Seal Evaporation Losses

Seal Evaporation Loss Calculations

Below are vapour loss calculations showing the Rim Seal losses for an external floating roof tank with the following conditions :-

  • 100ft (30.48m) diameter tank x 50ft (15.24m) high
  • Product Stored : Gasoline
  • Average annual temperature : 68°F
  • Average wind speed : 10mph
  • Tank Construction : welded, inner shell : light rust, outer shell : painted white.
Evaporation Loss Chart


The results above illustrate the differences between the 3 primary seal types with and without a rim mounted secondary seal as classified in API Chapter 19. As shown in the results, a product mounted primary seal with a tight fitting rim mounted secondary seal is the most efficient type of sealing system available.

Seal Classifications

Vapour Mounted Seals : Any rim mounted seal with a vapour space between the product level & bottom of the seal. A standard graver foam seal is classed as a vapour mounted seal.

Mechanical Shoe Seals : Any type of mechanical seal with a shoe plate which is extended into the product. A SR1 pantograph seal or any other type of modern scissor shoe type seal are classed as mechanical shoe seals. Double compression plate type seals with a vapour skirt extending into the product also fall into this category.

Product Mounted Seals : Any type of seal which is mounted directly on top of the product without any space to let vapours escape. The most reliable type of product mounted seal is a liquid filled Tubeseal. Foam seals such as the graver foam seal can be adapted to become product mounted or there are certain types of foam seals specifically designed to be product mounted seals.

Secondary Seals : Any type of secondary seal which is mounted from the pontoon rim.