Seal Leakage

high severity

DEFINITION

Seal leakage is uncontrolled flow of high-pressure gas (or oil-gas mixture) past compressor discharge seals, allowing contaminated discharge gas into bearing oil cavity. Seal leakage degrades bearing lubrication by: (1) water vapor condensation (humid discharge gas → liquid water on cooling) entering oil; (2) oxidized gas byproducts dissolving in oil, reducing viscosity and increasing TAN; (3) small droplets of oil-gas emulsion entering bearing cavity, changing oil physical properties; (4) elastomer seal materials (nitrile, EPDM, FKM) swell and soften from exposure to discharge gas solvents and elevated temperature. Seal leakage initiates from contamination particles embedded in seal face (preventing full seating) combined with elevated discharge temperature (80-120°C) creating thermal stress on elastomeric components. Cumulative effect: bearing oil degradation 3-5× faster than normal operation, bearing wash-out 10-50 hours after seal leakage initiation.

KEY PARAMETERS

<20 ppm

Design water content limit (bearing oil)

50–100 ppm

Action level water concentration

>150 ppm

Critical water level

80–100°C

Nitrile elastomer max operating temp

01 /

Compressor Seal Design and Failure Mechanisms

FH/FG compressor discharge seals prevent high-pressure gas (15-25 bar) from entering bearing oil cavity. Two seal designs used: (1) Labyrinth seals (clearance-type) — interlocking grooves in rotating/stationary surfaces create pressure drop stages, each stage reduces pressure 1-3 bar, 5-7 stages achieve low leakage at design point; advantage: no wear (grooves do not contact), maintenance-free; disadvantage: high leakage (0.5-1.5 L/min at rated pressure) if tolerance stack-up creates excessive clearance or if seal groove contaminated with particles creating bypass path; (2) Carbon-face mechanical seals (contact-type) — rotating carbon ring (seal face) contacts stationary carbon face with <5 µm gap, spring-loaded pressure ~1.5 bar maintains contact; advantage: low leakage (<0.1 L/min), effective pressure blocking; disadvantage: seal faces wear (~0.1 mm per 2000 hrs), require periodic replacement, face separation/stiction possible if contamination particles jam faces. FH/FG design decisions: FH-series typically uses labyrinth seal (simpler, lower cost, acceptable leakage <1 L/min); FG-series often uses mechanical seal (tighter sealing needed for higher pressure 20-25 bar). Seal leakage initiation: (1) Particle embedding — if upstream discharge filter (usually 10 µm) becomes clogged/bypassed, 10-50 µm particles reach seal cavity; particles embed in labyrinth grooves (blocking groove path, forcing gas bypass) or jam mechanical seal faces (preventing tight seating); (2) Thermal distortion — discharge temperature 100-120°C causes elastomeric seal components (springs, secondary seal O-rings) to soften (~20% modulus loss per 20°C above design temp); seal clearance increases 50-100 µm from thermal expansion mismatch between seal body (aluminum/stainless steel) and elastomer; (3) Corrosion/erosion of seal faces — if discharge gas contains acid (from fuel oxidation products) or particles, seal face surfaces (carbon, stainless steel) can corrode/erode, creating micro-grooves and increasing leakage pathway. Combined effect: labyrinth seal leakage can increase from design 0.5 L/min to 3-5 L/min; mechanical seal leakage from <0.1 L/min to 0.5-2.0 L/min.

02 /

Oil Contamination from Seal Leakage

Seal leakage introduces three contaminants into bearing cavity: (1) Water vapor from humid discharge gas — compressor discharge air at 100-120°C contains dissolved water vapor (absolute humidity 40-80 g/m³); when vapor enters cooler bearing cavity (55-65°C), water condenses as liquid droplets; water concentration in oil increases 50-200 ppm within 100 hours of active seal leak; (2) Oxidized byproducts from discharge gas — fuel combustion byproducts (aldehydes, ketones, organic acids) in hot discharge gas dissolve in bearing oil, increasing TAN (Total Acid Number) 0.2-0.5 mg KOH/g faster than normal oil aging; (3) Emulsion formation — high-pressure gas bubbled through bearing oil creates oil-gas mixture (tiny oil droplets dispersed in gas, escaping as mist); re-condensed mist droplets mix with bulk oil creating emulsion; emulsion reduces oil viscosity 30-50% (viscosity-reducing additives from emulsifier surfactants). Time progression of seal leakage damage: (1) Hour 0-24 of active leak: oil appearance normal, viscosity unchanged, ISO cleanliness unchanged; water content <50 ppm (below sensor detection); (2) Hour 24-100: water content reaches 50-100 ppm (visible cloudiness in oil on inspection), viscosity reduced 10-15%, TAN increased 0.3-0.5 mg KOH/g, ferrous debris slightly elevated (seal wear producing particles); (3) Hour 100-200: water content 100-200 ppm (oil milky/opaque), viscosity loss 20-30%, TAN >1.0 mg KOH/g (bearing corrosion risk), ISO cleanliness degraded to 18/16/13, bearing temperature rises 5-10°C from increased friction (viscosity loss); (4) Hour 200-500: water emulsion separating at bottom of bearing cavity (free water visible on sight glass), viscosity loss >40%, film thickness marginal, bearing wash-out imminent; (5) Hour >500: bearing seizure within hours, oil completely degraded.

03 /

Temperature Synergy and Elastomer Degradation

Elastomeric seal components (O-rings, lip seals, springs typically nitrile or EPDM) have maximum operating temperature limits: nitrile 80-100°C, EPDM 120-150°C, FKM (Viton) 150-200°C. FH/FG compressors with discharge temperature 100-120°C operate at upper limit of nitrile/EPDM elastomers, leaving minimal safety margin. Temperature excursions above nominal (120-140°C from high ambient or compressor load spike) accelerate elastomer degradation: (1) Modulus loss — elastomer spring force decreases 2-3% per °C above rated temp; if design rated 100°C with spring force 1.5 bar, operation at 130°C reduces spring force to ~1.0 bar (30% loss); (2) Swelling — exposure to discharge gas solvents (CFCs, hydrocarbons) at elevated temperature causes swelling 10-20%; seal O-ring diameter increases, clearance reduces, sealing pressure increases but elastomer material weakens (swollen elastomer is softer); (3) Embrittlement — prolonged temperature cycling (cold startup 20°C → hot operation 120°C → shutdown 20°C) causes elastomer micro-cracking from differential thermal expansion; micro-cracks accelerate degradation rate, leading to seal failure within 1000-2000 hours. Combined temperature + contamination synergy: If seal operates at design 100°C with 50 ppm water (normal), elastomer lives 3000-5000 hours. If temperature rises to 130°C + water 150 ppm (from increased seal leakage cycling), elastomer life drops to 500-1000 hours (80-90% reduction). Real-world example: FG-series compressor in hot climate (ambient 40-45°C), discharge temperature design 110°C, but actual discharge 130-140°C from high inlet air temperature + high system load. Mechanical seal elastomers swell rapidly (FKM swelling reaches 15% within 500 hours), seal clearance increases from design 5 µm to 20-30 µm, leakage increases, bearing oil contamination accelerates, bearing seizure occurs at 800 hours (vs. design life 2000+ hours).

04 /

Seal Leakage Detection and Maintenance Strategy

Seal leakage is detectable before catastrophic bearing failure if monitored proactively: (1) Visual inspection — check bearing cavity sight glass monthly for water level change; baseline water settling at tank bottom (0-5 mm layer); if water layer visible as milky separation >10 mm height, seal leak active; (2) Oil sampling — monthly ISO 4406 count + water analysis (Karl Fischer titration per ASTM D6304); water content >50 ppm indicates seal leak (normal baseline <20 ppm); trending upward week-to-week confirms active leak; (3) Acid number (TAN) trending — elevated TAN >0.8 mg KOH/g + simultaneous water increase confirms seal leak (vs. normal oxidation which is slower); (4) Discharge temperature monitoring — sudden 10-15°C temperature rise at discharge (measured via thermowell) coupled with increased bearing cavity leakage indicates seal degradation; (5) Acoustic diagnosis — high-pressure gas leaking past seal creates audible hiss (distinct from compressor operating noise); technician trained to locate hiss source can pinpoint seal leakage; (6) Bearing sump temperature trending — temperature spike 5-10°C + trending upward + simultaneous oil contamination indicators (water, TAN, ISO code) confirms bearing damage cascade from seal leak. Maintenance action: when water content approaches 50-100 ppm OR TAN rises >0.5 mg KOH/g above baseline, schedule seal replacement within 1-4 weeks (before bearing wash-out risk accelerates). Preventive measures: (1) Upgrade compressor discharge air filter to 3-5 µm (vs. standard 10 µm) to prevent particle embedding in seal faces; (2) Use mechanical seals (carbon-face) instead of labyrinth on high-temperature FG units (mechanical seals tighter sealing, lower leakage); (3) Consider cooler upgrade if discharge temperature chronically exceeds design (100-120°C), sized for actual ambient vs. assumed design ambient; (4) Deploy offline kidney-loop on bearing cavity oil to continuously remove water + oxidation byproducts during idle periods.

05 /

FH/FG Compressor Seal Leakage Case Study: Heat-Stressed Facility

Industrial facility in hot climate (summer ambient 45-50°C), 4 FG-series compressors (15-20 bar discharge). Baseline problem: compressor bearing seizures every 6-12 months per unit (2-4 failures annually × 4 units = €300,000-400,000 annual cost including replacement + downtime). Investigation: reviewed bearing failure progression, found pattern — bearing oil samples 2-3 months before failure showed water content 80-150 ppm (vs. normal <20 ppm), TAN elevated 0.8-1.2 mg KOH/g, discharge temperature chronically 130-140°C (vs. design 110°C). Root cause: (1) facility ambient 50°C summer; compressor inlet air 50°C + high humidity 60-70%; (2) compressor discharge temperature = ambient + compression rise = 50°C + 80°C rise (15 bar gauge pressure) = 130°C; (3) high discharge temperature soften mechanical seal elastomers, reducing sealing; (4) humid discharge gas → water vapor condenses in cooler bearing cavity; (5) degraded seal cannot prevent leakage, water-laden gas continuously enters bearing oil. Implementation: (1) Upgrade compressor discharge air filter from 10 µm to 3 µm (prevent particles jamming seal faces); (2) Replace standard nitrile seal elastomers with FKM (Viton) rated to 200°C (margin for 130-140°C discharge); (3) Install cooler upgrade on discharge line (inlet cooler cools compressor discharge from 130°C to 85-90°C before entering bearing cavity seal area); (4) Deploy offline kidney-loop on bearing cavity oil with water-absorbing cartridge (removes water continuously). Results after 18 months: (1) Water content in bearing oil maintained <30 ppm (vs. previous 100+ ppm); (2) TAN remained <0.5 mg KOH/g (vs. previous 1.0+ mg KOH/g spike); (3) Bearing seizures reduced from 2-4/year per unit to 0/year (100% elimination); (4) Bearing oil change intervals extended from 500 hours (forced by contamination) to 1500-2000 hours; (5) Discharge temperature reduced to 100-110°C (within design range). Total investment: €60,000 (discharge cooler €30K, seal upgrades €15K, filter upgrade €8K, kidney-loop €7K); Savings: 4 units × 2 bearing failures/year × €80K per failure = €640K/year prevented (over 5 years €3.2M); Payback: <2 months. Lessons: seal leakage is rooted in temperature management failures; cooler sizing critical in hot climates; water absorption during idle periods (kidney-loop) prevents transient water condensation events from becoming chronic bearing contamination.

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