Preventing fire with tank seals
As atmospheric tanks are storing tonnes of both volatile and often very explosive or easily ignited products, fire fighting is always a concern. Infrastructure for both fire detection and fire fighting requires careful evaluation, installation and maintenance
As steel external floating roofs typically float in full contact with the liquid, the most common fire scenario is a rim fire. A rim fire occurs in the area between the floating roof and the tank shell. It is not considered a major incident, as the surface on fire is limited, but it definitely requires immediate extinguishing as it could jeopardise the integrity of the floating roof. Allowing a rim fire to continue could introduce various escalations, resulting in serious concerns on the safety and integrity of the direct environment of the tank terminal.
A full pool fire is a much more serious incident and is much more difficult to deal with. The impact on both the tank as well as the immediate environment of a tank farm is much more severe. A full pool fire can be the result of several causes, such as an extensive amount of product or combustible vapours being present on top of the floating roof and a sunken external floating roof.
Tank seals on external floating roofs play a vital role in safely operating tanks. Typically tank seals are divided into primary and secondary seals. A primary seal is installed in the area between the floating roof and the tank shell, and represents the first barrier against product evaporation.
As the primary seal is either in direct contact with the product or in contact with saturated product vapours it always has to deal with this product exposure. A secondary seal is a barrier against product evaporation, and is usually installed on top of the primary seal, but typically independent from this primary seal by being rim mounted.
The most common types of primary seals include shoe plate seals, compression plate seals, foam filled seals and liquid filled seals. Shoe plate and compression plate seals represent well over 90% of the primary seals in service worldwide, whereas foam and liquid filled seals have been phased out at many terminals in the past two decades.
