Although JP-5 does have a high flash point ( 140°F
minimum) when manufactured, if it is mixed with
other fuels that have a lower flash point, the liquid
becomes unsafe. Even with its high flash point, JP-5
is highly flammable on rags and clothing, which act
as a wick.
JP-5 is also an acceptable substitute for fuel, naval
distillate, F-76 (commonly known as DFM), for use in
diesels, gas turbines, and boilers.
JP-4 (NATO Code Number F-40) is a wide-cut
gasoline-type jet fuel having a low flash point, typi-
cally below )0°F (-17.8°C). It is used by the Air Force,
Army, and some Navy shore stations. It is volatile,
flammable, and dangerous. JP-4 mixed with JP-5
will lower the JP-5 flash point to an unacceptable level
for shipboard use.
JP-8 (NATO Code Number F-34) is a kerosene-
type jet fuel having a flash point of 100°F (37.8°C). It
is used by the Air Force in Europe and the British Isles,
rather than JP-4. JP-8 mixed with JP-5 also will lower
the flash point of the JP-5 to an unacceptable level for
The volatility of a petroleum fuel is usually meas-
ured in terms of vapor pressure and distillation. The
vapor pressure indicates the tendency toward vapori-
zation at specific temperatures, while distillation pro-
vides a measure of the extent to which vaporization
proceeds at a series of temperatures.
Vapor pressure is measured in a Reid vapor pres-
sure test bomb. In the test, one volume of fuel and four
volumes of air are contained in a sealed bomb fitted
with a pressure gage. The container and fuel are heated
to 100°F, shaken, and the pressure read on the gage.
The pressure shown on the gage is known as the Reid
vapor pressure (RVP) and is expressed in pounds per
square inch (psi).
The measurement for volatility by distillation is
done in a standard distillation apparatus. The fuel
in this test is heated to given temperatures with an
amount of fuel boiled off as each temperature is meas-
ured. The military specification for the fuel gives these
temperatures and the percentages of the fuel allowed
to boil off to meet the desired standard.
Any fuel must vaporize and the vapor be mixed in
a given percentage of air for it to burn or explode. For
gasoline vapors in air, the limits are approximately a
minimum of 1 percent and a maximum of 6 percent by
volume. other types of fuel vapors may have different
Volatility is an important factor in the proper
operation of internal-combustion piston engines. In a
piston engine, the fuel must vaporize and be mixed
with a correct volume of air to bum and deliver power.
If part of the fuel does not vaporize, it is wasted.
Furthermore, it can damage the engine by washing the
lubricant from the engine cylinder walls, which causes
rapid wear to the piston rings and cylinder walls.
Military jet fuels in use at the present by the Navy
include JP-4, which has a vapor pressure of 2 to 3 psi.
and JP-5, which has no specification for vapor pres-
sure. The vapor pressure for JP-5 is almost 0 psi at
normal room temperatures and at standard atmos-
Gasoline has a very strong tendency to vaporize
and, as a result, always has considerable vapors mixed
with the air over the surface of the liquid. In fact, in a
closed tank at sea level with temperatures approxi-
mately 10°F or higher, so much fuel vapor is given off
by gasoline that the fuel-air mixture is too rich to burn.
When fuel is in contact with air, the fuel continues to
evaporate until the air is saturated.
The amount of fuel vapor in the air above a fuel
can never be greater than the saturation value. Of
course, it takes time to saturate the air with fuel vapor,
so the actual percentage of fuel vapor may be consid-
erably below the saturation point, especially if the fuel
container is open to air circulation.
JP-5 fuel does not give off enough vapor to be
explosive until it is heated considerably above 100°F.
However, if the JP-5 fuel is contaminated with even a
small amount of gasoline or, more likely, JP-4, the
amount of vapor given off increases to the point where
it is in the flammable range at a much lower tempera-
ture. At room temperatures, 0.1 percent gasoline or
JP-4 in JP-5 results in a fuel that is unsafe to store
aboard ship since it fails the flash point requirement
for unprotected storage.
Because of the range of its vapor pressure, grade
JP-4 forms explosive vapors from minus 10°F to plus
80°F, its normal storage and handling temperatures.
This means that the space above the liquid almost
always contains an explosive mixture.