Come up to speed on turbine fluids and their care at the 7F Users Group meeting

The 2019 annual conference of the 7F Users Group, May 20-24, at the Renaissance Schaumburg (Ill), provides an opportunity to update your knowledge on turbine lube and hydraulic fluids—both their selection and maintenance. As of April 1, three lubricant suppliers had committed to participating in the vendor fair along with six aftermarket services providers offering fluid inspection, analysis, and reconditioning services.

Consider visiting with the following business partners of CCJ ONsite while at the expo:

A suggestion: Take a few minutes to think about what you need to know about turbine fluids and their care to help avoid operational hiccups at your plant. Arrive in Schaumburg prepared to maximize the benefits of participating in the world’s largest independent user group serving owner/operators.

In turbine lubrication, like most areas of power-generation technology, there are no panaceas, no one-size-fits-all solutions. You have to do your homework to determine the best lubricant for your situation and the best way to manage its condition to assure maximum life. Talk to colleagues and subject-matter experts, read articles from back issues of CCJ (the search function at www.ccj-online.com is convenient for this purpose), scour user-group websites for back presentations, etc.

There may be no better way to learn about what works and what doesn’t, in given situations, than from colleagues. Consider the following, shared by the operations manager (formerly the maintenance manager) at a plant with two 2 × 1 7FA-powered combined cycles. This user has been at the facility since before it began commercial operation in 2002, making his perspective and experience particularly valuable in an industry challenged by retirements of its most skilled personnel and high turnover rates in plant staffing.

The lubricant selected for all four gas turbines at the subject plant, which today start about 100 times during a typical year and run about 5000 hours, was a popular mineral-based product provided by a leading oil company that was said to meet or exceed the requirements of all turbine OEMs. “Exceptional” thermal/oxidation resistance was another claim.

IGV issues surfaced in 2004 with varnish inhibiting proper operation of the inlet guide vanes during cold-weather starts. First “fix” was to install heat tracing on all oil lines to keep the varnish in solution.

In 2007, the plant took a more aggressive approach to resolving the problem: Sump was pumped and flushed. All “sorts” of gunk was removed, the user recalled. Refill was with ACT’s EcoSafe®-TF-25. This was the second commercial use of the TF-25 product, the first being at a nearby plant a couple of weeks earlier. Budgetary considerations militated against converting the site’s other gas turbines to TF-25.

Bear in mind that while heat tracing is not necessary on fluid-system components handling TF-25, it continues in service at this plant for mineral oils.

During a major inspection in fall 2018, the TF-25 sump was revisited and found in near-pristine condition after a service run of more than 47,000 hours and more than 1600 starts (photo). Turbine bearings and seals also were inspected during the outage and found clean.

What makes this plant a particularly good one for gaining objective experience on lubrication practices is that the sister unit of the gas turbine running with TF-25 continues to operate on the original oil supplied with both engines. Varnish issues have been mitigated by use of a commercial skid-mounted system that relies on resin technology to remove dissolved varnish precursors.

The two turbines serving the second combined cycle at the site were switched about 10 years ago to an alternative mineral oil (different brand than the original oil) as part of a beta test. The units have operated since without a varnish removal system.

However, varnish recently has been viewed as a possible concern and plant personnel are considering treatment with EcoSafe®-Revive™ to extend the productive lifetime of the oil. In the ops manager’s view, “it does as advertised” based on his research.

How long does it last? One question on the minds of many users evaluating lubricants: “What’s the long-term performance?” To get that answer for TF-25, 10-year-old PAG sample was entered into an industry-wide study conducted by Laborelec (Sidebar 1), which developed a test protocol involving six thermal cycles to simulate turbine operation (Sidebar 2). Twenty turbine fluids were evaluated on a level playing field to help turbine owner/operators make better decisions regarding lubricant selection and treatment.

1. Who is Laborelec?

Laborelec, today officially known as ENGIE Laborelec, is one of the world’s leading centers for research on electric-power technologies. Its objectives are similar to those of EPRI, familiar to most subscribers. Laborelec was founded in France in 1962, a decade earlier than the Electric Power Research Institute launched in the US.

ENGIE is a French multinational electricity provider claimed to be the world’s leading independent power generator with more than 115 GW installed and another 10 GW under construction. The company, formed as GDF Suez in 2008 with the merger of Gaz de France and Suez, was renamed ENGIE in 2015. It has more than 150,000 employees and business interests in more than 50 countries.

Laborelec, one of nearly a dozen R&D centers under the ENGIE umbrella, has 240 highly specialized engineers and technicians working across the electricity value chain. It is organized as a cooperative with ENGIE and independent grid operators as shareholders.

Key takeaways from the tests included the following:

    • Ten-year-old PAG bested new conventional and thermally stable mineral oils in RPVOT tests.

    • Ruler results showed “used” PAG was at least as good as all new fluids evaluated.

    • Acid number for TF-25 was relatively constant across the six thermal cycles and below the maximum recommended limit.

    • Membrane Patch Colorimetry results were 8 or less.

    • Fluid density remained relatively constant across the six test cycles.

Test results thus far indicate TF-25 may last 30 years, or more. This means some plants might not have to change their turbine oils before decommissioning.

2. How Laborelec evaluates alternative turbine fluids

The first step for assuring top performance from your turbine fluid is to choose the optimal product for your engine based on OEM recommendations and the plant’s operating profile. Your experience, and that of industry colleagues, should be factored into the selection process, of course.

It also is necessary to implement a proper maintenance program to maintain your turbine fluid in good condition throughout its operating lifetime. However, it’s important to re-evaluate this program regularly—annually, perhaps—and factor in operational changes that can influence fluid condition—such as a shift from baseload to peaking service.

Laborelec’s experts point out that once a turbine fluid enters service, it starts oxidizing, a process that promotes the formation of degradation products. The solubility of degradation products, in the case of mineral oils, is temperature-dependent: the lower the lube-oil temperature the more likely the degradation products are to plate out on turbine parts and impede operation of servos, inlet guide vanes, etc.

Backgrounder. The stress experienced by a turbine lubricant contributes significantly to the ageing of petroleum oil, causing the non-polar fluid to oxidize. However, the resulting byproducts of decomposition are polar and insoluble in the base oil; they come out of solution as “varnish.” By contrast, leading alternative turbine fluids—such as polyalkylene glycol (PAG)—are polar in nature and their byproducts of decomposition are infinitely soluble in the base stock. The bottom line: No varnish is produced.

Laborelec engineers and chemists were of the opinion that information important to decision-making on the selection of an appropriate turbine fluid for a given plant was not provided on the manufacturer’s technical data sheets. For example, a prospective customer might not know the service conditions considered in the development phase of turbine fluids of interest. All users had, basically, were some results of various ASTM tests.

In 2012, the research organization began work on the design of a test protocol to compare different turbine oils/fluids on a level playing field. First step was to meet with lube-oil suppliers, maintenance companies, and turbine OEMs to discuss their test specs.

This effort was the foundation for the development, in early 2014, of the “Laborelec Cyclic Turbine Oxidation Test.” The LCTO test protocol combines the “Standard Test Method for Oxidation Characteristics of Inhibited Mineral Oils (ASTM D943)—a/ka/ Turbine Oxidation Stability Test (TOST)—and the dry TOST developed by Mitsubishi Heavy Industries.

Test program. The results below obtained from the testing of 20 turbine fluids were interpreted based on the standard practice used for in-service monitoring of mineral turbine oils for steam and gas turbines (ASTM D4378-13 and VGB-S-416-00-2014-08-EN). The fluids were grouped into four categories for comparison purposes by turbine owner/operators and test participants—this to keep information on specific products anonymous. The categories: mineral oil (MO), thermally stable mineral oil (TSMO), high-performance mineral oil (HPMO), and PAG.

Lube-oil suppliers, of course, have access to their data, enabling a comparison of their fluids to the group performance of competitive products regarding speed of degradation and the formation of degradation products.

An important aspect of the test protocol is that each fluid was stressed to simulate real-world operating conditions. This was done by thermally cycling the test samples. For the purposes of the LCTO test, samples were heated to a nominal 250F and held at that temperature for four days. Sample temperature then was reduced to 77F and held there for three days. This cycle was repeated six times. Data were taken for the fluid when new and after each cycle.

Results illustrating fluid condition were divided into three zones as illustrated in the figure:

    • Normal (white field), no specific actions are recommended and the fluid can remain in service.

    • Follow-up (yellow field), beyond the normal acceptable value. At this stage, the first indicators of oil oxidation become visible. Corrective actions are necessary, but oil generally can remain in service provided monitoring is increased.

    • Out of spec (red field), indicates an on-going severe oxidation process. Immediate response typically includes specific maintenance actions to protect equipment from mechanical problems. Fluid replacement should be considered.

More detail is provided in the thumbnails below for each parameter included in the evaluation:

Color, ASTM D1500. Sample color darkens as the fluid degrades.

Fluid density at 20C, ASTM D4052, increases significantly.

Viscosity at 40C, ASTM D7072, exceeds ±10% ISO-VG class.

Acid number, ASTM D-664. Most rust inhibitors used in the formulation of new turbine oils are acidic and contribute to the acid number of the fluid. As mineral oils age, they form solids that precipitate out the amines, making the acid number rise. A maximum increase of 0.2 to 0.4 mg KOH/g of initial value is tolerable. Above 0.4, known as the condemning limit, is detrimental.

The best performers regarding acid number are the TSMOs which had acceptable acid numbers through six cycles. Mineral oils jumped out of spec after two cycles, while PAG received a caution flag after two cycles; HPMO went yellow in the first cycle but road through six cycles without hitting the condemning limit.

Note that ASTM D4378 refers to condition monitoring of in-service mineral oils and can create false-positive results or require modification for non-petroleum chemistries. For example, the Total Acid Number for PAG starts at approximately 0.11 with a condemning limit of 2.0. The result after the sixth cycle on TF-25 of <0.40 represents a favorable result well within specification.

RPVOT, ASTM D2272, is a controlled, accelerated oxidation test to measure the remaining level of anti-oxidants in lube oil. When the RPVOT value (units are minutes) of the oil drops below 25% of its initial value, fluid replacement should be considered.

PAG was one of the two best performers in this category. After six cycles, its RPVOT was still 75% of the new-fluid value; HPMO stole the show, retaining 95% of its anti-oxidant package at the end of the test period. Mineral oils “failed” after the second cycle, TSMOs after the fourth.

Ruler, ASTM D6971. Like RPVOT, oil change is recommended when 25% of the initial ruler value is reached. PAG got the yellow flag after two cycles when its Ruler value hit 50%. However, it continued on under the caution flag until testing was complete. Mineral oil was out of spec before the first cycle was completed; TSMO and HPMO each lasted four cycles.

MPC, a/k/a Membrane Patch Colorimetry, ASTM D7843, is a varnish potential test that identifies the propensity of a lubricant to form solid deposits, thereby helping maintenance professionals avoid catastrophic failures. MPC values greater than 30 require immediate attention.

Other tests included filtration (0.8 µm), as specified in ASTM D4055, and deposition (on glass tubing and catalyst coil), both with “dangerous zones” beginning at 100 ppm.

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