What Is the Thoma Cavitation Factor & Plant Sigma Calculator?
The Thoma cavitation factor σ is the dimensionless ratio of net suction head to total net head across a hydro turbine.
Comparing plant sigma with critical sigma σ_c reveals whether cavitation inception will erode runner blades.
How Does the Thoma Cavitation Factor & Plant Sigma Calculator Work?
For safe operation, σ_plant must exceed σ_c by at least 15% to 20%.
The formula dictates the maximum elevation zs,max above or below tailwater at which the runner can be safely installed.
Thoma Cavitation Factor & Plant Sigma Calculator Formula & Variables
The core mathematical equation utilized by this calculator is expressed as:
Ratio of available suction head to turbine head compared against critical cavitation inception.
How to Use the Thoma Cavitation Factor & Plant Sigma Calculator
- Enter available suction head, total net head, and specific speed.
- Check plant sigma, critical sigma, and the allowable draft tube setting height.
Step-by-Step Example Calculation
Francis Hydro Turbine Setting
Input Values:
Understanding Your Result
If σ_plant > σ_c, the plant operates safely without cavitation pitting.
Lowering the turbine below tailwater (negative setting) increases suction head and prevents cavitation.
Factors That Affect the Result
- Higher specific speed turbines (high flow, low head) exhibit much higher critical sigma values.
When Should You Use This Calculator?
- Hydroelectric powerhouse design, pump-turbine feasibility studies, and draft tube sizing.
Assumptions & Limitations
- Standard IEC 60193 and USBR empirical cavitation guidelines for Francis and Kaplan runners.
Frequently Asked Questions
Calculation Accuracy & Reference Note
Standard IEC hydro turbine cavitation engineering correlation.