High Consequence Areas & Potential Impact Radius

1. Why HCAs exist

Gas-transmission integrity management (IM) is risk-based: PHMSA concentrates the most demanding assessment and repair obligations where a failure would hurt the most people. A High Consequence Area (HCA) is the formal definition of "where the stakes are highest." It is defined in 49 CFR §192.903 and is built on a single geometric idea — the distance over which a large ignited gas release could cause serious harm.

2. The Potential Impact Radius (PIR)

The PIR is the radius of a circle, centred on the point of failure, within which the heat from an ignited rupture could be life-threatening. §192.903 defines it by the formula:

where r = PIR in feet, p = MAOP in psig, and d = nominal pipe outside diameter in inches (equivalently, r = 0.69 · √(p · d²)). The coefficient 0.69 is specific to natural gas (methane-rich); §192.903 notes that a different value applies for gases of different heat content, and PHMSA / ASME B31.8S give the constant's derivation from the heat-intensity threshold of 5,000 BTU/hr·ft² (the level associated with a high probability of fatality for a 30-second exposure).

The "Potential Impact Circle" is the circle of radius r around any point along the segment. The PIR scales with diameter and pressure together — doubling pressure raises PIR by √2 (~41%), and a large-diameter high-pressure line can have a PIR of many hundreds of feet, which is why such lines sweep more structures into HCA consideration.

3. Method 1 vs Method 2 (§192.903)

§192.903 gives two equivalent ways to identify an HCA:

• Method 1 is class-location based. The HCA is any Class 3 or Class 4 location (per §192.5); or a Class 1/2 location where the PIR is greater than 660 ft and the potential-impact circle (PIC) contains 20 or more buildings intended for human occupancy; or a Class 1/2 location whose PIC contains an identified site (a place hard to evacuate — school, hospital, prison, licensed day-care, or a place of public assembly occupied by ≥ 20 persons on ≥ 50 days/year).
• Method 2 is independent of class location. The HCA is the area within the PIC that contains 20 or more buildings intended for human occupancy, or contains an identified site — applied regardless of the segment's class location.

Both methods reduce to the same intent; operators pick one and apply it consistently. The PIR is the common engine of both.

4. Moderate Consequence Areas (MCA)

The 2019 Mega Rule added the Moderate Consequence Area to §192.3. An MCA is, roughly, an area within the PIC that contains 5 or more but fewer than 20 buildings intended for human occupancy, or a portion of an interstate/other principal arterial roadway — i.e. populated enough to matter but below the HCA threshold. MCAs that are piggable pull non-HCA mileage into the periodic-assessment and (via §192.624(a)(2)) MAOP-reconfirmation programs, substantially expanding the regulated footprint beyond classic HCAs.

5. What an HCA triggers

HCA status is the gateway to the heaviest obligations in Part 192 Subpart O: a written integrity-management program (§192.911), baseline and periodic re-assessment on a defined interval (§192.937, generally not to exceed 7 years for HCA segments), prompt remediation under the §192.933 repair criteria, and — under §192.624 — MAOP reconfirmation, which is triggered both where records are not traceable, verifiable and complete (§192.624(a)(1)) and, separately, by a location/population basis (§192.624(a)(2): MAOP set under §192.619(c), at ≥ 30% SMYS, in an HCA, Class 3/4, or piggable MCA). In short, deciding whether a segment "could affect" an HCA is the first calculation that determines how much of the rest of the regulation applies.

6. References

• 49 CFR Part 192 §192.3 (MCA definition), §192.5, §192.903 (HCA & PIR), §192.905, §192.911, §192.933, §192.937 (current eCFR, 2026).
• ASME B31.8S — Managing System Integrity of Gas Pipelines (PIR derivation & 5,000 BTU/hr·ft² basis).
• PHMSA Gas Transmission Final Rule (Part 1) — 84 FR 52180, Oct 1 2019 (MCA addition).
• Stephens, M.J., "A Model for Sizing High Consequence Areas Associated with Natural Gas Pipelines," GRI-00/0189 (basis of the 0.69 coefficient).