Turbine Oil Oxidation Stability Test (TOST)

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When choosing the ideal type of lubricant for the maintenance of your assets, you might have come across the term Turbine Oil Oxidation Stability Test or TOST. Organizations use this test to describe the lubricating oil’s stability against oxidation or the breakdown caused by environmental stress and the elements.  

What is TOST?

The Turbine Oil Oxidation Stability Test (TOST) is an industry-standard test. It determines the stability of lubricating oils used in turbines and other high-speed machinery under elevated temperature conditions. The test measures the extent of oil oxidation over a specific period at elevated temperatures.

TOST results can be an indicator of a lubricating oil’s performance under real-world operating conditions. Additionally, TOST results can help determine the suitability of the oil for use and its expected service life.

How TOST Works

The standard method used for TOST was first developed in 1943 by the American Society for Testing and Materials (ASTM), with its most recent updates approved in 2004. In general, the test forces the lubricant through extreme conditions of high temperatures and contamination by air and water. Then, it measures the speed and extent of its breakdown.

To do this, testers take a sample of the lubricant and mix it with water. The mixture is then heated, agitated by blowing oxygen into it, and combined with reactants like iron and copper to speed up the oxidation. As oxidation occurs, testers measure the acid number (AN), which indicates the extent of oxidation. The test usually measures the AN of the mixture for one year.

The measured ANs and their trends will indicate the oxidation stability of the oil tested. This oxidation stability, however, does not account for other factors. Real-world issues such as contamination by dirt and other particulates and the normal wear of metals have an impact.

TOST Considerations

Although TOST is not a direct measure of a lubricant’s service life, it’s a valuable estimate you can use when comparing different lubricant types. Below are some of the things to remember when using TOST to evaluate lubricants.

  1. TOST can detect acidic additives – Some lubricating products contain acidic additives that have anti-wear properties. Because TOST measures the acids produced during oxidation, these products’ results may be lower than their actual oxidation stability due to the additives.
  2. TOST does not accurately simulate actual field conditions – The amount of oxidation during testing and in the actual application may be lower or higher than expected. Therefore, you should only use TOST results to screen out high-risk fluids.
  3. TOST is not applicable for hydraulic oils or gear oils – TOST does not account for oil breakdown due to contamination and wear. So, it is not advisable to use TOST to evaluate oils for hydraulic and gear applications.

TOST is Not Enough

When making decisions for your lubrication management, it is best not to rely on TOST alone. First, we recommend requesting a comprehensive product selection from your lubrication supplier. They can provide information that includes the ideal applications, costs per gallon, and a detailed specifications sheet.

Next, consider consulting a lubrication engineer. They can help you identify suitable and safe lubricants for your needs. Lubrication engineers have the expertise and experience in lubricant applications and are knowledgeable of the additives that may help or harm your assets.

Lubrication suppliers can offer you cheap products with good TOST values. But a lubrication engineer will know if there are hidden costs to using these products. For example, the hidden cost of a cheap lubricant can be more repairs and labor costs due to increased downtime and more waste disposal costs due to increased waste. An expert lubrication engineer can also provide essential information on optimizing the use and efficiency of your lubricating oil and lubricating system. With so much to track and consider for lubrication management, you may also add Lubrication Management Software to your program. All of these experts and systems can provide the right support to optimize your lubrication!

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