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API Publ 2377-1999: A Technical Publication for Evaporative Loss Estimation from Hydrocarbon Storage Tanks
Review of the Evaporative Loss Model and Guidance for Emissions Calculation
API Publ 2377-1999 is a key technical publication from the American Petroleum Institute (API) that provides a comprehensive review of the evaporative loss model for atmospheric storage tanks. It is widely referenced in the oil and gas industry for estimating hydrocarbon vapor losses from fixed roof and floating roof tanks. The publication updates earlier loss estimation methods with improved theoretical models and empirical data, enabling more accurate emission calculations for environmental reporting, emission inventories, and regulatory compliance.
Scope of API Publ 2377-1999
API Publ 2377-1999 focuses on liquid hydrocarbon storage tanks operating at or near atmospheric pressure. It covers the two dominant tank designs used in petroleum storage:
- Fixed Roof Tanks: Vertical, horizontal, and underground tanks equipped with cone, dome, or flat roofs.
- Floating Roof Tanks: Both internal and external floating roof designs, accounting for rim seals, deck fittings, and roof landings.
The publication addresses two primary types of evaporative losses:
- Standing Storage Loss (LS): Vapor losses through tank openings, seams, and roof seals during quiescent storage.
- Working Loss (LW): Losses caused by changes in liquid level during filling and emptying, including breathing losses and vapor displacement.
The scope excludes pressurized tanks, refrigerated storage, and liquids not exhibiting typical hydrocarbon vapor behavior.
Technical Requirements and Methodology
The evaporative loss estimation model in API Publ 2377-1999 integrates physicochemical and operational parameters into a set of semi-empirical equations. The key variables required for calculation include:
| Parameter Category | Examples | Influence on Loss |
|---|---|---|
| Liquid Properties | True vapor pressure, density, viscosity | Determines volatility and vapor generation rate |
| Tank Geometry | Diameter, height, roof type, seal design | Affects vapor space volume and effective area for evaporation |
| Operational Data | Annual throughput, turnover rate, liquid level | Governs working losses and saturation of vapor space |
| Meteorological Conditions | Ambient temperature, solar insulation, wind speed | Influences breathing losses and vapor dispersion |
| Maintenance Practices | Paint condition (color and reflectivity), seal quality | Modifies tank temperature and emissivity factors |
Standing Storage Loss Equation
The standing loss for fixed roof tanks is expressed as:
LL = K × Dn × Hm × Pva × (Patm − Ptank) × S
where K is a coefficient dependent on roof type, D is tank diameter, H is effective vapor height, Pva is vapor pressure, and S accounts for surface finish. For floating roof tanks, the rim seal and deck fitting losses are computed separately with specific factors.
Working Loss Calculation
Working loss is proportional to the total throughput and a weighting factor that depends on the vapor space saturation during filling. The model distinguishes between small and large breaths and includes a saturation factor that varies with product type and number of turnovers per year.
Tip: The publication provides comprehensive tables of values for common hydrocarbon liquids (e.g., gasoline, crude oil, benzene) under typical storage conditions. Whenever possible, use measured product properties rather than default values to improve accuracy.
Implementation Highlights
To implement the methodology from API Publ 2377-1999, engineers should follow a systematic procedure:
- Data Collection: Gather tank geometry, liquid composition (or representative vapor pressure), annual throughput, local weather data (average ambient temperature, solar radiation, wind speed), and tank condition (paint color, seal type).
- Loss Category Identification: Determine which loss species apply (standing, working, rim seal, and so on) based on tank type and operation mode.
- Selection of Model Parameters: Use tables in the publication to obtain coefficients (e.g., K, n, m) and correction factors (e.g., temperature correction for vapor pressure).
- Computations: Apply the equations manually or through software that encodes the publication’s algorithms. Validate results against historical tank gauging data if available.
- Documentation: Record all input values, assumptions, and intermediate results to support regulatory reporting and third-party reviews.
Success Strategy: Integrate the API Publ 2377-1999 methodology with electronic tank gauging and ambient condition sensors. Real-time data logging enables dynamic emission tracking and can reveal seasonal or operational patterns that static calculations miss.
The publication is frequently used in combination with EPA’s AP-42 Section 7.1, but it offers greater detail on vapor pressure modeling, saturation dynamics, and the influence of tank maintenance on emissions. Engineers working on international projects or with non‐US regulations may prefer API Publ 2377-1999 for its theoretical rigor.
Compliance and Regulatory Notes
API Publ 2377-1999 is a voluntary technical publication, not a mandatory standard. However, it is widely accepted as a sound engineering practice for estimating hydrocarbon losses from storage tanks. Regulatory agencies, including the EPA and local air quality management districts, often cite this publication as an acceptable method for emission inventory submissions, permit applications, and basin‐scale modeling.
Caution: Before using API Publ 2377-1999 for compliance purposes, verify with the relevant regulatory body that its methods are recognized in your jurisdiction. Some regions require the use of specific emission factors (e.g., European EN standards) that may differ from the API approach.
Key compliance considerations include:
- Accuracy vs. Conservatism: The publication’s models aim to provide realistic estimates. If a more conservative result is desired (e.g., for worst-case impact assessment), consider using upper‐bound values for parameters like vapor pressure and temperature.
- Record Keeping: Emissions calculations must be defensible. Maintain a clear trail of input data, version of the publication used, and any adjustments made for site-specific conditions.
- Audit Trail: Many environmental audits require confirmation that the selected loss model matches the actual tank type and that the seal and roof conditions are accurately represented.
- Updates: Although the 1999 edition remains widely used, API periodically releases revisions and new guidance. Check for any applicable updates to ensure current best practices.
Important: The evaporative loss equations in API Publ 2377-1999 are sensitive to input values. Overly optimistic assumptions (e.g., neglecting seal wear or ignoring high‐temperature periods) can lead to significant underestimation of emissions. Always perform sensitivity analysis.
Frequently Asked Questions
Q: What is the primary difference between API Publ 2377-1999 and the EPA AP-42 method for storage tanks?
A: API Publ 2377-1999 offers a more detailed theoretical review of the evaporative loss model, including explicit consideration of vapor space saturation, temperature effects on vapor pressure, and maintenance factors. AP-42 provides a simpler, tabulated approach suitable for routine regulatory reporting. API 2377 is often used for more accurate site-specific emission calculations or when a deeper understanding of loss mechanisms is required.
A: API Publ 2377-1999 offers a more detailed theoretical review of the evaporative loss model, including explicit consideration of vapor space saturation, temperature effects on vapor pressure, and maintenance factors. AP-42 provides a simpler, tabulated approach suitable for routine regulatory reporting. API 2377 is often used for more accurate site-specific emission calculations or when a deeper understanding of loss mechanisms is required.
Q: Is the 1999 edition still considered current?
A: Yes, API Publ 2377-1999 remains widely referenced in the industry, although API has issued later guidance on specific topics (e.g., updates for floating roof tanks). Users should always check API’s latest catalog for any amendments or replacement documents that may be applicable to their project.
A: Yes, API Publ 2377-1999 remains widely referenced in the industry, although API has issued later guidance on specific topics (e.g., updates for floating roof tanks). Users should always check API’s latest catalog for any amendments or replacement documents that may be applicable to their project.
Q: Can the methodology be applied to liquids other than hydrocarbons, such as chemical solvents or water?
A: The models are calibrated using empirical data from hydrocarbon liquids. For other organic or volatile compounds, the vapor pressure behaviors may differ significantly. Engineers should validate the approach with physical properties and, if necessary, seek guidance from technical literature or the API to avoid large errors.
A: The models are calibrated using empirical data from hydrocarbon liquids. For other organic or volatile compounds, the vapor pressure behaviors may differ significantly. Engineers should validate the approach with physical properties and, if necessary, seek guidance from technical literature or the API to avoid large errors.
Q: How can I access API Publ 2377-1999?
A: The publication can be purchased from the American Petroleum Institute (API) through its publication store or through authorized distributors. Some online libraries and technical databases may offer it for reference. Check with your company’s technical information center.
A: The publication can be purchased from the American Petroleum Institute (API) through its publication store or through authorized distributors. Some online libraries and technical databases may offer it for reference. Check with your company’s technical information center.
This article was prepared for technical guidance purposes. Always refer to the official API Publ 2377-1999 document for complete definitions, equations, and tables.