Power Plant · Flow
Turbine Stage-Group Flow (Stodola Ellipse)
Steam flow through a turbine stage group from its inlet and exit pressures, scaled from one known reference point with the Stodola ellipse law and exact IF97 specific volume — the standard way to infer extraction and throttle flows between heat-balance points.
Standard method — stated scope limits apply
Reference point (design, heat balance or test)
New operating point
The ellipse law fits groups of several stages; a single stage or nozzle group behaves differently near choking.
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About This Calculation
The Stodola ellipse law relates the flow through a turbine stage group to the pressures before and after it: flow is proportional to the square root of (p1² − p2²) divided by (p1·v1). It is scaled from one known reference point and is used to infer extraction and throttle flows between heat-balance conditions.
Frequently Asked Questions
When does the Stodola law not apply?
When the group is close to choked flow, when geometry changes (e.g. governing-stage nozzle groups opening), or in wet steam.
Where should the reference point come from?
A heat balance or acceptance test at a known load — the result is only as accurate as that reference.
What is the Stodola constant?
It is the flow divided by √[(p₁² − p₂²)/(p₁·v₁)] at your reference point. Once fixed, the same constant predicts flow at other pressures for the same stage group, so it captures the group’s effective flow area.
Why is specific volume taken at the inlet?
The law was derived for compressible flow through a series of stages, where the inlet density sets how much steam a given pressure ratio can pass. Using the actual inlet temperature through the IAPWS-IF97 specific volume keeps the scaling correct when temperature changes along with pressure.