The crude tower is the first separation on a refinery and sets the feed for everything downstream. It is a large-diameter column with multiple side draws and pumparounds, which makes its hydraulics considerably more complex than a simple two-product tower.

Capacity limits usually appear at a specific section rather than across the whole column — a flooded pumparound, or a wash zone that cannot handle the vapour load. Identifying which section is actually limiting is most of the engineering work.