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Scope and Historical Context
| Seal Type | Tank Type | Emission Factor (lb/yr-ft diameter) | Performance Level | |—|—|—|—| | Primary, Mechanical Shoe | External FR | X | Moderate | | Primary, Liquid-mounted Resilient | External FR | X | Good | | Primary + Secondary (Mechanical Shoe) | External FR | X | Best | | Primary + Secondary (Liquid-mounted) | External FR | X | Best Practice | | Vapor-mounted Resilient | Internal FR | X | High |
API Publication 2557, formally titled “Vapor Release from Floating Roof Tanks: A Literature Review and Evaluation of Emission Factors” (First Edition, 1993), represents a cornerstone in the environmental management of petroleum storage facilities. This publication synthesized decades of research to establish a rigorous, technically defensible methodology for calculating evaporative losses from floating roof tanks. For environmental engineers, facility operators, and regulatory compliance managers, understanding the technical depth of API Publ 2557 is essential for accurate emission estimation, regulatory reporting, and implementing effective vapor control strategies.
Scope and Historical Context
The primary scope of API Publ 2557 was to consolidate available research, field data, and theoretical models into a unified framework for quantifying vapor losses. Unlike typical standards, this publication served as a critical evaluation of existing emission factors, ultimately leading to the development of the widely adopted correlations found in EPA’s AP-42 Chapter 7 and later API MPMS Chapter 19.2.
The publication addresses three primary tank configurations:
- External Floating Roof (EFR) Tanks: Open-top tanks with a roof floating directly on the liquid surface.
- Internal Floating Roof (IFR) Tanks: Fixed-roof tanks containing an internal floating roof, offering protection from wind.
- Covered Floating Roof (CFR) Tanks: Essentially a hybrid with specific deck fitting considerations.
By evaluating hundreds of data points, the 1993 publication established the framework that defined the loss mechanisms: rim seal evaporation, deck fitting losses, deck seam losses, and withdrawal losses.
Tip: When referencing API Publ 2557 for historical data, users should verify against the most current API MPMS Chapter 19 editions to ensure calculations reflect modern tank hardware efficiencies and updated vapor pressure correlations.
Technical Requirements and Emission Source Analysis
API Publ 2557 deconstructs total vapor loss into four distinct, additive components. Each component must be calculated independently and summed to determine the total emission rate from a given tank.
Rim Seal Losses
The rim seal is the primary barrier between the stored liquid and the atmosphere. The publication provides distinct emission factors (usually in lb/yr-ft of tank diameter) for various seal configurations:
- Primary Seal Only: Mechanical shoe seals exhibit higher losses compared to liquid-mounted resilient seals. Vapor-mounted resilient seals generally show the highest loss rates among primary seals due to vapor compression.
- Primary and Secondary Seals: The installation of a secondary seal (rim-mounted or shoe-mounted) significantly reduces evaporative losses, typically by 60-80% over a primary seal alone.
Fitting and Deck Seam Losses
Losses from fittings (hatches, gauge wells, column wells, roof legs) depend on the specific design (bolted vs. welded, tight vs. loose). API Publ 2557 assigns fixed loss factors to each fitting based on its “tightness” classification. Deck seam losses apply primarily to pontoon and double-deck roofs with bolted or welded seams.
Withdrawal Losses
As the roof descends, a liquid film clings to the exposed tank shell and internal support columns. This film evaporates over the course of the filling cycle. The loss is a function of the product’s adhesion properties, volatility, and the number of turnovers (or level changes) per year.
Warning: A common pitfall in applying the methodology is the mismanagement of wind speed corrections for External Floating Roof Tanks. API Publ 2557 stipulates specific correlations for wind speed, typically imposing a minimum effective wind speed of 5 knots to prevent unrealistic calculations.
Comparative Emission Factor Table
| Emission Source | Typical Configuration (EFR) | Range of Emission Factors (lb/yr-ft) | Key Influencing Variable |
|---|---|---|---|
| Rim Seal (Primary Only) | Mechanical Shoe | 8.0 – 12.0 | Seal gap, shoe condition |
| Rim Seal (Primary + Secondary) | Mechanical Shoe + Wiper | 2.0 – 5.0 | Secondary seal integrity |
| Rim Seal (Primary + Secondary) | Liquid-mounted Resilient + Wiper | 1.0 – 3.0 | Liquid mounting fluid level |
| Fittings | Bolted Access Hatch | Constant (lb/yr) | Number of fitting, gasket wear |
| Deck Seams | Bolted Deck (5 ft spacing) | 0.1 – 0.5 (lb/yr-ft seam) | Seam length, bolt tightness |
Implementation Highlights and Best Practices
Successful application of API Publ 2557 requires a systematic approach to data collection and calculation.
- Data Quality: Accurate tank dimensions, seal type identification, and discrete fitting counts are mandatory. Assumptions should be minimized and documented.
- Turnover Events: The withdrawal loss component is heavily influenced by tank usage patterns. Proper logging of throughput and level changes is critical for annual emission reporting.
- Software Implementation: Many commercial emission calculation software packages embed the logic of API Publ 2557. Users must ensure the specific correlations and factors are correctly configured, particularly regarding the distinction between IFR and EFR tank calculations.
Success: Implementing a rigorous rim gap inspection program correlated with the seal loss factors in API Publ 2557 can validate emission reduction investments. A gap exceeding 0.25 inches may significantly increase the effective emission factor, justifying a seal replacement project.
Danger: Failing to account for the saturation factor (Ks) or the vapor space expansion factor in IFR tanks can lead to errors. The assumption that the vapor space is completely saturated (Ks = 1) is a default recommended by the publication but may not be accurate for low-vapor-pressure products.
Compliance Framework and Regulatory Impact
While API Publ 2557 is a technical publication, its influence on regulatory compliance is profound. The U.S. Environmental Protection Agency (EPA) relied heavily on the 1993 publication to formulate the emission correlations in AP-42 Section 7.1. Internationally, the methodologies are mirrored in environmental regulations across the EU, Asia, and the Middle East.
Compliance strategies must account for the correct application of this technical basis:
- Documentation: Maintain a detailed tank history file including seal type, installation dates, gap inspection results, and throughput data.
- Audit Path: The publication serves as the technical justification for the emission factors used in Title V operating permits or E-PRTR reporting.
- Evolution: Users of the 1993 publication must stay informed of subsequent updates, particularly API MPMS Chapter 19.2 (Evaporative Loss from Floating Roof Tanks) which expanded and refined the original work.
The enduring value of API Publ 2557 lies in its rigorous, data-driven approach to understanding a complex physical process. It remains a critical reference for any professional tasked with managing, reducing, or calculating vapor emissions from floating roof storage tanks.
Frequently Asked Questions
Q: What is the fundamental difference between API Publ 2557 (1993) and the later API MPMS Chapter 19.2?
A: API Publ 2557 was a literature review and evaluation of existing emission factors. API MPMS Chapter 19.2 is the formal, consensus standard that superseded and significantly expanded the emission estimation methodology. Publ 2557 provides the scientific context for the correlations in MPMS.
A: API Publ 2557 was a literature review and evaluation of existing emission factors. API MPMS Chapter 19.2 is the formal, consensus standard that superseded and significantly expanded the emission estimation methodology. Publ 2557 provides the scientific context for the correlations in MPMS.
Q: Does API Publ 2557 cover losses from fixed roof tanks?
A: No, API Publ 2557 strictly covers External, Internal, and Covered Floating Roof tanks. For fixed roof tanks, the relevant standard is API MPMS Chapter 19.1 (which supersedes API Bull 2518).
A: No, API Publ 2557 strictly covers External, Internal, and Covered Floating Roof tanks. For fixed roof tanks, the relevant standard is API MPMS Chapter 19.1 (which supersedes API Bull 2518).
Q: How sensitive are the emission factors in the publication to liquid properties?
A: Extremely sensitive. The true vapor pressure (TVP) of the liquid, its molecular weight, and the average ambient temperature are dominant variables. The publication provides rigorous methods for calculating TVP from RVP and converting vapor pressure to standard conditions for mass emission estimates.
A: Extremely sensitive. The true vapor pressure (TVP) of the liquid, its molecular weight, and the average ambient temperature are dominant variables. The publication provides rigorous methods for calculating TVP from RVP and converting vapor pressure to standard conditions for mass emission estimates.
Q: Can the methodology in API Publ 2557 be used for Leak Detection and Repair (LDAR) programs?
A: While LDAR programs (like EPA Method 21) focus on immediate leaks from components, the vapor loss equations from API Publ 2557 quantify the much larger “permissible” holistic emissions from the tank itself. Both are complementary for a comprehensive environmental management plan.
A: While LDAR programs (like EPA Method 21) focus on immediate leaks from components, the vapor loss equations from API Publ 2557 quantify the much larger “permissible” holistic emissions from the tank itself. Both are complementary for a comprehensive environmental management plan.
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