Flash point and Fire point

Flash point and Fire point
Flash point
It is lowest temperature at which the oil gives off enough vapour that ignite for a moment, when a small flame is brought near it.

Fire point
It is the lowest temperature at which the vapour of the oil burns continuously for at least 5 seconds, when a small flame is brought near it. Generally the fire point is 5-40°C higher than flash point.

Significance
A knowledge of flash and fire point is useful in providing protection against fire hazard during transport and storage. Lubricating oils of paraffinic base possess higher flash points than those of naphthenic base. Hence the determination of flash and fire points is helpful in identifying the type of lubricating oil.
A good lubricating oil should have flash and fire points higher than the operating temperature of the machine.

Determination
Flash and fire points can be determined using the same apparatus. An oil is heated at a prescribed rate in an open cup (Cleve Land’s) apparatus or closed cup (pensky Martin’s) apparatus of standard dimensions. A small test flame is periodically applied over the surface of the oil. The temperature at which a distinct flash is seen is the flash point. The heating and periodical application of test flame is continued. The temperature at which the oil vapour catches fire and burns continuously for 5 seconds is noted as the fire point.





8.9.4 Cloud and pour point

Cloud point
When an oil is cooled slowly the temperature at which the oil becomes cloudy in temperature is called its cloud point.

Pour point
The temperature at which the oil ceases to flow or pour is called its pour point.

Significance
Most of the petroleum based lubricating oils contain dissolved paraffin wax and asphaltic impurities. When the oil is cooled these impurities undergoes solidification which cause jamming of the machine. So the cloud and pour points indicate the suitability of the lubricants in cold condition.
A good lubricant must have cloud point and pour point.

Determination
An oil is taken ina flat bottomed tube enclosed in an air jacket and it is cooled in a freezing mixture (ice + CaCl2 ). Thermometers are introduced into the oil and freezing mixture. As the cooling takes place via the air jacket temperature of the oil falls. The temperature at which the cloudiness appears is noted as the Cloud Point. The cooling is furthered continued. The temperature at which the oil does not flow in the test tube for 5 seconds on tilting it to the horizontal position is noted as the pour point.

How to improve cloud and pour point
Pour point of lubricating oil can be lowered by
(i) Dewaxing
(ii) Adding a pour point depressant.
Example
Poly alkyl benzene called “para flow’ is a commonly used pour point depressant.

8.9.5 Oiliness
Oiliness is the capacity of a lubricating oil to stick on to the surface of the machine parts under heavy load or pressure.

Significance
Lubricating which have oiliness stay in between the lubricated surfaces, when they are subjected to high load and pressure. But lubricants with poor oiliness will be squeezed out of the machine parts under this condition.

A good lubricant should have good oiliness.




How to improve oiliness
Mineral oils have poor oiliness where as vegetable and animal oils have high oiliness. Hence oiliness of mineral oil is improved by adding vegetable oils and higher fatty acids like oleic acid, stearic acid. etc.