There were static pressures below absolute zero in the model

A Critical Warning message in AFT Fathom

Full Message Text

There were static pressures below absolute zero in the model. While this is mathematically correct, it is not physically realistic. Please check the model.

Explanation

The Solver is mathematically capable of calculating negative absolute pressures which satisfy the fundamental equations, but this of course is not physically realistic. This message notifies the user of this unrealistic situation so that appropriate changes can be made to the model. If the model is based on a real functioning system, then all input data should be double checked. Furthermore, it may be possible that the real system is experiencing cavitation where the static pressure is dropping below the vapor pressure of the fluid at one or more locations. There could be other discrepancies such as worn components, pumps with lower efficiency, or fouling of components that affects the internal pipe areas, velocities, and pressure drops.

Required Action

The user must either correct the model such that this message is not displayed, or understand that all results are not physically possible and the current configuration of the model is not achievable in reality. Something in the model is causing the pressures to be too low which could be from any number of reasons:

  • A pipe or junction is providing too much pressure drop.

  • The flow demand from an Assigned Flow or Control Valve junction is too high causing too much friction loss.

  • The boundary conditions do not provide enough pressure differential.

  • Laminar corrections or Non-Newtonian viscosity models may be causing large pressure drops.

One of the best ways to start diagnosing the root cause is to review the Model Data. If all inputs for Analysis Setup, pipes, and junctions has been verified, it is recommended to make various Profile Graphs to visualize how the pressure changes throughout the various flow paths of the model. You may find a specific junction or pipe that is causing the pressure to drop below absolute zero.