This publication contains an improved method for estimating the total evaporative losses or the equivalent atmospheric hydrocarbon emissions from fixed-roof tanks that contain multicomponent hydrocarbon mixture stocks (such as petroleum liquid stocks like crude oils) or single-component hydrocarbon stocks (such as petrochemical stocks like ethanol). The standing storage loss equation was improved in the second edition of API Publication 2518 [also identified as API Manual of Petroleum Measurement Standards@ Chapter 19.1 (API MPMS 19.1)] over that which appeared in the first edition of API Bulletin 2518. The working loss equation in the second edition of API Publication 2518 remained the same as that in the First Edition. This third edition utilizes the same equations as those in the second edition@ but presents simplified calculation procedures as well as additional information. The following improvements have been incorporated into this edition: a. Simplified forms of the emissions estimating equations for the common scenario of a low volatility liquid (i.e.@ true vapor pressure not greater than 0.1 psia) stored in a fixed roof tank with vents that are either open or have very low set points [i.e.@ not greater than 0.03 pounds (0.5 oz) per square inch]. b. Methods to estimate emissions from horizontal tanks. c. Methods to account for the vent setting when estimating emissions from tanks with vent settings greater than 0.03 pounds (0.5 oz) per square inch (the previous edition accounted for the vent setting when estimating standing storage loss@ but did not account for the vent setting when estimating working loss). d. Methods to speciate estimated emissions of individual chemicals from the estimate of total hydrocarbon emissions for a multicomponent hydrocarbon mixture. This publication was developed by the API Committee on Evaporation Loss Estimation. The equations presented are based on test-tank and field-tank data. The 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. Types of fixed-roof tanks and roof fittings currently available 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 average annual losses from uninsulated fixed-roof tanks for various liquid stocks@ stock vapor pressures@ tank sizes@ meteorological conditions@ and operating conditions. The equations are applicable to properly maintained equipment under normal working conditions. The equations were developed for nonboiling stocks@ although volatile liquid stocks with a true vapor pressure over 1.5 pounds per square inch absolute are not now typically stored in the U.S. in fixed-roof tanks. To calculate emissions from tanks that contain material at or above their boiling point or the point at which material starts to flash@ the API model EP Tank can be used. Without detailed field information@ the estimation techniques become more approximate when used to calculate losses for time periods shorter than one year. The equations are not intended to be used in the following applications: a. To estimate losses from unstable or boiling stocks or from petroleum liquids or petrochemicals for which the vapor pressure is not known or cannot readily be predicted. b. To estimate losses from fixed-roof tanks which have an internal floating roof. c. To estimate losses from fixed-roof tanks which have either roof or shell insulation. A complete guide for estimating evaporative stock loss or the equivalent total atmospheric emissions from volatile stocks stored in fixed-roof tanks is included in 19.1.2. Detailed equations are given in 19.1.2.1@ for vertical aboveground tanks storing liquid stocks of low volatility at nearly atmospheric conditions. In addition@ the following special cases are addressed in 19.1.2.1.4: a. Horizontal tanks. b. Higher volatility stocks (true vapor pressure greater than 0.1 psia). c. Higher vent settings [breather vent settings@ PBP and PBV @ beyond the typical range of ??0.03 pounds (0.5 oz) per square inch]. A description of how to determine specific values for the variables included in the equations is given in 19.1.2.2. 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 19.1.2 (Tables 2 and 3). When the procedures in 19.1.2 are applied to a fixed-roof tank storing a multicomponent hydrocarbon stock@ the result is an estimate of the total hydrocarbon emissions from the tank. Guidance for speciating total hydrocarbon emissions into the emissions of the individual components is provided in 19.1.2.3.1. A sample problem estimating total emissions is presented in 19.1.2.4. Typical fixed-roof tank construction is described in 19.1.3. The bases and development of the loss-estimation procedures presented in 19.1.2 are described in 19.1.4. The estimation procedures were developed to provide estimates of typical losses from fixed-roof tanks that are properly maintained and in normal working condition. Losses from poorly maintained tanks 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 fixed-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.