CONDITION MONITORING, ONLINE MONITORING, AND VISCOSITY MONITORING, AS WELL AS ROLLING ELEMENT BEARINGS
Mineral lubrication oils have been widely used for many years. Hydro-treated mineral oils have been employed for many types of turbomachinery because of low fluid solubility (1% to 4%), for example.
Synthetic lubrication oils have been used to address changing parameters, such as the rise in operating temperature and how much dilution is present.
Polyalphaolefin (PAO) oils, for example, have excellent water and oxidation resistance. Polyalkaline Glycol (PAG) oils, which do not readily absorb hydrocarbons or working fluids, have been used in applications where lubrication oil might be in contact with working fluids.
Condition monitoring can be used for lubrication systems as it can highlight developing problems at an early stage. This is akin to blood tests for humans. Lab testing is one approach. Oil samples are collected and sent to a laboratory
A more popular approach is online monitoring. Online sensors or instruments, meters in most cases, are installed in an oil-lubrication system that is circulating to continuously monitor conditions. They measure particulate, moisture, conductivity and other paraments.
Some data parameters have only upper limits, such as particle counts or wear-debris levels. A few data parameters employ lower limits like flash point and oxidation stability.
Many data parameters, such as viscosity and additive elements, use both upper and lower limits. Rate of change is important and is applied to some parameters, such as particle counting, elemental wear metals, ferrous density and degradation of additives.
Special care should be taken for smaller equipment as online monitoring of lubrication oil is usually not feasible. Lab testing should be performed with great care. When sampling small reservoirs, such as those in machines below 200 kW, a complete oil change may be needed. This can be expensive for small machinery.
The condition monitoring of lubrication oils is critical for the monitoring and assessing conditions and health of oil-wetted parts and components. An important element in overall reliability is the condition and life expectancy of bearings.
Monitoring and management of the viscosity of lubrication oil can prevent breakdowns. The viscosity of lubrication oil also plays a major role in energy efficiency; demand for more efficient turbomachinery is driving the use of lower-viscosity lubrication oils.
For some turbocompressors and pumps where lubrication oil comes in contact with light hydrocarbons, viscosity can break down quickly. This increases the risk of problems, failures, and oils becoming too thin or too thick.
Managing the viscosity of lubrication oil is critical in maintaining machinery health. Although viscosity is a function of temperature, it does not work the other way around. Real-time temperature monitoring is inadequate when monitoring the viscosity of lubrication oil.
A traditional practice has been to monitor viscosity once a week or once a month by sending a sample to a lab. But rapid changes in viscosity can occur. Therefore, this old fashion method is often ineffective.
It is better to institute real-time viscosity monitoring of lubrication oil via an online viscometer. This can help to prevent bearing wear and turbomachinery failures.
Many factors can affect the viscosity of lubrication oil. These include oxidation, dilution, contamination, bubbles, and temperature changes. Continuous online monitoring of viscosity can detect faults in their early stages.
Modern online viscometer systems allow the operator to select and operate characteristics, such as electronic controls, self-cleaning sensors, built-in temperature detection, multiple output signals, automatic viscosity control, data logging, quick change memory settings, security, and alerts. For an automatic viscosity control, a sensor system that is pre-set but reconfigurable is preferred.
Rolling element bearings
Rolling element bearings are used in many small and medium-size centrifugal pumps and compressors. They have limited life and can suffer a variety of failure modes. The selection and operation of these bearings requires great care, otherwise high rates of failure can be expected.
Incorrect lubrication, contamination, loss of oil, over-lubrication and other problems can occur. Failures can be expected due to the use of the incorrect lubrication oil or oil deterioration.
Lubrication oil is the lifeblood of bearings. It provides an oil film that prevents harsh metal-to-metal contact between rotating elements and races. Bearing troubles account for around 50% to 70% of all failures associated with small and medium size turbomachinery using rolling-element bearings.
Poor lubrication practices account for most of troubles. Some estimates are that more than 50% of all bearing failures are related to lubrication. Good maintenance procedures, planning and the use of the correct lubrication oil can significantly improve overall operation and reliability.
Amin Almasi is senior rotating equipment consultant in Australia. He is chartered professional engineer from Engineers Australia and IMechE and registered professional engineer in Australia and Queensland (M.Sc. and B.Sc. in mechanical engineering). He specializes in rotating equipment, condition monitoring and reliability.
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