Aus der Praxis für die Praxis The Varnish Threat A sticking servo valve can cause a gas turbine to trip or shutdown in an instant. Taking power generation capacity off line is a costly event: • Lost generating capacity deprives the operator of revenue • Replacement power must be purchased at high spot rates • Unplanned maintenance expense is incurred • Intervals between costly scheduled maintenance are reduced Varnish has become an increasingly important issue in the gas turbine power generation industry because of 60 Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 PAG’s - Non-Varnishing Alternatives for the Power Generation Industry K. P. Kovanda, R. Schulze* * Kevin P. Kovanda, President American Chemical Technologies Inc., Fowlerville, MI, USA Rudolf Schulze, General Manager ACT/ Schulze & Partner, PGmbH, Eupen, Belgien Mineralöle haben die Energieerzeugungsindustrie von Anfang an dominiert. Jedochverursachten in den letzten 20 Jahren Ausfallzeiten, die mit Lackauftrag in Verbindung gebracht wurden kostspielige Verzögerungen in der Gasturbinenindustrie. Dieser Lackauftrag ist teilweise das Ergebnis des Verlusts der Polarität (Löslichkeit) des Schmieröls, da die großen Ölgesellschaften ihre Raffination aus Gruppe I nach Gruppe IV-Basismaterialien entwickelt. Dies äußert sich in schneller Lackauftrags-Bildung und in Additiv Tropfenbildung. Mit jeder „Turbinenschnellabschaltung“, die eine 4 Stunden Ausfallzeit und Neustart bedeuten, sind die Kosten für die Industrie enorm. Polyalkylenglykole (PAG) stellen die einzige Klasse von Basismaterial dar, die nicht die Bildung von Lackauftrag unterstützt. Sie sind dadurch einzigartig, dass aufgrund ihrer Polarität die Nebenprodukte der Oxidation in einem PAG löslich bleiben. Das macht sie zum idealen Kandidaten für den Einsatz in Gasturbinen. Diesers Artikel wird die Eigenschaften von Polyalkylenglykolen beschreiben, sie mit mineralölbasierten Turbineölen vergleichen und wie sie in den Hildebrandlöslichkeitsparameter passen. Die Arbeit wird sechs Jahre praktische Erfahrung in der Gasturbinen- Nutzung abdecken. Darüber hinaus wird eine zum Patent angemeldete PAG Flüssigkeit als Mineralöl-Enhancer für in Betrieb befindliche Turbinenöl behandelt. Sechs Jahre Praxiserfahrung in mehr als 36 Gasturbinen werden diskutiert. Der Erfolg von PAG Turbinenflüssigkeiten gipfelte im April 2013 mit der Revision des Papers von General Electric - „Lubricating Oil Recommendations for Gas Turbines with Bearing Ambients above 500 °F (260 °C) GEK32568 h“. Schlüsselwörter PAG, Polyalkylenglykol, Öl, Lackauftrag, Turbine, Zuverlässigkeit, Schmiermittel Varnish build-up in today’s more powerful gas turbines is the root cause behind many power station shutdowns and results in lost power generation capacity. A small amount of varnish can cause close tolerance servo valves to stick and automatically „trip“. A shut down, can cost tens of thousands for a partial load trip and the cost of a full load trip event can be even higher. The threat of varnish has become greater with the use of more highly refined turbine oils. Moving beyond petroleum, the field success of poly alkylene glycol (PAG)-based turbine fluids offers operators an alternative solution to the varnish problem. PAGbased turbine fluids eliminate varnish formation in large frame gas turbines with more than 7 years and 60,000 hours of service history. PAG’s improve starting reliability, eliminate unplanned shutdowns, ensure long-term fluid service, and extend equipment operating life, all while reducing maintenance and repair expense. We will explore the threat varnish poses to large frame gas turbines in power generation service, how conventional hydrocarbon oils contribute to varnish formation, and how PAG-based turbine fluids and a new PAG-based turbine fluid additive can enable operators to put an end to varnish. Keywords PAG, Poly alkylene glycol, Oil, Varnish, Turbine, Reliability, Lubricant Kurzfassung Abstract T+S_1_16 21.12.15 10: 54 Seite 60 Aus der Praxis für die Praxis trends in turbine design, evolving operating practices, and changes in hydrocarbon-based turbine oils. More Powerful Turbines Today’s gas turbines continue to get larger with greater power generating capacity. Their size and strength places more stress on the unit itself and the lubricating fluid that protects it. As a turbine oil ages, varnish formed in the lubricant can precipitate out of solution and adhere to critical surfaces. If varnish invades the tight tolerances surrounding servo valves, it can impede valve movement. When valve movement is hindered the default action is to trip the turbine. This results in unplanned downtime, lost megawatt production, and eventually high repair expense. The last chance filters installed ahead of servo valves offer no guarantee of protection. As shown in the accompanying photo, these filters often become plugged. Without continuous maintenance, loose varnish can break through and plug the valve. The result is an increase in the potential for varnish formation and other problems that affect the reliability of gas turbine operations. Solvency - Group I lubricant base stocks were not as susceptible to varnish formation. However, lubricants made from Group I oils were high in sulfur and aromatic content. As the products of the petroleum industry were held to standards of environmental protection and worker safety, their properties changed. The aniline point of various lubricant base stocks demonstrates the change in solvency properties of various types of lubricant base stocks. The aniline point of an oil is defined as the minimum temperature at which equal volumes of aniline (C 6 H 5 NH 2 ) and the oil are miscible (form a single phase when mixed). Lubricants with a lower aniline point have a greater ability to dissolve polar materials such as oxidation by-products and performance additives. Figure 1 shows the superior solvency of PAG base stocks compared to other base stocks. Changes in available base oils affected every industry where they were used. In automotive applications, equipment fill amounts are relatively small. Therefore, oil change intervals can be relatively short and the degradation of the oil is matched to other use factors. However, in the power generation industry, 3,100 gallons of lubricant are required to fill a GE7EA turbine and 6,100 gallons are needed to fill a GE7FA. Operators need the lubricant to last six to 10 years or more before the oil needs to be replaced. In practice, the more highly refined hydrocarbon oils cannot approach these service life expectations without significant maintenance support. Operators invest heavily in services to predict and remove varnish in turbine units as they attempt to extend the lifetime of their oil. Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 61 Last chance filter clogged with hydrocarbon varnish Figure 1: Aniline Points of Base Oils Demands of „Peaking“ Service Turbines that operate only during spikes in power, „peaker“ turbines, have system demands that are greater than base-load units. Start-and-stop peaker service is more stressful on the turbine and the lubricant. As the units cool down, oxidation by-products that have formed precipitate out of solution and form varnish. This directly impacts the reliability of startup the next time the turbine is needed. Changes in Hydrocarbon Oils The properties of today’s hydrocarbons are perhaps the single greatest contributors to varnish formation in gas turbine power generation. There has been a gradual shift to more refined hydrocarbon base oils. This has reduced the solvency of turbine oils, lowered their conductivity and affected air release properties. T+S_1_16 21.12.15 10: 54 Seite 61 Aus der Praxis für die Praxis Reduced Conductivity - Another property that changed with the refining of hydrocarbons is conductivity. Low electrical conductivity of the lubricant results in electro static discharge (ESD). ESD is an event that accelerates the degradation of turbine oils. These spark discharges rapidly oxidize turbine oil resulting in premature varnish formation. „Hot spots“ can form on equipment surfaces, contributing to dielectric breakdown and the presence of wear metals in the oil can also contribute to static discharge. The evolution of hydrocarbon base oils has reduced oil conductivity, increasing the potential for ESD. Extended Air Release - Varnish build-up in the lubricant prolongs air release times. These entrained air bubbles contribute to micro-dieseling, pump cavitation, premature oxidation and varnish formation as well as component wear. Micro-dieseling is a thermal degradation mechanism that occurs when small air bubbles entrained in the turbine oil are compressed, explode and burn the oil. Why PAG-based Fluids Eliminate Varnish The non-varnishing performance of PAG-based turbine fluids has now been documented in large-frame gas turbines in power generation service for more than seven years and 60,000 hours. As a result, turbine OEM General Electric has included a PAG-based turbine fluid specification in its June 2013 gas turbine lubricating oil recommendation document GEK 32568h (now GEK 32568j). Unlike refined hydrocarbon oils, polyalkylene glycols are full synthetic materials where oxygen is every third atom of the polymer backbone. These versatile, thermooxidatively stable products are used in a wide range of lubrication applications including fire resistant hydraulic fluids, environmentally acceptable lubricants for marine vessels, synthetic compressor fluids, and turbine fluids. PAG-based fluids are clean lubricants, with natural detergency. It is the oxygen in the polymer that is the origin of its unique chemical properties. The polarity allows for a wide degree of solvency towards many materials including oxidation by-products and performance additives. Additive and by-product solubility is important when assessing the varnish formation potential of hydrocarbon oils versus PAG-based fluids in gas turbine operations. Polar Solubility Figure 2 shows what happens when decomposition byproducts are formed in conventional hydrocarbon turbine oils, which are non-polar substances. The by-products produced by oxidation of the hydrocarbon oils are polar. Under typical temperature, system operating cycles, and the passage of time, these polar by-products form insoluble varnish that precipitates out of solution and adheres to system surfaces. Additives are included in hydrocarbon oils formulations in an effort to increase oxidative stability but with only partial success. In contrast, the PAG-based turbine fluid, a more stable and polar material, produces a smaller amount of low molecular weight polar by-products which remain in solution, even under prolonged exposure to typical turbine operating conditions. To summarize, the PAG-based turbine fluids eliminate varnish because any by-products they produce remain soluble in the fluid and will not precipitate out to form sludge or varnish in the system. Low Potential for Electrostatic Discharge Electrostatic discharges (ESD) within turbine systems create high temperature „hot spots“ (10,000 °C) that contribute to thermal degradation and premature varnish formation. As hydrocarbon oils have become more highly refined, purity has increased and conductivity has gone down. Lower conductivity increases the potential for ESD. Figure 3 compares the conductivity of Group V PAG-based base oils to the three hydrocarbon oil groups as well as Group IV polyalphaolefin (PAO) synthetic base oils. PAG-based fluids are higher in conductivity and have less potential for static discharge than the hydrocarbon oils and PAO. Note that spark discharge will occur at conductivity (pS/ m) levels of 400 or less and may occur between 400 and 1,100 pS/ m. PAG fluids provided more protection against ESD than Group I hydrocarbon oils, which previously provided the industry benchmark for acceptable ESD performance. 62 Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 Last chance filters after 6 years and 55,000 hours of service with PAG-based turbine fluid Comparing the photo above, to the clogged last chance filter shown preciously, dramatically demonstrates the difference in varnish accumulation with hydrocarbon oils and PAG-based turbine fluid in last chance filters ahead of servo valves in large frame gas turbines. Operators of GE7FA units report that after changing filters every 3 to 6 months while operating with hydrocarbon oils, their last chance filters have never required replacement in the years since the turbines were converted to PAG-based turbine fluid. Unique Polar Chemistry What are PAG’s and why are they so effective in eliminating varnish and related turbine operating problems? T+S_1_16 21.12.15 10: 54 Seite 62 Aus der Praxis für die Praxis Reduced Potential for Micro-dieseling The persistent presence of entrained air in turbine oils is a primary contributor to micro-dieseling, a thermal degradation mechanism that occurs when small air bubbles in turbine oil are compressed, explode and burn the oil. Figure 4 compares the length of time required by hydrocarbon turbine oils and PAG-based lubricants to get to 0.2 % entrained air volume. The GEK 32568F OEM Spec allows for a maximum of 5 minutes to reach the target 2 % level of entrained air. Even in the presence of water contamination, PAG fluids demonstrate excellent release of air, outperforming hydrocarbon oils and reducing the threat of varnish and other damage caused by micro-dieseling. PAG-based Fluids Meet or Exceed Industry Performance Requirements The properties of PAG-based turbine fluids compare very favorably to those of hydrocarbon oils as shown in Figure 5. Note that the higher viscosity index of PAGs allows the use of a lower viscosity grade to achieve the same absolute viscosity as an ISO 32 hydrocarbon oil. Using a lower viscosity grade reduces friction, and increases overall system efficiency while reducing thermal demand on bearings. The higher viscosity index of PAGbased turbine fluid also enables it to retain excellent viscosity characteristics over a wider temperature range than hydrocarbon Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 63 Figure 3: Electro Static Discharge Comparison Figure 4: Entrained Air Comparison (ASTM D 3427, 50 °C) Lubricant Minutes to 0.2 % Entrained Air Volume PAG-based Fluid (neat) 0.4 PAG-based Fluid with 2000 ppm water 0.7 PAG-based Fluid with 4000 ppm water 1 Hydrocarbon Oil A 4 Hydrocarbon Oil B 5 Figure 5: Properties Comparison Typical PAG-based Hydrocarbon Properties Turbine Fluid Turbine Oil ISO Viscosity Grade 25 32 Viscosity Properties cSt cSt @ 40 °C (104 °F) 26.23 32.44 @ 100 °C (212 °F) 5.19 5.56 Viscosity Index 132 109 Thermal Conductivity @ 40 °C watts/ m °K 0.145 0.1 Four Ball Wear Scar mm 0.63 0.65 Air Release Results ASTM D 3427 Minutes to 0.2 % Entrained 0.4 4.0 - 5.0 Air Volume Biodegradable Yes - Hydrolytic No Reaction Forms Acids, Stability with Water Degrades Figure 2: Why PAG-based turbine fluids will not form varnish T+S_1_16 21.12.15 10: 54 Seite 63 400 pS/ m Aus der Praxis für die Praxis oils. PAG-based fluid has a low pour point for cold weather startup of peakers and offline base load units. Figure 6 shows the viscosity of PAG-based turbine fluids versus hydrocarbon oils across a broad temperature range. Energy Efficiency - Heat Capacity Figure 7 shows the greater heat capacity of PAG-based fluids, which can translate into additional protection for turbine wear surfaces. Owners and operators of gas turbines report 5° to 10° lower bearing temperatures when PAG-based turbine fluids are used in place of hydrocarbon oils. Four Ball Wear Testing PAG-based turbine fluids provide excellent wear protection and are tolerant of water contamination. The data in Figure 8 show hydrocarbon oils do not offer the same level of production, even when PAGbased fluids contain up to 2 % water. Hydrolytic Stability While hydrocarbon turbine oils degrade and form acids when exposed to water, PAG-based turbine fluids are inert to water as show in Figure 9 which shows the effect of water contamination on total acid number (TAN) in PAG-based turbine fluid. A Two-Step Approach to PAG Conversion PAG-based technology from American Chemical Technologies (ACT) can be used to convert turbine units immediately to PAG-based turbine fluid or to extend the life of conventional hydrocarbon fluids by removing existing varnish and reducing the potential for future varnish problems. This offers owners and operators a two-step approach to incorporating the non-varnishing performance of PAG chemistry in current turbine units. Step 1 - Repair or Prevent Varnish in Existing Oils EcoSafe ® Revive™ is a patented fluid additive exclusively available from ACT that enables owners and operators to put an immediate end to varnish problems and scale back costly varnish management expense while continuing to use their current hydrocarbon oil until it reaches the end of its target service life. When added to conventional turbine oil, EcoSafe ® Revive™ eliminates varnish, improves lubricity, and extends the life of the 64 Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 Figure 7: Turbine Fluids Heat Capacity Comparison Figure 8: Four Ball Wear Comparison (ASTM D 4172 conditions: 40Kg, 1200 RPM, 1 hour, 75 °C) Lubricant Scar Diameter, mm PAG -based Turbine Fluid, neat 0.65 PAG -based Fluid with 7,500 ppm water 0.67 PAG -based Fluid with 20,000 ppm water 0.66 PAG-based Fluid with 2,900 pp water after 10,200 operating hours in a GE 7FA Turbine 0.77 ISO 46 Hydrocarbon Base Oil 0.83 Hydrocarbon Turbine Oil 0.75 - 0.80 Figure 6: Viscosity vs. Temperature Comparison T+S_1_16 21.12.15 10: 54 Seite 64 Aus der Praxis für die Praxis oil. Addition of EcoSafe ® Revive™ is predicted to double the life of typical hydrocarbon oils, greatly reducing the cost of operation and protecting against the need to replace the fluid prematurely. Recommended for use at an addition rate of 10-20 %, EcoSafe ® Revive™ is designed to repair existing oils identified with high varnish potential ratings or to change the solubility of newer oils so that varnish does not occur. The treatment changes the solubility of hydrocarbon oils by shifting polarity, so thermooxidative by-products remain in solution and do not form sludge or deposits on equipment surfaces. Existing varnish is also re-solubilized and phenolic and aminic antioxidants are regenerated. EcoSafe ® Revive™ is 100 % soluble and is compatible with all major commercial hydrocarbon base oils. No conversion process is required. Step 2 - Convert to PAG-based Fluid to Eliminate Varnish Entirely To take full advantage of PAG-based turbine fluid technology, owners and operators can replace conventional hydrocarbon fluids entirely, installing EcoSafe ® TF-25 PAG-based Turbine Fluid and eliminating varnish once and for all. EcoSafe ® TF-25 has been proven to eliminate varnish and protect large frame turbines in more than seven years and 60,000 hours of power generation service. The fluid complies fully with the PAG fluid specification within General Electric’s gas turbine lubricating oil recommendation document GEK 32568 h (now GEK 32568j). EcoSafe ® TF-25 is non-sludge or varnish forming to significantly reduce maintenance requirements and the need for costly varnish abatement products and services. The excellent low foaming and air-release properties of the fluid reduce the potential for micro-dieseling and related damage. The higher conductivity of EcoSafe ® TF-25 also reduces electrostatic discharge (ESD) and the hot spots that cause premature varnish formation. Available exclusively from ACT, EcoSafe ® TF-25 takes advantage of the inherent lubricity of PAGs to provide excellent wear protection. The fluid is hydrolytically stable and will not break down and react with water, minimizing fluid degradation and acid formation that can damage equipment. High temperature stability is also excellent and the higher viscosity index of EcoSafe ® TF-25 allows use of a lower viscosity grade to achieve the same absolute viscosity as ISO 32 grade hydrocarbon oil at typical operating temperatures. This reduces friction and increases overall system efficiency while reducing thermal demand on bearings. EcoSafe ® TF-25 is compatible with commonly used seals, hoses and metals. It is classified as readily biodegradable and environmental impact is low in the event of a spill or leak. EcoSafe ® TF-25 also satisfies stringent criteria for toxicity, to support a safe and healthy working environment. Conclusion Over more than 7 years and 60,000 hours of power generation operation, PAG-based turbine fluids have been proven to eliminate varnish formation in large frame gas turbines, delivering improved starting reliability, eliminating unplanned shutdowns, ensuring long-term fluid service, and extending equipment operating life, all while reducing maintenance and repair expense. As a result of this performance track record, General Electric’s gas turbine lubricating oil recommendation document GEK 32568 h (now GEK 32568j), released in June of 2013, now includes a PAG-based fluid specification. Now with access to ACT’s two-step approach, owners and operators of gas turbines can select the PAG adoption method that best matches their needs. Either route enables them to quickly put an end to the costly varnish problems, improving operating reliability, eliminating unnecessary maintenance costs, and protecting their significant equipment investment. Tribologie + Schmierungstechnik 63. Jahrgang 1/ 2016 65 Figure 9: PAG’s are inert to water T+S_1_16 21.12.15 10: 54 Seite 65