The Difficulty of Establishing Fluid Quality
After five decades working in the lubricants industry, I could be excused for thinking I know all about base oil quality. Meeting a colleague who is as long in the tooth as I am disabused me of that belief. He had a completely different set of criteria for fluid quality – I talked about viscosities, viscosity indices, saturates and sulfur while he spoke of distillations, boiling points and volatile organic compounds. But what we have in common is an understanding of using the best-suited tests to determine the right fluid for a specific application.
How best can we define the quality of a base fluid in terms of the tests we run? This is particularly important when users of the oils, such as lubricant blenders or base oil marketers, need to understand the nature of the oil they are purchasing.
A good example of this would be the level of aromatic hydrocarbons in a fluid. One simple procedure, the ASTM 2140 test, provides data on the composition of different carbon types: aromatic, paraffinic and naphthenic.
For an oil that is going to be used in a food-grade industrial lubricant, the level of aromatics should be close to zero, sometimes referred to as trace. The fluid has to be at this level if it is to be able to claim performance against food-grade quality test requirements. However, for base oils used in engine lubricants, aromatic molecules are not so critical for performance but the levels of paraffinics and naphthenics are of greater interest.
When recently reviewing the specification of a high-quality, low-viscosity base fluid, which could be used in a range of applications, I noted 31 properties listed, some of which were duplications. Some were due to the same type of test being run at different temperatures. Other information was provided for both centigrade and Fahrenheit. And there were four results for density or specific gravity.
All this was very comprehensive, but did it help me to determine whether the oil was acceptable for the application? Probably not. It would have been possible to determine its utility with around half of these properties.
Among the parameters listed were a number that initially seemed to be fairly simple as regards quality, including color, density and viscosity. However, all these properties can be defined in several ways.
The petroleum industry uses a variety of test methods developed over the years to measure color, depending on the nature of the oil and the color that is anticipated. One of the earliest is the Saybolt chromometer, established as the ASTM D156 test in 1923. This covers water-white to dark yellow oils, withSaybolt numbers ranging from plus 30 down to minus 16. Colors at the darker end of the spectrum can also be evaluated using another the ASTM D 1500 test, which measures colorsfrom yellow through todark brown, on a scale from 0.5 to 8. The bottom end of the Saybolt scale crosses over with the top
There are several others systems in addition to the two mentioned already. The most common of which is the platinum-cobalt method, designated ASTM D1209 and established in 1952, much later than the other systems. ASTM D1209 is recommended for clear oils. The convention now is that tests for darker oils for most lubricants use the ASTM method, while clear oils, where a light color is important for the application use the Saybolt method.
Then there is density, which is another simple test that compares the amount of matter an object has with its volume. But is it in fact so simple? Is that density at 15 degrees Celsius or 60 Fahrenheit? Is it measured in kilograms per liter or per cubic meter or even pounds per gallon? Could it be specific gravity or relative density (which are the same anyway) or should it just be API gravity? These all appear on base fluid specifications depending on what part of the world they originate from.
Most base oil specifications now have at least density at 15 C in kilos per cubic meter, but as mentioned previously, many specification sheets have this plus one or two more. And where to begin when it comes to viscosity? Kinematic or dynamic; Saybolt Universal Seconds; centistokes or millimeters squared per second; at 100 F or 40 C? At least not Redwood No 1 or Engler degrees!
After an entire career spent in the industry, it is still possible to get confused by all this, so imagine what it is like for a new recruit. With this in mind, the clarification of parameters in base oil specifications becomes even more important.
When a lubricant blender buys a base stock, they expect it to be fit for the intended purpose and of consistent quality every time it is purchased. When setting up a base stock supply agreement, the contract will have a set of specifications for each performance parameter. The purchasing specification needs to be established, agreed by both parties, where not only property limits but testprocedures are accepted. The receiving party needsto be able to run tests as part of their quality inspection procedure.
This is where there must be a clear understanding of the specification requirements, including those simple parameters of color, density and viscosity. There can be major financial consequences if there is a disagreement between the manufacturer or supplier and the blender over the quality of an incoming delivery of base oil – perhaps due to a misunderstanding of the specification – and the product is not accepted.
To make things easier for the entire industry, there needs to be a levelof conformity with a standard accepted test method and parameter nomenclature. Although things have improved a lot over the years, complete standardization of test methods and definition of base oil properties may never