The Froude number is a measure of whether vapor bubbles can disengage from liquids as they are flowing in a downward direction (in this app, assuming a normal pipe).
The Froude number is, in layman's terms, a measure of determining whether vapor bubbles will disengage from a liquid while flowing in a downward direction. If the Froude number is below 0.305 - 0.31, the bubbles will be able to disengage, or "self-vent", assuming that the upward path of the bubbles is free and open, and the fluid velocity decreases as the bubble travel back up to a larger area to be liberated from the liquid. The vapor density is often not needed, especially in low pressure systems, but can become significant if the Froude number is close to the 0.305 limit. Also, not good for highly visoucous liquids (e.g., syrups). When would you want to use this?
There are a multitude of places that too high of a Froude number can cause issues with lines surging in flow (possible symptoms that you'll see is swinging flow with flow controllers, or swings in pressure on the vessel that the fluid is leaving from); burping flows; on thermosiphon rebillers, perceived vapor lock (when really it's the issue of self-venting flow determined by the Froude number from where the liquid is coming); if a line is not "vented" adequately, it could be almost any system with downward liquid flow where vapor bubbles could get entrained.
My URL site listed is going to be updated with more examples, so if you're not sold, check that out as well for more descriptions and examples.
The app takes inputs of liquid flow (assuming downward direction only); liquid density; and pipe diameter you're flowing through. Vapor density can also be input but is not necessary. If the vapor density is low (e.g. atmospheric conditions), then you need not input a value. If the vapor density is high (e.g. hydrocarbons at 500 psig or higher), then you want to include the vapor density term, especially if you're close to 0.31 limit of Froude number.
The velocity of the liquid is first calculated by the app and reported. Simultaneously, then, the Froude number is calculated by dividing the velocity by the following term: (g * (pl - pg)/pg * D)^0.5.
g = gravitational constant, or 32.2 (ft/(s*s))
pl = liquid density, in (lb / ft3)
pv = vapor density, in (lb / ft3)
D = pipe diameter, input in program as inches
All the unit conversions are handled internally also.
Error messages are built in for bad data input, or flagging when you may have input a density wrong.
I have used this quick calculations in many places for many systems, and more than half the time this has been the "guilty part" for process upsets when I see downward flow direction from a vessel or other system upstream where vapor bubbles are capable of entering the down-pipe.
Have this in your hip pocket, you won't have to research it later or figure it out. Whip out the almighty iPhone during a meeting and run this quick calculation, and you can tell people immediately whether the system is susceptible to burping, surging flow, or other issues.
Notes are included in the app for quick reference.