Changing the vapor pressure limit to compensate for altitude or fluid type

  Applies To 
  Product(s): HAMMER
  Version(s): V8i, CONNECT Edition
  Area:  Modeling
  Original Author: Jesse Dringoli, Bentley Technical Support Group


How can I change the vapor pressure limit to compensate for significant altitude, or for a different fluid or temperature? How does this affect the transient simulation results?

Solution - Fluid or Temperature

The default of -1 bar is for water at 20 C. If you're using a fluid other than water, the vapor pressure limit can be changed according to the values in the Liquid Engineering Library. To do this, click on the Vapor Pressure field in the properties of the Transient calculation options, then click the ellipsis button (...) next to it. Expand the liquid library, select the appropriate liquid, then click the Select button. You will notice that the Vapor Pressure field will change accordingly.

Solution - Altitude

In some cases you may be modeling a system at an extreme altitude, where the different in vapor pressure limit could be significant. Note that this question is independent from the software package and it is ultimately your decision as the engineer.

Model pressures are gage pressures, so the default of -14.35 psi is -0.977 atm, which is an absolute pressure of zero. (at an assumed elevation of roughly 600 ft) At an altitude of zero feet, the atmospheric pressure is 14.70 psi, which is exactly 1.0 atm, in which case the gage pressure would need to drop exactly 1.0 atm below zero (-14.7 psi) in order to reach zero absolute pressure and vaporize. At a higher altitude, atmospheric pressure is reduced; for example 11.4 psi vs 14.4 psi at 7000 ft. This means that gage pressure only needs to drop 11.4 psi below zero in order to reach an absolute pressure of zero and cause water to vaporize.

Impact on Transient Simulation

As for how this affects the transient simulation: when the pressure at any given point in the network drops below the vapor pressure limit set in the calculation options, a vapor pocket will form. So, if subatmospheric pressures are occurring in your system, selecting the right vapor pressure limit may be important for an accurate simulation.

For example, if your pressure drops to -1.01 bar and your liquid has a vapor pressure limit of -1.1 bar, then you will see a vapor pocket form with the default vapor pressure (-1.0), but will not see one form with your liquid's vapor pressure limit. This can certainly affect the minimum and maximum transient pressures, because vapor pockets can cause severe upsurge pressures when they collapse.

If you need to add a new liquid to the Liquid library, go to Components > Engineering Libraries, expand Liquid Libraries, right click LiquidLibrary.xml, choose "add item" and fill in the fields. This will now be available to choose from. You can also edit the vapor pressure values of existing liquid library entries if needed.

Of course, you always have the option of simply manually entering the appropriate vapor pressure limit, but you should also select the liquid in the Steady State/EPS Solver calculation options, so that the Initial Conditions are calculated correctly according to that liquid.

Note: as of version, the Vapor Pressure value reported in the transient Calculation Summary may not show the same value that you entered in the calculation options. This is due to a rounding issue and is only a reporting problem - the calculations will still use the exact value you entered in the calculation options. This will be resolved in a future version of HAMMER. Reference Defect #1095738. See: How to receive alerts on new version availability

See Also

Modeling fluids other than water

Air Pressure at Altitude Calculator