Tank Spacing Using Radiant Heat Modeling

How Realistic are the Models?

Kris ThorsteinssonΔΔP Risk Inc.P Risk Inc.

www.deltaprisk.com

210/Jan/2007

Pool Fire Models Based on Fire Tests

Flames from large hydrocarbon fires are modeled as a mainly smoke-obscured sooty portion with a luminous lower band and periodic luminous spotting throughoutLuminous flame areas have a surface emissive power (SEP) of about 140 kW/m² and smoke-covered areas a much lower SEP of about 20 kW/m²Researchers noted the ”unintuitive” effect that as the diameters of medium / heavy hydrocarbon pool fires increase the radiative fraction or % of luminous flame to total flame surface decreasesThey developed formulae relating diameter to radiative fraction (Fs), e.g., a 15m crude oil pool fire has an Fs of 17% and an average SEP of 40 kW/m2

ΔΔP Risk Inc.P Risk Inc.

310/Jan/2007

Pool Fire Model SEP CalculationsCalculations using Mudan & Croce formulae result in hydrocarbon pool fires with diameters of 30 m and higher modeled as essentially all smoky, with very little luminous flame.40m and larger fires modeled with <1% luminous flameAverage SEP is close to the value for smoke of 20 kW/m², very much less than the value for luminous flame of 140 kW/m²

D(m) SEP (kw/m2) Fs (radiative fraction) % flame % soot or smoke

1 126.4 0.90 88.69% 11.31%2 114.4 0.82 78.66% 21.34%3 103.7 0.74 69.77% 30.23%5 85.9 0.61 54.88% 45.12%10 56.1 0.40 30.12% 69.88%15 39.8 0.28 16.53% 83.47%20 30.9 0.22 9.07% 90.93%30 23.3 0.17 2.73% 97.27%40 21.0 0.15 0.82% 99.18%50 20.3 0.14 0.25% 99.75%70 20.0 0.14 0.02% 99.98%100 20.0 0.14 0.00% 100.00%

ΔΔP Risk Inc.P Risk Inc.

410/Jan/2007

Large Tank Fires Accepted fire models for these large tanks would calculate a radiative fraction of <1%i.e. <1% of flame surface on average is comprised of luminous flame, or >99% of flame is hidden by smoke on averageThese photos suggest a more realistic Fs of 30%+Also noted is the upwind effect of a higher % of luminous flame, possibly outweighing the flame tilting effect

ΔΔP Risk Inc.P Risk Inc.

510/Jan/2007

Significance of Model Accuracy

Thermal radiation modeling for tank layout and spacing is recommended by insurance / risk management (e.g. GE GAP) and industry (e.g. Energy Institute / Institute of Petroleum) guidelinesEmergency response plans, fixed and portable fire-fighting provisions and tertiary containment systems may be based on reliable modeling

ΔΔP Risk Inc.P Risk Inc.

610/Jan/2007

Singapore 1988 Straight Run Naphtha Tank Fire – Escalation by Radiant Heat Tank #1 full surface fire underway, little or no windTank #2 at left, 21m from Tank #1 starting to burn at rim seal area after 2 hoursA Mudan & Croce based model (using default SEP and radiative fraction values) indicates only 4 kW/m2 at Tank #2, and 8 kW/m2 at 5m distance from Tank #1 Other models and observation suggest 25-30 kW/m2 radiation level at Tank #2 which is more realistic given the fire spreadThe Singapore Tank Fire Protection study indicated adjacent tank fire escalation at 0.5 Diameter separation in 2.8 hours

ΔΔP Risk Inc.P Risk Inc.

710/Jan/2007

Possible Explanations for Higher Observed SEP in Tank Fires v Pan Test Fires

Heat feedback from the fire to the flammable material has a great effect on vaporization rate and fire intensityTank shell heating up and radiating back into the fire would progressively increase with timeA partial chimney effect may occur increasing turbulence and overall oxygenation above the tank shellThe rate of burning eventually becomes more like volatile light end hydrocarbons which typically do not demonstrate reduced Fs with increasing diameter

ΔΔP Risk Inc.P Risk Inc.

810/Jan/2007

References

“Fire Hazard Calculations for Large Open Hydrocarbon Fires” Mudan & Croce, 1984TNO Yellow Book “Methods for the Calculation of Physical Effects” CPR 14E 3rd Edition 1997“Fire Precautions at Petroleum Refineries and Bulk Storage Installations” Energy Institute / IP, 2nd Edition 2007GE GAP 2.5.2 “Oil & Chemical Plant Layout & Spacing”, 2001“Investigation of a Naphtha Storage Tank Fire” Baker Engineering & Risk, 2004“Thermal Radiation from Large Pool Fires” NIST 2000

ΔΔP Risk Inc.P Risk Inc.