Mineral oils are cost effective and widely used for water pumps. Hydro-treated mineral oils are employed for their low fluid solubility (usually 0.3 percent to 3 percent). Synthetic lubricants are sometimes used for special water pumps depending on the temperature and how much dilution is present. PAO (Polyalphaolefin) oils, for example, have excellent water and oxidation resistance. PAG (Polyalkaline Glycol) oils, which do not readily absorb water/liquids, are used in special applications where the oil can be in contact with the water.
One of the main disadvantages of using grease over oil is the annoying tendency of grease to cake and dry out. Usually, the lubrication oil is preferred for lubrication of a medium/large water pump. There are four primary components to precision lubrication oil of water pump bearings/components:
- The lubrication oil selection
- The lubrication oil system
- The flow, pressure and operating conditions of lubrication oil
- The monitoring, control and condition monitoring of lubrication oil system.
The condition-based lubrication should be part of any world-class operation/maintenance program and the subject of condition-based lubrication/operation is gaining more and more interest in the water pump industry.
The lubrication oil condition monitoring should examine lubricant properties, contaminants and various kinds of wear debris to determine water pump health. This is comparable to a blood test on the human body. More than 65 percent of all water pump failures are lubricant related.
Lubrication condition monitoring
The oil condition monitoring could be in two major types, the lab testing and the online testing. For the first option, the lab testing, lubrication oil samples are sent to a laboratory, which could be an out-of-site lab or an on-site lab. Online testing instruments, which are meters, are installed usually in an oil circulating system in order to monitor continuously lubrication oil conditions. Particularly particle counters, moisture meters and dielectric instruments should be mentioned as good examples for online monitoring. Proper lubrication condition monitoring should be selected for critical water pumps.
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 like viscosity and additive elements use both upper and lower limits. Rate-of-change limits are effectively applied to some parameters such as particle counting, elemental wear metals, ferrous density and others. They also can be effectively applied to monitor abnormal degradation of additives.
Special care is needed for small water pumps. For a small water pump, the lubrication oil online monitoring is usually not feasible. The lubrication oil lab testing should be performed with great care. When sampling small reservoirs, such as those in small water pumps below 200 kW, following the flush portion and then sampling, a complete oil change would have occurred on every small pump. Considering the increased lubricant consumption coupled with the additional cost of testing oil samples, the overall costs would be significant.
Oil viscosity monitoring
The condition of lubrication oil is a critical factor in extending a water pump’s bearing/component life and overall reliability. Monitoring and managing the lubrication oil viscosity can prevent costly breakdowns because of the bearing failure. The viscosity of lubrication oil also plays a role in the energy efficiency, as demand for a more efficient pump is driving the use of lower-viscosity lubricants.
For pumps where lubrication oil could come in contact with pumped fluids, the lubrication oil’s viscosity can break down much more quickly, increasing the risk of failure. Clearly, managing lubricant viscosity is critical to maintaining pump health.
Many operators also have found that the real-time temperature monitoring is inadequate to monitor lubricant viscosity. It is a common practice to monitor lubricant viscosity of many large water pumps once a month by sending a sample to a lab for testing. Rapid changes in viscosity can occur, and results can be severe. It is recommended to install the real-time monitoring of lubrication oil viscosity (the inline viscometer) in critical (large) water pumps. Monitoring the lubrication oil viscosity is the best way to prevent bearing wear and pump failure.
Many factors can affect the lubrication oil viscosity. These include oxidation, dilution, contamination, bubbles, temperature changes and others. The continuous monitoring of the lubrication oil viscosity can show traces/effects of all above-mentioned faults. An operator can look and select certain characteristics for a modern inline lubricant viscosity measurement system, such as menu-driven electronic controls, built-in temperature detection, multiple output signals, automatic viscosity control, data logging, quick change memory settings, security and alerts. A self-cleaning sensor uses the inline fluid to clean the sensor as it is taking measurements. For an automatic viscosity control, a sensor that is pre-set but reconfigurable is always preferred.
Oil contamination monitoring
One of the most important factors in the selection of a lubrication oil contaminant monitoring instrument is the ease of operation/application. The “ease of use” is as important as technical specifications (such as the precision and other requirements). The lubrication oil contaminant monitoring instrument should be frequently used, but a moderate precision is usually sufficient.
Wear debris analysis
The “wear debris analysis” refers to the extensive array of wear particle technologies and tactics that can help reveal the true tribological condition of a water pump. These techniques should be used in combination, since individually they cannot be effective. Even an individual technique/measurement, sometimes, can be misleading. There have been many false detection reports, because people attempt to draw a premature conclusion from an incomplete/individual piece of information in the wear debris analysis.
Common important technologies used for screening purposes include the ferrous density analysis, the elemental spectroscopy and the particle counting and patch testing. Complementary methods used for proper and correct monitoring/fault-detection include: The filter debris inspection; the magnetic plug analysis; the sump sediment analysis; the ferrography methods; the acid-dissolution spectroscopy; the particle heat treatment; the particle impaction testing; the chemical microscopy; the digital shape profiling; the rotrode filter spectroscopy; the gravimetric analysis; the ultracentrifuge (separation of soluble metal fraction); the pore blockage particle counting; and many more.
Again, individually, the above mentioned methods might let a damage go unnoticed or a misleading conclusion. However, when two, three or even four trend lines move and show the same result, there can be a reliable signal that could be used for a further monitoring/investigation or even a corrective action.
The clean oil can strengthen the signal-to-noise ratio in a wear debris analysis. Without the background noise of dirty oil, even the weakest signals sometimes can be detected. The correct position should be selected for sampling points in a wear debris analysis.
Particle debris generating sources and potential damage locations should be properly identified and correct sampling points should be defined. By sampling downstream of the wear generating source and upstream of filters/reservoirs, the data are not stripped by filtration or muted by dilution.
The size and shape of particles play an important role in the wear debris analysis. The most important indications are particles in their original size and shape as produced from their generating sources such as bearings, gears and other particle generation points. Circulated particles, which produced some time ago and circulated in the lubrication oil system, cannot be useful.
Often, circulated particles cannot be properly employed for monitoring purposes because they have been reworked by the pump and its environment through crushing, laminating, corrosive action or others and the identification of wear mode and the location of particle generation are nearly impossible. Some good places to find particles in the original size and shape are filters, sump sediment, magnetic plugs, chip collectors and similar sources.
Lubrication and reliability
Usually, there is a risk to stop a water pump for repair (or overhaul) based on only one abnormal monitoring signal. In other words, without having the benefit of multiple technologies coming to the same conclusion, the reliability of a water pump cannot be properly assessed.
There are usually the following three monitoring arrangements for a critical water pump: The vibration analysis, the oil analysis and the infrared thermography.
At least one technology should be used on every major piece of equipment. If an anomaly is observed, other technologies should be applied to evaluate the water pump. If at least two monitoring methods show there is an issue, a proper action should be taken. The vibration analysis is a commonly used method and the other two can help to confirm any detected issue.
For example, the online vibration monitoring is usually employed for the continuous monitoring of pump’s bearings. If there is an abnormal vibration, the oil analysis can be used to further evaluate bearings. If there is a slight spike in lead, tin or aluminum, it could be concluded that there is a bearing issue.
As another example, the online vibration monitoring of bearing and casing are employed for many gear units, which are sometimes used in water pumps. If there is a high vibration recording, there could be the oil analysis for further reliability assessment of the gear unit. A slight increase in iron could be considered as the confirmation of a problem in the gear unit. Without having the benefit of two or even three technologies that confirm there is a reliability issue, there could be a risk to stop and open the pump and find no issue.
Fake signals of vibrations have been reported for many water pumps. There are numerous cases that pumps have experienced a high vibration, but still healthy and can work for a relatively long time without any problem. Many water pumps have experienced high vibrations, even two or three times than their normal (baseline) vibration amplitudes, without any serious problems.
A case study is presented here to show the importance of oil analysis to confirm a reliability issue. For a medium size pump, the measured vibration has been tripled (three-times of the baseline value) during four months. During this timeframe, the oil analysis showed significant increases in bearing material and other contaminants. The pump was stopped at the first opportunity. Detailed observations showed the pump seal was damaged, allowing contaminants to invade the bearing, which caused both high vibration (as result of bearing damages) and oil analysis abnormalities.
Lubrication cleanliness
The contaminant-free lubrication oil and clean lubrication oil system can extend the operating time and increase the overall reliability of water pumps.
The International Organization for Standardization (ISO) has developed a cleanliness code that is the primary specification of cleanliness for many industrial oil systems. The general trend in the industry is to give credit to the value of ISO cleanliness code.
The cleanliness has caused many issues in water pump lubrication systems. More than 75 percent of all lubrication oil skid delivery/commissioning delays have been caused by cleanliness related problems.
Rolling-element bearings
Rolling-element bearings used in many water pumps (generally in small/medium water pumps) potentially have different failure modes. Particularly, many failures can be expected if an incorrect lubrication strategy is implemented. Serious failures can be originated from incorrect lubricant selection, contamination, loss of lubricant and over-lubrication.
Most small water pumps are designed with anti-friction, rolling-element bearings. The lubrication is the lifeblood of these bearings, providing an oil film that prevents harsh metal-to-metal contact between rotating elements and races.
Bearing troubles account for around 60 to 80 percent of all failures associated with the small/medium size water pumps using rolling-element bearings. Poor lubrication practices account for most bearing troubles (over 50 percent). Good maintenance procedures, planning and the use of correct lubricant can significantly increase productivity by reducing bearing issues.
Amin Almasi is a rotating machine consultant in Australia. He is a chartered professional engineer of Engineers Australia (MIEAust CPEng – Mechanical) and IMechE (CEng MIMechE) in addition to a M.Sc. and B.Sc. in mechanical engineering and RPEQ (Registered Professional Engineer in Queensland). He specializes in rotating machines including centrifugal, screw and reciprocating compressors, gas turbines, steam turbines, engines, pumps, condition monitoring and reliability. Almasi is an active member of Engineers Australia, IMechE, ASME, Vibration Institute, SPE, IEEE and IDGTE. He has authored more than 80 papers and articles dealing with rotating equipment, condition monitoring and reliability.
