Relationship Between Heat Dose and the Onset of Secondary Fires

In the Netherlands, focus areas have been introduced around activities involving large amounts of hazardous substances. A focus area is defined as the effect zone where an incident can have life-threatening consequences for people inside buildings that are situated outside the plant boundary (“external safety”). There are three different focus areas, for toxic clouds, explosions (overpressure) and fire (heat radiation). Focus areas help policymakers to decide whether additional protective measures are needed. This paper investigates whether a dose relationship can be used to determine the fire focus area. In a dose relationship, both the heat flux and the duration of exposure are taken into account. First, the criterion for life-threatening consequences was established to be the onset of secondary fires. Next, a literature search was done to evaluate experimental data on the ignition of materials used in and around buildings. Based on these experimental data, a relationship between the heat radiation dose and the ignition of materials was determined. Finally, four accidents of high-pressure natural gas pipelines were used to calibrate the dose relationship. Based on this study, it is proposed to use the dose relationship ? (R(t) - C)B dt = A for the onset of secondary fires, where R is the heat flux in kW/m2, C = 5 kW/m2 is the critical heat flux, B = 2, t is the time in s, and A = 28150 s·(kW/m2)2 is the reference dose.