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Lubricant Analysis in Steam TurbinesBeatriz Graça, INEGI Jorge Seabra, FEUP Pinto Sousa, Portucel Tags: oil analysis, contamination control, Case Studies
Machinery Lubrication (10/2011)
Turbine oils are subjected to a wide range of conditions extreme heat, entrained air, moisture,contamination by dirt and debris, inadvertent mixing with different oil, etc. that degrade the integrityof the hydrocarbon base stock and deplete the additive chemistries, causing irreversible molecularchanges. There are two primary degradation mechanisms in turbine applications oxidation and thermaldegradation.
Oxidation is a chemical process where the oxygen reacts with the oil molecules to form a number ofdifferent chemical products, such as carboxylic acids. The rate at which this occurs depends on anumber of factors. Temperature is perhaps the most critical one, since the rate of oxidation doubles forevery rise of 10 degrees C. The temperature above which this occurs is influenced by the oxidationstability of the oil and the presence of catalysts and prooxidant conditions such as water, air, certainmetals, fluid agitation and pressure.
Thermal degradation is the breakdown of the oil molecules by heat (high temperature), forminginsoluble compounds that frequently are referred to as soft contaminants. Typically, thermaldegradation occurs as a result of microdieseling, electrostatic spark discharge and hot spots. Microdieseling is the combustion of imploding air bubbles creating adiabatic compressive heat (oftenexceeding temperatures above 1,000 degrees C). Electrostatic spark discharge results from the internalmolecular friction that generates highvoltage electric charges such as where oil passes through verytight clearances at high flow rates, producing temperatures over 10,000 degrees C.
Over time, it has become clear that the oxidation performances of the different base stock classes arequite different. The high natural oxidative resistance of Group II turbine oils combined with specificantioxidants employed (usually based in phenol and amine compounds) provide a nonlinear behavior interms of their molecular degradation over time. As a result, the majority of standard oil analysis testsoffer little to no warning as the lubricant starts to degrade and generate system deposits. Instead ofdegradation occurring in a linear and predictable fashion, many of the modern turbine oils fail rapidly.
Changes in the oil’s molecular structure due to additive depletion and the development of insolubleparticulates are among the first oil degradation conditions that affect equipment performance. Thesequential process will be the formation of sludge and varnish, which are common occurrences in turbogenerators. Besides these oxidation and thermal degradation byproducts being the main contributors forthe development of varnish and deposit problems in turbines, they interfere with other importantproperties in steam turbine lubricants, such as demulsibility and the detrainment of air. Therefore, it isvital that appropriate diagnostic analysis be performed to detect these conditions in critical andsensitive lubrication systems.
Analytical ferrography deposited patterns
Degradation trend of different base stock oils
Ferrography AnalysisFerrography is a technique that provides valuable information about wear evolution in machinerythrough analysis of a representative lubricant sample. Developed by Vernon Westcott at the U.S. Navyin the 1970s as a conditionmonitoring technique, it has been applied by hundreds of worldwide users toall kinds of lubricated systems.
The potential of ferrography is not only limited to predictivemaintenance strategies. Its important contribution totribology studies, by assisting in a better understanding ofthe wear mechanisms and of the lubricant effects on thecontact surfaces, turns this versatile technology into one ofthe most powerful diagnostic tools to assess machine health,providing valuable information about the past, present andfuture condition of the machine’s lubricated components.
The test procedure is lengthy and requires the skill of a welltrained analyst. As such, there are significant costs inperforming analytical ferrography not present in other oilanalysis tests. However, if time is taken to fully understandwhat analytical ferrography uncovers, most agree that thebenefits significantly outweigh the costs and elect toautomatically incorporate it when abnormal wear is
encountered.
In analytical ferrography, the solid debris suspended in a lubricant sample is separated and thoroughlydeposited onto a glass slide while passing across a bipolar magnetic field. When the sample flow iscompleted, a solvent “wash” cycle removes any lubricant remaining on the substrate, resulting in a“ferrogram,” where the particles are all arranged by size and permanently attached to the slide foroptical analysis using a biochromatic microscope. The particles are then examined and classified bysize, shape, concentration and metallurgy. This information carried by the wear particles is valuable forthe identification of the wear mode and mechanism.
Steam Turbine Monitoring
This case study is about the condition monitoring of the lubricant in a steam turbogenerator at a localcellulose industry plant. The turbine is a 26 MW Siemens G 8002. It has been in service for 22 years,operating continuously, with a lubricating oil reservoir holding 8,500 liters of ISO VG 46 oil to lubricateand cool bearings, gears and oil shaft seals and to act as a hydraulic medium for operation of thegovernor and steam control valves.
Since its initial operation in 1988, this turbine worked with solventrefined base stock oil (Group I).However, due to a manufacturer upgrade, this oil was replaced by a hydrocracked base stock (Group II)in 2002. In the meantime, about 6,000 liters of makeup fluid was added, along with a few periodic oilreservoir fillups, making the circulating fluid a blend of these two base stocks.
The turbogenerator was operating and performing normally, and no occurrences of anomalousfunctions of the lubricated components had been recorded. Nevertheless, a close monitoring of the oilcondition was ensured by analyzing the turbine oil periodically.
A steam turbogenerator at a cellulose industry plant
Turbine Oil Analysis
A lubricant analysis program was applied quarterly, taking two samples from the oil reservoir andsending it to independent laboratories. The standard methods used at one of the laboratories to assessthe condition of the turbine oil were:
Kinematic viscosity at 40 degrees C (ASTM D445)Water by Karl Fisher (ASTM D6304)Insoluble particulates (ASTM D4898)Acid number (ASTM D664)Neutralization number (ASTM D974)Elemental spectroscopy (ASTM D5185)Rust (ASTM D665A)Demulsibility (IP 19)Foam (ASTM D892)Flash point (ASTM D92)Air release (DIN 51636)
Cleanliness code (ISO 4406)Linear sweep voltammetry (LSV), (ASTM D6971)
Simultaneously, at another laboratory, ferrography and Fourier transform infrared (FTIR) analysis wereperformed along with other techniques. These analyses allowed a complemented diagnosis not only ofthe condition of the oil but also of the turbine wear rate conditions.
In this case study, among all the standard test results obtained, those that showed some indications offluid degradation were the demulsibility, air release, particle count and LSV. As can be seen in the tableabove, the oil viscosity and acid number are within the range over the time period. Water contaminationand foam tendency are maintained low. However, the particle contamination is high for all theevaluated period, the phenolic content falls below critical in some samples and the demulsibility is alsoaffected significantly.
The sequential events in the oil degradation produces an eventual depletion of the antioxidant additives.The aminic/phenolic antioxidant mixtures actuate as a complex system. The aminic inhibitor works toneutralize the free radicals that cause oil oxidation, but it is then regenerated by phenolic, which is agood freeradical trap. When phenolic levels fall below a critical level, the oil is in danger of rapiddegradation, resulting in the formation of soft contaminants and varnish. Soft contaminants are typicallyless than 2 microns in size and cannot be removed through standard mechanical filtration. They areinsoluble and polar in nature, and are unstable in a nonpolar oil environment, such as hydrocrackedbase oil (Group II).
Analytical results from standard oil tests show the oil viscosity and acid number are within the range
over the time period.
The high ISO Codes obtained, mainly in terms of small particles (less than 4 microns), can be relatedwith this turbine oil degradation process. Demulsibility is also compromised by the presence of polarcontaminants.
For the lubrication of turbogenerator bearings, the cleanliness level with respect to particles in the oilis of the utmost importance. Consequently, a proactive action is taken through periodical online oilpurification (filtration during 24 hours) to achieve the system cleanliness in accordance with OEMrecommendations (ISO 18/16/12). However, a swift increase of the ISO Codes is consistently verifiedduring the operation of this turbine.
The ferrography analysis completed for the same period revealed valuable information on the oil’s solidcontamination. In all ferrograms, the presence of soft contaminants that resulted from oil thermaldegradation and additive depletion was observed. This information is essential to identify the reason forthe persistent high ISO Codes obtained in particle counting. Although soft particulates are not harmful in
Figure 2. Particles aligned on the ferrogramto the magnetic field
terms of wear, they contribute to the generation of surface deposits, as detected through ferrography.
Figure 1 shows two photomicrographs of these particles deposited on a ferrogram as observed underwhite/green light and polarized illumination. The polarized light allows the identification of nonmetallicparticles (crystalline and amorphous materials, for instance) by the brightness of light reflected. Notethe brown pattern evidenced by some of these particles.
Figure 1. These two photomicrographs show turbine oil crystalline contaminants (1,000x magnification).
The particles in the ferrogram of Figure 2 are very small in size, and due to polarity, they easily alignedalong the magnetic field of the ferrograph. These particles have the tendency to form agglomerates,which when overstressed with the oil, form a large coherent structure by a molecular polymerization.
The varnish buildup seems to be a consequence of thisphysicochemical process, as can be realized by thephotomicrographs in Figure 3, obtained in different oilsamples. All these kinds of particulates have polaraffinities and high molecular weight and tend to beadsorbed onto dipolar metallic surfaces as a stickingmatter, which in turn captures hard contaminants as theyflow within the system. They are capable of shutting downa turbine or causing serious damage, which is frequentlyrelated to bearings and servo applications.
Another technique employed to monitor the oil conditionwas FTIR, which is used to measure organic molecularcomponents, monitor additive depletion (antioxidants) andidentify organic degradation byproducts (oxidation). The
monitoring of specific antioxidant depletion in used lubricants is still considered a relatively newresearch area. However, some studies show that the rate of antioxidant depletion is related to lubricantdegradation or affected by the antioxidant mix or base stock type used to produce the lubricant.
Used oil samples are complex mixtures of different chemicals, including compounds derived from theformulation of the base oil and its additives, and from oil degradation products and contaminants. As aresult, a used oil spectrum is complex and essentially the net sum of the spectra of all the individualcompounds making up the sample. In fact, because of this complexity, the used oil spectrum alone is oflimited value and must be compared against the spectrum of the unused oil to be of significantanalytical value.
Figure 4 shows transmittance spectral snapshots of the new and used turbine oil. The black spectrum isthat of the new oil (new base stock – Group II), while the red spectrum is from the blend oil in service,which still contains a small percentage of Group I base stock oil. Nevertheless, the spectra revealedidentical functional groups.
In analyzing the spectrum overlays, you can clearly see relative molecular changes in the oxidationpeaks, as well as thermal degradation of the oil through the signs of nitration. Another molecular
alteration is observed where the phenolic antioxidants are characterized. The type of decompositiondetected in the used oil spectrum is commonly observed in FTIR analysis of fluids where thermalbreakdown took place.
Figure 3. Ferrogram photomicrographs of the turbine oil particles in different samples (1,000x
magnification)
Figure 4. FTIR spectra in transmittance/wavenumber (cm1) of new and used turbine oils
Filter Analysis
Staticgenerated sparks are very common incidents in the filtering systems of turbogenerators. This isa phenomenon of molecular friction occurring as oil flows through small clearances, such as the filtermedia. Since oil and filter media are both dielectric, this electrical energy builds until a limit is reached,and then sparks are released in the lubrication system in the direction of the ground. These electricalarcs can have an extremely high, localized temperature (about 20,000 degrees C), instantly crackingthe hydrocarbon molecule.
Figure 5. Plugged filter from the turbogeneratorand filter mesh with black and brown shiny residue (200x magnification)
Since spark discharges generated on filters and otherlocations are a key root cause of varnish, and someof the previous oil analysis results confirmed that(through additive depletion and high particle counts),one of the duplextype filters was dismantled andanalyzed through an optical microscope.
Evidence of electric discharge can be easily seenthrough microscopic inspection of the filter media,filter core, filter meshes and from debris carriedaway from the filter.
Figure 5 shows one of the plugged filters changed ina periodic maintenance action due to a plugged filteralarm, with a microscopic view of the filter mesh. Ascan be seen, black and brown shiny deposits (sludgeand varnish) are present in high concentrations,clogging the filter mesh.
The solvent used for cleaning the filter mesh wascollected and used to prepare a ferrogram where significant amounts of ferrous spherical wear particleswere identified (Figures 6 and 7). One source of spherical ferrous debris is the erosion wear activatedby electrical discharges. The high temperatures attained by the sparks on the steel surface thermallyliquefy the steel debris, which acquires a spherical shape due to rapid cooling under the action ofsurface tension.
The microscopic analysis of the filter core surface showed several small, circular burned holes left bythe hightemperature spark discharges on the metal surfaces.
In conclusion, turbine oils must be wellmaintained to extend their service life and simultaneouslyprovide the maximum turbine performance. However, the recent upgrade in the turbine oil formulationshas caused some controversy. The older analytical techniques are no longer the predictive tools able tomonitor the real condition that they once were.
Figure 6. Photomicrograph showing high concentration of ferrous spheres (1,000x magnification) to the
magnetic field
Figure 7. Photomicrographs of small burned holes on the surface filter core (200x and 1,000x
magnification)
The generation and presence of soft contaminants are among the main consequences of the actualturbine oil degradation process. There are four likely reasons for this:
Unlike oldgeneration base oils (Group I), the type of base oil currently used (Group II) does nothold varnish precursors in suspension. These insoluble particles may form deposits.Group I and Group II turbine oils possess significantly different oxidation properties and failuremechanisms.The antioxidants precipitate as they are preferential oxides generating insoluble particulates.The new generation of antifoam additives has less effective airrelease characteristics, and thesesmall air bubbles are adiabatically compressed, causing varnish to appear.
In this case study, it was recognized that only the following techniques used to monitor the condition ofthe turbine oil were efficient in predicting eminent problems related to the generation of varnish andsludge:
Particle counting (ISO Code) was effective in monitoring particle contamination. This was in spiteof the fact that most particle counters are not sensitive to the small size of the polar particles(less than 2 microns). The reason for their efficiency was that the particles have a tendency toform agglomerates, increasing the size of the particulates and thus allowing particle counting todetect them.The demulsibility of the oil was a critical characteristic to evaluate since it is affected by thepresence of polar particles. The alteration of this property could be a signal of extreme particlecontamination.The LSV technology and FTIR are both already recognized as important techniques to monitor thecondition of modern turbine oils. They efficiently monitor the condition of the antioxidant packageand the creation of soft contaminants.Analytical ferrography was effective in the detection of soft contaminants and in the identificationof their nature. In the hands of a skilled analyst, analytical ferrography is a powerful technique toidentify turbine oilrelated problems, providing a root cause based on the morphology andcharacteristics of the insoluble particles, as well as monitoring the progressive mechanism ofvarnish formation.
