(49a) Fire Modeling Using a Solid Flame Approach Including Obstructions, Thermal Shielding, and Evacuation Path Thermal Dose Integration

Fire modeling in the process industries is usually performed using simplified models as implemented in consequence modeling packages like PHAST which model the fire as a geometric shape (cylinder for pools, cone for jet fires) followed by the calculation of radiation contours using analytic or numerical view factors. A more detailed approach is the use of computational fluid dynamics (CFD) which models the fire using site specific geometry and allows for a more physically realistic fire progression. CFD is also capable of including shielding effects of large objects which can reduce the thermal radiation along evacuation paths and ultimately reduce the vulnerability to building occupants who are evacuating.

ABS Consulting has developed an intermediate approach between the simplified models and CFD. Our solid flame modeling approach still utilizes geometric simplifications for the fire dimensions while also performing several refinements to the vulnerability predictions for evacuating personnel. These refinements include using specific release directions based on obstructions, thermal shielding of evacuation paths from equipment and buildings, and an evacuation thermal dose integration for the calculation of vulnerability.

Line-of-sight obstructions (e.g., buildings, tanks, equipment) providing thermal shielding and/or blockage of the fire. When inside the flame impingement distance for jet fires, these objects can physically block the fire or redirect it into a shorter and wider cone. Using the FACET3D consequence modeling software, we show the impact of obstructions on the predicted thermal radiation contours and provide our methodology for modifying fire geometry based on obstructions. Comparison of the result to CFD predictions using FLACS are provided.

Thermal shielding of evacuation paths due to the obstructions or buildings can have a significant impact on the calculated vulnerability. FACET3D uses numerical view factor calculation procedures that include the occlusion effects of obstructions on the calculated thermal radiation. The shielding effects will be shown to greatly reduce the thermal radiation during the initial evacuation period when traditional (non-occluded) methods predict the highest thermal flux and therefore the highest thermal dose contribution to the vulnerability.

The vulnerability of personnel during evacuation is a function of the thermal radiation they are exposed to and the exposure duration, the combination of which is the thermal dose. Thermal dose can be determined by either assuming an exposure duration for a static thermal radiation value at the building exit or by integrating the thermal radiation over the duration of the evacuation. The latter method has the advantage that the thermal radiation will be decreasing as personnel move away from the fire. We will show the impact of using an evacuation approach to vulnerability predictions and show the impact of combining evacuation with thermal shielding. Finally, the evacuation route can play a role in the final vulnerability. Including known evacuation routes in the dose integration allows for higher evacuation speeds due to the lack of obstacles compared to generic evacuation routes away from the fire which may encounter obstacles in the process units necessitating a lower evacuation speed.