Formula used
Cavitation sigma (σ) compares available pressure above vapor pressure to a reference pressure term. Two common forms are supported:
- Velocity-based: σ = (P − Pv) / (0.5 ρ V²)
- Head-based: σ = (P − Pv) / (ρ g H)
How to use this calculator
- Select velocity-based if you have flow velocity (or can compute it).
- Enter static pressure (P) at the component or location you are checking.
- Choose a vapor pressure method: temperature-based (water) or direct entry.
- Provide density (ρ). For non-water fluids, use datasheet density.
- Enter V, or enter Q and D to compute velocity automatically.
- If using head-based sigma, enter H instead of velocity.
- Click Calculate to view results and download PDF/CSV reports.
Example data table
| Scenario | P (kPa) | Pv (kPa) | ρ (kg/m³) | V (m/s) | σ (velocity-based) | Comment |
|---|---|---|---|---|---|---|
| Intake line, cool water | 260 | 2.3 | 998 | 2.5 | ~33.1 | Large margin; cavitation unlikely. |
| Control valve, higher velocity | 180 | 3.2 | 997 | 6.0 | ~9.9 | Monitor; verify local losses and transients. |
| Suction region, warm water | 120 | 12.3 | 992 | 4.0 | ~13.6 | Temperature raises Pv; margins reduce. |
| Near-vapor condition | 20 | 19 | 998 | 2.0 | ~0.5 | High risk; increase pressure or reduce velocity. |
Why cavitation sigma matters on construction sites
Cavitation sigma (σ) expresses how much absolute pressure is available above vapor pressure at a point, normalized by either dynamic pressure (½ρV²) or reference head (ρgH). When σ drops, vapor bubbles form and collapse, causing noise, vibration, pitting, and reduced capacity. For water near 20 °C, Pv is about 2.34 kPa, so suction losses can erode σ faster than most teams expect.
Key inputs and practical benchmark ranges
The calculator uses P, Pv, ρ, and either V (or Q and D) or H. As a rule of thumb, many components behave calmly at σ above 2–4, while σ near 1 signals elevated cavitation likelihood under transient conditions. Treat σ as a screening metric: compare scenarios, rank hotspots, and decide where to measure pressure more accurately during peak demand events.
How velocity and diameter choices shift risk
Velocity strongly influences σ because it sits in the denominator. If pressures stay constant, doubling V reduces σ by about four. With known flow, V = 4Q/(πD²). A 20% reduction in diameter increases velocity roughly 56% and can cut σ by about 60%. Use the Q and D fields to test “value engineering” options before procurement.
Interpreting results with field measurement data
Use absolute pressure at the location, not gauge. If you only have gauge readings, add local atmospheric pressure (≈101.3 kPa at sea level) to avoid overstating σ. At higher elevations, atmospheric pressure can be 5–20 kPa lower, reducing σ. Pv is temperature dependent; warmer fluid raises Pv and lowers σ even if the piping and pump are unchanged.
Mitigation steps when sigma is low
Improve suction conditions by shortening runs, reducing fittings, cleaning strainers, and increasing suction diameter. Lower velocity where possible, or raise inlet pressure with elevation, submergence, or priming improvements. For pumps, compare site conditions against manufacturer NPSH data and add margin for startup and valve closures. Save CSV/PDF reports to support design reviews, commissioning, and maintenance planning.
FAQs
1) What is a “safe” cavitation sigma?
There is no universal safe number. Many systems target σ above 2–4, but acceptable values depend on equipment, material, and transients. Use manufacturer guidance and field observations to set limits.
2) Should I use gauge or absolute pressure for P?
Use absolute pressure. If you only have gauge pressure, add atmospheric pressure for the site. Using gauge values directly can overstate σ and hide cavitation risk.
3) Why does temperature change sigma so much?
Vapor pressure increases with temperature. Higher Pv reduces the numerator (P − Pv), lowering σ. Warm water, hot process fluids, or heat soak during shutdown can push marginal locations into cavitation.
4) When should I use head-based sigma?
Use head-based σ when you track energy in meters of fluid head, such as open-channel structures, intakes, and draft tubes. It is also helpful when velocity is uncertain but head is known.
5) Can this calculator replace NPSH checks for pumps?
No. σ is a useful indicator and comparison metric, but pump selection should still follow NPSHa versus NPSHr with margin, including temperature, altitude, and transient allowances.
6) What if my sigma is negative?
Negative σ means Pv exceeds local absolute pressure, indicating vapor formation is likely. Recheck units and absolute pressure first, then address losses, elevation, temperature, or operating point immediately.