Nearly half of the power generated in the US every year is produced by gas turbines, and their importance with respect to meeting the world’s energy needs is growing as less efficient coal-fired power plants are phased out. Industrial gas turbines are, therefore, critical infrastructure and their reliability is paramount. Thanks to their sophisticated engineering, many gas turbines now boast reliabilities that exceed 99%. GE’s family of 176 MW Frame 7F turbines, for instance, are reported to have an industry-leading 99.3% reliability.

The True Cost of Gas Turbine Varnish

Despite these impressive claims, a 99.3%-reliable system will, nevertheless, be down 0.7% of the time. On an annual basis, this represents a production loss of 61 hours. EPRI has reported that 54% of “forced (turbine) outage hours” are the result of oil contamination.4 Even the 99.3% reliable Frame 7F turbine will, therefore, experience an average of 33-hours of avoidable oil-related downtime annually. At a conservative average wholesale price of $40/MWh,5 this amounts to an average annual loss of $232,320, which, in turn, represents up to $5.8 million in avoidable losses over the turbine’s 25-year lifetime.

As the most common cause of oil-related downtime in gas turbines, varnish is the single greatest contributor to these losses. As a result, numerous varnish-removal or mitigation strategies have been developed. These include: aftermarket chemicals (AMCs), depth filtration, and ICB® ion-exchange filtration. Despite the high costs associated with varnishing, many gas turbine operators fail to recognize the value that varnish-removal and/or mitigation can add to their operations; Sadly, a $45,000 capex for an effective varnish-removal system is viewed as a luxury, while a $232,320 annual loss is seen as normal.

To fully appreciate the economic benefit of varnish mitigation/removal, this paper will outline the total cost of ownership for three leading technologies and compare them with the costs of failing to address varnish over a turbine’s complete 25-year lifespan. Given the magnitude of the reliability-related losses described above, it will come as no surprise that the costliest thing any gas turbine operator can do is nothing at all.

Explore the data, science, and lifetime economics behind ICB ion-exchange filtration and lubricant chemistry management, and request the full technical paper below.

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Users often complain that before using Group II base-stock oils, they didn’t have a problem with varnish. As a result, Group II oils have a reputation for being varnish prone, leading users to search for alternatives. The market has responded, with many companies offering low-varnish versions, co-base stock oils, synthetic oils or even after-market additives for the more adventurous.

The truth is, Group II oils do have less capacity to hold breakdown products in solution than Group I. This is because breakdown products are polar in nature and on the spectrum of polarity, Group II base stocks are less polar (more non-polar) than Group I base stocks. Therefore, because “like dissolves like,” polar breakdown products are more likely to precipitate out of solution from more non-polar Group II oils than Group I resulting in varnish deposits.

To understand how these oils breakdown over time, we ran thermo-oxidative breakdown experiments in the lab subjecting each oil sample to peak turbine operating temperature (150°C), air and a copper catalyst. The results presented include acid number, LSV amine and phenol antioxidants and MPC varnish potential (Fig. 1 & 2). 

In the Group I sample, we see a rapid increase in varnish potential achieving a critical value in <5 hours. Acid number remains stable until approximately 80 hours when a rapid increase is observed corresponding with depletion of the amine anti-oxidants. While a Group I user may not change this oil because of MPC value, they would be forced to because of the significant increase in acid number. In the Group II sample, varnish potential values increase gradually, not hitting a critical value until 400 hours. Strikingly, we see no increase in acid number with amine life extended to 700 hours – 10x that of the Group I sample. 

Overall, we see a Group I base-stock oil that is sensitive to oxidation that quickly reaches critical varnish potential values and a Group II oil extremely resistant to oxidation with no acid production over the life of the test, even after the antioxidants are consumed. While some users may discount this lubricant because of the varnish potential numbers observed, in actuality the performance is exceptional. Considering that resin-based varnish removal systems are very effective, the perceived limitations of Group II oils are misunderstood.

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Users often complain that their turbomachinery lubricant is not performing well because the varnish potential (MPC) number is elevated, or their oil is oxidizing when additives are still present. This drives users to search for a different lubricant or after-market additives when the problem lies with the expectation being placed on the oil – namely that little to no maintenance should be required.

The misconception is that only poor quality lubricants oxidize, and that varnish is a result of improperly formulated oils. While lubricant quality is a key criteria, in reality all lubricants break down due to oxidation – even with additives – and increasing varnish potential over time is normal when dissolved oxidation products are allowed to accumulate.

Rather than discussing more effective varnish removal systems, vendors are now moving the discussion toward synthetic oils and after-market additives to extend the operating range of lubricants with high MPC values. While there is a place for advanced lubricant technologies, simple and more cost effective solutions are available using conventional lubricants to permanently remove dissolved oxidation products and maintain ultra-low MPC values.

EPT Clean Oil published a technical paper that discusses varnish formation in depth. Oxidation starts on the very first day a lubricant is put into service. The oxidation process creates a dissolved breakdown product, or varnish precursor, that will accumulate in the oil over time until it becomes saturated. Once a saturation point is achieved, these varnish precursors physically convert from the dissolved form into solid deposits.

The saturation point varies throughout the mechanical system due to differences in temperature and pressure, which dictates where varnish deposits will form first. For this reason, it is important to keep the lubricant in an unsaturated state by removing the varnish precursors. By doing so, you can prevent the lubricant from becoming saturated at any temperature or pressure present in the mechanical system and prevent varnish formation.

The below short video animation, offers further information on the varnish formation cycle.

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Matthew G. Hobbs, PhD, Peter T. Dufresne Jr.

Paper Abstract

Despite the critical nature of electrohydraulic control (EHC) systems, many steam turbine operators’ oil analysis programs fail to detect fine insoluble contaminants that are responsible for costly downtime and failures.  With minor modifications, however, gas turbine oil tests can be made suitable for use in steam turbine EHC applications.  A modified version of the industry-standard membrane patch colorimetry (MPC) test, in particular, allows for the identification of “hidden” contaminants that are often responsible for costly EHC failures.  By exploiting MPC analysis to its full potential, this method can be used to qualify application-specific problems like varnishing and dieseling in addition to quantifying their severity.  Once these previously “hidden” problems have been identified, effective EHC fluid maintenance practices can be developed so that steam turbine performance and reliability may be ensured.

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Peter T. Dufresne Jr., Matthew G. Hobbs, PhD

Paper Abstract

While gas turbine oil maintenance is recognized as being essential, many programs lack the basic tools to maintain their lubricants within specification.  Existing lubricant maintenance is typically reactive in scope, uses filtration technology from the 1970s and does not target the root cause of many lubricant and mechanical failures: varnish.

This paper discusses a modern approach to lubricant maintenance that offers technical advantages to gas turbine owners.  This approach maintains lubricant quality in ideal condition on a consistent basis, improving performance and eliminating common issues.  Furthermore, this approach can eliminate the cause of lubricant failures, extending the fluid’s operating life to 20 years or more.

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Matthew G. Hobbs, PhD, Peter T. Dufresne Jr.

Paper Abstract

Steam turbines are responsible for up to 80% of global electricity production. Because steam powers these units, ingression of water into steam turbine oils is a common concern. As the saying goes: “Oil and water don’t mix.” When they do, failures and costly downtime may result. A lubricant’s ability to separate from water is, therefore, paramount to the generating unit’s reliability.

While turbine oils are engineered to readily separate from water, their ability to do so (demulsibility) often becomes impaired during service. The factors which impact demulsibility are many; fortunately, the solution to this problem remains the same regardless of its root cause. Indeed, well-engineered ion exchange-based treatment systems have shown considerable promise when it comes to the restoration of turbine oil-water separability. In addition to restoring demulsibility, these resins also allow for the removal of oil breakdown products (varnish etc.) which present a further challenge to power generators.

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Matthew G. Hobbs, PhD, Peter T. Dufresne Jr.

Paper Abstract

Turbine lubricants employ a variety of base fluids. Mineral oil-derived base stocks remain the most prevalent, however, synthetics are becoming increasingly common. These fluids are exposed to severe oxidative stress during service, leading to breakdown and varnishing (the primary cause of downtime in gas turbine applications). Recently, PAGs have been positioned as varnish-free alternatives to more conventional mineral turbine oils. Our findings suggest that this claim is overstated.  PAGs possess several advantages relative to conventional oils, however, well-formulated mineral oils proved almost as resistant to varnishing. Moreover, the risks associated with mineral oil varnishing can be effectively mitigated using established varnish-removal technologies.

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Paper Abstract

Varnish is produced by breakdown of hydrocarbon lubricants.  Its deleterious impact on equipment performance and reliability is well-documented.  Varnish has traditionally been defined as an insoluble deposit, however, it also exists in an often-overlooked soluble state.  While soluble varnish forms as the result of an irreversible chemical reaction, the conversion between soluble and insoluble varnish is often a physical process; importantly, this process is reversible.  Like other interconvertible states of matter, the relative amount of soluble and insoluble varnish in a system is dictated by a dynamic equilibrium.

Numerous oil treatment strategies have been developed to mitigate the harmful effects of varnishing.  In order to be successful, these strategies must address the effect that their use will have on the soluble-insoluble varnish equilibrium.  Most rely on filtration, which focuses on the removal of insoluble varnish particles.  Unfortunately, these systems have little impact on varnish’s soluble form; once they remove insoluble varnish, soluble varnish persists and will re-establish an equilibrium leading to the formation of new particles/deposits.  Filtration, therefore, addresses only half of the varnish problem.  Resin-based soluble varnish removal (SVR®) systems are engineered to remove dissolved varnish at the molecular level.  As the soluble-insoluble varnish equilibrium is re-established, insoluble varnish then begins to re-dissolve.  SVR systems exploit the soluble-insoluble varnish equilibrium and provide a total solution by removing both varnish states. Their use can, therefore, mitigate the risks associated with lubricant breakdown.

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