Counting and repairing leaks is only half of an LDAR program. The other half is converting that field data into mass-based emissions estimates for emissions inventories, Title V annual reports, TRI Form R submittals, GHG Reporting Program filings, and NSPS OOOOa/OOOOb compliance reports. EPA's "Protocol for Equipment Leak Emission Estimates" (EPA-453/R-95-017) defines four progressively more accurate approaches. This guide walks through each one, when to use it, and how the underlying math works.
The Basic Equation
Every fugitive emissions calculation, regardless of approach, has the same general form:
The differences between EPA's four methods come down to which emission factor you use and how much monitoring data you have to refine it.
EPA's Four Estimation Methods
Method 1 — Average Emission Factor Approach
How It Works
The simplest method. EPA publishes a single default emission factor for each combination of component type (valve, connector, pump seal, compressor seal, pressure relief device, open-ended line, sampling connection) and service category (gas/vapor, light liquid, heavy liquid). You count components, multiply by the factor and time in service, and sum.
When to Use
- Initial emissions inventories before any monitoring data exists.
- Components in service categories where Method 21 monitoring is not required.
- Quick screening calculations for permit applications and PTE estimates.
Pros and Cons
Pro: No monitoring data needed; defensible because the factors come straight from EPA tables.
Con: Conservative. The factor is an average across many real-world facilities, so well-maintained sites end up over-reporting and poorly maintained ones under-report.
Method 2 — Screening Ranges (Stratified) Approach
How It Works
EPA publishes two emission factors for each component/service combination: one for components that screen at or above 10,000 ppm with Method 21 (the "leakers") and a much lower one for components that screen below 10,000 ppm. You divide your component population into the two categories using your monitoring data and apply the corresponding factor to each.
When to Use
- Sites with Method 21 monitoring data but not detailed enough for the correlation approach.
- An intermediate step when moving from average factors to a more refined estimate.
Pros and Cons
Pro: Rewards programs that maintain low leak rates — the population of "leakers" stays small, so the total emissions estimate is correspondingly lower.
Con: Still treats every leaker the same regardless of whether it screens at 10,001 ppm or 100,000 ppm.
Method 3 — EPA Correlation Equation Approach
How It Works
For each component that has been screened with Method 21, you plug the actual ppm reading into an EPA-published regression equation that correlates the screening value to the actual mass leak rate (kg/hr). The equations are component- and service-specific. Default zero values and pegged values (when the instrument reads off-scale) are handled with separate factors specified in the protocol.
When to Use
- Programs with comprehensive Method 21 monitoring data on each component.
- When more accuracy is needed than the screening ranges approach can provide.
- Annual emissions inventories at refineries, chemical plants, and other large process facilities.
Pros and Cons
Pro: Significantly more accurate than the average or stratified factors. Differentiates a 1,000-ppm leak from a 50,000-ppm leak, which the stratified approach does not.
Con: Requires Method 21 readings on every component to apply the correlation broadly. OGI-only programs cannot use this method without supplemental Method 21 data.
Method 4 — Unit-Specific Correlation Approach
How It Works
The site develops its own correlation equation by bagging a statistically representative sample of components, measuring the actual leak rate at each (typically with a low-flow chamber or vent gas measurement), and pairing each measurement with the corresponding Method 21 screening value. Regression analysis produces a site- or unit-specific correlation that often fits the actual data better than the generic EPA equation.
When to Use
- Large refineries and chemical complexes where the accuracy gain justifies the testing investment.
- Sites with unusual process streams where the EPA correlation may not fit well.
- Demonstration projects under specific consent decrees or permit conditions.
Pros and Cons
Pro: Most accurate of the four methods when properly developed.
Con: Significant up-front cost (component bagging is expensive), requires EPA approval of the correlation methodology, and the correlation must be periodically revalidated.
From Total VOC to Individual Species
Most fugitive emissions calculations produce a total VOC mass. To support TRI Form R, HAP reporting, and toxic-air-pollutant inventories, that total has to be split into individual chemical species using the weight percent of each species in the process stream.
Worked Example
Suppose a unit's process stream is, by weight:
- 80% methane (not a VOC under most definitions)
- 15% benzene (HAP)
- 3% ethylbenzene (HAP)
- 2% other VOCs
If the calculated total fugitive hydrocarbon mass is 10,000 lb/yr, then the VOC portion is the 20% that is not methane, or 2,000 lb/yr. Of that VOC mass, 75% is benzene (1,500 lb/yr), 15% is ethylbenzene (300 lb/yr), and 10% is other VOCs (200 lb/yr).
Time-in-Service and Special Cases
Time in Service
Most factors are expressed in mass per component per hour. Multiply by 8,760 hours for a full year, or by the actual operating hours if the unit was down for any portion of the year. Components that were added or removed mid-year should be prorated.
Heavy Liquid Service
Many rules exempt heavy liquid components from monitoring, but they still emit. Apply the average emission factor approach to these components even when no monitoring data exists.
Difficult-to-Monitor and Unsafe-to-Monitor Components
Components that cannot be safely or practically monitored should still be included in the inventory using the average emission factor for their type and service. Document the reason they are not monitored in the LDAR plan.
Where the Numbers Go
Reporting Destinations
- Title V annual compliance certification: Total VOC and HAP emissions per emission unit.
- TRI Form R: Annual mass of each listed chemical released as fugitive emissions.
- State emissions inventories: Annual updates submitted to state air agencies.
- EPA Greenhouse Gas Reporting Program (40 CFR 98): Subpart W requires fugitive methane emissions from oil and gas systems.
- NSPS OOOOa/OOOOb annual reports: Fugitive emissions data submitted electronically through CEDRI.
- Permit applications: PTE estimates for new construction and modifications.
Common Calculation Mistakes
Ecesis Software Solutions for Emissions Calculations
LDAR Software
Component inventories, Method 21 readings, and emissions calculation in one system.
Air Emissions Management
Stack tests, fugitive emissions, and source-level inventories tied to Title V reports.
Chemical Management
Track stream compositions, weight percents, and HAP designations for speciation.
Document Management
Calculation methodology, supporting data, and historical reports archived in one place.
Compliance Obligations
TRI, Title V, and GHGRP reporting deadlines tracked centrally.
Oil & Gas Compliance
Comprehensive EHS platform with Subpart W methane reporting built in.
