Control Valve Cavitation Index Calculator
The cavitation index (sigma) shows how close a liquid control valve is to cavitating. Compare the service sigma with the choked flow value from the valve’s FL.
- σ = (P1 − Pv) ÷ (P1 − P2) = 9.9683 ÷ 6 = 1.661
- σchoked ≈ 1 ÷ FL² = 1.235
Cavitation Index
σchoked ≈ 1 ÷ FL²
Higher σ means less risk. Manufacturers publish incipient, constant and damaging cavitation σ values for each trim (ISA RP75.23); the 2.0 guide used here is a typical rough threshold.
Worked example
Water at 25 °C from 10 to 4 bar(a): σ = (10 − 0.032) ÷ 6 = 1.66. With FL = 0.9 the choked value is 1.23, so the valve is cavitating but not choked.
Frequently Asked Questions
What is a cavitation index?
A ratio comparing the pressure available above vapour pressure with the pressure drop across the valve. Lower values mean more cavitation.
What is the difference between cavitation and flashing?
In cavitation the vapour bubbles collapse as pressure recovers; in flashing the outlet pressure stays below vapour pressure and the liquid leaves partly as vapour.
How can cavitation be prevented?
Reduce the drop per stage with multi stage trims, place the valve where pressure is higher, or use a valve with a higher FL.
Learn more on the blog
Control valve flow coefficient (Cv) explained
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article