cavitation-erosion

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).