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Quantifying Fugitive Emissions with API Publ 4675-1999: A Technical Guide
Understanding the Standard’s Scope, Methodology, and Compliance Applications for Air Permitting
Scope and Purpose of API Publ 4675-1999
API Publication 4675 (1999 Edition) is a cornerstone technical document developed by the American Petroleum Institute to provide standardized methodologies for estimating fugitive emissions from equipment leaks. Widely referenced in environmental regulatory frameworks, this publication focuses specifically on calculation procedures required for air permit applications, including Title V operating permits, New Source Review (NSR), and routine emission inventories.
The standard addresses a critical gap in environmental management: how to accurately quantify unintended releases of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) from thousands of individual components such as valves, pumps, connectors, and flanges without conducting exhaustive continuous monitoring on every source.
Tip: API Publ 4675 facilitates a tiered approach to emission estimation. Facilities can rely on default emission factors when monitoring data is scarce, and progressively refine estimates using site-specific leak detection and repair (LDAR) data.
By establishing a defensible, industry-accepted methodology, the publication enables operators, regulators, and engineers to create consistent emission profiles that are representative of actual facility performance.
Core Technical Requirements and Methodology
API 4675-1999 outlines three primary tiers of emission estimation, each requiring different levels of data input and providing varying degrees of accuracy.
Tier 1: Average Emission Factors (AEF)
Default emission factors are provided for general screening purposes. These factors are categorized by equipment type and service fluid condition (gas/vapor, light liquid, heavy liquid). They are best suited for initial permit applications or facilities without robust LDAR programs.
Tier 2: Stratified Emission Factors
For facilities with LDAR programs, stratified factors allow components to be categorized by monitored leak concentration ranges (e.g., ‘non-leakers’ and ‘leakers’). This provides a significantly more accurate picture than blanket average factors.
Tier 3: Correlation Equations
The most accurate method relies on EPA-developed correlation equations. These mathematical functions relate a measured leak concentration (in ppmv) from a portable hydrocarbon detector to a site-specific mass emission rate (kg/hr).
| Equipment Type | Service | Average Emission Factor (kg/hr/source) |
|---|---|---|
| Valves | Gas/Vapor | 0.00597 |
| Valves | Light Liquid | 0.00483 |
| Valves | Heavy Liquid | 0.00023 |
| Pump Seals | Light Liquid | 0.0199 |
| Pump Seals | Heavy Liquid | 0.00462 |
| Connectors | All | 0.00025 |
| Compressor Seals | Gas/Vapor | 0.636 |
| Pressure Relief Devices | Gas/Vapor | 0.104 |
Note: Values are representative examples. Refer to the specific tables in the publication for complete datasets and stratified factor groups.
Data Quality and Calculation Procedure Highlights
A significant technical highlight of API 4675-1999 is the explicit guidance on data quality objectives and the handling of monitoring data. The standard provides systematic equations for summing individual component emissions into total facility emissions.
Best Practice: The publication encourages the use of site-specific leak definitions. If a component is monitored and found to be above the regulatory leak threshold (e.g., 500 ppm or 10,000 ppm), the correlation equation is applied directly to its specific reading. Components below the threshold apply the ‘non-leaker’ factor, creating a powerful economic incentive for effective LDAR programs.
The calculation procedure typically involves five steps:
- Source Identification: Walk-down of the facility boundary to identify all potential fugitive sources.
- Categorization: Classifying each source by equipment type (valve, pump, connector) and service fluid (gas, light liquid, heavy liquid).
- Data Application: Selecting the appropriate emission factor from the publication based on the available monitoring data.
- Calculation: Multiplying the factor by the number of components in each category, or applying the correlation equation to individual readings.
- Aggregation: Summing all component-level emissions to derive total fugitive emission rates for the facility.
Important: Average emission factors are intended for screening purposes. Using site-specific data derived from a rigorous LDAR program typically results in more accurate and often lower emission estimates for permitting.
Compliance and Application in Modern Facilities
Although originally published in 1999, API 4675 remains the defacto industry standard for fugitive emission quantification. Its methodologies are embedded in EPA’s WebFIRE database and are explicitly referenced in numerous federal and state regulatory programs, including 40 CFR Part 60 (NSPS) and 40 CFR Part 63 (MACT standards).
Regulatory agencies often require permittees to submit calculations based on the procedures outlined in this publication. Failure to adhere to the tiered calculation hierarchy or applying incorrect emission factors can lead to significant discrepancies in reported emissions, potentially resulting in permit violations or enforcement actions.
Regulatory Alert: Ensure that the factor selection aligns with the specific requirements of the applicable regulatory program (e.g., EPA’s NSPS OOOOa for oil and gas production). Some regulations prescribe specific fractions or correlation equations that must be used over the default API averages.
Frequently Asked Questions (FAQs)
Q: What is the main difference between API Publ 4675-1999 and earlier fugitive emission documents like API 4589?
A: API 4675 focuses specifically on calculation procedures for air permits, incorporating refined emission factors and explicitly integrating EPA correlation equations and stratified emission factors based on LDAR data. It provides a more rigorous framework for generating defensible permit application numbers.
A: API 4675 focuses specifically on calculation procedures for air permits, incorporating refined emission factors and explicitly integrating EPA correlation equations and stratified emission factors based on LDAR data. It provides a more rigorous framework for generating defensible permit application numbers.
Q: Can API 4675-1999 be used for all types of hydrocarbon processing facilities?
A: Yes, the methodologies are broadly applicable to refineries, petrochemical plants, chemical production facilities, and upstream oil and gas production operations. However, users should note that the specific emission factors are most representative of typical refinery and chemical plant service conditions.
A: Yes, the methodologies are broadly applicable to refineries, petrochemical plants, chemical production facilities, and upstream oil and gas production operations. However, users should note that the specific emission factors are most representative of typical refinery and chemical plant service conditions.
Q: How do the correlation equations improve emission estimates compared to simple average factors?
A: Correlation equations provide a continuous function relating the measured leak concentration (ppmv) to a mass emission rate. This allows facilities to accurately quantify emissions from individual components based on their specific measured condition, rather than assuming every component in a category leaks at the same average rate.
A: Correlation equations provide a continuous function relating the measured leak concentration (ppmv) to a mass emission rate. This allows facilities to accurately quantify emissions from individual components based on their specific measured condition, rather than assuming every component in a category leaks at the same average rate.
Q: Is API Publ 4675-1999 a mandatory regulation or a recommended practice?
A: It is a publication (guidance document) and not a codified regulation itself. However, its methodologies are widely adopted and directly referenced in federal and state environmental regulations. Regulators generally accept data calculated using the procedures outlined in 4675 for air permit applications and compliance demonstrations.
A: It is a publication (guidance document) and not a codified regulation itself. However, its methodologies are widely adopted and directly referenced in federal and state environmental regulations. Regulators generally accept data calculated using the procedures outlined in 4675 for air permit applications and compliance demonstrations.
Last updated: 2026