Varnish issues still on the minds of users

Proper care of your turbine’s lube oil is critical to reliable operation. A significant contributor to unavailability, especially at plants powered by gas turbines relying on mineral-oil lubricants, is varnish, which can impede operation of servos in control systems, plate out on bearing journal surfaces and increase the severity of vibration, etc. Varnish formation is impacted by lubricant chemistry, the additive package, temperature, and other factors—some under your direct control, others not so much.

Perhaps the most practical and cost-effective way to mitigate the adverse impacts of varnish in any given steam- or gas-turbine lube-oil system is to continually remove it from the circulating fluid. There are several commercial solutions capable of doing this, some better than the others under certain operating conditions. One way to identify the solutions providers is to use CCJ’s online buyers guide; another is to type an appropriate keyword into the search-function box on towards the top of this page. If you want to “kick the tires,” attend one of the upcoming user-group meetings—Steam Turbine Users Group, Combined Cycle Users Group, CTOTF ™, 7EA Users Group —and walk the vendor fair.

That varnish is a significant concern in the user community was in evidence at the 501F/501G and 501D5-D5A meetings this spring with C C Jensen Inc’s Technical Manager, Axel Wegner, invited to speak at both conferences. The two presentations were much the same; however, a D5 user joined Wegner at the podium for the second meeting, giving his presentation a high level of credibility among attendees.

Wegner began both presentations with a review of a typical oil analysis incorporating data from RPVOT, FTIR, acid number, Ruler, Karl Fisher, and other tests. This material has been discussed by many presenters at user-group meetings over the years and is familiar, if not well understood, by most owner/operators with supervisory positions on the deck plates. Those who benefitted most from this segment of the presentation were first-timers and those new to the industry—typically about a third to half of the attendees. Access “Analysis and maintenance of turbine lubricants” for a quick review of information you should be aware of.

Regarding the presence of varnish in lube oil, Wegner said the most meaningful results for the majority of users are obtained from the Membrane Patch Colorimetry, or MPC, test, which recently was approved by ASTM (American Society for Testing and Materials). The value of ASTM certification is a well-defined and repeatable test process with results from different labs comparable. The test involves making a patch that isolates and agglomerates insoluble byproducts associated with varnish. The color of the patch provides a guideline as to the extent of varnish potential.

Wegner’s personal favorite is the UC test, because you can see varnish in the oil that could plate out in a turbine system. When relying on ultra-centrifuge test data, keep in mind that any result above “1”—the lowest value on the 1-8 scale—is unacceptable.

The next segment of Wegner’s presentation focused on choosing the proper oil conditioner for a riven application. He told the users, “Today we know the following three methods for removing varnish:

      • Physical filtration, including absorption and adsorption, removes only insoluble varnish. You can select from depth or surface filters, he said, with or without preconditioning step—such as electrostatic, balanced charge agglomeration, etc.

      • Chemical filtration removes both soluble and insoluble varnish. In use are cartridges with chemical bead compositions of different mixtures which can be adjusted to achieve better performance on different oil brands and machine types. This solution can be pricy. A disadvantage of it is there’s no increase in pressure drop to indicate varnish saturation: You have to carefully monitor MPC results to know when the beads are “full” and must be replaced.

      • Depth-filter absorption/adsorption with advanced agglomeration removes both soluble and insoluble varnish. This is the C C Jensen solution—VRU, for varnish removal unit (Fig 1A)—which Wegner said effectively preconditions the oil in a manner that soft contaminants fall out of solution (without chemical aids), agglomerate, and are removed by depth filter inserts with high dirt holding capacity (Fig 1B). The VRU removes particulate matter, varnish, and water from the oil. This was said to be the only method where high pressure drop indicates varnish saturation and when the filter inserts must be replaced (Fig 1C).

CCJensen Fig 1

The obvious question from a user: Which method should I choose? Wegner said, “That entirely depends on the efficiencies of alternative systems in your specific application.” He offered these general criteria:

      • Systems with oil operating temperatures up to about 100F can be treated by any one of the three options identified above.

      • In systems where lube-oil temperatures alternate from less than about 100F to more than 100F (such as daily-start engines), all three options generally work as well. However, the physical filtration method might prove less effective at higher temperatures unless the filter medium is upgraded, because more varnish will be in solution.

      • When lube oil is constantly over 100F, physical filtration is not viewed as an effective option for varnish removal, according to C C Jensen’s technical expert. Some of the chemical solutions also may be temperature limited, he said, although most can handle up to 160F. The VRU was said to not be limited by even the highest oil temperatures generally encountered in powerplant service.

CCJensen Fig 2

Wegner’s next topic: Methods to validate the effectiveness of an oil conditioning system. He offered the following:

      • Oil analysis. The VRU’s 90+% single-pass efficiency is best among the alternatives and offers a foolproof method for evaluating system performance right out of the box; chemical filtration generally ranges between 10% and 50% single-pass efficiency, while the range of numbers for physical filtration are lower. Trending of MPC levels in the turbine sump over time is another way to chart progress. Wegner recommended sampling the sump every two weeks until varnish is reduced to an acceptable level (less than MPC of 5). He added that the VRU typically halves the MPC value every two weeks until it drops to 5 or less (Fig 2).

      • Filter inspection is simple. Just visually check used filter inserts for varnish accumulation.

      • Visually inspect turbine components—such as servos, bearing journals, thrust-bearing shoes, etc—for varnish deposits.

      • Compare key performance indicators over time, including these:

      • Sump oil temperature. Lube-oil coolers are more effective without an insulating layer of varnish.

      • System oil pressure. Deposits can impede the delivery of oil to bearings, possibly reducing system pressure to less than that required for optimal lubrication.    

      • Vibration can increase because of varnish deposits on journal surfaces.

Fig 3 shows the positive impact of varnish removal graphically.

CCJensen Fig 3

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