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The End of Varnish: Advances in Turbine Fluid Technology to Improve Gas Turbine Performance


Varnish build- up in today’s more powerful gas turbines is the root cause behind many power station shutdowns and results in lost power generation capacity. A small amount of varnish can cause close tolerance servo valves to stick and automatically “trip.” A shut down can cost tens of thousands for a partial load trip and the cost of a full load trip event can be even higher.

The threat of varnish has become greater with the use of more highly refined turbine oils. Moving beyond petroleum, the field success of polyalkylene glycol (PAG)- based turbine fluids offers operators an alternative solution to the varnish problem. PAG- based turbine fluids eliminate varnish formation in large frame gas turbines and have more than 7 years and 60,000 hours of service history. PAG’s improve starting reliability, eliminate unplanned shutdowns, ensure long- term fluid service, and extend equipment operating life. The result is reduced maintenance and repair expense.

We will explore the threat varnish poses to large frame gas turbines in power generation service, how conventional hydrocarbon oils contribute to varnish formation, and how PAG- based turbine fluids and a new PAG- based turbine oil base stock modifier can enable operators to put an end to varnish problems.

By Kevin KovandaPresidentAmerican Chemical Technologies, Inc.

For the first time, General Electric’s gas turbine lubricating oil recommendation document GEK 32568h, released in June of 2013, includes a PAG- based fluid specification.



The Varnish Threat in Gas Turbine Power Generation

A sticking servo valve can cause a gas power generation turbine unit to trip or shut down in an instant. Taking power generation capacity off line is a costly event:

• Lost generating capacity deprives the operator of revenue

• Replacement power must be purchased at high spot rates

• Unplanned maintenance expense is incurred

• Intervals between costly scheduled maintenance are reduced

Varnish has become an increasingly important issue in the gas turbine power generation industry because of trends in turbine design, evolving operating practices, and changes in hydrocarbon- based turbine oils.

Larger, More Powerful Turbines

Today’s gas turbines are larger with greater power generating capabilities. This demand on the equipment is greater. Therefore, there is more stress on both the unit itself and its lubricating oil. As a turbine oil ages, varnish formed in the lubricant can precipitate out of solution and adhere to critical surfaces. If the varnish invades the tight tolerances surrounding servo valves, it can impede valve movement. When valve movement is hindered the default action is to trip the turbine. This results in unplanned downtime, lost megawatt production, and eventually a high repair expense. The last chance filters installed ahead of servo valves offer no guarantee of protection. As shown in the accompanying photo, these filters often become plugged. Without constant maintenance, loose varnish can break through and plug the valve.

FIGURE 1 – LAST CHANCE FILTER CLOGGED WITH HYDOCARBON TURBINE OIL



Demands of “Peaking” Service

Turbines that operate only during spikes in power, “peaker” turbines, have system demands that are greater than base-load units. Start- and- stop peaker service is more stressful on the turbine and the lubricant. As the unit cools down, oxidation by-products precipitate out of solution and form varnish. This directly impacts the reliability of start-up the next time the turbine is needed.

Changes in Hydrocarbon Oils

The properties of today’s hydrocarbons are perhaps the single greatest contributors to varnish formation in gas turbine power generation. There has been a gradual shift to more refined hydrocarbon base oils. This has reduced the solvency of turbine oils, lowered their conductivity, and affected air release properties. The result is an increase in the potential for varnish formation and other problems that affect the reliability of gas turbine operations.

Solvency – Group I lubricant base stocks were not as susceptible to varnish formation. However, lubricants made from Group I oils were high in sulfur and aromatic content. As the products of the petroleum industry were held to higher standards of environmental protection and worker safety, their properties changed.

The aniline points of various lubricant base stocks demonstrates the change in solvency properties of various types of lubricant base-materials. The aniline point of oil is defined as the minimum temperature at which equal volumes of aniline (C6H5NH2) and the oil are miscible (form a single phase when mixed). Lubricants with a lower aniline point have a greater ability to dissolve polar materials such as oxidation by-products and performance additives.

Figure 2 shows the superior solvency of PAG base stocks compared to other base stocks.

FIGURE 2 – ANILINE POINTS OF BASE OILS

Ani

line

Po

int

(ºC

)

Solv

atin

g P

ow

erGroup I (MO)

VI 95Group II (MO)

VI 102

Group II (MO) VI 135

Group IV (PAO) VI 136

Group V (PAG) VI > 200



Changes in available base oils affected every industry where they were used. In automotive applications, equipment fill amounts are relatively small. Therefore, oil change intervals can be relatively short and the degradation of the oil is matched to other use factors. However, in the power generation industry, 3,100 gallons of lubricant are required to fill a GE7EA turbine and 6,100 gallons are needed to fill a GE7FA. Operators need the lubricant to last for extended time intervals before the oil needs to be replaced. In practice, the more highly refined hydrocarbon oils have been reducing service life expectations without significant maintenance support, the more highly refined hydrocarbon oils are reducing service life expectations. Operators invest heavily in services to predict and remove varnish in turbine units as they attempt to extend the useful life of their oil.

Reduced Conductivity – Another property that changes with the refining of hydrocarbons is conductivity. Low electrical conductivity of the lubricant results in electro static discharge (ESD). ESD is an event that accelerates the degradation of turbine oils. These spark discharges rapidly oxidize turbine oil resulting in premature varnish formation. “Hot spots” can form on equipment surfaces, contributing to arcing. The presence of wear metals in the oil can also contribute to static discharge. The evolution of hydrocarbon base oils has reduced oil conductivity, increasing the potential for ESD and the problems it creates.

Extended Air Release – Varnish build-up in the lubricant prolongs air release times. These entrained air bubbles contribute to micro- dieseling, pump cavitation, premature oxidation and varnish formation as well as component wear. Micro- dieseling is a mechanical degradation mechanism that occurs when small air bubbles entrained in turbine oil are first compressed and then explode. The result is destruction of the oil.

Why PAG- based Fluids Eliminate Varnish

The non- varnishing performance of PAG- based turbine fluids has now been documented in large- frame gas turbines in power generation service for more than seven years and 60,000 hours. As a result, turbine OEM General Electric has included a PAG- based turbine fluid specification in its June 2013 gas turbine lubricating oil recommendation document GEK 32568h.

The comparison photos in Figure 3 dramatically demonstrate the difference in varnish accumulation with hydrocarbon oils versus PAG- based turbine fluid in last chance filters ahead of servo valves in large frame gas turbines. Operators of GE7FA units report that after changing filters every 3 to 6 months while operating with hydrocarbon oils, their last chance filters have never required replacement in the years since the turbines were converted to PAG- based turbine fluid.

FIGURE 3 – LAST CHANCE FILTERS WITHOUT AND WITH PAG TECHNOLOGY

Last chance filters after 6 years and 55,000 hours of service with PAG- based turbine fluid.

Last chance filter clogged with varnish and sludge produced by hydrocarbon turbine oil.



Unique Polar Chemistry

What are PAGs and why are they so effective in eliminating varnish and related turbine operating problems? Unlike refined hydrocarbon oils, polyalkylene glycols are full synthetic materials where oxygen is every third atom of the polymer backbone. These versatile, thermally stable polymers are used in a wide range of lubrication applications including fire-resistant hydraulic fluids, environmentally acceptable lubricants for marine vessels, synthetic compressor fluids, and turbine fluids.

PAG- based fluids are clean lubricants with natural detergency. It is the oxygen in the polymer that is the origin of its unique chemical properties. The polarity allows for a wide degree of solvency towards many materials including oxidation by-products and performance additives. Additive and by-product solubility is important when assessing the varnish formation potential of hydrocarbon oils versus PAG-based fluids in gas turbine operations.

FIGURE 4 – PAGS

The oxygen rich polymer provides polarity and non-varnishing performance in turbine systems.



Polar Solubility

Figure 5 shows what happens when decomposition by products are formed in conventional hydrocarbon turbine oils, which are non- polar substances. The by- products produced by oxidation of the hydrocarbon oils are polar. Under typical temperatures, system operating cycles and the passage of time, these polar by- products form insoluble varnish that precipitates out of solution and adheres to system surfaces. Additives are included in hydrocarbon oils formulations in an effort to increase oxidative stability but with only partial success.

In contrast, the PAG- based turbine fluid produces a smaller amount of low molecular weight polar by- products. The oxidation by-products remain in solution even under prolonged exposure to typical turbine operating conditions. To summarize, the PAG- based turbine fluids eliminate varnish because any by- products they produce remain soluble in the fluid and will not precipitate out to form sludge or varnish in the system.

FIGURE 5 – WHY PAG- BASED TURBINE FLUIDS WILL NOT FORM VARNISH

Non-Polar Turbine Oil

Polar Turbine FluidLow MW

® Polar By-Products

of Oxidation

*MW=Molecular Weight *PAG=Polyalkylene Glycol

while Depletion of Antioxidants are Occuring

(Amines and Phenols)

Will Not Precipate Out of Fluid at Temperature

Variations

that Precipitates Out of Solution and Adheres to Surface

EcoSafe® TF-25 acknowledged in G.E. GEK32568g

(revised December 2011)

Homogeneous in Solution

Polar By-Products of Oxidation

Agglomeration of Soft Contaminants Nearing Saturdaion Levels...

Temperature, System, Cycles, & Time

Additional Degradation May Occur, but By-Products Remain in Solution

Polar By-Products of Oxidation Grow and Form insoluble Varnish...

Petroleum-based Turbine Oil, including

Synthesized Hydocarbons (SHC) or

Semi-Synthesized

PAG-based Synthetic Turbine Fluid EcoSafe

®

TF-25 (100% PAG®

Turbine Fluid

CONDITIONS THAT SIMULATE VARNISH

FORMATION



Lower Potential for Electrostatic Discharge

Electrostatic discharges (ESD) within turbine systems create high temperature “hot spots” (10,000°C) that contribute to thermal degradation and premature varnish formation. As hydrocarbon oils have become more highly refined, purity has increased and conductivity has gone down. Lower conductivity increases the potential for ESD. Figure 6 compares the conductivity of Group V PAG- based base oils to the three hydrocarbon oil groups as well as Group IV polyalphaolefin (PAO) synthetic base oils. PAG- based fluids are higher in conductivity and have less potential for static discharge than the hydrocarbon oils and PAO. Note that spark discharge will occur at conductivity (pS/m) levels of 500 or less and may occur between 500 and 1,100 pS/m. PAG fluids provided more protection against ESD than Group I hydrocarbon oils, which previously provided the industry benchmark for acceptable ESD performance.

FIGURE 6 – ELECTRO STATIC DISCHARGE COMPARISON

The conductivity less than 500 pS/m Promotes ESD

Group I Group II Group III Group IV Group V



Reduced Potential for Micro- dieseling

The persistent presence of entrained air in turbine oils is a primary contributor to micro- dieseling, a thermal degradation mechanism that occurs when small air bubbles in turbine oil are compressed, explode and burn the oil. Figure 7 compares the length of time required by hydrocarbon turbine oils and PAG- based lubricants to get to 0.2% entrained air volume. The GEK 32568F OEM Spec allows for a maximum of 5 minutes to reach the target 2% level of entrained air. Even in the presence of water contamination, PAG fluids demonstrate excellent release of air, outperforming hydrocarbon oils and reducing the threat of varnish and other damage caused by micro- dieseling.

FIGURE 7 – ENTRAINED AIR COMPARISON*

Lubricant Minutes to 0.2% Entrained Air Volume

PAG- based Fluid (neat) 0.4

PAG- based Fluid with 2000 ppm water 0.7

PAG- based Fluid with 4000 ppm water 1.0

Hydrocarbon Oil A 4.0

Hydrocarbon Oil B 5.0



PAG- based Fluids Meet or Exceed Industry Performance Requirements

The properties of PAG- based turbine fluids compare very favorably to those of hydrocarbon oils as shown in Figure 8. Note that the higher viscosity index of PAGs allows the use of a lower viscosity grade to achieve the same absolute viscosity as an ISO 32 hydrocarbon oil. Using a lower viscosity grade reduces friction, and increases overall system efficiency while reducing thermal demand on bearings. The higher viscosity index of PAG- based turbine fluid also enables it to retain excellent viscosity characteristics over a wider temperature range than hydrocarbon oils. PAG- based fluid has a low pour point for cold weather start up of peakers and offline base load units. Figure 9 shows the viscosity of PAG- based turbine fluids versus hydrocarbon oils across a broad temperature range.

FIGURE 8 – PROPERTIES COMPARISON

Typical Properties PAG- based Turbine Fluid

Hydrocarbon Turbine Oil

ISO Viscosity Grade 25 32

Viscosity Properties cSt cSt

@40°C (104°F) 26.23 32.44

@100°C (212°F) 5.19 5.56

Viscosity Index 132 109

Thermal Conductivity @ 40°C watts/m °K 0.145 0.1

Four Ball Wear Scar mm 0.63 0.65

Air Release Results ASTM D 3427 Minutes to 0.2% Entrained Air Volume 0.4 4.0 – 5.0

Biodegradable Yes —

Hydrolytic Stability No Reaction with Water

Forms Acids, De-grades



FIGURE 9—VISCOSITY VS. TEMPERATURE COMPARISON

FIGURE 10 – TURBINE FLUIDS HEAT CAPACITY COMPARISON

Energy Efficiency – Heat Capacity

Figure 10 shows the greater heat capacity of PAG- based fluids, which can translate into additional protection for turbine wear surfaces. Owners and operators of gas turbines report 3-5°C (5-10°F) reduction in bearing temperatures and up to 12°C (22°F) reduction in system temperatures when PAG-based turbine fluids are used in place of hydrocarbon oils.

Absolute Viscosity

Vis

cosi

ty (c

P)

Temperature (Cº)

PAG based Turbine Fluid Petroleum-based Turbine Oil

Volumetric Specific Heat Cp.P

Vo

lum

etri

c he

at c

apac

ity

in J

/(cm

3 k)

Temperature (Cº)

PAG Oils

Traditional Oils

Ti Su SL PCX

PAG 46-2

Total HC 5W-30

PPG 32-2

PAG 40-4



Four Ball Wear Testing

PAG- based turbine fluids provide excellent wear protection and are tolerant of water contamination. The data in Figure 11 show hydrocarbon oils do not offer the same level of protection, even when PAG- based fluids contain up to 2% water.

FIGURE 11 – FOUR BALL WEAR COMPARISON*

Lubricant Scar Diameter (mm)

PAG- based Turbine Fluid, neat 0.65

PAG- based Fluid with 7,500 ppm water 0.67

PAG- based Fluid with 20,000 ppm water 0.66

PAG- based Fluid with 2,900 pp water after 10,200 operating hours in a GE 7FA Turbine 0.77

ISO 46 Hydrocarbon Base Oil 0.83

Hydrocarbon Turbine Oil 0.75 – 0.80

*ASTM D 4172 conditions: 40Kg, 1200 rpm, 1 hour, 75°C.



Hydrolytic Stability

While hydrocarbon turbine oils degrade and form acids when exposed to water, PAG- based turbine fluids are inert to water as shownv in Figure 12 which shows the effect of water contamination on total acid number (TAN) in PAG- based turbine fluid.

FIGURE 12 – PAGS ARE INERT TO WATER

Total Acid Number and Water content for AEPRES_O 1A

TAN

(mg

KO

H/g

)

Wat

er(p

pm

)

Total Acid Number

Water



A Two- Step Approach to PAG Conversion

PAG- based technology from American Chemical Technologies (ACT) can be used to convert turbine units immediately to PAG- based turbine fluid or to extend the life of conventional hydrocarbon fluids by removing existing varnish and reducing the potential for future varnish problems. This offers owners and operators a two- step approach to incorporating the non- varnishing performance of PAG chemistry in current turbine units.

EcoSafe® Revive™ is a patented base oil modifier exclusively available from ACT that enables owners and operators to put an immediate end to varnish problems and scale back costly varnish management expense while continuing to use their current hydrocarbon oil until it reaches the end of its target service life. When added to conventional turbine oil, EcoSafe® Revive™ eliminates varnish, improves lubricity, and extends the life of the oil. Addition of EcoSafe® Revive™ is predicted to double the life of typical hydrocarbon oils, greatly reducing the cost of operation and protecting against the need to replace the fluid prematurely.

Recommended for use at an addition rate of 10- 20%, EcoSafe® Revive™ is designed to repair existing oils identified with high varnish potential ratings or to change the solubility of newer oils so that varnish does not occur. The treatment changes the solubility of hydrocarbon oils by shifting their polarities, so oxidative by- products remain in solution and do not form sludge or deposits on equipment surfaces. Existing varnish is re- solubilized; phenolic and aminic antioxidants are regenerated and performance additives are reactivated.

EcoSafe® Revive™ is 100% soluble and is compatible with all major commercial hydrocarbon base oils. No conversion process is required.

To take full advantage of PAG- based turbine fluid technology, owners and operators can replace conventional hydrocarbon fluids entirely, installing EcoSafe® TF- 25 PAG- based Turbine Fluid and eliminating varnish once and for all. EcoSafe® TF- 25 has been proven to eliminate varnish and protect large frame turbines in more than seven years and 60,000 hours of power generation service. The fluid complies fully with the PAG fluid specification within General Electric’s gas turbine lubricating oil recommendation document GEK 32568h.

EcoSafe® TF- 25 is non- sludge or varnish forming to significantly reduce maintenance requirements and the need for costly varnish abatement products and services. The excellent low foaming and air- release properties of the fluid reduce the potential for micro- dieseling and related damage. The higher conductivity of EcoSafe®TF- 25 also reduces electrostatic discharge (ESD) and the hot spots that cause premature varnish formation. Available exclusively from ACT, EcoSafe® TF- 25 takes advantage of the inherent lubricity of PAGs to provide excellent wear protection. The fluid is hydrolytically stable and will not break down and react with water, minimizing fluid degradation and acid formation that can damage equipment.

High temperature stability is also excellent and the higher viscosity index of EcoSafe® TF- 25 allows use of a lower viscosity grade to achieve the same absolute viscosity as ISO 32 grade hydrocarbon oil at typical operating temperatures. This reduces friction and increases overall system efficiency while reducing thermal demand on bearings. EcoSafe® TF- 25 is compatible with commonly used seals, hoses and metals. It is classified as readily biodegradable and environmental impact is low in the event of a spill or leak. EcoSafe®TF- 25 also satisfies stringent criteria for toxicity, to support a safe and healthy working environment.

STEP 1: Repair or Prevent Varnish in Existing Oils

STEP 2: Convert to PAG- based Fluid to Eliminate Varnish Entirely



Conclusion

Over more than 7 years and 60,000 hours of power generation operation, PAG- based turbine fluids have been proven to eliminate varnish formation in large frame gas turbines, delivering improved starting reliability, eliminating unplanned shutdowns, ensuring long- term fluid service, and extending equipment operating life, all while reducing maintenance and repair expense. As a result of this performance track record, General Electric’s gas turbine lubricating oil recommendation document GEK 32568h, released in June of 2013, now includes a PAG- based fluid specification. Now with access to ACT’s two- step approach, owners and operators of gas turbines can select the PAG adoption method that best matches their needs. Either route enables them to quickly put an end to vcostly varnish problems, improving operating reliability, eliminating unnecessary maintenance costs, and protecting their significant equipment investment.

About ACT

American Chemical Technologies, Inc. (ACT) is at the forefront of synthetic lubrication technology. We create, supply, and technically support high performance solutions that extend equipment life, reduce operating expense and help protect the environment. Today, polyalkylene glycol (PAG) and other specialty lubricants from ACT are at work globally, meeting the varnish control/cleanliness, fire resistance, biodegradability, water solubility and other demands of industry leaders in energy, metals, marine, amusement, tunneling, and mobile equipment. ACT is headquartered near Detroit in Fowlerville, Michigan, USA.

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