How to Select and Service Turbine Oils

James B. Hannon, ExxonMobil
Tags: turbine lubrication, oil flushing, contamination control

The question “How long will this turbine oil last?” should be answered with the sound engineering response of “it depends.”

Turbine oil suppliers can give fairly wide-ranging estimates, say 5 to 15 years, in gas turbine applications. Any attempt to create a more exact estimate requires consideration of so many variables that it becomes somewhat useless. Water, heat, contamination, operating hours and maintenance practices will have a significant impact on turbine oil longevity.

There is no denying that properly tested and maintained, higher quality turbine oils will provide longer life than poorly tested and maintained, lower quality products. Following is a discussion of new turbine oil performance characteristics that will promote longer, trouble-free service.

Turbine

More than 100 tons of steel, rotating at 3600 rpm, is supported by plain bearings on a cushion of oil that is thinner than a human hair. In power plants around the world, the same fluid dynamics take place day-in and day-out without much notice.

Lost revenue at seasonal peaks can be counted in millions of dollars. An average utility sells electricity for about $50/MW hr during nonpeak periods, and as much as $1,000/MW hr during peak periods. Poor selection and maintenance of turbine oil can result in production losses exceeding $500,000 per day.

When selecting a turbine oil for steam, gas, hydro and aero-derivative turbines, oil supplier services and commitment to the customer should be evaluated as part of the selection process.

Have the Right Tool for the Job

It is important to have an understanding of the physical and chemical characteristics of turbine oils compared to other lubricating oils before embarking upon the selection process.

Steam, gas and hydro turbines operate on a family of lubricating oils known as R&O oils (Rust & Oxidation inhibited oil). Turbine equipment geometry, operating cycles, maintenance practices, operating temperatures and potential for system contamination present unique lubricating oil demands versus other lubricating oils like gasoline and diesel engine applications.

Utility steam and gas turbine sump capacities can range in size from 1,000 to 20,000 gallons, which drives the economic incentive for a long-life lubricating oil. Low turbine oil makeup rates (approximately five percent per year) also contribute to the need for high-quality, long-life lubricants. Without significant oil contamination issues, turbine oil life is primarily dictated by oxidation stability.

Oxidation stability is adversely affected by heat, water, aeration and particulate contamination. Antioxidants, rust inhibitors and demulsibility additives are blended with premium quality base stock oil to extend oil life. Lube oil coolers, water removal systems and filters are installed in turbine lubrication systems for the same purpose.

Unlike most gasoline and diesel engine oil applications, turbine oil is formulated to shed water and allow solid particles to settle where they can be removed through sump drains or kidney loop filtration systems during operation. To aid in contaminant separation, most turbine oils are not additized with high levels of detergents or dispersants that clean and carry away contaminants. Turbine oils are not exposed to fuel or soot and therefore do not need to be drained and replaced on a frequent basis.

Recommended Performance Characteristics of Turbine Oil Vary by Application Steam Turbines

A well-maintained steam turbine oil with moderate makeup rates should last 20 to 30 years. When a steam turbine oil fails early through oxidation, it is often due to water contamination. Water reduces oxidation stability and supports rust formation, which among other negative effects, acts as an oxidation catalyst.

Varying amounts of water will constantly be introduced to the steam turbine lubrication systems through gland seal leakage. Because the turbine shaft passes through the turbine casing, low-pressure steam seals are needed to minimize steam leakage or air ingress leakage to the vacuum condenser.

Water or condensed steam is generally channeled away from the lubrication system but inevitably, some water will penetrate the casing and enter the lube oil system. Gland seal condition, gland sealing steam pressure and the condition of the gland seal exhauster will impact the amount of water introduced to the lubrication system.

Typically, vapor extraction systems and high-velocity downward flowing oil create a vacuum which can draw steam past shaft seals into the bearing and oil system. Water can also be introduced through lube oil cooler failures, improper powerhouse cleaning practices, water contamination of makeup oil and condensed ambient moisture.

In many cases, the impact of poor oil-water separation can be offset with the right combination and quality of additives including antioxidants, rust inhibitors and demulsibility improvers.

Excess water may also be removed on a continuous basis through the use of water traps, centrifuges, coalescers, tank headspace dehydrators and/or vacuum dehydrators. If turbine oil demulsibility has failed, exposure to water-related lube oil oxidation is then tied to the performance of water separation systems.

Heat will also cause reduced turbine oil life through increased oxidation. In utility steam turbine applications, it is common to experience bearing temperatures of 120ºF to 160ºF (49ºC to 71ºC) and lube oil sump temperatures of 120ºF (49ºC). The impact of heat is generally understood to double the oxidation rate for every 18 degrees above 140ºF (10 degrees above 60ºC).

A conventional mineral oil will start to rapidly oxidize at temperatures above 180ºF (82ºC). Most tin-babbited journal bearings will begin to fail at 250ºF (121ºC), which is well above the temperature limit of conventional turbine oils. High-quality antioxidants can delay thermal oxidation but excess heat and water must be minimized to gain long turbine oil life.

Gas Turbines

For most large gas turbine frame units, high operating temperature is the leading cause of premature turbine oil failure. The drive for higher turbine efficiencies and firing temperatures in gas turbines has been the main incentive for the trend toward more thermally robust turbine oils. Today’s large frame units operate with bearing temperatures in the range of 160ºF to 250ºF (71ºC to 121ºC).

Next-generation frame units are reported to operate at even higher temperatures. Gas turbine OEMs have increased their suggested limits on RPVOT - ASTM D2272 (Rotation Pressure Vessel Oxidation Test) and TOST - ASTM D943 (Turbine Oil Oxidation Stability) performance to meet these higher operating temperatures.

As new-generation gas turbines are introduced into the utility market, changes in operating cycles are also introducing new lubrication hurdles. Lubrication issues specific to gas turbines that operate in cyclic service started to appear in the mid-1990s. Higher bearing temperatures and cyclic operation lead to fouling of system hydraulics that delayed equipment start-up.

Properly formulated hydrocracked turbine oils were developed to remedy this problem and to extend gas turbine oil drain intervals. Products such as Exxon Teresstic GTC and Mobil DTE 832 have demonstrated excellent performance for almost five years of service life in cyclically operated gas turbines where conventional mineral oils often failed in one to two years.

Hydro Turbines

Hydro turbines typically use ISO 46 or 68 R&O oils. Demulsibility and hydrolytic stability are the key performance parameters that impact turbine oil life due to the constant presence of water. Ambient temperature swings in hydroelectric service also make viscosity stability, as measured by viscosity index, an important performance criterion.

Aero-Derivative Gas Turbines

Aero-derivative gas turbines present unique turbine oil challenges that call for oils with much higher oxidation stability. Of primary concern is the fact that the lube oil in aero-derivative turbines is in direct contact with metal surfaces ranging from 400ºF to 600ºF (204ºC to 316ºC). Sump lube oil temperatures can range from 160ºF to 250ºF (71ºC to 121ºC).

These compact gas turbines utilize the oil to lubricate and to transfer heat back to the lube oil sump. In addition, their cyclical operation imparts significant thermal and oxidative stress on the lubricating oil. These most challenging conditions dictate the use of high purity synthetic lubricating oils. Average lube oil makeup rates of .15 gallons per hour will help rejuvenate the turbo oil under these difficult conditions.

Current technology turbine oils for land-based power generation turbines are described as 5 cSt turbo oils. Aero-derivative turbines operate with much smaller lube oil sumps, typically 50 gallons or less. The turbine rotor is run at higher speeds, 8,000 to 20,000 rpm, and is supported by rolling element bearings.

Synthetic turbo oils are formulated to meet the demands of military aircraft gas turbo engines identified in Military Specification format. These MIL specifications are written to ensure that similar quality and fully compatible oils are available throughout the world and as referenced in OEM lubrication specifications.

Type II turbo oils were commercialized in the early 1960s to meet demands from the U.S. Navy for improved performance, which created MIL - L (PRF) - 23699. The majority of aero-derivatives in power generation today deploy these Type II, MIL - L (PRF) - 23699, polyol ester base stock, synthetic turbo oils. These Type II oils offer significant performance advantages over the earlier Type I diester-based synthetic turbo oils.

Enhanced Type II turbo oils were commercialized in the early 1980s to meet the demands from the U.S. Navy for better high-temperature stability. This led to the creation of the new specification MIL - L (PRF) - 23699 HTS. In 1993, Mobil JetOil 291 was commercialized as the first fourth-generation turbo oil to satisfy present and advanced high temperature and high load conditions of jet oils. Improvements continue to be made in turbo oil lubricant technology.

Generator bearing sets typically use an ISO 32 R&O or hydraulic oil. The lower pour points of a hydraulic vs. an R&O oil may dictate the use of a hydraulic oil in cold environments.

Writing a Turbine Oil Procurement Standard

Steam, gas and hydro turbine oils are a blend of highly refined or hydroprocessed petroleum base oils, usually ISO VG 32 and 46 or 68. Lubricant suppliers have developed turbine oils to meet the varying demands of turbines in propulsion and power generation applications.

These formulations were developed to meet turbine OEM specifications. Many turbine OEMs have moved away from specific turbine oil brand name approvals due to enhanced technologies in their turbines and corresponding improvements in turbine oils. OEMs have identified suggested or recommended lube oil performance test criteria and typically stipulate that an oil known to perform successfully in the field may still be used even if all recommended values have not been satisfied.

Industry standard lube oil bench tests can provide great insight into the performance and life expectancy of turbine oils. However, turbine OEMs and oil suppliers generally agree that past successful performance of a particular oil under similar conditions is the best overall representation of quality and performance.

Regardless of the type or service of a turbine oil, the quality of the base stocks and additive chemistry will be a major factor in its longevity. High-quality base stocks are characterized by higher percentage saturates, lower percentage aromatics, and lower sulfur and nitrogen levels. The performance of additives must be extensively tested. They must also be blended into the oil in a tightly controlled process.

The key to a superior turbine oil is property retention. Some turbine oil formulations have been found to present good lab test data, but can experience premature oxidation because of additive dropout and base stock oxidation.

Again, lube oil laboratory analysis can support your efforts to determine turbine oil longevity, but direct field experience should take precedence. Note, turbine oil suppliers will offer typical lube oil analysis data to help assess predicted performance. Typical data is used because lubricating oils vary slightly from batch to batch because of minor base stock variations.

Utility steam and gas turbine oils can be either conventional mineral-based (Group 1) or hydroprocessed (Group 2). High-quality conventional mineral-based oils have performed well in both steam and gas turbine service for more than 30 years. The trend toward higher efficiency, cyclically operated gas turbines has spurred the development of hydroprocessed, Group 2, turbine oils.

Most hydroprocessed turbine oils will have better initial RPVOT and TOST performance than conventional turbine oils. This oxidation stability performance advantage is suited for heavy-duty gas turbine applications.

The oxidation performance advantages of a hydroprocessed turbine oil may not be necessary in many less demanding steam and gas turbine applications. Conventional mineral-based oils are known to have better solvency than hydroprocessed oils which can provide better additive package retention and increased ability to dissolve oxidation products that could otherwise potentially lead to varnish and sludge.

Compatibility testing between turbine oil brands should also be addressed when writing a turbine oil specification for systems not available for a complete drain and flush. Clashing additive chemistries or poor in-service oil quality may prohibit the mixing of different and incompatible turbine oils. Your oil supplier should provide compatibility testing to confirm suitability for continued service.

This testing should address the condition of the in-service oil compared to various possible blends with the proposed new oil. The in-service oil should be tested for suitability for continued service. Then a 50/50 blend should be tested for oxidation stability (RPVOT ASTM D2272), demulsibility (ASTM D1401), foam (ASTM D892, Sequence 2) and the absence of additive package dropout as witnessed in a seven-day storage compatibility test.

Turbine Lube Oil System Flushing

Turbine lube oil system flushing and initial filtration should be addressed in conjunction with the selection of the turbine oil. Lubrication system flushing may be either a displacement flush after a drain and fill, or a high velocity flush for initial turbine oil fills. A displacement flush is performed concurrently during turbine oil replacement and a high velocity flush is designed to remove contaminants entering from transport and commissioning a new turbine.

Displacement flushes using a separate flush oil are done to remove residual oil oxidation product that is not removed by draining or vacuum. A displacement flush is conducted by utilizing lubrication system circulation pumps without any modification to normal oil circulation flow paths, except for potential kidney loop filtration.

This flush is typically done based on a time interval vs. cleanliness (particle levels) to facilitate the removal of soluble and insoluble contaminants that would not typically be removed by system filters.

Most turbine OEMs offer high velocity flushing and filtering guidelines. Some contractors and oil suppliers also offer flushing and filtering guidelines. Often during turbine commissioning, these guidelines are scaled back to reduce cost and time. There are common elements of a high-velocity flush that are generally supported by interested parties. There are also some procedural concerns that may differ and should be addressed on a risk vs. reward basis.

Common elements of mutual agreement in high-velocity flushing are as follows:

Possible supplemental or alternative elements of a high-velocity flush are as follows:

Up-front planning and meetings with construction, start-up, oil supplier and the end user should be scheduled in advance to build consensus on these flushing procedures.

A good practice for turbine oil performance documentation is to take a 1-gallon sample from the supply tank and then a second gallon sample from the turbine reservoir after 24 hours of operation. The recommended testing is consistent with turbine oil condition assessment testing:

Past experience, turbine OEM recommendations, customer testimonials and oil supplier reputation are key elements to be considered in the selection of a turbine oil. Proper initial selection of turbine oil and continued conditioned-based maintenance should set the stage for years of trouble-free service. In many plants, Murphy’s Law strikes at the worst time. This is when you will truly appreciate a turbine oil with superior performance characteristics and an oil supplier with extensive technical support.

References
1. AISE Association of Iron and Steel Engineers. (1996). The Lubrication Engineers Manual - Second Edition. Pittsburgh, PA.

2. Bloch, H. P. (2000). Practical Lubrication for Industrial Facilities. Lithburn, GA: The Fairmont Press.

3. Exxon Mobil Corporation. Turbine Inspection Manual. Fairfax, VA.

4. Swift, S.T., Butler D.K., and Dewald W. (2001).
Turbine Oil Quality and Field Applications Requirements. Turbine Lubrication in the 21st Century ASTM STP 1407. West Conshohocken, PA.

5. ASTM. (1997). Standard Practice for In-Service Monitoring of Mineral Turbine Oils for Steam and Gas Turbines ASTM D4378-97. Annual Book of ASTM Standards Vol. 05.01.