I wonder if you are referring to "flows in long ducts" - a topic of any compressible flow text (e.g. "The Dynamics and Thermodynamics of Compressible Fluid Flow" - Ascher M. Shapiro)
Steam typical of many industrial applications is not superheated enough to be very close to a perfect gas. Any perfect gas analysis, such as in the text referenced above, will be in error, especially if the steam is a saturated mixture.
As TD2K has suggested, you should rephrase your question.