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 DESCRIPTION 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 DESCRIPTION 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 shipboard use. VOLATILITY 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 limits. 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- pheric pressure. 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. 3-2