Calculator inputs
Formula used
The calculator uses a point-source radiant heat approximation. Radiant power is estimated as: Qrad = HRR × χr.
Heat flux at distance r is approximated by: q'' = (Qrad × Fv) / (4πr²).
Solving for the separation distance with a safety factor SF: r = √((Qrad × Fv × SF) / (4π × q''allow)).
How to use this calculator
- Enter a design Heat Release Rate for your scenario.
- Set the radiant fraction and view factor for exposure conditions.
- Choose an allowable heat flux aligned with protection goals.
- Apply a safety factor to reflect uncertainty and consequence.
- Press Submit to view results above the form.
Example data table
| Scenario | HRR (MW) | χr | Fv | q''allow (kW/m²) | SF | Distance (m) |
|---|---|---|---|---|---|---|
| A | 3.00 | 0.30 | 1.00 | 12.5 | 1.20 | 2.62 |
| B | 5.00 | 0.35 | 1.00 | 12.5 | 1.20 | 3.66 |
| C | 8.00 | 0.35 | 0.75 | 12.5 | 1.30 | 4.17 |
| D | 5.00 | 0.25 | 1.00 | 4.7 | 1.30 | 5.25 |
Design heat-flux targets for separation
Common screening targets for incident radiant heat include 5, 12.5, and 37.5 kW/m². Around 5 kW/m² is used to limit pain and reduce ignition likelihood. About 12.5 kW/m² is frequently applied for preventing piloted ignition of materials with brief exposure. Values near 37.5 kW/m² represent severe exposure where robust barriers or large spacing are required. Select the criterion that matches your risk basis and local requirements.
Heat release rate inputs and scaling
The calculator uses heat release rate (HRR) as the primary driver. If you only know fuel mass, approximate peak HRR from test data or similar facilities. As a reference range, small equipment fires may be 500–2,000 kW, storage arrays can reach 5,000–20,000 kW, and large hydrocarbon spills can exceed 50,000 kW. Because distance scales with the square root of HRR, doubling HRR increases the separation distance by about 41%.
Radiative fraction and view factor
Only a portion of HRR is emitted as radiation. Radiative fraction χr is assumed between 0.20 and 0.40 for many flaming fires, while sooty fires can be higher. The view factor Fv represents how much of the radiant output “sees” the target; shielding, recesses, or orientation can reduce Fv below 1.0. For quick studies, use Fv = 1 for fully exposed targets, 0.5 for significant obstruction, and document the assumption. Use conservative values when information is limited.
Safety factor and uncertainty management
Engineering inputs vary, so the tool applies a safety factor (SF) to the radiant term. Typical screening SF values range from 1.1 to 2.0 depending on data quality and consequence. Wind, flame tilt, and transient burning can increase exposure beyond a static point-source assumption. If HRR and geometry are uncertain, consider SF ≥ 1.5 and compare results across low, nominal, and high cases to understand sensitivity.
Interpreting results for layout decisions
Use the computed separation distance as an initial layout check, not a final code distance. Compare the output against property lines, openings, egress routes, and critical equipment. If the required distance is impractical, reduce HRR through inventory limits, add shielding walls, or select lower exposure criteria for protected targets. After screening, confirm the design with applicable standards, fire modeling, and authority review.
FAQs
What does the calculator output represent?
It estimates the minimum separation distance where incident radiant heat meets your allowable heat-flux criterion, using a simplified point-source radiation model with your HRR, radiative fraction, view factor, and safety factor.
Which allowable heat-flux value should I use?
Pick a criterion aligned to your protection goal. Lower values are more conservative for people and vulnerable surfaces; higher values may suit protected or noncombustible targets. Always verify the selected limit against local rules and project standards.
How do I choose a reasonable HRR?
Use credible test data, literature values, or site-specific fire scenarios. If uncertain, run low/nominal/high HRR cases. Because distance grows with the square root of HRR, large HRR uncertainty can still materially change spacing.
What is the view factor in this tool?
It is a screening multiplier for geometry and shielding. Use 1.0 for full exposure, reduce it when barriers, orientation, or partial blockage limits what the fire “sees.” Document the basis so reviewers understand the assumption.
Why include a safety factor?
It provides margin for uncertainty in HRR, radiative fraction, geometry, wind, and simplified physics. Apply higher safety factors when consequences are high or inputs are speculative, and compare scenarios to understand sensitivity.
Can I use this for code compliance?
No. It is intended for preliminary engineering and option evaluation. Final separation distances may be governed by prescriptive code tables, performance-based analysis, detailed fire modeling, and authority approval.