Around the world, there are over 4,800 oil terminals responsible for storing nearly a billion cubic meters of crude oil and other combustible or flammable petroleum products (source: www.tankterminals.com). Many of these terminals use floating roof tanks (FRTs). Though FRTs are considered safer than traditional fixed-roof and open-top storage tanks, they still afford the possibility of gas or vapor leaks that can ignite due to lightning or other causes.
An FRT is an aboveground tank in which a metal deck or pan (the roof) floats directly on the surface of the liquid. The roof is flush with the wall of the tank and moves up and down with the level of the liquid, much like a piston in a cylinder. The purpose of the system is to minimise the volume of vapour present in the tank, as well as to reduce the emission of vapors from the tank thanks to a rim seal between the product fluid and the environment.
When a failure occurs on a rim seal, there is the potential for flammable vapours or products to escape. Escaped vapors are troublesome for two reasons: financial loss and the potential for fire. If a fire is not detected quickly, the impact can be disastrous in terms of life safety, facility downtime, loss of production and environmental impact. For all of these reasons, most locally observed codes and regulations require fire detection at the floating roof seal, where the risk of fire is greatest.
