Using Oil Analysis for Machine Condition Monitoring
EP Editorial Staff | September 1, 2000
Oil analysis can go far beyond simply revealing the condition of the lubricant. Advanced oil analysis techniques are being used to monitor equipment condition. Through the use of these advanced techniques, equipment reliability increases and unexpected failures and down time can be minimized. Many types of abnormal wear can exist inside a piece of machinery. However, there are only a few primary sources of the wear. Problems related to the oil itself may contribute to wear when the lubricant has degraded or become contaminated. Machine condition also can contribute to the generation of wear if a component is misaligned or improperly balanced. Improper use of the equipment, such as overload or accelerated heating conditions, also can generate wear. Here are some examples of types of wear.
- Abrasive wear is the result of hard particles coming in contact with internal components. Such particles include dirt and a variety of wear metals. Using a filtration process can reduce abrasive wear which will, in turn, ensure that vents, breathers, and seals are working properly.
- Adhesive wear occurs when two metal surfaces come in contact, allowing particles to break away from the components. Insufficient lubrication or lubricant contamination normally causes this condition. Ensuring that the proper viscosity-grade lubricant is used can reduce adhesive wear. Reducing contamination in the oil also helps eliminate adhesive wear.
- Cavitation occurs when entrained air or gas bubbles collapse. When the collapse occurs against the surface of internal components, cracks and pits can be formed. Controlling foaming characteristics of oil with an antifoam additive can help reduce cavitation.
- Corrosive wear is caused by a chemical reaction that actually removes material from a component surface. Corrosion can be a direct result of acidic oxidation. A random electrical current also can cause corrosion. Electrical current corrosion results in welding and pitting of the wear surface. The presence of water or combustion products can promote corrosive wear.
- Cutting wear can be caused when an abrasive particle has embedded itself in a soft surface. Equipment imbalance or misalignment can contribute to cutting wear. Proper filtration and equipment maintenance are imperative to reducing cutting wear.
- Fatigue wear results when cracks develop in the component surface, allowing the generation and removal of particles. Leading causes of fatigue wear include insufficient lubrication, lubricant contamination, and component fatigue.
- Sliding wear is caused by equipment stress. Subjecting equipment to excessive speeds or loads can result in sliding wear. The excess heat in an overload situation weakens the lubricant and can result in metal-to-metal contact. When a moving part comes in contact with a stationary part, sliding wear becomes an issue. Providing proper lubrication, filtration, and equipment maintenance can reduce much of the wear that occurs inside of equipment. Potential problems can be identified with predictive maintenance techniques such as vibration, infrared thermography, and oil analysis. By monitoring the equipment’s condition with oil analysis, a plant can identify various types of wear and take corrective action before failure occurs. In many cases, oil analysis can identify problems with rotating equipment even before vibration analysis detects it.
- When an oil analysis condition monitoring program is implemented, it is important to select tests that will identify abnormal wear particles in the oil. When components inside the equipment wear, debris is generated. Identifying the wear debris can establish the source of the problem. Here are some examples of laboratory tests that can help identify wear.
- Spectrometric analysis is the most commonly used technology for trending concentrations of wear metals. The main focus of this technology is to trend the accumulation of small wear metals and elemental constituents of additives, and identify possible contaminants. The results are typically reported in parts per million. This technology monitors only the smaller particles present in the oil. Any large wear-metal particles will not be detected or reported.
- Particle counting tracks all ranges of particles found in the sample. However, particle counting does not differentiate the composition of materials present. Its main focus is to identify the number of particles in the sample. The results are typically reported in certain size ranges per milliliter or per 100 milliliters of sample.
- Direct-reading ferrography monitors and trends the relative concentration of ferrous wear particles and determines a ratio of large to small ferrous particles to provide insight into the wear rate of the lubricated component. This method can be used as a tracking and trending tool, especially in systems that generate a high rate of particles.
- Analytical ferrography uses microscopic analysis to identify the composition of the material present. This technology differentiates the type of material contained within the sample and determines the wearing component from which it was generated. It is used to determine characteristics of a machine by evaluating particle type, size, concentration, distribution, and morphology. This information assists in determining the source and resolution of the problem.
Each laboratory test has limitations. A well-balanced test package will correctly identify potential problems in equipment. Many of the laboratory tests actually complement each other.
The purpose of an oil analysis program should not be to merely check the lubricant’s condition. The real maintenance savings from utilizing oil analysis occur when equipment problems are detected. Break-in wear, normal wear, and abnormal wear are the three phases of wear that exist in equipment. Break-in wear occurs during the startup of a new component. It typically generates significant wear-metal debris that will be removed during the first couple of oil changes. Normal wear occurs after the break-in stage. During this stage the component becomes more stabilized. The proportion of wear metals increases with equipment usage and decreases when makeup oil is added or oil is changed. Abnormal wear occurs as a result of some form of lubricant, machinery, or maintenance problem. During this stage the wear metals increase significantly.
When oil analysis is used routinely, a baseline for each piece of equipment can be established. As the oil analysis data deviate from the established baseline, abnormal wear modes can be identified. Once abnormal wear modes have been identified, corrective action can be planned.
Implementation of an oil analysis program with analyses consistent with the goals of the program significantly reduces maintenance costs and improves plant reliability and safety. Lubricant analysis for the purpose of machinery conditioning monitoring is at its best with a significant amount of historical data. It is important to establish a baseline for each piece of equipment. Certain analytical results may change with lubricant oxidation and degradation from normal use; the major changes occur because of contamination from environmental factors and machinery wear debris. The analytical costs of a properly implemented program should be covered by the extension of the lubricant change interval. Increased reliability and availability, and the prevention of unanticipated failures and downtime are added benefits. MT