Article Updated on 24th February 2021
Image Credits: A_Different_Perspective/pixabay.com
The pour point of an oil is an essential property that determines how it will flow at a given temperature. This is particularly useful for many systems where the temperature can fluctuate due to internal and external environments, such as in a car or aerospace engine. In this article, we look at what defines the pour point of an oil and how this property can be deduced.
What Is the Pour Point of An Oil?
The pour point of an oil is a very important property, and it is the temperature point at which the oil loses its flow characteristics, i.e. the lowest point at where it becomes too viscous and loses flow. It has many implications, especially within engines, as it can be used to determine what temperature ranges the oil, or petroleum, can be used in. It will also give a good indication of the temperature at which the oil will become too viscous that it will prevent the engine from starting.
However, it is much more complex than that. Because the pour point is relative to its viscosity, then the fluidic behaviour of the oil also contributes to the obtained pour point values, even if they are not being specifically deduced. The fluidic and viscous behaviour is usually down to two types of viscosity which are based around the shear stress and the shear rate of the fluid. The two types of viscosity that contribute to the oils fluidic properties, and the pour point, are dynamic viscosity and kinematic viscosity.
The loss of flow and fluidic properties at an oil’s pour point can be attributed to either the loss of waxes from the oil matrix (more common), or because of a strong influence from the viscosity properties of the oil (the latter is generally for more viscous oils). For residual fuels oils, there is sometimes another reason, where the loss of flow can be influenced by the thermal history of the oil, i.e. the amount and duration of heating and cooling that the oil has been subjected to in its lifetime. For the pour point of an oil that is involved in a spillage, the size and shape of the container, the head of the oil, and the physical structure of the oil can also affect the pour point value.
Once the pour point of the oil is known, it should never be stored at temperatures below this value. If it is stored below these temperatures, it can become significantly hard to handle and re-liquify, even when positive displacement pumps are used to transport the oil.
Analyzing the Pour Point of An Oil
There are a range of manual and automatic industry-adopted methods that are used to measure the pour point of an oil, which are upheld and approved by the international standards organisation, ASTM International. For measuring the pour point of petroleum-based oils, the standard tests are ASTM D97 and ASTM D5949. For crude oils, the standard test is ASTM D5853.
ASTM D97 is a manual method used to determine the pour point of any petroleum-based oil. In this method, the oil sample is placed into a vessel and pre-heated, and this is followed by a cooling stage so that the oil is at a temperature of 9 °C (48.2 °F) above its expected pour point. This enables the formation of paraffin wax crystals.
The sample is then inspected at this point and at every 3 °C (37.4 °F) intervals thereafter. The inspection takes the form of removing the vessel from the cooling apparatus and tilting it to see if there is any surface movement from the oil. 3 °C (37.4 °F) is added to the temperature point at which the oil doesn’t flow any more (as this is the last flowable point), and this is the oil’s pour point. This approach does mean that the pour point is a slight approximation, but with a maximum error of only 3 °C (37.4 °F).
However, the D97 method does have some drawbacks, in that it relies on human expertise, there is often a poor precision and reproducibility, it is a time-consuming process, and a large amount of sample is required to perform the pour point tests.
The ASTM D5949 method relies on the same principles as the D97 method, with the main difference being that it is an automated process, not manual. The automation of the apparatus provides a faster, more reliable, and more accurate pour point measurement and negates the possibility of human error. It also requires a much smaller sample than the manual D97 method and can usually provide a measurement at four times the speed.
This automated method applies a controlled burst of nitrogen gas onto the surface of an oil sample during cooling, and an optical device is used to measure any changes at the oil’s surface. The controlled cooling enables more accurate pour point values to be obtained and can be used without the need for any external refrigeration units or temperature baths
The ASTM D5853 method is a manual method specifically designed for use with crude oils, and is, in essence, an adapted extension of the ASTM D97 method. Because crude oil is composed of many different hydrocarbons, with varying hydrocarbon lengths, extent of branching and flow properties, this method has been designed to provide a pour point range for a crude oil sample. Within this range, the sample can appear as a liquid or a solid because the wax crystals form more readily during the heating stage.
To obtain this range, an upper and a lower pour point value is measured. The upper pour point is obtained through the same process detailed above for the D97 method. The lower pour point is obtained by pouring the crude oil sample into a stainless-steel vessel and heating it to above 100 °C (212 °F) using an oil bath. After a given time, the sample is then removed and cooled down, whereupon the pour point value is obtained.
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