This publication contains an improved method for estimating the total evaporative losses or the equivalent atmospheric hydrocarbon emissions from external floating-roof tanks that contain multicomponent hydrocrbon mixtures (such as crude oils and gasolines) or single-component stocks (such as petrochemicals). This publication was developed by the API Committee on Evaporation Loss Measurement. The equations presented are based on recent laboratory@ test-tank@ and field-tank data. These equations are intended to provide loss estimates for general equipment types@ since it is not within the scope of this publication to address specific proprietary equipment designs. Typical currently available types of floating roofs@ rim-seal systems@ and roof fittings are described for information only. This publication is not intended to be used as a guide for equipment design@ selection@ or operation. The equations are intended to be used to estimate annual losses from external floating-roof tanks for various types of tank construction@ floating roofs@ rim-seal systems@ and roof fittings@ as well as for various liquid stocks@ stock vapor pressures@ tank sizes@ and wind speeds. The equations are applicable to properly maintained equipment under normal working conditions. The equations were developed for liquids that are not boiling@ stocks with a true vapor pressure ranging from approximately 1.5 to less than 14.7 pounds per square inch absolute@ average wind speeds ranging from 2 to 15 miles per hour@ and tank diameters greater than 20 feet. Without detailed field information@ the estimation techniques become more approximate when used to calculate losses for time periods shorter than 1 year. The equations are not intended to be used in the following applications: a. To estimate losses from unstable or boiling stocks or from mixtures of hydrocarbons or petrochemicals for which the vapor pressure is not known or cannot readily be predicted. b. To estimate losses from tanks in which the materials used in the rim seal@ roof fittings@ or both have either deteriorated or been significantly permeated by the stored stock. Section 2 includes a complete guide for estimating evaporative stock loss or the equivalent total atmospheric emissions from volatile stocks stored in external floating-roof tanks. Note: The calculated pounds per year of total hydrocarbon losses may include both reactive and nonreactive compounds. To obtain reactive hydrocarbon emissions@ the weight fraction of reactive hydrocarbons in the vapor must be applied. Detailed equations are given in 2.1@ and a description of how to determine specific values for the variables included in the equations is given in 2.2. References are made to tables and figures that include information about the most common (typical) values to use when specific information is not available. The loss-estimation procedures are summarized in 2.3 (Tables 1 and 2)@ and a sample problem is presented in 2.4. Section 3 describes the typical equipment types covered in Section 2. Section 4 describes the bases and development of the loss-estimation procedures presented in Section 2. The estimation procedures were developed to provide estimates of typical losses from external floating-roof tanks that are properly maintained and in normal working condition. Losses from poorly maintained equipment may be greater. Because the loss equations are based on equipment conditions that represent a large population of tanks@ a loss estimate for a group of external floating-roof tanks will be more accurate than a loss estimate for an individual tank. It is difficult to determine precise values of the loss-related parameters for any individual tank. Equipment should not be selected for use based solely on evaporative-loss considerations. Many other factors not addressed in this publication@ such as tank operation@ maintenance@ and safety@ are important in designing and selecting tank equipment for a given application.