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API Publication 2514A (1981, Reaffirmed 1987): Evaporation Loss from Fixed-Roof Tanks – Technical Overview and Compliance Notes
Understanding the Methodology for Estimating Hydrocarbon Vapor Emissions from Fixed-Roof Storage Tanks
Scope and Purpose of API Publication 2514A
API Publication 2514A, originally issued in 1981 and reaffirmed in 1987, provides a standardized methodology for estimating hydrocarbon evaporation losses from fixed-roof storage tanks. The standard addresses both standing storage losses (due to vapor breathing from temperature and pressure changes) and working losses (generated during filling and emptying operations). It applies to vertical, cylindrical, fixed-roof tanks containing liquid hydrocarbons with true vapor pressures between 1.5 and 100 psia (approximately 10 to 690 kPa) and is intended for tanks operating at or near atmospheric pressure.
This publication is part of the API Manual of Petroleum Measurement Standards and is widely referenced by environmental agencies—such as the U.S. Environmental Protection Agency (EPA) in AP-42—for emission inventories and air permitting. The 1987 reaffirmation confirmed the technical content without revision, meaning that the guidance remains current for legacy systems and is often used as a basis for compliance demonstrations.
Technical Requirements and Emission Calculation Methodology
API 2514A presents two principal emission categories that must be quantified separately and then summed to obtain total annual evaporation loss:
1. Standing Storage Loss (Ls)
Standing storage loss, also called breathing loss or evaporation loss, occurs when vapor is expelled from the tank due to daily thermal expansion and contraction of the vapor space. The calculation uses tank geometry, vapor space volume, properties of the stored liquid, and local meteorological data. The basic formula is:
Ls = Vv × Wv × KE × N
Where:
- Vv = vapor space volume (ft³)
- Wv = vapor density (lb/ft³)
- KE = vapor space expansion factor
- N = number of turnovers (annual fill-and-empty cycles)
Vapor density is derived from the liquid’s true vapor pressure at the average liquid surface temperature, using Raoult’s law and the molecular weight of the vapor. The expansion factor KE depends on the daily temperature range and the vapor pressure of the product.
2. Working Loss (Lw)
Working loss occurs when liquid is pumped into or out of the tank, forcing vapor out. The standard provides separate equations for filling losses (which are usually larger) and emptying losses. The basic form for filling loss is:
Lw = Vf × Wv
Where Vf is the volume of liquid pumped into the tank during a given period. Notably, working losses during emptying are considered negligible because air is drawn into the tank without significant loss.
Tip: When applying API Publ 2514A, ensure that the liquid surface temperature is measured or estimated appropriately. The standard recommends using the average of the daily minimum and maximum liquid surface temperatures for breathing loss calculations. Many practitioners default to the average ambient temperature plus a 1–3°F increment for uninsulated tanks.
Key Input Parameters
| Parameter | Description | Typical Data Source |
|---|---|---|
| True vapor pressure (TVP) | Vapor pressure of the liquid at the tank’s average liquid surface temperature | Product specification or Reid vapor pressure converted using ASTM D2889 |
| Vapor space volume | Volume of the tank above the liquid level, including the roof cone | Tank strapping tables or direct measurement |
| Vapor density | Density of the hydrocarbon vapor at the average temperature and pressure | Calculated from vapor pressure and molecular weight |
| Daily temperature range | Typical daily minimum to maximum ambient temperature | Local meteorological records |
| Product throughput | Annual volume of liquid transferred into the tank | Operating records |