In industrial distillation, the term “weeping” is primarily used when the liquid in sieve-tray columns starts leaking through the holes or perforations as a result of insufficient counter-current vapor flow. Weeping is undesirable because the liquid should flow across the tray and through the downcomer. This sets the lower limit of vapor velocity for sieve-tray column operation. Conversely, the term "flooding" is used in sieve-tray columns where the excessive liquid build-up in the column leads to a flooding condition. This is because the liquid cannot move down due to high vapor velocity. This sets the upper limit of vapor velocity for sieve-tray column operation. This article presents the procedure for calculating both flooding and weeping velocities. MORE

When plants are built with excessive design margins, problems can arise when operating under turndown conditions. This article helps explain how overdesign tends to occur and how to avoid problems when operating at turndown Operating under turndown can be problematic, especially where the plant has well-intentioned, but excessive design margins. An “over-designed” plant can help meet future demands on the plant, but can fail to perform during a turndown requirement. This article helps to understand some of the causes, consequences and mitigation measures needed to properly manage plant design for a turndown requirement. Establishing design margins on plants and equipment is a common practice, and they are provided for a variety of reasons. Among these are anticipated downtime, future deterioration expected for equipment in plants built to last 20 years, and of course, standard engineering practice. Engineers generally love design margins, because using them makes them feel safer. MORE