Critical Importance of Annual Turbine Oil Analysis

Turbines are by far the single most-costly piece of equipment for many energy businesses, which highlights the importance of lubrication for these assets. Although turbine oil could last for more than a decade, it can start degrading long before that. Given this, and because oil is the lifeblood of turbines, regular testing is important.

Turbine oils, particularly those used in steam turbines, are expected to last 10 to 20 years. While monthly oil analysis delivers a basic view of the oil’s condition, there are additional tests that should be performed to monitor key performance characteristics of the oil and help prevent unexpected downtime.

1. Annual monitoring of a lubricating oil’s properties is essential to keep rotating equipment performing at peak levels. Many types of equipment can benefit from this monitoring, including diesel generators (shown here), turbines, hydraulic systems, gearboxes, pumps, and compressors. Courtesy: Eurofins TestOil

Annual monitoring of the oil’s physical and chemical properties, together with common contaminants such as water and solid particles, is essential for equipment in the power generation sector (Figure 1). This thorough analysis is also recommended for new oils that must meet rigorous performance specifications prior to selection and use in a new application.

Testing Lubricating Oils

The following tests/test methods should be included in an annual turbine analysis (Editor’s note: ASTM International, formerly known as American Society for Testing and Materials, is an international standards organization that develops and publishes voluntary consensus technical standards for a range of materials, products, systems, and services. ISO stands for the International Organization for Standardization, an independent, non-governmental international standard development group.):

Acid Number; ASTM D974. Acid Number (AN) is an indicator of oil serviceability. It is useful in monitoring acid buildup in oils due to depletion of antioxidants. Oil oxidation causes acidic byproducts to form. High acid levels can indicate excessive oil oxidation or depletion of the oil additives and can lead to corrosion of the internal components. By monitoring the acid level, the oil can be changed before any damage occurs.

Color; ASTM D1500. The ASTM D1500 color scale is used for contamination monitoring. If the fluid’s color is off specification, this can indicate contamination.

Demulsibility; ASTM D1401. Demulsibility measures an oil’s ability to release water. Water-shedding characteristics are important to lube oil systems that have potential to have direct contact with water. Demulsibility can be compromised by excessive water contamination or the presence of polar contaminants and impurities.

Foam; ASTM D892. The tendency of oils to foam can be a serious problem in systems such as high-speed gearing, high-volume pumping, and splash lubrication. Inadequate lubrication, cavitation, and loss of lubricant due to overflow can lead to mechanical failure. This test evaluates oils for such operating conditions.

Fourier-transform infrared (FTIR); Joint Oil Analysis Program (JOAP) Method, TestOil Turbine Method. Every compound has a unique infrared signature. FTIR spectroscopy monitors key signature points of a specific lubricant in the spectrum. These signatures are usually common contaminants and degradation byproducts unique for a particular lubricant. Molecular analysis of lubricants and hydraulic fluids by FTIR spectroscopy produces direct information on molecular species of interest, including additives, fluid breakdown products, and external contamination. It compares infrared spectra of used oil to a baseline spectrum.

Karl Fischer Water; ASTM D6304 procedure C. Low levels of water (0.5%) are typically the result of condensation. Higher levels can indicate a source of water ingress. Water can enter a system through seals, breathers, hatches, and fill caps. Internal leaks from heat exchangers and water jackets are other potential sources. When free water (non-emulsified) is present in oil, it poses a serious threat to the equipment. Water is a very poor lubricant and promotes rust and corrosion in the components. Water in any form will cause accelerated wear, increased friction, and high operating temperatures. Non-emulsified water poses a serious threat to the equipment, leading to rust and corrosion. Emulsified water will promote oil oxidation and reduce its load handling ability.

2. The Membrane Patch Colorimetry test is used to measure the color bodies of insoluble contaminants in lubricants. By monitoring the level of insolubles present in the lube oil, informed decisions can be made regarding the implementation of varnish mitigation technology. Courtesy: Eurofins TestOil   

Membrane Patch Colorimetry; ASTM D7843. The Membrane Patch Colorimetry test (Figure 2) is used to measure the color bodies of insoluble contaminants in lubricants. By monitoring the level of insolubles present in the lube oil, informed decisions can be made regarding the implementation of varnish mitigation technology, and costly downtime can be avoided. Continued testing once varnish mitigation has been implemented can be used to help evaluate the effectiveness of the technology in use.

Particle Count; ISO 4406.Using the Pore Blockage method, the sample is passed through a calibrated screen. As particles collect on the screen, the amount of flow decreases. This decrease in flow is measured and the particle count result is obtained. Higher levels of particulates in the sample may indicate machine health issues, a high rate of external particulate ingression, or filter inefficiency. High levels of particulates can lead to accelerated machine wear due to abrasive conditions. Maintaining lower levels of particulates can increase the operational life of lubricated equipment.

Rotating Pressure Oxidation; ASTM D2272. The Rotating Pressure Vessel Oxidation Test (RPVOT) is a test that determines the oxidation stability of an oil. RPVOT measures the actual resistance to oil oxidation, whereas, the other tests detect oxidation that is already present in the oil. Oxidation is a critical mode of lubricant degradation. As oil oxidizes it forms acids and insoluble oxidation products, which can lead to formation of sludge or varnish. These degradation products can coat bearing and oil cooler surfaces, preventing adequate cooling of the bearings. Areas with tight tolerances such as hydraulic control valves can also become coated causing operational issues.

Ruler; ASTM D6971. Linear sweep voltammetry, more commonly referred to as the Remaining Useful Life Evaluation Routine (RULER) test, measures hindered phenolic and aromatic amine antioxidant content. This test quantitatively analyzes the relative concentrations of antioxidants in new and used oils in order to monitor the depletion rates of the antioxidant protection package in the oil. Hindered phenols and aromatic amines are primary antioxidants used in many industrial oil and zinc-free turbine oil applications. By measuring the depletion and available reactivity of these antioxidant compounds while conducting other routine performance tests, the service life of used lubricants can be effectively monitored. The level of antioxidants present in the lubricating oil gives an indication of the ability of the fluid to resist oxidation. Regular monitoring allows the rate of depletion to be observed and can be useful in determining remaining useful life so that informed decisions can be made regarding oil changes.

Rust A; ASTM D665 procedure A. The Rust Preventing Characteristics Test (ASTM D665) is designed to measure the ability of industrial oils to prevent rusting under conditions of water contamination. The test can be performed with either distilled water or synthetic seawater. The test consists of stirring a mixture of 300 milliliters of the oil being tested with 30 milliliters of water, either distilled or seawater, at 140F (60C) for four hours. A special cylindrical steel test specimen made of #1018 cold-finished carbon steel is polished and then completely immersed in the test fluid. At the end of the four-hour test period, the specimen is removed, washed with solvent, and rated for rust. A lubricant’s ability to prevent rust is crucial for systems that have significant risk of water contamination. Testing a lubricant to determine if it has the capabilities to protect against rust is vital for maintaining the health of ferrous components within the equipment. In addition, rust particles can act as oxidation catalysts and can cause abrasive wear in journal bearings.

Spectroscopic Analysis; ASTM D5185. The Elemental Spectroscopy test uses a spectrometer to measure the levels of specific chemical elements present in an oil. Monitoring the concentration of metallic elements can provide important information regarding machine and lubricant condition. By monitoring wear metals such as iron, copper, tin, and lead, rates of wear can be observed and abnormal wear modes can be detected. Many contaminants have metallic components that can be monitored as well. Increases in contaminant metals such as silicon, aluminum, and potassium can indicate ingression of dirt, coolant, or process contaminants. Some additives also have metallic components. Monitoring additive metals can help indicate when a system has been topped off with the incorrect lubricant.

Viscosity; ASTM D445. The single most important property of a lubricant is its viscosity. It is the measure of the oil’s resistance to flow (shear stress) under certain conditions. It is an important criterion in the selection of a fluid. At low temperature, excessive viscosity may result in poor mechanical efficiency, difficulty in starting, and wear. As oil temperature increases, viscosity decreases, resulting in lower volumetric efficiency, overheating, and wear. Selection of the optimum fluid viscosity grade will provide the most efficient machine performance at standard operating temperatures, therefore minimizing lost time, and energy and fuel costs for the operator.

Understanding Results

The resulting annual turbine analysis report should be a complete rundown of each of the tests performed, along with a detailed explanation of each test result. The report should also contain a summary of findings, as well as recommended actions specified by analysts. Critical indicators commonly uncovered in an annual analysis include a failing foam test, a failing demulsibility test, low RPVOT, and high varnish potential.

Ideally, turbine oils should also be monitored monthly with routine oil analysis. The test slate might include:

■ Acid number

■ Elemental spectroscopy

■ FTIR spectroscopy

■ Viscosity

■ Particle count

■ Water content

Inherent in the power generation industry are distinct fluid challenges that ultimately decide the operational reliability of plant equipment. Whether the plant is powered by fossil fuel, nuclear energy, gas, hydro, or wind, turbine maintenance requires expertise to determine the condition of the turbine’s fluid and overall health. No turbine can perform optimally with degraded oil. With regular testing, that’s one issue plant managers can take off their plate.

Mary Messuti is president of Eurofins TestOil. The Strongsville, Ohio–based company offers lubrication testing, as well as fuel, coolant, grease, and associated tribology services.

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