External floating roof tank Advantages and Disadvantages

An external floating roof tank is a type of storage tank commonly used to store large quantities of petroleum products such as crude oil or condensate.It consists of an open-topped cylindrical steel shell fitted with a roof floating on the surface of the stored liquid.The roof rises and falls with the liquid level in the tank.In contrast to fixed roof tanks, there is no vapor space (void ratio) in floating roof tanks (except at very low liquid levels).In principle, this eliminates tank breathing losses and greatly reduces evaporation losses of the stored liquid.There is an edge sealing system between the tank shell and the roof to reduce edge evaporation.The roof has supporting legs that hang down into the liquid.At low liquid levels, the roof will eventually sink and a vapor space will form between the liquid surface and the roof, similar to a fixed roof tank.Support legs are usually retractable to increase the working volume of the tank.

Advantages 

For environmental or economic reasons, external roof tanks are often installed to limit product loss and reduce the potential for emissions of volatile organic compounds (VOCs) and other potential air pollutants.Typically, the top of the tank sits directly on the surface of the liquid being stored, leaving little room for vapor, so the risk of causing a fire in the rim space is much lower.

Disadvantages

Snow and rain can collect on roofs.Roof exposure to sunlight and weather can accelerate corrosion.

System functions

Water from the roof is usually drained through special hoses or other special drain systems that run from the sump in the roof and through the stored liquid to the drain valve on the tank's bottom shell.A hose is the shortest and fastest route, and other drainage systems come in both rigid and semi-rigid.These are named "articulated" because they use straight pipe sections with mechanical swivel joints or consist of steel pipe with flexible.