It may be prudent to be conservative given the uncertainty of two-phase prediction models. Although the piping may be sized for all vapor flow, liquid in the vent line should not be ignored. The piping design must account for its possible presence. Piping should include drains at low points, sloping to drain, knock-out pots and the practice of connecting each subheader into the top of a downstream header. The amount of liquid may be considerable.
Vertical pipes create a special problem.
Two-phase flow is influenced by gravity. In severe cases, there may be no flow at all due to the liquid component collapsing under its weight. To obtain the minimum total cost, a number of scenarios may be studied. Consider the effect of different failure assumptions. As with any piping system, the route chosen for running the pipe can have a major impact on cost.