In liquid storage tanks having floating roofs, the floating roof is constructed at a slightly smaller diameter than the side wall of the tank to define an annular space that permits some freedom of movement as the roof is raised and lowered by the fluid product stored in the tank. The annular space is sealed by a mechanical ring seal of which there are a number of designs such as those disclosed in Moyer, U.S. Pat. No. 2,696,930 and Lippiello et al., U.S. Pat. No. 5,103,992.
Seals are necessary in most installations where the liquid contents vaporize and cause loss of product and pollution. Improved seals have been developed that permit the storage of liquids that are more volatile than were previously stored in floating roof tanks resulting in considerable savings over fixed roof designs.
After extended periods of use, seals tend to break down and need to be replaced. In some older designs, certain components such as shoe plates were constructed from arcuate segments fastened together to form essentially one continuous ring around the tank. To replace or repair any ring seal component, the tank had to be taken out of service because environmental concerns prohibited use of a tank containing harmful vapor without a ring seal. Taking a tank out of service is time-consuming, hazardous and expensive.