Many upstream and midstream natural gas facilities rely on optical gas imaging (OGI) as a cornerstone of their leak detection and emissions monitoring programs. However, operators of certain glycol dehydrators may be overlooking a separate monitoring obligation within the current U.S. Environmental Protection Agency (EPA) National Emission Standards for Hazardous Air Pollutants (NESHAP) Subpart HH requirements.
For certain large glycol dehydrators that route emissions through a closed-vent system to a control device, the rule can require facilities to demonstrate “no detectable emissions” using EPA Method 21 instrument-monitoring procedures. Because that requirement is embedded within a series of regulatory cross-references, it can be easy to miss during compliance evaluations. As a result, some facilities may assume their existing OGI program satisfies all applicable monitoring requirements when additional Method 21 monitoring may be required.
Because glycol dehydrators are common at natural gas production and processing facilities across major producing regions, including the Marcellus, Utica, Piceance, and Haynesville plays, this issue may affect a significant number of Subpart HH-regulated operations.
This article focuses on the current rule, not proposed revisions, and walks through the regulatory pathway that can trigger EPA Method 21 requirements for glycol dehydrator closed-vent systems.
Does NESHAP Subpart HH Require EPA Method 21 Monitoring for Glycol Dehydrators?
Yes, for large glycol dehydrators that comply using a closed-vent system and control device, Subpart HH requires operators to demonstrate “no detectable emissions” using EPA Method 21 procedures. OGI monitoring can provide valuable emissions information, but it does not necessarily satisfy a Method 21 requirement under the current rule.
The step-by-step framework below explains how this requirement can arise and how facilities can determine whether it applies to their glycol dehydrators:
Step 1: Determine whether the glycol dehydration unit is subject to NESHAP Subpart HH
Glycol dehydration units at oil and natural gas production facilities may be subject to NESHAP Subpart HH requirements depending on the dehydrator compliance option used.
Use Table 1 to determine whether the glycol dehydrator is classified as a small or large dehydrator under Subpart HH. To be classified as a large dehydrator, the unit must exceed both the throughput and benzene-emissions thresholds shown below.
Table 1: Small vs. Large Dehydrators under Subpart HH
| Dehydrator Classification | Annual Average Natural Gas Throughput | Benzene Emissions |
| Small | < 85 Mscm/day (~3 MMscf/day) | < 0.90 Mg/yr (~1 ton per year) |
| Large | ≥ 85 Mscm/day (~3 MMscf/day) | ≥ 0.90 Mg/yr (~1 ton per year) |
If both annual average natural gas throughput and benzene emissions exceed their respective thresholds, the unit is classified as a large dehydrator under Subpart HH and should proceed to Steps 2 through 4.
For a regulated small dehydrator, EPA Method 21 monitoring is not required for compliance purposes.
Step 2: Verify Compliance with the Closed-Vent-System Performance Standard
For large dehydrators that use a control device, confirm that the closed-vent system meets the performance requirements of § 63.771(c). The system must route all gases, vapors, and fumes to the control device and operate with no detectable emissions.
Step 3: Determine the Required Inspection Schedule
Review § 63.773(c) to determine how the closed-vent system must be inspected and whether Method 21 monitoring is required. The rule distinguishes permanently or semi-permanently sealed connections from other closed-vent-system components and assigns inspection requirements accordingly.
Step 4: Perform Required EPA Method 21 Monitoring
When § 63.773 calls for a no-detectable-emissions demonstration, § 63.772(c) points to EPA Method 21 procedures, including instrument calibration and comparison of the measured instrument response with the regulatory criterion.
Determining whether Method 21 applies is only part of the compliance equation. Many facilities already perform OGI inspections or other leak detection activities, but those programs do not necessarily satisfy Method 21 requirements. Understanding the differences begins with a closer look at what Method 21 monitoring requires in the field and where compliance programs most often fall short.
What Does EPA Method 21 Monitoring Require in the Field?
EPA Method 21 is instrument monitoring at the potential leak interface; it is not an optical gas imaging survey, a soap-bubble check, or an audio-visual-olfactory inspection.
A suitable portable organic vapor analyzer is calibrated and the probe is traversed around the leak interface close enough to identify the maximum response. Background concentrations are addressed as required by the applicable procedure, and the resulting reading is compared with the Subpart HH definition of no detectable emissions. As a result, facilities that rely exclusively on OGI may create a compliance gap when Subpart HH specifically requires a Method 21 demonstration.
Where are EPA Method 21 Compliance Programs Most Likely to Break Down?
Even when facilities have established LDAR and OGI programs, Method 21 requirements can be missed. The issues below are among the most common compliance gaps observed when evaluating glycol dehydrator closed-vent systems under the current rule.
- Incomplete Closed-Vent-System Boundaries. The inventory starts at an obvious header and misses the piping, fittings, condenser connections, separator interfaces, or other components between the still vent and control device.
- Dehydrator Components Missing from the LDAR Program. A site may have a robust equipment-leak program, yet the closed-vent-system components are outside its database because they are governed by a different regulatory pathway.
- Confusing Visual Inspections with Method 21 Monitoring. That may address permanently sealed portions after the initial demonstration, but it does not erase instrument monitoring required for other components or after a sealed connection is opened, repaired, or replaced.
- Treating OGI as a Substitute for EPA Method 21. Optical gas imaging can be valuable operationally, but it does not satisfy a Method 21 requirement in the current rule.
How Can Operators Audit Glycol Dehydrator Method 21 Compliance?
Once a facility determines that Method 21 requirements may apply, the next step is confirming that required monitoring, documentation, and reporting elements are in place. The list below provides a practical framework for conducting that review.
- Build the Subpart HH Applicability File: Document facility status, affected-unit status, dehydrator size, compliance option, and the specific provisions relied upon.
- Map the Complete Closed-Vent System: Mark the emission point, every section of conveyance, each connection and component, any bypass, and the inlet to the control device.
- Classify Each Connection and Component: Separate permanently or semi-permanently sealed joints from components subject to recurring Method 21 monitoring.
- Verify the Initial EPA Method 21 Demonstration: Locate the original EPA Method 21 results and confirm they cover the full system that was operating at the time.
- Confirm Recurring Inspection and Monitoring Requirements: Confirm that visual and instrument inspections are occurring at the required frequencies for each component class.
- Reconcile EPA Method 21 Records and Compliance Reports: Inspection results, repairs, deviations, and required certifications should align with the Notification of Compliance Status, periodic reports, and the records maintained under § 63.774 and § 63.775.
Questions to Ask During Your Next Subpart HH Compliance Review
Because Method 21 obligations can be overlooked, periodic compliance reviews should include a focused evaluation of applicability, monitoring, and documentation requirements. The questions below provide a useful starting point.
- Which glycol dehydrators at our facilities are subject to NESHAP Subpart HH control requirements?
- Where does each closed-vent system begin and end?
- Can we produce the initial Method 21 demonstration for each system?
- Which components receive annual visual inspections, and which receive recurring Method 21 monitoring?
Key Takeaway: Review Your Subpart HH Closed-Vent-System Monitoring Program
If your compliance program cannot quickly identify the regulated glycol dehydrator, define the complete closed-vent-system boundary, and produce the supporting Method 21 records, the facility should reevaluate its Subpart HH compliance strategy. Those gaps can indicate that required monitoring obligations have been overlooked.
