Non-varnishing and tribological characteristics of polyalkylene glycol-based synthetic turbine fluid

Transcription

1 LUBRICATION SCIENCE Lubrication Science (2011) Published online in Wiley Online Library (wileyonlinelibrary.com)..165 Non-varnishing and tribological characteristics of polyalkylene glycol-based synthetic turbine fluid Govind Khemchandani*, The Dow Chemical Company, Freeport, TX 77541, USA ABSTRACT Varnish build-up in heavy duty gas turbines is a leading cause of costly unplanned shutdowns and resulting lost power generation capacity. The culprit is the conventional petroleum-based turbine oil, which breaks down to form varnish and sludge that cause servo valves to stick. Use of filtration to remove solid degradation by-products addresses a symptom but not the root cause of varnish formation: the petroleum-based turbine oils themselves. For the best protection against varnish-related shutdowns, a switch from petroleumbased turbine oil to non-varnishing polyalkylene glycol (PAG)-based synthetic turbine fluid was undertaken by four power plants in North America. PAG-based synthetic turbine fluid chemistry, its tribological characteristics and successful trials in GE 7FA heavy duty gas turbines (General Electric Company, Fairfield, CT, USA) are discussed in this paper. PAG-based synthetic turbine fluid has outperformed the hydrocarbon turbine oil in reduced sludge and varnish formation due to its polar nature and solvating power in plant trials as well as in a modified ASTM D 2893 test. Copyright 2011 John Wiley & Sons, Ltd. Received 14 August 2010; Revised 5 May 2011; Accepted 20 June 2011 KEY WORDS: non-varnishing; polyalkylene glycol; traction coefficient; static discharge; micro-dieseling; GE 7 FA turbines INTRODUCTION AND BACKGROUND Gas turbines have long been in industrial use to produce power since their inception in They have since evolved into more efficient and durable sources of energy and are widely used to supply industrial and residential power. Simply speaking, a gas turbine converts the chemical energy of a fluid (e.g. gas or air) into usable mechanical energy. 1 Most oils for lubricating turbine systems are based on mineral oil base stocks (groups I, II and higher). Group IV base stocks are also used for the formulation of turbine oils. It is necessary that today s gas turbine lubricant exhibit prolonged oxidative and thermal stability, in addition to exceeding turbine OEM requirements. These mineral oils, although less expensive than processed or synthetic fluids, have low viscosity indexes, limited operating temperature ranges and poor oxidative stability (sludge formation) resulting in decreased fluid life. *Correspondence to: Govind Khemchandani, The Dow Chemical Company, Freeport, TX 77541, USA. gvkhemchandani@dow.com Copyright 2011 John Wiley & Sons, Ltd.

2 G. KHEMCHANDANI Varnish build-up in heavy duty gas turbines is a leading cause of costly unplanned shutdowns and resulting lost power generation capacity. The culprit is the conventional petroleum-based turbine oil, which breaks down to form varnish and sludge that cause servo valves to stick. 2 Use of filtration to remove solid degradation by-products addresses a symptom but not the root cause of varnish formation: the hydrocarbon-based turbine oils themselves. For the best protection against varnish-related shutdowns, switch from petroleum-based turbine oil to non-varnishing polyalkylene glycol (PAG)- based synthetic turbine fluid is undertaken by four power plants in North America. This PAG-based synthetic turbine fluid is a formulation of PAG with a proprietary additive package. PAGs can be chemically designed to meet an extensive range of performance needs. No other base oil chemistry has such versatility. Generally speaking, a PAG (also known as a polyglycol, a polyol and a polyether) is prepared by reacting an initiator (alcohols) with one or more alkylene oxides under alkaline conditions and elevated temperatures. Within this reaction, there are four variables: the initiator, the oxides, the way the oxides are reacted on to the initiator (i.e. random or block additions) and the molecular weight. This gives PAG chemistry its versatility, which makes it possible to create tailor-made products that meet desired requirements. New non-varnishing PAG-based turbine formulation was designed for use in heavy duty gas turbines. Non-varnishing and tribological characteristics of this new PAG-based synthetic turbine fluid are discussed in the following sections of this paper. MATERIALS AND METHODS Tribology measurements Several bench tests can be thought of to investigate the frictional properties of base fluids and formulated products. One of the recently developed methods is a Mini Traction Machine (MTM) from PCS Instruments (London, UK). This machine measures Traction curves that provide useful information on the friction (traction) performance of fluids under different temperatures, pressures and operating slide/ roll ratios. The MTM rig is capable of measuring either constant or varying slide/roll ratios if required. Standard specimens are a 19.05mmdiameter ball as upper specimen and a 50mm diameter disc as lower specimen. Both are manufactured from AISI bearing steel. The type of specimen contact is shown in Figure 1. Four-ball wear test The four-ball wear test determines the wear protection properties of a lubricant. Three metal balls are clamped together and covered with the test lubricant, while a rotating fourth ball is pressed against them in sliding contact. This contact typically produces a wear scar, which is measured and recorded. Its self-aligning nature and ready availability of inexpensive high quality test specimens make it ideally suited to being an accurate and repeatable screening test. The most widely used wear test method for this apparatus is the ASTM D 4172 for lubricants. Air release, foam tendency and modified oxidation tests Most of the test methods used in this study are commonly used ASTM methods. The list includes ASTM D 2893, which was run for 4540h at 120 C instead of 13days at 95 C. Foam tendency was

3 SYNTHETIC LUBRICATION Figure 1. Mixed rolling sliding contact. Courtesy: Hugh Spikes, Imperial College London, SAE Powertrain measured by ASTM D 892, and air release tests were conducted per ASTM D Acid numbers and water content were measured by ASTM D 665 and Karl Fischer method ASTM D RESULTS AND DISCUSSIONS Physical property comparison Lubricating oil recommendations for gas turbines with bearing ambient above 260 C are displayed in Table I. 3 The physical property and GE specification performance comparisons for PAG-based synthetic Table I. Lubricating oil recommendations (gas turbines with bearing ambients >260 C). ASTM test Property Expressed value D 445 Viscosity 40 C, cst D 2270 Viscosity index 95 min D92 Flash point COC, C 215 min D97 Pour point, C 12 max D 665 Rust prevention-b Pass D 130 Copper corrosion 1b max D 974 Neutralisation no., mgkohg max D 3427 Air release 5.0 max D 892 Foam 50/0 D /0 D /0 D-287 Gravity, API D1500 Colour 2.0 max D 943 TOST (hours) 3000 min D 2272 RPVOT (minutes) 500 min D 2272 RPVOT modified (minutes) 85% min of original time Source: GEK 32568F. 3

4 G. KHEMCHANDANI turbine fluid and petroleum-based turbine oil are summarised in Table II. Petroleum-based turbine oil is widely used group II turbine oil. It is compared with water insoluble PAG-based synthetic turbine fluid in the present study. Gas turbines were of base load service and peaking operation type. Operating conditions and turbine hardware are described in the references. 4 6 Oil temperatures in reservoirs are commonly in the C range while bearings temperatures can be as high as 82 C. Pressures in the turbine system range from 1650 to 3150 psi. Some interesting differences can be seen in the physical properties of petroleum-based turbine oil and PAG-based synthetic turbine fluid. Besides better values for flash point, pour point and thermal conductivity, the significant difference is in viscosity grade of the fluids. Because PAG-based lubricants demonstrate improved viscosity indexes as compared with hydrocarbon-based oils, a lower viscosity grade can often be substituted in the application. To find the correct viscosity grade, the PAG and mineral oil viscosities were measured and compared and the various temperatures of the turbine cycle and beyond. The results are summarised in Figure 2. At the viscosity grade of 25, PAG-based synthetic turbine fluid offers a lower viscosity at start-up Typical physical properties* Table II. Physical property comparison. PAG-based synthetic turbine fluid Typical petroleum-based turbine oil Viscosity grade Kinematic viscosity at 40 C, cst (104 F, cp) (25.84) (27.90) (ASTM D445) Kinematic viscosity at 100 C, cst (212 F, cp) 5.19 (5.11) 5.56 (4.78) (ASTM D445) Viscosity index (ASTM D2270) Specific gravity (relative density) (ASTM D941) Pour point, C( F) (ASTM D97) 48 ( 55) 30 ( 22) Flash point, C( F), closed cup (ASTM D92) 242 (468) 215 (420) Specific heat at 40 C (104 F), Jg 1 K (ASTM E1269) Thermal conductivity at 40 C (104 F), Wm 1 K 1 (PLTL-73) *Not to be construed as sales specifications. Figure 2. Viscosity versus temperature.

5 SYNTHETIC LUBRICATION allowing for quick, efficient cold-starts while also providing a comparable viscosity profile to that of a 32 viscosity grade petroleum-based turbine oil. Using a lower viscosity grade reduces friction and increases overall system efficiency while reducing thermal demand on bearings. The higher viscosity index of PAG-based synthetic turbine fluid also allows use of a lower viscosity grade to achieve the same absolute viscosity as an ISO 32 mineral oil at typical turbine operating temperatures (Figure 3). Reduced static discharge Low thermal conductivity and resulting hot spots, dielectric breakdown and the presence of wear metals in the oil are believed to be important contributors to static discharge. With higher thermal conductivity than the ISO 32 petroleum-based turbine oil, PAG-based synthetic turbine fluid can provide better heat transfer, reducing the potential for static discharge produced by hot spots. The dielectric constant of PAG-based synthetic turbine fluid is approximately 5.7 compared with about 1.2 for the mineral oil, which suggests that the insulating capabilities of PAG-based synthetic turbine fluid are more likely to remain intact. The fluid exhibits excellent wear performance, minimising metal content in the oil. PAG fluid also shows lower potential for aeration and micro-dieseling. Micro-dieseling Micro-dieseling occurs when small air bubbles entrained in turbine oil are compressed, explode and burn the oil. The lower foaming tendency of PAG-based synthetic turbine fluid (Table III) suggests that minimal air is entrained in the fluid, reducing the potential for small air bubbles to become Figure 3. Absolute viscosity comparison. Table III. Foaming tendency and air release comparison. Specification PAG-based turbine fluid Petroleum-based turbine oil Foam tendency, ASTM D892, ml Seq. I 10/0/0 100/0/0 Seq. II 0/0/0 10/0/0 Seq. III 0/0/0 50/0/0 Air release, ASTM D3427 (minutes at 50 C)

6 G. KHEMCHANDANI compressed and explode. This reduces equipment wear and can also reduce a potential source of particle contamination. Polyalkylene glycol chemistry and varnish To be considered for use in gas turbines, the developed fluid has outperformed the hydrocarbon oil in reduced sludge and varnish formation due to its polar nature. Many references have described the polar nature of PAGs and their oxidation mechanism. 7 9 Sludge and varnish formation studies of hydrocarbon-based turbine oils are extensively discussed by many researchers One clear advantage of a PAG is its ability to hold polar oxidation products in solution, thus preventing sludge and varnish from developing. Two schematic diagrams explaining oxidation mechanism and polar nature of PAGs are shown in Figures 4 and 5. These diagrams are self explanatory. Hydrocarbons being non-polar in nature cannot hold polar varnish and sludge molecules in turbine peaking operations. The low molecular weight polar oxidation product remains attached to polar PAG molecule and do not form varnish or sludge during the operating cycles of turbines. A modified bench test ASTM D 2893 was run at 120 C instead of 95 C for 4540h to compare the oxidation stability of PAG-based synthetic turbine fluid and petroleum-based turbine oil. Acidity, ph and viscosity changes were observed at 500h intervals. The test data for 4540h are reported in Tables IV and V. Acidity, ph and viscosity change are the suitable parameters for monitoring PAG-based lubricants. 13 A viscosity comparison from Table IV shows a dramatic difference between PAG-based synthetic turbine fluid and petroleum-based turbine oil. Viscosity increase in the case of the petroleum-based turbine oil was also accompanied by varnish and deposit formation. The slight viscosity increase, even after extreme tests such as this, are typical of the results with PAGs such as PAG-based synthetic turbine fluid. It is interesting to note that acidity of PAG-based synthetic turbine fluid was only 0.82mg KOHg 1 versus 22mg KOHg 1 for petroleum-based turbine oil. Oxidation stability as measured by Figure 4. Chemical composition: polyalkylene glycol versus hydrocarbon oils.

7 SYNTHETIC LUBRICATION Figure 5. Why polyalkylene glycol (PAG)-based turbine fluid does not varnish. Table IV. Viscosity change 4540h (modified ASTM D 2893). Product PAG-based turbine fluid Petroleum-based turbine oil Initial at 40 C, cst Final at 40 C, cst Initial at 100 C, cst Final at 100 C, cst % Increase 100 C, cst Table V. Total acid number (TAN) change 4540h (modified ASTM D 2893). Product PAG-based turbine fluid Petroleum-based turbine oil Initial TAN, mgkohg 1 Final TAN, mgkohg 1 Initial ph Final ph TAN increase, mgkohg either of these metrics is quite interesting. High acidity, accompanied by the increase in viscosity, can result in corrosive wear, as well as is indicative of varnish formation in equipment. It is especially significant that the changes in the petroleum-based turbine oil happened quite rapidly later in the test. When the antioxidant in the hydrocarbon is depleted, varnish formation, acidity, and viscosity increase all occur quite quickly. Figure 6 shows the comparative sludging tendency of PAG and hydrocarbonbased fluid 14 after 2500h at 120 C. The tubes were partially emptied to demonstrate the difference in performance of polyalphaolefin (PAO) (synthetic hydrocarbon) versus PAG fluids.

8 G. KHEMCHANDANI Figure 6. Varnish versus sludging. The PAO showed significant deposits that could result in operational problems in hydraulic and lubrication systems. The PAG fluid was virtually free of deposits. This is the same visual result we have seen in all comparisons of the PAG-based turbine fluid with hydrocarbons and PAO in this test. Trials monitoring in GE 7 FA turbines Four GE 7 FA heavy duty gas turbines (General Electric Company, Fairfield, CT, USA) have been converted to PAG-based synthetic turbine fluid, two at American Electric Power (AEP) in Oologah, OK, one at Calpine in Broken Arrow, OK and one at Dow in Plaquemine, LA. A complete turnkey operation was performed to remove the old lubricant and varnish by-products. Fluid system flush and cleaning were performed on the turbine. All the turbines after conversions are running without any varnish formation and fluid monitoring (ASTM D 4378) data for water and acidity are shown in Figures 7 and 8. Tribological characteristics Although PAGs rate well on varnish/sludge formation, there are some challenges they face in overcoming customer perception. This includes water separability and hygroscopicity as a main cause of concern for the petroleum turbine oil users. It is known to them that relative to mineral oils, PAGs will Figure 7. Water content monitoring during trial, ASTM D 6304.

9 SYNTHETIC LUBRICATION Figure 8. Fluid monitoring: total acid number (TAN), ASTM D 664. readily absorb water from the atmosphere. Therefore, water ingress resulting from the hygroscopic nature of the PAGs poses a challenge for the petroleum turbine oil user who is willing to convert gas turbines to PAG-based turbine fluid. These engineers need to be assured that hygroscopic nature of PAG does not adversely affect the lubrication of the turbine system. This prompted the author to study important tribological characteristics of PAG-based turbine fluid as compared with petroleum-based turbine oil in the presence of water using two bench tests, namely, MTM machine and four-ball wear. It was also decided to see if the presence of water affects the air release properties of the PAG-based turbine fluid. It is to be noted that PAG-based turbine fluid can hold water without affecting the lubrication properties compared with petroleum-based turbine that shows haziness and, at higher levels, water separation that will be harmful for the bearings of turbines Figure 9. Coefficient of friction Traction performance testing on a PCS Instruments MTM showed that PAG-based synthetic turbine fluid demonstrates lower traction coefficients than petroleum-based turbine oil under specified test conditions (Figure 10). This low coefficient of friction can allow operating energy savings and has been confirmed by a decrease in temperature measurements of operating turbine bearings. Figure 9. Water absorbing and holding capacity of polyalkylene glycol-based turbine oil. Copyright 2011 John Wiley & Sons, Ltd. Lubrication Science (2011)

10 G. KHEMCHANDANI Figure 10. Traction curve of polyalkylene glycol (PAG)-based turbine fluid using Mini Traction Machine. Excellent wear performance The wear preventative properties of PAG-based synthetic turbine fluid in sliding contact were determined on neat and water-containing fluid per the ASTM D4172 standard test (Table VI). PAG-based synthetic turbine fluid shows excellent anti-wear performance, even with as much as 2% water in the fluid, as well as after over 1411h of operation in a turbine. Note that PAG-based synthetic turbine fluid passes the ASTM D665 rust prevention test, which is run with 10% water (100000ppm water). Air release Using the ASTM D3427 standard test, PAG-based synthetic turbine fluid shows excellent air release for both neat fluid and fluid contaminated with water (Table VII). These air release times are lower than those typical of petroleum-based and hydrocarbon-based turbine fluids. Prolonged air release times can lead to pump cavitation, micro-dieseling, premature oxidation and component wear. Figure 11 shows Fluid Table VI. Four-ball wear testing on polyalkylene glycol (PAG)-based synthetic turbine fluid. Scar diameter, mm PAG-based fluid neat 0.65 PAG-based fluid+7500ppm water 0.67 PAG-based fluid+20000ppm water 0.66 PAG-based fluid+2900ppm water after 1411 operating hours in GE 7FA turbine 0.66 Table VII. Air release characteristics. Fluid Temperature, C Minutes to 0.2% entrained air volume PAG-based fluid neat PAG-based fluid+2000ppm water PAG-based fluid+4000ppm water

11 SYNTHETIC LUBRICATION Figure 11. Case study no gel no varnish on lube oil filters. Polyalkylene glycol-based fluid produced no varnish (2years). that no gel or varnish was seen in lube oil filter assembly after 2years of operation at one of the GE 7 FA turbine. CONCLUSIONS Three years of successful operation of four heavy duty GE 7 FA gas turbines has not produced any varnish by using new PAG-based synthetic turbine oil. Tribological test results and oxidation test method have clearly shown better performance of these fluids compared with hydrocarbon-based turbine fluids. Future work will focus on interaction of additives and PAGs. REFERENCES Villforth FJ. Power generation industry II. Gas Turbines 1995; 81. Day L. The Secret s. Tribology and Lubrication Technology 2008; GEK 32568F. GE Power Systems Gas Turbines. Revised Feb (Tex) Leugner L. The Practical Handbook of Lubrication. Maintenance Technology International Inc., Canada Langston LS, Opdyke G. Introduction to Gas Turbines for Non-engineers. Global Gas Turbine News 1997; 37(2). Okazaki ME, Mesquita R, Sweeney PJ, Opstal T. Introduction to Gas Turbines for Non-engineers. Power Generation Lubricants: Gas and Steam Turbines. Chevron Products Company, CA, Rudnick LR. Synthetics, Mineral Oils, and Bio-based Lubricants, Chapter VI, 2006; Greaves M. Controlling Deposit Formation Using PAG Lubricants. Compoundings 2009; 59(4): Verbanic CJ. Fuels and Lubes International Quarter Two, Gatto VJ, Moehle WE, Cobb TW, Schneller ER. Oxidation Fundamentals and Its Application to Turbine Oil Testing. Journal of ASTM International 2006; 3 (NO.4 paper ID JAI 13498). Yano A, Watanabe S, Miyazaki Y, Tsuchiya M, Yamamoto Y. Study on sludge formation during the oxidation process of turbine oils. STLE/ASME International Joint Tribology Conference, Florida, Oct Livingstone GJ, Thompson BT, Okazaki ME. Physical performance and chemical changes in turbine oils from oxidation. Journal of ASTM International 2006; 4 Available from (No.1, Paper ID JAI ) Beatty D. Oil Analysis Boosts Compressor Reliability, Practicing Oil Analysis Magazine, Nov Beatty D. Lubricant Technology Center. The Dow Chemical Company, Business Communication, Freeport, Texas, Copyright 2011 John Wiley & Sons, Ltd. Lubrication Science (2011)