Carbon Graphite Labyrinth Seal Inserts for Centrifugal Compressors: 120℃/2.5 MPa Cracker Gas Compressor H₂/CH₄/C₂H₄ Mixed Gas Service

Centrifugal compressors are the core rotating equipment in petrochemical, natural gas processing and other industries. Labyrinth seals (also known as comb-type seals) are the most commonly used non-contact seals for inter-stage and shaft-end applications in compressors. The principle is to generate throttling pressure drop through the micro-clearance between multi-stage throttling teeth and the rotor, consuming gas kinetic energy stage by stage, thereby reducing leakage. However, traditional aluminum alloy and brass labyrinth teeth are prone to hard rubbing under rotor vibration and misalignment conditions, leading to fast tooth wear, rapid labyrinth clearance expansion, and exponentially increased leakage. Based on the application experience of Huoqiu County Huahao Sealing Co., Ltd. on multiple large centrifugal compressors such as ethylene cracker gas compressors and natural gas injection compressors, this article introduces key points for selecting carbon graphite insert labyrinth seals.

1. Service Characteristics

1.1 Typical Cracker Gas Compressor Parameters

• Medium: cracker gas (H₂ 28-32%, CH₄ 18-22%, C₂H₄ 22-28%, C₃+ 10-15%, CO 2-4%, trace H₂S and moisture)
• Pressure range: inter-stage 0.3-1.2 MPa, outlet 2.5 MPa
• Temperature range: first stage inlet 38-45℃, fourth stage outlet 110-125℃
• Speed: 6800-8500 r/min (gearbox speed increase drive)
• Shaft vibration: normal 25-40 μm, when passing critical speed 60-85 μm (API 617 alarm value 90 μm, trip value 120 μm)

1.2 Failure Mode Analysis

Traditional aluminum/brass labyrinth tooth failure statistics:

1.Rubbing wear (65%): Shaft vibration increases (> 60 μm) when the rotor passes the critical speed or under sudden load, hard rubbing between teeth and sleeve. Aluminum teeth (HB 95) rubbing against chrome-plated sleeve (HV 900) is like "egg hitting stone", tooth height wears 0.1-0.2 mm in a single critical speed pass, labyrinth clearance doubles every 3-4 months.

2.Gas corrosion (20%): Trace H₂S (30-200 ppm) + moisture in cracker gas condenses into H₂S aqueous solution in the low-temperature section, brass teeth (Cu-Zn) undergo dezincification corrosion, tooth strength decreases and brittle cracks occur.

3.Thermal deformation (10%): In the high-temperature section (> 100℃), the thermal expansion coefficient of aluminum teeth (23×10⁻⁶/℃) is much larger than that of carbon steel shell (12×10⁻⁶/℃), design clearance is "consumed" and continuous rubbing occurs.

4.Uneven installation clearance (5%): Manual feeler gauge measurement of each tooth clearance has error ±0.05 mm, local tooth clearance too small leads to early rubbing.

2. Carbon Graphite Insert Material Selection

2.1 Carbon Graphite Advantage Comparison

| Property | 6061-T6 Al | H62 Brass | 316L SS | M150Cu Bronze Impregnated Graphite |

|----------|-----------|---------|---------|--------------------------------------|

| Hardness | HB 95 | HB 85 | HB 187 | HS 80 (≈ HB 120) |

| Damaged part on rubbing | Al teeth wear | Brass teeth wear | SS damages sleeve! | Graphite wears, sleeve intact |

| H₂S corrosion resistance | Fair (pitting) | Poor (dezincification) | Good | Excellent (graphite not corroded by H₂S) |

| Thermal expansion coeff | 23×10⁻⁶ | 20×10⁻⁶ | 17×10⁻⁶ | 3.2×10⁻⁶ (close to steel) |

| Self-lubrication | None | None | None | Excellent (solid lubrication, effective at HT) |

| Machining precision | ±0.02 mm | ±0.02 mm | ±0.03 mm | ±0.005 mm (CNC grinder precision) |

| Service life (labyrinth) | 3-6 mo | 4-8 mo | 1-2 yr | 4-6 yr |

2.2 Huahao Sealing M150Cu Bronze Impregnated Graphite

Dedicated material for labyrinth seal inserts:

• Base material: fine-grain isostatic graphite (grain 8 μm, density 1.82 g/cm³)
• Impregnant: tin bronze CuSn10 (vacuum pressure impregnation, impregnation rate ≥ 95%, residual porosity < 1%)
• Key properties:
• Compressive strength 260 MPa, flexural 72 MPa, Shore HS 80
• Thermal conductivity 85 W/m·K (rubbing heat rapidly conducts out, avoiding graphite oxidation)
• Thermal expansion coefficient 3.2×10⁻⁶/℃ (paired with carbon steel 12×10⁻⁶, clearance increases rather than decreases at high temperature)
• Oxidation temperature: ≤ 450℃ in air (far above cracker gas 125℃ service)
• Medium resistance: fully stable in H₂S, CO, alkenes, alkanes; bronze impregnant slightly H₂S corrosion-resistant (no impact at H₂S content < 500 ppm)

2.3 Rotor Sleeve Requirements

Carbon graphite insert solution must be paired with high-hardness sleeves:

• Sleeve surface: hard chromium plating 0.10-0.15 mm, plating HV 850-950, roughness R_a ≤ 0.20 μm
• Plating requirements: chromium plating layer-substrate bonding force ≥ 60 MPa, micro-crack free (no cracks by electrolytic corrosion detection)
• Alternative: HVOF WC-Co-Cr coating, HV 1100-1200 (more wear-resistant but cost 3-4 times higher)

3. Labyrinth Seal Structural Design

3.1 Seal Type Selection

Comparison of two mainstream labyrinth seal structures:

| Structure Type | Schematic | Leakage (relative) | Rubbing resistance | Cost | Application |

|---------------|----------|-------------------|--------------------|------|-------------|

| Straight-through | All teeth on stator or all on rotor, tooth tips point to opposite smooth cylindrical surface | 100% (baseline) | Fair | Low | Low-pressure stage (< 0.5 MPa), low shaft vibration |

| Staggered | Teeth alternately arranged on stator and rotor, forming "Z-shaped" flow path, gas flow direction turns multiple times | 50-60% | Excellent (single stage contact during rubbing, energy dispersed) | Medium | Recommended! Medium-high pressure stage (> 0.5 MPa), large unit preferred |

Cracker gas compressor GB201 inter-stage pressure 0.3-1.2 MPa, recommend stages 2, 3, 4 use staggered type, stage 1 uses straight-through type.

3.2 Tooth Profile and Clearance Design

Tooth profile design (determines throttling effect):

• Tooth tip angle: 60° isosceles triangular tooth tip, tip radius R ≤ 0.03 mm (strong sharp edge effect, optimal throttling effect)
• Tooth height H: H = 8-12 mm (ensures tooth tip mechanical strength, avoids breakage during installation)
• Tooth pitch P: P = 10-15 mm, tooth height H / pitch P ≈ 0.6-0.8 (reasonable "vortex dead zone" proportion in flow channel)
• Number of teeth Z: 4-6 teeth per stage; when stage total pressure drop > 0.2 MPa, 8+ teeth required

Clearance design (balances leakage and rubbing risk):

• Diametral clearance 2c (cold state):
• High-speed stage (> 6000 r/min, shaft diameter 150-200 mm): 2c = 0.40-0.50 mm
• Medium-speed stage (4000-6000 r/min, shaft diameter 200-300 mm): 2c = 0.50-0.65 mm
• Hot state clearance verification: at 120℃ working temperature, sleeve (steel, 12×10⁻⁶) expansion vs graphite insert (3.2×10⁻⁶) expansion, hot clearance only changes -0.02 mm (clearance basically unchanged, no "seizure")
• Rubbing safety margin: cold clearance ≥ 2.5 × normal shaft vibration, ≥ 1.3 × critical speed shaft vibration (2c ≥ 1.3 × 85 μm ≈ 110 μm, actual 400+ μm is very safe)

3.3 Insert Fixing Method

Carbon graphite is brittle, cannot tap threads (prone to chipping), uses dovetail groove interference fit:

• Stator ring inner wall machined 60° dovetail groove (groove width slightly smaller than graphite insert dovetail bottom width, interference 0.02-0.03 mm)
• Graphite inserts slowly pressed into dovetail grooves at room temperature (15 MPa press, speed 1 mm/s, avoid impact)
• Both ends fixed by spot-welded thrust washers (weld points far from graphite, avoid thermal influence)
• Finally, CNC grinder finishes tooth tip circular runout ≤ 0.01 mm, each tooth height difference ≤ 0.005 mm

4. Huahao Sealing Case Study

An 800000 t/year ethylene plant under a petrochemical group, the cracker gas compressor (one of the "three compressors") (GB201, manufactured by Shengu, 6-stage centrifugal, 4-stage compression, power 15000 kW, speed 8200 r/min, 48 inter-stage labyrinth seal stages, max medium pressure 2.5 MPa, outlet temperature 122℃) originally used 6061 aluminum alloy labyrinth teeth, had serious problems:

1.Rapid tooth wear: each time passing the 1st order critical speed (about 4200 r/min) shaft vibration 75-80 μm, aluminum teeth hard rubbing chrome-plated sleeve, after 15-20 start-stops tooth height worn by half, labyrinth clearance expanded from design 0.28 mm to 0.7-0.9 mm

2.Inter-stage leakage exceeding standard: plant calibration leakage 2.8% (API standard should be ≤ 1.5%), hourly leakage of ethylene and other materials about 0.8 t, annual loss about 18 million CNY

3.H₂S corrosion: cracker gas contains H₂S 80-120 ppm, stage 3 low-temperature section (52℃) brass labyrinth teeth suffered dezincification corrosion, teeth embrittled and fractured (during 2017 overhaul 7 fractured teeth found)

During 2018 major overhaul, all 48 labyrinth seal stages fully replaced with Huahao Sealing's M150Cu bronze impregnated graphite inserts + staggered structure (stages 2-4) / straight-through type (stage 1) + dovetail groove insert + matching re-chrome plated sleeves solution:

• Leakage: after overhaul calibration 0.9% (better than API 1.5% standard), ethylene and other material leakage reduced 1.9 t/hour, annual material value saving about 12 million CNY (at ethylene 7500 CNY/t)
• Seal life: 2018-2024 experienced 6 start-stops (including 2 plant-wide overhauls), 2024 teardown inspection graphite tooth tip average wear only 0.012 mm (wear rate 0.002 mm/time), good for another 4-6 years (original aluminum teeth already replaced 4 rounds)
• Sleeve protection: chrome plating on each stage sleeve has no obvious damage, roughness still meets requirements, no need to replace (original plan each overhaul must replace sleeves, saved about 3.8 million CNY spare parts cost)

Selection core: shaft vibration magnitude determines tooth material (shaft vibration > 50 μm requires graphite inserts to avoid hard rubbing), medium corrosivity determines impregnant (H₂S-containing selects bronze impregnation), inter-stage pressure determines structure type (> 0.5 MPa selects staggered type), cold clearance designed at 2.5 times shaft vibration.