> While we are on the subject of temperatures, the autoignition
> temperature for av gas is 440 C, for diesel is 230 C and for polyester
> is 560 C. Motor oil should be similar to diesel. If anyone is
> interested...


Rob,


I (think that I) had read that the Navy does not want av gas on board
ship because it is more of a fire problem compared to jet fuel, and I
believe that jet fuel and diesel are interchangeable.  Are our turbine
brothers carrying around that much greater risk?


Robert M. Simon, ES-P N301ES

Robert,

 

Jet A is essentially kerosene and almost identical to No. 1 fuel oil. I think the main difference is that kerosene is a bit cleaner.

 

The big hazard with av gas over Jet A is in the flash point. For av gas the flash point is -50 F, for Jet A it is between 110 and 150 F.

 

If the fuel is significantly below the flash point, the liquid cannot generate flammable vapor concentrations. In this situation, a spark or hot surface cannot ignite the vapors. If the fuel is significantly above the flash point, a spark with sufficient energy or a surface exceeding the autoignition temperature will ignite the vapors if the air fuel ratio is right. At the flash point, ignition depends on the surrounding conditions.

 

Since the temperature aboard ship is never going to be below -50 F, any leak or open container of av gas is going to release flammable vapors which may come in contact with an ignition source. If the navy specifies their Jet A to have a flash point of say 130 F (I don't know what their spec is or if they even use Jet A), then as long as the temperatures are kept below 120 F there should be no flammable vapors generated. Just don't spill any on a hot exhaust manifold.

 

However, once gasoline and jet fuel/diesel fuel are vaporized in flammable mixtures, it is MUCH easier to ignite jet/diesel than gas.

 

The reason I presented those autoignition temperatures was to give some idea how much a fire scene must cool before the extinguishing agent is lost in order to prevent reignition from occurring.

 

As for whether the turbine drivers have an increased risk of fire, I don't know the answer. Certainly the risks are different. For instance, the head space in a gasoline fuel tank is generally too rich to burn at any altitude you might fly. However, the head space on a jet A tank generally starts out too lean to burn on the ground. But, as the altitude increases it will likely enter the flammable range. Remember the center fuel tank issue on TWA 007. Also note that the flash point drops with increased altitude.

 

 

Rob