April 2019

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Engineering Case Histories: Case 106: Delayed fireball type explosions

When a vessel containing a flammable liquid under pressure (such as those in an LNG road tanker truck) ruptures and ignites, a vapor fireball explosion can occur.

Sofronas, A., Consulting Engineer

When a vessel containing a flammable liquid under pressure (such as those in an LNG road tanker truck) ruptures and ignites, a vapor fireball explosion can occur. An internet search shows that many such explosions have occurred worldwide. The purpose of this article is to provide safety information for those involved with the transportation and handling of such products.

The key point is that these types of explosions may not occur immediately. A surrounding fire can weaken the metal to the rupture point, causing a delayed reaction. The fireball explosion can happen instantaneously, or several minutes after being engulfed in flames. Lethal radiated heat can be present within a distance of 200 m or greater. This makes rescue of personnel and extinguishing of fires complex tasks for emergency responders. Professional training in what to do under these situations is always recommended.1 Calculations will not provide the answer, since the science is not exact and the amount of product in the tanker truck is usually unknown.

The following analysis helps illustrate the effect of the many variables and dangers involved. The random flying fragments are always a concern; however, a fireball’s lethal thermal radiation effect is extensive.

How far away is a safe distance? Flying fragments from pressure effects may not have a reasonable safe distance. Some sources2 mention that 80% of the debris lands within 4 × Rmax and, in rare instances, up to 30 × Rmax (where Rmax is the calculated fireball’s maximum radius). The danger from the fireball’s heat radiation can be estimated for such blasts. Table 1 shows the damaging effects of this heat radiation energy on a surface area for a 10-sec exposure. Longer exposures can result in a heavier dose, and shorter exposures can lead to a lesser dose. Ten sec is typical for the mass explosion of a tanker truck.

Consider that an LNG tanker truck with a load of M = 19,000 kg (10,000 gal) of propane overturns, a fire erupts and the tanker explodes into a fireball after 10 min of being engulfed in a fire. The heat of combustion (Hc) for propane is 50,000 kJ/kg.

From experimental data, the fireball duration, t, can be approximately calculated as shown in Eq. 1:

t = 0.45 × (M)1/3 = 12 sec                                                                                                 (1)

At the end of the fireball growth period, t, it achieves its maximum radius (Eq. 2):

Rmax = 2.9 × (M)1/3 = 2.9 × (19,000 kg)1/3 = 77.4 m                                                   (2)

The surface area, A, of a sphere where the vapor has the correct air/fuel mixture and starts to burn can be calculated from Eq. 3:

A = 4 × π × Rmax2 = 4 × π × (77.4)2 = 75,282 m2                                                          (3)

Some experts4 claim that only 25% of the energy is transferred into heat radiation, and the rest is transferred into pressure development and other sources. The radiation energy on the surface of the fireball can be calculated from Eq. 4:

I = E/A = 0.25 × 50,000 kJ/kg × 19,000 kg ÷ 75,282 m2 = 3,155 kJ/m2                (4)

In terms of power, the surface of the fireball can be calculated from Eq. 5:

P = I/t = 3,155 kJ/m2 ÷ 12 sec = 263 kW/m2                                                                (5)

This radiant heat effect at ground level, at some distance x × Rmax, neglecting atmospheric conditions, can be estimated.

As the distance from this surface increases, as shown in Fig. 1, the surface intensity is decreased by the square of the distance. For example, when a person is 3Rmax away, the surface density of the area, A, decreases by nine times because of the geometry. For this example, at a distance of 2Rmax, or 155 m, Eq. 6 can be used to calculate the power development:

P = 263 × [(1 × Rmax)2] ÷ [(2 × Rmax)2] = 66 kW/m2                                                    (6)

Fig. 1. Fireball effect at a distance.
Fig. 1. Fireball effect at a distance.

From Table 1, this amount of radiated heat would probably be lethal. Being hit by flying fragments is a possibility, even at a distance of 30 × Rmax h 2,300 m.

     
    Table 2 
    compares observations at actual tanker fireball explosions with the calculation methods shown. These can be compared with Table 1 to help validate the assumptions made.

    Summarizing the results of Table 2, three things are obvious:

    1. The fireball type explosion can occur at any time when containers are engulfed in a fire
    2. The thermal radiation can be lethal within 200 m and beyond, and depends on the M in the explosion
    3. The range of debris from the explosion can be 400 m and greater.

    These examples show the importance of evacuating such accident areas quickly and training first responders well in what to do during these types of hazardous events.1 

    Observing pressure or thermal relief valves opening and closing does not mean the potential for an explosion becomes less. Other warning signs are also usually not reliable indicators. The common practice of looking for bulging, discoloration or noises can be misleading, since these events do not always occur.

    Even the practice of applying water is a complicated issue, since this requires being in proximity to the potential explosion zone. No guidelines are given here on safe distances or firefighting techniques, as it is important for first responders to arrive and assess the situation based on their training. HP

    Literature cited

    1. Center for Chemical Process Safety of the American Institute of Chemical Engineers, “Guidelines for consequence analysis of chemical releases,” 1999.
    2. Roberts, A. F., “Thermal radiation hazards from releases of LPG from pressurized storage,” Fire Safety Journal, Vol. 4, 1982.
    3. Hymes, I., “The physiological and pathological effects of thermal radiation,” UK Atomic Energy Authority, SRD R 275, 1983.
    4. Zhang, Q. and D. Liang, “Thermal radiation and impact assessment of the LNG BLEVE fireball,” Procedia Engineering, Vol. 52, 2013.

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