Modern Maintenance Strategies and Condition Monitoring for Industrial Lubricants
The formulation of modern lubrication fluids has changed and the importance of maintaining and monitoring the lubrication of turbines and compressors has requested new methods and technologies. Well-established tests have been developed to take proactive actions prior to any impact on a plant’s reliability.
OVER THE last decade, monitoring the life of industrial fluids has been carried out using well-established tests that provide early warning of problems and allow the user plenty of time to take proactive actions prior to any impact on a plant’s reliability. Things have changed dramatically over the last decade. Modern turbine oils are formulated differently than their cousins of recent past, starting with a switch to Group II (and beyond) base stocks and mixing in more complex antioxidant chemistries. Relying on decades old analytical techniques have caught many users off-guard, as these tests are no longer the predictive tool that they once were. The time has come to rethink how to monitor and maintain turbine oils to provide early warnings of incipient lubricant failure. The effort to optimize the life and performance of these critical fluids and equipment has called for introducing new monitoring and maintenance technologies.
New Lubricant Formulation Technologies
Before rethinking lubricant condition monitoring strategies, it is relevant to review the changes in lubricant (mostly turbine) technologies. Many lube oil formulations today are quite different than they were in the last decade. There have been many factors contributing to oil formulation changes. The three most influential factors on lube oil formulations have been increasing demands by for example turbine OEMs, upgraded lubricant refining technologies and an effort to reformulate lubricating oils to enhanced value propositions.
In general a shift from Group I oils to Group II and III oils has taken place, in combination with significant formulation strategies. Certain antioxidants are able to greatly enhance the oxidative stability of Group II lubricating oils as measured by the Rotating Pressure Vessel Oxidation Test (ASTM D2272) and Turbine Oxidation Stability Test (ASTM D943). As a result, newer lubricating oils have much higher oxidation stability values compared to older formulations.
It is now widely accepted by oil manufacturers that beyond a certain point, RPVOT values do not directly relate to field-performance. This is evident for several reasons, including:
- Some of the antioxidants that generate very high RPVOT values also form high levels of sludge and varnish upon depletion. The use of some antioxidant chemistries that contribute to very high RPVOT values has been one of the leading causes of turbine oil varnish problems in gas and steam turbines, as well as compressors. Clearly, there is no relationship between initial RPVOT values and field-performance. It is still surprising to the authors however, how often we hear people refer to turbine oil’s initial RPVOT values as having some relevance to field-performance.
This new generation of oils degrades differently to traditional formulated lubricants. The non-linear degradation of most modern lubricants is related to antioxidant selection as well as the oxidative stability of Group II and III base oils. Once the antioxidants have been depleted, Group II and III oils have less natural oxidative stability than Group I oils and degrade very rapidly. As a result, the majority of standard oil analysis tests provide no warning as to when the lubricant will start to degrade and generate deposits. Instead of degradation occurring in a linear and predictable fashion, many of these newer lubricants last longer, perform better but fail rapidly as depicted in Figure 1.
Figure 1: Heavy Deposits in Steam Turbines are a More Common Observation.
Figure 2: Degradation Trend of Traditional Industrial Oils versus Most Modern Lubricating Oils.
Figure 3: Results from the RULER instrument. The red line is the used sample and the grey line is the new oil reference sample. One can see that one of the primary antioxidants phenols, are 23% of the new oil references while another primary antioxidant amines, are 56% of the new oil reference.
Condition Monitoring Strategies
Oil analysis has been the primary tool to detect incipient lubricant failure for the last half century. The first step however in determining an appropriate test slate for any lubricant is to understand its mode of failure. The authors were part of a research project in 2005 (1) that heated five turbine oils in baths maintained at 120oC for 8 weeks to observe physical and chemical changes in the turbine oil. A wide range of analytical tests were performed in order to understand what tests would provide the highest value for detecting early turbine oil degradation and failure.
The traditional testing methodologies for monitoring used industrial oils are viscosity, acid number and RPVOT. It is clear from this research that these tests do little to reveal early lubricating oil degradation and nothing to identify the fluid’s deposit tendencies.
The Two Key Tests for Monitoring Industrial lubricating Oils – RULER and MPC
International Standard Organizations have been active in identifying testing methodologies in analyzing used turbine oils. Here is an overview of the most widely-used guidelines.
- ASTM D4378 – Standard Practice for In-Service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines
- DIN 51515, 1/2 – Lubricants and Governor Fluids for Turbines (Part I: Normal Duty; Part II: Higher Temperature Service) – this is a new turbine oil standard – Rather use: DIN VGB Guideline VGB-M 416 M – In-Service Monitoring of Turbine oils
- ISO 8068:2006-09 – Lubricants, industrial oils and related products (class L) – Family T (Turbines) – Specification for Lubricating Oils for Turbines. – ISO 11366 – Guidance for In-Service Monitoring of Lubricating oils for steam, gas and combined-cycle turbines.
ASTM D4378 is a widely-used guideline and provides some very valuable information. It is currently being updated to adequately reflect some of the changes that the industry has seen and to provide additional value in monitoring modern turbine oil chemistries.
Based on several bodies of research throughout the industry and our cumulative experience at analyzing thousands of used turbine oil samples, here is our view on the key tests to consider in setting up your condition monitoring programme.
Figure 4. Example Patches and values from the Membrane Patch Colourimetry (MPC) test. A value of 43,6 would be cause for concern in any servo-valve application and suggests that the lubrication system is already coated in varnish. A result of 11 would be considered normal.
Figure 5. A filter cartridge heavily covered with deposits from degraded lubricants.
Figure 6. Illustration of the Electrophysical Separation Process.
Individual Antioxidant Monitoring by RULER test (voltammetry)
Directly monitoring individual antioxidants has been demonstrated to be a very good predictive method to monitor antioxidant depletion and provides a more thorough understanding of how fluids degrade. FTIR analysis is a powerful tool to identify molecular changes in lubricants as they degrade, including some antioxidants. The RULER (ASTM D6971) is specifically engineered to track individual antioxidants and, unlike FTIR, the test is not influenced by other additive components. The RULER test identifies the type of antioxidants in the oil and by comparing the results to the new oil, allows you to determine how much of the antioxidants have depleted. An example of RULER results can be seen below.
Once the antioxidants in a lubricant start to degrade, the first physical impact to the lubricant is the generation of extremely small, sub-micron contaminants. These contaminants may consist of degraded base oils, but at the early stages of development, often consist of the degraded antioxidants. The test that has shown the most promise in identifying degradation by-products is referred to as Membrane Patch Colourimetry (MPC). This is a relatively straight-forward test. Fifty millilitres of sample are mixed with an equal amount of solvent (usually petroleum ether) and filtered through a 0.45-micron patch. The colour of the patch is then analyzed with a spectrophotometer and the total amount of colour is reported. Most laboratories report the result as CIE LAB ▲E, the same scale that is currently used in the draft standard being developed by ASTM (D02, C01, Work Item 13070).
Below are a couple of examples of patches and their corresponding MPC value.
The RULER and MPC tests are complimentary. RULER identifies the depletion of the antioxidants and provides critical insight to when a fluid will start to exponentially degrade. MPC measures the formation of these degradation products, allowing users to predict when deposits will start to settle out in their lubricating system. These two tests are appropriate tools to use for predicting the creation of deposits in modern lubricant formulations.
In order to establish appropriate test intervals, we recommend referencing ASTM D4378, consulting with your oil supplier, laboratory or other industry consultants.
Figure 7 Bearing before and after ESP cleaning.
Maintenance Strategies how to prevent reduction of lifetime and damages
An important part of Preventive Maintenance Strategies is Condition Monitoring and Inspections. By implementing a modern oil analysis regime with proper test methods and monitoring intervals, important decision data will be available at time. When acting according to these data you can achieve both reduction in number of oil changes and prevention of malfunctions or damage to the equipment.
Varnish is most commonly recognized as the brown/dark layers that occur on surfaces of bearings, valves, reservoirs, pipes, filters, mechanical seals etc. Some examples where varnish might occur in oil systems:
- Gold adherent films on valves
- Sticky-brown accumulations on filters
- Charcoal-like deposits on Babbitt sleeve bearings
- Black, burned deposits on mechanical seal surfaces and thrust-bearing pads
- Carbonized residue on mechanical surfaces
- Black encrustation on mechanical seals
- Brown, gold adherent films on reservoir surfaces, especially in the inlet area.
All these will cause malfunctions or breakdowns on components e.g. lubricants, bearings, sticking valves and seals. As a proactive approach it is important to prevent varnish from accumulating and to remove it before it sticks to surfaces. As a reactive approach it is important to remove already formed coatings or layers to reduce damage before breakdowns occur.
To be able to utilize the great advantage of an efficient condition monitoring setup it is necessary to install systems to correct the diverged conditions and stabilize it at an acceptable level.
A proactive approach is to remove deposits and varnish before it can harm lubricants and components.
There are more technologies that are in use to remove these soft components e.g. offline cellulose filters, electrostatic oil cleaning and chemical cleaning.
A state of the art technology that has proven successful to prevent degradation of lubricants and clean up contaminated systems is the Electrophysical Separation Process (ESP), illustrated in fig.7. This is an off-line system that circulates the oil in the reservoir during full operation. This system absorbs dissolved and suspended oil degradation products – which is the cause of varnish.
Conclusion
Industrial oils have undergone a dramatic makeover in the last decade. These newer formulations provide many performance benefits over lubricants in the recent past, however there is a different set of rules required to maintain them. Tests such as RPVOT, Acid Number and viscosity now provide little value in detecting incipient lubricant degradation. Most of these modern lubricants do not degrade linearly and have the potential to fail suddenly.
Recommended tests to maintain modern turbine oils are RULER to measure individual antioxidants and Membrane Patch Colourimetry to measure the formation of soft contaminants. Compatibility and water separability tests are also more important tests to be aware of, depending upon the application and situation.
Monitoring and trending the presence of other contaminants is also important – ISO Particle Count, Water and Metal Spectroscopy should be part of a routine oil analysis programme. Finally, the paper reviewed a wide range of other tests that may provide some value in a routine oil analysis programme as well as one of the states of the art technologies to prevent and remove soft contaminants as varnish to increase lubricant lifetime and prevent malfunctions and component breakdowns.
