To summarize, the total height of clear liquid in the downcomer is the sum of four factors:
• Liquid escape velocity from the downcomer onto the tray below.
• Weir height.
• Crest height of liquid overflowing the outlet weir.
• The pressure drop of the vapor flowing through the tray above the downcomer.
Unfortunately, we do not have clear liquid, either in the downcomer, on the tray itself, or overflowing the weir. We actually have a froth or foam called aerated liquid. While the effect of this aeration on the specific gravity of the liquid is largely unknown and is a function of many complex factors (surface tension, dirt, tray design, etc.), an aeration factor of 50 percent is often used for many hydrocarbon services.
This means that if we calculated a clear liquid level of 12 in in our downcomer, then we would actually have a foam level in the downcomer of 12 in/0.50 = 24 in of foam.
If the height of the downcomer plus the height of the weir were 24 in, then a downcomer foam height of 24 in would correspond to downcomer flooding. This is sometimes called liquid flood.
This discussion assumes that the cross-sectional area of the downcomer is adequate for reasonable vapor-liquid separation. If the downcomer loading (GPM/ft2 of downcomer top area) is less than 150, this assumption is okay, at least for most clean services. For dirty, foamy services a downcomer loading of 100 GPM/ft2 would be safer.