scope:

This publication contains recommended methods for estimating specific organic compound emissions from storage tanks, and marine vessel transfer operations handling multi-component hydrocarbon mixtures (such as crude oils and gasoline) associated with petroleum operations. This publication was developed by the API Committee on Evaporation Loss Measurement.

This document assumes that the user has access to API Publication MPMS 19.2, on Floating-Roof Tanks; API Publication MPMS 19.1, on Fixed-Roof Tanks; and API Publication 2514A, Marine Vessels, and will use those documents to calculate total hydrocarbon emissions. The methods in these documents are used to estimate annual losses from various types of tank construction including fixed and floating roofs, rim-seal systems and roof fittings, as well as various liquid stocks, stock vapor pressures, tank sizes, and wind speeds. They are also applicable for estimating annual losses from loading and unloading operations for various types of marine vessels, types and volumes of cargo, and compartment treatment.

The methodology contained in this document is applicable for properly maintained equipment under normal working conditions. The approach was developed for speciating vapor emissions from liquids with vapor pressure that has reached equilibrium with ambient conditions (i.e., not boiling), stocks with a true vapor pressure less than 14.7 pounds per square inch absolute (psia), for average wind speeds ranging from 2 to 5 miles per hour, and for tank diameters greater than 20 feet.

PURPOSE

It should be noted that quality of the vapor speciation calculation is directly dependent on the liquid composition profile approximation. Due to averaging effects, calculations based on standard profiles for a large number of sources achieve better accuracy. As the number of sources decreases and as the time frame is dropped to less than one year these calculations have the potential for less accuracy.

The approach is not intended to be used for the following applications:

a. To estimate losses from unstable or boiling stocks.

b. To estimate losses from hydrocarbons or petrochemicals for which the vapor pressure of some constituents is unknown or cannot be readily predicted.

c. 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.

METHODOLOGY

Methodologies for speciating emissions from petroleum tankage or marine operations are provided at three levels of detail and accuracy. The three levels are as follows:

a. Level 1 - Calculation of hydrocarbon composition of vapor based on analysis of a representative stock liquid sample or representative stock liquid profile. Speciated emissions are calculated from analysis of liquid stock samples or representative profiles of liquid stock using an appropriate vaporliquid equilibrium relationship.

b. Level 2 - Calculation of the hydrocarbon composition of vapor based upon representative vapor profiles. Speciated emissions are calculated using standard vapor profiles developed for representative stocks and can be used for similar stocks or blends.

c. Level 3 - Direct measurement of hydrocarbon composition of the vapor. Speciated emissions are calculated directly from analyses of vapor samples collected from the stock liquid in the storage tank.

CALCULATION PROCEDURE

The general procedure for determining evaporative emissions of specific hydrocarbons for the emission sources listed above is presented in Figure 1. Calculations of the hydrocarbon composition can either be made for emissions of the total vapor space involved, including the air in the vapor space (based on atmospheric pressure as the total system pressure), or on the hydrocarbon portion of the vapor space (based on the total hydrocarbon partial pressure of the vapor space as the system pressure). The best approach depends on how the results are to be used. These two approaches are discussed in detail in the sections that follow.

DOCUMENT ORGANIZATION

Section 4 provides a summary of the key product related variables needed for speciating hydrocarbon emissions from these sources. These variables are further discussed in Appendix B.

Section 5 provides a summary of the steps used in a typical calculation, and illustrates how the calculations could be set up in a spreadsheet program.

Section 6 provides an overview of the emission sources and the emission mechanisms from these sources.

Section 7 provides an overview of vapor-liquid equilibrium, and outlines the methodology for determining the gas-phase composition from liquid analyses. These concepts are further discussed in Appendix A.

Section 8 uses examples derived from API Publications 2514A, MPMS 19.1 and MPMS 19.2 as the basis for presenting the methodology to illustrate speciation.