Storage Tanks

Storage tanks are used to hold crude oil, produced water, and gas condensate for brief periods to stabilize flow between continuously producing wells and periodic pipeline or trucking transportation. These storage tanks are typically fixed roof design and operate at or near atmospheric pressure. Upon being introduced into a storage tank at above atmospheric pressure, crude oil and condensate experience pressure drops which cause gases dissolved in the liquid to vaporize or 'flash' out of the liquid phase and collect in the vapor space between the liquid and the fixed roof of the storage tank. These vapors, rich in methane, may be vented to the atmosphere to maintain atmospheric pressure in the storage tank.

A typical fixed roof tank consists of a cylindrical steel shell with a permanently affixed roof, which may vary in design from cone- or dome-shaped to flat. Such fixed roof designs cannot hold significant pressure or vacuum, and so are either freely vented or equipped with a pressure/vacuum vent. The latter allows the tanks to operate at a very slight internal pressure or vacuum to limit the release of vapors during small changes in temperature, pressure, or liquid level. Storage tanks that are fixed roof design typically have an access door, called a “thief hatch,” to allow an operator to gauge the hydrocarbon and water levels in the tank. A typical storage tank is shown in Figure 1.  

Crude oil, water, and gas condensate produced under pressure are saturated with light hydrocarbons (including methane and other volatile organic compounds [VOC], natural gas liquids [NGLs], hazardous air pollutants [HAP], and some inert gases). When the oil is brought to the surface much of the volatile lighter hydrocarbons are removed through a series of high- and low-pressure gas/liquid separators. The pressure in the last stage separator must be high enough to push the liquid into a full storage tank without a pump, and to provide an adequate suction pressure for a single stage compressor to move the gas into a gathering pipeline. Thus, the oil will experience a pressure drop upon entering the storage tank and remaining dissolved gases will flash out of solution as vapors into the space between the liquid and the fixed roof of the storage tank.  

The same principles apply for light hydrocarbons that may condense and accumulate in gas gathering pipelines. These gas condensate liquids are pushed through the gathering lines with foam plugs, called “pigs,” often to a fixed roof condensate tank in a gas processing plant. The recovered condensate, which contains dissolved methane and light hydrocarbons under pressure, will experience a pressure drop upon entering an atmospheric pressure storage tank, and the volatile hydrocarbons will flash out of solution may be vented to the atmosphere.  

Losses of methane and lighter hydrocarbons from crude oil and condensate stored in fixed roof storage tanks are categorized in three ways: 

• Flashing losses occur when the gas/liquid separator, operating above atmospheric pressure, dumps oil into the atmospheric pressure storage tank. This scenario occurs when a liquid saturated with volatile gas under pressure is subjected to a drop in system pressure that is no longer sufficient to maintain the gases in solution. The gases will then rapidly migrate out of the liquid, similar to carbon dioxide fizzing out of solution when a carbonated beverage container is opened. This rapid escape of gases from the mixture is referred to as flashing. 
• Standing losses occur with daily and seasonal temperature and barometric pressure changes. This scenario occurs as gas expands and contracts in the vapor space above the liquid due to diurnal temperature and seasonal barometric pressure changes, causing the vapors to be expelled and air drawn into the tank through the vent(s) on the fixed roof. If the venting is of a type that is closed by a weighted-pallet pressure-vacuum relief valve, then vapors may not be expelled until the pressure in the vapor space exceeds the set pressure of the vent. 
• Working losses occur when vapor is released from the changing of liquid levels and agitation of tank contents as fresh oil is circulated through the storage tanks. This scenario occurs as the liquid level increases and compresses the vapor space above atmospheric pressure to push vapors out of the tank vents.  

The magnitude of emissions released into the atmosphere due to storing crude oil and condensate in storage tanks that are fixed roof design depend on many factors. Lighter liquids (API gravity > 36°) flash more hydrocarbon vapors than heavier liquids (API gravity < 36°). Also, the operating temperature and pressure of the liquid prior to transferring it into the storage tank will affect the volume of flashing losses. In addition, in storage tanks where the crude oil, produced water, and/or condensate are frequently cycled, and the overall throughput is high, more working losses occur than in tanks with low throughput and where the liquids are held for longer periods.   

The composition of the emissions released into the atmosphere also varies, but the largest component is methane (typically between 40 and 60 percent). Other components may include more complex hydrocarbon compounds such as propane, butane, and ethane; natural inert gases such as nitrogen and carbon dioxide; and HAP such as benzene, toluene, ethyl benzene, and xylene (i.e., BTEX). 

Figure 1. Storage Tank

U.S. Environmental Protection Agency. (2020, June). Compilation of air pollutant emission factors, Volume I: Stationary point and area sources (AP-42), Fifth Edition, Section 7.1 (Organic liquid storage tanks). https://www.epa.gov/sites/default/files/2020-10/documents/ch07s01.pdf