Flash and fire point | Viscosity index

What is Flash and fire point?  What is the Viscosity index?


Flash and fire point :

Flash point : The flash point of an oil is the lowest temperature at which it gives off vapours that will ignite for a moment when small flame is brought near it.

Fire point : The fire point of an oil is the lowest temperature at which the vapours of the oil burn continuously for atleast 5 seconds when a small flame is brought near it.

Significance : Flash and fire point Flash and fire point

  • An ideal lubricating oil should have a flash point which is considerably above the working temperature of machine to ensure safety against fire hazards.
  • Flash point and fire point of lubricating oils help in knowing the highest temperature upto which an oil can be used as lubricant.
  • Knowledge of flash and fire points in lubricating oil aids in precautionary measures against fire hazards.
  • It is useful for identification and detection of contaminants in the oil.

Determination : The determination of flash and fire point can be made by using the following apparatus :

(i) Abel’s closed cup apparatus (ii) Pensky Marten’s apparatus (iii) Cleveland flash point apparatus

It consists of following parts :

(i)    Oil cup                                                              (ii) Shutter

(iii) Flame exposure device                                (iv) Air bath

  • Pilot burner.

Working :

  • The oil sample is filled up to the specified filling mark in the cup. Ÿ It is then covered and positioned properly in the stove.
  •  The thermometer is inserted in the sample.
  • The test flame is lighted while being stirred, the sample is heated at the rate of 9 0 F to 110 F per minute upto 220 0F.
  •  The test flame is applied at every 20 F rise in temperature, thereafter it is applied at every 50 F rise in temperature.
  • When a distinct flash occurs in the interior of the cup at the time of the flame application, the temperature reading on the thermometer is the flash point.

flash point apparatus

Viscosity index (V.I.) :

Viscosity is the property of a fluid that determines its resistance to flow. Viscosity index is an indicator of flowability of a lubricating oil. The lower the viscosity, greater the flowability.

 Significance : Viscosity helps in the selection of good lubricating oil. For instance, light oils have low densities and easy flowabilities. These oils when used on parts moving at high speed, promotes the formation of a good oil film. Moreover, light oils do not impose much drag on high‐ speed parts.

 Determination : For determining viscosity‐index, two types of standard oils are used as reference. One of these is pennsylvanian oil. The pennsylvanian oil consists mainly of paraffin and shows a small decrease in the viscosity with increase in temperature. These oils are arbitrary given a value of 100 for viscosity index. The other reference taken is naphthanic base, Gulf oils.

 V.I. = 100 ; Pennsylvanian oil

 V.I. = Zero ; Naphthanic base gulf oil

 For the determination of viscosity index, first the viscosity of the oil under test is determined at 100 0 F and 2100 F . Let it be U and V respectively.

Viscosity index

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