This chapter discusses the maintenance aspects of lubrication.
Detailed discussions cover: 'Essential Used Oil Properties' that must be retained to ensure adequate lubrication of equipment; 'Degradation of Lubricating Oils' and 'Degradation of Hydraulic Fluids'; 'Particulate, Water, and Biological Contamination'; 'Storage and Handling'; 'Safety and Hazardous Waste' impacts.
'Maintenance Scheduling' and the 'Relative Cost of Biobased Lubricants' are mentioned briefly.
15.2 Maintenance Schedules
Modern maintenance schedules are computer-generated, and are frequently referred to as Computer Maintenance Management Systems (CMMS). These systems are essential in organizing, planning, and executing required maintenance activities for complex manufacturing facilities. A complete discussion of CMMS is beyond the scope of this manual. The following discussion summarizes some key concepts of CMMS.
The primary goals of a CMMS include scheduling resources optimizing resource availability and reducing the cost of production, labor, materials, and tools. These goals are accomplished by tracking equipment, parts, repairs, and maintenance schedules.
The most effective CMMS are integrated with a Predictive Maintenance program (PdM). This type of program should not be confused with Preventive Maintenance (PM), which schedules maintenance and/or replacement of parts and equipment based on manufacturer's suggestions. A PM program relies on established service intervals without regard to the actual operating conditions of the equipment. This type of program is very expensive and often results in excess downtime and premature replacement of equipment.
While a PM program relies on elapsed time, a PdM program relies on condition monitoring of machines to help determine when maintenance or replacement is necessary. Condition monitoring involves the continuous monitoring and recording of vital characteristics that are known to be indicative of the machine's condition. The most commonly measured characteristic is vibration, but other useful tests include lubricant analysis, thermography, and ultrasonic measurements. The desired tests are conducted on a periodic basis. Each new measurement is compared with previous data to determine if a trend is developing.
This type of analysis is commonly referred to as Trend Analysis or Trending, and is used to help predict failure of a particular machine component and to schedule maintenance and order parts. Trending data can be collected for a wide range of equipment, including pumps, turbines, motors, generators, gearboxes, fans, compressors, etc. The obvious advantage of condition monitoring is that failure can often be predicted, repairs planned, and downtime and costs reduced.
15.3 Relative Cost of Biobased Lubricants
Cost is one of the factors to be considered when selecting lubricants. This is especially true when making substitutions such as using Renewable Biobased Lubricants, Fluids, and Greases in place of mineral oils and synthetics.
Biobased Lubricants lubricants are slightly more expensive than mineral lubricants. Therefore, justification for their use must be based on operating requirements for which suitable mineral and synthetic lubricants are not available.
15.4 Lubricating Oil Degradation
A lubricating oil may become unsuitable for its intended purpose as a result of one or several processes. Most of these processes have been discussed in previous chapters, so the following discussions are brief summaries.
Oxidation occurs by chemical reaction of the oil with oxygen. The first step in the oxidation reaction is the formation of hydroperoxides. Subsequently, a chain reaction is started and other compounds such as Acid, Resins, Varnishes, Sludge, and Carbonaceous deposits are formed.
Water and Air Contamination
Water may be dissolved or emulsified in oil. Water affects viscosity, promotes oil degradation and equipment corrosion, and interferes with lubrication. Air in oil systems may cause foaming, slow and erratic system response, and pump cavitation.
(1) Results of Water Contamination In Fluid Systems:
- Fluid breakdown, such as additive precipitation and oil oxidation
- Reduced lubricating film thickness
- Accelerated metal surface fatigue
- Jamming of components due to ice crystals formed at low temperatures
- Loss of dielectric strength in insulating oils
(2) Effects of Water On Bearing Life
Studies have shown that the fatigue life of a bearing can be extended dramatically by reducing the amount of water contained in a petroleum based lubricant.
(3) Effect of Water and Metal Particles
Oil oxidation is increased in a hydraulic or lubricating oil in the presence of water and particulate contamination. Small metal particles act as catalysts to rapidly increase the neutralization number of acid level.
(4) Sources of Water Contamination
- Heat exchanger leaks
- Seal leaks
- Condensation of humid air
- Inadequate reservoir covers
- Temperature drops changing dissolved water to free water
(5) Forms of Water In Oil:
- Free Water - Emulsified or Droplets
- Dissolved Water - Below saturation level
Typical oil saturation levels:
- Hydraulic - 200 to 400 ppm (0.02 to 0.04%)
- Lubricating - 200 to 750 ppm (0.02 to 0.075%)
- Transformer - 30 to 50 ppm (0.003 to 0.005%)
(6) Results of Dissolved Air and Other Gases In Oils:
- Slow system response with erratic operation
- A reduction in system stiffness
- Higher fluid temperatures
- Pump damage due to cavitation
- Inability to develop full system pressure
- Acceleration of oil oxidation
Loss of Additives
Two of the most important additives in turbine lubricating oil are the Rust Inhibiting and Oxidation Inhibiting agents. Without these additives, oxidation of oil and the rate of rusting will increase.
Accumulation of Contaminants
Lubricating oil can become unsuitable for further service by accumulation of foreign materials in the oil. The source of contaminants may be from within the system or from outside. Internal sources of contamination are rust, wear, and sealing products. Outside contaminants are dirt, weld spatter, metal fragments, etc., which can enter the system through ineffective seals, dirty oil fill pipes, or dirty make-up oil.
Lubricating oils are susceptible to biological deterioration if the proper growing conditions are present. Hydraulic oils are also susceptible to deterioration due to infections from biological contamination. Procedures for preventing and coping with biological contamination include cleaning and sterilizing, addition of biocides, frequent draining of moisture from the system, avoidance of dead-legs in pipes.
15.5 Hydraulic Fluid Degradation
Due to the hygroscopic nature of hydraulic fluid, water contamination is a common occurrence. Water may be introduced by exposure to humid environments, condensation in the reservoir, and when adding fluid from drums that may have been improperly sealed and exposed to rain. Leaking heat exchangers, seals, and fittings are other potential sources of water contamination.
The water saturation level is different for each type of hydraulic fluid. Below the saturation level water will completely dissolve in the oil. Oil-based hydraulic fluids have a saturation level between 100 and 1000 ppm (0.01% to 0.1%). This saturation level will be higher at the higher operating temperatures normally experienced in hydraulic systems.
Effects of Water Contamination
Hydraulic system operation may be affected when water contamination reaches 1 to 2%.
(1) Reduced Viscosity
If the water is emulsified, the fluid viscosity may be reduced and result in poor system response, increased wear of rubbing surfaces, and pump cavitation.
(2) Ice Formation
If free water is present and exposed to freezing temperatures, ice crystals may form. Ice may plug orifices and clearance spaces, causing slow or erratic operation.
(3) Chemical Reactions
(a) Galvanic Corrosion
Water may act as an electrolyte between dissimilar metals to promote galvanic corrosion. This condition first occurs and is most visible as rust formations on the inside top surface of the fluid reservoir.
(b) Additive Depletion
Water may react with oxidation additives to produce acids and precipitates that increase wear and cause system fouling. Antiwear additives such as zinc dithiophosphate (ZDTP) are commonly used for boundary lubrication applications in high-pressure pumps, gears, and bearings. However, chemical reaction with water can destroy this additive when the system operating temperature rises above 60 °C (140 °F). The end result is premature component failure due to metal fatigue.
Water can act as an adhesive to bind small contaminant particles into clumps that plug the system and cause slow or erratic operation. If the condition is serious, the system may fail completely.
(d) Microbiological Cntamination
Growth of microbes such as bacteria, algae, yeast, and fungi can occur in hydraulic systems contaminated with water. The severity of microbial contamination is increased by the presence of air. Microbes vary in size from 0.2 to 2.0 µm for single cells and up to 200 µm for multicell organisms. Under favorable conditions, bacteria reproduce exponentially. Their numbers may double in as little as 20 minutes. Unless they are detected early, bacteria may grow into an interwoven mass that will clog the system. A large quantity of bacteria also can produce significant waste products and acids capable of attacking most metals and causing component failure.
15.6 Essential Properties of Used Oil
Several important properties of used oil must be retained to ensure continued service, as discussed below.
New turbine oils are sold under the International Standards Organization (ISO) Viscosity Grade System. Oil manufacturers normally produce lubricating oil with viscosity of ISO-VG-22, VG-32, VG-46, VG-68, VG-100, VG-150, VG-220, VG-320, and VG-460.
The numbers 22 through 460 indicate the average oil viscosity in centistoke units at 40 °C (104 °F) with a range of ±10 percent. Most hydroelectric power plants use ISO-VG -68 or ISO-VG-100 oils.
(b) Oxidation Stability
One of the most important properties of new turbine oil is its oxidation stability. New turbine oils are highly stable in the presence of air or oxygen. In service, oxidation is gradually accelerated by the presence of a metal catalyst in the system (such as iron and copper) and by the depletion of antioxidant additives. Additives control oxidation by attacking the hydroperoxides (the first product of the oxidation step) and breaking the chain reaction that follows.
When Oxidation Stability decreases, the oil will undergo a complex reaction that will eventually produce insoluble sludge. This sludge may settle in critical areas of the equipment and interfere with lubrication and cooling functions of oil.
Most rust inhibitors used in turbine oils are acidic and contribute to the acid number of the new oil. An increase in acid number above the value for new oil indicates the presence of acidic oxidation products or, less likely, contamination with acidic substances. An accurate determination of the Total Acid Number (TAN) is very important.
Note: this test does not strictly measure oxidation stability reserve, which is better determined by the Rotating Bomb Oxidation Test (RBOT), ASTM Test Method D 2272.
(c) Freedom From Sludge
Sludge is the byproduct of oil oxidation. Due to the nature of the highly refined lubricant base stocks used in the manufacture of turbine oils, these oils are very poor solvents for sludge. This is the main reason why the oxidation stability reserve of the oil must be carefully monitored. Only a relatively small degree of oxidation can be permitted; otherwise, there is considerable risk of sludge deposition in bearing housings, seals, and pistons. Filtration and centrifugation can remove sludge from oil as it is formed, but if oil deterioration is allowed to proceed too far, sludge will deposit in parts of the equipment, and system flushing and an oil change may be required.
(d) Freedom From Abrasive Contaminants
The most deleterious solid contaminants found in turbine oil systems are those left behind when the system is constructed and installed or when it is opened for maintenance and repair. Solid contaminants may also enter the system when units are outdoors, through improperly installed vents, and when units are opened for maintenance. Other means of contamination are from the wearing of metals originating within the system, rust and corrosion products, and dirty make-up oil. The presence of abrasive solids in the oil cannot be tolerated since they will cause serious damage to the system. These particles must be prevented from entering the system by flushing the system properly and using clean oil and tight seals. Once abrasive solids have been detected, they must be removed by filtration or centrifugation, or both.
(e) Corrosion Protection
The corrosion protection provided by the lubricant is of significant importance for turbine systems. New turbine oil contains a rust-inhibitor additive and must meet ASTM Test Method D 665. The additive may be depleted by normal usage, removal with water in the oil, absorption on wear particles and debris, or chemical reaction with contaminants.
(f) Water Separability
Water can enter the turbine lubricating oil system through cooler leaks, by condensation, and, to a lesser degree, through seal leaks. Water in the oil can be in either the dissolved or insoluble form. The insoluble water may be in the form of small droplets dispersed in the oil (emulsion) or in a separate phase (free state) settled at the bottom of the container. Water can react with metals to catalyze and promote oil oxidation. It may deplete rust inhibitors and may also cause rusting and corrosion.
In addition to these chemical effects on the oil, additives, and equipment, water also affects the lubrication properties of the oil. Oil containing large amounts of water does not have the same viscosity and lubricating effect of clean oil. Therefore, turbine lubricating oil should not contain a significant amount of free or dispersed water. Normally, if the oil is in good condition, water will settle to the bottom of the storage tank, where it should be drained off as a routine operating procedure. Water may also be removed by purification systems.
Note: If turbine oil develops poor water separability properties (poor demulsibility), significant amounts of water will stay in the system and create problems.
The water separability characteristics of an oil are adequately measured using the ASTM Test Method D 1401 procedure. Insoluble water can be removed by filtration and centrifugation.
15.7 Other Properties of Used Oils
Other properties of lubricating oil that are important, but for which direct measurement of their quantitative values is less significant, are described below.
New turbine oils are normally light in color. Oil will gradually darken in service. This is accepted. However, a significant color change occurring in a short time indicates that something has changed. For example, if oil suddenly becomes hazy, it is probably being contaminated with water. A rapid darkening or clouding may indicate that oil is contaminated or excessively degraded.
(b) Foaming Characteristics
Foaming characteristics are measured by ASTM Test Method D 892. This test will show the tendency of oil to foam and the stability of the foam after it is generated. Foaming can result in poor system performance and can cause serious mechanical damage. Most lubricants contain antifoam additive to break up the foam.
(c) Water Content
Turbine oil should be clear and bright. Most turbine oil will remain clear up to 75 ppm water at room temperature. A quick and easy qualitative analysis of insoluble water in oil is the hot plate test. A small amount of oil is placed on a hot plate. If oil smokes, there is no insoluble water. If it spatters, the oil contains free or suspended water.
(d) Inhibitor Content
The stability of turbine lubricating oil is based on the combination of high quality base stock with highly effective additives. Therefore, it is very important to monitor the oxidation of the turbine oil. ASTM Test Method D 2272 (RBOT) is very useful for approximating the oxidation inhibitor content of the turbine oil. The remaining useful life of the oil can be estimated from this test.
(f) Wear and Contaminant Metals
Quantitative spectrographic analysis of used oil samples may be used to detect trace metals (and silica) and identify metal-containing contaminants. System metals such as iron and copper can be accurately identified if the sample is representative and the metals are solubilized or are very finely divided. A high silica level generally indicates dirt contamination.
15.8 Oil Operating Temperature
The recommended oil operating temperature range for a particular application is usually specified by the equipment manufacturer. Exceeding the recommended range may reduce the oil's viscosity, resulting in inadequate lubrication. Subjecting oil to high temperatures also increases the oxidation rate. As previously noted, for every 18 °F (10 °C) above 150 °F (66 °C), an oil's oxidation rate doubles and the oil's life is essentially cut in half.
Longevity is especially critical for turbines in hydroelectric generating units where the oil life expectancy is several years. Ideally the oil should operate between 50 °C and 60 °C (120 °F and 140 °F). Consistent operation above this range may indicate a problem such as misalignment or tight bearings. Adverse conditions of this nature should be verified and corrected.
Note: When operating at higher temperatures, the oil's neutralization (acid) number should be checked more frequently than dictated by normal operating temperatures. An increase in the neutralization number indicates that the oxidation inhibitors have been consumed and the oil is beginning to oxidize. The lubricant manufacturer should be contacted for recommendations on the continued use of the oil when the operating temperatures for a specific lubricant are unknown.
15.9 Lubricant Storage and Handling
Lubricants are frequently purchased in large quantities and must be safely stored. The amount of material stored should be minimized to reduce the potential for contamination, deterioration, and health and explosion hazards associated with lubricant storage.
Table 15-1 identifies the causes of lubricant deterioration and prevention during storage.
Although lubricant storage receives due attention, equipment that has received a lubricant coating and stored is frequently forgotten. Stored equipment should be inspected on a periodic basis to ensure that damage is not occurring.
Oil is stored in active oil reservoirs, where it is drawn as needed, and in oil drums for replenishing used stock. Each mode has its own storage requirements.
(1) Filtered and Unfiltered Oil Tanks
Most hydroelectric power plants use bulk oil storage systems consisting of filtered (clean) and unfiltered (dirty) oil tanks to store the oil for the thrust bearings, guide bearings, and governors. Occasionally the filtered oil tank can become contaminated by water condensa- tion, dust, or dirt. To prevent contamination of the bearing or governor oil reservoirs, the filtered oil should be filtered again during transfer to the bearing or governor reservoir. If this is not possible, the oil from the filtered tank should be transferred to the unfiltered oil tank to remove any settled contaminants. The filtered oil storage tank should be periodically drained and thoroughly cleaned. If the area where the storage tanks are located is dusty, a filter should be installed in the vent line. If water contamination is persistent or excessive, a water absorbent filter, such as silica gel, may be required.
(2) Oil Drums
If possible, oil drums should be stored indoors. Store away from sparks, flames, and extreme heat. The storage location must ensure that the proper temperature, ventilation, and fire protection requirements are maintained. Tight oil drums breathe in response to temperature fluctuations, so standing water on the lid may be drawn into the drum as it inhales. Proper storage is especially important when storing hydraulic fluids due to their hygroscopic nature. To prevent water contamination, place a convex lid over drums stored outdoors.
Alternatively, the drums should be set on their side with the bungs parallel to the ground. The bungs on the drums should be tightly closed except when oil is being drawn out. If a tap or pump is installed on the drum, the outlet should be wiped clean after drawing oil to prevent dust from collecting.
Grease should be stored in a tightly sealed container to prevent dust, moisture, or other contamination. Excessive heat may cause the grease to bleed and oxidize. Store grease in clean areas where it will not be exposed to potential contaminants, and away from excessive heat sources such as furnaces or heaters. The characteristics of some greases may change with time. A grease may bleed, change consistency, or pick up contaminants during storage.
To reduce the risk of contamination, the amount of grease in storage should not exceed a one-year supply. Before purchasing grease supplies, the manufacturer or distributor should be consulted for information about the maximum shelf life and other storage requirements for the specific grease.
15.10 Safety and Health Hazards
Safety considerations related to lubricants include knowledge of handling and the potential hazards. With this information, the necessary precautions can be addressed to minimize the risk to personnel and equipment.
Material Safety Data Sheets
When handled properly, most lubricants are safe, but when handled improperly, some hazards may exist. Occupational Safety and Health Administrtion (OSHA) Communi- cation Standard 29 CFR 1910.1200 requires that lubricant distributors provide a Material Safety Data Sheet (MSDS) at the time lubricants are purchased.
The MSDS provides essential information on the potential hazards associated with a specific lubricant and should be readily accessible to all personnel responsible for handling lubricants. The lubricant's MSDS should provide information on any hazardous ingredients, physical and chemical characteristics, fire and explosion data, health hazards, and precautions for safe use.
Risk of Fire, Explosion, and Health Hazards
(1) Non-Renewable Oils
Although Non-Renewable lubricating oils are not highly flammable, there are many documented cases of fires and explosions. The risk of an explosion depends on the spontaneous ignition conditions for the oil vapors. These conditions can be produced when oils are contained in enclosures such as crankcases, reciprocating compressors, and large gear boxes.
(2) Non-Renewable Hydraulic Fluids
Typically, Non-Renewable hydraulic systems are susceptible to explosion hazards. A leaking hose under high pressure can atomize hydraulic fluid, which can ignite if it contacts a hot surface.
Synthetic fluids are less flammable than mineral oils. Under normal circumstances, synthetic fluid will not support combustion once the ignition source has been removed.
(3) Bio Hydraulic Fluids
Use of fire-resistant Bio Hydraulic Fluids significantly reduces the risk of an explosion. By nature, Bio Hydraulic Fluids made from Stabilized High Oleic Base Stocks' (HOBS) are Low Volatility fluids. Low Volatility fluids do not off-Gasharmful vapors and therefore have high Flash Points and Fire Points typically 300 to 600 °F.
Non-Renewable Oil Health Hazards
Traditional, Non-Renewable lubricants also present health hazards when in contact with skin. Health hazards associated with Non-Renewable lubricants include:
- Toxicity -- Some additives contained in Mineral Oils may be toxic.
- Dermatitis -- May be caused by prolonged contact with neat or soluble cutting oil.
- Acne -- Mainly caused by neat cutting and grinding oils.
- Cancer -- May be caused by some Mineral Oil constituents.
Note: Contrary to these known health risks associated with the use and exposure of petroleum based oils, fluids, and greases, United Bio Lube's Biobased Lubricants have No Known hazardous materials; as stated on all product Material Safety Data Sheets.
Material Safety Data Sheets for products should be reviewed carefully by personnel to ensure that the proper handling procedures are used.