Method for comparison of different fire extinguishing technologies – Part 4.
To understand, why the increased foam application rate is so important, for the first we must understand the mechanism that makes the foam moving on the hydrocarbon surface.
Let us take a model, where the foam is flowing down on the inside surface of the tank shell.
Figure of foam spread on hydrocarbon surface.
Iv = the foam volume flow rate [m3/min],
A = the cross section of the spreading foam at L’ distance from the tank shell [m2],
L’ = distance of the examined foam cross section from the tank shell [m],
L = distance of the foam front (“foam leg”) from the tank shell [m],
F = the surface of the foam that is in touch with the hydrocarbon, not attacked by the flames [m2],
vp = the foam front spreading velocity away from the shell [m/min].
The manometer shows the pressure at the original level of the hydrocarbon in the tank.
Until there is only hydrocarbon in the tank, the manometer shows the atmospheric pressure at the liquid level.
Let us examine what happens when the foam flows in the tank, and reaches the liquid level.
The foam does not mix with the foam, because its density is lower than that of the foam. (If the expansion ratio is 5, the density is 0,2 [t/m3]).
The foam begins to float on the hydrocarbon surface. But since father Archimedes told us that the floating object has to submerge in the liquid until the displaced weight of the liquid will be equal with the weight of the object, we know that the foam will submerge into the hydrocarbon.
The depth of the submergence depends on the height of the foam piled up, and the rate between the foam and the hydrocarbon density.
If we continue our example: if the hydrocarbon has a density of 0,6 [t/m3], and the foam has a density of 0,2 [t/m3], the foam will submerge only to 1/3 part of its height piled up.
Anyhow, the foam submerges. At the original liquid level the hydrostatic pressure -measured by the manometer- will be higher than the atmospheric pressure. This overpressure has an effect to the foam material, produces a force which is pushing the foam away from the tank shell. The higher the foam piled up, the stronger the force will be. The result is the movement of the foam. The movement velocity (vp) depends on the weight of the piled up foam, and its viscosity.
For our examination it is enough to learn that the foam introduction flow rate has determining effect to the foam front movement velocity.
If we use a continuous foam curtain-like foam introduction pattern, we can calculate the Foam front movement velocity vs Foam volume flow rate per each meter of the perimeter of the tank, as it is shown on the next diagram.
If we wish to reach the full coverage of a hydrocarbon surface in one single minute, we can find the necessary “Foam introduction volume flow rate per each meter of the perimeter” of the tank, to achieve the necessary “Foam leg velocity”. If we have a storage tank of 60 [m] diameter, we need 30 [m/min] foam front velocity to reach the middle of the liquid surface in one minute. From the diagram we see that it is possible at 4 [m3/min/m] specific foam flow rate.
We must keep the application rate high, otherwise we will never be able to put out the fire. The next diagram shows the critical values, as they depend on the tank diameter.
We know that the extinguishing time is determined by the foam solution application rate, and by the diameter of the tanks.
The critical application rate varies with the tank diameters. This is the point where the traditional Tank Fire Extinguishing Standards fail. They do not see the importance of the tank diameter. This is the reason of several faulty operation of the Firemen.