After determining that RPVOT ratings are not ideal for choosing a natural gas turbine fluid, alternative ratings are needed to fill the gap. Over the years, several tests have surfaced as valuable, and experts recommend using a combination of both new and well-established tests when determining the right turbine fluid.
ASTM D7843 Test – Measurement of Lubricant Generated Insoluble Colour Bodies in In-service Turbine Oils using Membrane Patch Colorimetry (MPC)
Measures the level of insoluble degradation products present in the oil
This is a relatively new lab method of extracting the insoluble contaminants from an in-service turbine oil sample onto a membrane patch. The colour of the patch is analyzed by a spectrophotometer.
This test can be used as a guide on the formation of lubricant-generated insoluble deposits. This test is considered highly sensitive and reliable for detecting subtle changes in insoluble levels and offers the ability to predict varnish formation. The results are intended to be used as a condition monitoring trending tool.
ASTM D6971 – Measurement of Hindered Phenolic and Aromatic Amine Antioxidant Content in Non-Zinc Turbine Oils by Linear Sweep Voltammetry (LSV) or RULER®
Measures hindered phenol and aromatic amine antioxidants in lubricants
Voltammetry is often the technique of choice for measuring antioxidants, as it is indicative of field testing, not lab testing. Antioxidants are one of the first components of the turbine oil formulation to be impacted by thermal, oxidative and mechanical stress and provide early warning for incipient lubricant failure. When calibrated against the new oil, the remaining antioxidant concentration can be determined to estimate the lubricant’s remaining oxidative life.
Tests have been designed to measure oxidation reserve (the amount of protection remaining) and oxidation progress (the amount of oxidation that has occurred). Both testing methods have their advantages and the effectiveness of these tests depends on the operation of the in-service fluid. Understanding how the fluid is handling the oxidation problem can enhance the attempt in correcting the root cause of fluid oxidation. The lubricant specialist should be aware of the measuring tools available and what they may indicate. Therefore, one can address and potentially reduce this source of fluid oxidation.
Excellent indicator of varnish potential
The ultra-centrifuge (UC) test subjects a lubricant sample to G-forces that yield oil-degraded insoluble contaminants typically associated with varnish potential. Insoluble contaminants tend to have a higher density and will drop out during testing. This agglomerated material is compared to a rating scale to obtain a UC value (Rating 1 – 8).
ASTM D445 Test – Kinematic Viscosity of Transparent and Opaque Liquids
Measures the oil’s internal resistance to flow at a specified temperature (often cSt at 40°C)
This tried and true test is still invaluable. Viscosity is one of the most important oil properties because the oil film thickness under hydrodynamic lubrication conditions is critically dependent on the oil’s viscosity characteristics.
ISO 4406 – Method for Coding the Level of Contamination by Solid Particles (Cleanliness)
Quantifies particulate contamination levels per milliliter of fluid
This is an important test to perform and trend for all turbine fluids. It determines the overall cleanliness of the fluid.
ASTM D943 – Oxidation Characteristics of Inhibited Mineral Oils (Turbine Oil Stability Test – TOST)
Evaluates oxidation stability of the lubricant, testing “new” turbine oil properties
The TOST attempts to determine the expected turbine oil life and performance by subjecting the test oil to oxidative stress using oxygen, high temperatures, water and metal catalysts, all of which could increase sludge and acid formation. Because it is impossible to simulate actual in-service conditions in a lab, correlation between test results and actual field performance is difficult. As such, most turbine OEMs use TOST in their specifications to screen out high-risk turbine fluids. Also, this test does not account for other signs of deterioration such as sludge formation or catalyst coil corrosion. The use of ASTM D4310 is used for sludge measurements.
FTM-791-3462 Coking Tendencies of Lubrication Oils
Evaluates thermal oxidative stability of fresh turbine oil on hot surfaces
This test is run at a specified temperature and time, and at a consistent flow rate. The lubricant sample is dripped over the hot panel throughout the test period. Sludge and varnish will build on the plates and a visual comparison can be completed at the end of the test, as well as determining the weight of the varnish/deposit.
ASTM D3427 – Air Release Properties of Hydrocarbon Based Oils
Evaluates the ability of turbine fluids to separate entrained air
Some gas turbine OEMs specify air release limits in their new oil specification requirements. These limits are defined as the time for the entrained air in the fluid to reduce in volume to 0.2% under the conditions of the test and at the specified temperature. In turbines with small sumps and minimal residence time, entrained air mixtures could be sent to bearings and critical hydraulic control elements, causing film strength failure problems, loss of system control and an increased rate of oxidation.
In addition to looking at the above test results in comparing and selecting a new turbine fluid, it is also recommended that operators implement some of these tests in routine oil inspections. While traditional methodologies for monitoring the oxidative health of used turbine fluids – viscosity, acid number and RPVOT – are still beneficial, tests such as the MPC and LSV are more likely to reveal turbine oil degradation at earlier stages, as well as identify a fluid’s deposit tendencies.