Steam Turbine Blade Erosion: When Wet Steam Turns Destructive

Body Excerpt (Full 5,000+ words):

You know the sound. That faint metallic hissing during low-load operation, or worse – the occasional thump after a condenser level hiccup. It’s the sound of liquid water meeting turbine blades spinning at Mach 1. And if you ignore it? You’ll be pulling chunks of blade out of the exhaust hood next outage.

Why Last-Stage Blades Get Hammered:
When steam expands through those final LP stages, it crosses the saturation line. Suddenly, you’ve got 5-15% moisture flying around like microscopic bullets. Those droplets:

• Aren’t soft rain – they’re superheated water flashing on impact

• Hit like a sledgehammer – 500 m/s impacts generate local pressures over 3 GPa

• Aim for the throat – Leading edges near the blade root take the worst beating

The Erosion Progression You’ll Find During Inspection:

1. Stage 1: The “Orange Peel” Look (0-8k hrs): Surface hardening starts – blade feels rough like sandpaper under your glove. Micro-pits appear. Efficiency drops 0.5-1%.

2. Stage 2: “Craters on the Moon” (8k-20k hrs): Visible pitting develops. Material loss accelerates to 0.3mm/year. Vibration spikes at 2x running speed.

3. Stage 3: “Grand Canyon Edition” (20k+ hrs): Deep grooves (5mm+) carve paths along the blade. Airflow separates. Now you’re losing 3-5% heat rate AND risking blade separation.

Repair Strategies That Actually Work:
Option A: The Welder’s Art (For Local Damage)

• Good for: Isolated pits <3mm deep

• Process: Grind clean → Pre-heat to 300°F → TIG weld with ERNiCrMo-3 → Post-heat 1200°F

• Watch for: Cracking at HAZ if pre-heat skipped

• Real talk: Only buys you 2-3 years if erosion’s widespread

Option B: Armor Plating Your Blades (HVOF Coatings)

• The Contender: 88% Tungsten Carbide-Cobalt (WC-Co)

  • Pros: Rock-hard (1200 HV) – stops early erosion cold

  • Cons: Brittle as glass – chips during thermal cycling

  • Best for: Base-load units that never trip

• The Workhorse: Nickel-Chrome Matrix with Carbides

  • Pros: Tough (28 J/cm²) – laughs at thermal shock

  • Cons: Needs precise spray parameters

  • Field Verdict: Lasts 5x longer in cycling plants

Prevention Worth Its Salt:

• Moisture Separators: Not all MSRs are equal. If you’re still seeing >10% wetness after the separator, check:

  • Chevron plate spacing (should be ≤15mm)

  • Drain leg U-seals (they love to leak air)

• Leading Edge Stelliting: Old-school but gold. 1mm thick stellite weld overlay on L-0/L-1 blades. Lasts the life of the turbine.

• Operational Discipline: Ramp rates matter! >15 MW/min load changes flash condensate into blades.