| name | cavitation-erosion |
| description | Cavitation erosion — bubble collapse pressure, NPSH, erosion mechanisms, material resistance, pump cavitation, propeller cavitation, ultrasonic cavitation testing, ASTM G32, material selection. |
| metadata | {"priority":7,"promptSignals":{"phrases":["cavitation erosion","cavitation damage","NPSH cavitation","pump cavitation","cavitation pitting","bubble collapse"],"minScore":3}} |
Cavitation Erosion — Complete Skill
Cavitation Physics
Bubble formation:
Cavitation occurs when local static pressure drops below vapor pressure P_v(T)
P_local < P_v → vapor bubble nucleates
Rayleigh-Plesset bubble dynamics:
r d²r/dt² + (3/2)(dr/dt)² = (P_bubble - P∞) / ρ [r = bubble radius; P∞ = ambient pressure]
Collapse pressure:
P_max ≈ P∞ × (R_max / R_min)^3 [extreme pressure during collapse]
Typical peak pressure: 100–1000 MPa (1–10 kbar) at collapse → well above yield strength of most metals
Jet formation:
Asymmetric bubble near wall → microjet at collapse; velocity 50–500 m/s
Repeated impingement → fatigue failure → pitting and material removal
Cavitation Number
Cavitation index (σ):
σ = (P_ref - P_v) / (½ ρ U²) [dimensionless; P_ref = reference pressure; U = reference velocity]
σ < σ_inception → cavitation begins
σ << 1 → severe cavitation
Net Positive Suction Head (NPSH):
NPSH_available = (P_atm - P_v) / (ρg) + V_s²/(2g) - Z_s [m; Z_s = suction lift; V_s = suction velocity]
NPSH_required = from pump manufacturer curve [m]
Design rule: NPSH_available ≥ NPSH_required + 0.5 m (margin)
3% head drop criterion:
NPSH₃ = NPSH at which head drops 3% below non-cavitating value (standard manufacturer definition)
Erosion Rate
Erosion rate R_e:
R_e ≈ C × f_b × (V_jet - V_threshold)^n × (1/σ_y) [mm/year; f_b = bubble collapse frequency; V_jet = jet velocity; σ_y = material yield strength; n ≈ 2–3]
Material resistance factors:
R_erosion ∝ 1 / (UTS × ε_f^0.5) [combination of strength and ductility; Hutchings correlation]
Harder and tougher materials resist better
Pitting rate in early stage:
N_pits/m²/s = C × P_collapse × (P_collapse / σ_y)^n [number density of pits per second]
ASTM G32 (Ultrasonic Cavitation Test)
Test method:
20 kHz ultrasonic transducer with horn; vibration amplitude 50 μm; specimen immersed in distilled water
Duration: 2–5 hr; measure mass loss vs. time
Steady-state erosion rate (MDER):
MDER = mass loss / (density × time × area) [mm/hr]
Material ranking (relative MDER):
| Material | MDER [mm/hr] |
|---|
| Carbon steel | 1.0 (reference) |
| 304 SS | 0.5 |
| 316L SS | 0.3 |
| Stellite 6 (CoCrW) | 0.05 |
| Ni-Al bronze | 0.2 |
| AISI 431 SS | 0.4 |
| Ti-6Al-4V | 0.1 |
Correlation to field erosion:
Field erosion rate ≈ 0.1–10× ASTM G32 rate (depends on flow conditions, collapse energy distribution)
Material Selection for Cavitation Resistance
Austenitic stainless steel (316L, 304):
Work-hardens under cavitation → transforms to martensite → harder surface
Initial low resistance → improves with time (deformation hardening)
Cast iron (gray):
Poor resistance; brittle → early pitting and spalling; not recommended for pump impellers
Duplex stainless steel (2205, 2507):
Better than austenitic; σ_y higher; good cavitation resistance
Stellite (Co-Cr-W-C) hardfacing:
Excellent hardness + toughness; MDER = 1/20 of carbon steel
Use for: hydraulic turbine runner bands, pump wear rings, valve seats
Ni-Al Bronze:
Good for marine propellers; moderate cavitation resistance; corrosion resistant seawater
UNS C95800 standard
Elastomer coatings:
UHMWPE, PU rubber: absorbs impact energy; good for low-velocity cavitation (< 10 m/s)
Not suitable for high-velocity jets
Pump Cavitation
Onset indicators:
- Noise: clicking/crackling sound (bubble collapse)
- Vibration: broadband increase
- Head drop > 3% at given flow
- Erosion pitting on impeller suction side (leading edge of blade)
Locations of cavitation damage:
Suction side of impeller leading edge (most common)
Discharge side near trailing edge (recirculation cavitation)
Wear rings and balance holes
Remedies:
- Increase suction pressure (raise pump, lower suction tank elevation, increase suction pipe diameter)
- Reduce flow rate (operate at best efficiency point BEP)
- Use inducers (axial pre-impeller) to boost pressure before main impeller
- Suction-specific speed N_ss < 9000 (US units; metric: < 175) for cavitation-free operation
N_ss (suction-specific speed):
N_ss = N × Q^0.5 / NPSH_required^0.75 [N in RPM; Q in GPM; NPSH in ft] (US units)
Hydraulic Turbine Cavitation
Critical sigma (Thoma cavitation coefficient):
σ_c = (H_a - H_sv - Z_s) / H_n [H_a = atmospheric head; H_sv = vapor pressure head; Z_s = setting height; H_n = net head]
σ ≤ σ_plant: cavitation occurs; design to σ_plant > σ_plant_critical
Francis turbine: most susceptible; σ_critical from manufacturer
Kaplan turbine: similar but propeller geometry; tip cavitation
Standards
| Standard | Scope |
|---|
| ASTM G32 | Ultrasonic cavitation erosion test |
| ASTM G134 | Erosion of solid materials by cavitating liquid jet |
| ISO 2858 | End-suction pumps; NPSH measurement |
| Hydraulic Institute HI 9.6.1 | NPSH for pumps |
| IEC 60193 | Hydraulic turbines; acceptance testing |
Output
Provide: NPSH_available vs. NPSH_required [m] with margin, cavitation index σ, material selection for impeller/runner (hardness HV and ASTM G32 MDER [mm/hr]), erosion rate at operating conditions [mm/year], onset indicators (flow rate at cavitation onset [m³/s], head drop [%]), suction-specific speed N_ss (check vs. 9000 US limit), design remedies (suction pipe, inducer, material), and applicable standard (ASTM G32, HI 9.6.1).
