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1. Overview varnish 2. Varnish and its consequences3. Formation: Precursors and Varnish 4. Measurement 5. Varnish removal
How many of you have not experienced the following due to varnish? 1) Darkened color of lube oil 2) Increased temperatures due to cooler coatings 3) Plugged oil filters 4) Sluggish operations / sticking control valves 5) Accelerated rises in acid levels 6) Expensive system flushes and clean-out Reff: Analyst Inc .
Thin, insoluble film deposit that forms on surfaces inside the turbine lubesystem (pipes, tank, bearings heat exchanger, servo-valves, etc)Comprised of a wide range of oil additive and thermo-oxidativebreakdown, high molecular weight compounds. Varnish precursors arethe result of breakdown from mainly:◦ Oxidation: water, air and metallic contamination from wear◦ Thermal stress from extreme temperatures in cases of: static discharge Micro dieseling: adiabatic compression of air bubblesThe chemical compositions of varnish precursor vary depending on theturbine type, operating conditions and the oil type. Varnish precursors are>75% soft particles, <1 micronVarnish precursors are polar, and their solvency is temperature dependent◦ Over time migrate from the oil to machine surfaces - depending on system and oil conditions.Reff: Pall Corp. Varnish Precursors + + -- + - Varnish
Compressor/Turbine Shaft Main bearings ( roller or journal) Cooler Main filter Solenoid valve Main pump 3500 psi Control oil filter To Hydraulic control Lube Oil Reservoir circuit little/no flow at turning Control oil circuit Hydraulic circuit: High VPR impactLube circuit Low Reff: Analyst Inc .VPR impact Inlet Vane Control Fail open valve Gas valve
Contamination of critical GT control hydraulic components◦ Fuel control valves◦ Safety fail open valves◦ Vane control valvesInability to control operation, high maintenance cost for◦ Replacement of contaminated valves◦ Cost of chemical clean and flushFail to start condition: Loss of flexibilityTrip during operation: Loss of productionDowntime and lost production represent a large share of thecosts associated with poor varnish control in GT installations Reff: Pall Corp.
All turbine oils create varnish precursors under normal operatingconditions.The rate of generation is higher under severe / unusual operatingconditions.◦ Oxidation◦ Additive depletion◦ Filter related electrostatic discharge◦ Micro-dieseling, adiabatic compressionRecent 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 dischargeThe 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.
Measurement of varnish potential does not indicate the actualamount of varnish deposited on the surfaces of componentsIt measures varnish precursors in the oilA system can be considered “varnish free” when varnish depositshave disappeared, not necessarily when varnish potential is down.Removal of varnish precursors from the oil displaces the solvencyequilibrium 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 anextended 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 tank Oil 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 KPI Particles captured by adsorption, eg varnish precursors Particles captured by direct interception, eg wear metals Reff: Pall Corp.