Diesel fuel pour point testing

At lower temperatures, the crystals increase in size, and form networks that trap the liquid and hinder its ability to flow. The pour point is attained which can, depending on the diesel fuel, vary between -15 and -30°C. This characteristic (NF T 60-105) is determined, like the cloud point, with a very rudimentary device (maintaining a test tube in the horizontal position without apparent movement of the diesel fuel inside). 

The pour point (ASTM D-97, IP 15) is the lowest temperature at which the fuel oil will flow under specified conditions. The maximum and minimum pour point temperatures provide a temperature window where a petroleum product, depending on its thermal history, might appear in the liquid as well as the solid state. Pour point data can be used to supplement other measurements of cold flow behavior, and the data are particularly useful for the screening of the effect of wax interaction modifiers on the flow behavior of petroleum. The pour point should not be confused with the freezing point, which is an index of the lowest temperature at which the crude oil will flow under specified conditions. Test methods (ASTM D-2386, ASTM D-5901, ASTM D-5972, IP 434, IP 435) for the freezing point are not usually applicable to fuel oil but are more applicable to diesel fuel and aviation fuel. 

Pour point is the lowest temperature at which an oil sample will flow by gravity alone. The oil is warmed and then cooled at a specified rate. The test jar is removed from the cooling bath at intervals to see if the sample is still mobile. The procedure is repeated until movement of the oil does not occur, ASTM D97/IP 15. The pour point is the last temperature before movement ceases, not the temperature at which solidification occurs. This is an important property of diesel fuels as well as lubricant base oils. High-viscosity oils may cease to flow at low temperatures because their viscosity becomes too high rather than because of wax formation. In these cases, the pour point will be higher than the cloud point. 

P. Claudy, J.M. Letoffe et al., Diesel fuels determination of onset crystallization temperature, pour point and filter plugging point by differential scanning calorimetry. Correlation with standard test methods. Fuel 65,861-864 (1986)... 

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