What is meant by flashpoint of oil? What is meant by fire and ignition point? What is
meant by viscosity and by cetane number? Describe briefly the apparatus used to determine
the closed flashpoint of a fuel and how it is used?
Flashpoint: is the temperature at which the oil gives off a flammable vapour when heated.
When a naked light is applied the vapour flashes into a flame but does not burn. This only occurs
when there is air to mix with the vapour to form an explosive mixture.
Firing or ignition point: is generally about 40 degree to 50 degree above flashpoint. This is the
temp at which the vapours given off from the heated sample are ignited by flame application
and will burn continuously.
Viscosity: is a measured on a time basis. It is expressed as the number of seconds for the
outflow of a fluid quantity of a fluid through a specially calibrated instrument of a specified
temperature British praticier uses the Redwood viscometer. This redwood #1 is the flow time of
50ml of fluid up to 2000 seconds. Is an oils resistance to flow?
Cetane number: is an indication of the ignition quality of a fuel. Speed and cetane number can
be connected. The bridge speed engines, above 13.3 rev/sec a cetane number of 48 usually are
regarded as a minimum while for very slow running engines below 1.7 rev/sec a cetane number
of 15 is min.
To determine the closed flashpoint of oil, an apparatus known as the Pensky Martin Test can be
used.
• A fresh sample must be used for every test and can be taken from tank but caution must be taken that no trace of cleaning solvents is present in the oil cup.
• When the operating handle is depressed the shutter uncovers the ports. The flame element is depressed through one port above the oil surface. Starting out at a temperature 17 C below the judge flashpoint the flame is depressed raise again in a period of under two seconds at 1 C temperature intervals.
• Just below the flashpoint is reached a blue halo occurs around the flame. The flash is observed just after through the observation ports stirring being discontinued during flame depression.
• Oils with flashpoint below 22C are classified as dangerous (highly flammable such as gasoline )
• Flash points in the range 22-66C would relate to kerosene and vapouring oils
• above 66 safe and include diesel and fuel oils
