attention Bernd
Ok, let’s take a look at an empty tank on ground; let’s say it is 3 x 3 x3 meter 27 cubic m in volume with vent. As it is, it is full of air at the same atmospheric pressure say 101320 Pa (14.7 psig) so we have inside and out side pressure of that tank is 14.7psi. We are going to consider the major factors (mostly the physical properties listed in the MSDS).
Say we fill it up with 3cubic meter JP-8, we should have 24 cubic meters empty, but it is still occupied by air at the same pressure on ground (101320 Pa standard atmospheric pressure) and the remaining 3 cubic meter of air has been displaced by the fuel volume.
Let’s take that tank up to 13000 feet (flying fuel tank….!) now we have 50000Pa atmospheric pressure at that height in the tank acting on the surface of the fuel as well as all surfaces, so the rest of the atmospheric pressure (101320 Pa) has gone through the vent to the atmosphere leaving 24 cubic meters of less dense air inside the tank (less oxygen less other gases in the air) plus whatever fuel fume that would escape till the pressure equalize.
Now we have an enclosure with 3 layers of fuel in a liquid form, fuel vapor (heavier than air) and the rest is thinner air at that pressure / altitude / temperature combination. Say at that altitude you opened the filling cap of that tank and you dropped a burning wood stick (source of fire) in the tank, you will burn the air / fuel vapor mixture only in the area where it is combustible (the upper area of the vapor layer and the lower area of the air layer) and the fire well die very quickly leaving hot exhaust with insignificant amount of pressure escaping to the atmosphere.
That combustion is extremely inefficient producing inherently low energy yield because of improper fuel air ratio and also equally important the amount of air available in the tank that can support a significant fire, let alone explosion.
If you take a cup half filled with gasoline and you light it up with a lighter you will have a “poof” like fire and it will die quickly (please don’t try this at home) why the fire will not continue burning? Because the combustible layer is crudely atomized with air in unsustainable fashion.
fuel pump (booster, transfer or scavenge pump) are submerged under the fuel, even when that tank is almost empty, it is still well under the fuel vapor layer where no oxygen reaching that part of the tank even on ground because the vapor density of the fuel is higher than air, it will stay at the lower area of the tank.
The vapor pressure plus the evaporating rate of the fuel in the tank will significantly increase in low atmospheric pressure condition at that altitude, in effect, it will increase the pressure inside the tank and push the air out in proportion to the volume it create. Add to this the fuel temperature effect where further more increases the pressure as it rise. So at this point we have even less air volume in the tank and more fuel vapor layer because of the high vapor density.
Another major factor to consider is fuel atomization. When we talk about combustion we are referring to mixing fuel and air in a ratio that can produce a desired amount of heat out of burning, in real life fuel tanks are not designed for this. That's why Nick Charles is disputing the documentary he have seen and asking why those guys having air circulation fans in the tank that help atomizing fuel by creating air fuel mixture that can produce the desired results the producer want in his “hollywoodish” documentary, and in the process he insult the intelligence of the viewer, real life fuel tank is not like this.
It is very much the norm to conceder 1 part gasoline mixed (in a carburetor) with 14.7 part of air then subjected to compression (using piston) in hope to reach a critical mass where oxygen and fuel are well combined to produce a useful combustion in piston engine after ignition at sea level. This is just to give an idea of fuel/air ratio for something we know. For anybody who fly a Cessna (none fuel injected) he would be very much familiar with lean and rich fuel / air mixture, and when to lean out the mixture and when not. Basically it is adjusting the proper air/fuel ratio to optimize the combustion to produce useful power out of the engine when the altitude changes to accommodate the changing oxygen amount (actually oxygen density). Fuel tank doesn’t have pistons or atomization aid and it doesn’t change its volume.
Speaking of volume; so if we have 24 cubic meter of air at that altitude (thinner, low in oxygen content in comparison to sea level) and 3 cubic meter of JP-8 knowing that they are in layers with insignificant atomization, what would be the amount of energy that it can be produced to blow up the tank? Are fuel tank that frail?.
Fuel tank is very damp enclosure (water vapor mixed with fuel vapor) particularly in coastal area where the humidity is high. Aircrafts are known for having water collected in the tank due to the condensation of water vapor in the air that occupies the empty space in the tank. The accumulated water creates vapor (low pressure effect) making fire starting and propagation harder. In smaller aircraft, water level inspection in the fuel tank is part of the pre-flight procedure by actually taking sample.
In real life we are not anywhere near to have a significant fire in a fuel tank at that altitude neither the condition is conducive.
At this point, I will let you in a little secret:
B747 CWT vent is equipped with a float valve that blocks the vent in case of overfilling condition. Also, it will not allow fuel to come out when the aircraft is banking. The venting channels are interlinked with the other tanks (in the wing) and connected to a device called “surge box“at the wing tip and then to the atmosphere. This box regulates the pressure fluctuation and also keeps a little positive pressure inside the tank to reduce foaming. Further more, this will facilitate more fuel fume to escape to the wing tank and from there to the atmosphere during pressure changing, also allowing less air coming back to the center tank to equalize the pressure.
This design is approved by the FAA and Boeing engineers. As I said earlier about the inerting system, they didn’t see much of advantages for such a system, and the condition of the fuel tank designs are not notoriously life threatening knowing that jetliners are different breed of birds than military and aerobatics ones. There is so much DOT regulation on fuel and flammable vessels designs and safety that also didn’t change much the principle fuel tank design.
Reality dictate, hence the term “theory and practice”. You will be surprise of how many aircraft lost because of “fuel tank explosion” in mid air, search it, check out the FAA website (I don’t have the link right now) for the numbers. Safety shouldn’t be compromise for any reason and in my book any preventable loses of human life is not excusable.
The thing that troubled me the most is the elaborate effort using esoteric science to prove some thing scientifically not sound, tested over in real life for decades and proven it is no issue. At the end, people believe what they want.
Last edited by cyrus15; 20th October 2007 at 19:09.