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Pour point is defined as the temperature at which a liquid just starts to flow under certain conditions. Its value should be minimum in order to use the fluid at a very low temperature as per ASTM D97. Natural esters have pour point in the range of −6 to −25 °C which is higher than that of MO. This is because of the presence of triglyceride structure in the natural esters. When crude oil is trans‐esterified, these triglycerides form mono‐glycerides and support the flowability of the liquid and make it comparable to that of MO [54, 64]. Some comparison of the pour point values of different oils can be seen in Figure 2.7a.

Figure 2.7 Comparison of (a) pour point values of different oil samples and (b) flash point values of different oil samples.

The problem of higher pour point can also be improved with the addition of pour point suppressants, winterization, or blending with other fluids having lower pour points. In case of natural esters, operating at low temperatures becomes difficult as they have high pour points and tend to solidify faster than MO. So, the aspect of pour point must be taken into consideration when designing a transformer or other apparatus for operation in the colder regions. Blending of natural ester with some other compatible fluids may show lower pour points. Many transformer specifications require lower ambient temperatures of less than −20 to −25 °C. In general, the temperature of a running transformer is sufficient to keep the insulating liquid flowing. However, for outdoor installations, especially in colder regions, maintaining a free flow of the liquid becomes a concern as the temperature of the atmosphere drops below 0 °C. There are certain properties of the NEO, which determine the pour point like the acid chain length, level of unsaturation, and type of branching in the oil. Unsaturated fatty acids help in reducing the pour point and also the existence of aromatic groups in natural esters aids in maintaining a lower pour point value.