American Petroleum Institute

USE OF INTERNAL FLOATING COVERS FOR FIXED-ROOF TANKS TO REDUCE EVAPORATION LOSS
 N BULL 2519

Annotation

Introduction

Prevention of evaporation loss from petroleum products is becoming extremely important. In addition to conserving a valuable natural resource, reduction of evaporation loss, in many instances, provides a substantial economic savings.

The majority of the tanks in use today in the petroleum industry are the common fixed-roof type. Considerable progress has been made over the years in controlling evaporation loss from fixed-roof tanks. Gastight construction and fittings, proper venting, and reflective painting are all factors which wiII reduce losses from this type of storage. Nevertheless, even with these improvements, breathing and filling losses in the aggregate still are of considerable magnitude.

To reduce evaporation from a free liquid surface inside an existing fixed-roof tank, various internal floating covers have been devised to separate the liquid and the vapor region. These covers offer conservation practices similar to those of the floating-roof tank and are most economically suited for installation in existing fixed-roof tanks which store volatile products.

Installations of internal floating covers date from 1955 and include sizes from less than 10 ft to 84 ft in diameter. These installations are located in many countries and climates ranging from tropical to frigid. The stored prroducts include alcohols plus the full range of hydrocarbons—such as gasoline, naphtha, jet fuel, and toluene. Many oil companies have installed internal floating covers and, at the present time, are compiling performance data and studying the results.

This report summarizes to date the performance of internal floating covers as reported to the API Evaporation Loss Committee. Various tests have indicated that the efficiency of a floating cover, in protecting a liquid surface from evaporation, may be as high as 90 percent.