An external floating roof tank is a storage tank commonly used to store large quantities of petroleum products such as crude oil or condensate. It comprises an open- topped cylindrical steel shell equipped with a roof that floats on the surface of the stored liquid. The roof rises and falls with the liquid level in the tank. As opposed to a fixed roof tank there is no vapor space (ullage) in the floating roof tank (except for very low liquid level situations). In principle, this eliminates breathing losses and greatly reduces the evaporative loss of the stored liquid. There is a rim seal system between the tank shell and roof to reduce rim evaporation.
The roof has support legs hanging down into the liquid. At low liquid levels the roof eventually lands and a vapor space forms between the liquid surface and the roof, similar to a fixed roof tank. The support legs are usually retractable to increase the working volume of the tank.
Video External floating roof tank
Advantages
External roof tanks are usually installed for environmental or economical reasons to limit product loss and reduce the emission of volatile organic compounds (VOC), an air pollutant.
Normally (roof not landed), there is little vapor space, and consequently a much smaller risk of rim space fire.
Maps External floating roof tank
Disadvantages
Snow can accumulate on the roof; the roofs are designed to hold up to 10" / 255mm of water.
System Function
Water on the roof is usually drained by a special flexible hose or other special drain line system that runs from drain-sumps on the roof, through the stored liquid to a drain valve on the shell at the base of the tank. A hose is the shortest quickest route, Other drain systems are available both rigid and semi-rigid. These are named 'Articulated' as they use straight lengths of steel pipes with mechanical swivel joints or comprise of steel pipes with flexible sections. The main objective is to evacuate rainwater from the tank roof as it is heavier than the tank contents and will cause problems if above a given amount is accumulated (10" / 255mm). Also pumps are used to drain the roof water through outside the tank, not through the product.
See also
- storage tank
- fixed roof tank
Tanks and vessels come in many different shapes and sizes and many factors affect their design and manufacture. For example, pressure, temperature, and chemical properties are key factors that affect wall thickness, materials of construction, and shape. Tanks are container in which atmospheric pressure is maintained.
References
External links
- Use of floating roof storage tanks in Oil & Gas industry
- Compilation of Air Pollutant Emission Factors, Volume 1: Stationary Point and Area Sources. Chapter 7. Chapter 7: Liquid Storage Tanks
- www.we-couplings.com
Source of the article : Wikipedia
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