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API Publ 4631-1995: Methodology for Estimating Fugitive Hydrocarbon Emissions from the Petroleum Industry
A Technical Guide to the Scope, Emission Factors, and Compliance Application of API Publication 4631
API Publ 4631-1995 (often referred to as API 4631), titled Methodology for Estimating Fugitive Hydrocarbon Emissions from the Petroleum Industry, is a cornerstone publication developed by the American Petroleum Institute. Although published in 1995, it remains a widely referenced source for estimating uncontrolled fugitive emissions from equipment components in petroleum refining, production, and petrochemical facilities. This article provides an in-depth review of the scope, technical approach, implementation guidelines, and compliance relevance of API Publ 4631-1995.
Scope and Applicability
API Publ 4631-1995 provides a standardized methodology for estimating fugitive hydrocarbon emissions from process equipment in upstream and downstream petroleum operations. The publication specifically addresses emissions emanating from leaks in:
- Valves (gate, globe, ball, plug, check)
- Flanges and connections
- Pump and compressor seals
- Pressure relief devices
- Open-ended lines and sampling connections
- Other piping components subject to process fluid containment
The methodology is designed for use when direct measurement of emissions is not feasible. It is applicable to both existing facilities and new designs, enabling engineers to estimate baseline emissions, evaluate the effectiveness of leak detection and repair (LDAR) programs, and support air permitting activities. The publication explicitly excludes emissions from storage tanks, loading operations, wastewater systems, and combustion sources, which are covered by other API standards.
Important: API Publ 4631-1995 was developed for hydrocarbon service only. Its application to other chemical services or to facilities outside the petroleum and petrochemical sectors may yield inaccurate estimates. Users should verify the applicability of the emission factors to their specific process streams.
Technical Approach and Emission Factor Development
Correlation Methodology
The core of API Publ 4631-1995 is a set of empirically derived emission factors that correlate leak rates with screening values obtained from portable organic vapor analyzers (e.g., flame ionization detectors or photoionization detectors). The methodology divides components into two broad categories:
- Gas/vapor service: Components handling process fluids that are in the gaseous state at standard conditions.
- Light liquid service: Components handling liquids with a vapor pressure ≥ 0.3 kPa at 20°C.
For each equipment type and service, the publication provides average emission factors for three screening ranges: low (0–1,000 ppmv), medium (1,000–10,000 ppmv), and high (>10,000 ppmv). These factors are expressed in kilograms per hour per component (kg/h/component).
Emission Factor Table
| Component Type | Service | Low Range (0–1,000 ppmv) | Medium Range (1,000–10,000 ppmv) | High Range (>10,000 ppmv) |
|---|---|---|---|---|
| Valves | Gas/Vapor | 5.0 × 10−5 | 5.6 × 10−4 | 8.2 × 10−3 |
| Valves | Light Liquid | 1.2 × 10−4 | 1.1 × 10−3 | 1.5 × 10−2 |
| Pump Seals | Light Liquid | 4.5 × 10−4 | 3.9 × 10−3 | 6.2 × 10−2 |
| Compressor Seals | Gas/Vapor | 3.8 × 10−4 | 2.6 × 10−3 | 4.3 × 10−2 |
| Flanges | All | 1.1 × 10−5 | 4.7 × 10−5 | 1.6 × 10−3 |
These factors represent uncontrolled emissions (i.e., before any LDAR program). When a facility operates a periodic LDAR program, the publication provides adjustment factors to estimate the residual emissions following repair. The methodology assumes that all leaks are repaired within 30 days of detection unless technically infeasible.
Tip: For more accurate site-specific estimates, API Publ 4631-1995 encourages users to develop facility-specific emission factors by conducting correlation studies that relate measured leak rates to screening values. This approach is especially beneficial for components with unique design characteristics or service conditions.
Implementation Guidelines
Applying API Publ 4631-1995 requires a systematic approach:
- Component Inventory: Count and categorize all potential fugitive emission sources by type and service fluid.
- Screening Campaign: Conduct screening using an EPA Method 21-compliant instrument to record the highest observed concentration for each component.
- Assign Screening Ranges: Classify each component into low, medium, or high range based on its screening value.
- Calculate Emissions: Multiply the applicable emission factor (from Table 1 or facility-specific factors) by the number of components in each range, then sum across sources.
- Apply LDAR Adjustments: If a periodic LDAR program is in place, apply the relevant reduction factors for repaired leaks and for components awaiting repair.
The publication also includes guidance on handling components that are inaccessible for screening (“pegged” components) and on estimating emissions from pressure relief devices that discharge to the atmosphere.
Best Practice: Many facilities have successfully integrated API Publ 4631-1995 into their annual emission inventory reporting. When combined with a rigorous LDAR program, the methodology can provide defensible emission estimates that satisfy both internal sustainability goals and regulatory requirements.
Compliance and Regulatory Considerations
Although API Publ 4631-1995 is a voluntary industry publication, it has been widely accepted by regulatory bodies in the United States and internationally. The U.S. Environmental Protection Agency (EPA) references the API methodology in several guidance documents for estimating fugitive emissions under the Clean Air Act, including Title V permit applications and New Source Performance Standards (NSPS) subparts such as 40 CFR Part 60 Subpart VVa (chemical plants) and Subpart OOOO (oil and gas production).
Key compliance points:
- EPA Alternative: Facilities may use API Publ 4631-1995 as an alternative to default emission factors in EPA’s AP-42 when site-specific data are available.
- Difference from NSPS LDAR: The publication is not a LDAR regulation itself but provides the estimation method that complements LDAR programs required by NSPS Parts 60 and 63.
- State-Level Adoption: Several U.S. states (e.g., Texas, California, Louisiana) accept emission estimates developed using API Publ 4631-1995 for annual emissions reporting under their state implementation plans.
- International Acceptance: The methodology is cited in the IPIECA/API guidance on greenhouse gas and volatile organic compound emission estimation and is used as a reference in several national inventories.
Legal Notice: The use of API Publ 4631-1995 does not relieve a facility from complying with federal, state, or local LDAR regulations. Emission estimates must be consistent with regulatory definitions of “leak” and “repair.” Always consult current 40 CFR requirements and your permitting authority before applying this methodology for compliance purposes.
Frequently Asked Questions
Q: Is API Publ 4631-1995 still considered current, or has it been superseded?
A: API Publ 4631-1995 has not been formally withdrawn, but users should be aware that more recent publications such as API 4589 (on fugitive emission factors) and newer LDAR guidance exist. Nevertheless, 4631-1995 remains widely used in industry for historical baselines and in regions where more recent data are not required. For new projects, it is advisable to review the latest API standards and EPA guidance to ensure consistency with current best practices.
A: API Publ 4631-1995 has not been formally withdrawn, but users should be aware that more recent publications such as API 4589 (on fugitive emission factors) and newer LDAR guidance exist. Nevertheless, 4631-1995 remains widely used in industry for historical baselines and in regions where more recent data are not required. For new projects, it is advisable to review the latest API standards and EPA guidance to ensure consistency with current best practices.
Q: Can I use API Publ 4631-1995 for emission sources other than those listed in the scope?
A: The publication explicitly limits its methodology to the equipment types and services described. Applying it to other sources (e.g., loading arms, storage tanks, wastewater treatment) would require validation and is not supported by the original correlation data. Use dedicated estimation methods for those sources.
A: The publication explicitly limits its methodology to the equipment types and services described. Applying it to other sources (e.g., loading arms, storage tanks, wastewater treatment) would require validation and is not supported by the original correlation data. Use dedicated estimation methods for those sources.
Q: How does API Publ 4631-1995 account for the effect of temperature and pressure on emission rates?
A: The emission factors in the publication are based on field data from typical refinery and production operations; they implicitly reflect moderate ranges of temperature and pressure. For extreme conditions (e.g., high-pressure gas injection), the recommended approach is to develop site-specific factors following the correlation methodology described in the appendix of the publication.
A: The emission factors in the publication are based on field data from typical refinery and production operations; they implicitly reflect moderate ranges of temperature and pressure. For extreme conditions (e.g., high-pressure gas injection), the recommended approach is to develop site-specific factors following the correlation methodology described in the appendix of the publication.
Q: Does the methodology require the use of “pegged” component assumptions?
A: Yes, for components that are inaccessible or unsafe to screen (e.g., hot valve stems, high-pressure lines), API Publ 4631-1995 provides default emission factors for “nonscreened” components. Alternatively, users may conservatively assign a high-range factor to unscreened components if a regulatory authority requires it. The publication recommends that the number of pegged components be minimized and that appropriate justifications be documented.
A: Yes, for components that are inaccessible or unsafe to screen (e.g., hot valve stems, high-pressure lines), API Publ 4631-1995 provides default emission factors for “nonscreened” components. Alternatively, users may conservatively assign a high-range factor to unscreened components if a regulatory authority requires it. The publication recommends that the number of pegged components be minimized and that appropriate justifications be documented.
In summary, API Publ 4631-1995 provides a technically robust and regulatory-accepted framework for estimating fugitive hydrocarbon emissions from petroleum industry components. Its correlation-based approach, combined with clear implementation guidance, makes it a valuable tool for emission inventories, compliance reporting, and LDAR program evaluation. Engineers and environmental professionals should apply the methodology with attention to its scope, assumptions, and the need for site-specific adjustments where appropriate.
Updated 2026. This article is for informational purposes and does not replace the full text of API Publ 4631-1995. Always refer to the official publication for detailed requirements.