scope:

19.1.1.1 Scope

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 has been improved over that which appeared in the first edition of API Bulletin 2518, but the working loss equation is the same as that in the First Edition.

This publication was developed by the API Committee on Evaporation Loss Measurement. 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 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. 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.

d. To estimate losses from horizontal cylindrical tanks.

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. 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 19.1.2.1, and 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), and a sample problem 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.