scope:

This publication contains methods for estimating the total evaporative losses or the equivalent atmospheric hydrocarbon emissions from external floating-roof tanks (EFRTs) and freely vented internal floating-roof tanks (IFRTs), as well as for tanks with external-type floating roofs that also have a freely vented fixed roof. This type of tank is referred to as a covered floating-roof tank (CFRT) in this document.

The equations that appeared separately in API Publ 2517 [1] and API Publ 2519 [2] are combined in this document. The standing storage loss factors have been revised to reflect the results of the most recent testing.

This publication was developed by the API Environmental Technical Advisory Group. The equations and factors presented are based on recent laboratory, test-tank, and field-tank data and supercede previous publications. The equations are intended to provide loss estimates for general equipment types, since it is not within the scope of this publication to address proprietary equipment designs.

Typical currently available types of floating roofs, rim-seal systems, and deck 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 floating-roof tanks that contain multicomponent hydrocarbon mixtures (such as petroleum stock liquid like crude oils and gasolines) or single-component hydrocarbon stocks (such as petrochemicals). The equations are applicable to the various types of tank construction, floating-roof construction, rim-seal systems, and deck fittings, as described in Section 8, as well as for various stock liquid, stock vapor pressures, tank sizes, and wind speeds (EFRTs). 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 0.1 to less than 14.7 pounds per square in. absolute (but not greater than the atmospheric pressure at the tank location), average wind speeds ranging from 0 miles per hour (mph) - 15 mph (EFRTs), and tank diameters greater than 20 ft. The estimation techniques become more approximate when used to estimate losses for time periods shorter than one year. The estimation may be improved by using detailed field information, including climatic data and operational data for the appropriate time period.

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 tanks in which the materials used in the rim seal, deck fittings, or deck seams have either deteriorated or been significantly permeated by the stored stock.

c. To estimate losses from storage tanks that do not have a floating roof of any kind.

d. To estimate losses from closed internal or covered floating- roof tanks (that is, tanks vented only through a pressurevacuum relief vent, blanketed with an inert gas, vented to a vapor processing unit, or otherwise restricted from being freely vented).

The equations for estimating evaporative stock loss or the equivalent total atmospheric emissions from volatile stocks stored in floating-roof tanks are given in Section 4. The factors for the standing storage loss equation are discussed in Section 5 and for the withdrawal loss equation in Section 6. Sample problems illustrating the use of the equations are provided in Section 7.

These loss-estimating procedures are applicable only to EFRTs (as described in Appendix C of API Std 650 Welded Steel Tanks for Oil Storage ) [3], IFRTs of freely vented design (as described in Appendix H of API Std 650) [3], and CFRTs (as described in Appendix G of API Std 650) [3] of freely vented design (as described in Appendix H of API Std 650) [3]. Descriptions of the types of construction covered by this publication are given in Section 8.

The bases and development of the loss-estimation procedures presented in Sections 4 through 7 are described in Section 9.

The estimation procedures were developed to provide estimates of typical losses from floating-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 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.