Options for preventing, eliminating varnish in hydraulic, lube-oil systems

Lubricant varnish continues to be a topic of great interest at gasturbine (GT) user-group meetings. Reason: It is the primary cause of the servo-valve sticking/seizing in control circuits blamed for many starting problems and turbine trips.

One of the first presentations to this industry segment on the subject was by ISOPur Fluid Technologies Inc’s (Pawcatuck, Conn) Chuck Mitchell at the 2004 meeting of the 501D5/D5A Users in Hartford. Mitchell’s objective was to raise awareness regarding varnish and why it occurs. Obviously, he had a solution to the problem.

Mitchell stressed the importance of eliminating fine particulates from hydraulic and lubricating oils in systems equipped with standard filtration equipment. Conventional filters, he said, were effective for removing particles 10 microns and larger; fine filters could extend that coverage down to about 3 microns.

However, Mitchell continued, particle- size analysis of representative lube oils suggested that roughly half of the particulates present ranged in size from 0.1 to 5 microns. Given that clearances can be 1 micron in loaded bearings, many of the particles escaping removal by standard filters could wedge between the shaft and journal and do damage.

The ISOPur solution, he explained, relies on Balanced Charge Agglomeration ™ (BCA), which “grows” small and sub-micron particles to filterable size so they can be removed by existing filters in the system—thereby reducing wear and eliminating the source of varnish. Mitchell seemed to initiate a flood of presentations on varnish and how to deal with it.

A frequent participant in user-group meetings has been Greg Livingstone, formerly of Analysts Inc, Torrance, Calif, and now with Calgary-based EPT Inc. Analysts developed the QSA™ (quantitative spectrophotometric analysis) test to determine the presence or likelihood of sludge and varnish buildup on critical components; EPT offers filters and other solutions to remove contaminants from lube and hydraulic oil as well as related services.

Livingstone says a primary cause of lubricant varnish is auto-degradation, which he defines as the creation of soft contaminants in a static body of oil—such as a shut-down lube-oil system serving a cycling or peaking GT. Soft contaminants, he continues, often are more troublesome to remove than the hard particulates on which lubricant experts traditionally have focused.

Livingstone adds that varnishpotential tests—such as QSA—alone will not tell you if auto-degradation is occurring, though such tests remain essential to your overall oil-condition monitoring program. Similarly, electrostatic separators, seen by many as a one-step “cure-all,” will not eliminate the problem. Note that the term “electrostatic separators” as used by Livingstone includes electrostatic oil cleaning, BCA, electrostatic filtration, etc.

To fully understand if your lubricant is undergoing auto-degradation, he continues, you need to assess the antioxidant health of the fluid and examine it for specific types of degradation byproducts. If auto-degradation is identified, you’ll need a holistic approach that includes monitoring of initial oil quality, analysis and additive replenishment for inservice oil, and the installation of appropriate oil-cleaning technology to remove existing varnish and slow the degradation process.

The 7EA Users Group has identified varnish and other lube-oil issues as an area of significant interest to its membership. It should be. There are more than 1000 Frame 7s (model As through EAs) in operation worldwide, 70% of those in the US. About 60% of the total population is used in peak-power applications, meaning the majority of the units in the fleet are particularly susceptible to varnish formation based on Livingstone’s experience described above.

There was a varnish-related formal presentation at the organization’s 2006 meeting in San Diego.A representative of Pall Corp, Port Washington, NY, brought the group up to date on a new filter media designed to minimize the potential for electrostatic discharge in hydraulic, lubricating, and fuel systems.

Recall from the references suggested above that electrostatic spark discharge (ESD) from filters has been observed and documented in several powerplants worldwide. It is described this way: As oil flows through the small openings of a filter, molecular friction is produced and it creates static electricity. When the electrical charge in the fluid accumulates to a given point, the energy is released in the form of a spark, arcing from the sharp edges inside the filter housing (Fig 12).

The locally high temperatures produced by ESD oxidize the oil; the byproducts of this oxidation include varnish. It follows then that a filter medium capable of limiting ESD would have a positive impact on oilsystem health.

The speaker explained that the potential for electrostatic charging increases with decreased conductivity, increased flow rate or velocity, and the additive package. Also that fluid conductivity—which helps with charge dissipation—increases with temperature (lower viscosity), water content, additive concentration, and the amount of dust and other impurities in the oil.

Next, he described Pall’s test setup for measuring electrostatic charge, explained the charge collector, and presented the characteristics of the four oils tested. Regarding the last, the products evaluated were one turbine lube oil, two commercial hydraulic oils, and a hydraulic oil for the military. Additive packages included R&O, antiwear, and antiwear/antioxidant. Viscosities varied from 14 to 47 centistokes, dielectric strengths from 15.5 to 27.3 kV, and conductivities from 39 to 1460 picoSiemens/ meter (pS/m).

Results were presented as average charge generation in nanoamps for three filter materials: standard glass fiber, surface-modified glass fiber, and Pall’s new glass-fiber-based ESD. One set of tests was run on these materials after heat-soaking at 300F for one hour; a comparable set of tests without heat soaking prior to use. All tests were conducted with the oils at ambient temperature.

Here are the results of greatest interest to plant personnel:

  • Charge generation for the standard and surface-modified glass fiber materials was about two times greater after heat soaking. By contrast, charge generation for Pall’s new filter media was the same whether heat-soaked or not.
  • For the heat-soaked samples, charge generation for the new ESD media was a factor of 15 less than that produced by the standard glass fiber and six times less than that produced by the surface modified glass fiber media.

Field trials supported the test results. In sum, the new Pall filter substantially reduced charging—and eliminated all signs of noise, sparking, and filter damage—both in the laboratory and in field tests. Specifically:

  • In a manufacturing plant ’ s hydraulic system, the new filter media lowered the charge generated to a negligible amount and eliminated both noise and sparking.
  • In an injection molding hydraulic system, the new filter media eliminated noise and burn marks and reduced the charge generated by about 75%. It did the same in a paper-mill hydraulic system except that the reduction in charge produced was 98%.
  • In a powerplant lube-oil system a distinct clicking noise that was apparent before the change in filter medium disappeared.

In his conclusions, the speaker said that electrostatic charging can be a problem in hydraulic and lube-oil systems using any manufacturers’ standard glass-media filter—although it occurs relatively infrequently. Also, that grounding housings and pipes do not reduce the charge generated.

The editors followed up with the filter experts at Pall following the San Francisco meeting and learned that the company’s electrostaticdischarge- resistant filter media is now available commercially in various cartridge configurations and in several porosity grades. A company spokesperson said, “These filters have been employed in various industries and applications and have a track record of resolving the tough problem of electrostatic discharge and its associated damage while providing highly reliable fine filtration.”

Alternatives for varnish mitigation

A feature of the 7EA Users’ 2007 conference was a three-vendor panel describing alternative solutions for preventing varnish formation and for clean-up of existing deposits. It was developed by Julie Turner, plant manager of Progress Energy Florida Inc’s Intercession City facility.

The editors believe this was the first time a user group provided owner/operators the opportunity to compare the various offerings on a level playing field. Presenters were ISOPur; the Hilco Div of Hilliard Corp, Elmira, NY; and C C Jensen Inc, Tyrone, Ga. These companies, plus the participation by Pall last year and the availability of Kleentek Inc (Cincinnati) and EPT personnel at the vendor fair in San Francisco, allowed 7EA users to access information first-hand on perhaps all of the leading commercial varnish solutions.Analysts Inc and Chevron Lubricants were at the vendor fair as well to answer questions on test procedures and lubricant properties.

Mike Long, Hilliard’s product engineering manager, focused his presentation on the elimination of varnish root causes rather than its removal after formation. You knew where Long was headed from the get-go when he said, “Static discharge is not a fluid problem and not a cartridge problem. Its root cause is the use of API (American Petroleum Institute) Type II lubricant base stocks and low fluid conductivity—less than 35 pS/m.”

Long added that traditional staticdischarge control techniques—such as use of conductive filter elements or of large-diameter filter elements to reduce velocity through the screen and lower fluid shear—are not the complete answer because they do not address low fluid conductivity. Then he introduced his company’s new anti-static element for lube/ hydraulic-oil conditioning, which Long said was capable of raising fluid conductivity above 200 pS/m (Fig 13). It is designed for kidney-loop service.Note that fluid conductivity determines when to replace filter elements of this type, not pressure drop.

Next he discussed microdieseling, which contributes to varnish formation. It is caused by dissolved gases in the lube oil—mostly nitrogen and oxygen. When gas bubbles transition from a region of low pressure to one of high pressure, the gases implode, generating sufficient heat to thermally degrade the fluid. Oil analyses from three F-class machines from different areas of the country showed similar dissolvedgas compositions. A vacuum dehydrator/degasser removes both moisture and dissolved gases (Fig 14).

Thus an effective system for preventing varnish formation would combine an anti-static filter element and dehydrator/degasser. A threeweek trial of an F-class kidney loop equipped with both the dehydrator/ degasser and anti-static filter element produced these dramatic results:

  • Fluid conductivity increased from 19 to more than 500 pS/m before it began drifting backward. The parameter is measured in-situ by a digital conductivity meter that meets the ASTM D2624 test standard.
  • Moisture content of the oil was reduced by 80%.
  • Dissolved gases were reduced by more than 50% as confirmed by a third-party laboratory.
  • Improvement in ISO-4406 cleanliness codes from 20/18/15 to 18/16/14

Long estimated the cost of the varnish prevention system described at somewhere between $25,000 and $50,000 depending on throughput. Annual operation and maintenance— including electricity and consumables (filter elements, gaskets, etc)—would be less than $5000, assuming quarterly filter replacements.

ISOPur’s David Cummings told the group that key to preventing varnish issues are a good oil supplier, good filtration system, proactive user, and a good laboratory. Regarding filtration, he said, the BCA improves filtration efficiency by making both hard and soft particles larger (Fig 15), plus it prevents varnish buildup and removes existing varnish. System effectiveness is illustrated by Fig 16, which shows how average particle size increases with each pass of the oil through the kidney loop.

A free-standing oil conditioning skid that would be piped into a kidney loop off the main lube-oil reservoir is shown in Fig 17. It consists of a prefilter, charging/mixing unit, collection filter, and variable-speed gear pump. The ultra-clean oil produced acts as a solvent and pulls back into the oil the sludge and varnish hiding out in servos, gearboxes, sumps, etc.

Cummings (dcummings@isopur. com, 860-599-1872) agreed with Livingstone’s comment above that peaking systems do create a more difficult environment for varnish removal/ control. All of the conditions that create the precursors to varnish can increase when the turbine is on turning gear, he added. Time on turning gear and the level of antioxidants in the oil impact varnish production, removal, and control, he continued. For best results, the BCA system should remain in operation when the unit is on turning gear to remove oxidized material, extend oil life, and minimize varnish.

ISOPur conducted 19-week BCA performance tests in parallel on seven GE 7FA engines equipped with the system at Tampa Electric Co’s Bayside Power Station. Each of the units has a 6000-gal main lube-oil tank, meaning the 10-gpm kidney loop provides about 2.5 reservoir “turns” daily. Average varnish-potential rating dropped from 38 to 23 during the period, with the range of unit “end of test” VP ratings extending from 8 to 33. VP numbers at the start of the test were between 32 and 42.

Gravimetric analysis numbers were more tightly bunched at the end of the test—between 0.18 and 0.32 and averaging 0.25—after starting in the 0.4 to 0.6 range with 0.5 as the average. Total count of 0.2- to 2.0-micron particles averaged 200,000 at the start of the test program and all but one of the units (test stopped early) finished the program at 25,000. Likewise, water content of the oil averaged 40 ppm at the start and all but one unit with suspect numbers ended at about 10 ppm.

Cummings recommended that filter elements be inspected and replaced at frequent intervals. He offered a change-out plan for both prefilters and collection filters for older (used) oils and new. For units with used oil, your total Year One expense will be equipment capital cost and about $1650 in replacement filters; for new oil, the capital cost is the same but replacement filters should not cost more than about $1000 the first year. Replacement filters for both new and old oil every year after the first will run about $700.

Testing should include submicron particle count/distribution; VPI or QSA; FTIR (Fourier Transform Infrared Spectroscopy) to evaluate an oil’s condition and the presence of contaminants—such as water; and the so-called RULER test, to measure the concentration of antioxidants present in the oil—primarily phenols and amines. Based on initial findings, an ongoing retest program can be developed.

Cummings closed by saying that BCA technology has been validated by GE Energy and other OEMs. Specifically, GE TIL 1528-3 (Nov 18, 2005) stated, “GE has performed extensive studies to validate the use of BCA technology. . .this technology can be used to mitigate a current varnishing issue or to prevent the occurrence of it.” System components have GE part numbers and can be ordered online through GE PartsEdge.

Justin Stover, C C Jensen’s sales manager, closed out the program with a presentation on the value of cellulose filter media for adsorbing varnish. C C Jensen, a 50-yr-old company with Danish roots, is a relative newcomer to the US electric power industry. However, its filtration systems are particularly well known in the global marine and oil and gas industries. More recently, several manufacturers of wind generation systems have standardized on C C Jensen filtration packages for their gear-oil and hydraulic pitch-control systems. Tens of thousands of these currently are in service worldwide.

Stover began with the basics, including a review of adsorption physics. Recall from your formal education that adsorption is all about using solids to remove specific substances from gases and liquids; molecular attraction is what makes the process of absorption work. Specific to this discussion, when varnish passes by an adsorbent, it attaches to its surface (Fig 18).

Cellulose is particularly effective in this regard; its high polarity is well suited to attracting oxygenated molecules—such as varnish. Stover stressed that this was a “natural” process—no voltage required, no control system, etc. Capacity is determined solely by surface area. He said that just one gram of cellulose has a surface area of about 4000 ft2 and that a standard filter cartridge contains 3600 grams of cellulose; you do the math.

Exactly what happens inside the filter media is described in Fig 19. Here’s a more detailed explanation of the terms used in the drawing: Diffusion is the transport of matter (varnish in this case) from one point (the oil) to another (the filter media). Film diffusion describes how the varnish molecules are drawn to the boundary of the cellulose fiber by means of the inherent physical forces (polarization, electrostatic, and hydrogen bonding).

Once “inside,” the varnish molecules move among, or between, the cellulose molecules in open spaces. The spaces are large relative to the size of the molecules, hence the term macropore diffusion. Next, the varnish molecules come to rest on the adsorbent surface—that is, they diffuse from the fluid onto the cellulose molecule (micropore diffusion).

Stover said the filtration system is easy to operate and maintain and that it is installed in a kidney loop like the other offerings described above (Fig 20). To illustrate performance on a 7EA, he used centrifuge samples and color values of the oil taken between September 2006 and March 2007 (Fig 21). The color value at the beginning of the test was a 63. More specifically, the number of particles in the size range of 0.2 to 1 micron was more than 20 million. At the end of the test, color was 0 and the particle count was less than 3400.

The capital cost of a fixed filtration system serving a 7EA in a typical low-varnish environment is about $7000. Annual filter costs are a nominal $1000 for peaking turbines; less than half that for unit in base-load service. ccj