Fixed and Floating Roof Storage Tanks

Many industries rely on storage tanks to contain the liquid materials they handle. They are more famously used in the oil industry to store petroleum and hydrocarbon products, as well as in the chemical and petrochemical industry.

These tanks have to be carefully designed and their specifications are set depending on the type of product they contain and the hazards associated with it. For the most part part, they are typically designed and built according to the American Petroleum Institute (API) specifications. They are also generally installed inside containment basins called dikes that are themselves designed to contain spills in case an overfill or rupture of the tank takes place.

There are many different types of storage tanks, but the most famous are:

1. Fixed Roof Tanks

Fixed roof storage tank

Fixed roof storage tanks (credit: missrifka)

Fixed roof storage tanks are pretty straight forward, and are generally considered the minimum acceptable equipment for storing liquids. They are often the most practical and least expensive option when storing products with a low vapor pressure (close to atmospheric pressure).

A typical fixed roof tank consists of a steel shell with a cone or dome shaped roof that is permanently fixed to the tank shell. They are usually fully welded and designed to be both liquid and vapor tight. Some designs allow installing a breather valve (pressure/vacuum valve) that prevent the build up of excessive pressure or vacuum.

2. External Floating Roof Tanks

External floating roof tank

External floating roof storage tank (credit: AIR)

These tanks have a cylindrical steel shells equipped with a roof that floats on the surface of the stored liquid; rising and falling along with the liquid level. This floating roof system is comprised of a deck, fittings, and a rim seal system.

Floating roof decks are constructed of welded steel plates and have three main types:

  • Pan deck
  • Pontoon deck
  • Double deck

There is a annular space with seal gland between the floating roof and the tank wall, to separate air when stored liquid float up or down on the roof, greatly reducing evaporation loss in the storage process. Compared with fixed roof tank, using a floating roof tank to store oil product can reduce evaporation loss to 80% approximately.

3. Internal Floating Roof Tanks

Internal floating roof

Internal floating roof tank

Internal floating roof tanks have a floating roof that moves up and down above the product level, while at the same time include a fixed permanent roof above the tank's shell.

The internal floating roof tank (IFRT) was developed in the mid-1950s to provide protection of the floating roof from the elements, including lightning strikes to the floating roof. The tank vapor space located above the floating roof and below the fixed-roof includes circulation vents to allow natural ventilation of the vapor space reducing the accumulation of product vapors and possible formation of a combustible mixture.

Internal floating roof tanks provide the best option for controlling volatile losses compared to fixed and external floating roof tanks.

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