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Fixed roof tanks
Fixed roof tanks are common in production facilities to store hydrocarbons with vapor pressures close to atmospheric pressure. In this use, they should be equipped with pressure-vacuum valves and purged with natural gas to eliminate air intake into the vapor space. Product evaporative losses can be high especially when crude is added to the tank and vapors are expelled through the pressure vent valve.
Overview
In crude oil terminals and pumping stations, internal floating roofs may be added to the fixed-roof tank to reduce product vapor losses if the crude oil has been stabilized to vapor pressures less than 11 psia.
Examples of fixed-roof tanks are shown in Fig. 1.
Fig. 1—Typical fixed-roof tanks.
The most common fixed-roof design contains a shallow cone roof utilizing a single center column plus internal (or external) framing to support the roof plates. Intermediate columns are used for diameters greater than 120 ft. Designs may include a frangible roof joint for added protection in the event of a sudden increase in internal pressure. In this case the design pressure is limited to the equivalent pressure of the dead weight of the roof plates including structural rafters. Other fixed-roof designs such as the self-supporting dome roof or umbrella roof may be used if storage pressures exceed the capabilities of the cone roof design. Depending on the size (diameter) of the tank, API Standard 650, Appendix F designs can permit internal pressures up to 2.5 psig. If operating pressures exceed 2.5 psig, API Standard 620, Design and Construction of Large, Low-Pressure Storage Tanks provides design procedures for internal pressures up to 15 psig.
Gauge hatches
Fixed-roof tanks should have a quick opening gauge hatch in the roof, which allows the operator access to the tank to "gauge" the tank, determine if water is present, measure the height of the oil/water interface, and take samples of the crude oil. The gauge hatch can be weighted in such a way as to work as a backup pressure or pressure-vacuum relief device to the primary pressure-vacuum valve.
Standards for manual gauging of petroleum and petroleum products are given in the API Manual of Petroleum Measurement Standards, Chap. 3.1 A. "Gauging" includes measuring the amount of liquid contained in the tank as well as determining the temperature of the liquid and obtaining representative samples.
Filling or pumping operations
Normal tank filling and pumping operations also affect the vapor space of a fixed-roof tank. When product is removed from the tank, air is drawn into the vapor space as liquid is removed, creating a hazard. During the holding period before the next tank filling operation, evaporative breathing losses increase because of the increased volume of the vapor space. When product is added to the tank, the increasing liquid volume displaces the air/vapor mixture through the tank vent, resulting in significant evaporative emissions.
Gas blanketing systems
As long as the product vapor pressure is low (below 1.5 psia), it is considered safe practice to use a freely ventilated fixed-roof tank. For production tanks or other applications in which vapor pressure of the incoming liquid normally exceeds atmospheric pressure at normal ambient temperatures, a gas blanketing system is required to maintain a positive tank pressure and minimize the chance of air being drawn into the tank vapor space. During periods of no inflow, the tank breathing process alone could cause air to flow into the tank through the pressure-vacuum valve and form an explosive mixture.
A gas blanketing system includes a suitable supply of natural gas and a pressure regulator that operates as needed to maintain the tank pressure at a predetermined level. During the heat of the day, as pressure increases, the regulator closes. If pressure continues to rise, the pressure vent opens to relieve tank internal pressure by venting vapors (blanket gas + product vapor) to atmosphere or some downstream vapor recovery process. Note that a vacuum relief still must be used to protect the tank against vacuum should the gas blanketing system fail.
Fire exposure
Out breathing, resulting from fire exposure, may exceed the design venting rate based on normal operating conditions. In such cases, the construction details of the tank determine whether additional venting is required.
On fixed-roof tanks, where the roof-to-shell attachment is constructed in accordance with API Standard 650, Sec. 3.10.2.5.1, the roof-to-shell joint may be considered frangible, and in the event of excessive internal pressure may fail before failure occurs in the tank-shell joints or the shell-to-bottom joint. In tanks built in this manner, consideration need not be given to any additional requirements for emergency venting if the tank is isolated from other equipment and loss of the roof during emergency conditions is acceptable.
On tanks that do not follow the frangible joint details, design procedures are provided in the API Standard 2000 for calculating the required venting capacity for fire exposure.
General references
Evaporative Loss Measurement. In Manual of Petroleum Measurement Standards, Ch. 19, Sec. 2-E. 1997. Washington, DC: API.
API RP12R1, Setting, Maintenance, Inspection, Operation, and Repair of Tanks in Production Service, fifth edition. 1997. Washington, DC: API.
API RP575, Inspection of Atmospheric and Low-Pressure Storage Tanks, first edition. 1995. Washington, DC: API.
API RP651, Cathodic Protection of Aboveground Petroleum Storage Tanks, second edition. 1997. Washington, DC: API.
API RP652, Lining of Aboveground Petroleum Storage Tank Bottoms, first edition. 1991. Washington, DC: API.
API RP2003, Protection Against Ignitions Arising Out of Static, Lightning, and Stray Currents, fifth edition. 1991. Washington, DC: API.
API Spec. 12B, Bolted Tanks for Storage of Production Liquids, fourteenth edition. 1995. Washington, DC: API.
API Spec. 12D, Field-Welded Tanks for Storage of Production Liquids, tenth edition. 1994. Washington, DC: API.
API Spec. 12F, Shop-Welded Tanks for Storage of Production Liquids, eleventh edition. 1994. Washington, DC: API.
API Standard 650, Welded Steel Tanks for Oil Storage, tenth edition. 1998. Washington, DC: API.
API Standard 653, Tank Inspection, Repair, Alteration, and Reconstruction, second edition. 1995. Washington, DC: API.
API Standard 2000, Venting Atmospheric and Low-Pressure Storage Tanks (Nonrefrigerated and Refrigerated), fifth edition. 1998. Washington, DC: API.
API Standard 620, Design and Construction of Large, Low-Pressure Storage Tanks, tenth edition. 2002. Washington, DC: API.
See also
Vent system design for storage tanks
Controlling liquid leaks from tanks