Liquids for various industrial or domestic use need to be contained. Liquids are free flowing and they take the shape of any form they are placed in. For this reason, tanks are useful as they provide for proper storage and sealed transportation according to the requirement. Liquid storage tanks are built with special consideration since liquids can evaporate or leak through the smallest defect. These tanks could be made into different shapes and materials intended for varied purposes like - horizontal, vertical, steel, glass, plastic, aluminum, etc.
1. Fixed Roof Tanks
Fixed roof tanks are the least expensive to build among the other hefty budget tanks. These are cylindrically shaped with an affixed roof that is either dome or cone shaped. The breather valve holds certain pressure to the rather vacuum-less tank. Some older roof tanks do not have vacuum, so there is a possibility of vapor leak. The affixed roof is non changeable and has the gauge/hatch/sample well/manhole to steer through the liquid and in case of volatile access.
These tanks are smaller, no more than 40,000 gallons capacity. Since they carry only the required amount of liquid than other counterparts of storage tanks in similar categories.
2. External Floating Tanks
As the name suggests Floating roof tanks are of three broad sub types- external, internal and domed external. All of whose roofs are floating - meaning they can be set to rise or fall depending on the level of water in the tank. These external roof tanks are either pontoon fitted or double deck fitted steel containers. Their design is such that it prevents the evaporation of contained liquid to a minimum with only rim seal fitting standing storage - liquid loss and exposed tank wall - liquid loss .
The reason why the roof of floating tanks are more flexible is because of the evaporative loss caused in fixed roof tanks. This allows the storage tanks to hold more quantities of liquid. The builders generally specific floating tanks by their ability to carry load capacity, pontoon arrangement, roof variations and full liquid contact features.
3. Internal Floating Tanks
Internal floating roof tanks suggest that there is more than one roof in the tank. One is fixed whereas the other is free to rise or fall according to liquid level on /contact/non-contact decks. The newly built internal tanks can have both - supportive or independent roofs and are mostly non-contact deck fitted. Fixed roof tanks upgraded to floating versions have vertical support within the tank. And external tanks which have internal floating roofs are self-supporting. The internal floating roof has the function of lessening liquid loss due to evaporation. Hence there are vents which aid further to prevent any vapour accumulation in the tank.
4. Domed external Floating tanks
These are the newest innovation in the design line of floating tanks. They have evolved by retrofitting the external floating roof with a fixed roof tank system. It is almost identical to an internal floating roof tank with support structure within and a welded deck.
Aluminum domed external floating tanks have become popular due to the dome’s ability to keep wind particles and rain from entering into the otherwise flat roof tanks. They are also cost-effective containers against the steel tanks. The carbon footprints are reduced as the volatile organic compounds (VOC) are of limited discharge caused due to petroleum storage.
5. Pressure Tanks
Pressure tanks are used to store organic liquid and gases that need special pressure modulations. There are two different types of Pressure tanks along with recommended pressure / vacuum settings.
Low-Pressure Tanks: These are used to store liquids like crude oil which can emit working losses at ambient conditions. Low pressure tanks are equipped with a pressure / vacuum vent that reduces breathing loss from barometer, temperature or pressure changes.
High-pressure Tanks: These tanks are highly pressurized and are used to store liquids having pressure beyond atmospheric conditions. There is next to no evaporative or working losses of stored liquid in high pressure tanks.
6. Variable vapor space tanks
These tanks use expandable vapor reservoirs to account for changes to vapor volume resulting from temperature and pressure changes and can function as integrated components of vapor systems for fixed roof tanks. They may be either separate gasholder units or integral units mounted atop fixed roof tanks. Variable vapor space tank losses occur during tank filling when vapor is displaced by liquid. Loss of vapor occurs only when the tank's vapor storage capacity is exceeded. Variable vapor space tanks are equipped with expandable vapor reservoirs to accommodate vapor volume fluctuations attributable to temperature and barometric pressure changes. Although variable vapor space tanks are sometimes used independently, they are normally connected to the vapor spaces of one or more fixed roof tanks. The two most common types of variable vapor space tanks are lifter roof tanks and flexible diaphragm tanks. Examples of variable vapor space tanks are lifter roof tanks and flexible diaphragm tanks. These systems minimize VOC emissions from storage losses.
7. LNG Storage tanks
Liquefied natural gas storage tank or simply put LNG tank exclusively stores only Liquefied Natural Gas. LNG storage tanks are specialized in such a way that they can store the gas at very low temperatures(-162°C) which can be stored on and above ground.
Since LNG is volatile liquid, there is an extra layer with insulated material within the tank which stands at roughly 55m high and 75 m in circumference. LNG is a cryogen kept at low temperatures, if the pressure is kept constant the boil off gas will escape as a result of auto refrigeration. This is done because if LNG vapours are not released, the pressure and temperature within the tank will continue to rise.
8. Horizontal Tanks
Horizontal tanks are constructed for efficient space utilization. They are more preferred over vertical tanks since their easy to transport size and design. They have both above-ground and underground storage capabilities. Cathodic protection is a precautionary measure for underground horizontal tanks. It is done by fitting anodes inside. However, nowadays corrosion inhibitors have taken over Cathodic protection in most refinery and petroleum horizontal storage tanks.