1. Why RP 752 Is a Process, Not a Table
API RP 752 (Management of Hazards Associated with Location of Process Plant Permanent Buildings) governs the siting of occupied buildings — control rooms, MCC buildings, offices, operator shelters — relative to process hazards. Critically, it does not publish "a control room must be X feet from a compressor." It prescribes a consequence-based (or risk-based) evaluation: estimate the explosion, fire, and toxic consequences that could reach the building, then compare them against acceptance criteria for building damage and occupant vulnerability.
So the distance is an output of a consequence model, not a lookup. This guide and calculator implement the explosion-overpressure step of that evaluation — the most common governing hazard for a gas facility.
Screening, not a full study. Overpressure is one of several RP 752 hazards. A complete siting study also addresses fire (thermal radiation), toxic release, fragments / missiles, escalation, and the building's occupancy and risk tolerance.
2. TNT-Equivalence Overpressure
The simplest defensible screening model converts the flammable cloud to an equivalent TNT charge (Crowl & Louvar, Chemical Process Safety):
WTNT = η · M · ΔHc / ΔHTNT
η = explosion yield (fraction); M = flammable mass in the cloud; ΔHc = fuel heat of combustion; ΔHTNT = 4686 kJ/kg
The yield η is the fraction of the cloud's combustion energy that contributes to the blast — for unconfined / partially-congested vapour clouds it is typically 2–10 %. A higher yield gives a larger distance, so it is the conservative direction for siting. The credible flammable mass M comes from a release / dispersion analysis.
3. The Blast Curve (Kinney-Graham)
With the TNT charge known, the peak side-on overpressure at a distance follows from the scaled distance and the free-air TNT blast curve. We use the closed-form Kinney & Graham (1985) correlation (a validated analytical fit to the TNT curve), so every value is reproducible rather than read off a chart:
Z = R / WTNT1/3 [m / kg1/3]
pso/p0 = 808[1+(Z/4.5)²] / √{[1+(Z/0.048)²][1+(Z/0.32)²][1+(Z/1.35)²]}
To find a contour distance, the relation is inverted: pick a target overpressure (e.g. 1.0 psi), solve for Z, then R = Z · WTNT1/3. The curve cross-checks against the Crowl & Louvar TNT data (Z = 10 m/kg1/3 → ~1.45 psi; Z = 5 → ~4.2 psi).
4. RP 752 Appendix C Thresholds
API RP 752 Appendix C relates overpressure to building damage and occupant vulnerability for five building categories:
| Category | Building type | Damage onset | Collapse / major damage |
|---|---|---|---|
| B1 | Portable / temporary (trailers) | very low | very low — see API RP 753 |
| B2 | Steel-frame / metal pre-engineered | >1.5 psi (cladding) | >5 psi |
| B3 | Unreinforced masonry bearing wall | ~1.0 psi (walls) | ~1.5 psi |
| B4 | Steel/concrete frame, masonry infill | ~1.0 psi | ~2.5 psi (roof slab ~2.0) |
| B5 | Reinforced-concrete shear wall (designed ~2 psi) | ~4.0 psi | ~6.0 psi |
Onset of damage across all types is about 0.2 psi (≈ design wind pressure). Occupant vulnerability (Appendix C, Figure C-1) rises with overpressure — building collapse implies roughly a 0.6 probability of fatality, building destruction ~1.0. The 1.0 psi contour is the widely-used initial occupied-building screen because it brackets collapse of the more vulnerable building types.
A compressor / engine building is itself usually a B2 metal pre-engineered structure — relevant when the "occupied building" in question is a control room attached to or near such a building.
5. The TNO Multi-Energy Method
TNT-equivalence is a screening tool. Its weakness is that real vapour-cloud explosions derive their strength from congestion and confinement, not from total mass — an unconfined cloud produces far less overpressure than the TNT-equivalent mass implies, while a heavily congested one can produce more in the near field. The TNO Multi-Energy method (CCPS, Guidelines for Evaluating the Characteristics of VCEs, Flash Fires and BLEVEs) addresses this by basing the blast on the congested-volume combustion energy and a blast-strength class (1–10). It is the recommended method for a detailed siting study; TNT-equivalence is appropriate for screening and conservative bounding.
6. Worked Example
A control room near a gas compressor area. Credible flammable cloud: 8,000 lb methane, yield 5 %. The control room is a steel-frame metal building (B2), proposed at 250 ft.
Step 1 — TNT charge
M = 8,000 lb = 3,629 kg; ΔHc = 50,000 kJ/kg (methane)
WTNT = 0.05 × 3,629 × 50,000 / 4,686 = 1,936 kg (W1/3 = 12.47)
Step 2 — Contour distances
Inverting the Kinney-Graham curve at each target overpressure:
| Overpressure | Meaning | Distance |
|---|---|---|
| 1.0 psi | Occupied-building screen | ~550 ft |
| 3.0 psi | Severe damage | ~280 ft |
| 5.0 psi | B2 collapse | ~187 ft |
Step 3 — Verdict at 250 ft
At 250 ft the overpressure is ~3.0 psi — past the 1.0 psi screen and approaching severe damage, though still short of B2 collapse (5 psi). Verdict: API RP 752 detailed siting study required, with likely upgrades or relocation. To clear the 1.0 psi screen, the building would need to sit beyond ~550 ft, or be qualified as blast-resistant for ~3 psi.
Result
The grounded screening distance is ~550 ft (the 1.0 psi contour). 250 ft does not pass the screen — it triggers a full RP 752 evaluation. Yield drives the answer: at 2 % yield the 1.0 psi contour shrinks to ~400 ft; at 10 % it grows to ~700 ft.
7. Scope & Good Practice
- Screening only. Overpressure is one RP 752 hazard — also evaluate fire radiation, toxic, fragments, escalation, and occupancy/risk.
- Garbage in, garbage out. The credible flammable mass and yield dominate the result — derive them from a release / dispersion analysis, not a guess.
- Use TNO Multi-Energy for the detailed congested-VCE study; TNT-equivalence for screening and conservative bounds.
- Portable buildings (trailers) follow API RP 753 specifically and are the most vulnerable — site them well clear.
- Upgrading to blast-resistant design is a valid alternative to distance — qualify it for the overpressure at the actual location.
- Advisory / preliminary — confirm with a qualified process-safety engineer and a formal facility siting study.
Frequently Asked Questions
How far should an occupied building be from an explosion hazard?
API RP 752 does not give a fixed distance — it requires a consequence-based evaluation. You model the vapour-cloud-explosion overpressure versus distance (e.g. TNT-equivalence with the Kinney-Graham blast curve) and compare it against API RP 752 Appendix C building-damage thresholds. The 1.0 psi contour is the common occupied-building screening distance.
What overpressure damages an occupied building?
Per API RP 752 Appendix C: onset of damage is about 0.2 psi, an unreinforced masonry building collapses near 1.5 psi, a metal pre-engineered building collapses above 5 psi, and a reinforced-concrete shear-wall building near 6 psi. Building collapse implies roughly a 0.6 probability of occupant fatality.
What is TNT equivalence for a vapour cloud explosion?
TNT equivalence converts the flammable mass in a cloud to an equivalent TNT charge: W_TNT = yield × mass × heat of combustion / 4686 kJ/kg. The overpressure at a distance is then read from the TNT blast curve. VCE yields are typically 2 to 10 percent; higher is more conservative.
Is this a complete facility siting study?
No. Overpressure screening is one step. A full API RP 752 siting study must also address fire radiation, toxic release, fragments and missiles, escalation, and occupancy/risk. The TNO Multi-Energy method is recommended for the detailed congested-VCE analysis.