Designing piping for gravity flow

TLDR: In this paper, the authors consider the case of liquid flowing from the bottom of an absorption column through a pipe that has been sized for full liquid flow, and they show that when the liquid level in the column is low enough, the liquid entrains gas.

Abstract: Entrainment curtails liquid gravity flow from vessels by raising the pressure drop (above that for single-phase flow) through the outlet piping, and reducing the static head available for overcoming the pressure drop. A similar problem can arise when a liquid is near its boiling point, or contains dissolved gas, especially if the absolute pressure at any point in the piping falls below atmospheric pressure, as occurs in a syphon. This work considers the case of liquid flowing from the bottom of an absorption column through a pipe that has been sized for full liquid flow. When the liquid level in the column is low enough, the liquid entrains gas. The resulting increase in pressure drop and reduction of head restrict the flow rate, and the liquid level rises. Eventually, the level rises high enough to stop entrainment. However, gas still in the outlet pipe causes the level to continue to rise until the gas is all swept out. Now, the outlet pipe is running full flow (as was assumed in the design), but the static head, becoming higher than was assumed, creates excessive flow, which causes the level to fall until entrainment occurs again and the cycle is repeated.