Fluid is no longer in the vapor phase because the fluid temperature/pressure is...

A Warning message in AFT Arrow

Full Message Text

There are several variations of this Warning:

  • Fluid is no longer in the vapor phase because the fluid pressure is above the Saturation Pressure for the following:

    Pipe X Inlet/Outlet, Actual Pressure = Y, Saturation Pressure = Z

  • Fluid is no longer in the vapor phase because the fluid temperature is below the Saturation Temperature for the following:

    Pipe X Inlet/Outlet, Actual Temperature = Y, Saturation Temperature = Z

Explanation

The steady-state temperature and pressure conditions at the specified pipes are such that one or more gas components has reached saturation conditions. When the fluid property conditions fall below saturation the solution accuracy will be decreased since one of the fundamental Engineering Assumptions is that the fluid is single-phase and superheated. The Solver will attempt to extrapolate gas properties below the saturation line.

The variations of the Warning depend on which fluid library is used and different programming methods are required due to differences in the library sources.

  • Variation for "pressure is above the Saturation Pressure" is only applicable to AFT Standard or User Library Fluids.

  • Variation for "temperature is below the Saturation Temperature" is applicable to ASME Steam, Chempak, and NIST REFPROP fluids.

Required Action

Adjust the model such that the fluid properties remain superheated, single-phase, and outside the saturation conditions. If this is not possible, then a larger uncertainty in results must be accepted and the results may not be accurate depending on how far the properties extend into saturation.

If there is flexibility in adjusting the model, there are various changes that could be made depending on the goal of the analysis and what is acceptable:

  • Check for typos or incorrect inputs in the Model Data (including units), especially at the boundary condition junctions or any components with significant temperature or pressure changes

  • Increase or decrease the upstream boundary condition temperature (such as the temperature in a Tank junction)

  • Adjust the heat transfer in pipes, or consider adding insulation

  • Run a scenario using a different fluid library if the fluid can be found in the other Library sources, and compare the results