Hello everyone,
Attention: bsieker (Bernd)
Indeed, let’s call it Science 101
"Now let's see. Tanks have vents. Fluids (i.e. gases and liquids) stream from regions of higher pressure towards regions of lower pressure. What is the pressure inside and outside the tank?
If we go along with your premise that no air was in the tank to begin with (more on that later), the only thing inside the tank to create pressure is the fuel vapor pressure. The only value I have found is for JP-8 at 38 degrees Celsius: it is about 200Pa. Another document about Jet-A1 mentions "less than 10hPa", so, let's be generous and assume1000Pa.
Ambient air pressure at 13,000ft is around 50,000Pa, i. e. 50 times the inside pressure. To figure out which fluid will stream into which direction is left as an exercise to the reader."
I agree with your first paragraph.
The vapor pressure of JP-8 at 38 oC is correct but
not in this case.
Here’s the deal: check out the MSDS link for JP-8
http://www.hess.com/ehs/msds/JP8_HOV_4088_clr.pdf
Under basic physical properties section 9 it says:
BOILING RANGE: 280 to 572 oF (140 to 300 oC)
VAPOR PRESSURE: 0.029 psia @ 100 oF (38 oC)
VAPOR DENSITY (air = 1): AP 4.5
SPECIFIC GRAVITY (H2O = 1): 0.75 - 0.80
PERCENT VOLATILES: 100 %
EVAPORATION RATE: Slow; varies with conditions
SOLUBILITY (H2O): Negligible
The vapor pressure is 0.029
psia, that is equivalent to 200 Pascal (Pa) you mentioned but that is in
Absolute pressure reading (
almost in vacuum condition). Meaning; you can’t mix absolute pressure and atmospheric pressure readings without the proper conversion.
To do so, we should get the vapor pressure at the corrected temperature added to the ambient pressure at also the corrected temperature and
altitude to get the pressure inside the fuel tank at that altitude.
So, according to your figures (assuming it is temperature and altitude corrected) we should have 200Pa + 50000Pa to get the pressure inside the fuel tank at 13000 feet, that's why tank vent is needed to allow the 50000 Pa to get in the tank though in real life, the tank pressure is little
higher than ambient pressure because of the vapor pressure, that’s the correct solution to the exercise.
In the issued AD (after the accident) one item calls for adding small fuel quantity in CWT and should be considered as part of zero weight of B747 (very much permanent additional weight) to prevent the in-tank pump from running dry and overheat.
more on absolute, gauge and ambient pressure (PSIA/PSIG) reading can be found in this link;
http://www.iceweb.com.au/Technical/pressure_measurements_info_notes.htm
"The fuel pump as such was not found to be the ignition souce. Secondly, car fuel pumps are usually not submerged in the tank, but somewhere down the line, which their electrical parts away from the fuel line."
The first part of your paragraph is correct but not the second part, take a look at these websites:
See the picture of a typical automobile fuel pump housed in the tank at:
http://www.trustmymechanic.com/parts...nding_unit.htm
And here’s an article from popular mechanics December 1997 issue talking about replacing an in-tank pump.
http://www.popularmechanics.com/how_...e/1272236.html
Volkswagen Fuel pump (read the description)
http://replacement.vdubpartsdirect.com/parts/vdubpartsdirect/wizard.jsp?year=1998 &make=VW&model=PAS--004&category=E&part=Fuel% 20Pump&returnurl=null&dp=true
1997 Saturn fuel pump
http://replacement.vdubpartsdirect.com/parts/vdubpartsdirect/wizard.jsp?year=1997 &make=SN&model=SCSE--002&category=E&part=Fuel% 20Pump&returnurl=null&dp=true
Old cars (imperial 1981-1983) pump repair
http://www.imperialclub.com/Repair/Fuel/pump.htm
Go to"The Fuel Pump" paragraph at this link
http://www.picotech.com/auto/tutoria...-injection.htm
At the end it reads:
"Fuel passing across the fuel pump's armature is subjected to sparks and arcing; this sounds quite dangerous, but the absence of oxygen means that there will not be an explosion!
The majority of fuel pumps fitted to today's motor vehicles are fitted within the vehicle's petrol tank and are referred to as 'submerged' fuel pumps. The pump is invariably be located with the fuel sender unit and both units can sometimes be accessed through an inspection hole either in the boot floor or under the rear seat. Mounted vertically, the pump comprises an inner and outer gear assembly that is called the 'gerotor'. The combined assembly is secured in the tank using screws and sealed with a rubber gasket, or a bayonet-type locking ring. On some models, there are two fuel pumps, the submerged pump acting as a 'lift' pump to the external roller cell pump."
Also see the picture (Figure 1.2) in the same page.
"An additional thought: Why do you think many modern jetairliners have tank interting systems, which feed nitrogen-enriched air into the tanks, to reduce the amount of oxygen, to reduce the chances of creating a flammable fuel/air mixture."
Fuel Tank Inerting System is nothing new (since the 50’s); it is associated mostly with military aircrafts particularly when flying in combat zone. Up to now most modern jetliner including B747
do not have this system.
The FAA tested the system you mentioned back in the 70’s and didn’t make it mandatory because of the advantages vs. cost are not favorable, but recently, (after the accident) new developments brought back the possibility to incorporate this system in commercial wide body jetliners particularly the ones with center fuel tank. Meanwhile, we just wait for the final word from the FAA.