1. PTE Overview
Potential to Emit (PTE) is the maximum capacity of a stationary source to emit a pollutant under its physical and operational design. PTE represents the worst-case emission scenario and is used to determine regulatory applicability, including whether a facility is classified as a major or minor source under the Clean Air Act.
PTE vs. Actual Emissions
PTE and actual emissions are fundamentally different concepts. PTE reflects what a facility could emit if every emission unit operated at maximum capacity for 8,760 hours per year (24/7/365) with no downtime, using worst-case emission factors. Actual emissions reflect what the facility does emit based on real operating hours, throughput, and conditions. A facility with a PTE of 200 TPY of NOx might actually emit only 50 TPY based on normal operations. However, for regulatory classification purposes, PTE is what matters unless reduced by enforceable limitations.
Legal Definition
Under 40 CFR 70.2, PTE is defined as the maximum capacity of a stationary source to emit any air pollutant under its physical and operational design. Any physical or operational limitation on the capacity of a source to emit an air pollutant, including air pollution control equipment and restrictions on hours of operation or on the type or amount of material combusted, stored, or processed, shall be treated as part of its design if the limitation is enforceable by the EPA administrator.
When PTE Is Calculated
| Scenario | PTE Purpose |
|---|---|
| New facility construction | Determine permit type required (PBR, minor, Title V) |
| Facility modification | Assess whether modification triggers NSR or PSD review |
| Equipment addition | Re-evaluate facility-wide PTE with new equipment |
| Permit renewal | Update PTE for current equipment inventory |
| Regulatory compliance check | Confirm continued minor source status |
2. Calculation Methodology
PTE is calculated for each individual emission unit at the facility, then summed to determine the facility-wide PTE for each pollutant. The general formula applies to all emission unit types, though the specific inputs vary.
General PTE Formula
PTE = EF × A × H × (1 − CE) / CF
Where:
EF = emission factor (mass per unit of activity)
A = maximum activity rate (e.g., MMBtu/hr, gal/hr)
H = annual operating hours (8,760 unless limited)
CE = control efficiency (decimal, 0 unless enforceable)
CF = conversion factor (2,000 lb/ton for US tons)
Step-by-Step PTE Determination
| Step | Action | Key Consideration |
|---|---|---|
| 1 | Inventory all emission units | Include engines, tanks, fugitives, vents, flares, pneumatics |
| 2 | Identify pollutants for each unit | NOx, CO, VOC, SO2, PM, HAPs, GHGs |
| 3 | Determine maximum activity rate | Maximum rated capacity, not typical operating rate |
| 4 | Select emission factors | Use hierarchy: stack test > manufacturer > AP-42 |
| 5 | Apply operating hours | 8,760 hr/yr unless enforceable limit exists |
| 6 | Credit enforceable controls | Only controls with enforceable permit conditions |
| 7 | Sum by pollutant | Total all units for each pollutant |
| 8 | Compare to thresholds | Title V: 100 TPY; HAP: 10/25 TPY; PSD: 250 TPY |
Maximum Capacity Assumption
PTE calculations must use the maximum rated capacity of each emission unit, not the expected or typical operating rate. For example, a 1,000 HP compressor engine rated at 1,000 HP must use 1,000 HP in PTE calculations even if it normally operates at 800 HP. Similarly, a 10,000-barrel storage tank must assume maximum throughput at maximum vapor pressure conditions. Using actual or expected operations instead of maximum design capacity is the most common PTE calculation error and can result in enforcement actions.
3. Emission Factor Sources
The accuracy and regulatory acceptability of PTE calculations depend heavily on the emission factors used. EPA and state agencies have established a hierarchy of emission factor quality.
Emission Factor Hierarchy
| Priority | Source | Quality Rating | Application |
|---|---|---|---|
| 1 | Stack test data (site-specific) | A (highest) | Existing equipment with recent test data |
| 2 | Manufacturer guaranteed data | A–B | New equipment with emission guarantees at site conditions |
| 3 | CEMS data | A | Continuous monitoring data from same or similar equipment |
| 4 | AP-42 emission factors | B–E | Default when site-specific data unavailable |
| 5 | Material balance | B–C | Process vents, storage tanks, loading operations |
| 6 | Engineering estimates | D–E | Last resort; requires documentation of assumptions |
Key AP-42 Sections for Midstream
| AP-42 Section | Source Category | Pollutants Addressed |
|---|---|---|
| 3.2 | Natural gas-fired reciprocating engines | NOx, CO, VOC, PM, SO2, formaldehyde |
| 3.1 | Stationary gas turbines | NOx, CO, VOC, PM, SO2 |
| 1.4 | Natural gas combustion (boilers, heaters) | NOx, CO, VOC, PM, SO2 |
| 5.3 | Glycol dehydration units | VOC, HAPs (BTEX) |
| 7.1 | Storage tanks (organic liquids) | VOC, HAPs |
| 5.2 | Transportation and marketing of petroleum liquids | VOC, HAPs |
| 13.5 | Industrial flares | NOx, CO, VOC, SO2 |
AP-42 Quality Ratings
AP-42 assigns quality ratings from A (excellent, based on extensive testing) to E (poor, based on engineering judgment). For PTE calculations, lower-rated factors tend to be more conservative (higher), which may overestimate PTE and push a facility toward major source status. When AP-42 factors seem unreasonably conservative, site-specific stack testing can provide more accurate emission factors, potentially reducing PTE and maintaining minor source status.
4. Combustion Source PTE
Combustion sources (compressor engines, gas turbines, heaters, and boilers) are typically the largest contributors to PTE at midstream facilities, particularly for NOx and CO.
Engine PTE Calculation
PTE (tons/yr) = EF (g/hp-hr) × HP × 8,760 hr/yr × (1 lb / 453.6 g) × (1 ton / 2,000 lb)
Simplified: PTE = EF × HP × 0.009646
Example: Single Engine PTE
| Parameter | Value |
|---|---|
| Engine type | 4-stroke lean burn (4SLB) |
| Rated horsepower | 1,340 HP |
| NOx emission factor | 2.0 g/hp-hr (with SCR) |
| CO emission factor | 2.4 g/hp-hr |
| VOC emission factor | 0.68 g/hp-hr |
| Formaldehyde emission factor | 0.21 g/hp-hr |
| Pollutant | Calculation | PTE (TPY) |
|---|---|---|
| NOx | 2.0 × 1,340 × 0.009646 | 25.85 |
| CO | 2.4 × 1,340 × 0.009646 | 31.02 |
| VOC | 0.68 × 1,340 × 0.009646 | 8.79 |
| Formaldehyde | 0.21 × 1,340 × 0.009646 | 2.72 |
Multiple Engines at One Facility
PTE is additive across all emission units. A compressor station with four 1,340 HP engines would have a NOx PTE of 4 × 25.85 = 103.4 TPY, exceeding the 100 TPY Title V threshold. This illustrates how quickly PTE accumulates at multi-engine facilities and why synthetic minor limitations (such as operating hour restrictions) are frequently needed to maintain minor source status.
5. Fugitive Emissions PTE
Fugitive emissions from equipment leaks at valves, connectors, flanges, pump seals, and compressor seals contribute to both VOC and HAP PTE. These emissions are estimated using component count methods with EPA emission factors.
EPA Protocol Emission Factors
| Component Type | Service | Emission Factor (kg/hr/component) |
|---|---|---|
| Valves | Gas | 0.00597 |
| Valves | Light liquid | 0.00403 |
| Pump seals | Light liquid | 0.01990 |
| Compressor seals | Gas | 0.22800 |
| Connectors | All | 0.00183 |
| Flanges | All | 0.00081 |
| Open-ended lines | All | 0.00170 |
| Pressure relief valves | Gas | 0.10400 |
Fugitive PTE Calculation
PTEVOC = ∑ (EFi × Ni) × 8,760 × WFVOC × 2.205 / 2,000
Where EFi = emission factor for component type i (kg/hr), Ni = count of component type i, WFVOC = VOC weight fraction in stream, 2.205 converts kg to lb
Component Counts for New Facilities
For new facilities without detailed P&IDs, component counts can be estimated from typical equipment ratios. A typical gas compressor station with two units might have approximately 200–400 valves, 500–1,000 connectors, 50–100 flanges, and 2–4 compressor seals. More accurate counts from P&IDs should replace estimates as the design progresses. LDAR (Leak Detection and Repair) program implementation with enforceable monitoring can significantly reduce fugitive PTE.
6. Venting & Flaring PTE
Routine and non-routine venting and flaring contribute to VOC, HAP, and combustion pollutant PTE. Sources include pneumatic controllers, glycol dehydrator still vents, tank breathing and flashing losses, blowdowns, and emergency relief events.
Pneumatic Device PTE
| Device Type | Bleed Rate (scf/hr) | VOC PTE per Device (TPY) |
|---|---|---|
| High-bleed controller | > 6 scf/hr | 0.3–1.5 (depends on gas composition) |
| Low-bleed controller | < 6 scf/hr | 0.05–0.3 |
| Intermittent-bleed controller | Varies with actuation frequency | 0.1–0.5 |
| Zero-bleed (instrument air or electric) | 0 | 0 |
Storage Tank PTE
Storage tank emissions result from working losses (filling and emptying) and breathing losses (thermal expansion and contraction). PTE is calculated using EPA TANKS 4.09d software or AP-42 equations using maximum throughput and worst-case meteorological conditions.
| Tank Type | PTE Method | Key Parameters |
|---|---|---|
| Fixed roof | AP-42 Section 7.1 equations | Vapor pressure, throughput, tank dimensions, paint color |
| Floating roof (external) | AP-42 Section 7.1 equations | Seal type, wind speed, rim gap |
| Pressurized (flash emissions) | E&P TANKS or process simulation | Feed composition, separator conditions, tank pressure |
Flare PTE
Flare combustion emissions:
NOx = 0.068 lb/MMBtu × max heat input (MMBtu/hr) × 8,760 / 2,000
CO = 0.37 lb/MMBtu × max heat input (MMBtu/hr) × 8,760 / 2,000
Flare VOC destruction credit: 98% destruction efficiency (per 40 CFR 60.18)
7. Enforceable Limitations
PTE can be reduced below major source thresholds by accepting federally enforceable emission limitations in a permit. Only limitations that meet specific enforceability criteria can be credited in PTE calculations.
Enforceability Criteria
| Requirement | Description | Example |
|---|---|---|
| Legally binding | Condition in a state or federal permit, consent order, or SIP rule | Minor NSR permit condition limiting engine hours |
| Specific limit | Quantified emission limit or operational restriction | "Engine shall not operate more than 6,000 hrs/yr" |
| Monitoring | Method to verify compliance with the limit | Non-resettable hour meter; monthly fuel records |
| Recordkeeping | Documentation sufficient to demonstrate compliance | Monthly operating hour logs retained 5 years |
| Reporting | Periodic compliance reports to the regulatory agency | Annual compliance certification to TCEQ |
What Does NOT Reduce PTE
The following cannot be used to reduce PTE: (1) internal company policies or operating procedures, (2) economic constraints or market conditions, (3) historical operating data showing lower actual emissions, (4) physical inability to operate 8,760 hours due to maintenance (unless maintenance schedule is in a permit), (5) fuel supply limitations (unless contractually enforceable), or (6) voluntary emission reduction programs without permit conditions. Only conditions enforceable by the EPA administrator (or delegated state agency) count for PTE reduction.
Common Enforceable Limitation Strategies
| Strategy | Effect on PTE | Monitoring Required |
|---|---|---|
| Hour limits per engine | Reduces hours from 8,760 to permitted value | Non-resettable hour meters; monthly logs |
| Fuel usage cap | Limits heat input and proportional emissions | Fuel meters; monthly consumption records |
| Throughput limit | Caps production-related emissions | Flow meters; daily/monthly records |
| Emission control requirement | Credits control efficiency against uncontrolled EF | Initial and periodic stack testing; parametric monitoring |
| Equipment count limit | Caps number of emission units | Equipment inventory; change notification |
8. PTE Calculation Examples
The following examples illustrate PTE calculations for typical midstream facility configurations, showing how individual unit PTE is summed to determine facility-wide applicability.
Example: Gas Compressor Station
| Emission Unit | NOx (TPY) | CO (TPY) | VOC (TPY) | Formaldehyde (TPY) |
|---|---|---|---|---|
| Engine #1 (1,340 HP lean burn) | 28.4 | 31.0 | 8.8 | 2.7 |
| Engine #2 (1,340 HP lean burn) | 28.4 | 31.0 | 8.8 | 2.7 |
| Glycol dehydrator | 0.5 | 0.2 | 12.5 | — |
| Condensate tanks (2) | — | — | 5.2 | — |
| Fugitive emissions | — | — | 3.5 | — |
| Pneumatic devices (20) | — | — | 4.0 | — |
| Facility Total | 57.3 | 62.2 | 42.8 | 5.4 |
Threshold Assessment
In this example, no single criteria pollutant exceeds 100 TPY, so the facility is a minor source for Title V purposes. However, the combined HAPs (formaldehyde + BTEX from dehydrator + fugitives) should be checked against the 10/25 TPY HAP thresholds. If an additional engine were added, NOx PTE would reach approximately 85.7 TPY, approaching the 100 TPY threshold and potentially requiring synthetic minor limitations.