Enter cavitation inputs
Use local static pressure at the critical section and vapor pressure at the operating temperature.
Plotly graph
The chart shows how sigma changes with velocity while keeping the same pressure and density conditions.
Formula used
Cavitation Index: σ = (p − pv) / (0.5ρv²)
Where:
- σ = cavitation index, dimensionless
- p = local static pressure at the point being evaluated
- pv = vapor pressure of the fluid at operating temperature
- ρ = fluid density
- v = flow velocity
The numerator measures how far the fluid pressure stays above vapor pressure. The denominator is the dynamic pressure from flow motion. Lower sigma values usually mean the fluid is closer to cavitation onset. Threshold values vary with geometry, roughness, dissolved gases, and temperature.
How to use this calculator
- Enter local static pressure where cavitation is most likely.
- Enter the fluid vapor pressure at the actual operating temperature.
- Enter fluid density and flow velocity using any listed units.
- Add a reference sigma threshold from your design guide or test data.
- Submit the form and review sigma, pressure margin, and risk tendency.
- Use the chart to see how higher velocity lowers sigma quickly.
- Download CSV for data handling or PDF for reporting.
Example data table
| Case | Static Pressure (kPa) | Vapor Pressure (kPa) | Density (kg/m³) | Velocity (m/s) | Approx. σ | Comment |
|---|---|---|---|---|---|---|
| Cooling water line | 220.00 | 3.17 | 997 | 9.00 | 5.37 | Low tendency under these conditions. |
| Nozzle throat | 120.00 | 2.34 | 998 | 13.00 | 1.40 | Moderate watch range. |
| Pump inlet section | 60.00 | 2.34 | 998 | 11.00 | 0.95 | High cavitation tendency. |
| Venturi minimum area | 35.00 | 2.34 | 998 | 16.00 | 0.26 | Very high risk. |
| Light oil transfer | 180.00 | 0.60 | 850 | 8.00 | 6.60 | Low tendency for this example. |
These sample values are illustrative. Real cavitation onset depends strongly on equipment geometry and operating conditions.
Frequently asked questions
1) What does a lower cavitation index mean?
A lower sigma means local pressure is closer to vapor pressure after accounting for flow velocity. That usually indicates a higher chance of bubble formation, noise, surface damage, and unstable hydraulic performance.
2) Is there one universal safe sigma value?
No. Safe values depend on the device, geometry, roughness, Reynolds number, dissolved gas content, and testing method. Pumps, valves, propellers, and venturis can all have different practical thresholds.
3) Which pressure should I enter?
Use the local static pressure at the location most vulnerable to cavitation, not just the upstream line pressure. The critical point is often a throat, inlet eye, blade section, or sharp restriction.
4) Why does temperature matter in this calculation?
Temperature changes vapor pressure. As temperature rises, vapor pressure usually rises too, which reduces the pressure margin and lowers sigma. That makes cavitation more likely under otherwise similar flow conditions.
5) Why does increasing velocity reduce sigma so quickly?
Velocity appears inside the dynamic pressure term as v². When speed increases, dynamic pressure grows rapidly. That enlarges the denominator and drives the cavitation index downward, especially in narrow passages.
6) Can sigma become negative?
Yes. A negative sigma means the entered static pressure is already below vapor pressure. That often indicates flashing or severe cavitation conditions, and the operating point should be reviewed immediately.
7) Does this calculator replace NPSH analysis?
No. Sigma is useful for quick comparison and local flow assessment, but pump selection still needs proper NPSH available and required analysis. Use both methods when evaluating pump inlet conditions.
8) How should I use the threshold input?
Enter a reference sigma from design guidance, vendor data, experiments, or your internal standard. The calculator compares the computed value with that reference so you can screen the operating condition more consistently.