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1.Overview varnish2.Varnish and its consequences

3.Formation: Precursors and Varnish4.Measurement

5.Varnish removal

How many of you have not experienced the following due to varnish?

1) Darkened color of lube oil2) Increased temperatures due to cooler coatings3) Plugged oil filters4) Sluggish operations / sticking control valves5) Accelerated rises in acid levels6) Expensive system flushes and clean-out

Reff: Analyst Inc.

Reff: Pall Corp.

Thin, insoluble film deposit that forms on surfaces inside the turbine lube system (pipes, tank, bearings heat exchanger, servo-valves, etc)

Comprised of a wide range of oil additive and thermo-oxidative breakdown, high molecular weight compounds. Varnish precursors are the result of breakdown from mainly:◦ Oxidation: water, air and metallic contamination from wear◦ Thermal stress from extreme temperatures in cases of:

static dischargeMicro dieseling: adiabatic compression of air bubbles

The chemical compositions of varnish precursor vary depending on the turbine type, operating conditions and the oil type. Varnish precursors are >75% soft particles, <1 micron

Varnish precursors are polar, and their solvency is temperature dependent◦ Over time migrate from the oil to machine surfaces - depending on system and

oil conditions.

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Varnish Precursors

Varnish

Reff: Analyst Inc.

Lube Oil Reservoir

3500 psiControl oil filter

To Hydraulic control circuitlittle/no flow at turning

Solenoid valve

Control oil circuit

Inlet Vane Control Fail open valve Gas valve

Compressor/Turbine Shaft

Main bearings ( roller or journal)

Hydraulic circuit: High VPR impact

Lube circuit Low VPR impact

Main pump

Main filter

Cooler

Contamination of critical GT control hydraulic components ◦ Fuel control valves◦ Safety fail open valves◦ Vane control valves

Inability to control operation, high maintenance cost for◦ Replacement of contaminated valves ◦ Cost of chemical clean and flush

Fail to start condition: Loss of flexibility

Trip during operation: Loss of production

Downtime and lost production represent a large share of the costs associated with poor varnish control in GT installations

Reff: Pall Corp.

Reff: Pall Corp.

Reff: Pall Corp.

All turbine oils create varnish precursors under normal operating conditions.

The rate of generation is higher under severe / unusual operating conditions. ◦ Oxidation◦ Additive depletion◦ Filter related electrostatic discharge◦ Micro-dieseling, adiabatic compression

Recent increase in varnish related problems is attributed to:◦ Higher operating temperatures◦ Smaller reservoirs◦ More peaking and cyclic service◦ Highly refined base-stocks (Group II - lower solvency for varnish)◦ Finer filtration resulting in electrostatic discharge

The solvency of varnish in oil is temperature dependent◦ Transition point 54 - 57 °C ◦ Temperature falls below 54 - 57 °C in the hydraulic section

Reff: Pall Corp.

Oil has limited solvency for varnish◦ Majority of the varnish in a turbine lube system

is in the form of deposits◦ Small portion of the total is suspended in the oil ◦ As the oil is cleaned up, it dissolves more

varnish◦ Lube system is clean when all removable

varnish deposit is goneVarnish deposits cause◦ Restriction and sticking of servo valves◦ The cost of valve replacement due to varnish is

~$30,000 + turbine down time costs

Reff: Pall Corp.

Inside filter housing

Contaminated trip relay piston

Reff: Pall Corp.

Pencil Filter, contaminated with varnish deposits

Servo valve spool: heavily contaminated

Varnish formation on a turbine bearing

Have You Seen This?Have You Seen This?

Reff: Analyst Inc.

Shuttle Valve

Compressor Gears

Gas Turbine Bearing

Plugged Filters

Reff: Analyst Inc.

Oil test for visc., TAN, RPVOT, metals, foam, etc will not indicate varnish potentialVarnish forming potential of oil can be measured by:◦ FTIR - nitration procedure◦ Gravimetric analysis (Toluene soluble)◦ Ultracentrifuge (~17,500 RPM for 30 minutes)◦ 0.2 – 2.0 um Particle Count (Modified ASTM D312) ORColor patch - Quantitative Spectrophotometer Analysis (QSATM) –Analysts Inc)◦ Most commonly used – Draft stage ASTM procedure◦ Measures discoloration / stain on patch◦ Proprietary calculations determines varnish potential rating (VPR)◦ Significant variation in VPR values between labs◦ VPR of <35 normal - no action◦ VPR of 38-58 - active monitoring◦ VPR 60-79 - abnormal ◦ VPR >79 - critical - immediate action

Reff: Pall Corp.

Reff: Pall Corp.

Reff: Pall Corp.

Measurement of varnish potential does not indicate the actual amount of varnish deposited on the surfaces of componentsIt measures varnish precursors in the oil

A system can be considered “varnish free” when varnish deposits have disappeared, not necessarily when varnish potential is down.Removal of varnish precursors from the oil displaces the solvency equilibrium in the oil, forcing deposit to “redissolve” in the fluid, then removed by varnish removal units. To be free of varnish, varnish precursor measurements must be consistently low for an extended period.

Actual clean up time depends on◦ Efficiency of varnish precursor removal ◦ Amount of deposits already present in the system◦ Solvency behavior of varnish in the system (site dependent –

machine dependent – oil dependent…)

Reff: Pall Corp.

The electrostatic method (EST)Kidney-loop mode, off the main tankOil is subjected to electrical field causing varnish particles to:◦ charge / agglomerate to larger particles◦ captured by filter mat or ◦ attach to charged, disposable surface◦ As the oil is cleaned up, it lifts varnish deposits into the oil

phase, cleaning the surfaces

Reff: Pall Corp.

Chemical cleaning/flushing

◦ Lube system flushing with chemicals / solvents ◦ Softens and removes insoluble materials and the flushing

action suspends and helps remove the material by fine filters◦ Several hours to several days◦ System is flushed with appropriate flush fluid to remove

residual chemicals◦ Intensive & costly process. ◦ Allows quicker removal of deposits.◦ Continuous monitoring and turbine shut down◦ Cost of flushing: $50,000 to $60,000.

Reff: Pall Corp.

The adsorption method:◦ Utilizes large surface area, high void volume◦ Low fluxes allow proper residence time for adsorption◦ Electro-chemical affinity of the filter media for varnish

particles is a KPIParticles captured by adsorption, egvarnish precursors

Particles captured by direct interception, eg wear metals