By: Douglas J. Smith, IEng, Senior Editor
Sampling and analysis of lubricating oil is now becoming standard maintenance practice.
Electric generating plants with inadequate maintenance programs will invariably be unable to compete in today’s very competitive industry. With the advent of deregulation, power plants can longer rely on the rate base to cover their maintenance costs. For this reason, power plants are implementing a variety of technologies to reduce the costs of maintenance and to increase plant reliability and efficiency.
An area that is given little attention is the analysis and monitoring of lubricating oils. According to Kyle Jeffers, machinery management specialist, Bently Nevada, oil analysis can be a very effective tool in determining its condition. Unfortunately, oil analysis can sometimes be misleading as it implies that the primary area of interest is the condition of the lubricant. In reality, however, the primary goal is to monitor the condition of the equipment, says Jeffers. The focus of any oil analysis program should be to determine the presence and trending of any wear.
Even though the primary focus should be on monitoring for wear, determining the condition of the oil through analysis is still important. By analyzing the oil it is possible to establish the oil’s condition and its ability to continue protecting the equipment. An oil analysis program will indicate any changes in viscosity, oxidation and depletion of additives in the oil. It will also show if any contaminants are present.
Maintaining Oil Purity
A cost effective solution for reducing contaminants is to improve the storage and handling of the lubricating oil. Water is one of the most common contaminants, and if not eliminated, will reduce the life of bearings considerably, Figure 1. Water can be removed by a variety of methods including settling/evaporation, centrifuging, coalescing filters/screens and filter/dryers. Figure 2 shows a schematic for a typical oil analysis program.
Settling/evaporation to remove the water is inexpensive, but it only removes free water. Although centrifuging removes water, dirt and other solids, it does not remove entrained gases. Although coalescing filters remove free water in the oil, they are only effective within a narrow range of viscosity and specific gravity. Since centrifuging and coalescing filter/screen use physical separation, there is also the potential for removing additives in the oil, thereby reducing the protective properties of the oil.
Kansas City Power & Light’s (KCP&L) 1,450 MW LaCygne coal-fired generating station uses bulk storage for its lubricating oil. Because of coal dust, high humidity and high temperatures during the summer months, preventing oil contamination is a challenge. The plant samples the oil as it arrives on-site and at intervals throughout the year. In many instances, however, water has been found contaminating the oil in storage and the oil in use.
Studies of the plant’s equipment, including the fan reservoirs and gearboxes, confirmed the presence of water in the oil. After further analysis the plant decided to install a new type of oil filter/breather manufactured by Des-Case. Since installing the new filter/breathers on the power plant’s critical equipment and oil storage tanks, the moisture has been eliminated. In addition, LaCygne has been able to reduce the time between oil changes, due to water and particulate contamination, by 25 to 30 percent. A color indicator on the filter/breather indicates when the filter/breather needs replacing.
Oil Analysis Saves $1 Million
TexasGenco, a subsidiary of CenterPoint Energy, owns and operates the Cedar Bayou generating plant. The plant, located near Baytown, TX, has three gas/fuel oil-fired super-critical generating units with an aggregate net generating capacity of 2,260 MW. Unit 1, a 770 MW unit, was placed in service in December 1970. Units 2 and 3, both 770 MW units, went into commercial operation in March 1972 and December 1974, respectively.
In 1997, the plant initiated an oil analysis program for its critical equipment, using plant operators and laboratory personnel to take random oil samples. The samples were then sent to an independent off-site laboratory for testing and analysis. However, because of inconsistencies in the testing and analysis of the samples, Cedar Bayou decided in 1998 to assign a fulltime employee to oversee the program and make sure that the samples were being taken on a regular basis.
Once TexasGenco was convinced of the benefits and the value of the oil analysis program, they formed a central analysis group and installed an on-site testing facility at the Cedar Bayou plant. On-site analysis focused on determining particulate levels, moisture content and viscosity of the plant’s lubricating oils. The cost of equipment and personnel training at the plant was approximately $25,000-$30,000. After installing the on-site equipment, sampling of the oil was done on a time elapsed basis and not randomly as they had done previously.
In January 2001, as part of the normal startup routine on Unit 2, a sample of the main turbine lubricating oil return was taken. The subsequent analysis to determine the particulate count, absolute viscosity and moisture content revealed a dramatic increase in particulate count from a normal level of 14/11 to 22/19. This was well over the critical alarm limits. (The first number corresponds to the number of particles in the five-micron range in one milliliter of oil and the second number corresponds to the number of particles in the 15-micron range. The lower the numbers, the cleaner the oil.) A second sample verified the earlier results.
Further testing revealed a large concentration of metal particles. However, due to the absence of non-ferrous particles in the oil, it was concluded that the problem was not due to a bearing failure. Turbine vibration and temperature sensors also did not indicate any bearing problems. After discussing the problem, plant management decided to immediately begin filtering the oil in the reservoir to prevent any damage to the turbine’s bearings. Using a skid-mounted filter and a six-micron polishing filter, the plant was able to remove the particulates in the lubricating oil.
In parallel with filtering the oil the plant operators continued to investigate the reason for the oil contamination. During the subsequent investigation the plant operators heard a bumping sound coming from the steam turbine turning gear area that, in their opinion, indicated that the turning gear was not locked in the disengaged position. After manually operating the turning gear, the bumping disappeared.
From this it was concluded that the turning gear had not fully disengaged and thus was causing the turning gear pinion to strike against the shaft bull gear. After making sure that the turning gear problem had been rectified, management agreed that inspection of the turning gear could wait until the next scheduled outage. When the unit was eventually taken off-line, the ensuing turning gear inspection revealed that the pinion was damaged and needed replacing. However, because a replacement gear was not immediately available, operation continued until a new pinion gear arrived on-site.
“It is easy to see that not catching a problem of this type could lead to millions of dollars worth of damage and downtime,” says Robert Walker, oil specialist, Cedar Bayou power plant. It is estimated that catching the problem before it completely failed saved the plant $500,000 to $1 million in downtime, materials and man-hours. It also prevented damage to other parts of the steam turbine. In addition to preventing catastrophic failures, the oil analysis program has allowed the plant to extend the life of the lubricating oil.
Why Use Synthetic Oils?
Although synthetic oils are more expensive than mineral oils, they have better resistance to aging and a longer service life. Depending on the base oil the time between oil changes is three to five times that of mineral oils, assuming equivalent operating thermal conditions.
Synthetic oils have improved thermal and oxidation resistance, improved viscosity/temperature characteristics, better low temperature properties and lower evaporation properties. In some instances synthetic oil has less tendency to form residues. Some disadvantages of synthetic oil, however, include higher cost and incompatibility with certain metals. According to Ted Naman, technical coordinator, ConocoPhillips, a significant benefit of synthetic oils is they perform better than conventional oils at very low temperatures.
At the Cedar Bayou power plant, synthetic oils are now used in one of the plant’s cooling towers and in the bearings on the fuel forwarding pumps on Unit 3 and the air pre-heaters on all three units. According to Walker, the major reason for utilizing synthetic oil in the air pre-heaters is the synthetic oil’s better oxidation stability at higher temperatures. Even though the synthetic oils used are more expensive, in the particular applications where they are utilized at Cedar Bayou, they have proven to be more cost effective than mineral oils, says Walker.
Because of their higher operating temperatures, aero-derivative gas turbines invariably use synthetic oils. Rolls Royce only allows synthetic oils to be used in their aero-derivative engines. Because of the extended life of synthetic oils, power plants are able to reduce downtime and maintenance cost considerably, says Naman.
The physical and chemical properties of oils have a great effect on lubrication. Viscosity, the property that causes oil to resist flow, is a significant factor in predicting the performance and fatigue life of rolling element bearings and gears. As the temperature of the oil increases the viscosity decreases. Likewise, as the oil temperature decreases it increases the viscosity. Pressure also increases the viscosity of oil.
Lubricating oil contamination is a major problem and if not controlled can lead to catastrophic failures of power plant equipment. Some of the contaminants in lubricating oils include gases, liquids such as water, and solid particulates. Gases decrease viscosity and promote foaming that in turn reduces the lubricating properties of the oil.
To minimize undesirable properties in the oil, additives are often used. Antioxidant additives are added to increase the life and storage of oils, while antifoam additives help to reduce foaming. Other additives include: anti-wear, extreme pressure, pour point depressants, corrosion and rust inhibitors, de-mulsifiers, detergents and dispersants.
Depending upon the application, additives can comprise less than one percent to more than 25 percent of the composition of a formulated lubricant. Unfortunately, oil additives are expensive and as a result increase the final cost of the lubricating oil.