Oil Compatibility Testing – Is this really necessary?


Incompatibility of oils mainly occurs when the different additives of the oils chemically react with each other forming new compounds that might not be soluble. The product could be precipitating as sludge, varnish, gel etc. and as result of the chemical reaction the additive is no longer functional. Only after symptoms of incompatibility start to unfold in the plant the disaster starts, especially if a 100 000 litres of oil needs to be scrapped, but then it is too late as the process is irreversible.

Oil compatibility testing is a requirement that sounds easy to do as a tick-box-exercise. However, when the laboratory is approached to do ‘a quick compatibility test’ the customer is often sent back to the drawing board as the test will take at least 3 months to complete when two products are involved, when more than two products need to be tested the expected turnaround time, volume of sample required, and price of testing escalate logarithmically. Frequently asked questions are ‘what are the implications if this test is not done? Can’t results be issued sooner? Is it necessary to do compatibility testing?’

What are the symptoms of compatibility issues?

Below are illustrations of alarming conditions that could relate to mixing of incompatible oils:

Courtesy of testoil.com (Top) Courtesy of machinerylubricationindia.com  (bottom) Figure 1: Excessive foaming occurs when the anti-foam agent becomes inactive. On the right is the foaming test as performed in the laboratory.

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Courtesy of dataentrytsk.wixsite.com/

Figure 2: Water and oil form emulsions when the demulsibility additive fails to separate oil and water. Oil without demulsification additives (left) loses the ability to shed water.

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Courtesy of machinerylubrication.com

Figure 3: High differential pressure could indicate the presence of matter and/or gel are obstructing the oil flow through the filter.


Oil filter blocked but no debris is visible on the filter element.

Figure 4: A blocked oil filter with no visible debris on the filter element. This symptom is a challenge to the plant as there is no visible evidence of the problem. The only test that was found to identify possible gel formation is the Filterability Test (ref. ISO 13357).

Why have compatibility issues become more prominent recently in the power generation industry?

The expected life of turbine oils is about 20 years thus the plant is currently running on oil technology of twenty years ago. The following changes in technology and industry over the years are examples of factors that could have a major impact concerning compatibility if such oils are mixed with newer formulations:

  • Changes in base oil from Group 1 to Group 2, resulting in additives packages that were compatible with Group 1 base oils are not necessarily compatible with Group 2 base oils.
  • Although globally there was a changeover from zinc based to ashless oils, there are still power stations running on zinc-based oil, mixing those oils are prone to having compatibility issues.
  • Old generation additives were based on phenolic chemistry while new generation additives use amines and phenols in synergy.

Logistic challenges:

  • Transport via road tankers where tankers are not dedicated to transport for e.g., legacy zinc-based oil adds to the risk.
  • Procurement principles where oil supply is determined by the lowest cost in the market result in top-up oil from different brands are supplied.
  • Poor inventory control resulting in issuing incompatible product to the end user.

Why does a compatibility study take several months to complete?

Number of composites samples:

To perform the study, different scenarios must be simulated e.g., a system was not flushed properly with the new product prior to change over or where the system is topped up with a different oil (composite mix ratio 2:98), system sweetened with 50 % new product (composite mix 50:50) etc.

The number of composite samples increases logarithmically as the number of products increases that must be tested against each other:

For each pair of turbine oils that are tested for compatibility with each other a set of composite samples are prepared in the ratios 0:100; 10:90: 50:50; 90:10 and 100:0 for hydraulic oil two more composites is prepared i.e., 2:98 and 98:2. The composite samples are aged for 7 days in an incubator/oven at 70 °C before testing could start. (Reference: ASTM D7155 – Evaluating Compatibility of Mixtures of Turbine Lubricating Oils).

What tests are performed and what is the purpose?

Test Relates to
Elemental Analysis Identification of additive components (Zn, Ca, P) and the depletion thereof
Viscosity Identification of viscosity Grade e.g., VG 32, 46, 68 and detecting the thickening of oil due to oxidative reaction
Fourier Transform Infra-Red (FT-IR) Detecting formation of different organic components due to interaction of chemical components
% Moisture Water content, often a byproduct of chemical reactions
Filterability Test Blocking of filters possibly due to gel formation
Air Release Additive depletion: Changes in the property of the oil to release entrapped air
Foaming Test Additive depletion: Tendency to foam, establishing how stable is the foam that forms if antifoam agent has depleted
Demulsibility / Demulsification No Additive depletion: Ability of oil to shed water
Oxidation Stability (RPVOT) Depletion of antioxidant: Affecting the expected remaining life of the oil

Duration of tests – can the turnaround time be shortened?

Tests that need to be performed to determine the depletion of specific additives are physical tests, labour intensive and requires an average of 60 minutes per test per composite sample while the Oxidation Stability Test (RPVOT) can take between 5 and 13 hours per sample.

The answer is no. Compromising on the selection of tests and not ageing composite samples could defeat the purpose of compatibility testing.

Case Study – Low differential pressure:

Information received from the plant:

  • Unit kept tripping due to high differential pressure.
  • Filters blocked without visible debris.

Laboratory results obtained after sample analysis:

  • Oil from the unit failed Stage II of Filterability Test.
  • Sample from the bulk supply failed Stage I of the Filterability Test.
  • A clear, slimy product was produced that formed a film which resulted in blocked filters.
  • Air Release properties of the bulk oil supply failed.
  • Demulsibility of oil from the unit failed (formed an emulsion).


The root cause was that the bulk supply tank for oil was contaminated with oil that is not compatible with the brand of oil that was originally in the tank and that was running in the system.

Lessons Learnt:

  • Do not top up with different oil brands or types.
  • Not flushing the system thoroughly with new product before change over, results in small volumes left behind that cause compatibility issues – 98:2 composites are more likely to fail the compatibility test.
  • Filter blockage that happens as result of gel formation cannot be detected by visual inspection.

Figure 5: On the left is the membrane filter from the filterability test that has failed. On the right is the filtrate after filtering the oil through the 0.8-micron membrane, a white flock like suspension has formed.

Figure 7: The blocked oil filter with a clear gel-like substance.


Implications if this test is not done, is mixing of oils (topping up) that appear to be the similar concerning oil type and/or oil brand but have different base oils or additive chemistry that pose the risk of interaction causing depletion additive packages which is irreversible.

Not performing oil compatibility testing to save time and money might result in equipment downtime, damage to plant and wasteful expenditure.

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