Hydrocarbon vapors are drawn out of the storage (stock) tank under low pressure, typically between four ounces and two psi, and are first piped to a separator (suction scrubber) to collect any liquids that condense out. The liquids are usually recycled back to the storage tank. From the separator, the vapors flow through a compressor that provides the low pressure suction for the VRU system. (To prevent the creation of a vacuum in the top of a tank when oil is withdrawn and the oil level drops, VRUs are equipped with a control pilot to shut down the compressor and permit the back flow of vapors into the tank). The vapors are then metered and removed from the VRU system for pipeline sale or on-site fuel supply.

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