Basics of Oil Analysis
Structured practices/Programmes (PM=Preventive Maintenance/ Predictive Maintenance/Proactive Maintenance) are needed to control the maintenance costs. Whatever may be the strategy adopted, lubrication will always be an Integral part of these programmes.
Lubrication management covers a larger area right from selection of lubricants to handling and disposal. Oil analysis is an important link in achieving the goals prescribed in any lubrication management program. The main goal of Oil Analysis Programme is to increase the availability of the equipment while lowering the maintenance costs, thereby increasing the profits of the organisation (PM=Profit Maintenance).
Oil analysis is the most widely accepted and implemented proactive maintenance technology in the industry. At the start simple methods were used - like smelling the oil, visual check to see change of colour and for the presence of contaminants, using a hot plate to check the presence of moisture, etc., Foundation for oil analysis was laid way back in 1958 when US Armed forces developed a Joint Oil Analysis Program(JOAP) for detecting failures in Aircraft components and other machines. Many number of tests were developed over a period of time to check various parameters.
Focus of this paper is limited to the oil analysis tests usually conducted for oil condition monitoring to achieve the following three main objectives.
1)Monitoring oil cleanliness
2)Monitoring oil degradation
3)Monitoring wear debris in the oil.
Oil cleanliness / contamination monitoring
Majority of lubrication problems are caused due to contamination by solid particulates and moisture. Fluid cleanliness is critical in hydraulic and other systems (Vertical mill gear boxes in cement plants).Excessive fluid particulate contamination is a major cause of failure of hydraulic pumps, motors, valves, controls etc., Presence of very fine particles in the oil could be one of the reasons for varnishing on the components. Automated laser-based particle counters are used to test oils for particulate contamination/Particle Count according to IS 4406 and other reporting methods like NAS 1638,SAEAS 4039,MIL STD 1246-C etc., The ISO 4406:1999 Cleanliness Code references the number of particles greater than 4, 6, and 14 microns in each ml of fluid. A corresponding cleanliness code, such as 17/15/12, is then given to the fluid. Another major contaminant found in oil is moisture. Karl Fischer Moisture content as per ASTM D1744 in the lubricating oil is measured using an automated titrator and the results are reported in ppm.
High moisture levels in oil could lead to hydrolysis, oxidation, formation of sludge etc.
Limits and targets for Particle Count and Moisture are set according to the equipment being monitored and the experience/data gathered over a period of time.
Contamination control is better practiced by monitoring contamination, setting target cleanliness levels and taking actions to achieve target cleanliness levels.
Oil degradation monitoring
Basic tests conducted for monitoring the quality of the oil are Viscosity at 40°C and 100°C as per ASTM D445, Acid Number as per ASTM D664 /Base Number as per ASTM D4739 and FTIR Oxidation as per ASTM . High or low viscosity is a cause of concern as high viscosity leads to high energy consumption, generation of heat etc., while low viscosity could lead to excessive wear, film loss, etc. Acid Number is tested for all the equipment except for diesel engines. Diesel engine oils are tested for base number. FTIR oxidation is measured in an Infrared Spectrometer to know absorbance due to oxidation of the oil.
Foaming characteristics and demulsibility are the other routine tests run for condition monitoring of used lubricants.
Wear Debris Monitoring/Machine condition monitoring
The basic principle of Wear debris analysis is to monitor the size and concentration of wear particles which is an indication of the condition of the machine and its components.
Spectrometric analysis is widely used to detect wear elements such as Fe,Cu,Cr,Pb,Sn, etc., contaminants and additives. Inherent deficiency of Spectrometry is that the detection is limited to 7-8 microns and cannot identify the morphology or wear type. Ferrography is the result of the inefficiency of Spectroscopy in detecting large particles and in identifying wear modes/types. Ferrography is conducted in two stages.
1Direct reading ferrography to know wear particle concentration (WPC) using a direct reading ferrograph
2Analytical ferrogaphy to know the morphology (size, shape, Concentration etc.) of wear particles using a ferrogram maker and ferroscope. A Ferrogram(glass slide with particles from the oil sample) is made in a ferrogram maker and viewed under a Bi-chromatic Microscope with a digital camera(Ferroscope)to identify different types of wear and wear modes. Ferrous wear, nonferrous wear and contamination can be identified.Different wear modes can be classified as gear wear, bearing wear, sliding wear, fatigue wear, non-ferrous wear (Cu, Babbitt etc.), contamination(sand/dirt, etc.) Based on the results of the oil analysis tests, the oil analysis report can be tagged as normal, caution or critical.
1)Oil analysis is meaningless if the sample is not representative of the system being monitored. Use of proper tools, installation of sampling points is advised.
2)Oil analysis Lab writing recommendations should have adequate knowledge about the machine being monitored and lubricant characteristics.
Advantages and conclusion
To make sure that the right oil is being used and is within the specifications. To control contamination and to determine origin and arrest contamination ingress. To trend wear of components and take maintenance action to avoid breakdowns. Hence, Oil Analysis is considered to be a Proactive Maintenance tool as well as a Predictive Maintenance tool.
Why oil analysis programmes fail?
Many oil analysis programmes fail because the end users were not able to understand how to interpret/use the results, failed to send samples regularly, take bad samples etc.,. Many times it is seen that the maintenance personnel are reluctant to act on the recommendations due to lack of interest or in some cases not able to take the equipment for maintenance due to production demands. Other reason for failure of oil analysis programmes are poor communication between the people involved with oil analysis and the maintenance personnel/personnel using other condition monitoring technologies like vibration, thermography etc.
Maintenance personnel in the cement industry must consider the environment in which their machines are operating while selecting lubricants and lubrication management technologies. Because of diverse applications in a cement plant a wide range of lubricants from mineral oils to synthetics are being used. Experience and knowledge about the lubricants and the machines is essential for proper interpretation of oil analysis results.
Single test parameter in isolation can be misleading. Regular sampling to create a trend provides a more accurate and realistic judgment. Periodic Training of the maintenance engineer involved in the oil analysis programme is of utmost importance to get optimum benefit.
- Oil analysis is proactive and predictive maintenance tool.
- Always right oil is to be used with the specifications.
- Control contamination, source it´s origin, arrest contamination ingress.
- Trend wear of components and take maintenance action to avoid breakdowns.