Converting to Condition-Based Oil Changes - Part II

Raymond Thibault, ExxonMobil Lubricants & Specialties
Tags: oil analysis

Note from Editor
This paper was presented at Practicing Oil Analysis 2000 International Conference and Exhibition and was originally published in the proceedings. Because of the popularity of this topic, we chose to reprint this paper in the magazine. Due to length, it has been divided into two parts. This article is the second part of a paper published in the January - February 2001 issue of Practicing Oil Analysis magazine.

Condition Monitoring in the Power Generation Industry
The power generation industry uses both gas and steam turbines to generate electricity. Reservoirs can be as large as 20,000 gallons (76,000 L) and lubricant life can range from seven to thirty years. Therefore, oil condition monitoring is important to maintaining turbine reliability while extending oil life. An effective oil analysis program is needed to achieve this goal. Most turbine oils have rust and oxidation inhibitors and ISO viscosity grades of 32 or 46.

Gas and steam turbines are routinely sampled every 1,000 hours. Because demulsibility and oxidation life is important, RPVOT and demulsibility tests should be run every three years. As the oil approaches the end of its useful life, these tests should be conducted yearly. Water contamination is usually removed using centrifuges or dehydrators. Sweetening oil to improve the RPVOT number is also performed. If the oil loses its ability to shed water, in some cases, a demulsifier package can be added. A lubricant supplier should be consulted. Adding a demulsifier package may not be successful due to the complexity of blending certain additives and the degree of difficulty in certain reservoirs.

EHC synthetic phosphate esters are used in power generating plants because they are a fire resistant hydraulic fluid. Below are a few notes regarding EHC phosphorus ester fluids:

  • Many utilities keep their EHC water level below 300 ppm through onsite monitoring and conditioning.

  • When EHC fluids are used, leaks in the turbine oil are detected by measuring the phosphorus level. The oil is condemned if it has 4,000 ppm of phosphorus. At that level, problems occur when the ester attacks the paint in the reservoir.

  • EHC synthetic fluids are condemned if the AN exceeds 0.2 over the original value (typically 0.03). This is the key criterion for the fluid. Cleanliness is recommended not to exceed an ISO 18/15 and water not to exceed 2,000 ppm.

See Table 1

Case History # 4 - Timing Turbine Oil Change
A complete evaluation was conducted at a major Rocky Mountain utility on 26 large turbine reservoirs. Most of the oil had been in use for more than 15 years. The newest oil was five years old; the oldest was 25 years. This range revealed how the turbine oil deteriorated over the years.

  • Depletion of oxidative life and poor demulsibility characteristics would be the major reasons for condemning these oils.

  • Lack of demulsibility is not a problem unless water contamination is present.

  • AN increases significantly as RPVOT decreases until a certain condemning low level is reached.

See Table 2

Condition Monitoring for Heavy Duty Engine Oils
Oil analysis for diesel commercial vehicles is performed after failure to measure engine wear and contamination. Oil analysis conducted for off-road Mobile fleets is extensive. However, many large on-highway fleet operators change oil at predetermined intervals and don’t conduct oil analysis, although this trend is changing. Significant savings can be achieved by extending drain intervals on a large fleet. The following example is a program developed by a major lubricant supplier and illustrates the magnitude of the savings they experienced:

A small over-the-road fleet operator with 150 trucks decided to increase oil life by upgrading the lubricant and monitoring the lubricant’s condition through oil analysis. Samples were taken every 3,000 to 4,000 miles (4,800 - 6,400 km) until a reliable drain interval was established. Oil analysis was performed at the new drain intervals to determine wear metals and oil condition. Drain intervals were extended from 20,000 to 36,000 miles (32,000 - 58,000 km), resulting in savings of $28,000 per year.

In the next few years, major changes in emission standards will occur. New generation oils will be required to meet the stringent PC-9 requirements to be introduced in 2002. Retarded timing and exhaust gas recirculation will result in much higher soot loading in the oil, some predict as much as 10 percent. Also, higher BN will be required because of the acidic exhaust gas being recycled back into the engine to be burned. Higher temperatures will require better oxidation control in the oil. This will significantly change diesel engine oil formulations along with the condemning limits. The following limits are based on current engine requirements.

Condemning Limits for Diesel Engine Oils
These are general guidelines. Check with the OEM for specific guidelines. Viscosity change and coolant contamination are major reasons for condemning oil.

See Table 3

Condition Monitoring for Natural Gas Engine Oils
Natural gas engines are used to operate natural gas compressors, electric generators or cogeneration plants. These engines can be very large, approaching 10,000 hp with up to 16 power cylinders. Oil sump capacities usually range from 25 to 1,500 or more gallons. Most of these engines are turbocharged and have two- or four-stroke design. The compressor can either be joined by a crankshaft to the engine or be an integral type. The same oil is usually used to lubricate the compressor bearings and the engine. Oil is routinely changed only in four-cycle engines where sump capacity is approximately 100 gallons (378 L). Two-cycle engines burn oil and have much bigger sumps. Oil is changed only if major contamination occurs.
The formulation for gas engine oil is different from diesel engine lubricants. The cleaner burning fuel does not generate high soot levels, but instead can contribute to more oxidation and nitration.

Condemning Limits for Natural Gas Engine Oils
High oxidation, nitration and coolant leaks are the major factors in condemning the oil. Once oxidation and/or nitration reach a high level, for example: 30 absolute units/cm, the oil viscosity can increase dramatically. Viscosity does not normally decrease unless the VI improver shears.

See Table 4

Case History # 5 - Extended Gas Engine Lube Drains
A major Texas gas producer that had not yet implemented an oil analysis program, wanted to extend its drain interval from two to three months. The company selected 10 four-cycle engines with sump capacities of 100 gallons (378 L). It then successfully extended the oil life from two to three months using oil analysis data resulting in the following economics for each engine:

200 gallons/year @ $4.00/gallon = $800
Manpower savings two hours @ $30/hour = $120
Oil disposal costs @ $1.00/gallon = $200
Total saved per engine/year = $1120

This gas producer has adopted oil analysis and the longer drain interval for more than 300 four-cycle engines. This program produces annual savings of more than $300,000.


  • Oil analysis has achieved significant savings in oil life extension. Even greater savings have resulted from proactively detecting potential equipment problems caused by poor oil quality.

  • Trends should be monitored when measuring oil condition. If a significant change occurs in a sample, it should be retested to determine the validity of the sample and/or the testing procedure.

  • Limits are used as guidelines only. Lubricant suppliers and OEMs should be consulted to establish specific limits by lubricant and equipment type. The use of many different sources, both internally and externally, helped develop the limits presented in this article. Limits are based on oil and equipment type.

  • Many industrial companies use oil analysis and rely on the laboratory for recommendations. The more progressive plants understand oil analysis and the meanings of key tests and limits as it relates to their equipment. These plants also use results to make decisions on maximizing oil life without compromising equipment.

  • Industrial plants condemn oil mainly because of contamination. When both water and particulates cannot be removed economically.

  • Engine oils are normally condemned because of oxidation and coolant leaks.

  • Both caution and critical limits for industrial oils are presented in this paper while only critical condemning limits for engine oils are established. Industrial oils can often be reconditioned by removing contaminants through centrifuging, filtration and vacuum dehydration. Caution limits give early warnings to potential problems, allowing the plant to plan a more accurate reconditioning schedule.

  • Additive depletion, which is difficult to detect, is a less common reason for condemning oil. However, it occurs in paper machines where the lubricant is exposed to water. This can have a washing effect on the detergent package. Oxidation inhibitors in turbine oils deplete after many years. Because these inhibitors are ashless, their depletion is measured by using the RPVOT test that assesses the remaining useful oxidative life of the oil.

  • Oxidation is normally measured in industrial oils by AN and viscosity increase. New oils have an initial AN; therefore, the increase over the initial value measures oxidation.

  • FTIR is another effective method to measure oxidation, but only if referenced to a new oil sample. It cannot be used
    for synthetic oils containing esters because oxidation peaks and ester peaks are at the same frequency (wave number). Natural gas engine oils use FTIR as the primary test to measure oxidation and nitration.

  • Many different tests have been discussed, but onsite observance of oil appearance is still important and must not be neglected.


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