Method for comparison of different fire extinguishing technologies – Part 5.
How do we determine the foam blanket thickness that is necessary for the successful extinguishment in two minutes?
The Figure 5. of in Part 2. shows us that we are able to have influence the foam spread velocity. We must produce the appropriate foam volume flow rate through each meter of the tank perimeter, and then the foam blanket will close together in the middle of the fire surface in one single minute.
Hopefully it is easy to understand that the greater the specific foam flow rate per perimeter, the more foam has to be introduced to the tank.
The multiplication of the perimeter of the tank, and the specific flow rate, and the foam application time gives the total volume of foam that was introduced to the tank.
An example: if we have a tank of 60 m diameter, the needed penetration velocity to reach the middle of the fire surface is 30 [m/min]. From the Figure 5. we can read out that we must provide 4 [m3/min*m] specific foam flow rate throughout the prescribed 2 minutes full operation time via the entire perimeter of the tank (P60[m]).
Vtotal = Iv 30m [m3/min*m] * P60[m] * t [min],
Iv 30m (from Fig.5. diagr.) = 4 [m3/min*m]
P60[m] = D2 * π/4= 602 * 3,1415/4 [m2] = 2 827 [m2]
t = 2 [min]
Vtotal = 4 [m3/min*m] * 188,5 [m] * 2 [min] = 1508 [m3]
The foam blanket thickness is:
h = Vtotal/F = 1508 [m3] / 2 827 [m2] = 0,53 [m].
The 2 minutes foam introduction time is needed to create a thick, reignition-safe foam blanket.
THE FOAM BLANKET THICKNESS VS TANK DIAMETER FUNCTION
The necessary foam blanket thickness is increased by the diameter of the tank in square function.
Figure 8. Foam blanket thickness to be achieved in one minute vs tank diameter
Since the foam penetration velocity is increased by the Specific Foam Flow Rate, and the foam introduction time requirement is 2 minutes, the foam will be attacked by the fire only for 1 minute long period of time.
The larger the diameter of the tank, the higher foam blanket thickness requirement is.
In other words the foam blanket thickness is in linear function with the fire surface.
Figure 9. Foam blanket thickness to be achieved in one minute vs fire surface.
At larger tank diameters we must consider the effect of the possible wind on the foam movement. The foam introduction must be fast and aggressive to compensate the side wind forces.
Figure 9. also shows that the traditional recommendations (NFPA 11) need to use much more quantity of foam than the FoamFatale System does. They suggest to use lower foam solution application intensity, but for much longer time. And the low foam solution application rate jeopardizes the success of the extinguishing trial, because the fire “eats” our foam, and the foam blanket closing time will be too long, sometimes infinite.