What Are the Types of Roofs for Above-Ground Tanks?
Storage tanks can be designed with different types of roofs. A fixed roof tank, as the name suggests, has a flat, cone or dome-shaped roof permanently attached to the cylindrical steel tank shell. Some are designed with flattened conical roofs that provide a vapor reservoir between the liquid surface and the underside of the roof. Compared to cone roofs, geodesic domes can span a large area without needing structural support columns.
An external floating roof tank is an open-topped steel shell with a roof that floats atop the liquid in the container. Floating roofs typically consist of a deck, fittings and rim seal system. An internal floating roof tank usually has both a fixed roof and, beneath it, a floating roof atop the liquid.
What Are the Benefits of a Geodesic Dome?
In the mid-20th century, geodesic domes became popular construction options by being durable, easy to build and having visual appeal. The shape has high stability and strength for its weight, requiring a relatively low quantity of construction materials compared to other designs.
Aluminum domes were initially adopted by the U.S. petroleum industry to keep ice and snow off the top of external floating roofs. The lightweight design’s high strength-to-span ratio made it easy to add to existing tanks. Covering tanks with domes also increased working capacity while reducing product vapor loss due to evaporation.
More recently, geodesic domes have been increasingly valued due to their ability to reduce emissions, including wind-induced emissions, as well as improve fire safety.
Aluminum geodesic domes have fewer corrosion and coating costs compared to steel constructions.
What Are the Regulations for Above-Ground Tanks with Geodesic Dome Roofs?
The Environmental Protection Agency (EPA) requires many types of closed tanks to be vented to vapor recovery or vapor destruction systems, so dome roof applications may require internal pressure when used with sealed tank systems. Above-ground tanks also must meet the EPA’s SPCC requirements for spill prevention, control and countermeasures.
In addition, the American Petroleum Institute (API) standard requires that fixed roof storage tanks be equipped for normal and emergency vapor venting.
The American Water Works Association (AWWA) has a standard for the minimum criteria for designing, fabricating and erecting aluminum dome roofs on any size tank built in accordance with AWWA standards.
How Long Do Above-Ground Tanks Last?
The industry rule of thumb holds that ASTs will last approximately 20 years, depending on such factors as the construction of the tank (including thickness of walls and roof), the tank’s surrounding environment and the type of chemical contents of the tank. ASTs last longer than underground tanks due to the increased likelihood of corrosion due to contact with soil. Outdoor tanks last longer than indoor tanks due to the exposure to the elements and temperature extremes.
Regular above-ground tank inspection can help extend the life of a tank and its dome roof. Inspections can reveal whether dome fasteners are secured properly, whether sealants are adhering properly to the necessary surfaces and whether there are any signs of cracks or other damage that will require repair or replacement. It can also reveal whether thermal contractions and expansions have created gaps in the aluminum panels of the geodesic dome.
How Can You Repair Cracks and Leaks in a Geodesic Dome Roof?
Repair methods for geodesic dome roofs can include the replacement of aluminum panels and sealing gaps with tape.
Alternately, a dome coating system can be less expensive and more efficient. Dome coatings can cure quickly after being applied directly over the seal while proving stronger than tape and more chemically resistant to acids, fuels, leachate waters and their vapors. A flexible coating can repair cracks and leaks while reducing vapor loss and noxious odors, which can help ensure the AST is compliant with environmental requirements.
Applied properly, the coating can permanently seal panel gaskets, eliminating the need for potential replacements. In addition, a dome coating can be applied while the tank is in service, saving money by preventing down time.