Q. Define flash and fire points and its determination.

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Write short note on: Flash and fire points.

Ans. Flash and fire points: Flash point of oil is the lowest temperature at which the combustible liquid gives off sufficient vapours that spontaneously ignite for a moment when a small flame is brought near the vapour of lubricating oil. Fire point of oil is also the lowest temperature at which the vapours of oil began to produce a continuous flame. A liquid having flash point less than 140 degree centigrade is called a flammable liquid. Fire point of oil is 5 to 40 degree centigrade higher than its flash points. Flash and fire points do not have any bearing with the lubricating property of oil, but these are important when oil is exposed to high temperature services.

Following three standardized forms are available for finding out flash and fire points:

Abel flash point apparatus: It is used for the oil flash point below 120 degree centigrade.

Pesky-Marten's apparatus: It is used for the oils which possess flash point above 120 degree centigrade.

Cleveland's open cup apparatus: It is generally used for determination of flash point of fuel oils and other oils having flash point below 175 degree centigrade.

Pesky Marten's apparatus to determine flash and fire points of oil cup:

1.      The oil cup consists of brass and is of a standard size having a mark up to, which the oil has to be filled.

2.      The lid of the oil cups has four openings, which are meant for a special type of stirrer, thermometer, an air inlet and a device for introducing standard flame. The shutter provided at the top of the cup has a lever mechanism.

3.       The assembly is heated in a controlled manner by keeping it on an electric heater.

Working: Oil cup and other parts of the apparatus are cleaned thoroughly. Test oil is filled in the cup of the level of mark. The lid of the oil cup is covered and a thermometer, a stirrer device and a flame exposure are carefully inserted in its opening. Apparatus is heated homogenously when the temperature rises within the range of expected flash point at every 1 degree centigrade rise a flame is introduced for a moment with the help of a shutter. Flash point is recorded at the temperature when a flash appear in the form of a weaker sound and light. The producer is continued as before until the oil vapour catches fire, which stays up to a minimum for five seconds. This is fire point of oil.