External floating roof tank
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.
Normally (roof not landed), there is little vapor space, and consequently a much smaller risk of rim space fire.
Rain water and snow can accumulate on the roof; eventually the roof may sink.
Water on the roof is usually drained from a flexible hose or other special drain line system that runs from a drain-sump on the roof, through the stored liquid to a drain valve on the shell at the base of the tank. A hose often develops leaks and drains both water and product, Other drain lines do not leak. Also pumps are used to drain the roof water through outside the tank, not through the product.
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.