Turboexpanders and Oil Varnishing: Why a Small Bearing Can Decide the Reliability of a Petrochemical Plant

The bearing shown in your photo appears to be a small turboexpander bearing pad or shoe. The dark brown-to-black deposit on the working surface is consistent with severe oil-derived deposit formation: varnish, lacquer, thermal coking, or a combination of these. A photo alone cannot confirm the chemistry, but the visual pattern is a strong reason to investigate oil degradation, localized overheating, oil starvation, contamination, and bearing loading.

In turboexpanders, the bearing may be physically small, but its reliability importance is enormous. These machines operate at high speed, tight clearances, and demanding process conditions. A thin oil film separates the rotating shaft from the bearing surface. Once that film is disturbed by varnish, sludge, thermal deposits, low flow, wrong viscosity, or contamination, the machine can quickly move from normal operation to rising bearing temperature, vibration, thrust instability, rotor rub, trip, or catastrophic bearing damage.

1. What a turboexpander does

A turboexpander is a rotating machine that converts pressure energy from a gas stream into useful shaft work while producing refrigeration. In gas processing, LNG, petrochemical, ethylene, hydrogen-rich gas, air separation, and cryogenic refrigeration services, the expander is often used where a large pressure drop is required. Compared with a simple Joule-Thomson valve or restriction orifice, a turboexpander extracts work from the flowing gas and can produce significantly lower outlet temperatures; Gas Processing & LNG notes that turboexpanders can achieve isentropic efficiencies as high as about 90% in suitable applications. (gasprocessingnews.com)

In a petrochemical plant, the turboexpander is not just another rotating machine. It is usually tied directly to refrigeration balance, product recovery, cold-box performance, compressor loading, and plant throughput. A turboexpander trip can disturb downstream separation, increase flaring risk, force process recycle, or reduce production until the machine is inspected and restarted.

2. Why the bearing is so critical

Oil-bearing turboexpanders commonly use journal and thrust bearing arrangements to support radial and axial rotor loads. Industry literature describes oil bearings in turboexpanders as fixed-geometry or tilt-pad designs, requiring a clean and cool oil supply. Because the bearings are often mounted inboard of the wheels, they may be exposed to high-pressure process gas, so the lubricating-oil system can be more complex than a standard atmospheric bearing housing. (gasprocessingnews.com)

This is why a small bearing pad can become a critical reliability component. The hydrodynamic oil film depends on correct oil viscosity, pressure, flow, temperature, shaft speed, bearing geometry, and load. Varnish changes the surface condition of the bearing, reduces heat transfer, can disturb the oil wedge, and may reduce effective clearance. In severe cases, the deposit behaves like an insulating and abrasive layer rather than a harmless stain.

Seal-gas system health is also important. In cryogenic turboexpanders, process gas entering the bearing housing can contaminate or chill the oil, while oil entering the expander side can freeze inside the machine or downstream cold equipment. Turbomachinery Magazine notes that oil carryover into a typical cryogenic cold box is especially undesirable because it is very difficult to remove once it enters. (turbomachinerymag.com)

3. What oil varnish is

Oil varnish is an adherent, oil-derived deposit that forms on internal machine surfaces such as bearings, pipes, valves, reservoirs, strainers, filters, coolers, and servo valves. It is usually created from lubricant degradation by-products, especially polar oxidation products that become insoluble or semi-insoluble in the oil. One global filtration company describes varnish as an insoluble film deposit that forms inside turbine lubrication systems and can coat bearings, heat exchangers, tanks, pipes, and servo valves.

The main formation mechanisms include oxidation, thermal degradation, contamination, water, catalytic metals, additive depletion, electrostatic discharge, and microdieseling. A review article in Materials explains that varnish is primarily associated with oil degradation caused by oxidation, and that heat, water, and metals can accelerate the oxidation process. (MDPI)

In the field, the terms are sometimes mixed, but they are not exactly identical:

Varnish is usually a thin, sticky-to-hard, amber or brown organic film from oxidized oil.

Lacquer is often used for a hard, polished, brown deposit on bearing surfaces.

Sludge is softer, bulkier, and more mobile.

Coke or carbonized oil is usually black, hard, and associated with severe localized temperature, oil starvation, or stagnant oil exposed to hot metal.

The dark area in your bearing photo looks more severe than light varnish. It may represent varnish that progressed into lacquer or thermal coking at a localized hot zone. That distinction matters because light varnish may be managed with oil conditioning and deposit removal, while thermal coking usually points to a stronger mechanical or thermal root cause.

4. Why turboexpanders are vulnerable to varnishing

Turboexpanders are vulnerable because they combine high speed, small clearances, low oil-film thickness, high bearing heat flux, process-gas sealing complexity, and often severe temperature gradients. In cryogenic service, parts of the machine operate extremely cold while bearing compartments and oil systems must remain warm and stable. That creates a machine where both hot-zone degradation and cold-zone precipitation can occur.

Modern turbine oils are formulated for oxidation resistance, foam control, demulsibility, and long service life, but all oils degrade over time. The problem begins when degradation products remain dissolved at operating temperature but precipitate when the oil cools or passes into low-flow areas. Turbomachinery Magazine notes that varnish potential can change with operating cycles because degradation products can come out of solution and form deposits during offline periods. (turbomachinerymag.com)

In turboexpanders, varnish risk increases when any of the following are present: high bearing metal temperature, low oil flow, plugged oil jets or orifices, incorrect oil temperature, excessive thrust load, oil cooler fouling, high aeration or foam, poor air release, water contamination, process-gas contamination, incompatible top-up oil, dirty oil transfer practices, poor reservoir breathing, filter electrostatic discharge, or extended oil residence time in hot zones.

5. How varnish damages turboexpander bearings

Varnish on a bearing pad is not only a cleanliness issue. It is a heat-transfer and geometry issue. The bearing surface is designed to create a stable oil wedge. A deposit layer changes the surface profile and can create localized film collapse. It also acts as an insulator, causing the pad temperature to rise even when the load has not changed.

A Turbomachinery Magazine article on thrust bearings reported that varnish can cause thrust-pad temperatures to rise over time and can create large temperature differences between pads. The same article notes that rising pad temperatures and pad-to-pad temperature differences are typical behavior when varnish forms on thrust shoes. (turbomachinerymag.com)

Common consequences include rising bearing metal temperature, high drain oil temperature, increased vibration, rotor instability, thrust-position movement, reduced bearing clearance, oil starvation, plugged strainers, filter differential-pressure increase, cooler fouling, servo-valve sticking, and repeated trips. Machinery Lubrication lists varnish-related turbine problems such as sticking valves, increased wear, reduced heat transfer, plugged oil orifices and strainers, filter plugging, and journal bearing failure. (machinerylubrication.com)

6. What the bearing in the photo may be telling you

Based on the photo, the deposit appears localized rather than uniformly distributed. That is important. Uniform discoloration often points to general oil aging or long exposure, while localized blackening suggests a hot spot, restricted oil flow, load concentration, edge loading, or local film breakdown.

Possible root causes to investigate include:

1. Localized overheating caused by high pad load, high thrust load, misalignment, wrong clearance, or poor oil wedge formation.
2. Oil starvation caused by a plugged oil orifice, blocked spray nozzle, restricted supply line, incorrect oil pressure, wrong bearing assembly orientation, or low oil level.
3. High varnish potential caused by aged oil, oxidized oil, depleted antioxidants, poor varnish removal, or long-term operation with dissolved degradation products.
4. Poor heat removal caused by varnish acting as an insulating layer, low oil flow, high inlet oil temperature, cooler fouling, or poor oil drain path.
5. Contamination from water, process gas, seal-gas upset, dirt, wear debris, incompatible oils, cleaning chemicals, or residual flushing fluid.
6. Thermal coking if the black area is hard, carbon-like, and difficult to remove with solvent.

The key point: do not treat this as a cosmetic stain. In a turboexpander bearing, a deposit like this is evidence of a system problem until proven otherwise.

7. Oil analysis tests to confirm varnish risk

A normal oil analysis report is not enough. Varnish requires specific testing because many varnish precursors are soft contaminants or dissolved degradation products that may not show up clearly in standard particle counts.

For varnish investigation, the most important tests are:

Membrane Patch Colorimetry, ASTM D7843. This test extracts insoluble contaminants from in-service turbine oil onto a membrane patch and reports the result as a ΔE color value. ASTM describes it as a condition-monitoring trending tool for lubricant-generated insoluble deposits in turbine oils. (ASTM International | ASTM)

RPVOT, ASTM D2272. This evaluates oxidation stability of new and in-service turbine oils of the same composition and is used to assess remaining oxidation test life. (ASTM International | ASTM)

Linear Sweep Voltammetry, ASTM D6971. This measures remaining hindered phenolic and aromatic amine antioxidants in non-zinc turbine oils, helping determine how much antioxidant protection remains. (ASTM International | ASTM)

ASTM D4378 turbine-oil monitoring practice. ASTM D4378 is intended to help users maintain effective turbine lubrication and guard against oil degradation and contamination problems. (ASTM International | ASTM)

ISO 4406 cleanliness code. ISO 4406 provides a method for coding the level of solid-particle contamination in hydraulic fluids, and the same cleanliness-code concept is widely applied in circulating lube systems. (ISO)

A strong turboexpander oil-analysis program should also include viscosity at 40°C, total acid number, water by Karl Fischer, FTIR oxidation, particle count, ferrous debris, elemental spectroscopy, demulsibility, foam and air-release tendency, filter debris analysis, and comparison with the new-oil reference sample.

8. Correct sampling strategy

Sampling only from the reservoir can miss bearing-specific problems. For a turboexpander varnish investigation, take samples from the reservoir, downstream of filters, upstream of the bearing header, and from bearing return/drain locations where practical. Samples should be taken while the machine is operating at normal temperature and load, because shutdown samples may underrepresent dissolved degradation products.

For varnish, trend is more important than one isolated number. A single MPC value may not explain the machine condition. Compare MPC, bearing temperatures, oil temperature, filter differential pressure, varnish-removal performance, antioxidant depletion, RPVOT trend, and operating history. A system with heavy internal varnish can even show deceptively low varnish potential after an oil change if deposits remain inside the machine and piping. Turbomachinery Magazine specifically warns that replacing oil without flushing deposits can initially show low varnish potential even when deposits remain throughout the system. (turbomachinerymag.com)

9. Corrective actions

The first decision is whether the problem is mainly oil-condition driven, mechanically driven, or both. On a critical turboexpander, assume both until evidence eliminates one.

For the bearing itself, document the pad location, rotation direction, load zone, deposit location, and temperature-probe location. Photograph all pads before cleaning. Do a solvent wipe test and, for serious cases, send deposit scrapings for FTIR and elemental analysis. Inspect for wiping, babbitt fatigue, cracking, edge loading, polishing, embedded particles, and loss of geometry.

For the lube system, inspect oil supply strainers, bearing feed orifices, coolers, filters, filter elements, bypass valves, reservoir bottom, return lines, heater operation, breathers, oil mist or foam, and any varnish-removal unit. Check actual oil flow and temperature at the machine, not only at the console.

For the oil, avoid blind top-up and avoid mixing formulations unless approved by the OEM and lubricant supplier. Use the correct viscosity grade and approved turbine oil type. API Standard 614 covers lubrication, shaft-sealing, and oil-control systems and auxiliaries, while API Standard 617 covers axial and centrifugal compressors and expander-compressors; these are the relevant machinery-system standards to keep in view for oil-system design and reliability governance. (American Petroleum Institute)

For varnish removal, options include offline kidney-loop filtration, depth-media varnish removal, adsorbent media, electrostatic-type systems where suitable, chemical cleaning, and full system flushing. The choice should depend on oil type, OEM approval, varnish severity, reservoir volume, operating criticality, seal arrangement, and whether the machine can remain online. Simply changing the oil is often not enough when the internal system is already coated.

10. Prevention program for turboexpander varnishing

The best prevention strategy is simple in principle: keep the oil clean, cool, dry, chemically healthy, and compatible.

Maintain correct oil inlet temperature and adequate oil flow to the bearings. Keep coolers clean and verify that the control valve is not causing excessive temperature swings. Maintain reservoir heaters correctly; overheating stagnant oil in the tank can accelerate oxidation. Control water and process contamination aggressively. Use proper breathers or dry headspace management where applicable. Keep filter housings bonded/grounded and watch for signs of electrostatic discharge, especially with low-conductivity turbine oils and fine synthetic filter media; One global filtration company describes electrostatic discharge as a contributor to oil degradation and varnish formation in turbine lubrication systems.

The reliability team should trend bearing metal temperatures against load, speed, oil inlet temperature, and ambient conditions. A slow bearing-temperature increase at the same operating condition is one of the strongest field indicators of varnish or restricted heat transfer. A sudden increase is more likely to indicate flow restriction, load change, thrust issue, rub, or rapid damage.

11. Practical field checklist

For a bearing like the one shown, the immediate investigation should answer these questions:

• Was the deposit on the loaded zone, trailing edge, leading edge, or oil-supply side?
• Did bearing metal temperature increase before shutdown?
• Was there pad-to-pad temperature spread?
• Was vibration increasing at 1X, subsynchronous, or broadband frequencies?
• Was oil supply pressure normal at the console and at the machine?
• Were oil filters or strainers showing high differential pressure?
• Was there any recent oil top-up, oil change, flush, filter change, cooler cleaning, or seal-gas upset?
• What are the MPC, RPVOT, antioxidant, TAN, water, ISO cleanliness, and FTIR oxidation trends?
• Are the oil jets, orifices, and bearing feed passages clean?
• Is the deposit soft and sticky, hard and glossy, or black and carbon-like?

Conclusion

Turboexpanders are critical assets because they sit at the intersection of process performance, refrigeration, product recovery, and rotating-equipment reliability. The bearing may be one of the smallest parts in the machine, but it carries the rotor on a microscopic oil film at high speed. Varnish, lacquer, or coking on that bearing surface is a warning sign that the oil, the bearing environment, or the machine operating condition is no longer fully controlled.

“One of the smallest bearings on a critical turboexpander — and a clear reminder that varnish is not a cosmetic oil problem. In high-speed turbomachinery, oil deposits change heat transfer, clearances, vibration behavior, and ultimately plant availability.”