Definition of Atmospheric Tank

Candela Rocío Barbisan
Chemical engineer

An atmospheric tank is a container whose design is based on fulfilling the function of storage or processing, depending on atmospheric pressures or relatively low internal pressures (up to about 1 psig), this pressure being measured at the top of the tank. These tanks cannot logically be used for liquids at temperatures higher than their temperature boiling or, according to different definitions, for liquids with vapor pressures of 13 psia.

Typologies: floating roof and fixed roof (umbrella and dome type)

There are various types of tanks used in the industry, although the most typical are those with a floating roof and a fixed roof. One of the critical aspects in the design is the size of the tank, emphasizing that there are tanks of up to 55,000 m3, the largest in Latin America used for the storage of LPG (for its acronym in English: Liquefied Gas of Petroleum). These considerable sizes require body plate thicknesses that are large enough to support the structures, which also implies considering this effect on pressure. As in the case of containers, the cylindrical shape is the simplest and cheapest of design them today, however, the shape of the roof turns out to be the most complex component in the design.

Within the fixed roof tanks there are conical, umbrella and dome roof tanks. In cone-roofed tanks, the floor is flat while the depth of the roof is relatively shallow and shaped. conical and can be manufactured in pieces and assembled in situ, which implies many advantages, among them, minor costs. In umbrella-type tanks, the roof is inverted compared to those with a conical roof and, the advantage they offer, that this type of roof is known as self-supported, avoiding the need to install supports inside the tank, as is the case with the type Pussy. Although externally supported conical roof tanks are manufactured today.

Finally, dome-type tanks approximate the shape of the roof to a spherical surface, being similar in quality constructive to the sunshade roof.

Floating roof tanks, on the other hand, have a roof in the shape of a coin or disc and, as its name indicates, it is floating as long as it is operated within the pressure ranges, temperature and filling height stipulated by design. The ceiling raises and lowers its height depending on the fluid level inside. There are internal or external floating roofs. If the floating roof is external, the design pressure is the same atmospheric pressure. When the tank is open at the top, it is considered an external floating roof, while if the floating roof has a fixed cover, it is considered an internal floating roof. Now, the classic question is why we would cover a floating roof and this is done to avoid evaporative losses, the release of pollutants into the atmosphere.

If the roof is external floating it can be of the type with pontoon or double diaphragm. On the other hand, if it is of the internal floating type, it can be a pan type, a pan with an addition of a retaining wall, a honeycomb or diaphragm and pontoon roof.

Other structural aspects

Today, the material of construction It is very varied and depends on the cost-benefit ratio and the function of the tank. For example, in recent years the demand for aluminum geodesic dome tanks has increased, due to their excellent anticorrosive characteristics and its constructive advantages avoid the need for internal supports.

The bottoms of the tanks are also a fundamental piece, they must support the column of liquid contained and, specifically in the Oil & Gas industry, it is the part most exposed to corrosion due to the contact area and the speed nil, stagnant fluid. Tank bottoms can be flat, conical, or spheroidal.

In the case of the flat bottom, there may be a slight inclination inherent to the design, or it may be a cone with a small angle of elevation or sinking. The service conditions require the design of these tanks to provide for cleaning openings and "manhole" for the inspection and cleaning of the tanks, as well as connections for purges and drains, since microbiological corrosion and various types of corrosion associated with tight flow conditions are very common in these cases.

The design of atmospheric tanks follows numerous quality standards, and depending on the service will be your rule applicable design, such is the case of the code API 650 for example, which details the assessment of materials, welds, loads, seismic and wind conditions and others that must be taken into account at the time of design. Other aspects such as conditions of security They are a priority in manufacturing since, for example, the possibility of a roof blowing up in the event of an overpressure event may be a relevant parameter in the design.