Cavitation Index Calculator
Sigma method cavitation and flashing analysis — severity assessment with trim and material recommendations
Operating Conditions
Method: sigma = (P1 − Pv) / (P1 − P2), the service cavitation index per the ISA RP75.23 concept. Severity bands shown are generic baselines for globe-style control valves — high-recovery valves (ball, butterfly) cavitate at higher sigma values and need a larger margin.
Limits: indicative screening only. Valve-specific sigma coefficients (sigma_i, sigma_c, sigma_mr from the manufacturer) and pressure-scale/size-scale effects govern the final selection. Verify with a qualified engineer before purchase.
Sigma between 1.2 and 1.5 is the maximum-damage zone. Bubble collapse intensity peaks because large vapour volumes form and collapse close to metal surfaces. Standard trim can be destroyed in weeks. Noise exceeds 95 dBA and pipe vibration can damage instrumentation and small-bore connections. Operating any standard valve continuously in this zone is a maintenance emergency waiting to happen.
- Standard trim destruction in weeks, not months — this is the peak damage zone.
- Vibration can fatigue-fail small-bore instrument connections and drain valves nearby.
- Hardfacing alone is NOT sufficient — pressure staging is required.
- -Multi-stage anti-cavitation trim is mandatory (drilled-hole cage, stacked-disk, or labyrinth design)
- -Each stage must individually stay above sigma 1.7–2.0
- -Consider an angle valve with the drop taken in a multi-hole backpressure plate
- -Re-evaluate the system: can the pressure drop be reduced or split across two locations?
Sigma Severity Reference (Globe-Style Valves)
| Sigma Range | Regime | Damage Rate (Standard Trim) | Typical Mitigation |
|---|---|---|---|
| ≥ 2.0 | No cavitation | None | Standard trim |
| 1.7 – 2.0 | Incipient | Negligible (intermittent duty) | Stellite trim for continuous duty |
| 1.5 – 1.7 | Constant | Visible pitting in 3–12 months | Stellite + consider staging |
| 1.2 – 1.5 | Damaging | Trim destroyed in weeks | Multi-stage anti-cavitation trim mandatory |
| < 1.2 | Choked / super-cavitation | Body wall penetration possible | Severe-service valve + system redesign |
| P2 < Pv | Flashing | Smooth erosion, downstream pipe | Angle body, hard materials, larger outlet pipe |
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