Non-varnishing and tribological characteristics of polyalkylene glycol-based synthetic turbine fluid
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 petroleum-based 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.
Tribology and energy efficiency: from molecules to lubricated contacts to complete machines
The impact of lubricants on energy efficiency is considered. Molecular details of base oils used in lubricants can have a great impact on the lubricant’s physical properties which will affect the energy efficiency performance of a lubricant. In addition, molecular details of lubricant additives can result in significant differences in measured friction coefficients for machine elements operating in the mixed/boundary lubrication regime. In single machine elements, these differences will result in lower friction losses, and for complete systems (such as cars, trucks, hydraulic circuits, industThe impact of lubricants on energy efficiency is considered. Molecular details of base oils used in lubricants can have a great impact on the lubricant’s physical properties which will affect the energy efficiency performance of a lubricant. In addition, molecular details of lubricant additives can result in significant differences in measured friction coefficients for machine elements operating in the mixed/boundary lubrication regime. In single machine elements, these differences will result in lower friction losses, and for complete systems (such as cars, trucks, hydraulic circuits, industrial gearboxes etc.) lower fuel consumption or lower electricity consumption can result.rial gearboxes etc.) lower fuel consumption or lower electricity consumption can result.
The effect of a ZnDTP anti-wear additive on the micropitting resistance of carburised steel rollers
Improved Processing of High Alloy Steels for Wear Components in Energy Generation Systems, Transportation and Manufacturing Systems
Oak Ridge National Laboratory (ORNL), in Partnership with Avure Technologies Inc. and Carpenter
Technology Corporation, explored methods to improve durability of steels considered essential to
bearings in wind turbine applications by using hot isostatic pressing of steel alloy powders as a
The objective of the project was to evaluate the wear mechanism of selected high alloy tool steels,
440C, 440XH, and M62, fabricated by powder metallurgy (PM) techniques via the hot isostatic press
(HIP) process, and compare these results to two conventionally produced wrought alloys, 52100 and
M50. Samples from the five candidate metals were characterized microstructurally via optical and
scanning microscopy methods. A micro-pit wear testing machine was purchased and used to simulate
bearing loads in a wind turbine gear box The candidate samples were subsequently heat treated in
accordance with industrial protocols and machined into wear test specimens suitable for the micro-pit
wear testing machine. . Wear testing produced surfaces that were subsequently analyzed for wear
and damage using both optical microscopy and scanning electron microscopy methods.
Optimized rolling-sliding experiments with a relatively small slip ratio (5%) and higher load (650N)
exhibited signs of abrasive wear on all specimens, but not all specimens had the same degree of
micro-pitting damage. The conventional wrought 52100 steel had the most severe micro-pitting,
consisting of distorted craters, many of which were connected by micro-cracks. Cross-sectional
examination showed multiple micro-cracks extending into the alloy microstructure from the same pit.
The depth of the cracks was several times the depth of the micro-pit. Type 440XH had the least
amount of micro-pitting damage under similar test conditions. Micro-pits on 440HX were more
localized and the micro-cracks associated with them tended to avoid carbides during propagation.
Abrasive processes were observed on all the test specimens and were produced by debris from micopitting.
The combination of micro-pitting in conjunction with abrasive processes could work together
to worsen the surface damage, diminishing bearing service life.
The results suggest that there is a potential advantage of PM consolidated high alloy tool steels, such
as 440XH produced via hot isostatic pressing to better resist micro-pitting when compared to other
alloy types and wrought processing methods.
Experience with a Disc Rig Micropitting Test
The experimental work carried out was aimed at developing a test method that was able to consistently
produce micropitting damage and could discriminate between a good oil (i.e., one that rarely produces
micropitting in service) and a poor oil (i.e., one that does produce micropitting in service). The small–scale
3–Disc test rig that was used for this work employs 3 discs to apply the test load to a 12mm–diameter test
roller. This test geometry allows a large number of stress cycles (typically 600,000 to 800,000 cycles/h) to be
generated at the contact track on the roller.
The disc rig control system allows test parameters such as entrainment velocity, contact stress and slide/roll
ratio at the disc/roller contacts to be accurately and independently controlled. This enables the effect of key
parameters to be studied in isolation, which is something that cannot be easily achieved using conventional
gear test rigs.
The early work carried out using the disc rig was aimed at producingmicropitting damage by operating the rig
at contact conditions similar to those used in the FZG micropitting gear test method. These early tests
confirmed that the damage produced to the roller track exhibits characteristics that are typical of micropitting
damage, and showed that the severity of the micropitting produced was affected by the amount of running–in
carried out on the roller prior to applying the full test load.
A test procedure has been developed which provides a good level of repeatability and which allows
discrimination between oils which producemicropitting in service and those which do not. In addition, a study
of the effect of slide/roll ratio (SRR) has shown that the severity of micropitting damage increases as SRR
increased, whereas at 0% SRR nomicropitting occurred and, at negative SRRs,microcracking occurred but
not micropitting. This is the way that SRR seems to affect micropitting in gears.
Effect of lubricants on micropitting and wear
Micropitting was studied using a three-contact disc machine having a central roller in contact with
three harder, annular counter-discs (‘‘rings’’) of precisely controlled roughness. Roughness, running
conditions, base stock and additive concentration were varied. The response of the same lubricants in a
reciprocating sliding wear test operating in the boundary regime was also studied.
Results of experimental studies of the rolling contact behaviour of carburised steel rollers are
reported. All the tests with the additive present led to micropitting. However, severe micropitting wear
was only observed when the calculated film thickness exceeded 12% of the centre-line average
roughness of the rings.
It was found that there was an approximately inverse correlation between the micropitting damage
in the disc machine test and the mild wear in the reciprocating sliding test. This was attributed to the
tendency of anti-wear additives to prevent running-in of the rough surface.
Effect of oil rheology and chemistry on journal-bearing friction and wear
Legislation and market pressures are calling for increased engine power, reduced engine size, and improved fuel consumption. The use of low-viscosity lubricants is considered as a means to enhance fuel economy by reducing viscous friction, particularly in engine bearings. Journal bearings mostly operate under hydrodynamic lubrication with a thin film of oil separating the journal and bearing shell. There are, however, certain conditions, especially under high load or low speed, when the film thickness will be low enough to allow boundary lubrication to occur. In this study, the effect of lubricants with different viscosities, different types of viscosity modifiers, different additives, different types of dispersants, and different lubricant formulations have been studied under hydrodynamic and boundary lubrication regimes. For hydrodynamic conditions, a high-temperature high-shear viscometer, meeting the requirements of ASTM D4741 was used to measure viscosity at 106 s−1. In addition, a new ultra high-shear viscometer, from PCS Instruments, was used to measure viscosity at shear rates near to 107 s−1. Bearing weight loss and load bearing capacity were measured on a rig developed in-house using a specially designed half-bearing shell loaded against a rotating journal. A PCS journal-bearing rig was used to measure the bearing friction under transient load.