What is Leak Detection and Repair (LDAR)?

What counts as a leak

A fugitive emission is an unintentional release from an equipment component — a valve stem, a pump seal, a flange, a connector — rather than from a designed emission point like a stack or a vent. LDAR regulations require operators to measure component emissions using EPA Method 21 (typically a portable flame ionization detector) and compare the reading against a leak threshold specified by the applicable rule.

The two most common leak thresholds in US LDAR regulations are:

• 500 ppmv above background — applied under many NESHAP rules and under NSPS Subpart VVa for valves in gas service
• 10,000 ppmv above background — applied under older NSPS provisions and for certain lower-risk component types

The threshold that applies depends on the specific regulation, the component type, and the service classification (gas service, light liquid, heavy liquid). Applying the wrong threshold to the wrong component is itself a recordkeeping violation, even if the underlying measurement is accurate.

What components LDAR covers

LDAR programs cover equipment components that contact VOC or HAP process streams. The typical scope includes:

• Valves — by far the largest component category at most facilities
• Pump seals and compressor seals
• Flanges and connectors
• Pressure relief devices (PRDs)
• Sampling connections
• Open-ended lines
• Agitator seals (for reactor applications)

A large US refinery typically carries seventy thousand or more LDAR-regulated components. Each component must be tagged, located, classified by service, assigned a monitoring frequency under the applicable rule, and tracked through its lifecycle of inspection, leak detection, repair verification, and retirement.

The regulatory basis

LDAR is not a single regulation — it is a requirement that appears across multiple Clean Air Act programs:

• NSPS (New Source Performance Standards) under 40 CFR Part 60 — equipment leak subparts including VV, VVa, VVb, GGG, GGGa for SOCMI and petroleum refineries, and OOOOb/OOOOc for oil and gas facilities
• NESHAP (National Emission Standards for Hazardous Air Pollutants) under 40 CFR Part 61 — including the Benzene NESHAP at Subpart J
• NESHAP/MACT under 40 CFR Part 63 — including the Hazardous Organic NESHAP (HON) at Subpart H and the Refinery NESHAP at Subpart CC

A facility subject to both an NSPS and a NESHAP for the same equipment must generally comply with the more stringent standard.

The role of P&IDs in LDAR

LDAR compliance requires an accurate, current component inventory — and the master record of which components exist in which service is the governing P&ID. EPA's Leak Detection and Repair: A Best Practices Guide (October 2007) specifically identifies misalignment between the P&ID and the physical component inventory as a source of incomplete monitoring and audit findings.

Facilities that maintain their P&IDs as static PDFs routinely accumulate drift between the drawings and the field installation — undocumented field modifications, missing components, or components recorded but not installed. The LDAR program inherits this drift and carries it forward.